Steel Water Storage Tanks Design, Construction, Maintenance, And Repair

C H A P T E R 1Tank History, Typical

Configurations,Locating, Sizing,

and Selecting

Ira M. Gabin, P.E.Dixon Engineering

Richard A. Horn, P.E.CB&I

The rapid development and expansion of public water supply systemsat the beginning of the 20th century led to the establishment of publichealth standards for drinking water systems. An area of major con-cern for these systems was the storage facilities. Early steel reservoirsand standpipes were of riveted construction. Modern welded-steelreservoirs can be built to very large capacities with either domed orcolumn-supported roofs.

In the 1970s, it became common for smaller-capacity reservoirsand standpipes to use bolted construction technology, originally de-veloped for industrial and agricultural uses. Prefabricated panels andbolted connections reduced erection costs and made these structurespopular in rural areas. The advent of factory-applied ceramic coat-ings reduced future maintenance costs, adding to the tanks’ attrac-tiveness to water supply systems with limited financial resources.Bolted tanks with diameters greater than 30 ft (9 m) are often builtwith low-maintenance aluminum geodesic domed roofs, a technology

1

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Source: Steel Water Storage Tanks: Design, Construction, Maintenance, and Repair

2 C h a p t e r O n e

FIGURE 1-1 Geodesic dome on bolted-steel reservoir.

commonly found on wastewater plant storage tanks as well. Figure1-1 shows a geodesic dome on a bolted reservoir.

The earliest elevated storage tanks were constructed of wood inthe manner of water refilling stations for steam-powered trains. Somewere built on stone or brick columns. Limitations as to size and dura-bility, as well as public health concerns, led to steel becoming thematerial of choice for elevated tanks. Most steel elevated tanks con-structed before 1950 were riveted, their legs consisting of opposedchannels connected by latticework bracing. Roofs on most small tanksand many larger ones were the familiar cone or “witch’s hat” design(Fig. 1-2). Some larger elevated tanks had hemispherical or ellipsoidalroof designs.

Welded construction became the industry norm by the early 1950sand remains the standard for most elevated tanks. Legged tanks con-tinued to be built in great numbers; however, the lattice legs replacedtubular sections. Many larger-capacity legged tanks were of the radialarm design shown in Fig. 1-3. These have been phased out in favor ofthe toroelliptical legged tank style.

Early prototypes of single-pedestal tanks were developed in the1940s and became a common alternative to legged tanks by the 1950s.The more efficient shape of these structures provided the advantageof lower maintenance costs. In the 1960s, the fluted-column single-pedestal design was introduced, which provided a usable area in thecolumn for pumping equipment, storage, offices, and other municipaluses.

Legged tanks continue to be built primarily in sizes up to 1 milliongallons (mil gal) (3.8 million liters [ML]) as a lower-cost alternativeto single-pedestal or fluted-column tanks. Single-pedestal tanks arewidely specified from very small to large capacities. Larger capacities



Tank History, Typical Configurations, Locating, Sizing, and Selecting

3T a n k H i s t o r y , C o n f i g u r a t i o n s , L o c a t i n g , S i z i n g , S e l e c t i n g

FIGURE 1-2 Witch’shat roof design.

FIGURE 1-3 Legged tank with radial arm design.





(0.75 to 2 mil gal [2.8 to 7.6 ML] or more) are generally single-pedestalor fluted-column tanks. Some fluted-column tanks have even largercapacities.

In the late 1980s, composite-tank technology combined a concretepedestal with the steel-bowl geometry of the fluted-column tank. Thisaddressed one of the concerns of the fluted-column design—the largesteel surface area and resulting higher repainting costs. Built generallyto hold 0.75 to 2 mil gal (2.8 to 7.6 ML) of water, composite tanks arenow in use throughout the United States and Canada.

Other materials and technologies are available for specialized ap-plications. However, the steel, glass-lined steel, concrete, and com-posite tanks discussed in this chapter comprise the large majority oftanks currently in use and being specified for new construction.

ReservoirsA reservoir is a ground-supported, flat-bottom cylindrical tank with ashell height less than or equal to its diameter. Reservoirs are one of themost common types of water storage structure. They are used as a partof the distribution system as well as to hold treated water for pumpinginto the distribution system. Of the three types of steel water tanks, areservoir, because of its low height, is generally the most economicalto fabricate, erect, and maintain. See Figs. 1-4 and 1-5 for a photo anda cross-sectional view of a welded-steel reservoir; see Figs. 1-6 and 1-7for a photo and a cross-sectional view of a bolted-steel reservoir. Table1-1 gives typical sizes of welded-steel reservoirs, and Table 1-2 givescapacities of glass-coated, bolted-steel reservoirs and standpipes.

Storage reservoirs for potable water are covered by roof structures,which may be either column supported or self-supporting. Standardtank accessories may include shell and roof manholes, screened roofvents, inside or outside ladders, and connections for pipes as required.

StandpipesStandpipes are ground-supported, flat-bottom cylindrical storagetanks that are taller than their diameter. They are usually built wherethere is little elevated terrain and where extra height is needed to cre-ate pressure for water distribution. See Figs. 1-8 and 1-9 for a photoand a cross-sectional view of a welded-steel standpipe and Figs. 1-10and 1-11 for a photo and a cross-sectional view of a bolted-steel stand-pipe. Table 1-3 gives capacities and sizes of typical welded-steel stand-pipes.

Standpipe systems are often designed so that the water in the tank,until it reaches a certain low level, maintains the system pressure.When that low level is reached, pumps come on, valving is changed,





FIGURE 1-4 Welded-steel reservoir. (Photo: Gay Porter DeNileon, AWWA)

Capacity level

Roof manholesRoof vent

Approved ladder, cage, platform, or safety devices complying with Occupational Safety and Health Act

Roof rafters

Columnbases

Shell manholes(two required)

Overflowpipe

Splashpad

Inlet–outlet

(optional)

Base elbow or valve pit

Weir box(optional)

Column support

Tank bottomcrowned at center

Sand pad

Compacted backfill

Crushed rock or gravel Concrete foundation

12 in. (0.3 m)

3 ⁄4 in. (19 mm)

FIGURE 1-5 Cross-sectional view of welded-steel reservoir. (Source: AWWAManual M42, Steel Water-Storage Tanks)





FIGURE 1-6 Bolted-steel reservoir, glass fused to steel.

Approved ladder, cage, and platform

complying with Occupational Safety

and Health Act

Roof manway Gravity

ventilatorInternal

overflow

funnel

Overflowpipe

Splash

pad

Gradelevel

Inlet–outlet

24-in. (0.6-m) round

access door

Floor sloped toward

outlet pipe

FIGURE 1-7 Cross-sectional view of bolted-steel reservoir. (Source: AWWAManual M42, Steel Water-Storage Tanks)




Capacity Range of Sizes Available

Diameter Height to Diameter Height to(US gal) (m3) (ft [in.]) TCL (ft [in.]) (m) TCL (m)

50,000 189 19 [3] 24 [0] 5.9 7.3

60,000 227 21 [0] 24 [0] 6.4 7.3

75,000 284 23 [6] 24 [0] 7.2 7.3

100,000 379 23 [6] 32 [0] 7.2 9.8

27 [0] 24 [0] 8.2 7.3

125,000 473 26 [0] 32 [0] 7.9 9.8

30 [3] 24 [0] 9.2 7.3

150,000 568 28 [6] 32 [0] 8.7 9.8

33 [0] 24 [0] 10.0 7.3

200,000 757 33 [0] 32 [0] 10.0 9.8

38 [3] 24 [0] 11.7 7.3

250,000 946 37 [0] 32 [0] 11.3 9.8

42 [9] 24 [0] 13.0 7.3

300,000 1,136 40 [6] 32 [0] 12.3 9.8

46 [9] 24 [0] 14.3 7.3

400,000 1,515 46 [6] 32 [0] 14.2 9.8

54 [0] 24 [0] 16.5 7.3

500,000 1,893 46 [6] 40 [0] 14.2 12.2

52 [0] 32 [0] 15.9 9.8

60 [6] 24 [0] 18.4 7.3

600,000 2,271 51 [0] 40 [0] 15.6 12.2

57 [0] 32 [0] 17.4 9.8

750,000 2,839 57 [0] 40 [0] 17.4 12.2

64 [0] 32 [0] 19.5 9.8

1,000,000 3,785 66 [0] 40 [0] 20.1 12.2

74 [0] 32 [0] 22.6 9.8

1,500,000 5,678 80 [6] 40 [0] 24.5 12.2

90 [6] 32 [0] 27.6 9.8

2,000,000 7,571 93 [0] 40 [0] 28.4 12.2

104 [6] 32 [0] 31.9 9.8

3,000,000 11,356 114 [0] 40 [0] 34.7 12.2

127 [6] 32 [0] 38.9 9.8

4,000,000 15,142 131 [6] 40 [0] 40.1 12.2

147 [6] 32 [0] 44.9 9.8

5,000,000 18,927 147 [0] 40 [0] 44.8 12.2

165 [0] 32 [0] 50.3 9.8

7,500,000 28,391 180 [0] 40 [0] 54.9 12.2

201 [6] 32 [0] 61.4 9.8

10,000,000 37,854 233 [0] 32 [0] 71.0 9.8

208 [0] 40 [0] 63.5 12.2

Source: AWWA Manual M42, Steel Water-Storage Tanks.Note: TCL = top capacity level.

TABLE 1-1 Capacities and Sizes of Typical Welded-Steel Water-StorageReservoirs

7




Nom

inalH

eig

ht

(ft)

∗

15

19

24

28

33

38

43

47

52

57

61

66

70

75

79

84

89

93

98

102

107

112

116

121

Nom

inal

Dia

mete

r(f

t)∗

Capacit

yin

Thousands

ofG

allons†

14

16

22

27

32

37

44

49

54

59

65

70

75

80

86

91

96

10

11

07

11

21

17

12

21

28

13

31

39

17

24

31

39

47

54

63

70

78

86

93

10

11

08

11

61

23

13

11

39

14

61

54

16

11

69

17

71

84

19

21

99

20

33

43

53

64

74

86

96

10

61

17

12

21

37

14

81

58

16

81

79

18

91

99

21

02

20

23

02

41

25

12

61

27

2

25

54

71

88

10

51

22

14

21

59

17

61

93

21

02

27

24

42

61

27

82

96

31

33

30

34

73

64

31

81

10

71

32

15

81

83

21

22

38

26

32

89

32

03

40

36

53

91

41

64

42

36

11

41

49

18

52

20

25

62

92

32

73

63

39

84

34

46

95

05

42

15

11

99

24

62

94

34

13

88

43

64

83

53

15

78

50

21

82

86

35

54

23

49

15

59

62

86

96

62

32

63

26

42

85

30

63

27

34

83

6

70

42

15

53

68

58

16

94

8

81

56

77

44

92

11

,09

9

90

69

19

06

1,1

22

1,3

37

10

18

74

1,1

47

1,4

20

12

01

,24

71

,63

7

∗To

conv

ertf

eett

om

eter

s,m

ulti

ply

by0.

3048

.†

Cap

acit

yin

thou

sand

sof

gallo

ns.T

oco

nver

tgal

lons

tocu

bic

met

ers,

mul

tipl

yby

0.00

3785

4.So

urce

:AW

WA

Man

ualM

42,S

teel

Wat

er-S

tora

geTa

nks.

TA

BLE

1-2

Capaci

ties

ofG

lass-C

oate

d,B

olted-S

teelR

eserv

oirs

and

Sta

ndpip

es

8





FIGURE 1-8Welded-steelstandpipe withdecorativepilasters.

and water is pumped from the lower portion of the standpipe into thesystem.

As with reservoirs, steel standpipes are covered with a roof struc-ture and may be provided with ornamental trim. Standard accessoriesmay include shell and roof manholes, roof vent(s), a fixed outsideladder, and connections or pipes as required. Inside ladders are notrecommended in locations where freezing weather can be expected.

Roof Designs for Reservoirs and StandpipesThe emphasis on making steel water reservoirs and standpipes at-tractive as well as functional has led to the development of a widevariety of roof designs. Alternative roof styles for welded tanks in-clude conical, toriconical, umbrella, dome, and ellipsoidal designs.Some are column supported; others are self-supporting. Bolted-steeltanks are usually provided with conical roofs or may be furnishedwith an aluminum geodesic dome. Column-supported roof struc-tures are not usually used on steel standpipes taller than 50 ft (15 m).Whichever design is selected, it is particularly important to designany rafters, trusses, columns, stiffeners, and connections to minimizepotential corrosion sites. All interfaces and connections of such mem-bers should be analyzed for their corrosion potential, and protective





Approved ladder, cage, platform, or safety devices complying with Occupational Safety and Health Act

Painter’s trolley rail

Weir box(optional)

Capacity level

Concretefoundation

Sand pad

Crushed rock or gravel

Compacted backfillor undisturbed soil

Shell manholes(two required)

Tank bottomcrowned at center

Inlet–outlet(optional)

Base elbow orvalve pit

Splashpad

Roof ventRoof plate

Roof manholes

Overflow pipe

FIGURE 1-9 Cross-sectional view of typical welded-steel standpipe.(Source: AWWA Manual M42, Steel Water-Storage Tanks)

coatings should be applied to all surfaces deemed necessary from acost/benefit standpoint.

Column- and Rafter-Supported Cone RoofsThe column- and rafter-supported roof (Fig. 1-12) is generally the mosteconomical for a reservoir. The roof has a minimum slope for adequatedrainage and provides easy access to the manhole for interior inspec-tion. Column loads are spread to a safe limit by column bases, andconcrete footings under the columns are not usually required.

A modification of this design incorporates a transition from theshell plate to the roof plate that is a smooth curve rather than a sharpbreak. This transition, or knuckle plate, is a dished or rolled section thatusually requires a stiffener at the rafter attachment point (Fig. 1-13).




FIGURE 1-10 Bolted-steel standpipe.

Approved ladder, cage, and platform complying with Occupational Safety and Health Act

Roof accessGravityventilator

Internaloverflowfunnel

Overflowpipe

Splash

pad

Gradelevel

Inlet–outlet(optional)

24-in. (0.6-m) roundaccess door

Floor sloped towardoutlet pipe

Topelbow

Roof walkway and guard rail

FIGURE 1-11 Cross-sectional view of bolted-steel standpipe. (Source: AWWAManual M42, Steel Water-Storage Tanks)






Diameter Height to Diameter Height to

(US gal) (m3) (ft [in.]) TCL (ft [in.]) (m) TCL (m)

50,000 189 14 [9] 40 [0] 4.5 12.2

60,000 227 16 [2] 40 [0] 4.9 12.2

75,000 284 18 [0] 40 [0] 5.5 12.2

100,000 379 19 [0] 48 [0] 5.8 14.6

125,000 473 21 [3] 48 [0] 6.5 14.6

150,000 568 23 [3] 48 [0] 7.1 14.6

200,000 757 24 [10] 56 [0] 7.6 17.1

250,000 946 27 [9] 56 [0] 8.5 17.1

300,000 1,136 28 [5] 64 [0] 8.7 19.5

400,000 1,514 32 [10] 64 [0] 10.0 19.5

500,000 1,893 34 [7] 72 [0] 10.5 21.9

600,000 2,271 37 [10] 72 [0] 11.5 21.9

750,000 2,839 42 [6] 72 [0] 12.9 21.9

1,000,000 3,785 46 [4] 80 [0] 14.1 24.4

1,500,000 5,678 56 [9] 80 [0] 17.3 24.4

2,000,000 7,571 65 [6] 80 [0] 20.0 24.4

2,500,000 9,464 69 [10] 88 [0] 21.3 26.8

3,000,000 11,356 76 [6] 88 [0] 23.3 26.8

4,000,000 15,142 84 [6] 96 [0] 25.8 29.3

5,000,000 18,927 94 [6] 96 [0] 28.8 29.3

Source: AWWA Manual M42, Steel Water-Storage Tanks.Note: TCL = top capacity level.

TABLE 1-3 Capacities and Sizes of Typical Welded-Steel Standpipes

Self-Supporting Dome Roof and Umbrella RoofSteel self-supporting roofs are constructed of plates that are buttwelded, lap welded, or lap bolted. They are supported directly onthe top angle and shell plate. This type of roof is used where an un-cluttered interior and smooth exterior appearance are desired. Dome-roof sections are pressed to form a spherical shape. Umbrella roofs areformed to a radius in one direction only, forming chords like the clothbetween the spines of an umbrella (Fig. 1-14).

Structural stiffeners may be used internally on large-diameterroofs to avoid excessive plate thickness on welded or bolted tanks.Sometimes steel trusses may be used to support the roof, but these





Capacitylevel

Vent

Columnbase

Girders arerequired whenmore than onecolumn is used

12 in. (0.3 m)

3⁄4 in. (19 mm)

3 ⁄16-in. (4.7-mm) lap-welded roof plate

Channelrafters

1 ⁄4-in. (6.4-mm)

lap-welded

bottom plate

One or moresupportingcolumns

Top angle

Butt-weldedtank shell

FIGURE 1-12 Tank with column- and rafter-supported cone roof. (Source:AWWA Manual M42, Steel Water-Storage Tanks)

should be avoided if possible, because they may create corrosionproblems. In addition, the trusses should be kept above the waterline to prevent damage by ice and accelerated rates of corrosion.

A modification of the self-supporting dome is the toriconical roof.This consists of a rolled or pressed knuckle and a higher-pitched self-supporting center.

Aluminum dome roofs are sometimes erected on bolted-steel orwelded-steel tanks. These aluminum domes are usually constructed

Knuckle plate 12 in. (0.3 m)

¾ in. (19 mm)

Radius


Channelrafter

Columnbase

Capacity

level

One or moresupportingcolumns

3/16 in. (4.7-mm)

lap-welded roof plate

¼-in. (6.4-mm)lap-weldedbottom plate

FIGURE 1-13 Column- and rafter-supported roof with knuckle. (Source: AWWAManual M42, Steel Water-Storage Tanks)





Capacity level

Vent

3 ⁄16-in. (4.7-mm) minimum thicknesslap- or butt-weldedroof plate

1 ⁄4-in. (6.4-mm) lap-weldedbottom plate


Topangle

Spherical radius

= 1.2 D. max.

0.80 D. min.

Cap plate

FIGURE 1-14 Self-supporting dome roof or umbrella roof. (Source: AWWAManual M42, Steel Water-Storage Tanks)

of triangulated space truss (geodesic) panels. The dead weight of thesedomes is usually 3 lb/ft2 (143 N/m2) or less, compared with 3.8 lb/ft2

(181 N/m2) for a bolted-steel roof and 7.6 lb/ft2 (364 N/m2) for awelded-steel roof.

Self-Supporting Ellipsoidal RoofThe self-supporting ellipsoidal roof is not a true ellipse, but it is formedwith two radii yielding major- and minor-axis proportions of approxi-mately 2:1. The transition from shell to roof is a smooth unbroken curve(Fig. 1-15). This roof design is suitable for large- and small-diameterreservoirs and standpipes. On tanks 50 ft (15 m) in diameter or less,the roof is usually free of internal structural members. Larger-diametertanks usually have radial and circumferential stiffening members orrafters, which may be subject to corrosion problems if they are notproperly designed or maintained.

Self-Supporting Cone RoofAn inexpensive and very functional type of roof for small-diameterreservoirs and standpipes is the self-supporting cone roof without in-ternal structural members. This roof is usually too steep to walk on.Access to manholes and vents by a roof ladder or steps and handrailshould be provided. All means of access should be designed individ-ually and installed to comply with current standards.





Capacity level

Vent1⁄4-in. (6.4-mm) minimum thicknessand butt welded in areafilled with water

Area abovecapacity levelmay be lap welded

1 ⁄4-in. (6.4-mm) lap-weldedbottom plate


Knu

ckle

FIGURE 1-15 Self-supporting ellipsoidal roof. (Source: AWWA Manual M42,Steel Water-Storage Tanks)

Elevated TanksAn elevated steel water tank has two primary components: the tank it-self and its supporting structure. Such tanks are ordinarily used wherethere is insufficient elevated terrain to ensure distribution of water atsuitable pressure by gravity. These tanks are of welded construction.

Elevated tanks can be categorized into several different types. Thevarious diameters and head ranges for the tanks described in the re-maining figures and tables in this chapter are only representative andmay vary with individual fabricators. Specific diameter/head rangecombinations should be determined by the tank fabricator within thelimits indicated in the tables. Height should be specified by the pur-chaser as the dimension between the top of the foundation and the topcapacity level of the tank. Further dimensions, which are a function ofthe fabricator’s standard, should not be specified. To minimize cost,desired operating ranges should be specified to fall within standardavailable tank dimensions. However, individual operating needs maydictate nonstandard operating ranges.

Multiple-Column Elevated Tanks

Small-Capacity Elevated (Double-Ellipsoidal) TanksThe small-capacity multiple-column elevated (or double-ellipsoidal)tank has a cylindrical sidewall, an ellipsoidal bottom and roof, and a





FIGURE 1-16Double-ellipsoidaltank. (Photo: GayPorter DeNileon,AWWA)

top capacity level (TCL) in the roof several feet or meters above thetop of the cylindrical shell. Although in the past they were constructedin capacities up to 1 mil gal (3.8 ML), today, double-ellipsoidal tanksare typically constructed only in capacities of 200,000 gal (760,000 L)or less. See Figs. 1-16 and 1-17 for a photo and a cross-sectional viewof a small-capacity elevated (double-ellipsoidal) tank. Table 1-4 givescapacities and sizes of typical double-ellipsoidal elevated tanks.

Medium-Capacity Elevated TanksFor medium-capacity multiple-column elevated tanks, the toroellip-soidal design provides a lower initial cost by using the strength of steelmost efficiently. The features used (torus bottom and ellipsoidal roof)cause the central riser to support, as well as contain, a considerableportion of the stored water, while the major portion of the steel bottomacts as a membrane in tension. These tanks usually have a capacitybetween 200,000 gal (760,000 L) and 500,000 gal (1.9 ML). See Figs.1-18 and 1-19 for a photo and a cross-sectional view of a medium-capacity elevated tank. Table 1-5 gives capacities and sizes of typicalmedium-capacity elevated tanks.





Diameter

Headrange

As r

equired

Purc

haser

to s

pe

cify

6 in. (0.15 m) min.

Balcony orstiffening girder

FIGURE 1-17 Cross-sectional view of double-ellipsoidal tank. (Source: AWWAManual M42, Steel Water-Storage Tanks)

Large-Capacity Multiple-Column Elevated TanksLarge-capacity elevated tanks (>500,000 gal [>1,893 m3]) provide eco-nomical service for communities that need to store a substantial vol-ume of water. Lower operating and pumping costs are ensured be-cause of the low head range, which achieves minimum variation of wa-ter pressure throughout the system. See Figs. 1-20 and 1-21 for a photoand a cross-sectional view of a large-capacity elevated tank. Table 1-6gives capacities and sizes of typical large-capacity elevated tanks.

Pedestal Elevated Tanks

Small-Capacity Single-Pedestal TanksThe single-pedestal spherical tank is widely favored for smaller-capacity tanks when appearance is a concern. The gracefully flaredbase contains sufficient space for pumping units and other operatingequipment, a feature common to all pedestal-type vessels.






Diameter Head Diameter Head

(US gal) (m3) (ft) Range (ft) (m) Range (m)

25,000 95 18–20 12.5–15.5 5.5–6.1 3.3–4.7

30,000 114 18–20 15.0–16.5 5.5–6.1 4.6–5.0

40,000 151 22–23 15.0–17.0 5.7–7.0 4.6–5.2

50,000 189 22–24 18.0–20.0 6.7–7.3 5.5–6.1

60,000 227 22–25 19.0–23.0 6.7–7.6 5.3–7.0

75,000 284 26–30 16.0–24.0 7.9–9.1 4.9–7.3

100,000 379 23–30 20.0–25.0 3.5–9.1 6.1–7.6

125,000 473 30–32 23.0–28.0 9.1–9.7 7.0–8.5

150,000 568 32–34 24.5–29.5 9.7–10.4 7.5–9.0

200,000 757 36–38 28.0–29.5 11.0–11.6 8.5–9.0

Source: AWWA Manual M42, Steel Water-Storage Tanks.

TABLE 1-4 Capacities and Sizes of Typical Double-Ellipsoidal Elevated Tanks

FIGURE 1-18Medium-capacitywelded-steelelevated tank.(Photo: Gay PorterDeNileon, AWWA)





Balcony orstiffening girder

6 in. min.

HeadRange

Purc

haser

to s

pe

cify

As r

equired

FIGURE 1-19 Cross-sectional view of medium-capacity, torus-bottom welded-steel elevated tank. (Source: AWWA Manual M42, Steel Water-Storage Tanks)

Ladders to the container and roof are inside to protect againstunauthorized access. These tanks are usually constructed in capacitiesof 200,000 gal (760,000 L) or less. See Figs. 1-22 and 1-23 for a photo anda cross-sectional view of a small-capacity single-pedestal tank. Table1-7 gives capacities and sizes of typical small-capacity single-pedestaltanks.

Small-capacity elevated tanks are also constructed as various com-binations of cones and cylinders. An alternative design is shown inFig. 1-24.

Large-Capacity Single-Pedestal TanksThe tubular supporting pedestal gives the large-capacity single-pedestal tank a distinctively contemporary look. Large capacities (0.2to 2 mil gal [0.76 to 7.6 ML]) are provided by this low-head-rangespheroidal tank design. See Figs. 1-25 and 1-26 for a photo and across-sectional view of a large-capacity single-pedestal tank. Table 1-8






Diameter Height to Diameter Height to

(US gal) (m3) (ft) TCL (ft [in.]) (m) TCL (m)

200,000 757 36–38 28 [30] 11.0–11.6 8.5–9.1

250,000 946 38–40 28 [33] 11.6–12.2 8.5–10.1

300,000 1,136 43–45 28 [31] 13.1–13.7 8.5–9.4

400,000 1,514 46–50 30 [36] 14.0–15.2 9.1–11.0

500,000 1,893 50–56 29 [38] 15.2–17.1 8.8–11.5

600,000 2,271 51–0 40 [0] 15.6 12.2

57–0 32 [0] 17.4 9.8

750,000 2,839 56–65 34 [45] 17.1–19.8 10.4–13.7

1,000,000 3,785 64–65 45 [46] 19.5–19.8 13.7–14.0


TABLE 1-5 Capacities and Sizes of Typical Medium-Capacity Elevated Tanks

FIGURE 1-20 Large-capacity elevated tank. (Photo courtesy of LandmarkStructures, Inc.)





Purc

haser

to s

pecify

As r

equired

Headrange

Diameter

6 in. (0.15 m) min.

FIGURE 1-21 Cross-sectional view of large-capacity, multicolumn elevatedtank. (Source: AWWA Manual M42, Steel Water-Storage Tanks)




500,000 1,893 60–65 24–25 18.3–19.8 7.3–7.9

600,000 2,271 65–70 24–25 19.8–21.3 7.3–7.9

750,000 2,839 70–76 25–30 21.3–23.2 7.6–9.1

1,000,000 3,785 75–87 25–35 22.9–25.5 7.6–10.7

1,500,000 5,678 91–98 30–35 27.7–29.9 9.1–10.7

2,000,000 7,571 105–106 34–36 32.0–32.3 10.4–11.0

2,500,000 9,464 108–117 39–41 32.9–35.7 11.9–12.5

3,000,000 11,356 119–127 35–40 36.3–38.7 10.7–12.2


TABLE 1-6 Capacities and Sizes of Typical Large-Capacity Welded-SteelElevated Tanks





FIGURE 1-22Sphericalsingle-pedestaltanks give pleasantsilhouette. (Photo:Walter Baas,AWWA)

gives capacities and sizes of typical large-capacity single-pedestaltanks.

Modified Single-Pedestal TanksThe attractive modified single-pedestal tank has a central support col-umn (usually fluted to give structural rigidity) that encloses the riserpipe, overflow pipe, and access ladder to the tank roof. The supportcolumn may be constructed of steel or concrete. The space within thecolumn can provide multistory usable floor space for pumping, stor-age, and office facilities. Although available in all capacities, thesetanks are not usually constructed in capacities less than 500,000 gal(1.9 ML). See Figs. 1-27 and 1-28 for a photo and a cross-sectional viewof a modified single-pedestal tank. Table 1-9 gives capacities and sizesof typical modified single-pedestal tanks.

Composite Elevated TanksComposite elevated tanks are of an attractive design that uses thebest design features of steel and concrete. Concrete, which is excellentfor compression loads, is used as the support column for the steelbowl. The concrete has the advantage of requiring either no painting




Head

range

As r

equired

Purc

haser

to s

pecify

6 in. (0.15 m) min.

Diameter

FIGURE 1-23 Cross-sectional view of small-capacity spherical single-pedestaltank. (Source: AWWA Manual M42, Steel Water-Storage Tanks)




25,000 95 19–20 15–17 5.8–6.1 4.6–5.2

30,000 114 20–21 15–18 6.1–6.4 4.6–5.5

40,000 151 21–23 19–22 6.4–7.0 5.8–6.7

50,000 189 23–24 19–23 7.0–7.3 5.8–7.0

60,000 227 24–26 22–24 7.3–7.9 6.7–7.3

75,000 284 25–28 23–27 7.9–8.5 7.0–8.2

100,000 379 29–30 25–30 8.8–9.1 7.6–9.1

125,000 473 31–33 27–32 9.4–10.0 8.2–9.7

150,000 568 33–34 30–34 10.1–10.4 9.1–10.4

200,000 757 36–38 36–38 11.0–11.6 11.0–11.6


TABLE 1-7 Capacities and Sizes of Typical Small-Capacity Single-PedestalTanks

23





FIGURE 1-24 Alternative single-pedestal tank design.

or a low-cost exterior coating for aesthetic purposes. The steel bowlconstruction is similar to that found on the fluted-column tanks; thebowl can be built with either a cone or a domed roof. The most commondesigns use a domed concrete floor with a steel liner. Commonly builtto store between 750,000 gal and 2 mil gal (2.8 and 7.6 ML), these tanksprovide many of the benefits of a fluted-column tank with significantlyless area that requires painting, thereby reducing maintenance costs.The diameter of the concrete column is generally somewhat smaller(30 to 60 ft [9 to 18 m]) than for a fluted-column tank, so the area in thecolumn for other uses is reduced. See Figs. 1-29 and 1-30 for a photoand cross-sectional view of a composite elevated tank.

Locating, Sizing, and Selecting a Water TankLocating, sizing, and selecting a water-storage tank involve the eval-uation of several design considerations and require an awareness ofzoning and other regulations. The purpose of this section is to dis-cuss these considerations and to provide the reader with a checklistto work through in the effort to arrive at a reasonable solution.





FIGURE 1-25 Large-capacity single-pedestal elevated tank. (Photo courtesy ofTnemec/STI/SPFA)

Locating a Water TankGenerally, locating tanks depends on where people are living now andwhere future neighborhoods will be built within the area served bythe water system. In addition, numerous other conditions can signif-icantly influence the choice of a suitable site and therefore the overallcost of the tank project. Answers to the following basic questions mustbe determined and considered when selecting a location for a newwater-storage tank.

Hydraulics� What are the maximum and minimum pressures that youwant to provide the end users?� Is it better to pump or use gravity flow to provide the neededpressure?� What are the local utility costs of pumping during daily andpeak demand periods?

Proximity to Users� Where is the growth in the community taking place now andprojected to be in the future?� Is land available in the area of future growth?





Access

tube

6 in. (0.15 m) min.

Head

range

Diameter

As r

equired

Purc

haser

to s

pecify

FIGURE 1-26 Cross-sectional view of large-capacity single-pedestal elevatedtank. (Source: AWWA Manual 42, Steel Water-Storage Tanks)




200,000 757 40–42 27–30 12.2–12.8 8.2–9.1

250,000 946 43–47 25–32 13.1–14.3 7.6–9.7

300,000 1,136 46–48 30–33 14.0–14.6 9.1–10.1

400,000 1,514 50–53 30–40 15.2–16.1 9.1–12.2

500,000 1,893 55–60 30–40 16.3–18.3 9.1–12.2

750,000 2,839 64–66 38–42 19.5–20.1 11.6–12.3

1,000,000 3,785 74–78 35–40 22.5–23.8 10.7–12.2

1,250,000 4,732 76–80 40–45 22.9–24.4 12.2–13.7

1,500,000 5,678 85–90 45–50 25.9–27.4 13.7–15.2

2,000,000 7,571 90–95 50–55 27.4–29.0 15.2–16.3


TABLE 1-8 Capacities and Sizes of Typical Large-Capacity Single-PedestalTanks





FIGURE 1-27 Folded-plate design of a modified single-pedestal tank support.(Photo courtesy of Tnemec/STI/SPFA)

Acquiring Land� What is the cost of the tank site being considered? Is the landeven available?� What is the cost of connecting water mains and permanentelectrical power at each site being considered?

Zoning� Is a zoning map available, and are the potential sites zonedto allow a tank project?

Federal Aviation Administration (FAA)� Would the FAA allow a tank at the required height to be builton the potential site?� Are obstruction lights or FAA painting required on the tankat the potential site?

Size of SiteIs the site large enough for� Erection equipment, steel storage, staging operations, ground

assembly, and crane operations with a safe and adequate dis-tance for items that may be dropped from the tank duringerection?





Fluted

column

Diameter

Head

range

As r

equired

Purc

haser

to s

pecify

FIGURE 1-28 Cross-sectional view of a modified single-pedestal tank.




250,000 946 41–43 29–31 12.5–13.1 8.8–9.4

300,000 1,136 43–45 29–31 13.1–13.7 8.8–9.4

500,000 1,893 49–64 30–39 14.9–19.5 9.1–11.9

750,000 2,839 63–65 37–40 19.2–19.8 11.3–12.2

1,000,000 3,785 73–78 35–42 22.2–23.8 10.7–12.8

1,250,000 4,732 76–80 40–45 22.9–24.4 12.2–13.7

1,500,000 5,678 85–87 39–46 25.9–26.5 11.9–14.0

2,000,000 7,571 97–102 38–46 29.6–31.1 11.6–14.0

2,500,000 9,464 107–110 43–45 32.6–33.5 13.1–13.7

3,000,000 11,356 109–120 40–45 33.3–36.6 12.2–13.7


TABLE 1-9 Capacities and Sizes of Typical Modified Single-Pedestal Tanks





FIGURE 1-29 Composite elevated tank.� Maintenance of the tank and piping after completion?� Abrasive blasting and painting of the tank now and in thefuture?

TopographyDoes the site have—or can it be made to have—good drainageto ease construction operations and minimize standing wateraround the completed tank?

Access to Site� Is the site accessible on public roads by concrete and largesemitrailer tractor rigs?� Is there an access road or temporary easement to the site?Will permission be given to build a road? Who will pay for theroad? Will it be a permanent or temporary road? If temporary,will it be necessary to remove it at the end of the project?

Soil Conditions� Is the soil bearing strength at the bottom of the tank founda-tion adequate to support the tank without requiring expen-sive deep foundations?





Upper roofcone

Concretesupport dome

Low water line

Steel bottomplate

High waterline

Lower cone

Concrete column

Access tube

Note: Not to scale.

FIGURE 1-30 Cross-sectional view of composite elevated tank.

� Where is the water table? Will the foundation need to be de-watered during construction?� Is the earth firm enough to support construction equipmentduring normal weather conditions or will gravel, crane mats,and other earth-stabilizing methods be required?

Hazards and Construction� Are there power lines or other obstructions above or besidethe site or proposed access road that would interfere with thesafety of site traffic, construction, painting, or maintenanceoperations? Will a power line be closer than 40 ft (12.2 m)from the tank?� Are there underground obstructions such as gas lines, sewers,or buried electrical or telephone lines? Were there mines orburial grounds on this site?





� If pile driving is required, will it disturb or cause failure of ordamage to neighboring foundations or other structures?� Will pile driving, excavation, steel erection, or abrasive blast-ing cause noise unacceptable to a neighbor such as a school,hospital, or nursing home?� Will the tank be in an area frequented by small children orvandals and, if so, could this be mitigated by site fencing?

Environmental Assessment� Has an environmental assessment been completed on the site?� What agencies, forms, and permits may be required, and howlong will approvals take?

NIMBY (Not in My Backyard)� Will the tank obstruct the view of historical landmarks orother items of concern to the citizens?� How sensitive are the neighbors to having a tank in closeproximity?

Determining answers to these questions can help you to betteranalyze and compare costs of alternate sites, so you can select themost desirable location for your new tank. Additionally, you will wantto understand and consider the following criteria during your siteselection.

HydraulicsOther issues that affect site selection include the required pressureat hydrants and residences, the required site elevation, compatibilitywith the distribution system, the geographic size and location of thedemand area, and the tank’s proximity to the water supply. Rules ofthumb for required water pressure are shown in Table 1-10. Check thelocal standards or codes for more specific requirements.

One hundred sixty-two US and Canadian water utilities re-sponded to an AWWA network modeling survey that requested theactual minimum and maximum distribution system pressures that

Pressure

Location (psi/kPa) Comments

At hydrants

during fire flow

conditions

35/241 20-psi (0.138-kPa) minimum at

other fire hydrants not directly

serving the fire

Residential 50–75/ Higher pressures may need to

0.345–0.517 use a pressure-reducing valve

TABLE 1-10 Required Water Pressure





0

20

40

60

80

100

0

20

40

60

80

100

Minimum pressure Maximum pressure

Min. psi Max. psi

Perc

ent

Perc

ent62%

4%

10%

28%

34%

23%

1%

55%

10%

12%

25%

15%

14%

18%

6%

> 60 psi

50–59 psi

40–49 psi

30–39 psi

20–29 psi

< 20 psi

> 170 psi

150–169 psi

130–149 psi

110–129 psi

90–109 psi

70–89 psi

< 70 psi

FIGURE 1-31 Pressure ranges for utilities.

they provided. Figure 1-31 shows the percentage of utilities in eachpressure range. If the pressures provided are more than 75 psi (0.517kPa), it may be necessary to provide a pressure-reducing valve to pre-vent home appliances from being overpressurized.

The required pressure can be provided through pumping, gravityflow, or a combination of the two. How pressure is provided dependson the sites available and the type of tank to be used.

Pumping with Ground Storage TanksPumping will be required if a ground storage tank is used where thetopography is relatively flat throughout the service area and a higher-elevation site is unavailable.

Gravity Flow with Ground Storage and Elevated TanksThe required pressure can also be obtained by building a ground stor-age tank on a hill or at higher elevation above the demand area so thatgravity flow provides the pressure, much like a water cooler.

An elevated tank provides the required pressure by raising thewater storage height up to an elevation above the demand area sothat gravity can provide the pressure. Costs can be lessened if theelevated tank is also constructed on a hill site or at higher elevation.This not only lessens the necessary height of the tank but also canreduce its cost.





Gravity Flow and Pumping with a StandpipeWater could also be stored in a standpipe (a tall cylindrical tank) wherethe topography is relatively flat throughout the service area and a hillor higher-elevation site is unavailable. In a full standpipe, the upper-most one-third of water stored provides effective pressure for gravityflow. If the tank is two-thirds full, the upper half of the water wouldprovide emergency pressure. In a tank only one-third full, the waterprovides little or no pressure (i.e., ineffective pressure) and wouldhave to be pumped to be used.

Much like with an elevated tank, costs can be saved if the stand-pipe is constructed on a higher-elevation site or hill. This not onlylessens the necessary height of the tank but also can reduce its cost.

Gravity Flow Height CalculationsFollowing is an example of how to calculate the minimum height atwhich to store water to provide an assumed minimum pressure forresidential use through gravity flow. (Check your local standards orcodes.)

Height (for 50 psi [345 kPa] minimum)

= 50 psi [345 kPa]

(62.4 lb/ft3/144 sq in./sq ft)

= 115.4 ft [35.1 m] (≈ 115 ft [≈ 35 m])

orHeight (for 50 psi [345 kPa] minimum)

= 50 psi [345 kPa](62.4 lb/ft3/144 sq in./sq ft)

= 115.4 ft [35.1 m]

To this calculated height, add the additional height required tomeet the friction loss of the water in the distribution piping.

Alternatively, one can use a conversion chart to find the requiredheight at which to store the water to provide the pressure needed.Figure 1-32 shows how various types of tanks provide this pressureusing gravity flow.

Pumping Versus Gravity FlowPumping If a site with an increased elevation of at least 115 ft (35 m)above the service area cannot be found, the only option with a groundstorage tank is to use pumping to provide the required pressure. If youare going to pump, you should be aware that water demand variesthroughout the day. As such, you will have to use a variable-speedpump.

A typical water usage graph (Fig. 1-33) shows the filling of a tankduring the night and early morning hours when demand is low. The





115 ft (3

5 m

)

Elevated tank Standpipe

Reservoir

FIGURE 1-32 Providing pressure using gravity flow.

tank is emptied during the day; water demand peaks sometime be-tween 5 p.m. and 9 p.m.

Electric utilities charge more for electricity during their peak de-mand period (see sample rates in Fig. 1-34). By overlaying the sampleelectric rates on the water usage graph (Fig. 1-35), one can see that thepeak demands for electricity and water occur about the same time ofday. Using these data, one can make the following calculations:� Peak demand (5 p.m. to 9 p.m.) pumping costs: $0.1175/kWh

average utility cost to pump half of the daily water demandto end users.

Constant pumping rate

Filling tank

Usage rate

Peak demand

Midnight 6:00 A.M. Noon Midnight6:00 P.M.

Emptying tank

Time

FIGURE 1-33 Typical water usage.





$0.14

$0.12

$0.10

$0.08

$0.06

$0.04

$0.02

$0.00

Ave

rag

e c

ost

pe

r kilo

wa

tt-h

ou

r

Midnight 3:00 A.M. 3:00 P.M. 9:00 P.M. Midnight6:00 P.M.Noon9:00 A.M.6:00 A.M.

Time

FIGURE 1-34 Sample electric rates.

� Nonpeak demand pumping costs: $0.1080/kWh average util-ity cost to pump the other half of the daily water demand toend users.� Tank filling costs: $0.0675/kWh average daily utility cost tofill the tank by pumping.

In this case, utility costs during peak demand are almost 75 percentmore than the cost of the average rate used to fill the tank, while evennonpeak costs are about 60 percent more. These calculations shouldbe modified for your system using your local daily water usage andutility rates. Regardless of the local factors, pumping during peak

$0.14

$0.12

$0.10

$0.08

$0.06

$0.04

$0.02

$0.00

Ave

rage c

ost per

kilo

watt

-hour

Midnight 6:00 A.M. 6:00 P.M.Noon Midnight

Time

Constant pumping rate

Peak demand

Usage rate

Emptyingtank

Filling tank

FIGURE 1-35 Higher rates during peak demand.





electricity rates to meet peak water demand is usually more expensivethan gravity flow and can become quite costly over time.

Additionally, if you lack sites with hills or higher elevations andchoose to pump to meet the pressure and water demand, incorporatethe following initial and lifetime costs into your present-value analysisas follows:� The additional daily costs of pumping over and above gravity

flow (peak and nonpeak)� The added cost of a variable-speed pump (usually required;larger than the constant-speed pump used at night for agravity-flow tank)� Cost of a backup pump or pumps� Cost for additional piping and controls for the backuppump(s)� Cost of backup generator� Expense of enlarging the pump building to house the addi-tional pumps and piping� Cost to maintain and replace all of these as needed.

Often, when these additional costs are considered, it is most likelythat the extra initial costs to provide gravity flow may actually be amore cost-effective solution over time.

Gravity flow One can save these peak-demand electricity costs by peakshaving. To peak shave, start by locating a ground storage tank on theside of a hill, or build an elevated tank or standpipe. A smaller pumpcan then be used to pump the water up into the tank during the nightand early morning at a constant rate when electricity rates are muchlower. Then, during the demand period, water can be provided at theneeded pressure by using gravity flow. This avoids the much higherelectricity rates during this time period and allows use of a smaller,less costly pump.

Because of these advantages, gravity flow is the preferred methodof providing water pressure. If possible, place the tank on a hill orelevate it to take advantage of this method.

The ideal location: For any type of storage tank, the ideal locationis on a hill that is in the middle of the demand area and is owned bythe community.

Proximity to UsersWhen choosing a site for a new water-storage tank, the prospectivetank owner should consider the growth in residential demand (single-family, multifamily, and high-rise structures) and commercial demand(industry, schools, and hospitals). A new residential development on





the north side of the service area and a new tank on the south sidewould result in very little water pressure for residents of the newdevelopment. The ideal situation is to construct a new water-storagetank in the service area before the area experiences population growthand buildup. This way, you have a better chance to get the right pieceof land at the right time and at the right price.

Acquiring LandWhen acquiring land, the prospective tank owner must consider theavailability and suitability of the land for a tank project; the costs forthe land, required support utilities, and the length of connections tothe existing distribution system; and the surrounding conditions.

NIMBY (not in my back yard!) One of the biggest issues that a waterutility can face when attempting to locate a new water-storage tank isthe public concern of NIMBY! Despite these concerns, even the mostappearance-conscious communities can agree to a mutually beneficialsolution to this stumbling block. The following are some successfulapproaches to be used in overcoming public concern:

� Encourage community involvement: When choosing the style ofthe tank, let the citizens express their concerns and provide in-put. In some communities, citizen groups have used conteststo select the color scheme of the tank exterior or the letteringand logo design.� Educate the citizenry: Explain the reasons the new tank isneeded and the beneficial effect it will have on them person-ally (for example, improved water pressure and fire protec-tion). Demonstrate how improved fire flow will affect insur-ance rates, assure them of the safety record of water-storagetanks, and explain the anticipated maintenance cycle.� Help the public visualize the completed tank: Using an artist’s con-ception, computerized renderings, and a digital photographof the site, compile an image that shows the community whatthe finished water-storage tank will look like.

Zoning RegulationsOnce a site has been located, check on the zoning of the selected siteto ensure that it is currently zoned for this use or can be rezoned.Obtaining proper zoning for a water tank is typically more difficult ina residential area than in an industrial area or in an area near publicfacilities such as schools, government property, and airports.

Often, schools are built in the areas of population growth, andthe school yard may make a good site for a tank. There are manyaesthetically pleasing tank styles that limit access.





FAA ConsiderationsForms must be completed and filed with the Federal Aviation Ad-ministration (FAA) to establish whether a tank can be built on thechosen site at the required height. The FAA is concerned about anyobstruction to its airspace 200 ft (61 m) above ground level andany obstruction within an approach pattern to an airport runway.Lengths of approach pattern vary depending on the size of the air-port, the length of the runway, and the direction of the runway, asfollows:� Large airport: No obstruction that exceeds a 100:1 surface

within 20,000 ft (6.1 km) of an airport having at least onerunway >3,200 ft (>975 m).� Small airport: No obstruction that exceeds a 50:1 surface within10,000 ft (3 km) of an airport whose longest runway is <3,200ft (<975 m).� Heliport: No obstruction that exceeds a 25:1 surface within5,000 ft (1.5 km) of a heliport.

If the FAA determines that the tank will be in the approach pattern,the tank may have to be equipped with aviation lighting or paintedin a special aviation warning paint scheme. The most common of theaviation paint schemes is the red-and-white checked pattern foundon tanks near airports.

The following circulars, forms, and information regarding ob-struction evaluation and airport airspace analysis are available on theFAA Web site (https://oeaaa.faa.gov):� For information on proposed tank construction projects, con-

sult “Proposed Construction or Alteration of Objects that mayAffect the Navigable Airspace” (Advisory Circular 70/7460-2K).� Standards for marking and lighting tanks and other structuresare provided in “Obstruction Marking and Lighting” (Advi-sory Circular 70/7460-1K).� “Notice of Proposed Construction or Alteration” (Form 7460-1) should be completed by the tank owner before the site iseven purchased and certainly prior to construction. The formcan now be completed and submitted online at the FAA Website. Information required includes latitude, longitude, loca-tion marked on a US Geological Survey (USGS) map, elevationof site (mean sea level), and the greatest height of any part onthe tank, including handrails or antennas upon completion.Once the FAA has reviewed the information on the form, itwill make a determination on the proposed tank and location





and post it online. The determination may be one of the fol-lowing:� A tank can be built on this site at the height requested.� A tank cannot be built on the proposed site at all.� A tank can be built on this site but not at the height re-

quested.� A tank can be built on this site at the height requested,but will require an obstruction light and/or obstructionmarking.� A tank can be built on this site but not at the height re-quested and will require an obstruction light and/or ob-struction marking.� “Supplemental Notice of Actual Construction or Alteration”

(Form 7460-2) is usually completed by the tank contractor. Itmust be submitted 30 days prior to the start of tank erectionand requires information similar to that requested on Form7460-1, except for the following:� Indicate the start and completion dates of the construction.� Must indicate the greatest height of the tank or equipment

during construction. Often the tank contractor uses a der-rick with a boom to erect the tank. The height of the derrickand boom may actually exceed the maximum height of thetank on completion. The FAA will want to know this andmay actually require the tank contractor to install an ob-struction light at the tip of the boom to alert pilots to thetank’s location.

Size of SiteTank constructors recommend that the distance from the edge of thetank to the site boundary be a minimum of 50 to 75 ft (15.24 to 22.86 m).A tank may be constructed on a smaller site, but it will require extrahandling and planning to stage materials in a disciplined sequence.Eliminating space constraints enables the tank contractor to build thetank more efficiently and can reduce costs up to a point.

Take into consideration the space needed for the following:� Material storage during construction� Erection and painting operations� Support facilities such as pump houses, valve vaults, andparking areas� Future maintenance and repainting� Placement of tank at safe distance from private property andutilities.





TopographyThe tank site can have a major influence on the cost of constructionand on design details for the foundation. The site should allow gooddrainage away from the foundation(s), provide a level working sur-face for construction, and have some type of erosion protection. Stand-ing or ponding water on the site can add dewatering costs to the projectand may even require changes to the foundation design, leading toadded costs. Consider these added costs when evaluating sites.

Access to SiteAccess to the tank site is an important aspect of site selection. Devel-opers and residents often want the tank to be located in the back ofthe development, away from the streets or even in off-road remote lo-cations. This poses a problem getting the large trucks and equipmentrequired for construction to the tank site.

Other things that must be considered when assessing site ac-cess are the distance from paved roads, permanent versus temporaryroads, accessibility by large trucks, and securing temporary easementsfor site access during construction, if needed. The best site access isvia a permanent road up to the tank. The most economical meansof achieving this is to put the tank access road in with the originalsubdivision roads.

Soil ConditionsA full soil investigation should be conducted before the final site ischosen and certainly before it is purchased. The soil assessment willdetermine whether the soil is adequate to support the tank and itscontents and what type of foundation must be designed. Some sitesmay require deep foundations (piles or drilled piers) that could addsignificant costs to the design and construction of the tank.

The soil investigation will provide needed information about thefollowing issues:� Soil bearing capacity (how much of a load the can soil support)� Site classification for seismic design� Excessive or uneven settlement� Water table elevations� Rock elevations if present� Site history� Substrata conditions� Slope stability

The depth at which the required soil bearing is obtained to supportthe foundation along with the slope stability has implications for the





size of the site required. For example, with a 1:1 slope stability anda 50-ft (15.24-m)-diameter foundation with the required soil bearing15 ft (4.57 m) down, the minimum size of the hole for the foundationwould be 15 ft + 50 ft + 15 ft = 80 ft (4.57 m + 15.24 m + 4.57 m =24.38 m). To this, one would have to add room for digging equipmentand room to store the excavated material on site.

The results of the soils investigation can affect the design andcosts of both the foundation and tank to such an extent that one couldactually save money on the overall project by paying more for a sitewith better soil conditions. It is prudent to make the site purchase onlyafter you have received the results of the soil investigation.

Obstructions/HazardsObstructions that must be avoided include overhead power lines,underground utilities, and existing structures. OSHA (OccupationalSafety and Health Administration) and many tank contractors spec-ify safe minimum work distances required from power lines depend-ing on what voltage the lines carry. Construction hazards may in-clude abrasive blasting, painting, pile-driving vibration, noise, andfire.

Waves and energy produced by AM antennas comprise one ofthe least understood obstructions. AM antennas are typically the tall,slender, red-and-white antennas that do not have dishes or whip an-tennas hanging off of them; the entire structure acts as the broadcastantenna. On the electromagnetic spectrum, AM waves are the longestwaves generated and can be from 656 to 1,968 ft (200 to 600 m) long.These long waves carry energy. Metal objects used in tanks or tankconstruction such as rebar, steel plate, and even crane lines can actas receiving antennas that collect and store the AM wave energy. If agrounded worker touches these energized metal objects, the collectedenergy is released, possibly shocking the worker and making the worksite unsafe. Whether the AM antenna has any effect on your tank sitedepends on how far the antenna is from your tank, what power it isbroadcasting at, and whether it is a directional or nondirectional an-tenna. At the Federal Communications Commission (FCC) Web site(www.fcc.gov/mb/audio/amq.html), one can insert the latitude andlongitude of the centerline of the tank (also used in the FCC submit-tal) and use the “Stations within a Radius” input. The Web site willindicate if any AM antennas are present. If so, station details willindicate whether the transmission location is directional or nondirec-tional. Problems can be present for distances up to 0.6 mile (1 km) fornondirectional and 1.9 miles (3 km) for directional antennas. If youencounter an AM antenna that might be a problem and are seriouslyconsidering the site in question, you may want to hire a specialist tofurther examine the situation.





If the expense and risks of dealing with these obstructions andhazards adds enough costs to your project, you might be better offpaying more for a site that is free of such obstructions and hazards.

Environmental IssuesEnvironmental issues that come into play during tank construction in-clude the protection of vegetation, wildlife, wetlands, and floodplains;historical landmarks and burial sites; and local wind and snow. Moststates require that a permit request be submitted to the US Environ-mental Protection Association (USEPA) before construction to identifyany such environmental issues.

Sizing the Tank

DemandTank capacity One of the main purposes of a water-storage tank is toprovide storage to meet the water demands of the area it will service.As a rule of thumb, you can determine your new water-storage tankcapacity by making the following calculation:

Average daily usage (peak and nonpeak) + fire flow requirements+ added capacity to offset maintenance or pipe breaks+ additional capacity for future demand = tank capacity

Current average daily use This is the amount of water used on averagein a 24-hour period. Calculate this by determining the average waterusage currently per person and multiply this by the number of peoplethat the new area currently serves.� Peak demand: Peak demand typically occurs between 5:00 p.m.

and 9:00 p.m. and is usually half of the current average usage.� Off-peak demand: This comprises the other half of the averagecurrent daily usage.

Fire flow demand To the current average daily usage add an additionalone-half to one-third of the current average daily usage. This figurevaries depending on the local codes and standards. One should alsocheck the requirements of the Insurance Service Organization (ISO)(www.iso.com) and other local standards and codes.� Maintenance and piping breaks: As a contingency measure, con-

sider adding 10 percent, plus or minus, to provide extra stor-age if the service area distribution piping has leaks.� For future demand, project the future population for the ser-vice area and then multiply that by the current average dailywater use in gallons (liters) per person. An alternate method





0 to 2,000

2,000 to 5,000

5,000 to 10,000

10,000 to 20,000

20,000 to 52,000

Louisiana Florida

Idaho

Washington

Oregon

Nevada

California

NewMexico

Utah

Arizona

Nebraska

NorthDakota

Montana

Wyoming

Colorado

Oklahoma

Kansas

SouthDakota

Ark

ansa

s

Missouri

Georgia

Iowa

TennesseeNorth

Carolina

Wisconsin

os

en

niM

ta

India

na

Kentucky

Ohio

Virginia

Delaware

Massachusetts

Maine

New JerseyConnecticutRhode Island

Alaska

Hawaii

Puerto Rico

U.S. Virgin Islands

Mic

hig

an

Source: US Geological Survey Circular 1268

Water withdrawals in milion gallons per day

New Hampshire

Vermont

District of Columbia

Maryland

West

Virginia

South

Carolina

IllinoisOhio

Pennsylvania

New York

Mis

sssip

pi

Ala

bam

a

Texas

FIGURE 1-36 Average daily water usage per capita.

would be to check with the US Geological Survey to learn whatthe average daily usage is per person by state (Fig. 1-36).

Local standards and codes related to tank capacity should be con-sulted and complied with.

Turnover Tanks sized to meet peak demand must also have adequateturnover when demand for water is not at a peak. Unused water canbecome stagnant, generating unwanted tastes and odors. In cold cli-mates, lack of turnover can cause tank icing. Water turnover problemscan be solved by filling the tank to a lower capacity that matches thereduction in demand or by adding a recirculation system. Addition-ally, several mixing systems are available that can create a more uni-form residual chlorine content, reduce stagnation, and help preventthe generation of unwanted tastes and odors.

Volume/standard capacities For elevated tanks, the most economicalstorage is achieved by selecting a standard capacity and head rangeon the basis of the recommendations of the tank contractor. Typicalcapacity ranges of elevated tanks are given in Tables 1-8 and 1-9.

The largest-capacity elevated tank built to date is 4 mil gal (15,142m3). It may be possible to build larger capacity tanks, but they wouldbe the first of their kind.

Reservoirs and standpipes are more flexible in their height/diameter limitation. It was once thought that reservoirs could onlybe constructed in height increments of 8 or 10 ft (2.44 or 3.05 m). Steel





is now readily available in made-to-order heights (and widths) in ad-dition to these. An economical tank can be built to whatever diameterand height is required (Tables 1-1 and 1-3).

Diameter and Height SelectionFor a ground storage tank, three major factors influence the selectionof the most economical diameter and height. Of the following threefactors, soil bearing and earthquake design usually have the biggestinfluence.

Soil bearing The tank foundation and ultimately the soil must sup-port the weight of both the water and the tank. Each cubic foot of waterweighs 62.4 lb/ft3. The calculation of the weight of a 1-ft2 column ofwater from the bottom of the tank to its top capacity height can giveone an idea of the weight that must be supported. A sample calcula-tion for a 40-ft (12-m) column of water would be 40 ft × 62.4 lb/ft3 =2,496 lb/ft2 or about 2,500 lb/ft2. So, if a 2,500-lb/ft2 soil bearing is notavailable at the tank site, various foundation types could be evaluatedto support the column height of water needed. Deep foundations orlarge mats may increase costs to the extent that it may actually be moreeconomical to either change the height of the tank or evaluate othersites with higher soil bearing values.

Earthquake Typically, the taller and thinner the tank, the more thatearthquake may affect the design.

Wind The taller and wider a tank, the more wind may affect thedesign.

Here are some examples of diameters (D) and heights (H) ofground storage tanks with typical design conditions that might makethem more economical:� D = H: For decent soil bearing values of 4,000 psf/ft2, with

low earthquake factors and typical 90-mph wind design, atank in which diameter is equal to height may be the mosteconomical shape for small- and medium-size tanks.� D < H: For soil bearing values greater than 4,000 psf/ft2, withlow earthquake factors and typical 90-mph wind design, atank in which diameter is less than height may be the mosteconomical shape. In these tanks, there are fewer costs in thebottom and roof and more costs in the shell.� D > H: For soil bearing values less than 4,000 psf, with highearthquake factors and winds greater than 90 mph, a tankin which diameter is greater than height may be the mosteconomical shape. In these tanks, there are more costs in thebottom and roof and fewer costs in the shell.





It is important that you call your local tank contractor to helpdetermine the most economical diameter and height combination forthe design conditions at your site.

Selecting the Tank

Ground/Elevated StorageYour first decision is whether to build a ground storage tank or an el-evated tank. If higher-elevation sites are available that could providethe required pressure through gravity flow without the need for run-ning a lot of water main to reach the site, the more economical choicewould probably be a ground storage reservoir. If site elevations arenot available within the water system area or are not high enough toprovide the required pressure through gravity flow, an elevated tankor standpipe would probably be a better choice. Using a reservoir andpumping to meet the pressure and daily water demand will add dailypumping and peak demand charges throughout the life of your tank.See the previous section on hydraulics.

Again, it is important that you call your local tank contractor toprovide budget pricing for various tank options to evaluate the initialand lifetime costs of your new storage tank.

Aesthetics/AppearanceThe aesthetic appeal of a new water-storage tank is often one of themost talked-about elements of tank selection. The public may wanta tank that will blend into its surroundings, or be a highly visiblelandmark for the community, or match the system’s existing tanks.The tank owner and security personnel may want to place the tankon a more visible site that can be readily secured and monitored. Thisdecision must be handled on a case-by-case basis.

Ornamental TanksHighly stylized ornamental tanks can provide community or com-pany identity and advertisement, be more aesthetically pleasing, or re-solve NIMBY issues. Unique, decorative tanks have been constructedin many areas and, although more costly to construct, they are oftenlandmarks in which the community takes pride.

EconomicsAlthough the initial cost of constructing a tank has a significant eco-nomic impact, the tank’s operating cost, reliability, and maintenancerequirements must also be considered.

Special NeedsSometimes communities have special needs or desires; for example, acommunity may want to house the fire department in the base of the





tank. Multiuse tanks can be constructed to match the community’sneeds.

LiabilityTo limit liability, tank owners seek methods to control access. Somestyles of tanks—such as single-pedestal spheroid, fluted-pedestal, andcomposite elevated tanks—do not have exterior ladders, thereby effi-ciently limiting access. On legged or ground tanks, ladder guards canbe installed that limit access to the ladders.

Life-Cycle CostsAnticipated need for and scheduling of tank repainting and mainte-nance are important considerations. The style of tank, its surface area,and the type of surface all directly influence maintenance costs.




C H A P T E R 2Selecting and

SpecifyingAppurtenances

William B. Harper, P.E., Andre Harper,and Krista L. Harper, P.E.Harper and Associates

Appurtenances, or accessories, for tanks are vital for the function,operation, and maintenance of the tank system. Appurtenances arecovered in separate sections of American Water Works Association(AWWA) D100, Standard for Welded Carbon-Steel Tanks for WaterStorage; AWWA D103, Standard for Factory-Coated Bolted-SteelTanks for Water Storage; and AWWA Manual M42 Manual of WaterSupply Practices, Steel Water-Storage Tanks; and in regulatory docu-ments such as those issued by the Occupational Safety and HealthAdministration (OSHA). The majority of the appurtenances and at-tachments described for steel water-storage tanks are required by law,code, and industry standards to make the tank a safe and functionalfacility. Other accessories are optional and may be specified by theowner to improve the facility’s function or appearance.

Accessory items on the tank structure should generally be locatedwhere they are readily accessible from a fixed ladder or platform sur-face. An exception to this is the location of stub overflows, which,when used, are purposely located away from ladders to avoid laddericing. Specific tank accessories required by AWWA standards shouldbe shown but not detailed on the bid drawings unless a specific detailis required, because each manufacturer has proprietary componentsthat fulfill the intent of the standards. Such details may cause problemsif bidders are required to provide another constructor’s proprietary

47




48 C h a p t e r T w o

apparatus. Accessory details and orientation should be developed andincluded in the shop drawing submittals after the contract is awarded.The governing requirement should be that the accessories meet theminimum requirements of the regulations, referenced AWWA stan-dards, and the intent of the specifications. If the owner elects to in-clude components and operating systems in excess of those specifiedby AWWA standards, the engineer should provide specifications anddetails that clearly define the components required and the scope ofrelated work.

Ground-Supported Tanks

Welded-Tank Shell ManholesFor safety and ease of interior access during construction activities andmaintenance inspections, at least two opposing shell manholes arerequired on welded ground-supported tanks for ventilation duringinterior coating operations. On tanks more than 100 ft (30.5 m) indiameter, it may be desirable to have three or more shell manholes,keeping the maximum circumferential spacing at 100 ft (30.5 m) or less.AWWA D103 requires only one shell manhole on bolted tanks becausea tank panel can be removed to provide additional ventilation, but thespecifier may elect to provide additional manholes.

Sizes and TypesTypically, shell manholes are 24 or 30 in. (610 or 760 mm) in diam-eter to accommodate ventilating equipment and allow easy egress.Manholes larger than 30 in. (760 mm) in diameter are uncommon andmay require special design details for structural integrity. Single-boltinward-opening shell manholes or outward-opening shell manholeswith hinged covers are standard (Figs. 2-1 and 2-2). Outward-openingcovers may require reinforcing plates on the shell, whereas inward-opening manholes usually accomplish their reinforcement throughheavy plate necks. The heavy plate neck also provides the gasket sur-face to the cover. The inward-opening cover must be hinged to ensureproper operation. If the tank will be subject to severe icing conditions,an inward-opening manhole may not be desirable.

Welded-Tank Flush ManholesIf specified by the purchaser, flush-type cleanouts (Fig. 2-3) shall befurnished for ground-supported tanks. Flush rectangular manholes(rectangular manholes mounted flush with the bottom of the tank)having a minimum length of 24 in. (610 mm) in the short directionand a maximum length of 48 in. (1,220 mm) in the long direction arealso available. Such manholes are useful when a tank interior is be-ing cleaned. Refer to AWWA D100 and American Petroleum Institute



Selecting and Specifying Appurtenances

Front elevation

Exploded side elevation

Section A–A

1/4Typical

Typical

AA

3/8

3/8

FIGURE 2-1 Inward-opening shell manhole detail. (Source: AWWA Manual M42,Steel Water-Storage Tanks)

See Detail A

See Detail A

Detail A

Front elevation

Side elevation

Section A–AIsometric blowout

Bolt holesequally spacedto straddle centerline

Roll reinforcingplate to exterior tank shell radius

1/4-in. (6.4-mm)hole on horizontalcenterline

Machine flangeface full width

Weld beforemachining

Tank shell plate

Floor assembly plate

1/4

A

A

5 in. (127 mm) min.

1/4

B.C.d

iam

eter

9 in.(229 mm)

Tank shellplate

FIGURE 2-2 Outward-opening shell manhole detail. (Source: AWWA ManualM42, Steel Water-Storage Tanks)





Section A–A

Nearest horizontal weld

Shell

3 ft (0.9 m)Bottom plate

46 each 1-in.-

(25.4-mm)-diameter

bolts, equally spaced

(see API 650, Table 3-11)

Reinforcement plate15 in.

(381 mm) min.

Note: Refer to API Section 650, Figure 3-8 Flush-Type Cleanout Fittings and Tables 3-11, 3-12, and 3-13

FIGURE 2-3 Flush-type cleanout.

(API) Standard 650 for details and design requirements. Although theflush-type manhole is permitted in the AWWA standards, its use is notrecommended in high-seismic regions where the additional stiffnessof the reinforcing may cause stress concentrations or buckling in anearthquake.

ReinforcingThe shell plates where the manholes are located shall be reinforced tocomply with AWWA D100 Section 3.13.2.5, and all portions of themanholes (including reinforcing of the neck, the bolting, andthe cover) shall be designed to withstand the weight and pressureof the tank contents.

Bolted-Tank Shell ManholesOne manhole, unless otherwise specified, shall be provided in the firstring of the tank shell at a location to be designated by the purchaser.If any manhole cover weighs more than 50 lb (23 kg), a hinge shall beprovided.

Size and ShapeManholes may be either circular, 24 in. (610 mm) in diameter; square,24 × 24 in. (610 × 610 mm); or elliptical, 18 × 22 in. (457 × 559 mm),minimum size. Flush rectangular manholes with a minimum length




51S e l e c t i n g a n d S p e c i f y i n g A p p u r t e n a n c e s

of 24 in. (610 mm) in the short direction and a maximum length of 48in. (1,220 mm) in the long direction are also acceptable.

Flush ManholesFlush rectangular manholes (rectangular manholes mounted flushwith the bottom of the tank) having a minimum length of 24 in. (610mm) in the short direction and a maximum length of 48 in. (1,220mm) in the long direction are also available. Such manholes are usefulwhen a tank interior is being cleaned. Refer to AWWA D100 and APIStandard 650 for details and design requirements.

ReinforcingThe shell plates where the manholes are located shall be reinforced tocomply with AWWA D103 Section 3.11, and all portions of the man-holes (including reinforcing of the neck, the bolting, and the cover)shall be designed to withstand the weight and pressure of the tankcontents.

Pipe ConnectionsThe number of tank-bottom or shell-piping connections should bekept to a minimum. The earlier practice was to use a common in-let/outlet drain connection through the tank bottom or on the tankshell (Figs. 2-4 and 2-5). If a bottom connection is used, a removablesection of pipe 6 to 8 in. (150 to 200 mm) long may extend above theoutlet at floor level to serve as a silt stop. The drainpipe shall be re-cessed to aid in draining the tank. Recent requirements concerningminimum and maximum detention time during which the water re-mains in the tank may require separate inlet and outlet connections.Baffles and flow diverters are also used to control detention time.

1/4 (typ.)

FIGURE 2-4 Recessed inlet–outlet pipe bottom connection detail. (Source:AWWA Manual M42, Steel Water-Storage Tanks)





Plan

Elevation

FIGURE 2-5 Nonrecessed inlet–outlet bottom connection. (Source: AWWAManual M42, Steel Water-Storage Tanks)

Piping connections through the tank bottom or shell are normallyfurnished in steel pipe welded or bolted to the shell or bottom. Ductile-iron or cast-iron pipe connections must pass through a mechani-cal joint–type connection that is welded or bolted to the steel tankbottom.

Pipe connections shall be of the size specified by the purchaser andare usually attached to the tank bottom. The point of attachment shallbe designated by the purchaser. Connections to the tank or pipingfurnished by the tank constructor shall be made by the purchaser.

Silt StopIf a removable silt stop is required, it shall be at least 4 in. (102 mm)high, and the fitting or piping connection shall be flush with the tankfloor when the stop is removed. If a removable silt stop is not required,then the fitting or connecting pipe, or both, shall extend above the floorat least 4 in. (102 mm).

Drain SumpTo facilitate a more efficient and expedient removal of residual waterremaining on the tank bottom after draining the tank, a new drain





and a sump 3 ft (914 mm) in diameter by 6 in. (152 mm) deep shall beinstalled. The drain sump shall be covered with a slip-type or hinged,grated cover that can be easily removed or opened during mainte-nance intervals.

Shell ConnectionsShell connections are permitted as long as the purchaser makes ade-quate provisions to protect the pipe from freezing or vandalism andprovides adequate pipe flexibility to account for shell rotation and de-flections of the shell when the tank is filled and drained. These includesample taps, disinfection fittings, and fire-hose fittings for the interiorand exterior.

Piping FlexibilitySpecial piping flexibility to accommodate seismic movements and set-tlement in the piping system shall be provided to protect the connec-tion to the shell. AWWA D100 defines the distance from the shellintersection that through-the-tank-bottom piping connections may belocated on unanchored tanks designed for seismic conditions. Bot-tom connections shall comply with AWWA D100 Section 13.5.1 as aminimum. Underbottom connections are not recommended on tanksin high-seismic zones. Tank sliding or uplift may impose additionalstress on the connection and tear the bottom. Sidewall connections,which can readily be inspected after a seismic event, are preferred.

OverflowsA properly sized overflow is essential to protect the tank structurefrom excessive water levels caused by rapid variations in distributionsystem conditions. Exterior overflows are recommended. In colderclimates, ice buildup on an internal overflow may become a problemand eventually break the overflow pipe. Overflow waters should bedirected beyond the exterior perimeter of the tank to prevent damageto the tank grade or foundation during overflow. Most state stan-dards recommend that the overflow on elevated tanks be extendeddown the side of the tank to within approximately 12 to 24 in. (305to 610 mm) above grade. Extending the overflow pipe prevents waterdischarged from the pipe from freezing on the tower structure anddamaging it. In addition, most governing agencies require an air gapbetween the overflow tank piping and final drainage system to pro-tect against backflow. Figure 2-6 shows one type of overflow-pipe airgap. Most states require a screen or flap/gate arrangement over theend of the pipe connected to the tank and a removable grate on thebottom portion of the pipe. The valve shall be a flanged passive checkopening with 2 in. (51 mm) of water and shall be able to withstand zerobackpressure. As distribution systems and pumping capacities are in-creased, the vent and overflow capacities of existing tanks should be





FIGURE 2-6Overflow air breakwith flap valve.(Source: AWWAManual M42, SteelWater-StorageTanks)

reevaluated to ensure their adequacy to relieve potential pressure orvacuum conditions in the tank. Overflows should be easily accessiblefor maintenance, repair, and inspection.

Welded-Tank OverflowAn overflow protects the tank from overpressure, overload, and pos-sible catastrophic failure should the pumps or altitude valve fail toshut off when the tank is filled to capacity. A properly operated tankshould not overflow during normal operation. An overflowing tankis an emergency, and the malfunction causing the overflow should bedetermined and corrected as soon as possible.

The tank shall be equipped with an overflow of the type andsize specified by the purchaser. If a stub overflow is specified, it shallproject at least 12 in. (305 mm) beyond the tank shell. If an overflowto ground is specified, it shall be placed down the side of the tankshell and supported at proper intervals with suitable brackets. Theoverflow discharge shall be located such that it will not be obstructedby snow or ground clutter. The overflow to the ground shall dischargeover a drainage inlet structure or a splash block. It shall originate at thetop in a weir box or other appropriate type of intake. A top stiffenershall not be cut or partially removed. The overflow pipe and intake





shall have a capacity at least equal to the pumping rate as specified bythe purchaser. Where a side opening–type overflow is used, the headshall be not more than 6 in. (152 mm) above the lip of the overflowand in no case more than 12 in. (305 mm) above the top capacity level.The overflow pipe shall terminate at the bottom with an elbow. Un-less otherwise specified by the purchaser, the overflow pipe shall besteel pipe with screwed or welded connections if less than 4 in. (102mm) in diameter, or flanged or welded connections if 4 in. (102 mm)in diameter or larger. The purchaser shall specify the maximum flowrate in gallons or liters per minute, for which the overflow shall be de-signed. Overflow pipes may be either internal or external as specifiedby the purchaser. Minimum external overflow pipe thickness shallbe 3/16 in. (4.76 mm). Internal overflow pipes are not recommendedwhen tank usage and climatic conditions are such that ice may dam-age the overflow pipe or its attachments. When specifying an internaloverflow pipe, the purchaser should consider the consequences ofan overflow failure, which can empty the tank of its contents. An in-ternal overflow pipe shall be at least 0.25 in. (6.35 mm) thick. The endof the overflow pipe may be covered with a coarse, corrosion-resistantscreen equivalent to 3/8 in. (9.5 mm) or larger mesh. The end of theoverflow pipe may also be covered with a flap valve or other protectivecover as specified by the purchaser.

Bolted-Tank OverflowThe tank shall be equipped with an overflow of the type and size spec-ified by the purchaser. If a stub overflow is specified, it shall projectat least 12 in. (305 mm) beyond the tank shell. If an overflow to theground is specified, it shall be brought down the outside of the tankshell and supported at proper intervals with suitable brackets. Theoverflow to the ground shall discharge over a drainage inlet struc-ture or a splash block. It shall terminate at the top in a weir box orother appropriate intake. A top stiffener shall not be cut or partiallyremoved. The overflow pipe and intake shall have a capacity at leastequal to the pumping rate as specified by the purchaser, with a wa-ter level not more than 6 in. (152 mm) above the weir. The overflowpipe shall terminate at the bottom with an elbow. Unless otherwisespecified by the purchaser, the overflow pipe shall be steel pipe withscrewed or welded connections if smaller than 4 in. (102 mm) in di-ameter, or flanged or welded connections if 4 in. (102 mm) in diameteror larger. The external overflow pipe shall have a minimum thicknessof 3/16 in. (4.76 mm). The purchaser shall specify the maximum flowrate, in gallons or liters per minute, for which the overflow shall be de-signed. Internal overflows are not recommended but may be providedif specified by the purchaser. The internal overflow pipe shall have aminimum thickness of 0.25 in. (6.35 mm).





LaddersSafe access must be provided for authorized personnel who need toreach upper shell areas and the top of the tank facility.

Exterior Vertical LaddersExterior ladders, cages, and platforms designed to meet OSHA stan-dards are recommended (Fig. 2-7). Either the ladder should terminate

Cage elevation Ladder elevation

Tank shell

Flare out to

join top hoop

or platform

1 ft (0.3 m) to tank bottom 8 ft (2.4 m) to tank bottom

1/4

FIGURE 2-7 Exterior caged ladder details. (Source: AWWA Manual M42, SteelWater-Storage Tanks)





at least 8 ft (2.4 m) above grade, or a solid, locking door, providedto discourage unauthorized access to the tank, should be installed onthe lower 10 to 20 ft (2.4 to 6.1 m) of the exterior ladder. Certain areaswill require a locking door and anti-climb screening at the bottom ofthe ladder cage to discourage unauthorized access. The exterior lad-der, roof hatch opening, and interior ladder (if specified) should belocated close together to reduce the movement necessary by a climberon the tank roof. The ladder can be specified as painted carbon steel,galvanized steel, or stainless steel. If stainless-steel ladders are used,insulation (dielectric connections) must be included that separates thestainless steel from the carbon-steel tank, and all stainless-steel com-ponents must be coated to prevent corrosion from occurring on thecarbon-steel tank.

Exterior Circular StairwayWhen specified, the exterior stairway shall be of a semicircular de-sign meeting API Standard 650 Section 3.8.9, Platforms, Walkways,and Stairways (Fig. 2-8). The stairway shall be at a specified locationterminating at the exterior roof hatch. Minimum stair width shall be30 in. (762 mm). An expanded metal security enclosure with a hingedgate and lock system shall be installed around specified ground-leveltermination.

Hand railto matchroofhand rail

Tie into platform

30 in.(762 mm)

min.1.5-in. (38-mm)pipe handrailalong outer

stair perimeter

Roof

Shell

42 in. (1.06 m)

Platform

Plan view

1/4 in. × 6 in. × 6 in. (6.4 mm× 152 mm × 152 mm) plate

2.5 in. × 2.5 in. × 3/8 in.(63.5 mm × 63.5 mm × 0.38 mm) angle, two each

8 in.(203 mm)

9 in.(228.6 mm)

Stair treads 3/16 in.(0.1875 mm) plate,shape as shown30 in. (762 mm)wide (typ. 53)

0.25 in. × 12 in. (6.3 mm × 305 mm) FB stairway

runner inside and outside

Stairway brace

Wearpads 1/4 in. × 6 in. (1.3 mm × 152.4 mm)

11

FIGURE 2-8 Exterior circular stairway. (Note: FB = flat bar.)





Interior LaddersBecause of accelerated rates of corrosion and the potential for icebuildup in areas where freezing temperatures occur, ladders insidethe tank container are not recommended. Ice buildup on an interiorladder can impose loads on the tank wall plates that are sufficientto pierce or rupture the tank container. Even in temperate climates,corrosion can damage interior ladders, making them unsafe. The useof stainless-steel ladders must include insulation (dielectric connec-tions) separating the stainless steel from the carbon-steel tank, and allstainless-steel components must be coated to prevent corrosion of thecarbon-steel tank.

Ladders are installed inside dry risers and access tubes. There theyare not subjected to corrosive conditions, and the access doors maybe locked to deter access. In general, all interior ladders shall meetdesign criteria noted herein for exterior ladders.

Minimum requirements Minimum requirements for ladders, hatches,and so forth can be found in OSHA 29 Code of Federal Regulations(CFR) Part 1910, Occupational Safety and Health Standards, GeneralIndustry Standards.

Welded-Tank LaddersExterior tank ladder The contractor shall furnish a tank ladder on theoutside of the shell beginning 8 ft (2.4 m), or as specified, above thelevel of the tank bottom and located to provide access to the roofmanway. The minimum clear width of step surface for rungs shallbe 16 in. (406 mm), and rungs shall be equally spaced 12 in. (280mm) on center. The perpendicular distance from the centerline of therungs to the tank wall shall not be less than 7 in. (178 mm). Rungsize shall not be less than 3/4 in. (19 mm) in diameter or equivalentsection. The maximum spacing of supports attaching the ladder tothe tank shall not exceed 10 ft (3 m). The minimum design live loadshall be two loads of 250 lb (113.6 kg) each concentrated betweenany two consecutive attachments to the tank. Each rung in the laddershall be designed for a single concentrated load of 250 lb (113.6 kg)minimum. The design loads shall be considered to be concentratedat such a point or points as will cause the maximum stress in thestructural ladder member being considered. Side rails may be of anyshape having section properties adequate to support the design loadsand providing a means of securely fastening each rung to the side railso as to develop the full strength of the rung and to lock each rung tothe side rails.

Interior tank ladder Inside tank ladders are not recommended for coldclimates where ice may form. If an inside ladder is required, it shall





comply with the requirements for exterior ladders outlined in theprevious paragraph.

Roof ladder For tanks with roofs, unless otherwise specified, the man-ufacturer shall furnish access to roof hatches and vents. Such accessshall be reached from the outside tank ladder. A roof ladder is notrequired on portions of standpipe or reservoir roofs having a slopeless than 2 inches per 12 inches of rise (2/12). A roof ladder shall beprovided on roofs having a slope greater than 2/12. For roof slopesfrom 2/12 to 5/12, there shall be a nonskid walkway and a singlehandrail. For a roof slope greater than 5/12, a ladder or stairway shallbe provided.

Bolted-Tank LaddersExterior ladders, cages, and platforms designed to meet OSHA stan-dards are recommended. Either the ladder should terminate at least8 ft (2.4 m) above grade, or a solid, locking door, provided to dis-courage unauthorized access to the tank, should be installed on thelower 8 to 20 ft (2.4 to 6.1 m) of the exterior ladder. Certain areas willrequire a locking door and anti-climb screening at the bottom of theladder cage to discourage unauthorized access. The exterior ladder,roof hatch opening, and interior ladder (if specified) should be locatedclose together to reduce the movement necessary by a climber on thetank roof.

Exterior tank ladder The constructor shall furnish a tank ladder onthe outside of the shell beginning 8 ft (2.4 m), or as specified, abovethe level of the tank bottom and at a location to be designated by thepurchaser, preferably near one of the manholes. The side rails shallnot be less than 2 × 3/8 in. (51 × 9.5 mm), with a spacing betweenthem not less than 16 in. (406 mm). The nonskid rungs shall not beless than 3/4-in. (19-mm) round or square bars spaced 12 in. (305 mm)apart on centers.

Interior tank ladder Inside tank ladders are not recommended for coldclimates where ice may form. If an inside ladder is required, the siderails shall not be less than 2 × 3/8 in. (51 × 9.5 mm), with a spacingbetween them of not less than 16 in. (406 mm). Rungs shall not be lessthan 3/4-in. (19-mm) round or square bars spaced 12 in. (305 mm)apart on centers.

Roof ladder For standpipes and reservoirs with roofs, unless other-wise specified, the constructor shall furnish access to roof hatches andvents. Such access shall be reached from the outside tank ladder. Referto AWWA D103 Section 5.4 for minimum requirements for roof lad-ders based on the slope of the roof.





Minimum requirements Minimum requirements for ladders, hatches,and so forth can be found in OSHA 29 CFR Part 1910.

Ladder Safety DevicesIf federal or local laws or regulations require a safety cage, rest plat-forms, roof-ladder handrails, or other safety devices, the purchasershall so specify. None of these devices are advisable on the submergedportion of interior ladders in low-temperature climates.

Ladder platforms For tanks with a total height in excess of 20 ft (6.1 m)or as required by OSHA, ladders with offset rest platforms (secondaryplatforms) are required every 20 ft (6.1 m) or as required by OSHA forthe ladder assembly. This length may be extended to 30 ft (9.1 m) if anapproved safety cage is used.

If the ladder extends up the exterior of the tank, the ladder shouldbe equipped with a locked guard to prevent unauthorized access tothe tank exterior and roof.

Conduits, fixtures, pipes, valves, and other items should not in-terfere with the safety of the ladders or platforms.

Roof safety railings In addition to exterior ladder safety devices, themost commonly installed safety items are safety railings at the roofwhere the exterior ladder terminates (Fig. 2-9). These railings protectpersonnel on the roof near the roof hatch. New handrailing shall befabricated and installed to form a totally enclosed work area aroundthe roof hatch. Existing pipe sections may be used, provided theymeet current OSHA requirements. A self-closing hinged gate shall beprovided at the exterior ladder opening. Handrailing layout shall beverified in the field before handrails are fabricated. All safety railingsinstalled on the tops of tanks and ladders should comply with mini-mum OSHA requirements or local building codes. Consult with theapplicable agency in charge of tank location to determine the latestsafety requirements. Total-perimeter handrails are not recommendedin areas of high snow load.

Ladder safe-climbing rails or cables In lieu of intermediate platforms,approved safe-climbing rails or cables may be used. Figure 2-10 showsa typical safe-climbing rail. Some tank owners may desire supplemen-tary rest platforms in addition to the safety rail. Climbing rails can begalvanized or stainless steel. Painted carbon-steel rails are not recom-mended.

Safety belts and sleeves should be furnished for the ladder safetydevices. Safety sleeves should be checked for proper operation alongthe full height of the rails or cables. Any coating, deviations, orobstructions that prevent the free operation of the sleeve should beremoved. Special dismount sections are available to ensure that the





Exteriorladder

Interiorladder

3 ft(0.9 m)

Self-closinggate

To roof

Hatch

Typical handrailing layout

Plan view

Detail A

Partial

See Detail A Gate elevation

Self-closinghinges

Typical handrail post bottom

1.5-in. (38-mm)standard black pipe

1/4 in. (6.3 mm)1/4 in. (6.3 mm)

6-in. (152-mm)-square pad

1.5-in. (38-mm)-diameter pipe

4 in. × 1/4 in. (102 mm× 6.3 mm) plate

Rail elevation

42 in. (1.06 m)

FIGURE 2-9 Safety rail enclosure. (Note: For roof slope 2/12 or less, windy orwet conditions may require additional safety lines for areas outsideenclosure.)

climber does not fall from the tank when the climber is dismountingonto the roof.

Roof fall prevention cable assemblies To prevent personnel who areworking or walking on the tank roof from falling over the edge of theroof, furnish and install stainless-steel cable assemblies as shown inFig. 2-11. Cable shall be stainless-steel aircraft cable of 7 × 19 construc-tion.

Cable collar around center vent shall be 0.25 in. (6.35 mm) in diam-eter placed in a plastic sleeve to prevent paint damage and connected





FIGURE 2-10 Safe-climbing rail for an outside ladder. (Source: AWWA ManualM42, Steel Water-Storage Tanks)

with stainless-steel cable clamps. The cable shall allow a maximum of2 in. (51 mm) greater diameter than the center vent neck at the rooflevel. The cable shall rest freely on the roof after installation.

The stainless-steel cables that prevent personnel from falling shallbe 3/16 in. (4.76 mm) in diameter and shall be fabricated with a swagedloop at the center vent attaching point and a swaged carabiner double-locking attachment at the hold-down point adjacent to the roof hatchwork area. Each personnel cable shall terminate 42 in. (1,067 mm) fromthe outer perimeter of the roof when attached to center vent cable.

A hold-down eye shall be welded at the attachment point de-scribed previously. The size of the eye and pad shall be as determinedby the civil engineer and the structural engineer.

Roof OpeningsAt least two roof openings are required for personnel access and ven-tilation during maintenance and rehabilitation activities on welded-steel tanks.

Primary OpeningThe first (primary) roof opening should be located near the tank side-wall close to the exterior ladder. The previous minimum size for this





Attachment padSee Detail B

42 in.

(1.0

6 m)

Provide three carabiners compatiblewith the 1/2-in. (12.7-mm) round bar.Pad is for attachment when not inuse. Attach carabiners to cable witha swaged loop as per Detail A.

Cable loop constructionSee Detail A

Center vent

Fully e

xtended c

able

1/4-in. (6.3-mm) cable with a maximum 2 in. (51 mm)greater diameter than the center vent. Use swaged loop to connectthe two ends of cable.

Round bar 1/2 in. × 10 in. (12.7 mm × 254 mm) withlarge bend as shown 4 in.

(101.6 mm)3 in.

(76 mm)1/4 in.

(6.3 mm)

Safety cable attachmentDetail B

Install at exit to rooffrom enclosure area

Typical cable loopDetail A

Cable atcenter vent

Swaging tool usedat this point tosecure the cableto itself

6 in. × 6 in. × 1/4 in.(152 mm × 152 mm× 6.4 mm) carbon-steelpad for attachment

Existing wood roof

FIGURE 2-11 Safety cable system.

roof opening was 15 × 24 in. (380 × 610 mm), but OSHA now requiresa 30-in. (760-mm) square or round opening with a hinged cover andlocking hasp to facilitate access to the tank interior. With the adventof diving inspections in tanks, which necessitate the use of a rubberraft for inspecting the underside of the roof, it may be prudent to uselarger roof hatch assemblies. These can range in size from 48 to 60in. (1,220 to 1,524 mm) and can be constructed of aluminum covers.A curb at least 4 in. (100 mm) high and a 2.in. (50-mm) downwardcover overlap are mandatory on any roof opening to prevent rain orsnowmelt from entering the tank (Fig. 2-12). Bolted and gasketed roofmanways without the curb and overlap are allowed on bolted tanks.

Secondary OpeningThe second roof opening should be located near the tank center or180◦ circumferentially from the primary opening. Its diameter shouldbe at least 20 in. (500 mm). If the center vent is of adequate size, isnot obstructed, and has a removable cover, the vent may suffice as thesecondary opening. The secondary opening, whether the center ventor a separate opening, should be designed with a removable cover to





Typical

3⁄16

3⁄16

3⁄16

3⁄16

Curb elevation

Cover elevation

Plan view

FIGURE 2-12 Roof manhole assembly details. (Source: AWWA Manual M42,Steel Water-Storage Tanks)

accommodate a bolted ventilation fan. All removable covers shouldbe secured to the roof of the tank using hinges or chains. An additionalsafety rail may be required between the secondary roof openings andthe edge of the roof.

Roof manholes should be equipped with locks to prevent unau-thorized entry into the tank. Tanks that have access tubes leadingto the roof should have their roof manholes properly latched to pre-vent them from blowing open in a strong wind, and any access doorsto the tank ladders should be locked. Shell manholes should be prop-erly sealed to prevent leakage.

Additional Roof OpeningsCathodic protection handholes/covers, suspension insulators, and refer-ence cell access Where cathodic protection is to be installed in thetank interior, access handholes with covers and suspension insula-tors shall be provided through the roof at locations specified by thedesigner of cathodic protection. A fitting shall also be provided forthe cathodic-protection reference cell to be suspended into the tank.Access openings shall also be provided at designated locations to ac-commodate the electrical conduit for impressed-current systems. Ifalternate power sources are specified, they can be either wind- orsolar-powered units mounted on the roof at specified locations.

Liquid level indicator fittings If a gauge board consisting of a float and atarget board is present, holes for designated fittings shall be providedon the roof at locations specified by the designer.

Inlet stop/start controls Where a probe or transducer system con-trols the water level and advises operators of low water levels, probesrequiring waterproof flanged entries at the top of the tank shall beinstalled at locations specified by the designer.





Sampling hatch If specified by the owner, access through the roof fora sampling hatch shall be provided at the location specified by thedesigner. A suitable cover shall be provided over the access entry.

Overflow inspection hatch If specified by the owner, access throughthe roof shall be provided to enable observation and inspection of theinterior overflow opening. A suitable cover shall be provided over theaccess entry.

VentsFor closed-top tanks, venting must be provided to safeguard againstexcess pressure or vacuum buildup during the maximum inflow oroutflow of water. Structural failures of tanks can be caused by inade-quate venting. When the vents are being sized, the area to which theoverflow pipe contributes should not be considered part of the venti-lation area. A minimum of one vent is required; this should be locatednear the center of the roof. For larger-diameter tanks, several ventsshould be located around the periphery as well as at the center of thetank to facilitate crossflow ventilation.

The most common forms of tank vents are the mushroom, pan(Fig. 2-13), and 180-degree types. Vents with pressure- and vacuum-releasing pallets are recommended. A clog-resistant vent is shown inFig. 2-14. All vents should be screened to protect against the entryof birds, animals, and insects. The screening should be stainless steelor some other type of corrosion-resistant material. Some health au-thorities require that shields be installed to keep dirt and debris fromblowing into the tank. In areas of snow buildup, the vents should beprotected or elevated to prevent them from being clogged by snow.Special vent designs may be necessary to prevent vents from cloggingor freezing over, based on local conditions and operations.

AA

Plan view

Vent

diam

.

Cover diam

eter

(Outside diameter)

(Inside diameter)

(Outside diameter)

(Hole in roof)

Section A-A

Tank roof

3/16

FIGURE 2-13 Pan deck vent detail. (Note: diam. = diameter.) (Source: AWWAManual M42, Steel Water-Storage Tanks)





Support bars

Carbon–steel

body

Roof

PTFE gaskets (typical)

Air pressure

1/2 - no. 13 × 15 flattenedexpanded metal bird screen

Screen(brass material is normal)

Pressure palletVacuum pallet

Air vacuum

Install vent vertical 5 +

FIGURE 2-14 Typical clog-resistant vent detail. (Note: Pallets should beremoved during coating to prevent clogging of the screens. Periodicinspection and maintenance are required to keep in proper working condition.PTFE = polytetrafluoroethylene.) (Source: AWWA Manual M42, SteelWater-Storage Tanks)

Many older riveted tanks do not have vents; instead, they havefinial balls that provide limited or no ventilation area. These finial ballsshould be replaced with vents when maintenance or repair work isdone on the tank. As distribution systems and pumping capacities areenlarged, the vent and overflow capacities on existing tanks shouldbe reevaluated. Tanks have failed because of pressure or vacuum re-sulting from inadequately sized or improperly maintained vents andoverflows. The maximum withdrawal rate is usually assumed to beeither the value that occurs when the pipes at grade level break or themaximum rate pumped from low-elevation reservoir tanks.

Controls and Devices for Indicating Water LevelAn indicating system of some type should be provided on the tank sothat operators can easily determine the water level. The most commondevices used to measure water level are gauge boards and pressuretransducer readouts.

Each form of water-level indication has advantages and disad-vantages. Cost and the need for direct or remote reading, ease ofmaintenance, and performance in adverse weather conditions shouldall be considered when selecting an indicating system.

Gauge BoardsGauge boards are normally composed of a float and target board onwhich water level indication is accomplished by noting the positionof a target against a gauge board on the outside of the tank. The target





marker is controlled by a cable attached to the float. As the float rises,the target marker is lowered. At capacity or full conditions, the tar-get will be at the low end of the gauge board, making it potentiallyaccessible to vandalism or snow buildup. Half-travel gauge boardsare recommended to protect the target and float from vandalism andhigh winds. Float-type systems such as these are not recommendedin freezing climates.

Altitude ValvesIn many water distribution systems, altitude valves are used to con-trol the water level in tanks for which the high water level is at a lowerelevation than the pressure gradient of the system. Even some smallone-tank systems have been designed with an altitude valve on thetank inlet/outlet line. Both of these are examples of improper use of al-titude valves. Altitude-control systems can be designed and installedwith timers that force the altitude valve to open, allowing water toflow into and out of the tank and ensuring more frequent turnover.

Altitude valves may malfunction even in good weather. Freezingweather increases the likelihood of malfunction, with frozen pressure-sensing lines giving the altitude valve false signals. This usually causesthe tank to overflow, but it may also cause the valve to remain closed,keeping the water in the tank static. Putting electrical heat tape andinsulation on the control piping or heating the altitude valve enclosureminimizes these problems.

Remote ReadingsA pressure transducer in the tank can indicate the water level at aremote readout some distance from the tank facility. The pressuretransducer must be installed so that it is completely isolated from allinlet and outlet openings. Pressure transducers are sensitive enoughto sense pressure changes created by water movement through a linethat would cause a false reading. The pressure transducer can alsocontrol flow in and out of the tank by actuating pumps or valves.

Inlet Stop/Start ControlsA water utility may install a probe or transducer system to control thewater level and to advise operators of low water levels. Probes requirewaterproof flanged entries at the top of the tank. In addition, for radioor wired telemetry equipment, an insulated conduit from the tanktop to ground level must be installed to carry the electrical signal. Ifprobes are used in tanks that are subject to icing conditions, the probesystem should be designed to prevent damage from freezing.

Pressure GaugesIf freeze protection is provided, economical Bourdon pressure gaugesmay be connected directly to the tank or riser.





Emergency Fill/Withdraw ConnectionsSome tanks may require provisions for emergency filling or with-drawal. Typically, such provisions are needed when tanks are locatedin remote areas where firefighters may withdraw directly from thetank to fill pumper trucks or use stored water directly. Where required,the emergency valves and connections should be designed to matchthe emergency facilities of the agency that will use them. The designmust avoid cross-connections between the emergency system and thepotable water system. Connections should be protected from freezingand vandalism, and the tank venting and overflow systems should besized for these unusual fillings and withdrawals.

Cold-Weather Operations

Designing Tanks for Cold WeatherProper design of a tank will prevent most freezing problems and, iffreezing does occur, will allow personnel to follow operating proce-dures to easily deal with it.

Inside appurtenances Tanks located in an area where the lowest one-day mean temperature (LODMT) is −5◦F (−15◦C) or colder shouldnot be equipped with inside ladders or overflow pipes. As ice formsand moves up and down, it can exert tons of force on ladders andpipes, tearing them loose from their supports and possibly rippingor punching holes in the container. The resulting leak will occur ata very inopportune time. If an inside overflow pipe is broken, thetank will rapidly lose all water down to the break, creating a large icyarea on the ground below. If the vent is plugged with ice or snow, thetank roof may collapse when water evacuates the tank rapidly. It isacceptable to equip a tank with inside ladders and overflow pipes ifthe tank is known to have a high turnover of warm water. A ladder andoverflow can also be installed at the center of the tank and supportedby the access tube, as in single-pedestal tanks and extremely largecolumn-type tanks. The use of interior girders, roof bracing, painter’srails, or virtually any other protrusion below the high water line orwithin an area affected by floating or suspended ice is a poor designpractice for areas with an LODMT of −20◦F (−29◦C) or colder. Certainlocal conditions or tank usage patterns may cause equally severe icingproblems in warmer areas.

External features In addition to standard appurtenances and acces-sories discussed herein, several design issues for the tank exterior aresignificant for cold-weather operation.

Roof opening location Risers or inlet pipes should be directly belowroof vents or manholes, or an auxiliary opening should be provided.This arrangement will facilitate thawing the tank if required. No pipe





F(Normal threadengagement)

Padlock(by others)

H-max. opening

G

B-pipethread

E-max. opening

T-pipethread

J-lift

U-pipethread(Normal thread

engagement)

Screwed modelshave B-pipe threads

C-max. opening(normal thread engagement)

Flanged models haveK-150 LB ASA mounting flangeL-holes; M-dia. on S bolt circle

A-pipe thread

D

V

Size A B C D E F G H J K L M S T U V

2 in. 1 ⁄ 2 9 2 ⁄ 5 ⁄ 3 ⁄ 4 ⁄ 8 ⁄ ⁄ 3 4 ⁄ 6 1 ⁄ 1 ⁄ 5 ⁄

3 in. 2 ⁄ 3 8 ⁄ 2 ⁄ 5 ⁄ 4 ⁄ 5 ⁄ 7 ⁄ ⁄ 4 8 ⁄ 7 ⁄ 2 ⁄ 4 ⁄

4 in. 3 4 8 ⁄ 3 ⁄ 6 ⁄ 4 ⁄ 6 7 ⁄ 1 ⁄ 6 8 ⁄ 9 ⁄ 3 3 ⁄ 4 ⁄

FIGURE 2-15 Double-seating, internal-closing drain valve. (Note: lb = pounds;1 in. = 25.4 mm; 1 lb = 0.4 kg; ASA = American Standards Association [nowANSI].) (Source: American Water Works Manual M42, Steel Water-StorageTanks)

opening should have a protective discharge cap that would precludethe dropping of a probe into the inlet pipe to thaw the ice blockage.Any gratings over piping should not be so restrictive as to preventthawing lines or pipes from being lowered into the pipe. Gratingsalso conduct heat and promote freezing.

Static water projections Unless it is heated and insulated, pipingthat extends from the tank or from other piping should not containstatic water. Drain valves extending on nipples will easily freeze.Drain valves should be of the double-seating, internal-closing type(Fig. 2-15).

Additional outlets Side outlets on the riser pipe for use in pumpingin steam or warm water to thaw the riser may be included in thedesign. These outlets should be plugged at the pipe outer diameter toeliminate an unheated projection.

Frostproof vents Vents should be designed to avoid freezing over or toprovide for pressure or vacuum relief. Some tank manufacturers haveproprietary designs for this purpose. A unique freezing problem mayoccur when frost freezes solid over the fine screen in the vent and over-flow. This type of freezing usually occurs on the fine screen designedto keep insects out of the tank. Such freezing prevents the exchange





of air into the tank, resulting in a vacuum in the tank that can collapse(implode) the tank until there is a structural rupture to break the vac-uum. Preliminary research indicates that fiberglass screen material isresistant to freezing.

Systems to Prevent FreezingVarious types of systems or equipment can be used to prevent tankfreezing. The following information is included as the design mayinclude one or more of the noted items.

Heating Heating a community water supply tank is usually not eco-nomically feasible, though industrial sprinkler tanks for fire protectionhave been heated for many years. However, new insurance rate struc-tures and better community water supply systems have allowed manyfactories either to dismantle the fire protection tank or to discontinueheating it. In many cases, the insurance savings no longer offset theheating energy costs.

Air bubblers Air-bubbler systems have been used successfully inground storage tanks and in elevated tanks with large risers. A bub-bler system is shown in Fig. 2-16. Research on the use of these systems

Riser

Air compressor

Aeration line

Air compressor

RiserAeration line

FIGURE 2-16 Tank riser bubbler system. (Source: AWWA Manual M42, SteelWater-Storage Tanks)





indicates that a high-pressure compressor should be used that exertsjust enough pressure to overcome air-line friction, orifice friction, andthe hydrostatic head related to the water depth. There must also be aninflux of warmer water, because the bubbling action tends to removeall the heat from a confined volume of water. The compressor shouldbe equipped with an air filter and water and oil traps to minimize thelikelihood that contaminants will be pumped into the tank.

Circulating pumps Circulating pumps that do not heat the water havebeen successful on tanks with small-diameter (6 to 12 in. [150 to300 mm]) riser pipes in Iowa, Minnesota, North Dakota, and SouthDakota. A circulating system is shown in Fig. 2-17. A relatively small(1.5-hp [1.1-kW]) pump draws water from the base elbow, pullingwater down the insulated riser or from the connecting pipe. The pumpdischarges water into a line 1 in. (25 mm) in diameter that enters the

Bowl of tank

Drip ring

Insulated riser

Circulating line

Foundation

1.5-hp circulating pump

FIGURE 2-17 Pumped circulation system for small riser pipes. (Source: AWWAManual M42, Steel Water-Storage Tanks)





riser at the base of the tank and discharges into the tank container.This creates circulation in the riser.

Insulation In many tank designs, it is a common practice to insulateriser pipes. Many ground storage tanks and a few elevated tanks havealso had their exteriors sprayed with urethane foam to insulate them.This yields a rough appearance, however, and there may be problemsmaintaining adhesion between the foam and the steel. No matter howthick the insulation, the stored water will eventually freeze withoutheat input.

Additional Accessories and ExceptionsAny additional accessories required to be furnished shall be specifiedby the purchaser. Exceptions to the provisions of this section may bespecified by the purchaser to suit special situations.

Elevated Tanks

Steel RiserIn localities where freezing temperatures do not occur, the purchasermay specify a small steel riser. In other locations and unless a smallpipe is specified, a steel riser not less than 36 in. (910 mm) in outsidediameter (OD) shall be furnished. Where the riser pipe supports aconsiderable load, the riser diameter and thickness shall preferablybe determined by the constructor.

Cold ClimatesThe minimum riser diameter of 36 in. (910 mm) shall be increased incold climates unless the riser is heated to prevent freezing. The properdiameter depends on the extent of the tank’s use and the temperatureof the water supplied. In extremely cold climates, a minimum diameterof 72 in. (1,830 mm) is recommended.

ManholeLarge-diameter risers shall contain a manhole about 3 ft (0.91 m) abovethe base of the riser. The manhole shall not be less than 12 × 18 in.(305 × 457 mm), and the opening shall be reinforced or the riser plateso designed that all stresses are provided for around the opening.

Safety GrillA safety grill is intended to prevent a person from falling down theriser and shall be exempt from the design loads specified in AWWAD100 Section 3.1.6. When a safety grill is used in the top of the riserduring erection, it shall be removed if the tank is located in climateswhere freezing is likely to occur. When grills are left in place, they





shall be provided with a hinged door that is at least 18 × 18 in. (457 ×457 mm).

Expansion JointWhere the riser is not load bearing, flexibility to accommodate differ-ential movements of the tank and riser foundation must be included.This flexibility may be provided by an expansion joint or by riser lay-outs that have sufficient offset to be axially deformed without over-stressing the riser, tank, or foundation.

Pipe ConnectionThe pipe connection shall be of the size specified by the purchaser,and it is usually attached to the riser bottom at a point designated bythe purchaser. Connections to the tank or piping furnished by the tankconstructor shall be made by the purchaser.

Silt StopIf a removable silt stop is required, it shall be at least 6 in. (152 mm)high, and the fitting or piping connection shall be flush with the riserfloor when the stop is removed. If a removable silt stop is not required,the connecting pipe shall extend at least 6 in. (152 mm), and preferablyabout 2.59 ft (789 mm), above the riser floor.

Inlet ProtectionIn risers 36 in. (910 mm) in diameter or larger, the inlet pipe shall beprotected against the entry of foreign materials dropping from above.This shall be done by terminating the inlet pipe or the top of the silt-stop pipe with a tee, with the “run” of the tee placed horizontally, orby placing over the silt-stop or inlet pipe a circular plate 8 in. (203 mm)larger in diameter than the pipe and located horizontally above theend of the pipe or silt stop at a distance equal to the diameter of thepipe. The circular plate shall be attached to the pipe, silt stop, or riserbottom with a suitable bracket or welded bars. Adequate clearanceshall be provided between the ends of the elbow or from the edge ofthe circular plate to the wall of the riser pipe to permit proper flowof water through the inlet pipe. Pipe connections to the riser shell arepermitted, as long as adequate protection against freezing has beenprovided.

OverflowAn overflow protects the tank from overpressure, overload, and pos-sible catastrophic failure should the pumps or altitude valve fail toshut off when the tank is filled to capacity. A properly operated tankshould not overflow during normal operation. An overflowing tank





is an emergency, and the malfunction causing the overflow should bedetermined and corrected as soon as possible.

The tank shall be equipped with an overflow of the type andsize specified by the purchaser. If a stub overflow is specified, itshall project at least 12 in. (304 mm) beyond the tank shell. For tanksequipped with balconies, the overflow shall be extended to dischargebelow the balcony. If an overflow to ground is specified, it shall beplaced down the tank shell and supported at proper intervals withsuitable brackets. The overflow shall be located such that it will notbe obstructed by snow or ground clutter. It shall terminate at the topin a weir box or other appropriate type of intake. The top angle shallnot be cut or partially removed. The overflow pipe and intake shallhave a capacity at least equal to the inlet rate as specified by the pur-chaser, with a head not more than 6 in. (152 mm) above the lip ofthe overflow and in no case more than 12 in. (304 mm) above the topcapacity level, where a side opening–type overflow is used. The over-flow pipe shall terminate at the bottom with an elbow, which shallbe directed away from the foundation. Unless otherwise specified bythe purchaser, the overflow pipe shall be steel pipe, with screwed orwelded connections if less than 4 in. (102 mm) in diameter, or withflanged or welded connections if 4 in. (102 mm) or larger in diameter.Overflows may be either internal or external as specified by the pur-chaser. Minimum external overflow pipe thickness shall be 3/16 in.(4.8 mm). Internal overflows are not recommended when tank usageand climatic conditions are such that ice damage may occur to theoverflow or its attachments. When specifying an internal overflow,the purchaser should consider the consequences of an overflow fail-ure, which can empty the tank of its contents. Internal overflow pipeshall be at least 0.25 in. (6.35 mm) thick. The end of the overflow maybe covered with a coarse, corrosion-resistant screen equivalent to 3/8in. (9.5 mm) or larger mesh or with a flap valve, as specified by thepurchaser.

Ladders

Tower LadderA tower ladder shall be furnished with side rails no less than 2 in. ×3/8 in. (51 mm × 9.5 mm), with a spacing between side rails of not lessthan 16 in. (406 mm) and with nonskid rungs not less than 0.75 in. (19mm) round or square, spaced 12 in. (305 mm) apart on centers. Thetower ladder shall extend from a point 8 ft (2.4 m) above the groundup to and connecting with either the horizontal balcony girder or thetank ladder, if no balcony is used. The ladder may be vertical but shallnot in any place have a backward slope.





Outside Tank LadderIn all cases, a ladder shall be provided on the outside of the tank shellconnecting either with the balcony or with the tower ladder, if nobalcony is included. The outside tank ladder shall have side rails notless than 2 in. × 3/8 in. (51 mm × 9.5 mm), with a spacing between theside rails of not less than 16 in. (406 mm) and rungs not less than 0.75in. (19 mm) round or square, spaced 12 in. (305 mm) apart on centers.The tank ladder may be attached to the roof ladder.

Roof LadderUnless otherwise specified, the constructor shall furnish access to roofhatches and vents. Such access shall be reached from the outside tankladder or riser ladder on pedestal tanks according to the following:� For slopes 5/12 or greater, a ladder or stairway shall be pro-

vided.� Slopes less than 5/12 and greater than 2/12 shall be providedwith a single handrail and nonskid walkway.� Slopes 2/12 or less do not require a handrail or nonskid sur-face.

Ladder RequirementsMinimum requirements for ladders, hatches, and so forth can be foundin OSHA 29 CFR Part 1910. Note: Regardless of the access protectionprovided to tank roof hatches and vents, weather conditions on tankroofs are extremely variable, and workers and their supervisors areexpected to exercise good judgment in matters of safety. Among otherthings, this may include the use of safety lines when windy, icy, orother hazardous conditions exist.

Ladder Safety DevicesIf safety cages, rest platforms, roof-ladder handrails, or other safety de-vices in excess of OSHA requirements are stipulated by the purchaseror by state or local laws or other regulations, the purchaser shall sospecify. None of these devices are advisable on the submerged portionof interior ladders in low-temperature climates.

Roof Openings

Above Top Capacity LevelAn opening shall be provided above the top capacity level. It shallhave a clear dimension of at least 24 in. (610 mm) in one direction and15 in. (381 mm) in the other direction and shall be provided with asuitable hinged cover and a hasp to permit locking. The opening shall





have a curb of at least 4 in. (102 mm) high, and the cover shall have adownward overlap of at least 2 in. (51 mm).

Tank CenterAn additional opening with a removable cover having an openingdimension or diameter of at least 20 in. (500 mm) and a neck at least4 in. (102 mm) high shall be provided at, or near, the center of thetank. This opening may also be used for the attachment of exteriorpaint rigging. Where conveniently accessible to an outside balconyor platform, a shell manhole may be substituted for the additionalopening. If properly designed, the shell manhole may be placed belowthe top capacity level.

VentIf the tank roof is of tight construction, a suitable vent shall be fur-nished above the top capacity level, which shall have a capacity topass air so that at the maximum flow rate of water either entering orleaving the tank, excessive pressure will not be developed. The over-flow pipe shall not be considered a tank vent. Warning: An improperlyvented tank may cause external pressures to act on the tank that cancause buckling even at a low-pressure differential.

LocationOne tank vent shall always be located near the center of the roof, evenif more than one tank vent is required. For tanks with centrally locatedaccess tubes, a reasonable offset of the vent is permissible. The ventshall be designed and constructed to prevent the entrance of birds oranimals.

ScreeningWhen governing health authorities require screening against insects, apressure-vacuum screened vent or a separate pressure-vacuum reliefmechanism shall be provided that will operate if the screens frostover or become clogged with foreign material. The screens or reliefmechanism shall not be damaged by the occurrence and shall returnautomatically to operating position after the blockage is cleared. Note:The purchaser should clean the screens and check the pallets or reliefmechanism for operation at least once a year, but preferably eachspring and fall.

Additional Accessories and ExceptionsAny additional accessories required to be furnished shall be specifiedby the purchaser. Exceptions to the provisions of this section may bespecified by the purchaser to suit special situations.





Tank Mixing SystemsRecently, many improvements have been made to the efficiency ofmixing systems that allow homogeneous mixing of all water each timethere is a fill and/or draft cycle (Fig. 2-18). New designs are needed thataccommodate these systems, which include appurtenances and acces-sories not previously used. Because of differences among systems, thefollowing overview is offered to assist in determining appurtenancesand accessories to be used on a specific tank mixing design.

Placing the inlet and outlet nozzles of water-storage reservoirson opposing sides of the tank shell was for years considered to bethe optimum arrangement to achieve good water blending. Addingan interior elbow to direct the water in a circular flow pattern wasconsidered a good practice to help avoid short-circuiting through theoutlet. In recent years, the industry standard practice has included theinstallation of a hydraulic tank mixing system (TMS). A typical TMSconsists of an arrangement of piping placed internally in the reser-voir to allow a homogenous mixing of all water in the reservoir eachtime there is a fill and/or draft cycle. The TMS consists of a manifoldsystem in which both inflow and outflow waters pass through twosets of properly sized and placed check valves. One set of valves isused during the fill cycle, and the other is used to drain the tank. Themanifold can be a common inlet/outlet system connected to one shellnozzle designed per API Standard 650 or to a bottom-flanged nozzle

Plan-outlet crossInlets

Manifold

See N

ote

2

Blind flange

InletsOutlet cross Shell

Shell

Existing outlet

Existing inletManifold

FIGURE 2-18 Mixing system layout. (Notes: [1] Modification to inlet reservoirmay be required in order to install mixing system. Detail of existingpenetration must be provided. [2] Angle to be 30 degrees for water depth30 ft [9 m] or below. Angle to be 45 degrees for water depth above30 ft [9 m].)





connected to a 90-degree elbow. Alternate designs may include sepa-rate inlet and outlet manifolds positioned either horizontally or verti-cally depending on the physical height and diameter of the reservoir.

The size of the manifold piping and the placement, number, andsize of the inlet and outlet valves can be determined through calcu-lations resulting from hydraulic studies and/or computational fluiddynamics (CFD) modeling.

The types of materials used to construct the TMS manifoldsis largely a function of budget limitations. Materials may includepolyvinyl chloride (PVC), high-density polyethylene (HDPE), fusion-bonded epoxy, liquid epoxy, or cement-mortar-lined (CML) andepoxy-coated carbon- or ductile-steel pipe and fittings. At the upperend of the scale, 316-grade stainless-steel pipe and fabricated fittingscan give a lifetime of maintenance-free service. The choice of materialsalso depends on whether the TMS is being installed in a new tank (inwhich case the parts can be placed inside the shell before the roof goeson) or whether the project involves retrofitting an existing reservoirwith limited access.

Water Sampling StationsAs with TMSs, water sampling stations have become more sophisti-cated in recent years. Because systems differ, the following overviewis offered to assist in determining appurtenances and accessories tobe used for a specific sampling design.

Testing the quality of the water in water-storage reservoirs isbecoming increasingly important. The use of chloramines for disin-fection requires that the chlorine residuals in the water reservoir bechecked far more often. Placing water sampling points at various lo-cations and levels in the reservoir enables the sampling technician toalso check the water for possible problems resulting from stagnationand/or thermal stratification. These problems usually result from pe-riods of low water usage—that is, in the off-peak season or when thereare rapid changes in the weather. They can also result from incompletemixing of the reservoir contents when fresh water is introduced to wa-ter that has been in the tank for an extended period of time.

It is also important for the system operator to regulate the fill/draw cycle to match periods of high and low consumption. Numerousvariables enter into the formulations that keep these ratios at optimumconditions, and by sampling the water for pH, chlorine residual, andbacteriological levels, the operator gains valuable information to helpmonitor and control proper conditions.

The size of the reservoir determines the quantity, size, and locationof the sampling points. Small reservoirs with high turnover rates mayonly require one sampling port, while in most reservoirs with a singlecenter roof-support column, it is suggested that at least three collection





points be positioned vertically in a fixed position near the shell. Thisusually provides sufficient sampling data. The sampling points shouldbe set at elevations within the lower, middle, and upper-third of thecontent levels. Direct sunshine on the tank shell, especially duringsummer months, can raise the temperature of the water near the shell,thus making the water sample misleading. Therefore, it is suggestedthat the sampling points be placed at least 3 ft (0.9 m) away from theshell, preferably on the north side of the reservoir. Water samples takenat these points yield consistent levels and are more representativeof the tank water. For larger reservoirs, additional sampling pointsfrom the middle and intermediate points should be considered. It ispossible to locate sampling points at variable levels through lines thatare attached to brackets connected to the intermediate roof-supportcolumns. The final number, size, and location of sampling points mustbe carefully planned to allow sampling from all points within thereservoir. Lastly, in some cases it might also be beneficial to locate asampling point directly in front of the inlet water nozzle to collect datafrom the incoming water. This allows the operator to compare datafrom the fresh water with data from the stored water.

Generally, lines 0.75 in. (19 mm) in diameter permit sufficient sam-pling from all points within the reservoir. It is imperative that the linesbe constructed of noncorrosive materials such as PVC or 316-gradestainless steel. For the convenience of the system operator, it is best tohave all sampling lines pass through the shell and into one securedenclosure that is mounted to the shell at chest height. Samples canbe taken by opening control valves that are also made of 316-gradestainless steel. These valves should be conveniently located within theenclosure and positioned such that there is ample room beneath themfor the collection bottles. For security purposes, the enclosure shouldbe locked when not in use.

AntennasThe AWWA Steel Tank Committee has noted that the wireless commu-nication industry has been installing antennas on ground and elevatedtanks at an ever-increasing rate. The major tank contractors have allbut forfeited these installations to non-tank constructors. Guidelineswere added to the commentary for AWWA D100–05 (see AppendixA) to provide the owners and their consultants with additional infor-mation when addressing these accessories. The guidelines considerfunctional, structural, future maintenance, and safety issues havingto do with antenna and communication installations.

Health and SafetyRecommended safety precautions for radio frequency (RF) exposureof personnel maintaining the tank should be reviewed with the wire-less carriers.





Precautions shall be taken to prevent water contamination. Accessto the tank interior water compartment should not be permitted.

The paint system should be checked for hazardous metals. Wherehazardous metals are found in the paint system, the environment,potable water, and workers must be protected from contamination.

Fall protection should be provided for workers. This may consistof a safety rail around the installation or anchor points on the tank roofwhere safety lines can be attached. The addition of auxiliary laddersor safety lines for access to new equipment should be considered.

Antenna cables should be supported at regular intervals (about 4ft [1.2 m] on center) in exposed locations. Cables should be attached toladders, as they present a safety hazard. Cable ladders or other com-mercially available cable support systems are available and should beinstalled separately on the tank.

Manholes and other access ports should not be obstructed bythe cable routing. Where space is limited (e.g., small-diameter 36-in.[915-mm] access tubes in pedestal tanks), cables should be fitted tothe access tube well to maximize clearance.

Cables routed along balconies and platforms should be routed soas not to obstruct access. Consideration should be given to provideauxiliary painters-scaffold supports if the antenna installation rendersthe existing system unusable. Antenna cables should be raised off thetank surface to permit painting behind them.

BibliographyHarper, W. B. 1986. Designing a More Corrosion-Free Water Storage Tank.

In Proc. 1986 AWWA Annual Conference, Washington, D.C.; Denver, CO.:AWWA.

Matchett, B. 2006. Introduction to Improved Water Sampling Stations for SteelWater Storage Tanks. NACE International, Channel Islands Section semi-nar “Build a Tank in a Day,” Oxnard, CA.

Matchett, B. 2007. Introduction to Improved Tank Mixing Systems forWater Storage Tanks. NACE International, Channel Islands Section semi-nar “Build a Tank in a Day,” Oxnard, CA.




C H A P T E R 3ControllingCorrosion

Mike Bauer and Joe DavisTnemec

Anthony D. IppolitiSherwin Williams

Jeff RogCorrpro

The Nature of CorrosionThe corrosion of steel in aqueous solutions is an electrochemical pro-cess in which a current flows and a chemical reaction occurs. Corrosionis a natural process that follows the laws of science. All metals possessinherent levels of electrical energy. They must maintain those levelsto remain stable and thus not be subject to the degradation process ofcorrosion. These levels of electrical energy are measurable, and met-als can be either more or less reactive in various environments on thebasis of their inherent levels of energy or electrical potentials. Metalswith higher levels of electrical energy tend to be more reactive, andmetals with lower levels of electrical energy tend to be less reactive(more noble). Figure 3-1 illustrates how the various metals’ electricalpotentials compare.

The corrosion cell comprises four basic elements: anode, cathode,electrolyte, and closure path (Fig. 3-2).

The anode is the metal that corrodes—that is, metal ions leaveits surface and enter the electrolyte solution. The cathode is a metalfrom which no metal ions enter the solution. The electrolyte may beany solution, such as drinking water, that is capable of conducting

81




82 C h a p t e r T h r e e

Active

• Magnesium alloy

• Zinc

• Aluminum alloy

• Cadmium

• Mild steel (new)

• Mild steel (old)

• Cast iron

• Stainless steel

• Copper, brass, bronze

• Titanium

• Gold

Noble

FIGURE 3-1 Howelectricalpotentials ofvarious metalscompare.

electricity. The closure path, also called the return current path, isthe electrical conductor (usually metal) that connects the anode andthe cathode. If any one of these elements is missing, corrosion doesnot occur. For example, coating stops corrosion from occurring byproviding a barrier to the current that flows between the metal andthe electrolyte.

A dry-cell battery is a corrosion cell. When the battery’s anode(zinc) and cathode (carbon) are connected through a closure path (thelightbulb), the potential difference between the zinc and the carbon

Metallic path

Cathode

OH+

Electrolyte

H+

Anode

FIGURE 3-2 Elements of the corrosion cell.



Controlling Corrosion

83C o n t r o l l i n g C o r r o s i o n

Cathodicarea

(protected)

Anodicarea

(corrodes)

Tank wall (conductor)

Water

(electrolyte)

FIGURE 3-3 Anode and cathode in a steel water-storage tank.

produces a current flow. The current continues to flow until the zincanode is consumed by the corrosion process.

It is important to consider why the current flows in the directionit does. The direction of flow is determined by the metals selectedfor the dry-cell battery’s case and center post. If the center post wasmagnesium instead of carbon, the current flow would be reversed:The magnesium center post would be the anode (which corrodes),and the zinc base would be the cathode (which does not corrode).

The current can also be forced to flow in the opposite directionif the standard carbon/zinc battery is connected to an outside cur-rent source instead of the lightbulb. In this situation, the anode andthe cathode would also be reversed—that is, the battery case wouldbecome the cathode and would be protected from corrosion.

In a steel water-storage tank, some portion of the metal will be theanode and some portion will be the cathode (Fig. 3-3).

Which area takes on which function depends on impurities inthe metal; surface conditions; oxygen concentrations in the water; thepresence of any dissimilar metals; stresses caused by manufacturing,heat, or concentrated structural loads; and/or several other factors. Atthe anode, metal ions leave the surface, enter the water, and combinewith oxygen to form rust. Electrons released from the anode travelthrough the metal to the cathode. At the cathode, an ion exchangeoccurs, but no metal is lost and no corrosion occurs.

The presence of ladders, mixing systems, baffling systems, floats,or other accessories made of stainless steel that are electrically





continuous with the carbon-steel tank causes accelerated corrosionin steel exposed at holidays (voids) in the coating. In such cases, thestainless-steel components are the cathodes and the exposed-steel por-tions of the tank are the anodes. Care must be taken when designingsuch accessories to eliminate the galvanic or dissimilar metal corro-sion between the metals of different electrical potentials. Methodsfor addressing the corrosion caused by dissimilar metals are cathodicprotection, using a homogenous metal (coated carbon steel), using anonmetallic material such as fiberglass-reinforced plastic, coating thestainless steel to minimize the cathodic surface areas the carbon steelis reacting with, and/or electrical isolation of the dissimilar metals.A corrosion engineer should review situations in which dissimilarmetals are used in steel water tank fabrication to determine the mosteffective solution(s) for controlling corrosion.

Principles of Cathodic ProtectionCathodic protection systems are used to prevent or retard the cor-rosion that would naturally occur in a steel water tank. These sys-tems prevent or slow corrosion by altering the electrochemical envi-ronment so that the submerged tank shell becomes the cathode of acorrosion cell. Since the cathode of a cell does not corrode, the sub-merged metallic tank shell is protected. There are two basic types ofcathodic protection systems: impressed-current systems and galvanicsystems.

Impressed-Current SystemsIn an impressed-current system of cathodic protection, an outsidesource of electrical power forces current into anodes submerged in thestorage tank’s water. The current flows from the anodes through thewater (electrolyte) and onto the submerged walls of the tank, makingthe tank itself the cathode of the corrosion cell. An impressed-currentcathodic protection system (Fig. 3-4) consists of a manual or automaticalternating current/direct current (AC/DC) converter (i.e., a rectifier),feeder wires, and anodes inside the tank. The DC output voltage istypically adjusted and controlled automatically to account for a widerange of variables. To prevent damage to the coating, care must beexercised to ensure that the polarized voltage does not exceed a max-imum value as noted in the industry standards; otherwise the coatingmay be damaged. Because excessive current output may damage thecoating, manually controlled rectifiers without automatic adjustmentand potential limiting capabilities are typically not recommended forcoated steel. The precise maximum negative voltage is dependent onthe characteristics of the coating and other factors.





FIGURE 3-4Impressed-currentsystem.

Galvanic SystemsIn a galvanic system (Fig. 3-5), a block of specially selected metalcalled a sacrificial anode is immersed in the electrolyte and electricallyconnected to the metal of the tank. The metal of the sacrificial anode isselected so that it will become the anode of the corrosion cell, with thesteel tank being the cathode. Magnesium is the most common anodematerial employed for corrosion control in potable water. The anodesare typically of an extruded-rod type that are either suspended in thewater from the roof of the tank or suspended in the lower portionof the tank supported from the sidewalls or by supports on the tankfloor.

These anodes are fabricated with a copper lead wire connectedto the core of the anode and then attached to the steel tank. Whenthe connection is made and the anodes are submerged in the water,the current flow from anode to cathode (steel tank) begins; thus, themagnesium corrodes and the steel is protected. Galvanic systems havebecome increasingly popular because no electrical current is required.

Output of the sacrificial anodes may be monitored by using refer-ence electrodes permanently installed in the tank below the surface ofthe water. The anode lead wires and reference electrode lead wires maybe run into a test station installed at ground level to facilitate routine





FIGURE 3-5 Galvanic system.

monitoring. A test station may also be equipped with a rheostat tocontrol the output of the anodes by altering the circuit resistance. Inthe absence of a test station, routing testing is accomplished with aportable reference cell.

Protective CoatingsCathodic protection is normally used in conjunction with a well-coated tank surface. The coating reduces the rate of anode consump-tion and power use. Coatings typically have microscopic voids thatexpose the metal to the water and allow metal loss if cathodic protec-tion is also not in place. The ideal corrosion control system combinesa good dielectric coating (metallic coatings are not dielectric) anda properly designed, installed, and maintained cathodic protectionsystem.





Exterior Corrosion of Tank BottomCathodic protection systems are usually designed to protect the inte-rior wetted surfaces of a water-storage tank. In some cases, however,the exterior of a tank bottom or shell is in contact with corrosive soils.This is typically the case for flat-bottom tank styles such as reser-voirs and standpipes. In those situations, proper selection of the tankbase material or backfill may reduce corrosion, or a separate cathodicprotection system can be designed to protect exterior surfaces in con-tact with soil (refer to American Water Works Association [AWWA]Manual M27, External Corrosion: Introduction to Chemistry and Control,for details). In cases where cathodic protection will be used to pro-tect such surfaces, impressed-current systems are generally recom-mended. Sacrificial anodes can also be used, depending on factorsincluding the size of the tank floor to be protected, soil or sand resis-tivity, and whether the surface to be protected is coated or bare.

Cathodic Protection DesignIn designing a cathodic protection system, the engineer must con-sider the quality of the protective coating, tank geometry, surface area,obstructions, geographic location, temperature, turbulence, and thechemical composition of the water stored in the tank. Among itemsto be specified in the design are the AC/DC converter or alternativepower source, the anode materials, and the anode configuration andsuspension.

Automatically Controlled AC/DC Converter (Rectifier)The protective-current demands in a water-storage tank continuouslychange because of variations in water chemistry and temperature,fluctuations in water level, coating deterioration, and polarization ef-fects. Automatically controlled impressed-current cathodic protectionsystems (Fig. 3-6) are typically used in water-storage tanks to adjustfor these variations.

Reference electrodes are used to continuously monitor the pro-tective level and control the cathodic protection current delivered tothe structure by the system. Separate control circuits are used for riserpipes and other areas of the tank that may have different localizedconditions.

Anode Materials and Design LifeImpressed-current anodes are typically made of mixed-metal oxide orplatinum that can provide a nominal minimum 10- to 20-year life. Thedesign life of the anode system is based on the anticipated protective-current requirements, condition of the coating, percentage of bare steelto be protected, and the known consumption rates for the selected





FIGURE 3-6 Automatically controlled AC/DC converter (rectifier).

anode material. The approximate consumption rate is 17 to 20 lb/ampyear for magnesium anodes. For platinized niobium, platinized ti-tanium, or mixed-metal oxides on titanium, the approximate con-sumption rate is 0.00008 to 0.0013 lb/amp year (0.036 to 0.59 g/ampyear).

Anode Configuration and SuspensionDistribution of current from any anode configuration is affected bythe geometric shape of the tank, obstructions within the tank, interiorcoating, and chemical characteristics of the water.

The anode system may be installed vertically or horizontally. Ver-tically suspended anodes (Fig. 3-7a ) are installed by hanging the an-ode from an electrically insulated device at the tank roof adjacent toholes cut in the roof. Horizontally suspended anodes (Fig. 3-7b) arepositioned below the normal water level attached to the tank shellor access tube. In elevated tanks with an inlet/outlet riser pipe 30 in.(76 cm) in diameter or larger, a vertically suspended anode is used toprovide protection within the riser.

When holes are cut in the roof, the finished installation must bewatertight to eliminate openings for insects and runoff to enter thetank.

For tanks subject to icing, either vertical anode systems with exten-sible elements or horizontal suspension systems designed to minimize





A

FIGURE 3-7a Vertical system reservoir tank.

ice damage to the anodes should be considered. These suspension sys-tems can provide year-round protection and may eliminate the needfor annual anode replacement due to ice damage.

Maintenance of Cathodic Protection SystemFollowing the installation of a cathodic protection system, a tank-to-water potential profile is performed to ensure that the systemis providing optimal corrosion control. The level of corrosion con-trol achieved by the cathodic protection system can be determinedthrough electrical testing. Corrosion is under control when a copper/copper sulfate reference electrode is placed adjacent to, but not touch-ing, the submerged tank surface and a polarized tank-to-water poten-tial of −0.850 V or less is measured.

Automatic rectifiers continuously monitor the tank-to-water po-tential being maintained by the system and make adjustments to





B

FIGURE 3-7b Horizontal system reservoir tank.

control corrosion. Personnel responsible for operating and maintain-ing the cathodic protection system should refer to the designer’s in-structions to fully understand their responsibilities. They should con-sult with the manufacturer if necessary regarding the equipment’soperation and make certain that all responsible personnel are fa-miliar with its operation. A successful cathodic protection corrosioncontrol system will continuously operate according to establishedcriteria.

Annual inspection of the cathodic protection system by the man-ufacturer or by a qualified corrosion engineer is recommended. Ata minimum, this inspection should include an overall examinationof the entire cathodic protection system, replacement of all defectiveparts, a potential profile survey, a physical check of the anode place-ment, and a written report. Various annual service plans are availablefrom the cathodic protection companies or other service organizations.Cathodic protection systems should be regularly tested and inspectedto ensure that they provide the maximum level of corrosion control tothe surfaces of the submerged steel tank.





Paint (Coating) BasicsPaint is a generic term used to describe a protective or decorativecoating that can be applied to a surface. Protective coatings and liningsapplied to potable water tanks are formulated to protect the substratefrom corrosion on tank interiors as well as for corrosion protectionand aesthetic value on tank exteriors. The many types of protectivecoatings available vary in their intended use, formulations, methodsof application, and in how they dry or cure.

Components of PaintMost paints are made up of three primary components—the solvent,which is incorporated into the formula to lower viscosity and allowthe painter to get the paint out of its container and onto the substrate;the resin or binder, which binds the material together and, more thanany other component, determines the physical properties and perfor-mance of the cured film; and the pigment, which can provide color,hiding, or any number of other desirable properties in the film (e.g.,gloss control, sag resistance, or added film strength). The combinationof the solvent and the resin is called the vehicle (Fig. 3-8). The resinbinder and the pigments make up the protective dried film after thesolvent evaporates. Most paints also contain additives, which will alsobe covered in this section.

Vehic

le

Volatile solvents

Resins (binders)

Pigments

FIGURE 3-8 Primary components of paint: solvent, resin, and pigment.





Organic SolventsMost paints contain volatile organic compounds (VOCs). These or-ganic solvents are incorporated into paint formulas to lower viscosityand allow the painter to get the paint out of its container and ontothe substrate. Water acts as a solvent for some types of paint, but itis not an organic solvent and therefore is not considered a VOC. Or-ganic paint solvents generally fall into one or more of the followingclassifications:

Active solvent: An active solvent is a true solvent for the resin(binder) portion of the formula. It dissolves the resin and keepsit in solution.

Diluent: Although not a solvent for the resin portion of a partic-ular paint formula, a diluent can still be used in conjunctionwith a true (active) solvent, without causing precipitation orincompatibility (“kick-out”).

Latent (auxiliary) solvent: This is not a true solvent, but com-bined with an active solvent, it increases the strength (solvencypower) of the active solvent.

Solvent Properties That Determine UseBefore deciding what solvent or combination of solvents can be usedin a particular paint formula, two important solvent properties mustbe considered:

Evaporation rate Some solvents are more volatile than others: Thegreater the volatility, the faster the evaporation rate. Because of itseffect on application properties, it is important to consider the evap-oration rate when selecting a solvent or solvent blend for a coatingformula. For example, the use of a “fast” solvent may be appropriateif the coating is typically spray applied, but the result may not be asmooth, continuous film if the coating is applied by brush or roller.

Flash point Flash point is defined as the temperature at which thevapor directly above a liquid will ignite when exposed to a spark oran open flame. The faster the evaporation rate of the solvent, the lowerthe flash point. The U.S. Department of Transportation defines paintswith a flash point below 100◦F (38◦C) as flammable and paints with aflash point above 100◦F (38◦C) as combustible. A label picturing a redflame must be affixed to containers holding flammable liquids with aflash point below 100◦F (38◦C).

Solvent Types or FamiliesAliphatic hydrocarbons Derived from the distillation of crude oil,aliphatic hydrocarbons are considered weak solvents for most resintypes. In a limited number of paint formulas, however, they canbe active or true solvents. Examples are oil-based paints, or alkyds.Naphtha and mineral spirits are the most commonly used aliphatichydrocarbons.





Aromatic hydrocarbons Aromatic hydrocarbons are also derived fromdistillation of crude oil, but they are considered stronger solvents fora wider range of generic paints than the aliphatic hydrocarbons. Allaromatic hydrocarbons contain a benzene-ring molecular structure.In fact, benzene is a solvent and the base molecule for this family ofsolvents. Aromatic hydrocarbons are active solvents for generic paintsincluding alkyd, oil-based, chlorinated-rubber, certain epoxies, anda few others. They are also used extensively as diluents. Aromatichydrocarbons most widely used in paint formulations are toluene,xylene, #100 solvent, and #150 solvent.

Ketones This family of organic solvents has very high solvencypower for most generic paints. Acetone, often used for cleanup in or-ganic chemistry labs, is the base molecule for this family. Ketones mostoften used in paint formulas are MEK (methyl ethyl ketone), MIBK(methyl isobutyl ketone), MNPK (methyl normal propyl ketone), andMAK (methyl amyl ketone).

Esters Esters, like ketones, have high solvency power for mostgeneric paints and are used most often in lacquers and furniture fin-ishes. They have limited use in industrial coatings because of theirhigh cost and reactivity with certain resins. Ester solvents most oftenused in paints are ethyl acetate, butyl acetate, isobutyl acetate, andamyl acetate.

Alcohols Alcohols are not true (active) solvents for most genericpaints. Exceptions are found in vinyl wash primers and ethyl sili-cate inorganic zinc–rich primers. Certain water-soluble alcohols arealso used as co-solvents in water-based paints and in water-emulsionpaints. Alcohol solvents found in paints include ethyl alcohol (drink-ing alcohol), isopropyl alcohol (rubbing alcohol), butyl alcohol, andamyl alcohol.

Glycol ethers and glycol-ether acetates Glycol ethers are unusual in thatseveral are water soluble yet also have high solvency power. Becauseof their water solubility, they are often used as co-solvents in water-based paints. Glycol-ether acetates are strong solvents that are oftenused in urethane paint formulations.

Pigments and Their FunctionsNearly all paints contain pigments. Exceptions are certain high-glossclear coatings. Unlike dyes, pigments used in protective coatings areessentially insoluble in water and organic solvents. Their chemicalstructure can be either organic or inorganic depending on the pigmenttype. Most inorganic pigments are derived from minerals that aremined from the earth. Organic pigments are made synthetically andare typically much more expensive than inorganic pigments.





Paint manufacturing would be a fast, simple process if it was notfor the need to include one or more pigments in most paint formulas.Considerable energy and time are required to disperse pigments intothe liquid components of any paint formula. During initial manufac-turing, the pigment suppliers grind pigments to a very small parti-cle size. Regrettably, these small pigment particles agglomerate be-fore they are added to protective coatings. When dispersing pigmentsduring paint manufacture, the intent is to break apart the pigmentagglomerates into the individual particles originally produced by thepigment manufacturer. The amount of energy and time required toaccomplish this depends on the type of pigment. In general, organicpigment agglomerates are much more difficult to disperse than inor-ganic. Most inorganic pigments can be dispersed using high-speeddispersion equipment, which is generally the fastest and the mosteconomical method of pigmented paint manufacture. More time andenergy are required to disperse most organic pigments; other manu-facturing methods such as sand milling or ball milling are required.

To understand the important functions of pigment in paint, weneed to examine the three major classifications of pigments: primepigments, extender pigments, and corrosion-inhibiting pigments.

Prime pigments Prime pigments provide color and hiding power andcan be either organic or inorganic. Red iron oxide, yellow iron oxide,titanium dioxide (TiO2), lead molybdate (toxic), and lead chromate(toxic) are examples of inorganic prime pigments. Carbon black, ph-thalocyanine blue, phthalocyanine green, and quinacridone violet areexamples of the more expensive organic prime pigments.

Extender pigments Most extender pigments provide little, if any, colorand hiding. All extender pigments are inorganic minerals that aremined from the earth. These pigments are used in paint formulasto provide a variety of desirable properties including gloss reduc-tion, primer surface roughness for better topcoat adhesion, highersolids, film reinforcement, lower moisture vapor transmission (MVT),and thixotropy (sag resistance). Magnesium silicate (talc), barium sul-fate (barytes), mica, aluminum silicate (clay), calcium silicate (wollas-tonite), and silica (sand) are examples of extender pigments used inpaint.

Corrosion-inhibiting pigments Also known as active pigments,corrosion-inhibiting pigments are typically inorganic and have verylow solubility in water. They help control corrosion of steel when usedin certain generic primers (primarily alkyd or oil based). Their low re-activity with water produces an alkaline condition and/or passivatingions that interfere with the electrochemical process that causes corro-sion of steel. Lead tetroxide (red lead), zinc chromate, zinc phosphate,





calcium borosilicate, zinc molybdate, barium metaborate, and cementare examples of corrosion-inhibiting pigments used in paint. Over theyears, red lead and zinc chromate have served as excellent corrosion-inhibiting pigments for specific primer formulations, but they havebeen virtually eliminated because of their toxicity.

ResinsWith only a few exceptions, resins (binders) are organic polymers. De-pending on molecular weight and resin type, these paint constituentscan be either liquid or solid before they are added to the batch.

The physical properties and performance of a cured paint film arerelated more to the types of resins used in the paint than to any of theother ingredients. This is why most paints are classified according tothe resins (binders) they contain. Single-component paints can some-times contain more than one type of resin, but these paints are usuallyclassified by the resin that is present in the formula at the higher per-centage, with the other resin identified as a modifier. For example, apaint that contains 80 percent alkyd resin solids and 20 percent acrylicresin solids would be classified as an acrylic-modified alkyd. Two-component paints are generally classified using both types of resins.For example, a two-component paint that consists of epoxy resin inone component and polyamide resin in the second component wouldbe identified as a polyamide epoxy.

Curing Mechanisms Versus Resin TypesTo gain a basic understanding of the performance properties of themany generic paints available today, we will discuss the most commoncuring mechanisms—that is, the various methods by which a painttransforms from a liquid to a solid.

Lacquer cure Lacquer cure—the drying of a paint film by solventevaporation only—is the simplest curing mechanism. As raw mate-rials, resins that fall into this category are usually high-molecular-weight resins. Since no further polymerization takes place when thedried paint film is formed, the performance properties are already ex-hibited by the resin as a raw material. In general, the advantages ofpaints that exhibit this curing mechanism include fast dry, completecure at low temperatures, and fast recoat times. With the exceptionof coal tar pitch solutions, the major disadvantage is low solids (highVOC content), which requires application of multiple coats. In addi-tion, use of these paints is limited because of air pollution regulations.Another disadvantage for all of the generic paints that fall into thiscategory is poor solvent resistance.

Vinyls: Organic solvent-based vinyl paints are made from vinyl-chloride and vinyl-acetate polymers. They are characterized





by excellent acid resistance, good alkali resistance, and lowMVT. Historically, they have been used on the interior and ex-terior of steel potable water tanks and in applications wheregood chemical resistance is desired. Their use today, however,has become almost nonexistent because of air pollution regu-lations.

Chlorinated rubbers: These resins are formed by a reaction of rub-ber with chlorine. Like the vinyls, they have good chemicalresistance and low MVT. In the past, chlorinated-rubber coat-ings were used on the interiors of potable water tanks. Theyalso have excellent adhesion to concrete and have been usedfor swimming pools.

Coal tar pitch solution: Coal tar pitch is produced from the destruc-tive distillation of bituminous coal. A protective coating can bemade by simply dissolving hot pitch in xylene. The cured filmhas low MVT and good chemical resistance. Coal tar pitch so-lutions can be formulated with a higher solids content (lowerVOC) than the other lacquer coatings. These coatings have ex-cellent adhesion to steel and concrete and are typically usedfor sewage and wastewater immersion service. They are onlyavailable in black and have poor resistance to direct sunlight.

Nitrocellulose: This resin is produced by treating cellulose with amixture of nitric and sulfuric acids. Nitrocellulose lacquers arewidely used as clear and pigmented furniture finishes. Nitro-cellulose is also used as a film-forming material in flexographicand gravure inks. Sunlight resistance is poor, so these coatingsare most often used for interior applications.

Oxidation and polymerization Generic paints that exhibit the oxida-tion and polymerization curing mechanism include oil-based paints,alkyds, and epoxy esters. Following application of the wet paint filmand evaporation of the solvent, oxygen from the surrounding atmo-sphere facilitates the linking together (polymerization) of the resinmolecules. Paints that cure by this process have been available formany years.

Oil-based paints At the turn of the 20th century, the choice of genericpaints was very limited. Most paints were based on vegetable oils suchas linseed oil. These “oil-based” paints exhibited excellent wettingand adhesion to marginally prepared steel substrates and providedlong-term weathering and corrosion resistance when exposed to manyatmospheric conditions. The biggest disadvantage was very slow drytime.

Alkyds Alkyd technology was developed in the late 1920s. It was dis-covered through laboratory testing that by combining a trifunctional





alcohol with an acid salt and reacting them at high temperature, anester-type resin could be formed that had very fast dry time. How-ever, the film was too brittle to be considered as a paint binder. It wasthen discovered that by incorporating a vegetable oil such as linseedoil in the same reaction, resins could be produced that dried morequickly than straight linseed oil, yet had good enough film propertiesto be considered as paint binders. This development revolutionizedthe paint industry at that time.

Further testing revealed that film properties and dry times couldbe adjusted depending on the type and amount of oil included in thereaction. Terminology that came out of this discovery included alkyd,which stands for an alcohol/acid reaction, and short-oil, medium-oil,or long-oil alkyd resins. The short–medium–long terminology desig-nates the amount of oil the alkyd resin contains. Short-oil alkyds drymore quickly than medium- or long-oil alkyds, but are generally lessflexible and do not produce paints with as good weatherability andcorrosion resistance.

In general, paints that exhibit this curing mechanism producepaint films with a wide variety of properties depending on the typeand amount of oil in the resin. Overall, alkyd and oil-based paintsare less expensive than other generic types and can provide long-termcorrosion protection of steel as long as the exposure environment isnot too severe. Chemical resistance and color and gloss retention areconsidered fair to good, depending on the formulation. Alkyd paintformulations can be modified with other resins, such as acrylics andsilicones, to upgrade color- and gloss-retention properties.

Epoxy esters Epoxy esters are epoxy resins that have been “esteri-fied” with fatty acids, resin, and so on. Epoxy-ester paints have betterchemical resistance than oil-based or alkyd paints, but they chalk morereadily when exposed to direct sunlight.

The co-reacting curing mechanism Paints that exhibit the co-reacting(chemical cross-linking) curing mechanism usually have two or morecomponents. Examples are two-component epoxies, two-componentaliphatic urethanes, two-component fluorourethanes, and two-com-ponent polyureas. In this mechanism, a chemical reaction betweentwo differing resin types is involved. In general, paints that cure bythis mechanism produce cured films with high cross-linked density,excellent hardness, abrasion resistance, corrosion resistance, low MVT,and excellent chemical and solvent resistance. One disadvantage islimited pot life.

Two-component epoxies One component of two-component epoxycontains an epoxy resin with one or more chemically reactive sitesknown as epoxide rings. The second component (a curing agent)





contains an amine or amide-functional resin. When the two paint com-ponents are combined in the proper ratio and applied to a substrate,they chemically react with each other to form the cured paint film. Per-formance properties vary depending primarily on the type of curingagent used. In general, polyamide-cured epoxies have slightly betteradhesion characteristics, better flexibility, longer pot life, and less acidresistance than the amine-cured epoxies. It is also more difficult todevelop protective coatings with ultra-high solids (90 to 100 percent)using polyamide-cured epoxies than it is for those using amine-curedepoxies. With few exceptions, two-component epoxies chalk readilywhen exposed to direct sunlight.

Two-component epoxies can be modified with coal tar pitch, pro-ducing coal tar epoxy coatings. The coal tar pitch is usually addedto the polyamide or polyamine curing agent. Properly formulated,these coatings can be applied up to 20 mil (508 �m) dry film thickness(DFT) in a single coat. They typically cost less than unmodified two-component epoxies and have low MVT and good chemical resistance.They are most often used for protecting steel and concrete substratesfrom immersion in sewage and wastewater.

Two-component polyurethanes Urethane coatings are generally dividedinto aromatic and aliphatic types. They are noted for their over-all balance of high-performance properties. These include excellenttoughness, chemical and solvent resistance, hardness, and abrasionresistance. One component typically contains a hydroxyl-functionalpolyol resin that cross-links with a second component containing anisocyanate-functional polyisocyanate resin.

Aliphatic polyurethanes usually contain an acrylic or polyesterpolyol portion. When cross-linked with an aliphatic isocyanate, theyexhibit excellent color and gloss retention when exposed to directsunlight. They are expensive and are most often used as thin-filmtopcoats over two-component epoxies.

Aromatic polyurethanes generally have better adhesion to steeland are less expensive, but they tend to yellow when exposed to directsunlight. They are often used as direct-to-steel or direct-to-concretestand-alone coating systems. Some formulations can be used for im-mersion service.

Two-component fluorourethanes Fluoropolymer-based coatings areknown for their outstanding color and gloss retention when exposedto direct sunlight. Kynar r©-based coatings have been around for manyyears but require baking at high temperature for proper film forma-tion.

Two-component fluorourethane coatings are a relatively new tech-nology. These coatings exhibit the co-reacting curing mechanism atambient conditions. One component typically contains hydroxyl- and





carboxyl-functional fluoroethylene vinyl ether (FEVE) resin. Fluo-rourethane coatings generally exhibit the same performance proper-ties as two-component aliphatic polyurethanes, but they have muchbetter long-term color- and gloss-retention properties.

Two-component polyureas These coatings are usually 100 percentsolids, cure extremely quickly (sometimes within seconds of applica-tion), and have extremely short pot life. Specialized plural-componentspray equipment is required. One component contains either aromaticor aliphatic isocyanate-functional polyisocyanate resin; the other com-ponent contains an amine-functional resin that cross-links with thepolyisocyanate resin when the two components are mixed together inthe proper ratio.

Because they are 100 percent solids and exhibit an extremely fastcure response, these coatings can be spray applied in one coat at veryhigh film thickness. Depending on formulation, a wide range of per-formance properties can be achieved using this technology. Rigid orelastomeric (greater than 100 percent elongation) formulas are possi-ble. Chemical resistance is generally very good, and some formula-tions are suitable for immersion service. One hundred percent solids,two-component polyurethane coatings are also available. Althoughtheir properties are generally similar to those of the polyureas, theyare less flexible and have better chemical resistance.

Baking enamels These are coatings with two co-reacting resins pack-aged in the same container. One resin is typically a polyester or acryliccontaining one or more hydroxyl groups. The other resin is usually amelamine or urea formaldehyde. Cross-linking of the resin moleculesonly takes place at high temperatures (generally above 250◦F [121◦C]).Home appliances such as stoves, refrigerators, washers, and dryershave metal substrates that are protected with baking enamels. Com-ponents are painted in the shop and then sent through ovens, wherethe coating cures (usually in 10 to 15 minutes) at high temperature.Pot life is not a concern, since cure (co-reacting) will not take place atambient temperatures.

Moisture curePolyurethanes Some aromatic and aliphatic urethane resins are de-signed to react with atmospheric moisture to form a cured film. Theseresins are usually based on toluene diisocyanate (TDI) or dephenyl-methane diisocyanate (MDI). In the presence of atmospheric mois-ture (humidity), the resin molecules react with each other to formcured films. Single-package aromatic urethane primers, intermedi-ate coats, and topcoats containing aluminum or micaceous iron ox-ide pigments are available that cure by this mechanism. The aro-matic urethane types typically have excellent wetting and adhesion





characteristics, even to marginally prepared surfaces, and offer excel-lent barrier-protection properties. For better color and gloss retention,single-package aliphatic urethane topcoats are also available that cureby this mechanism.

CoalescenceEmulsion (latex paints) Water-based paints such as acrylic andpolyvinyl acetate (PVA) emulsions exhibit the coalescence curingmechanism. Acrylic emulsion resin (acrylic latex), for example, con-sists of small droplets of acrylic resin emulsified in water. The ap-plied wet paint film cures through solvent evaporation (mostly water)and coalescence. As the water evaporates, the emulsified acrylic resindroplets get closer together. When most of the water is evaporated, anorganic solvent known as a coalescing agent causes the resin dropletsto flow together, forming a smooth, continuous paint film. Water-based emulsions are often used as interior (PVA) and exterior (acrylic)house paints. They are also used for other architectural applications.Substrates that are coated with emulsion paints include masonry (con-crete, concrete block, plaster, and so on.), wood, and drywall. Speciallyformulated acrylic emulsion paints are also available for direct-to-metal applications.

Free-radical polymerization Polyesters and vinyl esters have been usedextensively for specialty applications since the 1960s. They are basedon unsaturated prepolymer resins that are dissolved in an unsaturatedmonomer such as styrene. By addition of peroxide catalyst, carbon-to-carbon double-bond sites react with each other to form the curedfilm. Although the monomer (styrene) is volatile in the liquid state, itacts as a cross-linking agent and is incorporated into the film.

The polyesters form hard, dense, chemical- and water-resistantfilms. They are used primarily as laminating resins and gel coats forthe manufacture of fiberglass boats, shower stalls, bathtubs, bowlingballs, and so on.

The vinyl esters have excellent acid resistance and are used pri-marily as fiberglass-reinforced and nonreinforced linings for steel andconcrete substrates that come in contact with strong acids. Because oftheir reactive nature, vinyl esters have poor package stability, resultingin short shelf life, especially at high storage temperatures.

Hydrolysis The primary use of ethyl silicates and polysilicates, whichare among the few resins that are not based on organic polymers, isin the formulation of inorganic zinc-rich primers. The curing mech-anism is similar to the curing mechanism for concrete (cement). Aninorganic zinc-rich primer contains considerable metallic zinc dust,typically more than 70 percent by weight in the cured film. When theliquid paint is applied and the solvent evaporates, moisture from the





surrounding air is absorbed into the film, setting up the hydrolysis pro-cess. In the case of ethyl silicates, complex chemical reactions occur,forming a silicon oxide (SiO) matrix in which a small amount of resinbinder holds together a large amount of metallic-zinc pigment. Uponcomplete hydrolysis (curing), the applied film is very hard, dense, andresistant to abrasion and solvents. Inorganic zinc-rich primers protectsteel from corrosion by galvanic action. The zinc becomes the anodein the corrosion battery and the steel becomes the cathode. The zincsacrifices itself to protect the steel in a process that is similar to thatexhibited by galvanized-steel substrates. Inorganic zinc-rich primersare resistant to high temperatures (up to 750◦F [399◦C]), are difficultto topcoat, and have poor acid resistance.

Additives In addition to the primary paint ingredients (resin, pig-ment, and solvent), most paint formulations contain one or more ad-ditives. Although they comprise a minor portion of the liquid paint(usually less than 2 percent by weight), additives play an essentialrole. Most are rheology agents that positively affect the properties ofthe paint in the can and during application. Following are a few of theadditives used to enhance paint properties.

Antisettling agents: There is probably more of this additive usedeach year than any other paint additive. Since pigment is theheaviest paint ingredient, gravity causes it to settle to the bot-tom of the container. Antisettling agents do not actually pre-vent pigment from settling, but they keep it from hard packingin the bottom of the container. This makes it much easier tostir the pigment into a homogeneous mixture with the otheringredients. The most widely used antisettling agent is hydro-genated castor oil (trade name: MPA).

Thixotropic agents: These additives are used to incorporate theproperty known as false body or high viscosity at rest. Thisproperty is essential to paints that are to be applied withoutruns or sags to vertical surfaces at high dry film thickness.

Defoamers: Defoamers are essential additives for water-basedpaints and certain solvent-based paints to prevent bubbles dur-ing manufacture and/or in the applied film. They are normallysilicone based and change the surface tension of water and or-ganic solvents.

Driers: For driers, metallic soaps are added to oil-based or alkydpaints to accelerate the oxidation and polymerization process.

Mildewcides: Paint films that contain natural (nonsynthetic) in-gredients promote mildew growth when they are exposed towarm, damp conditions. Mildewcides are used as additives inthese types of paints to prevent mildew growth. Most alkydand latex paints contain mildewcides.





Anti-skinning agents: These are antioxidants used in oil-based oralkyd paints to prevent skinning (drying at the surface) in thecan.

Pigment wetting agents: These additives are usually silicone basedand are incorporated into paint formulas to assist the resin insurrounding and wetting out each of the dispersed pigmentparticles.

Coating CalculationsThe spreading rate (coverage) of any gallon of paint depends on itsnonvolatile (solids) content. One gallon occupies a volume of 231 in.3

(0.0038 m3) or 0.1337 ft3 (0.0038 m3). If a gallon or liter of paint con-tained no volatile (solvent) and it could be applied without any losses,the spreading rate obtained applied at 1.0 mil (25.4 �m) would be1,604 ft2 (149 m2). This figure is expressed as the theoretical spreadingrate per gallon or liter.

If a gallon or liter of paint contains volatile and its percentageof total volume solids is known, its spreading rate, wet-to-dry filmratio, and cost per applied mil per square foot (per applied micronper square meter) can be calculated as follows:

Theoretical spread rate @ 1.0 mil DFT = Percent of volume solids× 1,604 ft2

Spread rate @ DFTs other than 1.0 mil = Percent of volume solids× 1,604 ft2/specified DFT

Wet film thickness = DFT/percent volume solidsCost per mil per sq ft = Cost per gallon/spread rate @ 1.0 mil DFT

ExampleLet us assume we have one gallon of paint that is 50 percent volumesolids. The specified DFT is 5 mil and the cost per gallon is $22. Withthis information, we can make the following calculations:

Theoretical spread rate @ 1.0 mil DFT = 0.50 × 1,604 ft2 = 802ft2/gal

Spread rate at specified 5.0 mil DFT = 0.50 × 1,604 ft2/5 = 160ft2/gal

Wet film thickness = 5.0/0.50 = 10 milCost per mil per sq ft = $22/gal/802 = 2.74 cents/ft2

Corrosion Protection of Steel Water Tanks withLiquid-Applied CoatingsA protective coating is a material that, when applied to a structure, iso-lates the structure from its environment. Properly applied protective





coatings are a cost-effective way to protect both exterior and interiortank surfaces. A coating applied to the interior wet surfaces of a tank isalso called a lining. Both exterior and interior coating systems must becarefully selected to provide the best protection value for the moneybased on coating life, effectiveness of protection, ease of application,and ease of adding coats in future years. Many protective coating sys-tems have become much more complex than the single-componentmaterials that were prevalent before 1970.

D102 is the AWWA standard for painting the interior and exte-rior of steel water tanks. The objective of this standard is to provideinformation about various coating systems for coating and recoatingthe interior and exterior of steel tanks used for potable water storage.Coating systems for new bolted-steel tanks are not covered by thisstandard.

AWWA D102 is not a specification. AWWA standards describeminimum requirements and do not contain all of the engineeringand administration information normally contained in specifications.Specifying engineers often reference specific interior and exterior coat-ing systems contained in D102 when writing detailed specificationsfor a steel water tank painting project.

D102—Past, Present, and FutureThe first edition of this standard was approved by the AWWA Boardof Directors on February 11, 1964. The second edition was approvedon January 28, 1978, and subsequently withdrawn on June 23, 1991.The standard was reissued and subsequently approved by the AWWABoard of Directors on February 2, 1997. The third edition was approvedin 2003.

Inside and outside coating systems contained in D102 havechanged substantially from one edition to the next (Table 3-1).

Following are the primary reasons that substantial changes havebeen made to inside and outside coating systems over the years:� Advances in coatings technology� State and federal regulations limiting the amount of organic

solvent contained in protective coatings (VOC regulations);these regulations essentially eliminated the use of chlorinatedrubber and vinyl coating systems� Restrictions placed on certain coating ingredients such as redlead and zinc chromate� Introduction of National Sanitation Foundation (NSF)/ANSIStandard 61 Drinking Water System Components–HealthEffects in the late 1980s, which virtually eliminated some in-side coating systems because of potential extraction of highlevels of harmful ingredients into drinking water.





Outside Coating

Standard Systems Inside Coating Systems

AWWA Alkyd Red lead/aluminum phenolic

D102–64 Vinyl Vinyl

Red lead/linseed oil, Zinc/phenolic

alkyd High-solids vinyl

Metallic aluminum Cold-applied wax

Hot-applied wax

Metallic zinc

Hot coal tar enamel

Cold-applied coal tar

Cold taste-and-odor tar

AWWA Alkyd Two-component epoxy

D102–78 Vinyl Vinyl

Alkyd/silicone alkyd Chlorinated rubber

Alkyd (two primer coats) High-solids vinyl

Chlorinated rubber/alkyd Hot-applied coat tar

Cold-applied coal tar

Metallic sprayed zinc

ANSI/AWWA Alkyd Two-component epoxy (two

D102–97 Alkyd (two primer coats) coats)

Alkyd/silicone alkyd Two-component epoxy (three

Vinyl coats)

Epoxy/epoxy/aliphatic Zinc-rich primer/epoxy/epoxy

urethane Vinyl

Zinc-rich primer/epoxy/ Hot-applied coal tar

aliphatic urethane Cold-applied coal tar

ANSI/AWWA Alkyd Two-component epoxy (two

D102–03 Moisture-cured coats)

polyurethane Two-component epoxy (three

Water-based acrylic coats)

emulsion Inorganic zinc/epoxy/epoxy

Zinc-rich primer/epoxy/

fluorourethane

100 percent solids

polyurethane

Epoxy/epoxy/aliphatic

polyurethane

Organic zinc/epoxy/epoxy

Zinc-rich primer/epoxy/

aliphatic urethane

Sources: AWWA standards D102–64, D102–78, ANSI/AWWA D102–97, ANSI/AWWA D102–03.Note: ANSI = American National Standards Institute.

TABLE 3-1 Changes in Inside and Outside Coating Systems Specified inVarious Editions of AWWA D102





Inside and outside coating systems contained in future editionsof D102 will be further restricted by the continued implementation ofmore stringent VOC regulations for both shop- and field-applied pro-tective coatings. Future editions of D102 will most likely only containinside and outside coating systems that are very high solids based orwater based.

Surface PreparationBefore a protective coating system can be applied to a steel or concretewater storage tank, appropriate surface preparation must be under-taken. The purpose of surface preparation is twofold: to clean thesubstrate of contaminants and to roughen or “profile” smooth sur-faces to ensure mechanical adhesion of the first (primer) coat. Weldsmay be ground, corners and edges may be smoothed, and voids maybe filled so that the applied coating system does not fail prematurely.

The Society for Protective Coatings (SSPC), established in 1950 andheadquartered in Pittsburgh, Pennsylvania, assesses and advancessurface preparation and its understanding by conducting researchand “developing standards, specifications, and guides covering tech-niques and materials of surface preparation.” NACE International isa professional technical society that provides education and commu-nicates information to protect people, assets, and the environmentfrom the effects of corrosion. It, too, develops surface preparation andother standards, provides education and certification, and publishesnumerous books and journals. Founded in 1943, NACE is the largestorganization in the world committed to the study of corrosion, with amembership consisting of 15,000 engineers, scientists, and researchersin 91 countries.

Together, these two organizations have issued joint standards thatare commonly referenced by those who need to specify proven sur-face preparation methods. For example, AWWA D102–03 Standardfor Coating Steel Water-Storage Tanks cites four SSPC/NACE sur-face preparation standards (SP10/NACE 2 Near White Blast Clean-ing, SP6/NACE 3 Commercial Blast Cleaning, SP7/NACE 4 Brush-Off Blast Cleaning, and SP11 Power Tool Cleaning to White Metal).Surface preparation methods vary and may not be appropriate for allmaterials of construction. Methods may use abrasive blast cleaning(SP10/NACE 2, SP6/NACE 3, and SP7/NACE 4, for example), handor power tools (SP2, SP3, SP11, and SP15, for example), or water underpressure (SP12/NACE 5, for example).

The surface preparation standards listed previously are primar-ily used for steel surfaces. Methods such as SP13/NACE 6 SurfacePreparation of Concrete exist for cementitious substrates. In addi-tion, SP13/NACE 6 further identifies surface preparation practices





FIGURE 3-9 Surface preparation under controlled conditions.

developed by ASTM International, formerly the American Societyfor Testing and Materials. Organized in 1898, ASTM International isone of the largest voluntary standards development organizations inthe world. ASTM International is a not-for-profit organization thatprovides a forum for the development and publication of voluntaryconsensus standards for materials, products, systems, and services.Surface preparation is so important to the successful completion of awater tank project that members of AWWA, ASTM, and other orga-nizations donate time to review and update industry standards andmethods and to develop new standards and methods as to surfacepreparation equipment improves and changes.

Surface Preparation—SteelFor welded-steel water-storage tanks, surface preparation completedin a fabrication shop before the first (primer) coats are applied is un-derstandably faster and easier than surface preparation that must becarried out after erection. Shop conditions are controlled, in that op-erations can be continued regardless of outside weather. Lighting andaccess to all areas of the structure being fabricated are generally su-perior to field lighting and access (Fig. 3-9). After cleaning, steel platesurfaces are abrasive blasted to remove mill scale and/or create a sur-face profile to which the applied coating will adhere. Abrasive blasting





FIGURE 3-10 Open-nozzle blasting.

operations may be performed within an enclosed chamber or booth orby an operator using an open nozzle. Speed and efficiency are two ofthe advantages of centrifugal blasting machines; open-nozzle blasting(Fig. 3-10) may allow an operator more time to prepare difficult-to-access areas.

AWWA D102–03 Section 1.2, Surface Preparation, paragraph4.5.2.1 states for outside surfaces of new tanks:

Exterior surfaces shall be cleaned in accordance with SSPC SP6/NACE 3.If specified, tanks located in coastal areas or industrial environments shallbe blast cleaned to SSPC SP10/NACE 2. Blast cleaned surfaces shall havea surface profile that is appropriate for the specific primer and coatingsystem as recommended by the manufacturer of the coating.

For outside surfaces of existing tanks, AWWA D102–03 Section1.2, paragraph 4.5.2.2, states:

When the. . . coating system will adhere to the existing coating, all corro-sion products and deteriorated coatings shall be removed by spot clean-ing to SSPC SP11 or SSPC SP6/NACE 3 and the remainder of the exteriorsurfaces shall be cleaned by SSPC SP7/NACE 4 or by washing with an al-kaline cleaner. . . to remove all dirt, dust, coating/paint chalk, and foreign





matter. When the new coating system is not compatible with the exist-ing coating, all existing coatings shall be removed and the surfaces blastcleaned to SSPC SP6/NACE 3 or, if specified, to SSPC SP10/NACE 2.

For interior surfaces of new tanks, AWWA D102–03 Section 1.2,paragraph 4.5.3.1, states:

The interior surfaces of new tanks shall be cleaned in accordance withSSPC SP10/NACE 2, excluding interior surfaces of dry risers and drypedestals. Interior surfaces of dry risers and dry pedestals shall be cleanedin accordance with SSPC-SP6/NACE 3. Blast-cleaned surfaces shall havea surface profile that is appropriate for the specific primer and coatingsystem as recommended by the manufacturer of the coating.

Finally, for interior surfaces of existing tanks, paragraph 4.5.3.2states:

When existing coatings have not deteriorated extensively and the newcoating system will adhere to and is compatible with the existing coating,all corrosion products and deteriorated coatings shall be removed by spotblasting to SSPC SP10/NACE 2 and the remainder of the interior surfacesshall be cleaned by SSPC SP7/NACE 4. When existing coatings have dete-riorated extensively or the new coating system is not compatible with theexisting coating, all existing coatings shall be removed and the surfacescleaned to SSPC SP10/NACE 2. Blast-cleaned surfaces shall have a sur-face profile that is appropriate for the specific primer and coating systemin accordance with the coating manufacturer’s recommendations.

Steel plate for bolted tanks may be prepared by first cleaningand rinsing in a hot alkaline solution followed immediately by hot-air drying. This process removes not only dirt but also hydrocar-bon contaminants that will prevent future coating adhesion. There-after, these plates are sent through a centrifugal blast machine wherean approximately 2-mil-deep (51-�m-deep) profile is made on thesurface in accordance with SSPC SP10 Near White Blast Cleaningprocedures.

AWWA D103 Standard for Factory-Coated Bolted Steel Tanks forWater Storage recommends that tank components to be protected withglass coatings receive either Near White Blast Cleaning (SSPC SP10)or, as an alternative, Pickling (SSPC SP8).

Surface Preparation—ConcreteSurface preparation for concrete water-storage tanks is different fromsurface preparation for steel. Most surfaces of concrete tanks are al-ready rough due to the way these vessels are constructed, so there islittle need to add a profile. This coarse surface characteristic allows forgood adhesion of paints and coatings. That does not mean, however,





that water tanks constructed of concrete are without their own surfacepreparation requirements.

What needs to be accomplished is the removal of areas of poorlyadhered concrete—flake-shaped fragments that detach from the sur-face of concrete in a process known as spalling. Laitance, a poorlyadhered layer of concrete, may also be found on concrete surfacesand must be removed. Although less severe and less damaging thanspalling, laitance comprises cement and tiny particles called fines thatmay be caused by improper vibration of concrete within forms. Leftunrepaired, spalling may continue and expose reinforcing bars to cor-rosion, causing damage to the water tank that is difficult to repair.One way to remove spalling and laitance is by mechanical means, inaccordance with ASTM D4259.

Roughening of concrete surfaces may be desired on cast-in-placeconcrete surfaces, such as composite tank columns. These smooth, as-cast surfaces may not be of uniform appearance, so abrasive blasting—again in accordance with ASTM D4259—helps to regulate the appear-ance of the completed pedestal tower. “Bugholes,” small irregularcavities uncovered by surface preparation procedures, should be filledwith an appropriate material so they will not trap airborne contami-nants and mold spores.

Occasionally, bolted tanks without bottoms are constructed andplaced on concrete slabs. These concrete slabs act as the floor and mustbe constructed of materials that will not leach out into the potablewater supply and thereby contaminate it. NSF International, foundedin 1944 as the National Sanitation Foundation, created ANSI/NSFStandard 61, a certification protocol that addresses these concerns.Regarding “bottomless” tanks resting on a concrete base, NSF mayrecommend that these concrete surfaces be constructed of ANSI/NSF61-certified cements and admixtures, for example, or coated with anANSI/NSF 61-certified coating before being placed in service. Theseconcrete floor surfaces would also require surface preparation prior tothe application of ANSI/NSF 61-approved paints and coatings. Onceagain, preparation routines in accordance with ASTM D4259 may beused.

Even though most concrete tank surfaces may need little to noroughening, they do require cleaning. The SSPC SP12 standard forlow-pressure water cleaning, WJ4, will accomplish this. This proce-dure usually removes loose shotcrete clusters and may remove de-bris left behind by, or concrete escaping through, placement forms.Placement forms are coated with release agents or compounds to pre-vent adhesion of concrete to these forms and thereby allow placementforms to be removed cleanly. These compounds may act as contam-inants, however, and prevent adhesion of paints and coatings. WJ4may be specified for this purpose. If non–water soluble or non–waterdispersible form-release compounds are used, low-pressure water





cleaning will not remove them, and abrasive blast cleaning throughthe use of ASTM D4259 may be required.

Concrete surfaces, because they are porous, also absorb moisture.Whether because of rain or the use of water when cleaning equipmentfor surface preparation, these surfaces must be allowed to dry beforecoating. The length of, and need for, time to dry are influenced bothby the temperature and humidity at the tank site and by the type ofcoating to be applied to the tank.

Coating SelectionTo the residents of the community or neighborhood closest to a water-storage tank, how the tank looks—its exterior color and design—maybe the most important characteristic. The city name or the name andmascot of the local high school may be emblazoned on the elevatedsteel tank for all to see. In contrast, residents living near ground con-crete storage tanks may not want them visible at all, desiring themto blend into the background landscape. Enhancements are made toboth steel and concrete tanks to increase their visual impact. Such en-hancements may be from fabrications such as pilasters or simply fromthe allure of carefully chosen and illustrated paints and coatings.

Understandably, residents want the exterior surfaces of water stor-age tanks to remain colorfast and appealing. But the interior surfaces,which few see, are in fact more important, because it is there wherebacteria can grow and corrosion can occur undetected. Coating sys-tems for both interior and exterior surfaces can be selected based oninformation provided by tank fabricators, by coating manufacturersand their representatives, by the owner’s or specifier’s preference, orby reviewing applicable AWWA standards.

Interior Surfaces—Welded-Steel TanksAn interior coating system in the wetted areas of the tank must with-stand constant immersion; it must be able to resist alternate wettingand drying in the upper portion of the operating range and high hu-midity above the top capacity level; it must be resistant to the actions ofice abrasion in cold climates; and, in some geographic areas, it mustbe able to withstand extreme temperature fluctuations. In addition,interior coating systems must be able to be both shop-applied andfield-applied, must be cost-effective, and must meet the minimumrequirements of ANSI/NSF 61 Standard for Drinking Water SystemComponents.

For interior surfaces of tanks, ANSI/AWWA D102 lists five interiorcoating systems (Table 3-2). Such coatings “shall have been evaluatedfor long-term fresh water resistance and the system shall have demon-strated satisfactory service in fresh water for at least 18 months. Anycoating that cannot meet these requirements, whether or not includedin this standard, shall not be used.”





ICS Number Description

1 Two-coat, two-component epoxy

2 Three-coat, two-component epoxy

3 Three-coat, zinc/epoxy/epoxy, interior nonimmersed

surfaces above TCL

4 One-coat polyurethane or polyureas

5 Three-coat zinc/epoxy/epoxy, interior surfaces above

and below TCL

Note: ICS = interior coating system; TCL = top capacity level.

TABLE 3-2 Five Interior Coating Systems

Interior Surfaces—Bolted-Steel TanksInterior surfaces of bolted-steel tanks are subjected to the same condi-tions as are welded-steel tanks and must also be protected by speciallydesigned coating systems. ANSI/AWWA D103 Section 10.3–10.6 al-lows four protective coating systems to be used: galvanized coatings,glass coatings, thermoset liquid (epoxy) coatings, and thermoset pow-der (epoxy) coatings.

Glass coating systems may begin with a primer coat of catalyticnickel oxide prior to the application of 6 to 19 mil (152 to 482 �m) ofthe glass coating, firing, and fusion of the glass coating to the substratein a furnace operating at temperatures above 1,200◦F (649◦C).

Thermoset liquid (epoxy) coating systems are generally appliedin two coats. The first coating is applied and heated to create a tacky,partially cured first coat followed by additional coating to achievea minimum 5-mil (127-�m) DFT. It is subsequently baked at 425 to525◦F (218 to 274◦C), to thermally cross-link the complete coatingsystem.

Thermoset powder (epoxy) coating systems are electrostaticallyapplied to achieve a minimum 3-mil (76-�m) DFT prior to oven curingand baking.

According to ANSI/AWWA D103, when galvanized coatingsare to be supplied, “zinc metal suitable for immersion in drinkingwater shall be applied to the tank parts after fabrication in accor-dance with the recommended practice of the American Hot Dip Gal-vanizers Association in compliance with ASTM A123 and ASTMA153.”

Exterior Surfaces—Welded-Steel TanksAs mentioned earlier, the exterior coating attracts more attention andmay cause more concern than any other aspect of a tank project be-cause it is all that the neighboring community sees.





OCS Number Description

1 Three-coat (optional four-coat) alkyd

2 Three-coat moisture-cured urethane

3 Three-coat water-based acrylic

4 Three-coat zinc primer/urethane/fluorourethane

5 Three-coat epoxy primer/epoxy/urethane

6 Three-coat zinc primer/epoxy/urethane

Note: OCS = outside coating system.

TABLE 3-3 Six Outside Coating Systems

For exterior surfaces of tanks, ANSI/AWWA D102–03 lists sixoutside coating systems (Table 3-3). According to the standard,“[p]roprietary formulations will be acceptable provided the coating isof the same generic type and that the performance of the formulationoffered meets or exceeds the performance of the formulation definedin the referenced coating standard and also is suitable for the specifiedservice conditions.”

Exterior Surfaces—Bolted-Steel TanksTanks fabricated in compliance with ANSI/AWWA D103 and pro-tected with thermoset liquid or thermoset powder coatings are top-coated with acrylic or urethane baking enamels to yield a minimumof 3-mil (76-�m) DFT.

Regarding the occasional repair and touch-up of welded- andbolted-steel tank coating systems, it is very important to rememberthat such activity does not require the full removal and replacementof the coating system. Repair and touch-up are only required in areasthat have been damaged, and it is only after many years of service thatwelded- and bolted-steel tanks require overcoating and/or completerepainting. Table 3-4 describes common water tank coating character-istics.

Application Techniques and EquipmentBy definition, a protective coating is a material that is applied to theexterior of the tank that acts as an insulator between the tank and itsenvironment. A protective lining has a similar definition, the majordifference being that the protective lining system is applied to theinterior of the tank and acts as a barrier between the tank and itscargo.




Generic

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113





Many considerations need to be taken into account when selectingthe proper interior and exterior protective systems, including govern-mental regulations, various environmental considerations, the effec-tiveness of the protection, the ease (or difficulty) of application, andthe anticipated service life of both the coating and the structure. Forinstance, an interior lining must be able to withstand constant im-mersion in water, varying water temperatures, alternate cycles of wetand dry periods, ice abrasion, high humidity and heat, and varyinglevels of chlorine and mineral contact. The lining materials must notpose a health risk to the general public and must be approved forsuch use by the appropriate state or federal regulatory agency. Alter-nately, an exterior coating system should take into consideration thetype of atmosphere to which it will be subjected, the expected ambienttemperatures, the areas surrounding the tank, and the desired overallappearance and aesthetic value of the coating.

In some cases, the entire coating system may be applied in a shopenvironment; in other cases, the coating system may be applied en-tirely in the field. Quite often, though, it is in a combination of thesetwo settings. Each method will be examined individually.

Shop ApplicationThe two major advantages in applying a protective coating in a shopenvironment are control and accessibility. Often the interior and ex-terior sections receive just a primer coating in anticipation of fieldapplication of the subsequent coatings specified. This is done to al-low fabricators to quickly clean, prepare, and prime the surface inaccordance with specifications while still allowing them the ability tocontinue working the plate and shipping it without compromising thesurface preparation. When this approach is taken, an area is generallyleft uncoated around the perimeter of each plate, commonly referredto as the margin. The margin area can vary by specification but is usu-ally between 4 and 6 in. (102 and 152 mm) wide. This allows for fieldwelding to be performed during the erection in the field. Of course,these areas will not meet the surface cleanliness requirements of thespecification, and they will need to be addressed in the field prior tothe continued application of the coating system. This will be coveredin more detail in the following section on field painting.

In some instances, the entire protective coating system is appliedin the shop—for example, with bolted tanks. The steel panels are gen-erally coated following roll forming and bolt-hole punching. In thissituation, a thermoset liquid coating may be applied and then bakedat a prescribed temperature, or a thermoset powder coating may beapplied and then baked according to the manufacturer’s instructions.Depending on the type of interior lining system, the bake tempera-tures can vary between 425◦F (218◦C) and (in the case of a glass lining)





1,600◦F (870◦C). Factory-applied lining systems are discussed in moredetail in AWWA D103. The factory-applied liquid baked-on exte-rior coating systems generally combine epoxy primer with an acrylicenamel topcoat or an acrylic polyurethane topcoat.

The basic premise of a shop application of the protective coatingsystem, whether it be the primer or the entire system, is that sinceapplication is all performed on the ground and under controlled shopconditions, the result will be (and most often is) a very uniformlyapplied and fully cured protective coating system. However, the coat-ing system is often damaged during loading, transporting, unloading,and erection of the plates. Depending on the extent of the damage, ma-jor field work may be needed for repair. Anticipating this situation, acombination of shop priming and field painting is often employed.

Field PaintingAlthough coating and/or lining systems applied in the field sharemany of the considerations we reviewed under shop-applied coat-ings, other factors specific to field painting need to be evaluated.Among these items are the type of lining that is environmentally com-pliant, tank heating and ventilating, dehumidification requirements,the landscape surrounding the tank, and the type of environment thatthe tank is subject to during the preparation, application, and cureof the lining and coating systems (e.g., chloride sources in a marineenvironment).

Generally speaking, the interior of the tank requires the highestdegree of surface cleanliness and preparation. Many of the protectivelining systems require a minimum surface cleanliness equaling anSSPC SP-10/NACE 2 Near White Metal Blast. In an effort to achievethis, painting contractors typically blast the bottom of the tank firstand then begin to blast the wall section by section. Each section (calleda drop) is blasted and coated during the work shift unless an environ-mental control such as dehumidification is needed. The abrasive usedin the cleaning process is allowed to fall to the floor of the tank and ac-cumulate there. This abrasive provides an insulation of sorts from theenvironment so that the initial blast on the floor is held or maintained.If the blast is lost, the contractor reblasts the floor area and coats it ashe is finishing the interior of the tank. Special care needs to be taken toensure that spent blast media is not billowed and deposited into thefreshly coated surface. Although this is a common approach to liningthe inside of storage tanks, it is not the only correct way to performthis task.

When a primer has been applied in the shop and the contractor isonly applying finish coats to the tank in the field, the surface prepa-ration specification is usually a bit different than that just described.Two concerns must be addressed: (1) the condition of the shop primer





and its preparation before receiving any topcoats and (2) the weldseam preparation. Typically, the shop primer is swept blast accordingto SSPC SP7/NACE 4 Brush-Off Blast Cleaning, while the weld seamsare prepared according to SP10/NACE 2 Near White Metal Blast. Insome cases the primer is reapplied, while in others only the finishcoat(s) are applied. Proper ventilation of the tank’s interior is criticalto ensure a thoroughly cured lining.

When coating the outside of the tank, the consequences of over-spray, dry spray, and ambient conditions must be considered. Adverseconditions during surface preparation, coating application, and cur-ing can (and most often do) lead to premature catastrophic failure ofthe coating system. Specifiers and contractors should also be aware ofthe areas surrounding the tank and the environment to which the coat-ing system will be subjected. For example, an elevated water tank in acongested urban area may require coating materials that can only beroller applied or that tend to “dryfall” if applied by a sprayer. By wayof environmental considerations, coastal regions may require a coat-ing system that has a higher film build and more barrier protection toprotect the tank from a chloride-rich environment. Depending on thetank and the contractor, the coating process is completed in differentways, but completing drops is still the most common way to ensurethat a properly cleaned surface is maintained. Typically, the specifiedcleanliness for the exterior of the tank would be an SSPC SP6/NACE 3Commercial Blast. For a shop-applied primer, the primer is swept blastaccording to SP7/NACE 4, while the weld seams require SP6/NACE3. Again, a primer may be reapplied if specified or the finish coatsmay be applied over the prepared existing primer.

Equipment and TechniquesMany shops that use liquid coatings apply them with spray equip-ment, which has undergone many improvements since the intro-duction of conventional spray units. This section briefly describesmethods and equipment for applying protective coating and liningsystems in both shop and field. In the field, paint contractors typi-cally use rollers or airless spray systems. However, because of newcoating technology and new environmental concerns and legislation,plural-component spray equipment is increasingly showing up ontank coating projects. The use of brushes and rollers in a shop envi-ronment is mostly limited. Brushes and rollers are typically used fortouch-up or to coat difficult or complexly designed areas.

BrushesBrushes are not as high tech as sprayers, and many consider them anoutdated way to apply paint. However, many situations still requiretheir use. A “stripe coat,” often specified for added protection of edges,





rivets, corners, bolts, and welds, is best applied by a brush, because itimproves the wetting capabilities of the coating by forcing the coatinginto areas that would be problematic if sprayed or roller applied. Aworker applying a coating by brush should hold the brush at an an-gle of approximately 45 degrees and, using the wrist and arm, spreadthe coating evenly and quickly onto the substrate. Once the coating isevenly spread, it should be smoothed by light brush strokes to elimi-nate any irregularities. When the next brushload of coating is applied,the final smoothing strokes should extend from the newly appliedcoating into the previous brush-applied adjacent area. This spreadsthe coating over the overlap between the two areas and provides asmooth, uniform coating over the whole surface while maintainingthe wet edge (the end of the stroke of the previous applied coating).As the next brushload of coating is applied, the painter should sweepthe coating from the substrate back into the wet edge of the previousapplication to help prevent lap marks. It is also important to brushout over an edge, not against it. Brushing against an edge creates anaction that pulls the coating away, causing a thin area on the edge.

RollersRollers have earned a bit more respect than brushes, but their produc-tion results still pale in comparison with results of spray application.Application by roller is faster than application by brush, but is notquite as fast as application by spraying. Because the roller cover holdsconsiderably more coating than a brush, a much larger area can becovered with one load. Rollers are excellent for large, flat areas—forexample, the tops or sidewalls of tanks. Rollers can be used whereverthe skill of a brush or spray application is not called for. Rollers canalso be used if spray applications are prohibited due to oversprayconcerns.

The procedure for using the roller is to immerse it in the coatingtray or bucket and roll it back and forth on either a tray ramp or bucketgrid on the inside of the bucket. This removes the excess coating fromthe roller and prevents excessive drip and spatter. Continue spreadingthe liquid coating onto the surface in the form of a W or an M overan area that one roller’s worth of coating will cover. After initiallyspreading the coating by this method, fill in the area by rolling theroller back and forth over the entire surface being covered. Finishby rolling the coating in one direction. This is called laying off, andit aids in developing a uniform finished appearance. Spraying andbackrolling is another example where the roller is used to ensure auniform application and finish.

Conventional Spray EquipmentMany shops apply the protective coatings with what is termed con-ventional air spray equipment. This method uses compressed air to





Fluid pressureregulator

Regulated air

Fluid

Pressure container

Regulated air

Regulator and moisture separator

FIGURE 3-11 Components needed to apply paint by air spray. (Courtesy ofSpray-Quip, Inc., Houston, TX.)

atomize the paint as it exits the spray gun. In other words, air is in-jected into a stream of paint through the air nozzle in front of the gun,creating the mist that is propelled out. The basic components neededto apply paint by air spray are an air compressor, paint pot or cup, oiland moisture separators, air supply hoses, material hose, regulators,and air spray paint gun (Fig. 3-11).

The most important element in air spray painting, as in other ap-plication methods, is the person operating the gun. Conventional airspray equipment affords the applicator a great level of control andresults in a high-quality finish. The applicator is responsible for ap-plying the paint correctly, using the best technique, and keeping theequipment in good working order. Generally, coating manufacturerslist the optimum pressures for applying their coatings. They also listthe type of gun and the correct sizes of paint nozzle, air cap, andneedle to produce the best-quality applied film. Typically, external-mix air caps are used. The space between the fluid nozzle and the airnozzle is called the annular ring. It provides a column of air aroundthe fluid stream. As the fluid and air leave the air cap, they begin toexpand and mix. As this mixed stream leaves the center of the noz-zle, it is further atomized with additional force from the holes on thehorns of the external-mix air cap. The biggest advantage of a con-ventional air spray system is the control the applicator has over thefinish; relatively easy adjustments to the fluid pressure and air pres-sure give the applicator tremendous flexibility and versatility. Thebiggest drawback is probably low transfer efficiency; conventionalair spray equipment has a transfer efficiency of approximately 25 to30 percent.





FIGURE 3-12 Standard setup of an airless spray system. (Source: Graco)

Airless Spray EquipmentBecause of the low transfer efficiency and slow production time asso-ciated with conventional spray equipment, airless spray equipmentwas developed and introduced (Fig. 3-12). Airless sprayers work bypressurizing paint fluid. The paint travels through a supply hose toan airless spray gun tip, and the coating atomizes into fine dropletsas it exits the tip. In airless spray, as the name suggests, air is not usedto atomize the paint. The basic components of an airless spray systemare a power source, a pressure pump, a paint container, high-pressurefluid hose, an airless spray gun, and an airless spray tip. The paintis pressurized by the pump and forced through high-pressure hoseto the airless spray gun. When the stream of high-pressure coating





FIGURE 3-13 Standard setup of an air-assisted airless spray system. (Source:Graco)

reaches the airless tip, it is atomized and shaped by the specific sizeof the tip. The only adjustment to an airless spray gun is to changethe tip. Coating manufacturers specify the correct tip size and properatomizing pressures needed to produce the best-quality applied film.

Although the airless spray applicator does not have the controlthat a conventional spray system affords, the trade-off is that greaterspeed and greater transfer efficiency value (35 to 50 percent) are pos-sible.

Some have found a way to combine the best features of conven-tional air spray with airless spray equipment and have created a newspray finishing capability. The process has been termed as air-assistedairless spray. This equipment (Fig. 3-13) uses a standard airless pumpand an airless spray tip to atomize the coating and shape it into a fanpattern. However, in contrast to normal airless operations, the fluidpressure in an air-assisted airless spray system is relatively low. Asexpected, a low fluid pressure (usually below 1,000 psi [6,894 kPa])





fails to produce an acceptable spray pattern, and the resulting patternhas heavy edges called tails. To eliminate the tails and assist in theatomization process, air is added at a low volume, typically 1 to 3 cfm(0.47 to 1.41 L/s), and at a low pressure between 10 and 20 psi (69and 138 kPa). To this coating stream, assisting air is directed into theairless pattern through horns on a special air cap. This results in agood appearance, very good transfer efficiency (50 to 65 percent), re-duced overspray, less tip wear, and longer pump life due to the lowerpressures.

Because of new VOC regulations as well as owners seeking“solvent-free” coating solutions in order to reduce or eliminateextractables, manufacturers have begun formulating coatings withmuch higher percentages of solids. Fabrication shops and fieldpainters have had to comply with tougher emission laws and havebegun looking for equipment that provides higher transfer efficiencyand higher solids protective coating material application capabil-ity. These concerns have led to the development and refining ofequipment known as electrostatic and plural-component spray equip-ment.

The main advantages of electrostatic spraying are the savings inmaterials and labor and the high transfer efficiencies of material to thesurface. In electrostatic spraying, a high-voltage electrical charge isimparted to the atomized paint particles via an electrode on the gun,causing the paint particles to be attracted to the substrate, which isgrounded. This virtually eliminates all overspray. Paint particles thatwould normally be lost because of overspray are instead attractedto the edges and even the back of the substrate. Transfer efficienciesobtained with electrostatic spray painting range between 65 and 98percent. The equipment typically used for electrostatic spraying is thesame as that used with conventional air spray equipment and withairless equipment. However, airless and/or conventional electrostaticsystems have electric power supplies to give the paint the negativecharge needed to draw it to all sides of the substrate being painted. Anelectrode at the tip of the gun adds a high-voltage electrostatic chargeto the atomized paint particles (Fig. 3-14). This technology lends it-self to the application of both liquid-applied coatings and powdercoatings.

In plural-component spray painting, two-component (or more)catalyzed coatings are proportioned, mixed, and applied by the sprayequipment. This method is generally for use with coatings that havea very short pot life (from a few seconds to a few minutes) and avery high solids content (typically 100 percent). The base resin andconverter are mixed at the spray gun, or at a mixing manifold pre-ceding the spray gun. The two components are then immediatelysprayed onto the substrate being coated. There are two basic types ofplural-component systems: fixed ratio and variable ratio. A fixed-ratio





FIGURE 3-14 Standard setup of an electrostatic spray system. (Source: ITWRansburg)

system provides only one ratio of volume for the multiple componentsof the coating. A variable-ratio system can be adjusted for differentcomponent ratios (e.g., 1:1, 2:1, 3:1). The equipment consists of two orthree airless pumps (feed pumps) attached to an air motor. The pumpsmove the coating components individually from their containers inseparate lines to a proportioning pump. The materials are then nor-mally heated and directed either to the spray gun tip or to a mixingmanifold assembly fitted with one or more in-line static mixers. Fromthe manifold, the mixed material travels through a whip hose to thespray gun.

Heaters play an important role with plural-component systems.They are used to reduce the coating viscosity, improve fluid flow, andoptimize the reactivity of the materials. Heaters are often installedin-line and are placed on the material containers. The material hosesare often heat traced and insulated, as well help maintain the desiredtemperature. Plural-component systems also use a solvent-fed purgepump that connects the container of solvent with the back of the mix-ing manifold. When an applicator shuts down the equipment, thevalve for the purge pump is opened, and a solvent flush is deliveredto flush out any material that could set up in the mixing manifold(Fig. 3-15).

The mixing manifold, when required, is critical for properly blend-ing and mixing the materials before they leave the spray gun. Themanifold usually contains a static in-line mixer that works by split-ting the coating stream and rotating it to 90 degrees. This is donenumerous times so that the components are mixed thoroughly whenthey exit the spray gun.





Mixer/manifoldSpray gun

Catalyst supply

and pump

Proportioning pumpBase supply

and pump

Solvent supply

and pump

FIGURE 3-15 Standard setup of a plural-component spray system. (Source:JPCL, October 1989)

Inspection of Linings

Independent Inspector’s DutiesCoating contracts usually involve a significant investment of both timeand money. The owner has a well-written job specification completedand conducts some type of bid process to select the coating contractor.The inspector provides the owner with written assurance that theproject has met the specifications and that the coating system willperform for its intended full life. The coating inspector is also viewedas providing additional assurance that the risk of catastrophic failureis significantly decreased or altogether eliminated. The inspector mostoften becomes the eyes through which the owner observes the finishedwork and determines whether the contract has been fully satisfied.

Many tests can be performed after the coating has been applied.It is often difficult to find deficiencies in the coating system, however.Once the job is finished, a poor job may look the same as a high-qualityjob. Therefore, it is important that inspections occur not only at theconclusion of a project, but also during coating operations. This willhelp determine that the coating specification was met.

During ApplicationDuring the application, the inspector may need to conduct a widearray of key tests and observations:





� Ensure that the proper environmental conditions exist. Withthe emergence of new coating technologies, the temperaturesand relative humidity constraints change from product toproduct, and it is always best to refer to the manufacturer’sdata pages. A good rule of thumb is to ensure that the steelsurface temperature is at least 5◦F (3◦C) above the dew pointand rising.� Ensure that the surface is free of dust, dirt, or any abrasiveresidue from the blast cleaning operation.� Ensure that coatings have been properly thinned ahead oftime. This includes being certain that the recommended thin-ner has been used in the recommended amount.� Ensure that the wet film is of the proper thickness. This canbe determined by conducting mathematical calculations ofthe surface area coated and/or by periodically taking directreadings while the coating is being applied.� Ensure that the coating is applied evenly, that the passes areoverlapped, and that there are no thin spots, discontinuities,dry spray, and so on.

The more often problems during application are addressed imme-diately, the greater the likelihood that runs, sags, and discontinuitiescan easily be brushed out and corrected.

After InstallationOnce the coating installation is complete, the inspector should checkthat the proper curing and drying conditions are being maintained.The inspector should also make certain that there has been no con-densation on the surface or that any type of contamination hasbeen deposited on the coating during the curing process. Over-spray, pinholes, runs, or any other imperfections not uncovered atthe time of application should be marked now and repaired beforeanother coat is applied. In many cases, other tests may be requiredonce the coating application is complete, including the followingpoints:

� Discontinuity (holiday) testing: low- or high-voltage type� Dry film thickness measurements: Type I, Type II, or TookeGage method� Adhesion testing: tape test or tensile adhesion tests� Degree of cure using durometers or the solvent-resistancemethod





Inspection ToolsIndividuals responsible for quality control should be familiar withbasic inspection tools including those listed here. This is not to beconstrued as an exhaustive list.

WFT gauge: The wet film thickness gauge is used to measure thethickness of paint being applied at the point of application.Two common gauges used are the interchemical gauge and anotch-type gauge.

DFT gauge: The dry film thickness gauge is used to measure thethickness of paint after it has been applied and, preferably,cured.

Type 1A magnetic DFT gauge: Commonly called a banana gauge,this is a single-point lift-off gauge (Fig. 3-16). It measures non-magnetic coatings over a magnetic surface. It operates by mag-netic contact and resistance of the magnetic force to the surfaceby the coating thickness. Calibration assurance in the field isstrongly recommended.

Type 1B magnetic DFT gauge: Commonly called a pencil pull-offgauge, this is a single-point lift-off gauge. It measures nonmag-netic coatings over a magnetic surface. It operates by magneticcontact and resistance of the magnetic force to the surface by thecoating thickness. Calibration assurance in the field is stronglyrecommended.

Type 2 electromagnetic DFT gauge: This gauge measures noncon-ductive coatings over a ferrous metal surface. It operates byelectromagnetic contact and resistance of the electromagneticforce to the surface by the coating thickness. Calibration assur-ance in the field is strongly recommended.

Eddy current gauge: The eddy current gauge (Fig. 3-17) measuresnonconductive coatings over a nonferrous surface. It operatesby emitting an eddy current and measuring the difference be-tween the emitted signal and the return signal. This differ-ence in time is affected by the coating thickness. Calibration

FIGURE 3-16 Type1A magnetic DFTgauge (bananagauge). (Source:KTA-Tator)





FIGURE 3-17 Type 2electromagneticDFT gauge.(Source: KTA-Tator)

assurance in the field according to the gauge manufacturer’sinstructions is strongly recommended.

Probing knife: This knife is useful in determining adhesion of coat-ings. It can be used in conjunction with adhesive tape.

Magnifiers/magnified light scopes: These are useful for closer ex-amination of substrates and evaluation of pits, contamination,pinholes, etc.

Mirrors and telescoping mirrors: Mirrors are helpful when the appli-cator needs to check behind hard-to-reach areas such as nutsand bolts—wherever direct viewing is impossible.

Surface comparators: These are effective in evaluating varioussurface profiles obtained using various abrasive materials.

Replica tape and spring micrometers: Replica tape and micrometersare often used to determine surface profiles.

Sling psychrometer and U.S. Department of Commerce WeatherBureau Psychrometric Tables: These are used in conjunction todetermine relative humidity values and dew point tempera-tures.

Surface temperature gauges and infrared noncontact temperaturegauges: These gauges aid in determining when the surface isapproaching the dew point and when surface temperature is





excessive. Either situation may be problematic and can lead toblistering.

NACE/SP0178 Design, Fabrication, and Surface Finish Practices forTanks and Vessels to Be Lined for Immersion Service (also known asthe weld replica standard): This is quite effective in determining ifthe welds have been prepared properly and are in a conditionto receive a protective coating or lining.

Holiday detectors: Both wet-sponge low-voltage and high-voltageDC units are effective in examining a lining to ascertain thenumber of discontinuities (holidays) within it. The low-voltageunit is effective if the lining is less than 20 mil (508 �m) DFT.The delimiters on the high-voltage equipment are the coating’sthickness and its dielectric strengths; voltages can range from500 to 200,000 V. Neither unit is recommended for coatingsthat contain conductive pigments (e.g., aluminum, zinc, andgraphite).

Adhesion testers: These reveal possible problems with adhesion bydefining a numerical adhesion value in pounds per square inchor kilopascals and revealing where the break has occurred.

Tooke Gage: Commonly called a paint inspection gauge, the TookeGage is known as “the referee,” so named for its ability to ex-amine individual coatings within a multicoat system as wellas the system in its entirety. It is highly accurate in determin-ing film thicknesses up to coating films of 50 mil (1,270 �m).Testing with the Tooke Gage is destructive, so repairs will berequired.

Soluble-salt testing (chloride and ferrous): Wide arrays of tests can beperformed to obtain information determining the presence ofinvisible contaminants that will be detrimental to the lining.Two common extraction methods are the swabbing methodand the Bresle patch method:� In the swabbing method, the salts are extracted by using

distilled water and medicinal-grade wool or cotton swabs. Adefined surface area is then swabbed with the cotton swabsthat have been saturated with the distilled water solutionand dried with additional dry cotton swabs. The wet anddry swabs are then placed back into the beaker containingthe distilled water and stirred for several minutes.� In the Bresle patch method, the salts are extracted by us-ing distilled water in conjunction with a plastic patch self-adhesive cell and syringe. Distilled water is injected into thecell, allowed to dwell for 20 seconds, and then drawn backinto the syringe. The same solution is then reintroduced intothe cell, and the process is repeated. This process is con-ducted a total of three times.





It is generally accepted by the industry that neither extractionmethod removes all of the soluble salts, but only up to a maximum ofapproximately 45 to 50 percent. Once you have obtained your solution,there are several methods for measuring the amount of soluble saltsobtained, including the following:� Potassium ferricyanide test� Conductivity test� Kitagawa tube test� Quantab strip test� QuantoFix test

BibliographyBauer, M. 1988. Changing Regulations on Coatings for Contact with Potable

Water. Journal of Protective Coatings and Linings (JPCL) 5(12):27–33.Bauer, M. 1996. Organic Zinc Rich Primer for the Interior and Exterior of

Potable Water Tanks. In Proc. SSPC Expanding Coatings Knowledge World-wide, Charlotte, N.C. (Nov.). Pittsburgh, PA.: SSPC.

Crist, M. T. 1996. The Cost and Timing of Water Tank Maintenance. In Proc.SSPC Technologies for a Diverse Industry, Charlotte, N.C. (Nov.). Pittsburgh,PA.: SSPC.

Dromgool, M. B. 1996. Maximizing the Life of Tank Linings. Journal of ProtectiveCoatings and Linings (JPCL) 13(3):62–74.

Dubcak, T. O. 1995. Inspecting Water Tank Linings: The Importance of the FirstAnniversary. Journal of Protective Coatings and Linings (JPCL) 12(9):60–8.

Finch, D. 1996. Protecting Water Storage Tanks in an Era of EnvironmentalCompliance. WATER/Engineering & Management, Nov.

Huffman, L. R. 1997. Going with the Flow: A Sampling of Water Tank Mainte-nance Painting Programs. Journal of Protective Coatings and Linings (JPCL)14(5):38–46.

Ippoliti, T. 2000. Waterborne Coatings for Water and Wastewater TreatmentPlants. PWC (Nov./Dec.):92–7.

Ippoliti, T. 2002. Polyurea Coatings Win Place in Water, Wastewater Facilities.WaterWorld (Nov.):16–8.

Kapsanis, K. 1990. A Water Tank Update: Issues and Practices in RemovingLead-Based Paint. Journal of Protective Coatings and Linings (JPCL) 7(5):50–6.

Knoy, E. 1992. When to Repair Pits in Steel Water Tanks. Welding Design andFabrication (Oct.):51–2.

———1993. Maintaining Aged Steel Water Tanks: What to Look for and Why.Journal of Protective Coatings and Linings (JPCL) 10(5):61–5.

Maronek, A. H. 1988. Evaluating Acceptability of Potable Water Tank Coatings.Journal of Protective Coatings and Linings (JPCL) 5(7):40–5.

Munger, C. G., and D. V. Louis. Corrosion Protection by Protective Coatings. 2nded. Houston, TX: NACE International.





O’Donoghue, M., R. Garrett, and V. J. Datta. 1998. Optimizing Performance ofFast-Cure Epoxies for Pipe and Tank Linings: Chemistry, Selection, andApplication. Journal of Protective Coatings and Linings (JPCL) 15(3):36–51.

O’Toole, D. 1997. Overcoating: An Effective and Economical Solution for Wa-ter Storage Tank Exteriors. In Proc. 1997 SSPC Technologies for a DiverseIndustry, San Diego, Calif.; Pittsburgh, PA.: SSPC.

Preparation of Protective Coating Specifications for Atmospheric Service.2000. Joint Technology Report SSPC-TR 4/NACE 80200. In Proc. Planningand Specifying Industrial Protective Coating Projects (2004). Pittsburgh, PA.:SSPC.

Public Works. January 1995. Specifications for Coating Water Storage Tanks areEvolving.

Roetter, S. P. 1993. Liability Enormous for Lead-Based Paint Removal. Opflow(March).

Schubert, R. 1999. Construction and Operation of Water Storage Tanks inRural Alaska. In Proc. 1999 AWWA Engineering and Construction Confer-ence, Orlando, Fla. Denver, Colo: AWWA.

Shannon, G. B. 2003. Selecting Coatings for an Elevated Water Tank in aDensely Populated Business Park. Journal of Protective Coatings and Lin-ings (JPCL) 20(7):85–7.

Smith, L. M., ed. Generic Coating Types: An Introduction to Industrial Mainte-nance Coating Materials. SSPC 95–08. Pittsburgh, PA.: Technology Publish-ing Company.

SSPC Painting Manual Volume 1: Good Painting Practice. 4th ed. 2004. Pittsburgh,PA.: Society for Protective Coatings.

SSPC Painting Manual Volume 2: Systems and Specifications. 8th ed. 2000. Pitts-burgh, PA.: Society for Protective Coatings.

Stein, G. R. 1994. Community Acceptance of Lead Paint Removal Projects.Journal of Protective Coatings and Linings (JPCL) 11(5):119–25.

Zienty, D. 2002. Tanks Pull Double Duty. WATER/Engineering & Management149(2):9–13.

Zienty, D., and L. Dornbusch. 2002. Painting for Antenna Installations on WaterStorage Facilities. Journal of Protective Coatings and Linings (JPCL) 19(9):73–9.




C H A P T E R 4Contractual

Considerations

William J. Dixon, P.E.Dixon Engineering, Inc.

This chapter includes general discussions of the most important topicsto be investigated and detailed in preparing the specifications for anew steel water-storage tank. Some topics are common to steel watertank repair and/or repaint. Repair and repaint comments are madefollowing the new-tank comment in some cases. This chapter describesthe content and purpose of each of the documents involved in bidding,and it notes the duties of owner, engineer, and constructor with respectto specifications, bidding, design, construction, and inspection.

Competitive BiddingThe water industry in the United States has been serving municipalclients for more than 200 years. In that time, the industry has de-veloped competitive bidding practices, which are required by law inmost states and by governmental subdivisions. Many variations tothe standard construction project (designed and bid by the engineer)and some old practices such as maintenance contracts have resur-faced with new twists. Different methods of contract administrationhave been developed, the most common of which is usually calleddesign/build. Newer practices include computer bidding and whatis called a reverse or negative auction. These alternative methods havedifferent benefits depending on the project and on whether you are apublic owner or a private owner.

It is necessary to fully understand the benefits of the selectedmethod. The closed competitive bid process was developed to elim-inate fraud and political influence in the awarding of contracts and

131




132 C h a p t e r F o u r

generally led to the project being awarded to the lowest responsi-ble and responsive bidder. Responsible refers to the company’s abilityto successfully complete the project within the required time frame,and responsive refers to the thoroughness of the bid submittal—thatis, whether the bidder properly completed the bid, included all biddocuments (e.g., noncollusion of contractor with other contractors;minority and/or women and/or disadvantaged participation forms),and submitted the required bonding.

Negative Bidding/Reverse AuctionIn the negative or reverse auction, the bidder submits the bid on anopen auction Web site. The bid is open for a set time, generally a coupleof days. Other bidders then submit lower bids. The first bidder is freeto resubmit his or her bid, undercutting successive bidders. If biddingis active, the owner can extend the bid time in his or her interest. In the-ory, this procedure may work well for industry or private firms—buta low bidder, in a common industry joke, is the bidder who made thebiggest mistake. It is in everyone’s interest that the constructor makea fair profit. When a constructor, after submitting what he calculatesis his best competitive price, is squeezed to cut his bid further, he en-ters the project in makeup mode: During the project, he will either belooking to cut corners or looking for extras to increase the scope of theproject. Negative bidding, therefore, requires a very thorough set ofcontract documents and full-time third-party inspection.

The closed or sealed public bid process was designed to eliminatethe awarding of projects under political influence, and, in general, ithas worked. Remember, the issue is more than a fair and open publicbid process; it is also the appearance of a fair and open public bidprocess. With the ability to extend the closing time, it is possible toclose bids after the preferred bidder has submitted his bid. His priceis the lowest, but if a significant change order develops, at least theappearance of favoritism is there.

Roles of the Owner, Constructor, and Engineer inStandard Municipal ContractingThere are usually two contracts—the construction project contract andthe contract between the engineer and the owner. Figure 4-1a bestexemplifies the standard roles of the owner, constructor, and engineerin the competitive bid process.

In the construction project contract, the constructor and the ownerhave vertical privity of contract that is extended downward to sub-constructors, suppliers, and subcontractors’ suppliers (Fig. 4-1b). Itis important to insulate the owner from subcontractors and sup-pliers through contract protective clauses (safety, indemnification,insurances, etc.). In the second contract, that between the engineer



Contractual Considerations

133C o n t r a c t u a l C o n s i d e r a t i o n s

Owner

Constructor

Engineer

Horizontalprivity

Lawsuits

Lawsuits

Verticalprivity

(c)

Owner

Constructor

Subcontractor

Subcontractor supplier

Tier-2 subcontractor

Engineer

Supplier

(b)

(a)

Owner

Constructor

Engineer

Horizontalprivity

Verticalprivity

FIGURE 4-1 Roles of owner, constructor, and engineer in the competitive bidprocess. (a) Privity means a direct relation throughout the contract. (b) In thissituation, there is still no privity between the constructor and the engineer.There is vertical privity from the second-tier subcontractor all the way up tothe owner. Contract clauses and performance and payment bonds insulatethe owners as much as possible from lawsuits by subcontractors andsuppliers. (c) Because the engineer and the constructor are not third-partybeneficiaries of each other’s contract, neither can sue the other. The owner isunder privity both ways, so if he or she is sued by the constructor, the ownercan bring the engineer in by filing a claim against the engineer.

and the owner, the engineer is hired to prepare specifications andcomplete project management and inspection services. This is knownas horizontal privity of contract between the engineer and the owner.On a repaint or rehabilitation, the engineer is hired to also do a prelim-inary bid inspection to establish the scope of the project. The ownerdoes not intend the constructor to be a third-party beneficiary of thiscontract. The owner is the sole beneficiary of the services performed.





That is why in Fig. 4-1b there is no connection to the constructor inthe horizontal privity line.

The lack of contractual privity and the owner’s intention that nothird-party beneficiaries are to be part of the owner/engineer contractare represented in Fig. 4-1c by the jagged lines between the engineerand the constructor. The arrows indicate that the engineer does notcontrol the constructor and that the constructor cannot rely on (and,in most cases, cannot sue) the engineer. The arrows further indicatethat the engineer cannot interfere with the constructor’s “ways andmeans” or “means and methods.” If the engineer, who might be moreexperienced, starts giving directions or advice to the constructor, heor she has crossed the line. The engineer assumes the risk for thisadvice, which has broken down the wall between vertical and hori-zontal privity. The engineer’s job is to review, report, and interpret thecontract documents. Sometimes the owner’s staff members providetheir own specifications and inspection services. In this situation, adefinition is needed so that the owner’s inspections do not substitutefor the constructor’s own quality control responsibilities.

Design/BuildThe design/build procedure puts the responsibility of engineeringdesign on the constructor. This procedure works well in new-tankprojects but has conflicts with rehabilitation projects. The benefit ofdesign/build lies primarily in expediting the project. On new-tankor new-tower projects, the owner supplies the bidder with soilinvestigations, establishes capacity and high water level, and designsstandards and a time schedule to follow. The bidder can properlyprepare costs and submit the bid. If the procedure is followed prop-erly, there should be no unknowns and no cost increases. Third-partyinspection is still necessary.

On tank repair/repaints, the extent of repairs and the condition ofinterior coatings and corrosion may not be known to the tank ownerbecause of the complexities of removing the tank from service for in-spection. It is not recommended that the party completing the workbe allowed to establish the scope of the work. New construction withno unknowns and larger budgets may be competitively bid as de-sign/build. Design/build is not practical on lower-budget rehabili-tation projects, because the unknowns cannot be competitively bid.A pre-bid independent tank inspection can eliminate the unknownson repair/repaints. But to ensure competent work and competitivebidding, a full set of specifications is still necessary, because the con-structor’s idea of what constitutes a proper repair and coating systemwould be different than what the owner expects. Third-party inspec-tion would be necessary for quality assurance, but without specifica-tions and a contract, what would the inspector inspect?





Project AdministrationFor a further discussion of all of the major methods of project ad-ministration, see American Water Works Association Manual M47,Construction Contract Administration.

Pre-Bid Inspections—Existing TanksSealed competitive bids are only as accurate as the bidding docu-ments that all bidders review. For rehabilitation projects, the biddingdocuments are only as good as the pre-bid inspection report they arebased on. Pre-bid inspections are discussed in Chap. 10. The pre-bidinspection report is intended to provide the owner with a thoroughunderstanding of existing conditions and repaint options. The pre-bidinspection report is not intended for use as the bid document. If theowner permits the pre-bid inspection to be performed by a constructorinstead of an inspection firm with no vested interest, that inspectionshould be the total scope of the constructor’s contract. The constructorshould never be awarded an emergency repair or repaint change orderbased on his or her inspection. With the exception of vent screens andopen cathodic protection holes, most repairs can wait to be completedthrough the competitive bidding process.

The practice of some inspection/paint firms to sell emergency re-pairs has led some state regulatory agencies to develop or expandpermit requirements. Most states require permits for all work on atank’s interior; some states also require permits for exterior work toverify compliance with air quality laws. Permits are issued on the ba-sis of specifications submitted to the regulatory agency and on thecondition that work is to be completed according to those submittedspecifications. New or relocated tanks require location and elevationreview and may require a permit from the Federal Aviation Adminis-tration (FAA). If, in the tank owner’s opinion, the emergency repairsdetailed by the constructor’s pre-bid inspection are truly emergencies,contact the regulatory agencies to determine whether an emergencypermit is needed and whether proposed repairs meet state standardsor codes.

The pre-bid inspection report can be included in the bidding docu-ments. It is important that it be labeled an “appendix for informationonly” so that its language does not conflict with the specifications.The appendix report should state that it is a service only and that fieldverification by site visit is required.

Contract DocumentsThe terms contract document and bidding document are often incor-rectly thought to be interchangeable. Bidding documents used to so-licit project bids traditionally include all the information needed to





prepare and submit a bid, including the advertisement, informationfor bidders, general conditions, supplementary general conditions,addendums, technical specifications, drawings, and proposal pages.Contract documents include all the bidding documents as well assigned contract or agreement forms, bonds, insurance certificates, no-tice of award, notice to proceed, and submittals. As the project pro-gresses, change orders and field orders are added. Most contractspermit the inclusion of minutes from preconstruction and progressmeetings.

Design Standards

Incorporation of StandardsStandards prepared by national associations have been incorporatedinto technical specifications for decades. Standards are beneficial be-cause the bidder is using the same terminology as used by the owner/engineer and for the most part knows what is expected. The Amer-ican Water Works Association (AWWA) standards incorporate othernationally recognized standards—American Welding Society (AWS),Society for Protective Coatings (SSPC), National Association of Cor-rosion Engineers (NACE), American Petroleum Institute (API), andconcrete standards, to name a few. The benefits are obvious, but thereare pitfalls.

Using standards requires the engineer’s understanding of the in-cluded standards. He or she must decide whether to include the entirestandard or just portions of it. An example would be the use of AWWAD100 Standard for Welded Carbon-Steel Tanks for Water Storage.D-100 defines how many X-rays are to be taken. If you want moreX-rays or a different selection process for X-ray locations, the spec-ifications must detail the variance. The potential for a conflict mayoccur more in the incorporation of other standards within the spec-ified standards (i.e., when a second tier of standards is incorporatedby reference within the specified standard).

Most standards are submitted to the American National StandardsInstitute (ANSI) for certification. One condition for ANSI certificationis that the standard be reviewed and formally updated at least every5 years. Because of the constant updating and the long bid process,specifications must identify which standard is being incorporated, ei-ther by date or by clause in the section that outlines general conditions.This also applies to standards incorporated within the standard spec-ified. Industry standards recognize the standard in use when bids areopened. If a standard is updated during a nonbid situation, a negoti-ated contract, the negotiating parties should define which standard isto be used.





Policy Changes of AWWA StandardsAfter review by AWWA attorneys and officers and by the AWWA Stan-dards Council, AWWA has made two major changes to the standards.First, they are to be minimal standards that establish the minimal ac-ceptable level of performance. Second, all contractual language is tobe removed from all AWWA standards. These significantly changeboth the content of the standards and the effect of incorporating theminto the specifications.

Minimal Acceptable Level of PerformanceAWWA D100, AWWA D103 Standard for Factory-Coated Bolted-SteelTanks for Water Storage, and the proposed standard for composite el-evated tanks assign specific responsibilities to the owner (or to theowner’s agent), the engineer, and the constructor for the construc-tion of steel tanks. AWWA D102 Standard for Coating Steel Water-Storage Tanks is the painting standard used for both new and ex-isting tanks. AWWA D104 Standard for Automatically Controlled,Impressed-Current Cathodic Protection for the Interior of Steel WaterTanks is the cathodic protection design standard for new and existingtanks. The design requirements for water, snow, wind, and live loadsin D100, D103, and future composite-tank standards are the minimumallowable values. The design methodology has changed, permittingthinner steel.

D100 and the proposed composite-tank standard have changedthe approach to design of allowable loads for the buckling of conicaland dished sections. The owner/engineer must choose one of threemethods for the design. All three are safe, but the owner/engineermust select the level of conservatism preferred. When specifying ac-cording to these minimal standards, construction tolerances are morecritical. These standards do not carry a default design for this partic-ular choice if the specifier fails to select the design methodology. Theresult may be a less conservative design than the owner would prefer.The owner and the engineer are expected to be aware of all local codesand ordinances and should provide the required loading information,design information, or both to the constructor if local requirements aremore stringent than those found in the applicable AWWA standard.Note: Contractual language often shifts the responsibility for knowinglocal codes to the constructor.

This change to a minimal standard is not in itself a bad practice.The new tank still will be designed per the International Building Codeand will perform as needed. The trade-off in connection with the newminimal standard is the higher degree of attention to maintenance,which is still the responsibility of the owner. The same level and costof maintenance exist, regardless of the change in the standard: Thedifference is that there is no time cushion and so no opportunity todelay that maintenance.





The old tanks were behemoths and used more steel than the tanksdesigned today. Water towers were once designed by engineers us-ing slide rules, and steel thicknesses were rounded up to the nextone-eighth of an inch (0.125 in. [0.3 cm]). To this, the early engineersadded another 0.125 in. (0.3 cm) for corrosion allowance. When weld-ing replaced riveting, welds had to have only 66 percent penetration.Whereas earlier, steel was one size going down the stem until thenext fraction was needed, now every stem section could be differ-ent, because current steel manufacturing allows steel with differencesin size, measured by 10 mil (254 �m), to be purchased. Computer-assisted design, cutting, and rolling permit the new thinner de-sign methodologies for conical and dished sections. Welding require-ments have increased to 100 percent penetration welds. Corrosion al-lowances are still the option of the design engineer, but they are seldomspecified.

The excess steel—not maintenance or the coating systems—is thereason some of those older towers are still standing. The interior coat-ing of old towers was two coats of red lead primer protected by awax (grease) coating. Lead was good from a durability standpoint butobviously now is out of favor because of health effects. The greasecoating had a very short life, particularly in cold climates. Pit weld-ing was standard in maintenance projects, whether it was needed ornot. The modern epoxy, urethanes, and polyureas, as well as cathodicprotection, offer a far superior and cleaner method of protection. Main-tenance painting and cathodic protection are more critical now thatnew designs have no corrosion allowance. There is a safety factor indesign calculation, but that is not the same thing as a cushion factorfor steel loss.

Removal of Contractual Language in the D100 StandardThe second change was the removal of contractual language fromAWWA standards. It used to be that the specifier would incorporatethe D100 standard, which would then speak for itself (“the constructorshall design . . . , shall fabricate . . . , shall erect . . . , shall test welds,”and so on). It is still necessary to incorporate the various sectionsof the standard or the whole standard, but the contractual languagemust now be in the specifications. Failure to use contractual languagecould make complying with the incorporated standard optional forthe constructor.

Contractual language has also been removed from the D102 stan-dard for painting projects. D102, however, always required the se-lection of an interior or exterior coating system. The new standarddoes require input by the specifier (questions found in the appendix)defining some responsibilities.





Assignment of Responsibilities of the Engineer, Owner,and ConstructorIn standards, a foreword poses a series of questions that need to beanswered by the owner and the engineer. Their answers will define theresponsibilities of the owner, engineer, and constructor on the project.The constructor needs this information to prepare the bid.

Rebuilt Tanks—Going Where No Standards Have Gone BeforeThe other pitfall of design by standard is specifying the building of,or a repair to, a product to which no standard applies. Examples arespecifying structural repairs according to D102 or rebuilding a tankaccording to D100. D102 concerns painting only; D100 covers the de-sign loads of and construction methods for new tanks, not the designof rebuilt tanks. Rebuilt tanks were built earlier according, it is hoped,to some standard in existence at that time. It would be the exception,not the rule, if a rebuilt tank met current D100 requirements for newtanks. The D100 committee had struggled with the concept of whetherto establish a subcommittee to develop a standard for rebuilt tanks,but the complexities involved and potential liabilities may preventany effort to establish a standard for rebuilt tanks.

If you live in seismic zone 0, for example, it does not matter wherethe rebuilt tank came from. If you live in seismic zone 2, you mustknow what zone the tank was originally built in, what the code re-quired when the tank was bid, and preferably the original designcalculations and as-builts (original construction drawing revised toreflect any construction changes). Without this information, you haveto do reverse engineering—measuring every section of steel, account-ing for corrosion losses, and then designing according to current stan-dards and codes to see whether the old tank meets them.

Remember, older tanks may have been built under a standard thatrequired only 66 percent weld penetration. Old riveted tanks shouldnever be rebuilt. A rebuilt tank should be built from only one tank,not from a collection of parts from more than one tank. If you chooseto allow a rebuilt tank, your specifications should be very thorough.Specify only the pertinent sections of D100 and D102.

Exercise caution in evaluating a used tank that is bid as an alterna-tive to a new tank. The initial cost should not be the only consideration;total-life-cycle maintenance costs should also be projected. An ownerconsidering a used tank should require the following:� A copy of an up-to-date inspection report of the structure

provided by a qualified, registered professional engineer.� A signed document from the present owner stating that thetank is available for sale.





� A detailed proposal of any remedial coating or repair workthat will be done to bring the structure to the equivalent stateof the proposed new tank.

It is also important to know the specifications and standards underwhich the tank was originally built, as well as the wind and seismicloadings for which it was designed and constructed. An additionalcaution is the possibility that lead or other hazardous materials are inthe coating of the tank being considered for reuse.

Tanks that were designed and used for fuel oil or a liquid otherthan nonpotable water may not have the desired wall thickness forwater storage, having been designed to contain a liquid with a dif-ferent specific gravity. Additional problems in converting a tank fromhydrocarbon storage to potable water storage have to do with howclean the steel that contacts the water can be made, and how thataffects adhesion and the purity of interior coatings.

New-Tank DesignsNew-tank designs are another example of industry outpacing thespeed at which standards are developed. For example, the compos-ite tank (concrete pedestal/steel tank) was built without an AWWAstandard for more than 15 years before a standard was developed.Other smaller associations, including the Steel Plate Fabricators, wereable to develop standards more quickly, permitting the composite-tank industry to grow until the more comprehensive AWWA stan-dard could be developed. A hybrid—a glass-lined bolted-steel tankon a pedestal—was developed and applied for inclusion in AWWA’sproposed composite-tank standard, but it failed to make the standardbecause of timing.

The caution is not to avoid new products but to understand thestandards before you specify something that is not included in them.

Factors in Competitive Bidding

Construction Time FrameThe time frame during which you expect a constructor to build anelevated tank is inversely proportional to the amount of money youhope to spend. A tower can be designed, fabricated, and erected in6 months, but it would be expensive. A year and a half—540 days—isa reasonable time frame. (In the northern half of the country, becauseof the shorter painting season, the tank can be constructed over thewinter, but painting will be delayed for a few months; allowing 540days enables painting to take place during warm weather.)





The construction process involves design by the constructor andsubmittal of design to the engineer for review. Sixty days may elapsebefore the engineer turns it around. After the design drawings areaccepted, steel is ordered, cut, rolled, and fabricated in the shop. Af-ter shop drawings are approved and before foundation work is evenscheduled, the constructor prioritizes the project (based on comple-tion schedule and the schedule of other projects). The foundation isconstructed and allowed to cure for 28 days. Steel is usually deliv-ered during this cure time. The start of concrete work is dependenton timely review of submittals. Actual time to erect steel in the fieldranges from 6 to 20 weeks, depending on weather, site conditions, andtank size. The painters need 30 to 60 days, again, depending on tanksize, containment requirements, and, particularly, weather. Includinga break for winter, it is not hard to see why 540 days and schedulingflexibility are needed.

On new-tank projects, the existing tank or operating system isnot removed until the new tank is operational. On repair and repaintprojects, time restrictions are critical as there may be no backup system.Again, the shorter the project time, the more the project may cost, butconditions may justify the cost. Weather, summer demands on thewater supply, extra time for containment projects, and even the startof the owner’s budget year may delay the project start. Money canbe saved by specifying a maximum out-of-service time and the latestcompletion date. This way, the constructor knows that it is a 60-dayjob and can schedule on the basis of the crew’s availability. The dayscan be scheduled anytime, as long as work starts 2 months before thespecified latest completion date.

Pre-Bid MeetingA pre-bid meeting is beneficial and useful for discussing specific non-technical portions of the project, the timing of subcontracting require-ments, and forms in the bid documents (e.g., noncollusion, minority,affidavits, subcontractor lists, lien waivers, etc.). This meeting can bemandatory or optional. If it is mandatory, interest in the project canbe gauged by how many constructors are present, which could affectthe contract price. As a minimum, job-site visits should be required.On repaint projects, if the tank is empty, it should be made availableto the painter for inspection. If the tank is elevated, prospective bid-ders should provide proof of insurance before they climb it. To limittime infringements on the owner, limit the days the site is availablefor inspection.

Prequalification of BiddersTo shorten the time between bidding and awarding of the contract,prequalify the constructors. Tank constructors specialize in the design,





fabrication, and erection of welded-steel, bolted-steel, or compositeconcrete/steel tanks. Tank painters are also very specialized. Con-structors and painters rely on extensive safety training, specializedequipment (some that they have manufactured themselves), and waysand means procedures developed during years of experience. Tankconstructors and tank painters should be prequalified on the basisof their experience, ability to supply bonds, financial condition, andsafety and environmental record. For future discussion, see Chap. 4of AWWA Manual M47, Construction Contract Administration. Cau-tion: Competitive bidding laws in some states restrict prequalifi-cation.

Constructor AssistanceGenerally, tank constructors and manufacturers are more than willingto discuss individual tank needs and to assist the owner and the en-gineer by providing standard design information for a project. For anelevated tank of a given capacity, each manufacturer has different ge-ometric parameters. These dimensions normally do not vary enoughto cause difficulty if one constructor is selected for a project on thebasis of the information supplied by another constructor.

To become familiar with current industry standards and practices,the owner is advised to contact prospective bidders and discuss aproject before issuing an invitation to bid. Most manufacturers arewilling to provide copies of preliminary specifications developed fortanks of varying styles and capacities. The owner must be careful tomake every effort to write a specification that is open and does notexclude bidding by any qualified manufacturer or supplier. In par-ticular, a given manufacturer’s proprietary design details should notbe included in a project’s contract document; this would create aninequitable bidding situation for other qualified suppliers or manu-facturers.

Forced Use of SubcontractorsSome communities have requirements to hire a set percentage of mi-nority or disadvantaged subcontractors. Some communities call forhiring of local employees or subcontracting of local firms in the bidpackage. Although these are laudable goals, care must be taken fortank constructing and painting on how and by what part of the con-tract these provisions are enforced. Concrete foundations are a goodexample. Many firms are capable of forming and placing the concrete,but the foundation is just that—a foundation that supports millionsof gallons or liters of water. The AWWA standard requires the con-structor’s engineer to design the foundation. For liability reasons, theconstructor should design and be responsible for the foundation andall structural items.





Painting projects consist primarily of abrasive blast cleaning,painting, and containment construction, some metal repairs, but nota lot of subcontract work. Also, remember that these constructors andpainters have had extensive safety training and that they are respon-sible for every worker, including subcontractors and forced hires, onthat tank.

Elements of tank construction, for example, electrical controls, siterestoration, and waste disposal, can be separated and bid locally. Infact, it is good practice for the local firm or employees responsiblefor future maintenance and controls to be familiar with their instal-lation. Telemetry must interface with the owner’s master system. Ifthe mechanical system requires piping and several valves inside thetank, this work can be bid to local mechanical constructors. On paintprojects, it may be possible to tie another project into the paint workto accommodate the subcontracting.

Bid SecurityBonds are required on most publicly funded projects over a minimumdollar amount. This dollar amount under the Miller Act is $50,000;many states have a “Little Miller Act” that may have lower limits.Some municipalities have lower limits yet. The Miller Act requiresperformance and payment bonds. Bid bonds are generally requiredby state or local statute.

The bidding process is time-consuming and involves significantexpense for both owner and constructor. In a tank-painting contract inthe northern states, a bidder failing to honor his or her bid could delaythe project into the next season. In that case, the second constructorwould still have to honor his or her bid but, thinking the job had beenlost, may have taken another project. Painting constructors usually bidand complete all of their contracts within the same weather-restrictedseason. They traditionally fill their season and do not leave openingsfor jobs for which they came in second. Fairness may require adjustingthe project schedule into the next year if the first bidder cannot meetthe agreement.

The bid bond, intended to cover the increased cost of rebiddingor awarding to the higher bidder, has been traditionally set at 5 per-cent of the bid. This amount should be sufficient on a $500,000 new-tank contract ($25,000), but it may not be sufficient for a $50,000 paintproject ($2,500). If the constructor defaults and the bond is collected,the constructor could lose the ability to purchase bonds. In the coat-ing industry, with its limited number of qualified constructors, limitedseasons, and wide range of bids, a painter who has received a morelucrative contract may buy his or her way out of the smaller job. Forthis reason and others, many engineers require a bid bond higher than5 percent or for a set amount.





General Conditions and SupplementaryGeneral Conditions

Common parlance at the beginning of a bid document, referred toas boilerplate, includes information for bidders concerning generalconditions and supplementary general conditions, or supplementals.This is standard text that remains virtually unchanged for all projects.Rewriting the general conditions and the supplementals is not neces-sary on all projects, but they should be reviewed each time. AWWAM47 recommends modifying the general conditions or supplementarygeneral conditions to be more job specific.

The general conditions, if prepared by the engineer, are gener-ally standard documents prepared by the Engineers Joint ContractDocuments Committee (EJCDC). This is a coalition of engineering as-sociations with endorsements by the Associated General Constructorsof America and the Construction Specifications Institute. Even moreimportant is another EJCDC document, “Guide to the Preparationof Supplementary Conditions.” This guide follows the general condi-tions, explaining what they really say and listing some of the standardexceptions. Most bidders do not review the general conditions everytime, but they should read them at least once. The general conditionsare generic to construction work; supplementary general conditionsthat are tank specific should be clauses describing the following localrequirements:

� Bonding� Warranties, guarantees� Maintenance contracts� Insurance� Indemnification� Prevailing wage and documentation� Use of local or union labor� Payment application, change order procedures� Steel or concrete cost escalation� Dispute resolution� Safety� Meetings—preconstruction, progress, final punch list� Severability� Schedule of values� Termination





BondingBid bonds, discussed in the previous section, are usually free. Theconstructor pays roughly 1.5 to 2.5 percent depending on the work-load and contract default experience for the actual performance andpayment bonds.

A combined performance and payment bond for 100 percent ofthe project bid meets the requirements of the Miller Act, but it maynot be sufficient to cover a project gone sour. The performance bondis used to ensure that all aspects of the project are completed. If theoriginal constructor is unable to meet the terms of the contract, thebonding company brings in another constructor to finish the project,at least to the extent of the bond. A payment bond is used to payall legitimate subcontractors and suppliers if the constructor fails tomake payments.

Problems generally start with the lowest bid process. If the projectis awarded to the lowest bidder, who turns out not to be responsible(is incapable), a 100 percent combined performance payment bond isinadequate. Remember, the bond is for an amount that all of the otherbidders thought was too low. Out of that amount, you must pay off allunpaid subcontractors and suppliers and bring in another constructorto finish.

Separate performance and payment bonds, each at 100 percent ofcontract amount, are the current requirements of the EJCDC generalconditions. That and control of partial payments should be sufficientto fund completion of the project.

Bonding companies have a contract with the constructor, who usu-ally must personally guarantee the bond. The bond names the owneras the intended beneficiary. Consider the bond a product supplied asa condition precedent to award of the contract to the constructor. Aword of caution: There is no contract between the surety and the owner,but consider the relationship an obligation. The owner is required tocontrol payments to the constructor and to receive waivers from allsubcontractors and suppliers; this is called a waiver of surety. If theowner pays the constructor too much and there is insufficient moneyleft to complete the project, the surety has an affirmative defense toavoid payment.

Another caution: Verify that the surety company can be served pro-cess to force payment. Offshore bonding companies have attractivebonding rates. Offshore companies are offshore for several reasons;they enjoy tax advantages and are untouchable for claim enforcement.The surety should be licensed in the state of the project.

Another aspect of the payment bond is the notice requirementsof the Miller Act and the Little Miller Act. The prime contractor’ssubcontractors and suppliers do not require notice of hire. Their con-tract with the prime contractor, or prime, is evidence of notice. If the





second-tier subcontractors or suppliers and subsequent tiers fail togive notice to the prime at the start and again within so many daysof the last workday, the surety does not have to pay the claims un-der the Miller Acts. The Miller Acts and surety also limit coverage:Third-tier subcontractors and suppliers to second-tier subcontractorsare not protected. Although they have to draw a line somewhere, thisis unfortunate, because those companies are usually the locals. Al-though local companies are not protected by surety, the owner canstill hold payment until they are paid if the owner has received noticeof nonpayment.

The fourth type of bond is a maintenance bond. Maintenancebonds are usually free, but a separate maintenance bond is reassur-ance that any work due under the warranty will be completed. Be-cause a warranty is a contractual requirement, it technically is alreadycovered by the performance bond. However, a separate maintenancebond with the time frame defined is recommended for all warrantiesexceeding the standard 1-year/13-month warranty. A maintenancebond should also be considered for the full term of a multiyear main-tenance contract (extended maintenance bonds would not be free).Performance bonds or maintenance bonds expire unless the ownergives notice to the bonding company that work is needed.

WarrantyThe standard construction warranty period is 1 year. The painting war-ranty is also 1 year, but the D102 standard allows a 13-month periodin which to complete the paint warranty. The time extension recog-nizes the difficulty some communities have in isolating their tank.Also, weather may interfere with draining the tank within the speci-fied time. There is also a trend toward specifying longer warranties, apractice that theoretically raises project costs. Constructors prefer towrap up a project in 1 year. Their bonds are then released; they haveless unknown potential liability and can bid other jobs.

A multiyear warranty on tank construction has little benefit un-less full use of the tank must be delayed. Most problems with weldedsteel are evident within a year. Extended warranties are specifiedmore often on painting of new tanks or repainting contracts. Con-tractual problems of long maintenance periods can be resolved bythe use of a maintenance bond. The problem is that the constructoris giving a warranty on a product that deteriorates with age, weath-ering, ablation, ultraviolet (UV) degradation, and so on. There is nostandard against which to hold a 2- or 5-year paint project warrantycondition.

Unless specified differently, D102 limits holiday testing (direct-current voltage testing for coating pinholes) to the high waterline anddown on the wet interior surface. If there are coating breaks in that





area after 1 year, repairs under the warranty are justified. Small coatingbreaks, rust staining at the lap seams of the wet interior roof or pinholerusting on the dry interior surface or on the exterior, are not warrantyissues. If the specifications did not require a holiday-free coating, awarranty cannot require a holiday-free system at 1 year. At 2 yearsor 5 years, enforcement can only be to whatever expected coatingcondition at warranty was set in the original specifications.

Also, a 5-year warranty has to consider normal wear and tear. Ifyou are looking for annual maintenance, bid the maintenance contractinto the construction bid as a separate cost; do not try to complete thework under a warranty clause. Trying to have work completed in5 years for no pay is tricky—it is difficult enough just rememberingthat there is a 5-year warranty. The second concern is whether theconstructor is still in business. The owner needs to pay only whenscheduled work is completed.

Maintenance ContractsMaintenance contracts are for a set period of time. Generally, the ac-tual painting takes place during the first year, and touch-up and re-pairs happen in subsequent years. Some maintenance contracts areattractive because they begin with an enticing finance offer, in whicha company finances the initial high cost of the first painting over thefirst couple of years. As always, some good constructors and some badones offer these services. To differentiate, follow the money—or, bet-ter yet, control the money. Work including maintenance proceduresshould be controlled by specifications prepared by your engineer. Thework should be inspected annually by a third-party inspection firm.The financial portion should be written or at least reviewed by yourattorney. Your attorney should also offer an opinion if competitive bid-ding of the entire maintenance project is required by local ordinanceor state statute. Some of the cost advantage is in the constructor pro-viding financial and engineering services, but the owner must decidewhether the savings are in the owner’s interest or are in the vestedinterest of the maintenance constructor.

When the dust settles, the good constructors will be there; theones with prices too good to be true will be the missing parties. Main-tenance contracts are not new. In the 1960s and early 1970s, annualmaintenance contracts were a major portion of the painting market.Most contracts were for 10 or 12 years and had the same annual pay-ments. Painting on the interior would take place during years 5 and11, and the exterior would be painted during years 6 and 12. Essen-tially, the major work was paid in advance. A painter had enoughother contracts at varying stages and so could finance the work whenit was due. When the gas crunch came in 1973 and gas and paintprices skyrocketed, most constructors merely walked away with the





up-front money or went bankrupt, and the owners were left with paidbut uncompleted projects.

Insurance RequirementsRisk managers or insurance consultants for the owner should establishlimits and types of coverages. Sometimes, consultants are nervousbecause the project requires elevated work or may involve exposureto lead paint. Most tank constructors carry large limits of liability,and some are self-insured. But smaller paint constructors have lowerlimits. The limits and types of insurance should match the owner’sexposure to risk. Excess coverage sometimes cannot be purchased bysmaller constructors. Purchasing special insurance for one job drivesproject costs up, particularly if some constructors are eliminated frombidding.

Workers’ Compensation insurance levels are standard and protectthe constructor’s workers. General liability insurance protects site vis-itors other than employees. Auto insurance is recommended even ifall work is on one enclosed site, because the constructor still will runerrands. An umbrella policy would be in addition to these policies.

The owner and the engineer require the constructor to namethem as additional insureds. Any claim against the additional insuredother than gross negligence is covered under the constructor’s policy.Over the years, attorneys have expanded the gray area between theconstructor’s and the owner’s insurance liability. Some insurancecounselors now require the constructor to furnish a separate owner’sprotective policy.

Professional liability insurance covers errors and omissions andis associated with the engineer. If the constructor acts as an engineerin a design/build contract, this insurance may still be needed. Somestates have strict liability laws for accidents involving gravity, falls, orinjuries from dropped objects. Under contract terms, this liability canbe covered under an owner’s protective policy paid by the constructor.But with strict liability laws, responsibility is automatic and liabilitycannot be avoided. The owner should consider his or her own policyand consider making the constructor’s policy a primary-pay policy.

The EJCDC documents require the owner to supply a builder’srisk policy. As noted, the EJCDC documents were prepared by profes-sional engineers’ associations and endorsed by a constructors’ associ-ation; the owner’s municipal associations were not involved. Ownersprefer that the constructor provide a builder’s risk policy. This policyprovides insurance for the project during construction (e.g., if a tor-nado blows over an unfinished tank). The policy covers the cost toreplace the tank.

Some owners require submittal of the constructor’s entire policy,but most owners prefer just a certificate. The certificate warrants that





the constructor will not cancel the policy without giving a specifiednumber of days’ advance notice. The expiration date, additional in-sureds, deductibles, and all policy exclusions should be checked.

IndemnificationIndemnification clauses are often points of contention and are some-times contract killers. The owner’s attorneys want contractual lan-guage that protects the owner from all claims—suits that might origi-nate as a result of this project regardless of who is liable. Constructorshave attorneys who are as smart as the owner’s attorneys, and the in-surance companies, especially, have their share of attorneys. Althougha constructor may agree to any clause to get a contract, it does not meanhis or her insurance policy covers the indemnification. A constructorwho has no insurance policy does little to offer true indemnification.Rather than proceed under the false assumption of coverage, boththe owner’s and constructor’s policies should carry comparative li-ability coverage. Each party pays on the basis of each one’s share ofliability.

Both bonds and insurance are conditions precedent to contractaward. We are a litigious society and, as with bonds, insurance is nowa requirement before the owner even signs a contract.

Prevailing Wage, Local/Union Labor, and Local RestrictionsIf prevailing wages are required, the required pay scales should be in-cluded in the supplementary general conditions. Constructors shouldbe aware that unlike the standards, which are fixed at the date ofbid opening, wage rates can change within the project time frame.Supplying certified pay records or other pay documentation is anoverhead cost that the constructor should be made aware of throughthe supplementary general conditions section when preparing his orher bid.

The use of local labor or union labor should be detailed, as wellas local time or noise ordinances. Can the constructor work late or onweekends? Time restrictions to painters are critical because of weatherrestrictions.

Payment, Change OrderThe general conditions and supplementary conditions detail how tofile the application for payments, how the applications are reviewedand by whom, denial of partial payment, reasons for denial, deter-mination of any denied amount, and the corrective actions needed torecoup lost payments. The change order process and other ways ofchanging the contract are included here.





Steel/Concrete Escalation ClauseMost painting projects are completed within 1 year. Project costs are forfuel, abrasive material, paint, equipment, and labor. Abrasive materialand paint are (or can be) purchased and stockpiled soon after projectaward. Equipment is a known cost, and labor cost increases are alsoforeseeable, as prevailing labor rates have expiration dates. On new-tank construction, concrete costs for composite tanks and steel costsfor steel and composite tanks are a major portion of expenses. Gas ordiesel for their equipment is also a major expense. Their costs cannotbe fixed immediately upon award. Steel is not bought until after thedesign is approved. Concrete for composite tanks is not purchaseduntil it is time to start construction. Gas or diesel is purchased on-site.

We have previously discussed the length of time between bidaward and steel purchase. What we did not discuss was the delayin award. Working from the basic premises that a constructor whoruns a project efficiently is entitled to a fair profit and that materialcosts fluctuate, constructors are either covered by a safe cost cush-ion (overpricing the project) or construction contracts should have asteel escalator clause and possibly a concrete escalator clause. Thesecosts are beyond the constructor’s control and can rise rapidly, as wasdemonstrated in 2006, when increased demand drove the price of steelup 30 percent almost overnight; it continues to drive the price upward.An escalator price should require the constructor to specify quantitywith the bid and to tie that quantity to a certain appropriate index.Calculation of the index should be by an independent third party. Theowner either pays for the inflated bid price or pays the exact increase,if any.

Dispute ResolutionThe best defense for disputes is to detail in the specifications a clearprocedure for dispute resolution. The two primary alternatives arethe courts and arbitration. Owners, being deep pockets without a face(usually a municipality), prefer the courts. In fact, a future trend bymunicipalities will be to contractually require the constructor to waivethe right to a jury trial. The owner wants a decision based strictlyon the law, with no human element, whereas the constructor prefersarbitration by certified arbitrators.

The three alternative methods of dispute resolution are negotia-tion, mediation, and arbitration. There are many variations and hybridmethods of resolution (minitrials, for example). Negotiation—directtalking between the contract parties—is the first step in all contractdisputes. Mediation, generally the second step in the process, bringsin a third party. The EJCDC’s general conditions make the engineerthe mediator. He or she both hears arguments and tries to get theparties to resolve the problem. Because the argument is usually about





specifications that the engineer wrote, he or she, as mediator, is giventhe final interpretation of the specifications. The engineer’s decision,while final, is usually not binding. If either party is still dissatisfied,it may proceed to court or to arbitration, whichever method is namedin the contract documents.

Arbitration can be binding or nonbinding. Recent legislation per-mits contracting away the right of appeals to the court system. Thecourts are attempting to unload their dockets, and to appeal a binding-arbitration award to a court has very little chance, barring the allega-tion of fraud, capriciousness, or a significant procedural error.

SafetyEngineering associations have always maintained that the engineeris not responsible for job-site safety. If the engineer and the ownerassume such responsibility, they are potentially liable for extensivedamages. If the engineer (and ultimately the owner) was responsibleand thus liable, the employee can sue through Workers’ Compen-sation and collect a much larger settlement. So far, most cases havedismissed the engineer because he or she had horizontal privity withthe owner and no contractual relation with the constructor. The ex-ceptions were when the on-site inspector or project manager took onthe constructor’s role by directing some of the work or giving adviceon ways and means.

No one questions the need for safety, but constructors are contrac-tually responsible for safety. Constructors have developed their ownconstruction procedures and ways and means. Constructors have de-veloped safety programs and trained their personnel accordingly. Theengineer’s personnel should follow the constructor’s safety programin addition to the engineer’s safety program.

MeetingsThe number and types of meetings required, and who must attend,should be listed as accurately as possible. If home office personnelare required to attend, it is a cost issue. Most repaint contracts havea preconstruction meeting and a final punch list meeting with thejob superintendent. Interim progress meetings are usually attendedby the on-site superintendent but not by the project manager unlessthere are problems. On new-tank projects with larger budgets, it ismore common to require the project manager to attend.

Severability and TerminationEvery contract should have a severability clause. This clause says thatif a court finds one or more clauses to be illegal, they can be severedfrom the contract. The rest of the clauses and the contract still remain.





Contract termination clauses describe the procedure for givingnotice to, and terminating the contract by, the owner for convenienceor cause and by the constructors for cause.

Schedule of ValuesNew-tank projects are usually bid as lump-sum contracts. Extras, suchas complicated logos, are bid as deductible alternates. On a project thatis spread over a year or 18 months, partial payments are in order. Toavoid paying too much money up-front or not enough, a schedule ofvalues is submitted early. This schedule can be negotiated until it isacceptable to all parties. The engineer has the final say. Following is asample schedule:

Design phase 10 percent

Foundation 10 percent

Fabrication 25 percent

Erection 30 percent

Painting 15 percent

Electrical 5 percent

Site work and punch list 5 percent

Repaint and repair projects can be bid with line items for repairitems and unit prices for painting different areas—wet interior, dryinterior, and exterior. If the project is bid lump sum, a schedule ofvalues should be included on a bid form that assigns costs to wetor dry interior and exterior. Payment is then figured on percentageof work completed (e.g., wet interior abrasive blast cleaning 40 per-cent, prime coat 20 percent, intermediate coat 20 percent, and topcoat20 percent).

Technical Specifications—New TanksTechnical specifications, while ultimately the heart of the entireproject, are only briefly discussed in this chapter. Technical specifica-tions for almost all new-tank projects are performance based. Discus-sions about siting, type of tank to select, foundations, appurtenances,and other topics appear in other chapters.

Tank Water Testing and DisinfectionTank disinfection procedures are usually performed in accordancewith AWWA C652 Standard for Disinfection of Water-Storage Fa-cilities or the more stringent requirements of local health agencies.





Specifications should define responsibilities for this work. AWWAD100 requires that the tank be tested before coating and that the ownerfurnish the water to fill the tank and provide a means of disposal fol-lowing testing. D100 permits the coating to be applied before watertesting if the tank constructor and the owner have specifically agreedto that. Tank testing and disinfection phases of the project could thenbe concurrent, thus saving the owner the cost of producing and dis-posing of a large quantity of water. However, water testing the weldedtank before applying the coating has the following advantages:� It allows identification and correction of distortions caused

by anticipated or unanticipated foundation settlements beforecoating.� It allows the identification and correction of any leaks thatmight have been temporarily covered by the interior coatingsystem.

Valves and piping should be tested in accordance with AWWAC600 Standard for Installation of Ductile-Iron Water Mains and TheirAppurtenances.

The constructor may be required by contract to provide and dis-pose of the testing and disinfection water. However, the owner willultimately bear these costs, plus the constructor’s overhead and profit.

Federal Aviation AdministrationThe owner or the owner’s engineer should file notification of construc-tion with the FAA before construction of tall standpipes or elevatedtanks. The FAA will determine whether the site is acceptable and, ifso, the requirements for temporary and permanent tank markings andlighting.




C H A P T E R 5Foundations

Sayed Stoman, Ph.D., P.E., S.E., M.L.S.E., and Kevin Gallagher, P.E.Caldwell Tanks, Inc.

The proper design and installation of the foundation is essential to allwelded-steel tanks for water storage. Before any water tank is erected,it is vital to ascertain the suitability of the site and the feasibility of theproject from engineering, construction, cost, and safety perspectives.The site soils must be capable of supporting all loads imparted on orgenerated by the water tank system with appropriate factors of safetyagainst bearing, uplift, and lateral stability and with reasonable totaland differential settlements. (The factor of safety [FS] is a function ofthe foundation type, the source of loading, and whether the loads arelong-term or transient.) Most importantly, conditions at the site mustconstitute a safe working environment for the construction crew.

The ideal sites for erecting water tanks are relatively large andlevel, dry, and easily accessible. The ideal bearing soils are sandy soilsthat range in relative density from medium to dense, to very dense,and clayey soils with consistency ranging from stiff to very stiff, tohard. These characteristics are well suited to shallow foundations,with a minimal amount of settlement and full-foundation stability. Theideal site should be large enough to accommodate the constructionequipment, provide ample lay-down area, and be free of obstructions,especially high-voltage electric power lines.

Appropriate Foundation TypeThe foundation type is governed by the soil characteristics in the ef-fective zone of influence of the bearing soils, the loading, the size ofthe property, topography, site location, and the presence or absenceof structures and facilities within the site premises. Future expansionplans and modifications are also factors in the selection of the appro-priate foundation type.

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156 C h a p t e r F i v e

The foundation system should provide for adequate factors ofsafety for both strength and stability. Since the water-storage systemis very sensitive to settlement, care must be exercised to limit the totaland differential settlements so that they do not adversely affect thetank and piping system. As a practical rule, it is preferred to limit thetotal and differential settlements to a maximum of 2 in. (5 cm) and1 in. (2.5 cm), respectively. If high settlements are expected, specialpiping or piping joints may be necessary to allow more flexibility inthe system. While settlement is a major design consideration, eachsystem is unique and must be examined on its own merits.

Generally, for economic reasons, shallow foundations are pre-ferred. As long as the bearing soils have reasonable capacity, providefor tolerable settlements, and, when excavated, are properly suited forbackfill around the footings, shallow foundations are possible. A netallowable bearing capacity of about 3,000 psf (144 kPa) or an ultimatebearing capacity of 9,000 psf (430 kPa) or better would be most ap-propriate for shallow foundations. Shallow foundations can be builtwhere the allowable bearing capacity is lower, but the resulting foot-ings would be much larger and the settlements possibly higher.

Deep foundations are more appropriate where the bearing soilsare composed of loose sands or soft clays or where tests have detectedthe presence of sinkhole cavities, sandy layers that are prone to liq-uefaction, or silty soils with high moisture content that are likely toconsolidate under loading. The type of deep foundation is influencedby cost, availability of piles, and local practice.

Regardless of the foundation type used, grade beams intercon-necting the individual footings may be necessary where the horizontalshear resulting from either wind loads or seismic loads is too large fora single footing to resist. These compressive elements are designed asbeam-columns on elastic foundations.

Location/OrientationAside from accessibility, site location is crucial for several reasons.Specific site location and accurate determination of property lines arevital, as disputes resulting from even minor infringements onto ad-jacent property can cause major delays in construction and possiblyeven cancellation of the project. Precise staking of the foundation foot-print early in the project can eliminate orientation concerns and facili-tate establishing the proper grade elevations and boring locations forgeotechnical investigations. Ownership of the site property is essen-tial, as building a storage tank on land owned by others can be costly.

Orientation of the tank on the site must also be considered withrespect to the piping layout, existing utilities, and other obstacles. Tofacilitate the connection to the inlet piping as well as to accommodate



Foundations

157F o u n d a t i o n s

the flow of overflow water away from the footing area, situating thetank in the proper orientation, is essential. Tank and piping orientationshould be clearly shown on the site plan with reference to reliableexisting benchmarks.

Sites in low-lying areas and floodplains, areas with sinkholes, andareas with underground shafts, tunnels, or fault lines are not recom-mended, nor are areas containing substantial fills. Similarly, coastalregions are not recommended—they are generally subject to signifi-cantly higher wind loading and may require design for tidal waves.The loose sandy soils in these areas could require additional stabiliza-tion measures.

Establishing Existing and Final Grade ElevationsBecause system hydraulics depend on the attained potential energy,it is very important to accurately establish the high water line andlow water line so that proper pressure and flow can be maintainedat all water levels. When the water delivery system has several tanksalong the line, each with precise overflow and low water line eleva-tions calibrated, it is extremely important that all elevations, includinggrade elevations, be accurately established for the system to functionas intended.

Grade elevations must be established with respect to verifiedbenchmarks, and a clear distinction should be made between existingand final grade elevations. Grading across the tank footprint is alsocritical. The final grade should provide for drainage away from thetank foundations, but the slope should not be so steep that excessivefooting exposure is required. The footing exposure above grade levelmust be properly considered and accounted for, especially when thesite work involves major cuts and fills. These considerations will cir-cumvent expensive consequences, which can and must be avoidedwith proper planning and attention.

Minimum Depth and Projection Above GradeThe proper minimum depth at which a foundation should be placeddepends on several factors. The slope and drainage of the site, thelocation of the frost line, the magnitude of the uplift and lateral loads,the potential for erosion, the presence and location of surface wateror groundwater, and the type of soil all affect the depth at which afooting is set to bear. Similarly, these factors also influence how far thefooting should project above the final grade level.

Generally, geotechnical reports recommend a minimum depth onthe basis of regional experience and familiarity with local conditions.However, it is preferred to bear the individual spread footings below



Foundations


the frost line at a minimum depth of about 4.5 to 5 ft (1.4 to 1.5 m).A minimum projection of 6 in. (15 cm) above the final grade shouldbe sufficient to protect the column base plates from ground moisture.On sites where standing water or settlement is a concern, higher pro-jections would be appropriate to further protect the base plate andanchor bolts from corrosion.

Excavation RequirementsBefore any excavation begins, it is essential that all aspects of job-site safety are well understood. All excavations, particularly confinedexcavations, must be performed in strict compliance with the latestfederal Occupational Safety and Health Administration (OSHA) stan-dards.

It is recommended that excavated slopes, including those for shal-low foundations, be laid back at a maximum of 2H:1V (horizontal tovertical) slope. Permanent slopes of 3:1 may be used on fill slopes thathave been placed on suitable subgrade. Grass should be planted orother measures taken for erosion control.

Vertical cuts for shallow foundations are not recommended unlessall the requirements for job-site safety and foundation stability canbe assured. Such cuts are not possible in dry, sandy soils, but theycan be made to a critical height in an undrained soil where the porepressures are negative. If vertical cuts are used, however, one mustensure that clear and achievable compaction requirements for the soilwedge alongside shallow foundations are well defined.

Site Access and DrainageWater tanks are often erected in remote rural areas where site accesscan be difficult. Even in urban areas, with everyone competing forprime space, site conditions can be challenging and access to the jobsite arduous. As noted previously, the job site requires access by large,heavy construction vehicles and lay-down and work areas for materialand construction equipment. Thus, a suitable access road is vital tothe entire construction effort.

Preparing the site for work often requires tree removal, clearing,and grubbing. Where rain and mud can prohibit site access, buildingsome basic roadwork is necessary before all other activity can begin.Geotechnical investigation and reconnaissance activities also requirefull access to the site. Where there is standing or seeping water, wa-ter removal measures will be required. As the presence of water canseverely complicate construction efforts, site drainage must always beaddressed for all phases of construction, including postconstruction,to facilitate proper maintenance.



Foundations


Water Table and Perched WaterThe presence of groundwater is always a major construction concern.A high groundwater table is also a significant design considerationdue to the buoyancy effects, increased settlement and consolidation,and potential liquefaction associated with ground seismic activities.Likewise, the presence of perched water can be a challenge duringconstruction and will require creative dewatering measures.

Where water is present, various dewatering techniques are avail-able to drain the excavations and to dry the soils before pouring thefoundations. Pumping water out from established well points is acommon method of dewatering. In situations where the bearing soilsare or could be under water, a layer of crushed stone can be placedon the bearing soils to allow for seepage and flow of water to the wellpoints for pumping during construction. Alternatively, where possi-ble, a mud mat 4 to 6 in. (10 to 152 cm) thick of low-strength (2,000 psi[14 MPa]) concrete can be placed on the bearing soils to contain thewater below or to protect the bearing soils from rain.

As buoyancy reduces the effective unit weight of soil, it may benecessary to place the footing deeper to maintain lateral and upliftstability in shallow foundations. Similarly, in pile foundations, pileuplift and lateral capacity may be affected by the presence of water.During a seismic event, clean, submerged sands may liquefy, causingdown-drag on piles and loss of lateral support in the liquefied zone.A high water table also creates design challenges when drilled piersor auger-cast piles are used.

Soils and Geotechnical InvestigationsOnce the suitability of the site is established on the basis of a visualreconnaissance, a subsurface investigation must be performed to de-termine the geotechnical composition and engineering properties ofthe bearing soils and to identify suitable foundation types based on theprevailing conditions. Although there is some uniformity in geotech-nical investigations, substantial differences exist in the scope of workperformed and in the extent of data included in the final reports. Toensure that all relevant information is included in a well-documentedreport, a clearly defined scope of work for the investigation is essential.

The number, location, and depth of borings should be speci-fied before commencing work. Geotechnical professionals who arethoroughly familiar with their regions can offer specific guidance indefining the scope on the basis of their field experience and past ex-plorations. Generally, the shallow-foundation option is pursued forreasons of economy and ease of construction. When unsuitable soilsare encountered and/or the required bearing depths become exces-sive, other alternatives should be considered. When there are feasible



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alternatives, the geotechnical report must include all the engineeringdata necessary for each design option.

Exploration, Sampling, and TestingExplorations for water tank foundations should include a minimum ofthree borings that preferably extend to depths of 30 to 50 ft (9 to 15 m).When details about the average soil properties in the upper 100 ft(30 m) of the site profile are required by code for site class determina-tion, one boring may be drilled down to 100 ft (30 m) or to the bedrock,if it is encountered sooner. To ensure that sound rock has been reached,it is recommended that rock cores of 5 to 10 ft (1.5 to 3 m) or deeperbe obtained. Additional borings may be required if the subsurfaceconditions so dictate. Sinkhole cavities, liquefiable soils, rock lenses,and boulders are examples of conditions that may warrant furtherinvestigation.

In all cases, boring locations must be precisely staked out and doc-umented with reliable measurements to well-established benchmarksor other dependable references. The presence of adjacent structuresand facilities affecting, or affected by, the project must be noted andrealistically evaluated. The borings should be spread out over the foot-print of the water tank. For leg tanks, it is preferable to perform twoborings on the column circle 180 degrees apart and a third boring atthe center of the tank. Generally, the deepest boring is located at thecenter of the tank.

The boring logs should carefully reflect the geological profile en-countered in the borings as well as the groundwater observations.Standard-penetration blow counts resulting from split-spoon sam-pling must also be reflected on the boring logs. Whether the boringswere advanced by dry augering or by rotary-wash drilling should benoted. Soils encountered during drilling should be examined and clas-sified in the field. All factual, inferred, and interpretive informationincluded in the final report must be unambiguously noted.

It is important to recognize that geotechnical investigations per-formed on the basis of a few borings provide only a limited view ofthe overall conditions at the site. Given this large margin of uncer-tainty, the designer should anticipate and be prepared to accommo-date changes as they are encountered. Often, to lessen uncertainty,field testing is performed to verify the proposed soil properties, theallowable bearing, and other geotechnical concerns.

As borings are performed, soil samples are generally taken atabout 2-ft (0.6-m) intervals in the top layers and at 5-ft (1.5-m) inter-vals subsequently. These intervals may be shortened where noticeablevariations are observed. Collected samples of disturbed and undis-turbed soil must be properly handled, correctly labeled, and care-fully transferred to the soil laboratory for testing. As the boreholes



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Specified Test ASTM Designation

Classification of soils D2487

Cone penetration D3441

Consolidation

One dimensional D2435

Undrained triaxial D4767

Direct shear D3080

Modified proctor (compaction) D1557

One-dimensional swell D4546

Seismic refraction method D5777

Standard penetration D1586

Standard proctor (compaction) D698

Triaxial compression D2850

Unconfined compression D2166

Note: ASTM = American Society for Testing and Materials.

TABLE 5-1 ASTM Test Designations

are occasionally left open overnight to permit measurement ofgroundwater level the next day, preventive measures should be takento ensure safety during the night. In most states, regulations requirethat the boreholes be filled with grout or plugged on completion ofthe investigation.

The standard penetration test and cone penetration test are thecommon tests used to determine properties of soils. The subjectiveseismic refraction method is also used in subsurface explorations.To obtain an estimate of consolidation settlement in saturated clayeysoils, consolidation tests are run in the laboratory. The unconfinedcompression test, direct shear test, and triaxial compression test arealternative test procedures for determining the shear strength of soils.The unconfined compression test is primarily suited for cohesive soils.The magnitude of potential swell in clays is determined by the unre-strained swell test or the swelling pressure test. Table 5-1 describessome of the American Society for Testing and Materials (ASTM) stan-dards that can be referred to for further details regarding geotechnicaltest procedures.

Similarly, test procedures are available for checking the designcapacity of piles. The axial compression load test (ASTM D1143), thepullout load test (ASTM D3689), the lateral load test (ASTM D3966),and the dynamic pile load test (ASTM D4945) are among some of thecommon methods used.



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Engineering PropertiesSoils that are well suited for foundation design can be cohesive (pri-marily clayey) or cohesionless. Cohesion, the characteristic that en-ables the soils to bind or stick together, gives added shear strength tothe soil. Shear strength defines the suitability of the soil for the typeof foundation work being considered. Cohesionless soils are predom-inantly sandy soils having particles that lack cohesion. Cohesionlesssoils draw their shear strength from sliding friction and interlockingof grains.

Some soils combine cohesive and cohesionless characteristics andare classified according to which is dominant. Silty soils have beenclassified as cohesionless by some authors and cohesive by others,depending on the soil’s clay content. Silty soils, however, are not con-sidered good foundation material due to their compressible naturewhen wet. Similarly, topsoil and organic soils are unsuited for foun-dations. Sandy and clayey soils with high bearing capacities and lowplasticity are best suited for foundations of water-storage tanks.

Engineering properties of soils are defined by the soil grain sizedistribution. For cohesionless soils, this distribution is determined bysieve analysis, which involves sifting the soil through sieves havingopenings of different sizes arranged in descending order from coarseto fine. The amount of soil retained in each sieve after sieve agita-tion serves as the basis of measurements and plots used in defininggrain-size distribution. Particle-size distribution in fine-grained cohe-sive soils is determined by hydrometer analysis based on sedimenta-tion of soil particles in water over a given length of time.

Another measure of cohesive soil consistency is called theAtterberg limits. The Atterberg limits refer to the moisture content atwhich a given volume of a cohesive soil changes consistency from onestate to another. These states are defined as solid state, semisolid state,plastic state, and liquid state. The moisture content at which the soiltransitions from the solid state to each successive state is referred toas, respectively, the shrinkage limit (SL), the plastic limit (PL), and theliquid limit (LL). The difference between the LL and the PL is definedas the plasticity index (PI). The PI is also a measure of the expansivepotential of the soil. Soils with high PI values (PI > 35) have severeshrink/swell characteristics and require additional consideration indesign, as is discussed later in the chapter.

Soil ConsolidationAll structures are subject to foundation settlement. Given the loading,footing size, and properties of soils, these settlements can be evaluatedwith reasonable accuracy. Consolidation is time-dependent settlementthat can be significant in saturated soils. It occurs when the soil under-goes compressive deformation under the loading from the structure.



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Water is extruded from the voids in the soil as the soil rearrangesits grains to accommodate the increased pressure. The process occursrapidly in granular soils because of their permeability, whereas in co-hesive soils or fine-grained silty soils, consolidation can take a longtime.

Consolidation is also a function of footing size. The larger the foot-ing, the greater the depth of the soil affected by the loads on the footing.The affected depth is also referred to as the effective bearing depth ofthe foundation. Depending on the thickness of the compressive layeror layers and the depth(s) at which they occur, the resulting settle-ments can be substantial and could create serious consequences forthe structure. Therefore, to ensure structural stability, it is important toinclude consolidation testing in the work scope where deformation-prone soil layers are present within the effective bearing depth of thefoundation.

Total and Differential SettlementAll water tank foundations undergo settlement. For reasons of stabil-ity and serviceability, it is necessary to minimize these settlements totolerable limits. The extent or severity of the settlement depends onthe foundation type, the magnitude and direction of loading, and theproperties of the bearing and supporting soils. In addition to the over-all settlement, a foundation undergoes relative or differential settle-ment, which, in essence, is the settlement of one part of the foundationwith respect to another. Although all settlements must be evaluatedfor their effects on the system, differential settlements must be ex-amined more closely, as they are critical to foundation strength andoverall system stability.

As noted in the section “Appropriate Foundation Type,” at thebeginning of the chapter, for water tanks, it is preferable to limit thetotal and differential settlements to a maximum of 2 in. and 1 in.(5 cm and 2.5 cm), respectively. If the use of shallow foundationswill cause excessive settlement, deep foundations can be used tofurther limit these settlements. Special piping and fittings are avail-able that can offer flexibility in the system when high settlements areexpected.

Settlement is a major design consideration, and its effects on theentire water tank system must be carefully evaluated. In addition tothe direct vertical settlements, the foundations are also subject to hori-zontal displacements under the influence of lateral loading. The extentof the horizontal displacement depends on the amount of foundationmovement that is necessary to activate the surrounding soils’ passiveresistance against the lateral loads. Therefore, geotechnical reportsmust fully address all settlement and lateral displacement considera-tions.



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Required Design InformationThe subsurface investigation report must be prepared by qualified,registered geotechnical engineers who are experienced in designingwater tank foundations. The number, location, and depth of the bor-ings should be provided by the foundation designer and confirmed bythe geotechnical engineer. All data supporting the recommendationsfor each feasible foundation type must be included in the geotechnicalreport, which should establish the following basic requirements:

For all foundations� Site classification� Site topography and site preparation� Description of the soil and its engineering properties� Classification of soil strata� Liquefaction potential and its consequences under dynamicloads� Presence of rock, rock lenses, and boulders� Potential for and consequence of shrink/swell� Replacement or remediation of shrink/swell soils� Anticipated total and differential settlements� Drainage considerations� Elevation of groundwater and dewatering requirements� Minimum recommended bearing depth of foundation� Excavation and backfill requirements� Suitability of site soils for backfill� Compaction and compaction testing requirements� Seismic design parameters for American Water Works Asso-ciation (AWWA) and/or other applicable codes

For shallow foundations� Soil ultimate and net allowable (FS = 3.0) bearing capacity� Soil carrying capacity for lateral load based on soil passiveresistance� Extent of overexcavation, if necessary, and backfill recom-mendations

For pile and caisson foundations� Anticipated pile/caisson type, size, and length� Required pile/caisson spacing� Pile/caisson axial load capacity—compressive and pullout(include values for end bearing and skin friction separately,as appropriate)



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� Minimum required reinforcement for caisson (as per localpractice)� Pile/caisson allowable lateral load capacity� Pile/caisson bending-moment diagram for appropriate endconditions� Pile or caisson safety factor for long-term and transient load-ing� Pile testing required and type of test� Special installation considerations� Appropriate uplift connection recommendations

Typically, laboratory analysis of selected samples includes vi-sual classification, cohesive shear strength tests, determination ofAtterberg limits, grain-size analysis, determination of field moisturecontent, and the following additional parameters:

Soil unit weight �

Unit cohesion of soil c

Coefficient of soil active pressure Ka

Coefficient of soil passive pressure Kp

Standard penetration resistance values N

Angle of internal friction �

Coefficient of friction, if different than tan � f

Modulus of subgrade reaction ks

All the required seismic parameters—including the mapped max-imum considered earthquake acceleration at short period Ss and at1-second period S1—should be specified. Where required, site-specificgeotechnical investigation and dynamic site response analysis shouldbe performed to determine the appropriate values.

Problem SoilsExperience indicates that certain types of soils pose special challengesin design and require remedial measures before they can support wa-ter tank foundation loads. Among these are expansive soils, whichare prevalent in many areas of the United States and elsewhere inthe world. The expansive clays with very high plasticity index valuesare not suited for shallow foundations unless remedial measures aretaken that include lime mixing, prewatering, use of water barriers, orsoil replacement. All of these measures are costly and require strictquality control.



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Foundation considerations in shrink/swell soils depend on thedepth of the active zone and the swell potential of the soil. Whena thin layer of these soils is present near the surface, it can be re-placed with more suitable low-plasticity soils. When deeper layersare encountered, the expansive soils under the foundation can simi-larly be replaced and properly compacted. An approach that is oftenrecommended is to place the shallow footing below the depth sus-ceptible to shrink/swell and to replace or remediate the surroundingsoils to make them suitable for backfill. However, the footing can beplaced within the active zone as long as the uplift forces caused by theswelling of the soils are taken into account in design and as long asthe structure can tolerate the resulting movements in the foundation.It is important to note, however, that as long as moisture is preventedfrom entering the soil, the shrink/swell volumetric changes cannot oc-cur. Therefore, pouring a concrete slab over the footing area or placingwaterproof barriers around the footing are alternate remedial options.Further detailed options are discussed later in the chapter.

When materials such as organic soils, fills, or any other type ofloose soil are encountered at the bearing level, they should be undercutand replaced with suitable soils. The replacement soils may be whatis commonly known as select structural fill, sand, or crushed stone.Select structural fill consists of uniformly graded sands to silty orslightly clayey sands, free of organics and other deleterious material,with less than 30 percent passing through a no. 200 sieve. Select fillis also recommended for backfill around the footings and pile capswhen unsuitable soils are present.

Structural fills are commonly placed in thin (6- to 8-in. [15- to20-cm]) lifts and compacted to 95 to 98 percent of the soils’ modifiedproctor maximum dry density (ASTM D1557) or other ASTM criteria.They may require some manipulation of the moisture content (wettingor drying) to achieve the required compaction. Flowable fill is anothermaterial that can be used for this purpose. Of course, replacing theundercut soils with low-strength concrete is always an option.

Structural ConcreteWater tank foundations are primarily constructed of reinforced struc-tural concrete. Concrete is a mixture of hydraulic cement with fineand coarse aggregates and water in appropriate proportions. Sand,gravel, crushed stone, and, in some cases, iron blast-furnace slag con-stitute the aggregates. The governing properties of hardened con-crete are defined by the quality of the cement paste, ratio of waterto cement, and the properties of the aggregates. Structural concreteis concrete mixed to a uniform distribution of materials on the basisof a precise mix/design and satisfactory quality control for required



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durability and compressive strength. The design and construction ofwater tank concrete foundations follow the Building Code Require-ments for Structural Concrete (ACI 318-05) and Commentary (ACI318R-05) (see “Bibliography”).� Building Code Requirements for Structural Concrete, ACI

318-05.� Building Code Requirements for Structural Concrete, Com-mentary, ACI 318R-05.

Concrete is strong in compressive strength, but its tensile strengthis limited to a small fraction of its compressive strength. Therefore, inflexural design applications such as for foundations, steel reinforce-ment is provided on the tension side of a member to resist the ten-sile stresses. Similarly, as shrinkage and temperature reinforcementnormal to flexural reinforcement, or wherever tensile stresses can de-velop, steel reinforcement is added to provide tensile resistance. Re-inforcement bar sizes and details as well as criteria for determiningthe amount of reinforcement needed are all outlined in ACI 318-05.

MaterialsStructural concrete materials include cement, aggregates, water, andadmixtures. The reinforcing steel used in water tank foundations com-prises deformed bars ranging in diameter from 3/8 in. (9.5 mm) toabout 13/8 in. (3.5 cm). Two larger bar diameters of 13/4 in. (4.5 cm)and 21/4 in. (5.5 cm) are also available but are seldom used in watertank foundations. Welded wire fabric is another form of reinforcementoften used in floor slabs.

Cement is a powdered substance produced from a burned mixtureof clay or shale and limestone. Portland cement is the most commontype, grayish in color, consisting chiefly of calcium and aluminumsilicates. Portland cement is manufactured to various designationson the basis of the physical and chemical requirements as definedby ASTM C150. Type I designation represents the general-purposecement for foundations subject to normal exposure. Where sulfateattack from soil or water is a concern, if high strengths at an earlyperiod are required, or if hydration heat needs to be minimized,other ASTM cement types would be better suited and should bespecified.

Aggregates are generally classified into fine and coarse categorieson the basis of their particle size. Fine aggregates consist of sands thatpass through a no. 4 sieve, meaning that their maximum particle sizeis less than 1/4 in. (6.4 mm). Some references include a particle sizeup to 3/8 in. (9.5 mm) in fine aggregates. Coarse aggregates constituteany material larger than 3/8 in. (9.5 mm). The most common aggre-gate size is about 3/4 in. (19 mm). However, the maximum coarse



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aggregate size is governed by the space limitations between the re-inforcing bars. As the aggregates constitute up to 75 percent of thetotal volume of concrete, it is important not only that the proper sizebe selected carefully, but that the aggregates be properly graded andhave the requisite strength, durability, and weather resistance for theexposure environment.

Uniformity and workability of concrete are affected by the aggre-gate gradation or particle size distribution within the aggregate. Aproperly graded aggregate has a balanced distribution of particle sizethat remains consistent from batch to batch. Aggregates with smallergradation minimize the number of air voids and result in more dense,stronger, and better concrete.

Water is a necessary ingredient that initiates the hydration processof cement. The mixing water should be clean potable water that isfree of oils, acids, alkalis, salts, and other organic materials that aredetrimental to concrete or the reinforcement steel. Likewise, the watershould be free of high concentrations of dissolved solids.

Admixtures should be used only when required by design. Theyimprove the workability of plastic concrete and enhance the proper-ties of hardened concrete. The admixtures include air-entraining toincrease resistance to freezing, water-reducing admixtures to reducethe quantity of water needed to maintain a certain slump, retardingagents to slow the setting of concrete, accelerators to hasten strengthdevelopment at an early age, and fly ash and ground, granulated blast-furnace slag to improve the plastic or hardened properties of cementconcrete.

DurabilityBecause water tanks are erected in varied climates and locations, theenvironmental effects on their concrete foundations can be harsh andmust be taken into consideration. Conditions that can profoundly af-fect the service life of the foundations include extreme temperaturefluctuations, freeze/thaw cycles when exposed to water, and exposureto chemicals, salts, deicers, etc. Durability, in essence, refers to the ca-pability of concrete to withstand these exposure conditions withoutdamage, distress, or deterioration.

ACI 318-05 provides detailed durability requirements for im-proving the performance of concrete. These requirements includeair-entraining recommendations for concrete exposed to freez-ing/thawing or deicing chemicals, maximum water-to-cement ratiosand minimum strength values for concrete exposed to special condi-tions, maximum percent of total cementitious material by weight forexposure to deicing chemicals, and criteria for resistance to sulfate-containing solutions and soils. This reference also provides require-ments for corrosion protection of the reinforcing steel.



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Quality Control, Mixing, Placing, Finishing, and CuringFor reinforced-concrete foundations to perform and function as in-tended, it is essential to adhere strictly to specific requirements ineach phase of construction. These requirements govern the strength,durability, mixing, conveying, depositing, workability, and curing ofconcrete. The quality of concrete is mostly defined by the quality of thecement paste, its proper mixing with the aggregates, and the aestheticquality of the finish.

Structural concrete is proportioned to achieve an average con-crete strength based on a mix design and an anticipated exposure.The proportions are established to provide workability and consis-tency, resistance to special exposures, and conformance with strengthtest requirements as outlined in ACI 318-05. The mix design must befollowed precisely to produce concrete that satisfies structural perfor-mance requirements.

Placing and compacting are also important to the quality of con-crete. Concrete must be placed and vibrated properly to avoid segrega-tion, honeycombing, settling, and separation of the heavier aggregatesfrom the rest of the mix. Concrete should be placed continuously inlifts or layers using multiple discharge locations. This eliminates ag-gregate separation caused by the horizontal flow of concrete withinthe formwork and the need for concrete to be moved into its finalposition. During placement, samples of the plastic concrete should betaken for field testing of unit weight, slump, and air content to en-sure compliance with mix specification. The tests can be performedaccording to the appropriate ASTM specifications.

Unless cured by accelerated curing techniques, poured-concretefoundations should be maintained at a temperature above 50◦F (10◦C)and in moist condition for at least 7 days after placement. Schedule-driven activities often dictate backfilling around the footings soonerthan 7 days. In no case should backfilling be started before the con-crete has gained sufficient strength to withstand the loading inducedon the footing by the pressures resulting from the backfill and theconstruction equipment.

Required Strength, fc′

Structural concrete can be proportioned to a wide range of designstrengths and characteristics. The design of water tank foundations isbased on a specified design strength for concrete. The design strengthrefers to the compressive strength gained by concrete after 28 days ofcuring and is referred to as f ′

c. For water tank foundations, a minimumdesign strength of 3,000 psi (20.7 MPa) is recommended. The preferredspecified strength, however, is 4,000 psi (27.6 MPa). A common prac-tice in the industry is that when concrete strength in excess of 3,000 psi(20.7 MPa) is required by the specifications, a design strength that is



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500 psi (3.4 MPa) less than the specified strength is used in compu-tations. This allows some flexibility in situations where strict qualitycontrol measures cannot be maintained due to the long hauls to remotejob sites.

Evaluation and acceptance of concrete for a given design mix canbe made on the basis of actual testing. Adjustments to the mix designcan be made to improve the resulting strength as necessary. To en-sure that the concrete furnished meets the specified design strengthrequirements, fresh concrete specimens can be prepared at the jobsite for testing in the laboratory. ACI 318-05 provides criteria for con-crete sampling and testing and for acceptance of concrete compressivestrength: Concrete strength is considered acceptable when the aver-age of any three consecutive strength tests equals or exceeds f ′

c andno individual test (average of two cylinders) falls below f ′

c by morethan 500 psi (3.45 MPa).

For the test results to be meaningful, it is critical that the samplesbe taken, handled, and cured in strict compliance with the applicableASTM standard. Testing of cylinders that are mishandled or ignored atthe job site may not be truly representative of the concrete furnished.Also, it is important that qualified personnel test all specimens, as theoutcome of the tests determines the acceptability of the foundations.

If the strength test results fail the acceptability criteria, hardenedconcrete can be tested by taking core bores in accordance with ASTMC42. Experience indicates that unless the requirements of ASTM C42are strictly adhered to, the core bore test results will underestimatethe true strength of the hardened concrete. ACI 318-05 also providesspecific criteria for core drill testing.

Reinforcing SteelThe reinforcement steel used in water tank foundations is generallydeformed bars conforming to the ASTM A615 specification and hav-ing a minimum yield strength of 60 ksi (414 MPa). Where weldabilityis a requirement, low-alloy steel deformed bars conforming to ASTMA706 can be used. As noted previously, welded wire fabric reinforce-ment can be used in floor slabs. These fabrics conform to ASTM A185for plain wire and ASTM A497 for deformed wire. Epoxy-coated barsor wires are not necessary for water tank foundations.

ACI 318-05 provides detailed criteria for determining the amountof reinforcement necessary in design as well as requirements for thespacing, cover, development length, and splice length. Requirementsfor the development length of bars should be carefully reviewed, espe-cially for the horizontal bars. If horizontal bars are so placed that morethan 12 in. (30.5 cm) of concrete is cast in the member below the bar,the required development length is 1.3 times the normal developmentlength.



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Nonconforming ConcreteOn occasion, it is possible that concrete mixed and furnished under aspecification fails to meet the minimum design strength requirement.As a result, contractual obligations may require the foundation con-tractor to remove the nonconforming foundations and reinstall thefoundations according to the stated requirements. This can signifi-cantly delay project completion and can be very costly. Therefore, it isimportant that serious attention be paid to quality control during themixing, conveying, placing, and curing phases of the concrete foun-dation construction.

Contractual obligations notwithstanding, not every nonconform-ing condition warrants the removal of the footings. If testing confirmslow strength, calculations can be performed to check the adequacyof the furnished concrete with reduced strength and the actual rein-forcement provided. The nonconforming condition may be acceptedif the calculations confirm that the load-carrying capacity of the foun-dation is not significantly reduced and that the design intent has beenmet.

Formwork and RemovalConstruction of water tank foundations requires the use of formwork.Proper formwork ensures that the foundations conform to the shapeand dimensions shown on the drawings. The formwork also preventsmoisture loss from concrete, and it facilitates curing, especially whenthe top surface of concrete is kept moist. To be effective, the formworkmust be strong enough to withstand the loads and pressures frompouring concrete and any other loads that are present. Leakage ofplastic concrete must not occur.

Formwork should remain in place for as long as possible, espe-cially in cold weather. Formwork must not be removed until the con-crete has gained sufficient strength to withstand its dead load andany other construction loads. When properly cured, general-purposeconcrete gains about 500 psi (3.45 MPa) strength in 24 hours; within1 week of placement, it reaches nearly 70 to 75 percent of its maximumcompressive strength. Although many contractors are in a hurry to re-move the formwork so that they can complete backfilling around thefootings, it is best to keep the formwork in place based on achievinga defined minimum strength.

To simplify formwork, some contractors take the liberty of adjust-ing footing thickness or other dimensions. This should not happenwithout the explicit consent of the engineer of record. The formworkrequired for water tank foundations is relatively simple and shouldeasily facilitate the required shape and dimensions shown on the en-gineering drawings.



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Top offootingelevation

Head r

ange

Overflow elevation

Tank diameter

Centerline tocenterline offoundation

Column (typ.)

Center riser

Plan

Diameter at

centerline of

foundation

Hig

h w

ate

r lin

e

Low

wate

r lin

e

Elevation

FIGURE 5-1 Typical leg tank elevation and shallow foundation plan.



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Shallow FoundationsThe shallow foundation is the most cost-effective foundation for waterstorage tanks. Shallow foundations typically include isolated footingsor mat or raft foundations placed just below the columns on the lowestpart of the structure, as applicable. Footings can be placed as shallowas possible as long as the bottom of the footing is below the frost line,the resulting bearing pressures are within the allowable limits, theoverall settlements are tolerable, and the stability requirements aremet. However, for water-storage tanks, shallow foundations can betypically placed as shallow as 4.5 ft (1.4 m) and as deep as 10 ft (3.0 m)below grade.

Shallow foundations transfer structural loads to the bearing soilor rock strata occurring below the base of the footing. Shallow foun-dations for multicolumn elevated storage tanks typically consist ofisolated piers with footings (Fig. 5-1). For ground storage tanks andelevated single-pedestal tanks, the foundation may take the form ofa ringwall, a ring-tee, or a ring-slab. These ring-type foundations arefurther discussed later in the chapter. Ground storage tanks may alsobe founded on a slab or a granular berm. Figures 5-2a, 5-2b, 5-2c, and5-3 show several common types of shallow foundations.

Based on tank geometry, site conditions, and specific environmen-tal loading effects, various foundation alternatives should be evalu-ated. Typically a shallow foundation is the preferable option. If poorsoil conditions, high settlement expectations, or low bearing capac-ities dictate, deep-foundation alternatives must be considered. Lowbearing capacities generally result in large footings, causing the adja-cent footings to encroach upon each other. As a result, the overlappingof the pressure bulbs from the individual footings can exacerbate thebearing stresses and magnify settlements. Therefore, when the netallowable bearing pressure falls below 2,000 psf (96 kPA), the deep-foundation alternative should be pursued.

Loads and Load CombinationsWater-storage tanks are subjected to a variety of loads. The gravityforces consist of the weight of the tank metal, appurtenances, andthe liquid. Common appurtenances include roof-mounted cellularantenna systems, platforms, floors, walkways, ladders, and piping.Snow loading consists of the weight of snow on the tank balcony andthe tank roof where the roof slope with the horizontal axis is flat tomoderate. The tank roof may also be subjected to live loading that is inexcess of the snow loads. Lateral forces on the tank and tower consistof loads resulting from wind pressures or earthquake ground motion.

The AWWA D100-05 Standard for Welded Carbon-Steel Tanks forWater Storage states that a unit weight of 62.4 lb/ft3 (1,000 kg/m3) forwater, 490 lb/ft3 (7,850 kg/m3) for steel, and 144 lb/ft3 (2,310 kg/m3)



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Dowels

Hoops

Bars

Bars

Top of footingelevation

Anchor bolts

Radial centerline

Plan

Elevation

ExposureTop of grade

Centerline offoundation

Diameter at centerlineof anchor bolts

Cente

rlin

e o

f fo

undation

Cente

rlin

e o

f anchor

bolts

Cente

rlin

e o

f fo

unda

tion

Cente

rlin

e o

f anchor

bolts

(a)

FIGURE 5-2a Typical shallow foundations: sloped slab



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Dowels

Hoops

Bars


Anchor bolts

Plan

Elevation

ExposureTop of grade

Centerline offoundation

Cente

rlin

e o

f fo

undation

Cente

rlin

e o

fanchor

bolts

Diameter at centerline of anchor bolts

Radialcenterline

Cente

rlin

e o

f fo

undation

Cente

rlin

e o

f anch

or

bolts

(b)

FIGURE 5-2b Flat slab



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Slab reinforcing

U-bars

Dowels

Dowels

U-bars

Bars

Bars

on bolt circleAnchor bolts


Anchor bolts

Hole forinlet/outletpipe

A

A

Plan

Elevation

Exposure

Vault forpiping

Cente

rlin

e o

f fo

undation

Centerline of foundationC

ente

rlin

e o

ffo

undation

(c)

FIGURE 5-2c Riser.



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Coarse stone or

coarse gravel Thoroughly compacted fill of

gravel, coarse sand, or other

stable material

Compacted crushed stone,

screenings, fine gravel, clean

sand, or similar material

Coarse stone or

coarse gravel

Slopedown

11.5

11

To drain

away

Extend 3–5 ft

(0.9–1.5 m)

beyond tank 2 ft (0.6 m)

minimum

Tank plate

Top of bermelevation

Mat top

reinforcement bars

Mat bottom

reinforcement bars

Top of footing elevation

Exposure

Centerline of

tank and foundation

Centerline of

tank and foundation

(a)

(b)

FIGURE 5-3 Examples of shallow mat and berm foundations: (a) typical squaremat foundation and (b) typical granular berm foundation for flat-bottom tanks.

for concrete should be considered in the design of tank structures andfoundations. The standard also recommends consideration of a min-imum allowance of 25 lb/ft2 (1,205 N/m2) for the pressure resultingfrom the design snow load on the horizontal projection of the tankroof surfaces with slopes not exceeding 30 degrees. A reduction ofthis allowance is permitted in warmer regions where snow loadingis smaller. However, D100-05 limits the minimum roof design load to15 lb/ft2 (720 N/m2).

AWWA D100-05 has adopted the American Society of Civil Engi-neers (ASCE) standard 7-05 for wind loading criteria. However, it re-tains the minimum design pressures to be 30 Cf lb/ft2 (1,436 Cf N/m2),with the force coefficient Cf being 1.0 for flat surfaces, 0.60 for cylin-drical or conical surfaces with apex angle <15 degrees, and 0.50 fordouble-curved or conical surfaces with apex angle ≥15 degrees. Forseismic design, the AWWA standard has essentially adopted ASCE7-05 criteria with some variation in the minimum design acceleration.ASCE 7-05 provides detailed criteria for both wind and seismic load-ing. Proper determination of the period of oscillation of the water tanksystem is necessary in all seismic evaluations.

The wind or seismic forces can originate from any direction. Struc-tural analysis indicates that the greatest uplift force in a multicolumnelevated water tank occurs in the column that is situated exactly up-wind of the lateral force. The maximum uplift force generally occurs



Foundations


when the tank is empty. For taller and shorter towers in areas of highseismic risk, it would not be unusual to find that the maximum upliftoccurs under seismic loading when the tank is full. Similarly, the great-est downward force occurs in the column situated exactly downwindof the lateral force. The downward load will be a maximum when thetank is full. The direction of the lateral force that will cause the greatestuplift may not be the same as the direction of the lateral force that willcause the greatest downward force. Hence, all column foundationsare candidates for the worst-case orientation.

Typically, structural loading required for the design of the founda-tions is determined from the analysis of the elevated tank and toweror pedestal. The resulting reactions, shears, and overturning momentsdue to the gravity loads, wind loads, and seismic loads are all defined.These loads, in addition to the foundation dead loads and other loadsemanating from soil pressure or swells, constitute the design loads.

Foundations are generally designed according to ACI 318-05 andits commentary, ACI 318R-05. This building code for structural con-crete stipulates that the foundations be designed to have designstrengths at all sections at least equal to the required strength basedon factored loads in defined load combinations. Although ACI 318-05still retains the classical factored load combinations in its AppendixC as an alternative, in its 2005 edition it has adopted the ASCE 7-05factored load combinations for design. In the seven load combinationsstated for determining the required strength U, loads not present canbe eliminated from the load combinations:

U = 1.4(D + F) (5-1)

U = 1.2(D + F + T) + 1.6(L + H) + 0.5(Lr or S or R) (5-2)

U = 1.2D + 1.6(Lr or S or R) + (1.0L or 0.8W) (5-3)

U = 1.2D + 1.6W + 1.0L + 0.5(Lr or S or R) (5-4)

U = 1.2D + 1.0E + 1.0L + 0.2S (5-5)

U = 0.9D + 1.6W + 1.6H (5-6)

U = 0.9D + 1.0E + 1.6H (5-7)

where

D = dead loadsF = load due to weight of fluidsLr = roof live loadL = live loadR = rain loadS = snow loadE = load effects of seismic forcesW = wind loadH = loads due to weight and pressure of soil, water in soils,

or other materials



Foundations


T = cumulative effect of temperature, creep, shrinkage, differentialsettlement, and shrinkage-compensating concrete

It is the authors’ view that the load due to the weight of the flu-ids F should be included in the strength design load combinations(Equations [5-4] and [5-5]) with a load factor of 1.2—that is, 1.2F . Oth-erwise, the case of the full tank under wind or seismic loading maynot be appropriately considered. Interestingly, the ASCE 7-05 basicload combinations for allowable stress design correctly include the Floads in load combinations for wind and seismic loading. Likewise,the weight of fluids should be included in the seismic load combina-tions (Equations [5–7] and [5–10]), as the seismic uplift can be moresignificant in areas of high seismic risk or in the case of tall elevatedtanks when the tank is full. The uplift for an empty tank is generallygoverned by the wind load combinations.

Alternatively, the classical ACI 318 Appendix C load combinationsmay be used:

U = 1.4(D + F ) + 1.7L (5-8)

U = 0.75(1.4D + 1.4F + 1.7L) + (1.6W or 1.0E) (5-9)

U = 0.9D + (1.6W or 1.0E) (5-10)

U = 1.4D + 1.4F + 1.7L + 1.7H (5-11)

Where structural effects (differential settlement, creep, shrink-age, expansion of shrinkage-compensating concrete, or temperaturechange T) are significant, U should not be less than the larger of thefollowing equations:

U = 0.75(1.4D + 1.4F + 1.4T + 1.7L) (5-12)

U = 1.4(D + T) (5-13)

Regardless of the load combination selected for design, the loadfactor 1.6 on wind load can be reduced to 1.3, where W has not beenreduced by a directionality factor. Also, the factor 1.0 on the seismicload should be increased to 1.4, where E is based on service-levelseismic forces. Refer to ACI 318-05 for other specifics in using theseload combinations.

It should be noted that in foundations design, AWWA D100-05Section 12.1.1 requires the water load to be considered as live load,with appropriate factors for live load. Furthermore, the standard doesnot require the inclusion of snow loading in the load combinationsthat include wind or seismic loads.

An additional AWWA D100-05 requirement that is associated withthe ductility of the bracing rods states that foundations should bechecked for stability at a lateral seismic force equal to yielding ofbracing rods. For A36 rods, the actual yield stress may be as muchas 1.33 times the minimum published yield. Hence, the anchor boltsmust also be checked to ensure load transfer.



Foundations


Bearing CapacityBearing capacity refers to the ability of the soil strata below the footingto safely resist the structural loading on the foundation with reason-able safety and tolerable settlements. The loads described earlier mustall be transferred to the bearing strata through the foundation system.The pressure resulting from the structural loading at the interfacebetween the foundation and the bearing strata is referred to as thebearing pressure. The bearing pressure must always remain belowthe ultimate bearing capacity of the bearing soils. The ultimate bear-ing capacity may correspond to a general shear failure or a punchingshear failure in the soils. However, in design, safety factors are appliedto further limit the bearing pressures to levels commonly referred toas the allowable bearing capacity.

The determination of the ultimate bearing capacity followsTerzaghi’s equations (Terzaghi and Peck 1967). Based on equilibriumanalysis and experimentation, Terzaghi expressed the ultimate bear-ing capacity in semiempirical forms that can be expressed as

qult = �c Nc + qNq + �� BN� (5-14)

where

� = 1.0 for strip foundation= 1.3 for square and circular foundations= (1+ 0.3B/L) for rectangular foundation

� = 0.5 for strip foundation= 0.4 for square foundation= 0.3 for circular foundation= 0.5 × (1 − 0.2B/L) for rectangular foundations

� = unit weight of soilq = surcharge, or � times the bearing depth of the foundationB = width or diameter of the footingL = length of the footingc = cohesion of soilNc , Nq , N� = bearing capacity factors determined on the basis of

the angle of internal friction of soil �

Refer to a soil mechanics textbook for further details (Terzaghi andPeck 1967 or Smith and Pole 1981).

Water tank foundations are designed using the net allowable bear-ing capacity. The net allowable bearing capacity is determined by sub-tracting the effective surcharge or the overburden pressure from theultimate bearing capacity and dividing the result by a factor of safety(FS). The FS included in the recommendations of the geotechnicalreport is critical and should be reviewed carefully. Typically, the FS



Foundations


ranges from 2.5 to 3.0. However, AWWA D100-05 prescribes specificvalues for various bearing conditions:� A safety factor of 3.0 shall be provided on the basis of calcu-

lated ultimate bearing capacity for gravity loads.� A safety factor of 3.0 shall be provided on the basis of calcu-lated ultimate bearing capacity for gravity loads plus windload, excluding overturning toe pressure caused by shear atthe top of the footing, unless specified otherwise. The safetyfactor may be reduced to 2.25 when specified in the geotech-nical report.� A safety factor of 2.25 shall be provided on the basis of calcu-lated ultimate bearing capacity for gravity loads plus seismicload, excluding overturning toe pressure caused by shear atthe top of the footing, unless specified otherwise.� A safety factor of 2.0 shall be provided on the basis of calcu-lated ultimate bearing capacity for gravity loads plus windor seismic load, including overturning toe pressure caused byshear at the top of the footing.

Therefore, the geotechnical investigation must identify the net al-lowable bearing capacity using an FS of 3.0 against the ultimate bear-ing capacity. If a different FS value is specified in the geotechnicalreport, the bearing pressures must be corrected to an FS of 3.0, asrequired by AWWA D100-05.

Design of Isolated Spread FootingThe isolated spread footing for water-storage tanks consists of a pieror pedestal sitting on top of a flat or sloping slab. The dimensions ofthe pier are generally specified to be compatible with the column ori-entation and size and the base plate bolting requirements. The heightof the pedestal is a function of the bearing depth recommended by thegeotechnical engineer. The pedestal height is also affected by lateralstability of the footing.

Determining the footing size is an iterative process. The base areaof the footing is initially determined from unfactored forces and mo-ments by maintaining the resulting bearing pressures below the al-lowable bearing pressure. With this determination, the footing is thensubjected to the load combinations defined previously. Appropriateadjustments to the slab dimensions, thickness, or bearing depth aremade to satisfy both equilibrium and stability (lateral and uplift) re-quirements. Once the footing is sized, the concrete sections and rein-forcement requirements are selected in accordance with ACI 318-05Ultimate Strength Design Method.



Foundations


The footing thickness is based on the concrete flexural and shear-strength requirements of ACI 318-05. The slab bending moment re-quirements are checked at the faces of the pedestal. Checks are alsomade on strain compatibility to ensure that the failure mode will beby yielding of the reinforcement, not by the crushing of concrete inthe compression zone. Beam or flexural shear is checked at the criticaldistance d from the face of the foundation pier. The punching shearis checked at the critical distance d/2 from the face of the pier. The di-mension d represents the distance from the extreme compression fiberto the centroid of longitudinal tension reinforcement in the slab, at theface of the pedestal. When checking the punching shear or flexuralshear capacity of sloping slabs, caution is warranted to use the actualdepth at the location under consideration, not the depth at the face ofthe pier.

For the multicolumn tank, the tower typically consists of tubularcolumns with a base plate at the top of the footing. The column pieror pedestal is sized to accommodate the column and base plate inaddition to providing adequate embedment depth and edge distancefor the anchor bolts. The pier can be circular, square, or—in the case ofa battered column—rectangular. The rectangular pier allows the lineof action of the column axial force to be centered on the footing, thusavoiding creation of eccentric moments at the base. The depth of thefoundation below grade, the pedestal projection above grade, and thethickness of the footing determine the required height of the pier.

The designer must ensure that the pier does not become slender.Otherwise, it would have to be designed as a column. ACI 318-05 limitsthe ratio of the pedestal height to its average least-lateral dimensionto a maximum of 3. By this limitation, the ACI building code providesspecific reinforcing requirements that are lighter than those typicallyrequired for columns.

The minimum reinforcement of flexural members relevant to thefoundation slabs is defined in ACI 318-05 Section 10.5.1. Section 10.5.3allows this minimum reinforcement requirement to be waived if thecalculated area of the reinforcement is increased by one-third. For foot-ings of uniform thickness, Section 10.5.4 specifies that the minimumarea of tensile reinforcement in the direction of the span should be thesame as that required for shrinkage and temperature defined in Sec-tion 7.12. The upper bound on the flexural reinforcement ratio shouldsatisfy the requirements of Section 10.3.5 or Section B.10.3.3. Shearfriction should be checked at the pedestal/slab interface according toSection 11.7.4.

For cast-in-place pedestals, ACI 318-05 Section 15.8.2.1 specifiesthat an area of reinforcement across the interface should not be lessthan 0.005 times the gross area of the pedestal. For shorter pedestals,this value can be arguably reduced by as much as 50 percent accord-ing to industry practice. However, the value should be maintained



Foundations


for taller pedestals. The vertical dowels constituting the pedestal re-inforcement resist the uplift and lateral forces that exist at the upwindcolumns when the tank is empty and the design wind load is fullyactive. They also strengthen the pier in resisting compressive loads.The dowels will require lateral ties as per ACI 318-05 Section 7.10.5.

As was noted earlier, once the foundation system is sized basedon all other requirements, it is necessary to check for vertical (uplift)and lateral (sliding) stability. When the uplift is severe, it will controlthe embedment depth and the size of the footing. To ensure stabilityagainst uplift, the foundation can be placed deeper or made largerto maximize the dead weight of the soils directly above the footing.This will be especially necessary when the water table is high andwhen buoyant weights are used in design. Similarly, footing depthor slab dimensions can be adjusted to provide stability against lateralsliding. When such adjustments are made, it will be necessary to revisitfinal reinforcement requirements for code compliance. (Typically, forbuoyant weights, 60 lb/ft3 [961 kg/m3] for soil and 82 lb/ft3 [1,314kg/m3] for concrete are used in design.)

AWWA D100-05 requires the weight of the pier (footing) plus theweight of the soils directly above the pier to be sufficient to resist themaximum net uplift occurring when the tank is empty. The lateralstability is provided by the passive resistance, cohesion, and adhesionprovided by the soils. It is recommended that a minimum FS of 1.3 bemaintained against the working load uplift by including the weightof a 25-degree soil wedge and that a minimum FS of 1.5 be maintainedagainst lateral sliding.

In elevated tanks, the riser carries a major portion of the waterweight. For torus-bottom tanks, this loading may be equivalent to theweight of water within half the diameter of the tank times the tankhead range. The loading and the requirement for pipe entry and exitat the base of the riser footing make the riser foundation unique. Thepipe pit design differs considerably from the column pedestal designin that it has a top slab that can support a considerable load.

The pipe pit often has an open front so that support for the top slabis provided by just three walls. The slab, treated as a two-way slab, issupported on the front edge by a deepened girder or non-deepenedband beam across the open face of the pit and on the other three edgesby the walls of the pit. The load transferred to the top slab by the riserconsists of two parts: one part comprises the direct loads from the tanktransferred by a compressive axial stress in the riser pipe walls, andthe other part is due to the water column that bears on the riser floorin wet risers. Another design consideration for the top slab is whetherthe diameter of the riser pipe is less than or greater than the clear spanin the pipe pit below. If the riser diameter is smaller than the clearspan, the reinforcement in the slab must be attuned to account for theadditional bending moment in the slab.



Foundations


The walls of the pit are reinforced in accordance with ACI 318-05Sections 14.3.2 and 14.3.3, unless required otherwise. The walls aretypically 12 in. (30.5 cm) thick. They are checked for compressive stressas well as flexural bending stress caused by soil active pressure, andtheir thickness is adjusted as required. Where the water table is highor when the soils are saturated, soil lateral pressures will be affected,and their effect must also be considered when specifying design lateralpressures.

Sand cushionTop of footingelevation

Hoops Verts.

Crushed stone orconcreteslab

Clean drysand

Concretethrust block

Inlet or outlet pipe

Section A-A

Detail X

Compactedbackfill

A

Outside diameter ringwall

In

side diameter r

ingwall

Plan

Centerlineof tank andfoundation

Compacted regularor gravel fill

Compactedregular or gravel fill

Expansionmaterial

Exposure

Centerline oftank and foundation

A

3 in. (76 mm) at wall 6 in. (152 mm) minimumat center


6–12 in. (152–305 mm)crushed stone

See Detail X

Compactedbackfill

FIGURE 5-4 Typical ring-wall foundation plan for a flat-bottom tank. (Verts. =vertical reinforcement dowels.)



Foundations


Ringwall, Ring-Tee, and Ring-Slab FootingsAside from the standard isolated shallow footings, there are othertypes of shallow foundations. A ringwall foundation is a cylindricalwall footing of defined thickness and height. Ringwalls are typicallyused for flat-bottom storage tanks, commonly known as reservoirs andstandpipes (Fig. 5-4). Ringwall foundations allow the tank contents tobe supported directly on the soils at grade as long as the allowablebearing capacity of soil is not exceeded. The ringwall itself supportsthe weight of the tank container and significant appurtenances, a smalltributary weight of the water, the roof loads, and pressures resultingfrom the effects of wind and seismic loads.

Foundation systems consisting of ring-tees or ring-slabs are typ-ically used for single-pedestal tanks such as pedespheres, fluted-column tanks, or composite elevated tanks. A ring-tee foundation isessentially a ringwall supported on a footing (Fig. 5-5). When thebearing capacity of the soil is exceeded under a ring-tee foundation, aring-slab foundation is used. The ring-slab comprises a ringwall sup-ported on a slab whose diameter is larger than the diameter of theringwall (Fig. 5-6).

Ringwall, ring-tee, and ring-slab footings—as with the singlefooting—are sized for the load combinations defined previously.These footings may be symmetrical about the ringwall or asymmetri-cal. Asymmetrical footings are used to balance the shear or bendingmoments along the two faces of the ringwall. This minimizes tor-sional moments on the ringwall. Footings containing a ringwall arealso subject to hoop stresses from the soils or the surcharge that mustbe considered in design.

As with all foundation systems, it is necessary to check for stabilityagainst overturning as well as lateral sliding.

Backfill and Lateral StabilityBackfill is an essential component of properly designed foundationsystems. The geotechnical investigations generally determine the suit-ability of the in-situ soils for structural backfill. If the on-site soils aredetermined to be unsuitable, recommendations for alternate backfillmaterial are made in the report. Backfilling in accordance with thegeotechnical recommendations ensures lateral stability of the founda-tion and stability against uplift.

Backfill soils must be capable of providing the necessary passiveresistance to stabilize the foundation against horizontal sliding andto eliminate the possibility of water accumulation and buildup. Asnoted earlier in the chapter, select structural fills that consist of uni-formly graded sands to silty or slightly clayey sands are well suitedfor backfill. However, they must be free of organics and other dele-terious material, and it is preferable that less than 30 percent passes



Foundations



Anchor bolts

Hoops

Verts.

B

B

Section A-A

Bars

Pipe sleevefor inlet-outlet

Section B-B

HoopsRadial bars

ConcreteThrust block

Do not groutpipe. Bend barsaround opening

Hoops(special top bars)

Verts.

Gravel floor

Roughenedconstruction joint(or a shear key)

Suitable bearingstrata

Regular orgravel fill

Inside diameter ringwall


Outside diam

eter slab

Radial bars

Hoops

Chord

Chord

Anchor boltson bolt circle

A

A

Plan




Insid

e d

iam

ete

r slab

FIGURE 5-5 Typical ring-tee foundation plan for a single-pedestal tank. (Verts.= vertical reinforcement dowels.)

through the no. 200 sieve. Soils with high values of liquid limit and/orplasticity index should be avoided. A liquid limit in the range of 30 to35 and a plasticity index of less than 15 are commonly preferred.

Backfill may also be required to replace unsuitable bearing soils.The backfill material in this case may consist of well-compacted struc-tural fills as defined above, clean-washed crushed stone (e.g., no. 57stone), or a lean-concrete mud mat with a compressive strength of



Foundations



Anchor bolts

Verts. Verts.

B

B

Section A-A

Hoops

Radial bars

Gravel floor orconcrete slab

Concretethrust block

Do not grout pipe.Bend bars aroundopening

Hoops

Slab bottomreinforcementbars

Slab topreinforcementbars

diameter

Compactedregular orgravel fill

Compacted soil or ground fill

Compactedbackfill

Inlet-outlet pipe

Bearing elevationRoughened

construction joint (or a shear key)

Inside diameter ringwall


Outside diameter slab

Chord spacing

Radial bars Hoo

psp

acin

g

Hoops

Chord

spacing

Anchorbolts on

bolt circle

A

Plan

Outside

Centerlineof tank and foundation

A Centerline of inlet-outlet pipe

Centerline oftank andfoundation

Centerline of tank and foundation

Bars

Pipe sleevefor inlet-outlet

Section B-B

FIGURE 5-6 Typical ring-slab foundation plan for a single-pedestal tank. (Verts.= vertical reinforcement dowels.)

about 2,000 psi (14 MPa). The geotechnical consultant must providespecific recommendations as to the appropriate backfill material andrequired compaction.

Resistance to sliding is generally derived from the passive resis-tance of the soils acting against the foundation. Cohesive soils also



Foundations


draw passive resistance from the soil cohesion. In addition, the shear-ing resistance at the base of the footing resulting from internal frictionin the soil may contribute to the lateral resistance. The coefficient offriction at the base of the footing ranges from about 0.3 for silty soilsto 0.5 for coarse-grained cohesionless soils.

The backfill around the footings is commonly placed in 6- to 8-in.(15- to 20-cm) lifts and is compacted to 95 to 98 percent of the soil’sstandard proctor maximum dry density (ASTM D698) or modifiedproctor maximum dry density (ASTM D1557) criteria. Manipulationof the moisture content of the backfill material may be necessary toachieve the required compaction. Flowable fill is an alternative back-fill material that is simple to place and does not require elaboratecompaction.

SettlementAll structural foundations are subject to settlement. As long as the set-tlements are reasonably small and uniform, their effect on the structureis relatively small. However, if the settlements become large and thedifferential settlements excessive, there can be serious consequencesthat could lead to failure. Therefore, it is absolutely essential that thesettlement of all foundations is estimated and that its effect on thestructure as a whole is examined before construction proceeds.Geotechnical consultants are expected to provide proper assessmentof the total and differential settlements.

As was stated earlier, for water tanks it is preferable to limit thetotal and differential settlements to a maximum of 2 in. and 1 in. (5 cmand 2.5 cm), respectively. If shallow foundations will cause excessivesettlement, deep foundations can be used to further limit these set-tlements. The effect of the settlement on the piping should also becarefully examined. Special piping and pipefittings are available thatshould be used when flexibility in the system is required.

Settlement of foundations bearing on rock is not a concern aslong as all individual footings bear on rock. However, the rock layermust be thick and strong enough to support the loads without be-ing crushed (as might be the case with a rock lens). The designershould be very cautious of situations in which the foundation bearspartly on rock and partly on soils, as high differential settlements canresult.

Pile FoundationsWhen the bearing soils are weak or prone to excessive settlements,deep foundations—of which one type is the pile foundation—arenecessary. Piles transfer the structural loads deep into the stronger



Foundations


underlying soil strata or rock formations. They also transfer the lat-eral loads to the surrounding soils and maintain lateral stability.Given the complex nature of the resisting soils, competent advicefrom a qualified geotechnical engineer should always be sought onthe basis of a thorough subsurface investigation to assess the ap-propriate pile type, length, and other characteristics necessary fordesign.

Pile foundations consist of long, slender structural members thatare either driven into the soil or poured in place after drilling. Whethera pile develops its capacity from end bearing or side friction dependson how deep it is embedded and on the properties of the soils sur-rounding it. Piles driven to and bearing on hard rock or very denselayers of soil are primarily end bearing, as they axially transmit theloading to the bearing strata. Piles driven to shallower depths and notresting on hard and dense layers transmit the loading mainly by skinfriction and, hence, are referred to as friction piles. Generally, pilesdevelop resistance through a combination of both end bearing andskin friction. The resistance varies on the basis of the pile length andthe relative density and consistency of the soil layers.

The lateral capacity of the pile is a function of the soil characteris-tics near the surface. A simulation technique called “beam on elasticfoundation” can be used to assess the lateral resistance capacity ofthe pile. The spring constant necessary for the evaluation can be de-termined from the elastic or shear modulus of the soil. Pile lateralload is also a function of the flexural capacity of the pile itself. Pro-fessional advice must be sought in determining pile lateral capacitiesand load-displacement characteristics.

Pile TypesPiles can be driven or cast in place. Available driven pile types in-clude timber, precast, prestressed concrete, steel pipe, and H-piles.All have certain advantages and disadvantages. Where the resultingvibrations from pile driving can be a problem, cast-in-place piles maybe more suitable. Auger-cast piles, drilled piers, or caissons are alter-natives often preferred over pile driving because of their lightweightequipment.

Although pile selection depends on many factors—among themcost, availability, and load test requirements—there are advantages tousing a particular pile type for a given job. The common pile types arelisted in Table 5-2 (ASCE 1993b).

The main disadvantage associated with timber piles is the diffi-culty of achieving a high-strength connection between the pile andthe pile cap. Similarly, prestressed-concrete piles can pose a challengein achieving uplift connection. Dowels can be embedded into thepile head for transfer of tensile load, but because of physical space



Foundations


Type of Pile Description

Timber piles Best suited for short-embedment, low-capacity, and

low-cost applications. They are also appropriate

for application in regions with corrosive

groundwater. Timber piles are difficult to splice

and very susceptible to decay if untreated.

Precast concrete Usually prestressed, these durable and

corrosive-resistant piles are well suited for higher

capacities. Although precast concrete piles are

available in long lengths, splicing them is difficult

and can be a problem because water tank

foundations resist significant tensile (uplift) and

lateral loads.

Steel pipes Open- or closed-ended steel pipes pose some

difficulties in driving but offer high-resistance

capacity, especially when filled with concrete.

Steel pipe piles are well suited for splicing, but

they do have the potential for corrosion.

H-piles Available in a wide variety of sizes and lengths,

H-piles can be driven easily and spliced rather

conveniently. They offer high axial load carrying

capacity and flexural bending resistance.

Corrosion can be a problem, as with all steel,

which can be alleviated with preventive measures.

Source: ASCE 1993b.

TABLE 5-2 Common Pile Types

constraints, developing a connection for moment transfer can be dif-ficult for smaller piles.

Capacity and Driving FormulasPile capacity evaluation has evolved significantly in recent years, ashave requirements for the design of piles. Criteria endorsed by manyrecent building codes reflect the National Earthquake Hazards Reduc-tion Program (NEHRP) Recommended Provisions for Seismic Regu-lations for New Buildings and Other Structures (NEHRP 2003). Theserequirements are indeed very different than the common design prac-tices employed by the design community thus far. The differencesare more pronounced for regions of high seismic risk. Therefore, thegeotechnical engineer’s role is extremely important in the design ofpile foundations for elevated water tanks.

Geotechnical engineers generally provide the allowable pile loadeither as a set of recommendations or in the form of raw soil borings



Foundations


that must be interpreted. (The latter approach is not preferred.) Therecommendations may also include options for other pile types andcapacities for flexibility in design. A pile’s load-carrying capacity de-pends on its type, size, and depth of penetration. Geotechnical consult-ing engineers also provide pile lateral and uplift capacities, a center-to-center spacing recommendation, and anticipated settlement underthe governing loads. The minimum center-to-center spacing shouldbe at least three times the diameter or side dimension of the pile forend-bearing piles but larger for friction piles, especially when manypiles have been driven in a group.

Pile capacity may be limited either by the pile’s internal struc-tural capacity or by the external capacity offered to it by the resistingsoils. For the pile to be able to furnish the full resistance offered bythe surrounding soils, the internal capacity must exceed the externalcapacity. Pile capacity is basically determined on the basis of an ap-proved pile-driving formula, wave equations, or load test methods.IBC-2006 limits the allowable compressive load on any pile to 40 tons(356 kN) when it is determined on the basis of a driving formula alone.For allowable loads above 40 tons (356 kN), it recommends the use ofthe wave equation method of analysis and verification of this allow-able load by a load test in accordance with ASTM D1143 Test Methodsfor Piles Under Static Axial Compressive Load and ASTM D4945 TestMethods for High-Strain Dynamic Testing of Piles. Similarly, IBC-2006provides criteria for allowable frictional resistance and uplift capacity.

IBC-2006 and/or ASCE 7-05 provide detailed criteria forlongitudinal-reinforcement and transverse-confinement reinforcingsteel for precast, prestressed piles as a function of site class and seismicdesign category. ACI 318-05, in its Chapter 21, also provides designcriteria for piles, pile caps, and foundations that resist earthquake-induced forces.

The ultimate capacity of a pile that derives its resistance from boththe side friction and the end bearing is given by

QUltimate = QSide Friction + QTip Bearing (5-15)

The evaluation of Q depends on whether the pile is driven insand or clay. Refer to any textbook on pile foundations for appropriatemethodologies for determining pile capacity.

AWWA D100 requires a minimum FS of 2.0 for gravity loads andan FS of 1.5 for gravity loads plus wind or seismic loads. Other refer-ences define factors of safety on the basis of whether load tests havebeen performed.

Various theoretical pile-driving formulas can be used to estimatepile load-carrying capacity. These formulas do not correlate well withtest results and are historically inaccurate. However, they are help-ful in establishing when to stop driving a pile to achieve a capacity



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equivalent to that of the load test. The reader is once again referred toa textbook on pile foundations (Prakash and Sharma 1990). The equa-tion that is often used by the industry is known as the Engineering-News formula (Liu and Evett 1987):

Qa = 2Wr HS + C

(5-16)

or

Qa = 1000 Wr H6(S + C)

(in SI units) (5-17)

where

Qa = allowable pile capacity (lb [kN])Wr = weight of ram (lb [kN])H = height of fall of ram (ft [m])S = amount of pile penetration per blow (in./blow [mm/blow])C = 1.0 for drop hammer; 0.1 for steam hammer (25 or 2.5)

There is a built-in FS of 6 associated with Equations (5-16) and(5-17).

Pile Driving and Load TestsBefore any pile driving begins, the geotechnical engineer must ap-prove the equipment and method of pile driving. This, along withmonitoring the penetration resistance, will ensure safe driving of thepiles. When the engineer authorizes pre-boring to a certain depth, thediameter of the hole must be smaller than the pile diameter or side di-mension. A limit of two-thirds times the diameter or side dimension isrecommended. Larger or oversize holes cause loss of the skin frictionand, consequently, a reduction in the pile axial and lateral load capac-ity. If pre-boring is necessary, then auger-cast piles or drilled shaftsmay be a better option.

The geotechnical engineer must provide guidance on pile drivingand criteria for pile length or depth of penetration. Such guidance canbe in the form of limiting the penetration resistance based on a par-ticular hammer and rate of energy or recommendations for dynamictesting using a Pile Driving Analyzer (PDA) to ensure pile structuralintegrity and adequate load-carrying capacity.

Unless geotechnical recommendations for pile capacity are basedon previous experience in the site vicinity, most recommendations aretheoretical and only an estimate of the carrying capacity of the pile.Load tests are performed to determine or to confirm those theoreticalcapacities. Test piles should be driven where the soil conditions areknown. Test piles must be the same as the actual piles being used



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for the job. Also, the pile-driving techniques on test piles must be thesame as those that will be used for the production piles.

For foundations for elevated water tanks, the most commonmethod of testing piles under static axial compressive loading is thatperformed under ASTM 1143. But this method is costly and requires aminimum waiting period of 7 days for piles in granular material and14 days for piles in cohesive soils for dissipation of excess pore wa-ter pressure after test pile installation and before load testing begins.More recently, the use of PDA is gaining much acceptance over thestatic load tests in view of its fast pace and the more quickly avail-able resulting data. PDA is also used to assess pile capacity and pilestresses from measurements of the applied force and acceleration atthe head. Refer to ASCE 1993b and ASCE 7-05 for further details.

Pile Spacing and Group EfficiencyAs stated previously, the minimum pile center-to-center spacingshould be three times the pile diameter or side dimension for end-bearing piles. For friction piles, however, a larger spacing (three tofive times the pile diameter or side dimension) may be required, asdetermined by the geotechnical consultant. A larger center-to-centerspacing makes the pile cap larger and heavier and can increase the re-quired number of piles. However, a smaller center-to-center spacingbetween piles may reduce the group efficiency of the piles. Some ref-erences allow smaller spacing, but the geotechnical engineer shouldcarefully review any reduction in the spacing.

As the spacing between piles decreases, the group capacity ofpiles may not equal the sum of the individual pile capacities in thegroup. Pile group efficiency must be evaluated on the basis of a rationalevaluation that considers the overlapping effects of individual piles.The geotechnical engineer should provide the required pile spacingand group efficiency along with all other pertinent information.

Auger-Cast PilesAuger-cast piles are piles that are installed by pumping grout underpressure into holes drilled to required depth by continuous-flight,hollow-stem augers. The common diameters of these piles range from12 to >30 in. (30 to >76 cm), with lengths from 50 to 100 ft (15 to 30 m)and compressive capacities that can exceed 125 tons (1 MN). Auger-cast piles are reinforced by inserting a single reinforcing bar and/or areinforcing cage through the unset grout. The cage extends to a definedlength based on the structural requirements of the pile in resistingtensile, compressive, and lateral loading. The single reinforcing bar atthe center of the pile typically continues to the bottom end of the pile.The grout mostly consists of portland cement, sand, and water.



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Where vibrations due to pile driving can cause damage to otherstructures or entities in the near vicinity of the job site, auger-cast pilesoffer a better alternative. As with all other piles, pile axial load testsare performed in accordance with ASTM 1143. Pile installation recordsare maintained as required. More information on these piles can befound in the Augered Cast-in-Place Piles Manual prepared by the DeepFoundations Institute (DFI 1990).

Lateral Loads and Bending MomentsIn addition to the gravity loads, foundations for elevated water tanksmust resist vertical and lateral loads caused by wind or seismic load-ing. These loads are transferred through the pile cap to the resistingpiles. Depending on pile head fixity and the characteristics of the sur-rounding soils, the lateral shears can cause significant bending mo-ments, in addition to the axial loads acting on the piles. Therefore,both the pile and the surrounding soils should be analyzed and inves-tigated for strength and stability.

Along with the allowable load capacity for the pile, geotechnicalengineers must provide allowable lateral load capacities for variouspile head fixity conditions. If the pile head extends into the pile capand is anchored by uplift connections, there is very little, if any, pilehead rotation. But due to the movement of the pile cap, lateral trans-lation of the pile head is possible. For design purposes, however, itis helpful to define the pile head boundary condition to locate wherethe maximum bending moment occurs. With the pile head restrainedagainst rotation, the maximum bending moment in the pile generallyoccurs at the restrained end, at the pile cap. Otherwise, it occurs atsome distance below the pile cap.

Geotechnical engineers often provide bending-moment curves asa function of the lateral loads and an assumed pile head fixity condi-tion. These curves are very helpful in design and should be includedin all geotechnical reports that recommend pile foundations. Theyshould be carefully reviewed for assessing pile structural capacity aswell as the effect of pile lateral displacement on the elevated watertank system.

Pile Caps and Uplift ConnectionsPile caps are reinforced-concrete structural elements that resist directvertical and lateral forces and transfer them to the supporting piles.Pile caps are sized based on the number of piles required for a towercolumn loading. Depending on their thickness, pile caps can be rigidor flexible. Typically, pile caps join the pedestal from the top and thepiles from the bottom. Piles usually extend 4 to 6 in. (10 to 15 cm) intothe pile cap.

The size of the pile cap is a function of the number and center-to-center spacing of piles required for a footing. The larger and thickerthe pile cap, the heavier it is. Thus, the dead weight of the pile cap



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S

S

S

S

S S

S

S

S

S

S S

SS

S S

S

Four piles Five piles Six piles

Nine pilesEight pilesSeven piles

FIGURE 5-7 Typical pile group patterns for single foundations. (S = spacingbetween piles.)

itself can cause an increase in the number of required piles. Fig-ure 5-7 provides typical pile layouts for four-, five-, six-, and seven-pile footings. Pile caps must accommodate pile spacing as well as edgedistance requirements. The pile cap edge distance, measured from thecenterline of the outer piles to the edge of the pile cap, is generally afunction of the pile diameter. Typically, for piles with a diameter ofabout 1 ft (30 cm), a distance of 1 ft. 3 in. (38 cm) is used.

The thickness of the pile cap should be checked for punching shearcaused by piles exterior to the critical section. The critical section istaken to be at a distance of d/2 from the pedestal, where d is the currentdepth to centroid of tensile steel in the pile cap. The punching shearshould also be checked around the individual piles at a critical sectiontaken a distance d/2 from the face of the pile, and the pile cap thicknessshould be adjusted, if necessary.

The pile cap thickness is also dependent on flexural shear bothtangentially and radially at a distance d from the face of the pedestal.Flexural reinforcement, in both the tangential and radial directions,should also be checked at the faces of the pedestal. The flexural rein-forcement determined should then be compared against the minimumflexural reinforcement requirements of ACI 318-05 Section 10.5.1 andadjusted, if necessary.

IBC-2006 provides specific criteria for pile connection to the pilecap. For prestressed piles, uplift anchorage to pile cap can be achieved



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by exposing and developing the reinforcing strands at the top of thepile. This approach, however, is not permitted by other model codesin areas of high seismic risk. IBC-2006 permits the option, providedthat the reinforcing strand results in a ductile connection. As an alter-native, reinforcing dowels can be grouted into the top of the pile. Intimber, steel, or pipe piles, a reinforcing bar can be inserted throughthe member and bent upward into the pile cap. The pile cap mustbe thick enough to accommodate the anchoring mechanism. Refer toFigures 5-8a and 5-8b for typical pile foundations.

Radial centerline

Tangential bars

Prestressed

concrete piles

Hoops

(a)

DowelsTop of footingelevation

Aggregate

Projection

Anchor bolt

Exposure

Typical upliftanchor

Radial bars

Prestressed

concrete piles

Pile layout plan view

Centerline of foundation

Column foundation with piles elevation view

Centerline of foundation

FIGURE 5-8a Typical pile foundations: typical pile foundation for a singlecolumn



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Outs

ide d

iam

ete

r sl

ab

Prestressed, precastconcrete piles Anchor bolts

on bolt circleA


Insid

e dia

met

er sl

abInside diameter ringwall

Cho

rd

Hoops

Radial bars

Cho

rd

A

Concreteslab

Reinforcedconcrete

thrustblock

Hoops

Radial bars

Subgrade

Prestressedconcrete piles

Verts.

Hoops

Radial bars

Hoops(specialtop bars)

Anchorbolts

Expansionjoint

Verts.Hoops


Expo

sure

Typical upliftanchor


Plan

Section A-A


(b)

FIGURE 5-8b Typical ring-tee pile foundation). (Verts. = vertical reinforcementdowels.)



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The pedestals for pile foundations are usually compressive ele-ments with a ratio of unsupported height to least lateral dimensionnot exceeding 3.0. As noted in the section on shallow foundations,although ACI 318-05 Section 15.8.2.1 recommends a minimum rein-forcing ratio across the interface (between the pedestal and footing) of0.5 percent of the gross area of supported member (i.e., the pedestal),a smaller reinforcement ratio can be justified for the tower columnpedestals due to their large size. For taller pedestals, the 0.5 percentreinforcement ratio should be maintained. However, the reinforce-ment furnished must be sufficient to meet the requirements for upliftas well as flexural requirements necessitated by the lateral shears onthe pedestal.

Pile Stability and SettlementPiles are required to be laterally braced in all directions. Piles intercon-nected by a rigid pile cap may be considered braced for lateral stabil-ity provided they are situated as defined in IBC-2006 Section 1808.2.5.Elsewhere, the surrounding soils furnish lateral stability along pilelength. In regions where the piles extend vertically through voids orholes, the piles should be analyzed as columns.

Settlement is an important aspect of design for all water tank foun-dations, including pile foundations. The geotechnical engineer shouldevaluate pile settlement as well as potential differential settlements forfull consideration in design. As discussed previously in the section onsoils and geotechnical investigations, settlement not only affects thestructural behavior but also the piping systems and the interconnec-tions among the various components and appurtenances.

Drilled-Pier (Caisson) FoundationsDrilled piers, caissons, or shafts offer an alternative design option indeep foundations. Drilled piers are cast-in-place reinforced-concreteshafts with or without a bell at the bottom. They are installed bydrilling a hole of predefined diameter and depth at the design locationand then filling the excavation with concrete and reinforcement.

Drilled-pier construction is relatively easy and can be accom-plished with rotary drilling equipment. Depending on the soil con-ditions, casings or laggings may be needed to prevent the soils fromfalling or caving into the hole. Typical diameters of drilled piers forwater tank foundations range from about 3 to 6 ft (0.9 to 1.8 m). Largerdiameters may be needed for higher-capacity tanks, depending onavailability of large-diameter drill bits. Otherwise, several smaller-diameter piers will be used, which would require a larger pier cap.



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There are many advantages to using drilled piers for watertank foundations. Because of its size and capacity, a drilled piercan replace a group of piles and eliminate the need for a pile cap.Installation of drilled piers does not generate much noise or vibration,as pile driving does, and drilled piers can be set up in hard-to-accessplaces. A primary advantage of drilled piers is that they can sustainlarge axial loads with minimal settlement when bearing on bedrock.

Pier Bearing CapacityDrilled piers draw their structural capacity from the reinforced-concrete shaft. They develop their bearing capacity from side resis-tance, generated by skin friction, and base resistance, generated byend bearing. In equation form, the ultimate static load capacity of apier can be expressed as

Qu = Qbu + Qsu − Wp (5-18)

where

Qu = pier ultimate resistance (kip [kN])Qbu = pier ultimate end-bearing resistance (kip [kN])Qsu = pier ultimate side friction resistance (kip [kN])Wp = pier dead weight (kip [kN])

An FS of 3.0 is applied for allowable-stress design application.Some references may apply an FS of only 2.0 on the ultimate resistancedue to side friction Qsu in service-load design application. Others mayalso apply a load factor on the shaft dead weight Wp. Refer to Fig. 5-9for a typical belled, drilled pier configuration.

A very useful and relevant reference on drilled piers is a re-port entitled Drilled Shafts: Construction Procedures and Design Methods,issued by the US Department of Transportation, Federal HighwayAdministration (Reese and O’Neill 1988). This reference provides de-tailed analysis and design, fabrication, and quality control criteriafor drilled-shafts foundations. The Bearing Capacity of Soils, preparedby the American Society of Civil Engineers (ASCE 1993a), is anothersource for criteria regarding analysis and design of drilled shafts.

Pier Side or Skin FrictionPier side resistance offered by the skin friction along the pier shaft isa function of the undrained shear strength of clay soils as determinedby testing. Shear strength varies with depth and soil strata and isempirically related to the shaft load transfer in side resistance. The



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Reinforcing steel

Bell—may be usedwhen required.Size varies—no larger than three times shaft diameter at base. Underreamangle θ is 45° or 60° typically.

Diameter depends on loading and depth

of excavation

Depth

can v

ary

per

desig

n

Q base resistancebu

Q side resistancesu

load

Axial load

Base resistance

WPθ

Lateral

Exposure

Hoops

Additional reinforcement,if required

FIGURE 5-9 Typical drilled shaft. (Wp = pier dead weight; Qsu = pier ultimateside friction resistance; Qbu = pier ultimate end-bearing resistance) (Source:Reese and O’Neill 1988.)

shear resistance offered by sands or cohesionless silts, however, is afunction of the soil angle of internal friction.

The resistance capacity offered by side friction can be very sig-nificant. For piers socketed into bedrock, it is possible that the entireresistance capacity is emanating from the side resistance furnished bythe socket.

It is a recommended practice within the drilled-pier design com-munity to ignore the contributions of side friction and passive re-sistance in the top 5 ft (1.5 m) or 1.5 diameters of the shaft when



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evaluating lateral stability. The reason is that lateral movement bythe pier causes a wedge of soil to move up and out, resulting in aloss of side friction and passive resistance. Similarly, in clay soils, theside friction in the periphery of the bell or the bottom of the straightpier within 1 diameter of shaft length is ignored in determining theresistance capacity of the pier. The reason is that movement of thebase of the pier can result in the development of a tensile crack inthe soil, which in turn can cause a lateral stress at the base of the pierand, consequently, a reduced load transfer in side friction (Reese andO’Neill 1988).

Design of PiersThe diameter of the pier is a function of the soil characteristics withinthe profile, the location of the water table, and the presence of lateralloads and/or moments. The design of the concrete mix and its strengthare also of critical importance.

The geotechnical profile of the soil dictates not only how far downto extend the pier, but also the method of construction to be em-ployed, the need for casing and/or dewatering, and the need forunder-reaming. Special characteristics of soils—shrinking/swellingof plastic soils, occurrence of boulders, remains of abandoned foot-ings, presence of debris or other unsafe materials, and so on—all re-quire that certain measures be taken into full consideration. Asidefrom the basic structural design, the most important consideration isthe amount of the expected settlement of the pier foundation and itseffects on the elevated water tank system.

Under-reaming, where possible, helps increase the pier bearingsurface and consequently the bearing resistance. Under-reaming canalso be used interchangeably with socketing where required. The lon-gitudinal reinforcement for drilled shafts depends on the many fac-tors noted previously, but as a minimum, industry practice has beento provide at least 0.5 percent of the cross-sectional area and at least sixbars, forming a cage of equally spaced bars. This minimum is actuallybased on ACI 318 Sections 10.8.4 and 10.9.1. Section 10.9.1 requires aminimum reinforcement of 1 percent. For regions of low-to-moderateseismic risk, Section 10.8.4 states that, for compression members withcross-sectional areas larger than required by consideration of loading,it should be permissible to base the minimum on a reduced effectivecross-sectional area not less than half the total area.

Additional reinforcement may be required where heavy tensileloading or bending moments are present. The longitudinal reinforce-ment cage may extend a partial depth or the full length of the pierwhen required. Hoop or spiral reinforcement is also used for drilledshafts. Hoops are more economical, but from a performance stand-point spirals are preferred. Figure 5-10 illustrates pier reinforcement.



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Length of cage dependson lateral designrequirements and maynot require extending to full length. Extend cage to full length in highly plastic (expansive) soils.

Ties—spacing as perACI 318 Sections 7.10and 7.11. Refer toACI 318 Chapter 21 forspecial provisions forseismic design.

FIGURE 5-10 Drilled shaft reinforcing cage.

In regions of high seismic risk, special reinforcement requirementsmay also apply, including confinement steel near the interface regionswith the pedestal or grade beams. Refer to ACI 318-05 Chapter 21 forspecial provisions in seismic design.

Where belled bottoms are needed, the base diameter is generallylimited to less than three times the shaft diameter. The under-reamangle is typically in the range of 45 degrees to 60 degrees, with 60degrees often used for water tank foundations. Also, a toe height ofabout 1 ft (30 cm) is maintained at the base.

Lateral and Uplift StabilityLateral stability of the drilled pier depends on the shaft length andflexibility. It also depends on whether the pier is drilled in cohesive orcohesionless soils. Lateral stability of piers should be carefully exam-ined, especially where the piers are not socketed into bedrock.

Similarly, uplift stability is a function of pier length, geometry,and side friction. Where the uplift forces are relatively small, the re-sistance provided by the side friction along a straight shaft pier may



Foundations


be adequate for uplift stability. But when the loads are severe, belled,or under-reamed, piers are necessary. Detailed criteria on the stabilityof drilled piers and a discussion on the potential collapse of the bellin loose soils during construction can be found in the study by Reeseand O’Neill (1988).

Socketing into BedrockPiers drilled into rock derive their load-bearing capacity from endbearing and side friction offered by the length of socket in the rock.A roughened socket length equal to 1 diameter into rock with highermodulus than the pier shaft enables the pier to carry 50 percent of theload by side friction, whereas a 4-diameter roughened socket lengthinto the bedrock allows the shaft to transfer nearly all the load byside friction (Wyllie 1992). Hence, the depth of socketing should be afunction of design requirements and not established arbitrarily.

The socket drilled into bedrock also provides end fixity, allow-ing the pier to develop moment resistance at its base. Geotechnicalengineers generally provide a simplified, uniform, unit side frictionvalue along with the end bearing for design. This information may bepresented as ultimate capacity or service load capacity. The referencespreviously noted provide further information on the subject.

Design Considerations in Plastic SoilsPlastic soils can be found in many parts of the world. In the UnitedStates, Texas, Oklahoma, and the upper Missouri Valley area havehighly expansive soils. Increases in moisture cause swelling in theseclayey soils, and, as a result, foundations are subjected to rather largeuplift forces. These forces can be large enough to pull the drilled pierout of the ground unless it has been properly designed. Similarly,if the pier shaft is not adequately reinforced, it could break apart fromthe base because of the tensile forces caused by swelling soils alongthe shaft.

Piers in shrink/swell soils should terminate in bells that bear deepin soil layers not in the zone of seasonal activity and movement. Thereinforcement cage in these belled piers should extend the full heightto allow the belled segment to anchor the uplift forces in the upperareas of the shaft. The tensile reinforcement needed is in addition tothe reinforcement needed for normal tensile loading.

Load TestingA clear way of establishing the structural integrity of a drilled pier isby load testing. However, due to the high costs and logistical difficul-ties associated with the arrangement of reaction shafts, such testingis rarely performed. If it is absolutely necessary that a load test be



Foundations


performed, the pier (as with piles) must be able to sustain withoutexcessive settlement a load that is at least twice the working load.

The best way to ensure the structural integrity and intended per-formance of drilled piers is to follow a credible quality control programof inspection and installation procedures. Recent studies have shownthat minor construction flaws that may not be detectable by commonnondestructive evaluation methods can lead to significant capacityreduction in drilled piers. Such flaws include the presence of smallvoids, soil inclusions, misaligned cage or other reinforcement steel,weak concrete, or corroded reinforcing bars. Refer to Sarhan et al.(2004) for further details.

SettlementSettlement concerns associated with drilled piers are similar to thosedefined for piles. Typically, if the drilled pier is bearing on or socketedinto the bedrock, settlement caused by direct loading is negligible.Under other installation conditions, it is important that a proper set-tlement analysis be performed by the geotechnical engineer to ensurethat the expected settlements are tolerable from operations and per-formance perspectives and from the standpoint of structural design.

Reservoir and Standpipe FoundationsReservoirs and standpipes are considered flat-bottom tanks. The de-sign of foundations for flat-bottom tanks follows the criteria de-fined previously for shallow and deep foundations. AWWA D100-05provides detailed guidance on various foundation types for reservoirsand standpipes.

The bearing pressure induced by the water at the base of a flat-bottom tank is equivalent to the height of the high water line H timesthe density of water. For a flat-bottom tank to be supported on theground, with or without a ringwall, the bearing soils must have anallowable bearing capacity of at least 62.4 H lb/ft2 (9.81 H kN/m2).Pile foundations may be necessary if the induced bearing is in excessof the allowable bearing capacity of the resisting soils. Therefore, it isimportant to reiterate that a formal geotechnical investigation mustbe performed to verify that the bearing soils can carry the resultingloads.

AWWA D100-05 also provides criteria for grading the interfacebetween the bottom of the tank and the supporting base and the useof oiled or clean sand, crushed rock, or asphalt road mix. The standardalso provides information on granular berms, grout, foundation toler-ances, anchor bolts, etc. Refer to Fig. 5-11 for a typical granular-bermfoundation for a flat-bottom tank (Fig. 5-3b).



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Coarse stone orcoarse gravel Thoroughly compacted fill of

gravel, coarse sand, or otherstable material

Compacted crushed stone, screenings, fine gravel, clean sand, or similar material

Coarse stone orcoarse gravel

Berm

Flat-bottom water-storage tank

Subgrade

Slopedown

11

2 ft (0.6 m)minimum

Extend 3–5 ft (0.9–1.5 m)beyond tank

To drainaway

FIGURE 5-11 Typical granular berm foundation.

Slab FoundationWhere the bearing soils are strong or when the water tank capacity issmall, flat-bottom tanks can be supported by a mat or slab foundation.The slab is uniformly loaded by the pressure head in the tank. Theresulting bearing stress under the slab is the pressure due to the weightof the tank and its contents added to the uniform pressure caused bythe thickness of the concrete slab. Overturning moment resulting fromwind or seismic loading also contributes to the bearing stress.

The reinforcement requirements of the mat or slab foundationare based on the loading and deformation characteristics of thefooting. Often the minimum reinforcement requirement defined byACI 318-05 will control. Anchorage and stability requirements shouldbe investigated when the tank is full and when it is empty. Slab exte-rior edges supporting the tank wall may be thickened, if necessary, toaccommodate the additional bearing stress caused by wind or seismicoverturning moments. Consideration should also be given to the frostdepth in determining slab thickness and bearing elevation.

Ringwall FoundationAs discussed in the section on shallow foundations, ringwall founda-tions are used when the bearing pressure under the tank shell exceedsthe allowable bearing pressure of the soil near grade. The ringwallcarries the loads deeper and distributes the pressure over a widerarea. When the overturning moments are severe and anchor bolts arerequired for stability, a ringwall foundation best accommodates theseanchorages. The design must consider hoop stresses caused by theinternal soil pressure resulting from the weight of the tank and itscontents.



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Ring-Tee and Ring-Slab FoundationsWhen the bearing soils are weak or when the tank is large, a ring-tee ora ring-slab foundation may be required. The foundation system is thendesigned for the direct bearing loads and the overturning momentscaused by wind and seismic loading for the load combinations definedearlier. The ringwall requires vertical and horizontal reinforcementand full consideration of the hoop stresses. The tee or slab portionrequires radial and tangential reinforcement.

Uplift is a loading condition that must be considered for wind withthe tank empty and for seismic with the tank full. Hence, for upliftstability, the dowels must be developed within the slab by hooks at theends. It is possible that piping has been routed through the ringwall. Atsuch locations, additional reinforcing must be provided to strengthenthe opening periphery.

Deep FoundationsWhere the bearing soils are weak or the settlements are excessive,flat-bottom tanks require deep foundations. Driven piles and auger-cast piles are typically used under flat-bottom tanks. In regions ofhigh seismic risk, special reinforcement requirements also apply, as isdiscussed subsequently in this chapter.

Anchor Bolts (Rods)Load combinations governing the design of foundations for elevatedwater tanks were defined in Equations (5-1) through (5-13). The loadcombination causing the maximum uplift and shear in the bolt gener-ally governs the design of the anchor bolts (more recently also referredto as anchor rods). Because of the significant forces imposed on ele-vated water tanks, it is critical to properly design all anchor bolts tosafely transmit these forces to the foundation.

Flat-bottom tanks may or may not require anchor bolts. All el-evated water tanks require anchor bolts. Cast-in-place anchors arethe most common type of anchor bolt for water tank foundations, al-though post-installed anchors have their uses. For all bolt or anchortypes, the embedment length, center-to-center spacing, edge distance,and group action should be evaluated for the design loads, with ap-propriate factors of safety.

Of the many references addressing structural design of anchorbolts, Appendix D of ACI 318-05 is entirely devoted to anchors inconcrete. This reference requires anchors and anchor groups to bedesigned for critical effects of factored loads as determined by elasticanalysis.



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Minimum Embedment and ProjectionThe minimum embedment length should be such that the anchor boltis capable of developing the required uplift strength and shear re-sistance for all loads in the design load combinations. Most manu-facturers of post-installed anchors recommend an embedment lengthfor their proprietary anchors. However, cast-in-place anchors requireunique design based on the governing loads or as determined bytesting or past evaluations.

Bolt projection is especially important when settlements are ex-pected to be large. In such cases, the projection should be long enoughto accommodate shimming, as required. To provide for variations infoundation elevations, AWWA D100 further requires a projection ofthe anchor bolts’ threaded ends an additional 2 in. (5 cm) beyondthe anchor nuts. Typically, a 7-in. (18-cm) projection above the top ofconcrete is sufficient.

Allowable Tension and ShearAWWA D100 recommends sizing the bolts for tension using the rootarea and a basic allowable tensile stress of 15 ksi (103 MPa), with aone-third increase for the wind load combination. The minimum boltdiameter specified is 1.25 in. (31.8 mm), and the maximum center-to-center spacing required is 10 ft (3 m).

For the wind load combination, tension–shear (linear) interactionis checked using a basic allowable tensile stress of 15 ksi (103 MPa)(as per AWWA D100-05) or 19.1 ksi (as per Table I-B in AllowableStress Design, American Institute of Steel Construction [AISC 1989]) forA36 anchors in tension. For shear, AWWA D100 recommends 7.5 ksi(51.5 MPa) for unfinished bolts, and AISC (1989) recommends 9.9 ksi(68 MPa). An interaction value less than or equal to 1.33 renders thedesign acceptable.

For the seismic load combination, AWWA D100-05, in Section3.3.3.2, provides a higher allowable tensile stress for mild steel an-chors based on the lesser value of 0.8Fy or 0.5Fu, where Fy and Fu

refer to the anchor bolt yield and tensile stresses, respectively. ForA36 steel, this means 28.8 ksi. For concurrent shear, the AWWA D100-05 allowable stress of 10 ksi (i.e., 1.33 × 7.5 ksi) or the AISC allowableshear stress of 13.2 ksi (1.33 × 9.9 ksi) is used. An interaction valueof 1.0 renders the design acceptable. Note that in this case, the ten-sile allowable stress is increased by a different multiplier than 1.33, sothe increases are taken directly in the denominator of the interactionequation for the seismic load combinations.

For single-pedestal and ground-supported flat-bottom tanks, thedesign tensile load in the anchors is calculated from Equations (3-41)and (3-42) in AWWA D100. For all styles of tanks, when checkingbolt interaction under seismic loads, the resistance offered by friction



Foundations


forces may also be taken into consideration, in the authors’ view. Somecodes specifically disallow this, but for water tanks it is justified sincenearly the entire mass is considered effective in the formation of allseismic loads, including the seismic overturning moments. This mustbe done with attention to signs (load direction), since a column underuplift cannot generate frictional resistance.

Bolt InteractionInteraction can be checked by means of a simple equation. For thecombined effects of tension and shear, the following linear interactionequations may be used in design:

TTallowable

+ VVallowable

≤ 1.33 (wind) (5-19)

TTseismic-allowable

+ V1.33Vallowable

≤ 1. (seismic) (5-20)

where

T = tensile loadV = concurrent shear load on bolt

The allowable tensile and shear stresses are as defined previouslyfor mild steel. For other types of steel anchors, refer to AWWA D100or other applicable codes for all allowable stresses.

If high-strength or stainless-steel bolts are required, D100 allow-able tensile stress for these bolts is based on the lesser of 0.4 timesthe minimum published yield stress or 0.25 times the published ten-sile strength. The calculated bolt size may need to be adjusted whencorrosion allowance is required in design. AWWA D100 discouragesthe use of J and L bolts because of their tendency to straighten out, asobserved in pull-out tests.

Quality control in placement of bolts is essential. Given the sizeand embedment length required, bolt relocation may not be possible,and remedial measures can be expensive. Therefore, proper bolt place-ment, including correct embedment and projected length, is critical toproper design.

Bearing Stress Under Base PlatesThe design bearing strength of concrete is defined in Section 10.17 ofACI 318-05. Typically, under service load conditions, the allowablebearing stress Fp is 0.35 f ′

c when the entire area of concrete support iscovered (AISC 1989). Otherwise, when the supporting surface is wideron all sides, the bearing stress is based on 0.35 f ′

c√

A2/A1 ≤ 0.7 f ′c. An

additional one-third increase may be taken for wind or seismic load



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combinations. Refer to ACI 318-05 for determination of A2 and A1

areas.

Foundations in Regions of High Seismic RiskSeismic design has undergone a tremendous evolution over the pastdecade. For years, seismic design of elevated water tanks was primar-ily based on the fixed-percentage method, in which the total weightof water and structure was multiplied by a specified coefficient basedon seismic risk zones. This was subsequently changed to the pseu-dodynamic approach, but design was still governed by seismic riskzones. Concurrently, some building codes required seismic design tobe performed using the velocity-based acceleration Av and the effec-tive peak acceleration Aa.

AWWA D100-05 has entirely eliminated language regarding de-sign on the basis of seismic zones (0, 1, 2, 3, and 4). Instead, the AWWAstandard has essentially adopted the ASCE 7-05 criteria (based onNEHRP [2003]) with some variation with respect to the minimum de-sign acceleration. These requirements are substantially different fromthe procedure thus far used by older AWWA standards.

The International Building Code in Section 1613.1 invokes the re-quirements of ASCE 7-05 for the design and construction of elevatedand flat-bottom water tanks to resist the effects of earthquake motions.

Special Design ProvisionsAs per ASCE 7-05, seismic design involves a procedure in which spec-tral response acceleration parameters for the maximum consideredearthquake ground motions are determined from figures and thenmodified for local site effects with site coefficients. The resulting ac-celerations are then scaled down to design values. ASCE 7-05 alsopermits the use of site-specific procedures in design and mandatesthis procedure where provisions specifically require it.

ASCE 7-05 classifies sites based on shear wave velocity and otherfeatures. Depending on soil consistency ranging from hard rock to stiffsoils, site classifications A, B, C, and D are defined. Site classificationE involves any profile with more than 10 ft (3 m) of soil having highplasticity index or high moisture content, or low shear strength asdefined in the reference. Site class F involves soils that are vulnerable topotential failure or collapse, highly organic soils, very high-plasticitysoils, and very thick, soft/medium clays. Site class F soils require site-specific evaluations.

With the site classifications defined, the 5 percent damped designspectral acceleration at short period SDS and at 1-second period SD1

are determined. The elevated water tank system is then assigned toa seismic design category (SDC) based on these accelerations and on



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the appropriate seismic use group (I, II, or III). All structures havingSDS ≥ 0.5g or SD1 ≥ 0.2g (where g is the acceleration of gravity) are as-signed an SDC of D. Also, seismic use group III structures with 0.33g≤ SDS < 0.5g or 0.133g ≤ SD1 < 0.2g are assigned an SDC of D. Seismicdesign category E is assigned to seismic use group I and II structureslocated on sites with mapped maximum considered earthquake spec-tral acceleration at 1-second period S1 equal to or greater than 0.75g.Similarly, seismic use group III structures at these accelerations areassigned an SDC of F. Structures assigned to categories C, D, E, orF require special attention to quality assurance during construction.Structures assigned to category E or F are prohibited from being sitedwhere there is a known potential for an active fault.

The seismic importance factor IE significantly affects seismic de-sign. (This factor is defined in ASCE 7-05 Section 11.5.1, Table 11.5-1.)Values of the importance factor range from 1.0 to 1.5, depending onthe seismic use group category assigned to the elevated water tanksystem. AWWA D100-05 assigns a default value of 1.5 to IE unlessotherwise specified by the purchaser, but it allows the use of 1.0 forsystems not supplying water for fire protection.

Reinforcement CriteriaIn regions of high seismic risk, ACI 318-05 requires structures to com-ply with requirements defined in Sections 21.2 through 21.10. Thesesections define maximum and minimum flexural and transverse re-inforcement, maximum spacing for hoops and crossties, bar develop-ment length, and other requirements. Section 21.10 provides criteriafor the design of foundations. Footings, mats, piles, pile caps, piers,and caissons are all required to be designed under this section.

ASCE 7-05 refers to ACI 318-05 for design and construction ofconcrete foundations assigned to seismic design categories D, E, andF. ASCE 7-05 requires individual pile caps or drilled piers in thesecategories, as well as in category C, to be interconnected by ties. Like-wise, spread footings founded on site class E and F soils are requiredto be interconnected by ties. The design strength for ties in tension orcompression is required to be greater than 10 percent of SDS times thelarger pile cap or column factored-dead plus factored-live load, withsome exceptions. There are also rigorous requirements for the designof piles in site class E and F soils.

Precast Prestressed and Cast-in-Place Concrete PilesIBC-2006 provides detailed criteria for the design of foundations, piles(including precast prestressed piles), and pile cap connections. It spec-ifies a 28-day compressive strength f ′

c of 5,000 psi (34.5 MPa) andrequires the prestressing strands to conform to ASTM A416. For pre-stressed piles, IBC-2006 also specifies a minimum volumetric ratio of



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spiral reinforcement, defines the ductile region of the pile as a functionof its length, and establishes bounds for the center-to-center spacingof the spirals or hoop reinforcement. Similarly, IBC-2006 also estab-lishes criteria for the design and detailing of cast-in-place pile and pierfoundations.

The IBC-2006 requirements are very similar to the ASCE 7-05 re-quirements. Either reference can be used as required, individually orin conjunction with ACI 318, in designing elevated water tank foun-dations.

Foundation StabilityDesign for stability is critical in regions of high seismic activity. Foun-dations must be designed to withstand all design loads with adequatefactors of safety. Foundations must also be stable against all forcescausing uplift, lateral sliding, and overturning. The safety factors re-quired for stability and strength are defined in various ways by differ-ent codes. It is important to appreciate the reasoning and philosophyassociated with these factors to ensure structural integrity, safety, andstability.

Lateral stability in saturated soils, settlement evaluation in satu-rated or high-moisture-content silty soils, and potential liquefactionin sandy soils are all conditions that require competent evaluation andassessment before elevated water tanks are built on sites with thesecharacteristics. Piles designed for fixity at the pile head must be prop-erly connected to or embedded deep into the pile cap to develop upliftand moment capacity.

Backfill around spread footing and pile foundations must be con-sistent with the geotechnical engineer’s recommendations. Specialrecommendations made in terms of moisture content, maximum looselifts, or soil remediation measures must be followed. All nonconform-ing conditions must be brought to the attention of the engineer ofrecord for evaluation and disposition.

Special ConsiderationsDesign of elevated water tank foundations requires close coordinationwith the project geotechnical engineer and with the construction teamat the job site. All parties must clearly understand the design require-ments and must appreciate what is essential to quality design andconstruction. Structural engineers must not assume that all geotech-nical requirements defined in the subsurface evaluation report willbe routinely implemented during construction. In today’s fast-pacedconstruction, it is not unusual to see the forms removed the followingday, or long before the concrete has achieved its specified 28-day com-pressive strength f ′

c. Backfill placement could be started immediately



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thereafter. Therefore, the engineer of record (EOR) must be specificin defining any special design and construction, including formwork,requirements.

Thus, as the concrete strength and backfill procedures are criticalto the various phases of construction, strict quality control measuresmust be put in place to ensure that the correct concrete strength isachieved before any backfill activities commence. Site inspection andtesting will be necessary to ensure proper compaction or soil remedi-ation.

It is possible that the site soils are unsuited for backfill, and so suit-able soils must be imported. It is also possible that topsoils containingorganics and other deleterious material could get mixed with other sitesoils during backfilling around the footing. Some states have definedcertain soils or soil mixtures as “select fills” that are recommended forspecific structural fills or backfill. All of these issues are important infoundation design, and the necessary quality control steps should betaken before starting any construction activity.

Vertical Versus Sloped ExcavationsSome foundation contractors prefer making footing excavations nolarger than is required to place the footings. Where possible, theseexcavations are vertical unless they are deep enough that OSHA reg-ulations mandate them to be sloped. Unless backfill compaction isclearly specified and required by explicit notes on drawings, it is pos-sible that backfill compaction in these excavations will not occur. Ifcompaction of soils within the 25-degree to 45-degree wedge was alsoincluded in lateral stability consideration, those soils would not becompacted if the excavations are vertical.

Therefore, it is crucial to clearly define all requirements for excava-tion, backfill, backfill compaction, moisture content, and dewateringwhere necessary. Curing procedures, minimum concrete strength be-fore backfill can be placed and/or compacted, and the extent of com-paction beyond the footings must be defined as well. These require-ments must be delineated precisely by concise notes on the foundationdrawings.

Backfill CompactionGeotechnical engineers generally specify compaction in terms of max-imum thickness of loose lifts and standard proctor maximum dry den-sity unit weights per ASTM D698 or modified proctor maximum drydensity tests per ASTM D1557 (see Annual Book of ASTM Standards).These requirements are usually specified to be 95 to 98 percent ofthe maximum dry density; even higher percentages are specified forsubgrade compaction. Soil compaction is accomplished by the use ofhand tampers and sheepsfoot or pneumatic rollers.



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Backfill compaction not only improves lateral and uplift stabil-ity by improving soil shear strength, but it also reduces permeabilityin cohesive soils. In cohesionless soils, surface drainage must be ac-complished by proper grading to avoid basin or boat effects aroundfoundations. A typical compaction note reads:

Backfill material should be placed in 6 to 8 in. maximum loose lifts andcompacted to at least 95% of the Standard Proctor maximum dry density(ASTM D698).

As a rule, for water tank foundations, a minimum unit weight of100 pcf (15.7 kN/m3) should be achieved. Rock fragments and stoneslarger than 3 in. (7.5 cm) in diameter should not be used in the vicin-ity of the footings. Any soft or loose material should be removedfrom the bearing areas before concrete is placed. Thus, it is recom-mended that a geotechnical representative be present before and dur-ing the pouring of the foundations as well as during placement ofbackfill.

Water- and Moisture-Control MeasuresDesign in areas of high water table should consider buoyant unitweights of soil and concrete for lateral and vertical stability. If thewater table is likely to be encountered during excavation or construc-tion, proper measures must be taken to dewater the bearing areas toa minimum depth of 3 ft (0.9 m) below the bottom of the founda-tion. When rainfall is imminent or when the excavation must remainopen overnight, a 4- to 6-in. (10- to 15-cm) mud mat of lean concrete(2,000 psi [13.8 MPa]) should be poured over the bearing soils, with thetop of concrete being at the required bearing elevation. If water doesenter the excavations or if unsuitable soils are encountered, softenedsoils should be completely removed and excavation brought back tobearing grades with a mud mat or no. 57 stone. The geotechnical en-gineer should approve this activity.

It is best to maintain soil moisture content as close to (within 2to 3 percent of) the optimum moisture content. Plastic soils are notideal for fill or backfill, but when the geotechnical engineer approvestheir use, they should be placed with a higher moisture content of±5 percent of the optimum. These levels of moisture content facilitatecompaction and help accomplish the desired unit weight.

Shrink/Swell SoilsExpansive soils and challenges associated with shrink/swell soilswere discussed earlier in the chapter in the section on design con-siderations in plastic soils. Expansive soils are generally plastic clays,also known as fat clays, that swell with increases in moisture content.



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U

U U

FIGURE 5-12 Typical foundation in shrink/swell soils. (Uw = vertical uplift dueto wind; Us/w = vertical uplift due to shrink/swell.)

They are classified as CH clays in the ASTM’s Unified Soils Classi-fication Chart. The depth of the expansive soil active zone can varyfrom a few feet or less than a meter to perhaps more than 15 ft (5 m).Foundations constructed in these soils can potentially be subjected tovery large uplift and possibly destabilizing forces if they are not de-signed properly. Refer to Fig. 5-12 for a typical foundation in plasticzone with an effective active zone of 12 ft (3.7 m).



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The plasticity index (PI) is generally used as a measure of swellpotential in plastic soils. Soils with low swell potential are those withPI values below 25; a PI value of 25 to 35 indicates moderate swell po-tential; PI values exceeding 35 correspond to soils with very high swellpotential. Foundations for elevated water tanks sited on soils with lowswell potential can be constructed using the standard practices, butfoundations sited on soils with PI values exceeding 25 (see ASCE 7-05or IBC-2006) require precaution, remedial action, and special designconsiderations (Das 2006).

Typically, when placing foundations in plastic soils, geotechnicalengineers recommend bearing the footing deep in nonplastic soils orbelow the active zone. To mitigate the effects of the uplifting forces,the use of select structural fill and backfill, soil stabilization with lime,and/or inclusion of clearly defined uplift forces in the design havebeen recommended. Das (2006) provides recommendations for reme-dial measures as well as a procedure for estimating the uplift forcescaused by the swelling forces.

Typical remedial options often recommended for shallow footingsinclude the following:

1. Bear the footing below the active zone and replace the backfillwith select structural fill.

2. Bear the footing in the active zone, replace 3 to 5 ft (0.9 to 1.5 m)of soil below the footing with select structural fill, and useselect structural fill for backfill.

3. Use site soils for backfill, but ensure that soil moisture contentis greater than the plastic limit and that the moisture contentis 3 to 5 percent above the optimum moisture.

4. Use a polyethylene or bitumen material on the vertical facesof the footing.

5. Same as point (2), but use site soils for backfill with lime mix-ing.

6. Same as point (1) or (2), but use site soils for backfill, andconsider the uplifting forces in the design of the footing and inits stability. Use J-voids where necessary to allow room for thesoil to swell without imposing any forces on the foundationelement.

7. In soils where piers are recommended, geotechnical engineersshould recommend belled piers, with emphasis on the rein-forcement requirements for resisting the uplift forces at thejunction of the shaft and the bell.

In summary, when site soils involve clays prone to swelling, spe-cific geotechnical guidance must be sought. The geotechnical report



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must clearly define the active zone, the potential for swell, uplift,or adhesion forces that need to be considered in design, the bearingdepth at which the foundations are to be placed, and suitability ofsite soils for backfill or recommendations for imported soil. If im-ported soils are to be used for backfill, clear criteria must be providedregarding the nature of the soil, its Atterberg limits, compaction re-quirements, and guidance on local availability of the recommendedsoils.

ConclusionFoundations are critical to the design, construction, operation, andperformance of welded-steel tanks for water storage. Therefore, foun-dation design and construction require attention to detail and properunderstanding of all criteria and requirements.

Sites that are relatively dry, level, and easily accessible and thathave good soils properties are ideal locations for erecting elevatedwater tanks. The suitability of sites must always be established bya qualified geotechnical engineer. Grade elevations and site bound-aries must be established carefully to achieve the proper overflow andfoundation elevations.

Geotechnical investigation reports must provide all the neces-sary information for design. This includes detailed soil propertiesand other characteristics defined in this chapter. Certain soils exhibitshrink/swell or other characteristics that require extra measures andprecautions in design. All of these characteristics must be clearly de-fined and appropriately addressed in the report. Site classificationand settlement evaluation must also be included in the geotechnicalreport.

Generally, isolated spread footings or shallow foundations arethe most economical foundation type when suitable to site condi-tions. Otherwise, deep foundations are necessary. Detailed criteriaare provided herein to assist the designer in selecting the most suit-able foundation type and to assist the designer with the design, be itshallow footings or deep foundations using piles and drilled piers. Inregions of high seismic activity, special design requirements apply thatmust be incorporated into the foundation design. Both the logic andphilosophy for these requirements are explained in this chapter, andfurther resources are provided in the bibliography at the end of thechapter.

Requirements for the quality control, mixing, placing, finishing,and curing of concrete have also been defined here. These require-ments are critical, as they govern the strength, durability, and work-ability of concrete foundations. Also, criteria have been introduced



Foundations


for sizing anchor bolts and for allowable bearing capacity under baseplates.

Requirements for backfill compaction and lateral and uplift sta-bility are defined. It is further emphasized that to ensure safety, allexcavations must be performed in full compliance with the latestOSHA construction standards.

Foundation Design Example

Problem StatementTo illustrate the design of a shallow foundation, the loading resultingfrom the analysis of a typical 500,000-gal (1,893-m3) elevated torus-bottom water tank will be considered. The tank has a diameter of50 ft (15.24 m), head range of 37 ft (11.28 m), and a high water line of116 ft (35.4 m). It is supported by six columns, similar to the tank shownin Fig. 5-1. The design service loads on the footing are as follows:

Vertical Loads Horizontal Loads

Dead load D = 37.0 kip

Water load F = 520.0 kip

Snow load S = 7.0 kip

Wind load W = ±105.0 kip Wind shear WS = 30 kip

Seismic load E = ±142.0 kip Seismic shear ES = 28 kip

Assume the live load to be zero, and assume that the wind loadhas been reduced by a directionality factor so that the 1.6 load factorapplies.

Use a net allowable bearing pressure of 3,000 psf (144 kPA) at aminimum embedment depth of 5.5 ft (1.68 m) below existing grade anda concrete compressive strength f ′

c of 4,000 psi (27.58 MPa). Assumethe pedestal to be 4 ft × 4 ft (1.2 m × 1.2 m) with a 1-ft (0.3-m) projectionabove grade. Refer to Fig. 5-13.

Footing Design

Step 1: Governing Load CombinationsThe load combinations were defined in Equations (5-1) through (5-7).A quick examination of these equations reveals that only load combi-nations (5-1), (5-4), (5-5), and (5-6) are governing. After simplification,these equations are as follows:



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Pedestal width pw

Footing width B

Em

bedm

ent depth

d2

d

t d

Flexuralshear

Punchingshear

V

U

Footingexposure

Grade

FIGURE 5-13 Shallow footing example.

Vertical Loads

U1 = 1.4 (D + F ) = 780 kip (5-1)

U4 = 1.2 (D + F ) + 1.6 W + 0.5 S = 840 kip (5-4)

U5 = 1.2 (D + F ) + (1.4 × 1.0) E + 0.2 S = 869 kip (5-5)

U6 = 0.9 D + 1.6 W = −135 kip (5-6) (uplift)

Corresponding Horizontal Loads

V 1 = 0.0 (5-1)

V 4 = 1.6 WS = 1.6 × 30 = 48 kip (5-4)

V 5 = (1.4 × 1.0) ES = (1.4 × 28) = 39 kip (5-5)

V 6 = 1.6 WS = V 4 = 48 kip (5-6)

Step 2: Size Footing Using Service LoadsCorresponding to the load combinations in Step 1, as per ASCE 7-05Section 2.4.0, the service (vertical) loads are as follows:

Service U1 = (D + F ) = 557 kip (5-1)

Service U4,Wind = (D + F ) + W = 662 kip (5-4) (governs wind)

Service U5,Seismic = (D + F ) + EService = 699 kip (5-5) (governs seismic)

Service U6,Wind = 0.6D + W = −83 kip (5-6) (uplift)



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Required footing area:

AWind = 662 kips3000 psf

= 220 ft2 (governs)

ASeismic = 699 kips1.33 × 3000 psf

= 175 ft2

AWind governs and a square footing of 15 ft × 15 ft (4.6 m × 4.6 m)provides the required bearing area. The net bearing pressure at thetoe of the footing is

fbearing-Wind = 662 kips

152 ft2 + 30 kips × 6.5 ft

(15 × 152/6) ft3

= 3,290 psf < 4,500 psf (using FS = 2.0)

fbearing-Seismic = 699 kips

152 ft3 + 28 kips × 6.5 ft

(15 × 152/6) ft3

= 3,430 psf < 4,500 psf (using FS = 2.0)

Although the increase in the net overbearing pressure resultingfrom the differential weight of concrete is not yet accounted for, theinitial size selected is reasonable.

Step 3: Check Punching Shear at a Distance d/2 from the PedestalPunching shear is checked at a distance d/2 from the face of thepedestal as shown in Figs. 5-13 and 5-14a . The maximum punchingshear is caused by the load U5 of 869 kip (3.87 MN) and the factoredweight of the pedestal. Assuming a depth d of 17 in. (43.2 cm) for theslab, the critical perimeter is given by

b0 = 4(pw + d) = 4 × [4 ft × 12 (in./ft) + 17 in.] = 260 in.

where pw is the width of the square pedestal.For a bearing depth of 5.5 ft (1.68 m) and a slab thickness t of ap-

proximately 20 in. (50.8 cm), the pedestal height will be 4.83 ft (1.47 m).The factored weight will be

Dpedestal = 1.2 × (4 ft × 4 ft × 4.83 ft) × 0.144 (kip / ft3) = 13.36 kip,

which results in a new factored U5 of 882 kip.



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d2

Footing width B

Critical perimeter = 4(pw + d )(square footing)

Pedestal width pw1

Pedesta

l w

idth

pw

2

d2

d2

d2

(a)

Footing w

idth

B

Footing width B

Pedestal width pw d

(B – pw)

2– d

(b)

FIGURE 5-14 Design shear and bending moment evaluations: (a) punchingshear, (b) flexural shear, and (Continued)



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Footing w

idth

B

Footing width B

Pedestal width pw

(B – pw)

2

(c)

FIGURE 5-14 (Continued) (c) flexural bending.

The nominal punching shear capacity as per ACI 318-05 Section11.12.2.1 is

Vc = 4√

f ′c.b0.d = 4 ×

√3, 500 × 260 × 17 = 1,046 kip

Here, f ′c is reduced by 500 psi (3.45 MPa) for reasons described in the

section on structural concrete. Assuming no contribution from slabreinforcement, using a shear reduction factor of 0.75 as per ACI 318-05 Section 9.3.2.3, the nominal punching shear capacity is

�Vn = 0.75 × 1,046 = 784 kip

The punching shear caused by U5 is

Vu = 882 kips

152 ft2 [152 ft2 − (4 + 1.42)2 ft2) = 767 kip



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Since �Vn is greater than Vu kip, a slab depth d of 17 in. (43.2 cm)satisfies punching shear requirements.

Step 4: Check Flexural (Beam) Shear at a Distance dfrom PedestalFlexural shear will be checked at a distance d from the face of thepedestal, as shown in Figs. 5-14b and 5-15. The bearing pressures for

pw

d

(Concrete—pedestal)

(Concrete—slab)

(soil)

W3

3,7

40 p

sf (1

79 k

Pa)

3,9

20

psf

(18

8 k

Pa

)

4,0

40 p

sf (1

93 k

Pa)

4,1

25 p

sf (1

98 k

Pa)

4,3

70

psf

(20

9 k

Pa

)

}450 psf

(22 kPa)

B

B/2

W1

W2

pw

2

pw

2d + (B – pw)

2

FIGURE 5-15 Flexural shear and bending moment evaluation. (psf = poundsper square foot, kPa = kilopascal.)



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the factored wind and seismic load combinations are as follows:

fbearing-Wind = (840 + 13.36) kips

152 ft2 ± 48 kips × 6.5 ft

(15 × 152/6) ft3 = 3.79 ± 0.55

= 4,347 (psf) maximum < 9, 000 psf (ultimate bearing)

= 3,238 (psf) minimum

fbearing-Seismic = (869 + 13.36) kips

152 ft3 + 39 kips × 6.5 ft

(15 × 152/6) ft3 = 3.92 ± 0.45

= 4,370 (psf) maximum < 9,000 psf (ultimate bearing)= 3,470 (psf) minimum

The bearing stress due to seismic loading governs. From Figs.5-14b and 5-15, the bearing pressure at a distance d from the face ofthe pedestal is

fbd = 3, 920 + (2 + 17/12)(15/2)

(450) = 4, 125 psf

The resulting flexural shear at the same location is

VFlex = 12

(4, 125 + 4, 370)(15)[

(15 − 4)2

− 1712

]= 260 kip

The flexural shear capacity of the footing slab as per ACI 318-05Section 11.3.1.1 is

Vcf = 2√

3, 500(15 × 12)(17) = 362 kip

Vnf = 0.75 × 362 = 272 kip

Since Vnf > VFlex, the 17-in. (43.2-cm) depth selected is adequatefor shear.

Step 5: Determine Required Flexural ReinforcementThe bearing pressure at the face of the pedestal is (Figs. 5-14c and 5-15)

fbp = 3, 920 + (4/2)(15/2)

(450) = 4,040 psf

The resulting ultimate bending moment at this location is

Mu =(

12

)(4, 040)

(15 − 4

2

)2

(15)+(

12

)(4, 370 − 4, 040)[(

23

) (15 − 4

2

)2

(15)

]Mu = 967 ft · kip



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Determine the reinforcement needed to carry this bending mo-ment. The minimum reinforcement required as per ACI 318-05 Section10.5.1 is

As, min = (200)(15 × 12)(17)(60,000)

= 10.20 in.2

Try a steel area of 14.0 in.2 (PCA 1999):

� =[

14(15 × 12)(17)

]= 4.58 × 10−3

�Mn = �As fyd[

1 − 0.59 · �fy

f ′c

]�Mn = (0.9)(14)(60)(17)

[1 − 0.59(4.58 × 10−3)

603.5

]�Mn = 1,021 ft · kip

�Mn > Mu

Therefore, fourteen no. 9 bars each way, 14.5 ft (4.42 m) long, willsuffice.

Step 6: Reinforcement for the PedestalACI 318-05 Section 15.8.2.1 requires a minimum pier reinforcementof 0.005Ag , where Ag is the gross area of the pier across the interface.Thus,

Aspedestal = (0.005)(48 × 48) = 11.52 in.2

As noted in the section on design of isolated spread footing, for theshort pedestal, this reinforcement area can be reduced per industrypractice. But this is a matter of decision by the designer. If this rein-forcement is to be maintained, sixteen no. 8 bars will provide 12.64 in.2

(81.55 cm2).Under the combined action of the 135-kip uplift U6 and the 48-kip

shear V6, the resulting stress in the concrete pedestal will be

�ped = 135(4 × 4)

+ (48 × 4.83)(4 × 42/6)

= 210 psi

This stress is less than the concrete modulus of rupture fr where,as per ACI 318-05 Section 9.5.2.3,

fr = 7.5√

3500 = 444 psi

As per ACI 318-05 Section 11.5.6.1, assuming a 3-in. (7.62-cm) coverfor the pedestal dowels, since

0.5�Vc = 0.5(0.75)[2 ×√

3500(48)(45)] = 96 kip > U6 = 48 kip,



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no shear reinforcement is needed. However, since the dowels will bein compression due to the other load combinations, No. 4 ties at aspacing of 12 in. (30.5 cm) on center are recommended.

Step 7: Check Uplift StabilityAWWA D100 requires the concrete weight along with the weight ofthe soils directly above the footing to be greater than the service loaduplift. The weight of the concrete (W1 + W2 = 65 kip) and the soil (W3

= 80 kip) amounts to 145 kip total, which is in excess of the 105-kipwind uplift. Refer to Fig. 5-15. Therefore, uplift stability is maintained.

Other StepsThe anchor bolts should be designed for tension and shear interac-tion. Lateral stability should be checked on the basis of the active andpassive pressures and the cohesion, if any, of the backfill soils. Backfillcompaction is a function of the stability requirements. A compactionto 95 percent standard proctor maximum dry density, as discussedin this chapter, may be recommended. The embedment depth can beadjusted, if necessary, to achieve additional passive resistance.

Refer to Fig. 5-16 for the reinforcing details. Note that, in gen-eral, strain compatibility must be checked to ensure that a balancedcondition prevails and that the footings are not over-reinforced. Also,note that a nominal top mat reinforcement can be added as requiredfor uplift or shrinkage control.

4 ft (1.2 m)

15 ft (4.5 m)

1 ft(0.3 m)

16 #8 dowels

#4 ties @ 12 in. (305 mm)centerline to centerline

14 #9 each way

No. 5'S–nominal–each way (only if required

for uplift or shrinkagecontrol)

Anchor bolt

5.5

ft

(1.6

5 m

)

20 in

. (

508 m

m)

FIGURE 5-16 Shallow footing design example.



Foundations


BibliographyAmerican Institute of Steel Construction (AISC). 1989. Manual of Steel Con-

struction, Allowable Stress Design. 9th ed. Chicago: AISC.American Petroleum Institute (API). 2008. Design and Construction of Large,

Welded, Low-Pressure Storage Tanks, 11th ed. API Standard 620. Washington,D.C.: API.

American Society of Civil Engineers (ASCE). 1993a. Bearing Capacity of Soils.Technical Engineering and Design Guides as Adopted From the U.S. Army Corpsof Engineers, No. 7. New York: ASCE Press.

American Society of Civil Engineers (ASCE). 1993b. Design of Pile Foundations.Technical Engineering and Design Guides as Adopted From the U.S. Army Corpsof Engineers, No. 1. New York: ASCE Press.

Bowles, J. E. 1995. Foundation Analysis and Design. 5th ed. New York: McGraw-Hill.

Das, B. M. 2006. Principles of Foundation Engineering. 6th ed. Florence, KY: CLEngineering.

Deep Foundations Institute (DFI). 1990. Augered Cast-in-Place Piles Manual. 1sted. Englewood Cliffs, NJ: DFI.

Kosmatka, S. H., and W. C. Panarese. 1990. Design and Control of Concrete Mix-tures. 13th ed. Skokie, IL: Portland Cement Association.

Liu, C., and J. B. Evett. 1987. Soils and Foundations. 2nd ed. Englewood Cliffs,NJ: Prentice-Hall.

National Earthquake Hazards Reduction Program (NEHRP). 2003. NEHRPRecommended Provisions for Seismic Regulations for New Buildings and OtherStructures (FEMA 450). Part 1: Provisions. Washington, D.C.: NEHRP.

Nilson, A. H., D. Darwin, and C. Dolan. 2004. Design of Concrete Structures.New York: McGraw-Hill.

Peck, R. B., W. E. Hanson, and T. H. Thornburn. 1974. Foundation Engineering.2nd ed. New York: John Wiley & Sons.

Portland Cement Association (PCA). 1999. Notes on ACI 318–99 Building CodeRequirements for Structural Concrete, with Design Application. 7th ed. Skokie,IL: PCA.

Prakash, S., and H. D. Sharma. 1990. Pile Foundations in Engineering Practice.New York: John Wiley & Sons.

Reese, L. C., and M. W. O’Neill. 1988. Drilled Shafts: Construction Proceduresand Design Methods. US Department of Transportation, Federal HighwayAdministration FHWA-HI-88-042, ADSC-TL-4. McLean, VA: US Depart-ment of Transportation Federal Highway Administration; and Dallas, TX:ADSC, the International Association of Foundation Drilling.

Sarhan, H. A., M. W. O’Neill, and S. W. Tabsh. 2004. Structural Capacity Reduc-tion for Drilled Shafts with Minor Flaws. ACI Structural Journal 101(3):291–297, May/June.

Smith, G. N., and E. L. Pole. 1981. Elements of Foundation Design. New York:Garland STPM Press.

Terzaghi, K., and R. B. Peck. 1967. Soil Mechanics in Engineering Practice. NewYork: John Wiley & Sons.

Woodward, R. J., W. S. Gardner, and D. M. Greer. 1972. Drilled Pier Foundations.New York: McGraw-Hill.

Wyllie, D. C. 1992. Foundations on Rock. New York: E. and F. N. Spon, an imprintof Chapman and Hall.



Foundations

C H A P T E R 6Construction of

Welded-SteelWater-Storage

Tanks

Jim Noren, P.EAdvance Tank Construction

Donita Fredricks, P.E.CB&I Constructors

Steel FabricationTank constructors have developed specialized equipment, tools, andprocedures for the construction of ground storage tanks and elevatedwater tanks. In the construction of nearly all steel-welded tanks, thesteel components are fabricated in a shop environment and shippedto the tank site, where the fabricated components are fit and weldedinto the finished tank by field construction crews. Steel plate layoutsare developed by the constructor, which minimizes welding and max-imizes the use of the ordered plate, with consideration to the size andweight restrictions for shipping. For the composite elevated tank, spe-cialized forms and equipment have been developed and are used inthe construction of the concrete components.

MaterialPlate material may be purchased from a steel warehouse or directlyfrom a steel mill. Steel warehouses stock plate material in most of the

227




228 C h a p t e r S i x

grades used in welded-steel tanks. Material that is required to con-form to supplementary requirements (e.g., silicon-killed, fine-grainpractice, normalized, or ultrasonically inspected material) is gener-ally not available from a warehouse.

Material Purchased from Steel WarehouseSteel plate from a warehouse is typically available in standard platewidths ranging from 48 to 96 in. (1.2 to 2.4 m) in 12-in. (0.3-m) in-crements. Plates are normally stocked in 20-ft (8.8-m) lengths, thoughsome warehouses have the capability to cut coiled plate to length.Stock plates commonly used in welded-steel water-storage tanks areavailable in the following thicknesses: 3/16 in. (4.8 mm), 1/4 in.(6.3 mm), 3/8 in. (9.5 mm), and in 1/4-in. (6.3-mm) increments forthicknesses between 1/2 in. (12.7 mm) and 2 in. (51 mm). Stock mate-rial grades, sizes, and thicknesses vary from warehouse to warehouse,so availability of a specific plate size and thickness must be verified inthe design phase of the project. Delivery time for warehouse materialis shorter and minimum tonnages normally do not apply, but the costis higher than for material purchased directly from the mill.

Material Purchased from a Steel MillPlate material purchased from a mill may be ordered to the customer-specified width, length, and thickness. Material conforming to specificsupplementary requirements is available from most mills. Steel millstypically require a minimum order, and delivery times are signifi-cantly longer than for warehouse-purchased material.

Regardless of whether a plate is purchased from the warehouse ormill, conformance to the American Society for Testing and Materials(ASTM) requirements for the ordered plate should be confirmed bythe constructor on receipt of the material. This can be accomplished byreviewing material test reports or certificates of compliance furnishedby the supplier. If the plate cannot be traced to a material test reportor a certificate of compliance, testing by a qualified testing laboratorymay be used to verify that the plate complies with the chemical andmechanical requirements of the specified ASTM standard.

On receipt of the plate, measurements should be taken to verifythat its width, length, and thickness are consistent with the orderedplate size. Permitted variations in dimensions are outlined in ASTMA6. Visual examination of the plate should be performed to verify thatthe material is free from injurious defects and has a workable finish.Thick plate should also be checked along the edge for lines that wouldindicate a possible lamination.

CuttingSeveral methods are available for cutting plates to size in the shop,including thermal cutting by either oxy-fuel gas torches or plasma



Construction of Welded-Steel Water-Storage Tanks

229C o n s t r u c t i o n o f W e l d e d - S t e e l W a t e r - S t o r a g e T a n k s

FIGURE 6-1 Cutting plate by oxy-fuel torches.

arc. Thermal cutting methods are commonly used in the shop forirregularly shaped plates (Fig. 6-1). Oxy-fuel gas torches may be au-tomated by either setting up a track burner or by use in a numericallycontrolled burning bed. Plasma arc cutters are used in a numericallycontrolled burning bed. Using either method, the plate may be simul-taneously cut to size and the edges tapered and beveled for weldingby using multiple burning heads. The finished edges should closelyfollow the detailed plate dimensions to ensure good fit-up in the field.The edges should be uniform and smooth and cleaned of slag accu-mulation when necessary.

Machining and shearing are other methods of cutting plates tosize. Rectangular plates that are ordered with minimal trim allowancemay be trimmed and squared by machining the edges using an edgeplaner. Shearing is another option for straight edges that are less thanthe width of the constructor’s plate shear. American Water Works As-sociation (AWWA) Standard D100 limits plate thicknesses for shearingto 1/2 in. (12.7 mm) or less if the joint is to be butt welded. Edges thatwill be lap welded are limited in thickness only by the capacity ofthe plate shear. Sheared edges should be square and burrs removedbefore welding.

FormingSingle-curvature plates for welded-steel tanks are typically cold rolledin the shop to the appropriate curvature using a plate roll. Plate widths





Plate Thickness

Minimum Maximum

(in.[mm]) (in.[mm]) Minimum Plate Diameter (ft[m])

>3/8 [>9.5] ≤3/8[≤9.5] 40 [12]

>1/2 [>12.7] 1/2 [12.7] 60 [18.2]

>5/8 [>15.8] 5/8 [15.8] 120 [36.6]

Must be rolled for all diameters

TABLE 6-1 Minimum Diameter for Plates Not Rolled

may be limited by the fabricator’s plate roll capacity. AWWA D100makes provisions for plates that need not be rolled on the basis of theminimum diameter and plate thickness as outlined in Table 6-1. Single-curvature plates are frequently used to construct a double-curvaturesurface if the radius is large enough. One example of this is pie-shapedplates in a dome roof.

Double-curvature plates are cold pressed using repeated blowswith a mortar-and-pestle-shaped die (Fig. 6-2). Typical examples of adouble-curvature plate include the flare and ball of a pedestal tank.

Press breaks are used to form sharp bends in a plate—for example,the fluted plate in a fluted-column-style tank. Press breaks and pressescan also be used to simulate a rolled plate by repeatedly hittingthe plate with a straight die, allowing for short spaces between hits(Fig. 6-3). This method can also be used to form cone-shaped platesand is particularly useful for thick plate.

Angle rolls are commonly used to roll structural angles for welded-steel tanks. With all forming operations, it is important to have

FIGURE 6-2 Pressing double-curvature plate.





FIGURE 6-3 Forming fluted plates in a press break.

adequate dimensions on the shop drawings to verify the accuracyof the formed plate.

Shop SubassembliesFabricated plate may be subassembled and welded in the shop(Fig. 6-4). This is done to maximize the welding that can be performed

FIGURE 6-4 Shop assembly of cut and formed plates for a dome roof.





in the controlled environment of the shop using optimal welding pro-cesses and configurations. Shipping restrictions dictate the extent ofshop subassembly that can be executed.

When plate sections are sized such that the shop-welded sub-assemblies are not an option, complete or partial shop assembly maystill be warranted to ensure proper fit-up in the field. This is especiallyuseful for complex geometries with double-curved surfaces.

Blast and PrimeThe life of a coating system depends on the surface preparation. Asmooth, regular surface with the proper steel profile will provide agood basis for the application of a protective coating system. Thefabricator should ensure that weld contours are smooth and that anyunacceptable weld undercutting is eliminated. Weld flux and weldspatter should be removed and the sharp edges ground smooth.

Most tank constructors recommend that tank components be abra-sive blast cleaned and primed in the climate-controlled atmosphere ofthe shop. Exterior surfaces and interior dry surfaces should be cleanedto a commercial finish per Society for Protective Coatings (SSPC) SP6as a minimum. Inside wet surfaces should be cleaned to a Near WhiteBlast finish per SSPC SP10. Blast-cleaned surfaces should have a sur-face profile that is appropriate for the specified primer and coatingsystem per recommendations of the coating manufacturer.

Blasting may reveal small laminations or pitting in the plate sur-face not previously apparent. If these imperfections are large enoughto produce holidays in the coating system, they should be removedby grinding. Occasionally, deeper laminations may require weldingor further testing.

The prime coat should be applied immediately after surface prepa-ration, before the occurrence of any surface rusting or accumulation ofdust or moisture. The type and thickness of primer should be definedin the customer’s specifications. AWWA D102 Coating Steel Water-Storage Tanks may be referenced for interior and exterior coating sys-tems. Prime coats may be applied using any method recommendedby the coating manufacturer, except that rolling should not be used forthe prime coat on interior wetted surfaces unless required for roughpitted surfaces. An unpainted margin approximately 4 in. (102 mm)wide should be provided around all plate edges that will be fieldwelded.

ShippingShipping from the shop to the job site is almost exclusively by truck(Fig. 6-5). The current weight limit is 80,000 lb (36,287 kg) gross forthe truck, trailer, and load, resulting in a net load capacity of approx-imately 45,000 lb (20,412 kg), depending on the weight of the truck





FIGURE 6-5 Shipping formed plates by truck.

and trailer. Rules for oversized loads vary from state to state. Specialpermitting, routing, and escorts may be an option when oversizedloads cannot be avoided or are deemed to be more economical. Platelayouts are often dictated by shipping limitations. Site access shouldalso be considered in planning shipping loads.

Material should be sufficiently blocked, braced, and tied down tosecure the components to the trailer and maintain the fabricated shapeduring shipping.

WeldingIn the 1950s, welding replaced riveting as means of connecting tankjoints. Welding can be performed in all climates and in a variety of po-sitions. Over time, the technology has improved, leading to increasesin productivity. To convey the correct welding information from thetank designer, weld symbols in accordance with AWS Standard Sym-bols for Welding, Brazing, and Nondestructive Examination shouldbe used on the fabrication and erection drawings.

Welding ProcessesThe primary welding processes used in the shop and field are shieldedmetal arc welding (SMAW), submerged arc welding (SAW), and fluxcored arc welding (FCAW). All are arc welding processes that usean electric arc generated by an electric current between the tip of theelectrode and the base metal. Heat from the arc melts the electrode





and adjacent base metal which then combines, cools, and solidifies toform the weld bead.

Welding may be performed manually, semiautomatically, auto-matically, or by machine welding. Manual welding requires the welderto manually maintain the proper positioning and arc length and re-place the electrode as it is consumed. Semiautomatic welding is per-formed with a handheld gun that continuously feeds the electrodeand flux. Automatic welding is accomplished with equipment that iscapable of performing the welding operation without a welding op-erator. This type of welding is more commonly used in assembly lineoperations. In machine welding, specialized equipment performs thecomplete welding operation; however, the welding equipment mustbe monitored by a qualified operator who is responsible for position-ing the steel components, starting and stopping the weld, setting thespeed, and adjusting the controls.

Shielded Metal Arc WeldingSMAW, also referred to as stick welding, utilizes a stick electrode—typically 9 to 18 in. (229 to 457 mm) long—a solid metal wire core thatconducts electric current and provides filler metal for the joint. Themetal core is coated with a material that provides arc stability and ashielding gas or a flux coating as the electrode is consumed. Shieldinggases are needed to eliminate oxygen from the molten weld metal.The fluxing agents allow the molten metal to wet the surfaces of thebase metal and remove impurities from the weld metal.

SMAW is one of the most versatile weld processes and is widelyused in tank construction. The equipment is relatively simple andportable. SMAW can be used in a wide range of positions and in areaswith limited access. It is also less sensitive to wind than either SAW orFCAW. Shielded metal arc welding is limited to manual welding and,consequently, has one of the lowest deposit rates. The electrodes arerelatively short and frequent stops are required to replace them. Whenpresent, slag must be removed before restarting. As a result, SMAWis the least efficient welding process for long production welds.

Submerged Arc WeldingThe electrode for SAW is a continuous bare wire inserted into a wire-feeding mechanism that automatically feeds the electrode toward thejoint at a controlled rate. The weld is submerged in a blanket of gran-ular flux that is continuously deposited ahead of and around the elec-trode. During welding, some of the granular flux is melted and servesthe same purpose as the electrode coating in SMAW welding. Thisweld process can be used in a semiautomatic, automatic, or machinemode.

SAW has one of the highest deposit rates because of the continuouswire feed. The process is limited by joint position and accessibility.





SAW is typically limited to the flat position for butt welds and theflat and horizontal positions for fillet welds. With specialized equip-ment to contain the flux, SAW may be used for lap and butt jointsin the horizontal position. Moving the bulky wire-feeding mecha-nism for the continuous-feed electrode may make SAW a less desir-able option for inaccessible areas. Good joint fit-up is also critical forSAW.

Flux Cored Arc WeldingLike SAW, flux cored arc welding is a continuous-feed wire weldingprocess. The electrode has a center core of flux encased in a tubularmetal sheath. Two types of FCAW exist: gas shielded and self shielded(Fig. 6-6). Gas-shielded FCAW uses a gas envelope, usually CO2 orargon/CO2, to protect the molten metal from the air. This method isnot suitable for use when the weld cannot be protected from the wind.For self-shielded FCAW, shielding is provided by gas emitted by theflux as it vaporizes and by a slag blanket that covers the molten metal.Self-shielded FCAW is no more sensitive to wind than SMAW, so it iscommonly used in the field.

FCAW is a semiautomatic process in which a handheld weld gunis used. Deposition rates are typically higher than for SMAW but lowerthan for SAW. FCAW is versatile, in that it can be used in all positionsfor all the basic joint types. Like SAW, FCAW requires moving thewire-feeding mechanism; consequently, it may not be the best optionfor inaccessible areas in the field.

FIGURE 6-6 Shop flux cored arc welding.





Weld Procedure Specification and ProcedureQualification RecordConstructors are required to develop weld procedure specifications(WPS) that define the welding parameters to be used in the fabricationand erection of the tank. Each WPS must be qualified in accordancewith the rules in American Society of Mechanical Engineers (ASME)Boiler and Pressure Vessel Code, Section IX, or American NationalStandards Institute (ANSI)/AWS B2.1, Standard for Welding Proce-dure and Performance Qualification. To qualify a WPS, the constructorwelds test coupons and tests the specimens per ASME Section IX orANSI/AWS B2.1. The weld parameters and test results are recordedin a document known as a procedure qualification record (PQR). Theconstructor is required to certify that he or she has qualified each WPSwith a PQR. Constructors maintain standard WPSs and PQRs for weldparameters that are routinely used.

The constructor may elect to use an ANSI/AWS standard weldingprocedure to justify a weld in lieu of performing an independent qual-ification. If this option is selected, it is necessary to comply with allthe rules in AWS B2.1 that govern the use of the ANSI/AWS standardwelding procedure.

Welder Qualification and IdentificationWelders in both the shop and the field are required to demonstratetheir ability to perform acceptable welds. Weld testing shall be in ac-cordance with ASME Boiler and Pressure Vessel Code, Section IX, orAmerican Welding Society (AWS) B2.1 Standard for Welding Proce-dure and Performance Qualification. The tank constructor is respon-sible for testing all welders for the specific weld processes that theindividual welder will use. Records of the testing dates and test re-sults must be maintained by the tank builder.

Each qualified welder is assigned a number, letter, or symbol thatis stamped on the tank to identify the weld operator employed for eachjoint. The stamp is placed adjacent to and at intervals not exceeding3 ft (0.9 m) along the weld. Alternately, the tank constructor may keepa written record of the welders employed on each joint and omit thestamping. This record must be certified by the tank constructor andincluded in the inspection report when specified by the purchaser.

GrindingSome grinding of welds may be required to ensure that the finishedweld contour is suitable for cleaning and painting and will not be detri-mental to the life of the coating. Grinding should be used to removeweld slag, weld spatter, burrs, and any sharp surfaces along welds.If the purchaser requires special grinding, it should be noted in the





contract specifications, and a recognized standard addressing weldprofiles should be referenced to clearly define the extent of grindingrequired.

When lapped plates are joined with fillet welds that are less thanthe full thickness of the plate, the exposed sharp edge of the plateshould be removed by grinding. This will minimize the potential fora paint failure at the edge of the plate.

Construction

Scheduling IssuesIn addition to the production capacity and workload of the tank con-structor, construction schedules are affected by the style and size of thetank, the availability of material, the time of year, and daily weatherconditions. The purchaser may specify either the number of weeksto completion after award of contract or a set calendar date for com-pletion, or the purchaser may allow the tank builder to propose ascheduled completion. If a specified completion date is critical to theowner, the purchaser may specify liquidated damages to be chargedon a daily basis if the work is not completed on time.

If the purchaser chooses to set a construction schedule, consider-ation should be given to mill delivery time to allow the constructor touse material from the mill. In northern climates, a schedule should bespecified that allows painting to be performed at an appropriate timeof year. Tanks with very short construction schedules are typicallypurchased at a premium.

Weather can significantly influence the field schedule. Wind, ex-treme temperatures, rain, snow, and sleet can affect what work canbe performed and how productive the crew is. Even moderate windscan make it unsafe to pick up and place steel plate.

Weather and temperature conditions must also be appropriate forwelding. Welding is not permitted when the parts to be welded arewet from rain, snow, or ice, or during periods of high wind, unless thewelder and the work are properly protected. No welding is allowedwhen steel is wet. The protection is typically an enclosure to block thewind. Welding is not allowed if the base metal temperature is lowerthan 32◦F (0◦C) unless the base metal is preheated to at least 100◦F(38◦C) through the thickness and maintained for a distance along theweld of four times the thickness of the parts to be welded. If base metaltemperatures fall below 0◦F (−18◦C), welding is not recommended.If welding is performed, low-hydrogen electrodes or low-hydrogenprocesses must be used, and the base metal must be preheated to 200◦F(93◦C) in accordance with AWWA D100.





Unless protection is provided, concrete should not be placed inrain, sleet, snow, or extreme temperatures. American Concrete Insti-tute (ACI) 306.1 and 305.1 provide guidelines for concrete placed incold and hot weather.

Site IssuesThe engineer’s drawings typically show the site layout superimposedon a topographic drawing. This gives some indication of the relativeslope of the site and of potential access problems. If, during the bidstage, questions arise regarding the site, a site visit may be warranted.

AccessThe owner should provide a suitable right-of-way for access fromthe nearest public road to the tank site. The access should be ableto handle a semitrailer tractor rig with a trailer that is 53 ft (16 m)long and that weighs 80,000 lb (36,287 kg) under ordinary weatherconditions. Side clearance needs to be adequate to accommodate themaximum shipping width for the job. The access road should be freefrom underground and overhead obstructions that could be damagedby the truck traffic. A minimum vertical clearance of 14 ft (4.2 m) isrequired.

Site SizeFinal property lines should be located sufficiently far from the founda-tion footprint to permit construction operations. During construction,additional clearance is required for steel delivery, storage, staging, andsubassembly. If a permanent site of adequate size is not available, theowner should provide an adequate temporary construction easement.As a minimum for the construction operations, a site clearance fromthe center of an elevated tank to the site limits should be equal to theheight of the tank. For a ground tank, it is preferable to have at least20 ft (6 m) clear around the entire tank so that a crane can be usedaround the full circumference of the tank.

The site should also be big enough to permit abrasive blastingand painting without impacting neighboring property, both after ini-tial construction and during future recoating operations. Clearancerequirements between the tank and the neighboring property varywith the prevailing wind conditions, type of paint application, andconsequence of damage. Sites should be evaluated on a case-by-casebasis, but as a general rule, a clearance of approximately 100 yd(91.5 m) is suggested. If adequate clearance cannot be provided, itmay be necessary to shroud the tank during initial and future paint-ing operations. Shrouding the tank is costly and should be avoided ifpossible.





DrainageThe tank site should have good drainage during construction. Storage,staging, and subassembly areas should be free from standing water.For sites with poorly draining soils, the bearing surface for a shal-low foundation should be protected from becoming saturated priorto concrete placement.

Power LinesOverhead or buried power lines present a significant safety risk fortank construction. Sites having power lines within 40 ft (12 m) of thetank or tank foundation are unacceptable.

SecurityAccess to the tank should be blocked when the tank is left unattended.If the location is remote or subject to frequent vandalism, additionalmeasures to ensure site security may be required. The additional mea-sures may include fencing and full- or part-time security.

Power During Construction

Power SupplyThe purchaser should indicate whether electrical power is availableat the site. If power is available, the purchaser should indicate thevoltage and whether it is direct current or alternating current (if al-ternating current, what cycle and phase). The specifications shoulddefine who will furnish the power to the site and who is responsiblefor the associated costs.

Tank constructors frequently provide their own power supply inthe form of generators.

Power RequirementsPower requirements in the field vary depending on the types of toolsthat will be used on the job. If the purchaser is furnishing power tothe site, the power requirements should be coordinated with the tankconstructor.

Construction of Welded-Steel Ground Water-Storage Tanks

Anchorage and GroutIf the tank is not anchored, the interface between the tank bottom andthe concrete foundation can be either grouted or filled with the place-ment of asphalt-impregnated cane fiberboard. If the tank is anchored,the use of grout is recommended, since the fiberboard may deteriorateover time. This deterioration would cause vertical movement of thetank, which would require subsequent tightening of the anchor bolts.





Tank-Bottom ConstructionThe bottom of a ground-supported reservoir or standpipe is essen-tially a nonstressed membrane, the purpose of which is to contain theproduct inside the tank and transmit the water-bearing load directlyto the foundation. The minimum thickness of the bottom plate is 1/4in. (6.3 mm); it may be thicker if a corrosion allowance is specified.

The tank bottom should be crowned up from the shell to the cen-ter with a minimum slope of 1 in. (25.4 mm) vertical to 10 ft (3 m)horizontal.

Layout The typical plate layout for bottom plates is a “rect-and-sketch” layout, which refers to rectangular plates with sketch plates atthe outside cut to a radius. The outside radius must be a minimum of2 in. (50 mm) outside of the shell. Additional projection may be pro-vided to compensate for shell out-of-roundness and weld shrinkage inthe bottom welds. With a lap-welded bottom, there will be three platelaps at the corners of the rectangular plates. Three plate laps must beat least 1 ft (0.3 m) from the shell. Refer to Fig. 6-7 for an example of arect-and-sketch bottom layout.

Annular ring An annular ring may be required under two possiblescenarios. First, if the shell uplift due to seismic overturning is large, athickened annular ring will increase the uplift resistance of the shell.This strategy is used to eliminate tank anchorage. For this circum-stance, butt-welded sketch plates may be substituted for an annular

Inside shell

1.5 in. (38 mm) typical

(1 in. [25.4 mm] minimum)

Cut radius 1 ft. (0.3 m)

minimum

1 ft.

(0.3

m)

min

imum

FIGURE 6-7 Typical rect-and-sketch layout for bottom plates.





Shell

Annular plate

2 ft. (0.6 m)

minimum

inside shell

2 in.

(51

mm

)

outsi

de p

rojec

tion

1.5 in. (38 mm) typical

(1 in. [25.4 mm] minimum)

FIGURE 6-8 Tank bottom with annular ring.

ring. Second, if the tank is designed in accordance with AWWA D100Section 14 and is greater than 150 ft (45.7 m) in diameter, an annularring is required.

If an annular ring is provided, the minimum inside projectionshall be 2 ft (0.6 m) or the minimum width required for seismic upliftresistance, whichever is less. The bottom plate is lap welded to theannular ring and has a rect-and-sketch layout. Refer to Fig. 6-8 for anexample of a tank bottom with an annular ring.

Welding sequence The welding sequence for the bottom plates shallminimize out-of-plane distortion. A general sequence for bottomwelding is described as follows:

� Weld the sketch plate to sketch plate joints.� Weld the rectangular short side joints.� Weld the rectangular long side joints.� Weld the rectangular plate to sketch plate welds.

Lap welded versus butt welded Bottom plates can be welded by eitherlap welds or butt welds. For bottom plate thicknesses up to 3/8 in.(9.5 mm), the plates are typically lap welded from the top side only.The weld is a full-thickness fillet weld and, for thicknesses equal toor greater than 5/16 in. (8 mm), the fillet is typically a two-pass weld.If it is necessary to seal the underside of the bottom or if the bottom





Shell plate

Bottom plate

FIGURE 6-9Breakdown oflapped areabeneath shell andprojected outsideof tank.

plate thickness is greater than 3/8 in. (9.5 mm), butt welding of thebottom plates is appropriate.

For lap-welded bottoms, the lapped area that is beneath the shelland projected outside of the tank must be “broken down.” The pur-pose of the breakdown is to provide a smooth transition at the lap onthe top side so that there will be no gap at the shell-to-bottom connec-tion in the region of the lap. Refer to Fig. 6-9 for an illustration of abreakdown.

Annular ring splice welds must be butt welded. The welds maybe either single butt welds with backup bars or double butt welds.Commonly, the fabrication shop will subassemble annular ring sec-tions with double butt welds, and the subassemblies will be weldedin the field with single butt welds.

Butt-welded bottom plates can be either a one-sided weld witha backup bar or a double butt welded. Single butt welding is thepreferred method since the bottom can be laid out and the weldingperformed from the top side. Double butt welding is difficult for largetanks due to the inaccessibility of the underside of the bottom. Forsmall tanks, however, the initial weld pass can be performed down-hand and the bottom can be flipped over so that the welding can becompleted down-hand.

Shell-to-bottom junction The connection of the shell to the bottomplate shall be a continuous fillet weld on both sides of the shell. Table18 in AWWA D100-05 gives the minimum size of the fillet welds to beused on the basis of the thickness of the shell plate. If the fillet weld is5/16 in. (7.9 mm) or larger, the weld shall be two-pass minimum.

The weld should be inspected for watertightness using dye pene-trant, penetrating oil, or diesel fuel. The inside fillet weld is completedfirst, and indicator is sprayed on the weld. If any indicator is visibleoutside of the shell after a wait period, a leak is indicated and shouldbe repaired. Once there are no indications of leakage, the outside weldcan be completed.





Shell ConstructionCrane versus jacking Normally, there are two basic methods for erect-ing the tank shell. The more common method is to start from thebottom ring and use a crane to place each individual shell plate foreach successive ring until the shell is complete. However, for tall tanksa method using hydraulic jacks may be more economical. Using thismethod, the top two shell rings and roof are erected on temporary jackstands. Once this is complete, the shell is jacked using hydraulics, andthe next shell ring is placed. This process is repeated until the shell iscomplete. This method reduces the crane requirements since there areno high picks. Also, temporary scaffolding for the shell is not requiredsince all shell erection and welding activities are performed at groundlevel.

The shell is the critical component of a storage tank. It is the pri-mary stressed membrane that contains the liquid. Therefore, great caremust be exercised in laying out, fitting, and welding the shell.

Listed here are general steps for layout and fit-up of a tank shell.This procedure varies among contractors; however, the general stepsare the same.

� Check the elevation of the tank bottom.� Scribe the radius of the inside of the shell on the bottom. Thiswill be used as a guide for setting the first ring.� Mark the chord dimensions for the first ring.� Install erection nuts or lugs on the bottom that follow theoutside radius of the first ring.� Set the first plate starting at the first chord mark and followingthe circular scribe mark.� Set the second plate and use fit-up gear to align the verticalseam for welding.� Set the remainder of the first ring plates and check for level.� Weld the vertical seams.� Weld the first ring to the bottom.� Hang the second and subsequent rings using fit-up gear onthe horizontal and vertical seams.

Shimming and the tub ring After the bottom plate is laid, the first shellring, or “tub ring,” is set. Care must be taken in setting the tub ring,since the roundness of the tub ring is the basis of whether the rest of theshell will be round. An essential part of making the shell round is thatthe tub ring must be level. If the tub ring is not level, tank roundnessis difficult to achieve. The tub ring is leveled by using shims betweenthe bottom and the foundation.





Wind stability During shell erection, the shell is susceptible to windloads. The shell is designed to be stable and resist buckling due towind when the tank is complete. However, during construction theshell can buckle easily, even during moderate winds. Therefore, theshell must be braced to prevent a “blow-in” incident. One method ofbracing the shell is with the erection scaffolding. The scaffolding isnormally set 3 to 4 ft (0.9 to 1.2 m) below the top of the ring beingerected and consists of brackets and scaffold boards or planks span-ning between the brackets. If the boards overlap at the brackets andare tied down securely, the scaffolding itself acts as a ring stiffeneron the shell. Because of this phenomenon, incomplete scaffolding isnormally not allowed to be left overnight.

Partial- versus complete-penetration welding The shell vertical weldsare always complete-penetration welds. The horizontal welds may beeither complete-penetration or partial-penetration welds. When theshell thickness of the thinner of the two plates being joined is greaterthan 3/8 in. (9.5 mm), the horizontal weld can be a partial penetration.The finished weld must have at least two-thirds the strength of acomplete-penetration weld. Partial-penetration welds are not allowedin the shell plates for Section 14 designs.

Weld clearances Weld clearances for shell vertical joint offset, perma-nent attachments, and shell penetrations should meet the require-ments of AWWA D100 and good industry practice. Section 14 ofAWWA D100 prescribes the requirements for weld clearances. Thebase code does not describe any weld clearance requirements; how-ever, good practice indicates the following weld clearances:

� Vertical shell plate offset = 12 in. (305 mm) minimum� Permanent attachments = 3 in. (76 mm) (horizontal) and 6 in.(152 mm) (vertical)� Shell penetrations = 3 in. (76 mm)

Construction openings A construction opening in the shell is normallyprovided to allow easy access to the interior of the tank. This openingis usually in the form of a short plate that is removed in the first shellring. The short plate left out of the shell, called a door sheet, is 6 to12 ft (1.5 to 3.6 m) wide and has a height equal to the width of the firstshell ring. If a crane must be driven inside of the tank for roof erec-tion, a taller opening is usually required. This is accomplished witha first and second ring door sheet, which may or may not includethe full height of the second ring. Temporary stiffening must be pro-vided around the door sheet to bridge the vertical loads during tankerection.





Shell penetrations Shell penetrations are required for access to theinterior of the tank and for an overflow to prevent overfilling. Theinlet and outlet piping may go through a shell penetration but morecommonly goes through the tank bottom. A minimum of two man-holes are required for access. The penetrations have a neck plate that iswelded to the shell, and additional reinforcing in the form of a circularreinforcement plate may be required.

Post-weld heat treatment Post-weld heat treatment is only requiredfor shell penetrations that are 12 in. (305 mm) in diameter or greater inshell plate that is thicker than 1 in. (25.4 mm). The penetrations shouldbe prefabricated in the shell and stress relieved before shipment. Thisrequirement applies only to Section 14 designs.

Roof ConstructionRoof configuration can be either supported on structural framing orself-supporting. The self-supporting roofs can be unstiffened, or stiff-eners can be welded to the roof plate.

Method of roof erection—crane versus air raised For tanks with struc-turally supported roofs, the typical method of construction is to usea crane to lift the various components into place. The roof framing iserected after the bottom and shell are in place, and the roof plate isplaced after the framing is complete.

For tanks with self-supporting roofs, there are more options. Theroof can be built in place using a crane and temporary support forthe roof or the roof can be built on temporary supports outside of thetank and the entire roof can be lifted into place. The latter methodis advantageous for tall tanks, roofs requiring seal welding, and insituations where a crane with enough capacity can be used econom-ically. Another option is to have the roof erected on the floor of thetank and to lift it into place after the shell erection is complete. This isaccomplished by sealing the outside edge of the roof to the shell andpressurizing the underside of the roof to lift it to its final position. Thismethod is economical for large-diameter tanks that are relatively tall.Surprisingly, the pressure required to air-raise a roof is on the orderof a water column of 3 to 6 in. (76 to 152 mm). The roof can be raisedusing high-velocity fans bolted to the shell manholes.

Subassembly For self-supported roofs, to minimize the number ofcrane picks and reduce the need for welding in place, the field crewmay elect to subassemble some of the roof sections. This may alsodecrease the amount of time the crane needs to be on-site, thereforereducing costs.

Roof-to-shell junction The roof-to-shell junction can be configured inseveral ways. For cone- and dome-type roofs, an angle can be either





butt or lap welded to the top of the shell. The roof plate laps ontothe top of the angle. This arrangement can be advantageous sincefabrication and erection variances can be tolerated. The top angle isused to aid in keeping the tank shell round. As an alternative to usingan angle, a bar may be used.

A double-curved transition may be used for either a supported orunsupported roof. This type of transition can be more visually pleasingfor taller tanks and any tank for which aesthetics are important.

Seal welding Seal welding may be specified to reduce rust bleedingfrom the inaccessible plate lap areas. If seal welding is required, thetype of roof must be considered. If the roof plate is supported on struc-tural members, the surfaces mating the tops of the structural mem-bers to the underside of the roof plate will also be inaccessible. Sealwelding the roof plate lap welds will solve only part of the problem.However, if the roof framing is to be seal welded to the roof plate,the tank designer must consider the effects of thermal expansion andcontraction caused by temperature differentials on the roof framing.Also, the amount of welding required to seal all of the roof framingmight induce additional weld distortion in the roof plate. If the roofis self-supported, any framing will be welded to the roof plates bydesign.

Ponding For supported cone roofs, the minimum roof slope is a 3/4-in. (19-mm) rise in a 12-in. (305-mm) run. This is a very shallow roofslope and it may therefore produce ponding if the roof plate is builtwith excessive distortion. In many local jurisdictions, ponding is notallowed by law. The easiest solution to potential ponding is to increasethe roof slope.

Construction of Elevated Steel Water-Storage Tanks

Method of Erection—Crane Versus DerrickThe constructor should select the type of crane to be used to constructan elevated water tank, basing the decision on the tank geometry,schedule, equipment availability, and cost.

Elevated tanks are frequently erected using a derrick. A derrick isa fixed-mast, guyed crane that is positioned at the center of the tank.Placement of a derrick is optimal for construction of circular tanks,because the boom is capable of a full 360-degree swing. On the basisof years of construction experience, tank constructors have developedguyed derricks specifically designed for tanks.

Mobile or tower cranes are also options for erecting an elevatedtank in the field. Because of rental expenses, mobile or tower cranesare usually limited to use on a smaller-capacity tank of limited height





with a short field-construction schedule. Use of a mobile crane requiresmore site clearance around the tank.

Whether using a derrick, tower crane, or mobile crane, adequatesupport is critical. For either a derrick or stationary tower crane, sup-plemental groundwork or a support pad should be provided if re-quired before setting the crane. A mobile crane requires a reasonablylevel surface around the tank and site conditions capable of support-ing the loaded crane.

General RequirementsField subassemblies Shipped plates are frequently subassembled onthe ground in the field. The subassemblies are planned on the basisof the maximum weight and size feasible to lift and fit into place.This erection practice allows the welding to be performed close to theground in more favorable positions.

Construction aids Specialized erection equipment developed by thetank constructor is used to aid in lifting, fitting, aligning, and spacingplates with the appropriate weld gaps. Maintaining the proper gaps,alignment, and overall dimensional accuracy is critical for subsequentplate placement. Some construction aids may be permanently left inplace, while others are temporary and are removed after the plate issecured. Temporary attachments need to be removed without damag-ing the plates, and the remaining weld should be chipped or groundsmooth before painting. Dimensional accuracy is maintained by con-sistently checking dimensions. Levels and transit levels may be usedto verify elevations, check angles, and to verify that a component isplumb.

Access to the tank Safe access to the tank and tower is required forwelders, inspectors, and painters. Temporary scaffolding is commonlyused in conjunction with permanent and temporary ladders for access.Aerial lifts such as a boom lift or scissor lift may also be used. A workbasket or chair hung from a spider line is another frequently usedoption. This option requires a secure anchor point above the area tobe accessed. Regardless of the method of access, fall protection needsto be considered and special measures taken to ensure the safety ofthe workers.

Fluted-Column-Style TankThe fluted tower rests on butt-welded base plates that are seated onshims and fixed to the foundation by the anchor bolts. It is critical thatthe base plate is level before erecting the fluted plates. After the tankhas been erected, but before it is filled with water, the space betweenthe base plate and foundation is grouted.





FIGURE 6-10Erection of tower,fluted-column-styletank.

The vertical joints in the fluted tower are lap welded, and thehorizontal joints are butt welded. Water-bearing plates in the tankare welded with full-penetration butt welds. The roof is typically lapwelded on the top side only. When specified by the purchaser, theoverhead laps in the roof are also seal welded (Figs. 6-10 to 6-12).

Access to the inside of the fluted tower is required at all times andis typically provided at the opening for the overhead door. A bottommanhole provides access to the inside of the tank. Tank constructorshave developed specialized equipment to enable safe access to dif-ficult areas such as the outside of the cone or the underside of theroof.

The constructor must be cognizant of the stability of the structureat all times, but especially when the structure is left overnight. Re-gardless of what component the crew is erecting, the crew should notleave the tank unattended until all the plates in a given ring are inplace and adequately secured. Provisions should be taken to stiffenunfinished sections of the tank in case of high winds. This may includeproviding stiffening or continuous scaffolding at the upper limits ofconstruction or guying the structure to the ground.




FIGURE 6-11Erection of coneplate, fluted-column-style tank.

FIGURE 6-12Erection ofcylindrical shellplate, fluted-column-style tank.

249





FIGURE 6-13 Erection of spherical plate, pedestal-style tank.

Pedestal-Style TankComplete-penetration butt-welded construction is used for all thecomponents of the pedestal tank and tower except the roof. The roofis usually lap welded on the top side. At the request of the purchaser,the underside may also be seal welded or, alternately, the roof may bebutt welded (Figs. 6-13 and 6-14).

The base cone of the tower of a pedestal-style tank sits on a thickbase plate that is welded with complete-penetration butt welds, seton shims, and fixed to the foundation by the anchor bolts. As with thefluted-column-style tank, it is critical that the base plate be level beforeerecting and welding the base cone. The base cone is fillet welded tothe base plate. As with the fluted-column-style tank, grout is placedunder the base plate after the tank is completely erected but before itis filled with water.

Multicolumn-Style TankErection for a multicolumn-style tank typically begins with one bentin the first panel, consisting of a pair of columns, one bolted strut,and loosely connected cross-bracing that will either be welded or





FIGURE 6-14 Erection of roof plate, pedestal-style tank.

bolted to the columns. Base plates are welded to the bottoms of thecolumns in the first panel. The base plates of the first bent are seton shims placed on the foundation pedestals and are fixed using theanchor bolts. Additional bents are erected by sequentially adding acolumn, strut, and cross bracing around the tower. After all the bentsare in place in the first panel, the cross braces are adjusted to length,as required, to ensure that the panel is square and true before pro-ceeding to the next panel. Subsequent panels are erected in a similarmanner with the columns of the upper panel welded to the lowercolumns.

Typically, the portion of the tank that is welded to the upper col-umn is welded to the column before the tank is erected. After thetower is complete, the intermediate plates are fit-up and welded withcomplete-penetration butt welds. It is critical to maintain dimensionalaccuracy of the tower for proper fit-up of these plates. The tank jointsbetween water-bearing plates are joined with complete-penetrationbutt welds. The roof plates may be lap welded with or without sealwelding, or they may be butt welded.

An alternate construction sequence is to construct the upper bentsand tank without the lower columns in place. The advantage of thisconstruction method is that the tank and upper tower can be con-structed and painted when closer to the ground. After this portion ofthe tank is complete, it is lifted by cranes and the lower columns are





FIGURE 6-15Multicolumn-styletank.

set underneath (Fig. 6-15). The structure is then lowered to its properheight and seated on the columns.

After the tank is complete, but before it is filled, the final tighteningor welding of the cross-braces is done. At this stage, the grout is alsoplaced under the base plates.

Construction of Composite Elevated Tanks

Concrete Support StructureThe concrete support structure for a composite elevated tank is castin place. Jump forms are commonly used. Constructors of this styleof elevated tank have specialized forms conforming to their standardgeometries. Forms have horizontal and vertical rustications built intothe exterior face to provide architectural relief and help mask formpanel joints and construction joints (Fig. 6-16).

Wall reinforcing is placed and tied before the forms are installed.Special reinforcing is required around the overhead, mandoor, andother significant openings. After the rebar is placed, the forms areset and prepared to receive concrete. Large openings in the tower areblocked out, while smaller openings may be cut or drilled after con-crete placement. The concrete is delivered to the tops of the forms





FIGURE 6-16 Towerconstruction,compositeelevated tank.

either by pumping or by bucket. Hand compaction and power vi-bration are performed in accordance with ACI 309 to ensure propercompaction, minimize segregation, eliminate air voids, and to ensureclose contact with the reinforcement and forms.

After the first ring is poured and has had time to cure, the forms areremoved. The sequence of placing reinforcement, jumping the forms,and pouring the concrete is repeated until the support tower is com-plete. Forms are set for the dome and ringbeam or the flat slab. Similarto the tower forms, the tank constructor will have developed special-ized forms for their geometry and construction practice. Reinforce-ment is placed and tied and embedments secured before the concreteis poured.

Concrete MixThe concrete mix should be suitable for the method of placementand the weather conditions. The proportions of the mix should beadjusted to provide adequate workability and the proper consistencyfor placement.

For each tank, the material should be from a consistent sourceand the mix design number verified upon delivery. The arrival oftrucks should be sequenced to sustain a pour without long delays.Retempering of the concrete should be controlled to maintain the mix





FIGURE 6-17 Rooferection,compositeelevated tank.

parameters. Concrete testing should be in conformance with ACI 318,Building Code Requirements for Structural Concrete.

Welded-Steel Container for the Composite Elevated TankWelding for the cone, shell, and roof of a composite elevated tank issimilar to that for a fluted-column tank (Fig. 6-17).

Method of Erection—Hoisted Versus Crane or DerrickConstruction methods for the steel container on the composite ele-vated tank and the fluted-column tank are similar if a crane or derrickis used. One additional construction technique, hoisting, has beensuccessfully employed for composite tanks. This method allows theconstructor to erect and weld the cone, shell, and upper cone rooftransition as a complete unit around the concrete shaft at grade. Afterthe welding is complete, the container is hoisted into position usinga series of cables and hoists. It is welded into place, and the roof isinstalled. (Fig. 6-18a and b).

Liner PlateAn interior liner plate is placed over the dome or flat slab. The liner islap welded on the top side only. For tanks with a dome, formed linerplates may be used and constructed so that the liner lies directly on




(a)

(b)

FIGURE 6-18 (a) CET hoisted tank erection as the tank is being raised. (b) CEThoisted tank erection with the tank in the final position.

255





FIGURE 6-19 CETliner plate formedto fit dome withderrick-erectedcone and shellplates.

the dome. Alternately, unformed steel liner plates that do not matchthe shape of the dome may be used if the space between the plateand the dome is completely filled with flowable grout after welding(Fig. 6-19).

Inspection and Testing

Foundation TolerancesBefore construction is started of the tank’s support structure, the foun-dation should be checked to verify that it is within specified toler-ances. AWWA D100 provides minimum requirements for foundationtolerances for multicolumn tanks, single-pedestal tanks, and ground-supported flat-bottom tanks.

Minimum tolerances for anchor bolts need to be maintained forinstallation of the base plates for elevated tanks and anchor chairsfor ground storage tanks. AWWA D100 anchor tolerances for all tankstyles are as follows: Anchors should be within ±1/4 in. (±6.3 mm)of the theoretical location and plumb within 1/8 in. in 12 in. (19 mmin 305 mm). The anchor projection above the top of the foundationshould be within ±1/4 in. (±6.3 mm).





Foundations and anchors that are not within tolerances shouldbe identified and addressed before the tank is erected. A design pro-fessional representing the tank constructor should evaluate whetherthe structural capacity of the foundation has been compromised andshould provide details for remedial action when required.

Tolerances for Concrete Support StructureDimensional tolerances for the concrete support structure of a com-posite tank are outlined in ACI 371R and repeated here. Fabricationand placement tolerances for rebar should be in compliance with ACI117.

Dimensional tolerances for the concrete support structure are asfollows:

Variation in thickness� Wall: −3.0 percent, +5.0 percent� Dome: −6.0 percent, +10.0 percent

Support wall variation from plumb� In any 5 ft (1.5 m) of height: 3/8 in. (9.5 mm)� In any 50 ft (15 m) of height: 1.5 in. (38 mm)� Maximum for total height: 3.0 in. (76 mm)

Support wall diameter variation� 0.4 percent (not to exceed 3.0 in. [76 mm])

Dome tank floor radius variation� 1.0 percent

Level alignment variation� From specified elevation: 1.0 in. (25.4 mm)� From horizontal plane: 1/2 in. (12.7 mm)

The offset between adjacent pieces for formwork facing materialshould not exceed the following:� Exterior exposed surfaces: 1/8 in. (3 mm)� Interior exposed surfaces: 1/4 in. (6.3 mm)� Unexposed surfaces: 1/4 in. (6.3 mm)

The finish tolerance of troweled surfaces should not exceed thefollowing when measured with a 10-ft (3-m) straightedge orsweep board:� Exposed floor slab: 3/8 in. (9.5 mm)� Tank floors: 3/4 in. (19 mm)� Concrete support for suspended steel floor tank: 1/4 in.

(6.3 mm)





Maximum Diameter (ft[m]) Radius Tolerance (in. [mm])

40 [12] ± 0.5 [±12.7]

150 [45.7] ± 0.75 [±19]

<250 [<76.2] ± 1.0 [±25.4]

≥250 [≥76.2] ± 1.25 [±32]

TABLE 6-2 Roundness—Cylindrical Shells

Welded Tank TolerancesTank constructors are responsible for maintaining the quality of theirwork. AWWA D100 outlines some specific shell tolerances for plumb-ness, roundness, peaking and banding, and localized flat spots forground storage tanks. The maximum out-of-plumbness from top tobottom should not exceed 1/200th of the total shell height of the shell.Single shell plates should also meet the flatness requirements of ASTMA6. To check the roundness of the shell, measurements should betaken 1 ft (0.3 m) above the shell to bottom seam and not exceed thetolerances in Table 6-2. Peaking, defined as the out-of plane distor-tion across a vertical weld seam, should not exceed 1/2 in. using a36-in.-long sweep board. Banding, the out-of-plane distortion acrossa circumferential weld, is also limited to 1/2 in. using a 36-in.-longsweep board.

Industry-wide tolerances for plumbness, roundness, peaking andbanding, and localized flat spots for elevated tanks do not exist.AWWA D100 indicates that the tanks are to be “trued up” after erec-tion and prior to grout placement. To guard against fit-up problemsas field erection progresses, most individual constructors have self-imposed tank and tower tolerances to ensure that the tank is builtplumb and within fairly tight construction tolerances.

AWWA D100 provides erection tolerances for plates designed forstability. If the compression allowables of AWWA D100 are used inthe design, the maximum local deviation from the theoretical shapemust be less than

ex = 0.04√

Rt

Lx = 4√

Rt

where

Lx = gauge length to measure local imperfectionex = local deviation from theoretical shape

t = shell thicknessR = radius of exterior surface of the shell, normal to the plate at the

point under consideration and measured from the exteriorsurface of the plate to the axis of revolution.





Subject to Subject to

Thickness Primary Stress Secondary Stress

(in. [mm]) (in. [mm]) (in. [mm])

0 < t ≤ 5/8 [15.8] 1/16 [1.6] 1/8 [3.1]

>5/8 [15.8] Lesser of 0.10t or Lesser of 0.20t or

1/4 [6.3] 3/8 [9.5]

t = Nominal thickness of the thinner plate at the joint.

TABLE 6-3 Maximum Allowed Offset for Butt-Welded Plates Subject to Primaryor Secondary Stress

Alternately, if a plate thickness is based on a buckling analysis per-formed by the tank constructor, measurements must be taken to verifythat the deviations assumed in the analysis have not been exceeded.

AWWA D100 also provides tolerances for plate alignment for lap-welded joints and butt-welded joints. Lap joints should be held inas close contact as possible, with no plate separation exceeding 1/16in. (1.6 mm). When plate separation is present, the weld size shouldbe increased by the amount of separation. The maximum allowedoffset for butt-welded plates subject to primary or secondary stress isdefined in AWWA D100 and repeated in Table 6-3.

Welded Tank InspectionShop welds that will carry stress from the weight or pressure of thewater are typically inspected by the tank constructor before the com-ponent is shipped. Some purchasers may choose to visit the tank con-structor’s shop during fabrication to observe the fabricating practicesand operations. Per AWWA D100, either the constructor or a qualifiedinspector hired by the owner is required to check the quality of thefield welding.

Visual inspection by an individual who is competent to performthe inspection is to be performed on all welds. Competency for vi-sual inspection can be obtained either by training or by experience. Aweld shall be repaired or replaced if any of the following defects arefound: a crack, lack of fusion, unfilled craters, overlap resulting fromthe protrusion of weld metal beyond the weld toe or root, undersizedweld, and porosity in butt joint subjected to primary stress. Weldsexhibiting any of the following defects in excess of the AWWA D100–specified limits also need to be repaired or replaced: excess butt jointreinforcement, fillet-weld convexity, undercut, porosity, and plate mis-alignment. All welds should also be visually inspected to ensure theremoval of all weld spatter, sharp surfaces, overlaps, and unacceptableundercuts that would be detrimental to the coating life.





The most common method of evaluating complete-penetrationbutt-welded joints for weld quality is by radiography. Radiographicinspection is to be performed in accordance with ASME Section V,Article 2 by Level II radiographers. Section 11 of AWWA D100 pre-scribes the number and location of radiographs for tank shells, risers,and single-pedestal columns. The radiographic inspection should beperformed as the work progresses. The purchaser may participatein the selection of the specific radiograph locations. AWWA D100 hasradiograph inspection standards that must be used for evaluating dis-continuities and defects present in the radiograph film. A weld shallbe repaired or replaced if cracks, incomplete fusion, or inadequatepenetration are noted. In addition, welds with inclusions or roundedindications that exceed the limits specified in AWWA D100 need to berepaired or replaced.

The welds in bottom plates of a ground storage tank are requiredto be tested for watertightness by magnetic-particle testing or by air-pressure or vacuum testing.

There are additional inspection requirements for ground storagetanks built to AWWA D100 Section 14. The weld between the shelland the bottom should be inspected for watertightness using dye pen-etrant, penetrating oil, or diesel fuel. The inside fillet weld is completedfirst and indicator sprayed on the weld. If any indicator is visible out-side of the shell after a wait period, a leak is indicated and it should berepaired. Once there are no indications of leakage, the outside weldcan be completed. Section 14 requires more extensive radiographicexamination of butt welds in the shell and annular plate. It also re-quires that all welds attaching manholes, nozzles, and other pene-tration be inspected for cracks by either the magnetic-particle or thedye-penetrant method by a qualified inspector.

HydrotestWater testing is typically performed on the completed tank after it ispainted and disinfected. The purchaser is responsible for furnishingthe water to the site with sufficient pressure to fill the tank. Watershould be filled to the top capacity level, and weld seams should beinspected for any signs of leakage. If leaks are found, the water mustbe lowered at least 2 ft (0.6 m) below the point of repair, and the defectmust be repaired and rewelded. If no leaks are found, the tank can beput directly into service, which eliminates the need to dispose of thetest water.




C H A P T E R 7Construction of

Bolted-SteelWater-Storage

Tanks

Keith McGuire, P.E.Columbian TecTank

Richard Field, P.E., S.E.Engineered Storage Products Company (retired)

Bolted-steel tanks are factory coated and conform to American WaterWorks Association (AWWA) D103, Factory-Coated Bolted CarbonSteel Tanks for Water Storage.

Erection of the TankEquipment required to erect a bolted-steel tank varies among tankmanufacturers. When jacks are used, a complete ring of shell sheetsis assembled and raised by the jacks so that the subsequent ring ofsheets may be assembled under them. The tank is assembled from thetop down while the construction crew operates from a location at ornear ground level.

Tank walls consist of shell sheets or panels carefully packed atthe factory to prevent damage to the factory-applied coating duringshipment. Shell sheets and panels typically have varying bolt-holepatterns designed to resist the increased hoop loading as tank heightincreases (Fig. 7-1). The roof may consist of factory-coated roof panels,deck plates supported by structural members, or a self-supported alu-minum dome.

261




262 C h a p t e r S e v e n

FIGURE 7-1 Bolt-hole patterns vary. (Photo courtesy of CST Industries.)

Various lengths and strengths of structure bolts, washers, and nutsare used to assemble the tank and appurtenances. Each manufacturerspecifies the degree to which bolts must be tightened to properly as-semble the tank and its components. Typical instructions indicate aspecified torque or a visual observation of gasket deformation thatmust be achieved.

Coated areas that are damaged in the erection process must berepaired in accordance with the tank manufacturer’s written proce-dures. In addition to wrenches and drift pins, proprietary equipmentmay be used to ensure that the sheets are properly joined and aligned.

Sealant and GasketsTo ensure watertightness, gaskets and/or sealer are used to seal alljoints. Sealer is dispensed from tubes or sausage packs via hand-heldcaulking guns. The most common sealant materials used are urethaneand silicone.

Gaskets to seal along sheet seams are supplied in strips. Stripgasket is pre-punched and furnished in rolls, and it must be cut to therequired length during erection. When splices are necessary, lap jointsare required, and a bead of sealant is applied along the joint. Circulargaskets for items such as nozzles, manways, and collars are furnishedin one piece or are in several pieces that are joined to form rings.

Special gaskets are sometimes required to seal certain areas of thebolted tank panels, depending on the type of construction. Lap gas-kets, radius fillet gaskets, and other types of gaskets may be furnishedby the tank manufacturer for use at specific tank locations during as-sembly.



Construction of Bolted-Steel Water-Storage Tanks

263C o n s t r u c t i o n o f B o l t e d - S t e e l W a t e r - S t o r a g e T a n k s

FloorIf the tank will have a concrete floor, specially sized sheets and panelswill be embedded into the floor, forming a ring of foundation sheetson which the remainder of the tank will be erected. If the tank willhave a steel floor, factory-coated floor segments/panels will be used.

Unloading and StorageTank components and materials are usually shipped to the site onskids or special racks on open trailers. Before unloading, the materi-als should be checked for shipping damage. During unloading, it isrecommended that a forklift truck or crane be used to avoid damage totank components and boxed items. An inventory should be performedto ensure that all items shipped agree with the bill of lading.

The materials should be stored near the tank location on dryground, out of the way of other construction activities, and securefrom pilferage and theft. All materials should be covered to preventdamage from the weather.

Concrete Floor ConstructionFooting and floor concrete placement are usually performed in twoseparate pours (Fig. 7-2). Leveling plate assemblies are installed in

FIGURE 7-2 Footing and floor concrete placement.





FIGURE 7-3 Leveling plate assemblies.

the top of the footing pour. These assemblies are used to position theembedded tank foundation sheet (Fig. 7-3). The foundation ring is seton the leveling plate assemblies, then leveled and rounded to specifiedtolerances (Fig. 7-4). Floor sumps are then installed, and other pipingis stubbed off above the floor line (Fig. 7-5).

FIGURE 7-4 Foundation ring set on leveling plate assemblies.





FIGURE 7-5 After floor sumps are installed, other piping is stubbed off.

Reinforcement is placed in the floor area and around the curb tothe outside of the foundation ring in accordance with the engineer-ing drawings (Fig. 7-6). Forms are usually used for creation of thefoundation floor curb around the entire perimeter of the foundation

FIGURE 7-6 Reinforcement placed in floor area and around curb.





FIGURE 7-7 Floor and curb concrete placed and finished.

assembly. Forms may be installed during or after placement of thefloor and curb reinforcement steel. A water-stop seal strip is installedonto the foundation sheet prior to concrete placement. The floor andcurb concrete is placed, finished, and allowed to cure before furthertank construction takes place (Figs. 7-7 and 7-8).

FIGURE 7-8 Concrete allowed to cure.





Steel Floor ConstructionFor a tank with a steel floor, a concrete ring footing should be installedthat contains properly prepared soil and provides support for the tankshell, steel floor, and the stored liquid load. Special soil conditions andother requirements may dictate the use of a concrete slab to properlysupport the tank. Embedded anchor bolts, if required, are installed tothe specified projection above the top of the footing (Fig. 7-9).

If the tank does not require anchor bolts, and depending on thesite conditions and tank loadings, then tanks with steel floors maybe supported on a prepared granular base or berm in lieu of a concretefoundation. A steel perimeter ring at least 1 ft (0.3 m) larger than thetank radius may be used to contain the compacted base.

The steel floor is constructed in accordance with the tank man-ufacturer’s instructions. Assembly of pie-shaped segments begins ata circular plate at the center of the tank, and segments are assem-bled from the center outward until the outside of the tank is reached(Fig. 7-10). For floors using rectangular segments (Fig. 7-11), workersstart at a designated point along the tank perimeter and work theirway across the tank to the other side.

FIGURE 7-9Embedded anchorbolts are installedto specifiedprojection.





FIGURE 7-10 Assembly of pie-shaped segments.

FIGURE 7-11 Floors using rectangular segments.

Tank Construction

Jacking MethodSpecially designed jacks are used to build the remainder of the tank.The jack assemblies are anchored to the tank floor, one at each sheet





FIGURE 7-12 Jack assemblies are anchored to tank floor.

location around the tank perimeter (Fig. 7-12). After all jacks are in-stalled, the faceplates to which the wall sheets will attach are leveledwith each other.

As the first ring of sheets is placed on the jacks, sealer is placedin the overlapping vertical joints, and the sheets are bolted together.Before tightening the bolts, a special tool is used to spread the jointapart, simulating the loading that will be placed on the vertical jointfrom the stored liquid load. As the joint is held in its spread condition,the bolts are tightened to their specified torque.

At this point, the roof segments are bolted and attached to the topof the first ring of sheets (Fig. 7-13). The center of the roof is temporarilysupported while the roof segments are bolted into place. When theroof is complete, the temporary support is removed and the entirestructure is jacked up to the next level.

After the first ring of sheets is completed and tightened, the jacksare energized and the structure is raised to a height that allows thenext ring of sheets to be installed in the same manner as the firstring (Fig. 7-14). The second ring of sheets rests in supports attachedto the foundation sheet while being bolted to the ring above it. Oncompletion of the second ring of sheets, the jacks are disconnectedfrom the first ring, lowered to a specified location in the ring justcompleted, and connected to that ring. The jacks are energized, andthe structure is raised to the next level. Tank erection continues inthis manner until the last ring of sheets is installed and tied into thefoundation sheets. Tanks taller than 120 ft (36.5 m) have successfullybeen erected in this manner.

The last sheet of the bottom ring is typically left out to providean easy means of access for other work inside the tank and for theremoval of the jacks.





FIGURE 7-13 Roof segments are bolted and attached.

Wind stiffeners, ladders, ladder cages, and platforms, as applica-ble, are attached to the side of the tank as successive rings are assem-bled and jacked to the next level (Fig. 7-15).

Scaffold MethodExterior scaffolding similar to that shown in Fig. 7-16 is required. Thequantity and length of scaffold planks required are determined by

FIGURE 7-14 The jacks raise the structure.





FIGURE 7-15 Wind stiffeners, ladders, ladder cages, and platforms areattached.

FIGURE 7-16 Exterior scaffolding.





FIGURE 7-17 Scaffolding bracket.

the size of the tank and the width of the tank panels. Scaffolding ismoved from ring to ring as work progresses, meaning that the min-imum quantity of planks is enough to encircle the tank at one level,plus a few extra. Some crews prefer to leave all scaffolding in place un-til the shell and roof are complete, thus requiring two planks for everyshell segment in the tank. Scaffold planks are supported by scaffold-ing brackets (Fig. 7-17). Normally, the brackets consist of steel angles2.5 in. × 2.5 in. × 0.25 in. (63.5 mm × 63.5 mm × 6.35 mm). Ac-commodation for safety line uprights must be provided. The bracketshown in Fig. 7-17 will receive a tubular upright that fits over the 6-in.-(152-mm)-long plank-retaining rod at its end. Any plank-retaining de-vice should have a minimum height of twice the scaffold plank thick-ness. The safety line upright must be 42 in. (1 m) in height, measuredfrom the top of the scaffolding board. Common practice is to leave allscaffolding brackets in place until the shell and the deck are complete.This requires one bracket for each shell segment in the tank. All boardsmust be secured.

If interior scaffolding is not used, a hooked drive-out ladder sim-ilar to the one shown in Fig. 7-18 is required during erection. Pointsthat bear on the shell should be padded to protect the interior finish ofthe tank. Hoisting equipment capable of lifting components weighingas much as 1,000 lb (453.6 kg) is required. A variety of devices canbe used, but gin poles similar to the one shown in Fig. 7-18 are the





FIGURE 7-18 Drive-out ladder and gin pole.

most common. Made of steel pipe or tubing, they are supported fromscaffolding brackets and extend well above the top of the ring underconstruction. The tank shell must be protected from gin-pole bearingpoints.

Roof InstallationAt least three types of roofs are commonly installed on factory-coatedbolted-steel tanks. These types are self-supported (Fig. 7-19), center-supported (Fig. 7-20), and self-supported aluminum domes (Fig. 7-21).

Self-Supported RoofsSelf-supported roofs consist of one-piece pie-shaped panels that aretemporarily supported in the center while the panels are lapped ontoeach other and bolted together. The roof slope is typically 20 degrees,and the outer end of the panel is formed in a rounded shape, creating aknuckle, which adds stiffness to the panel. This outer edge bolts to thetop of the tank wall (Fig. 7-13). After all roof panels are assembled andattached to the tank wall, the temporary center support is removed,allowing the roof to support itself.

Low-profile self-supported roofs are typically sloped at a 1:12 pitchto allow for rain runoff. One or two horizontal support members spanthe tank diameter, and an elevated collar is located at the mid-span toset the roof pitch and accept the radial rafters or stiffened roof sheets.





FIGURE 7-19 Self-supported roof.

Center-Supported RoofsCenter-supported roofs are supported by a center pole that extendsfrom the tank floor. A common system consists of a center supportcolumn of prefabricated pipe with a base plate at the bottom, a rafter-bearing plate at the top, and radial rafters (see Fig. 7-22 for properarrangement of parts).

FIGURE 7-20 Center-supported roof.





FIGURE 7-21 Self-supported aluminum dome roof.

Self-Supported Aluminum DomesSelf-supported aluminum domes are spherical structures conform-ing to the dimension of the tank. It is a clear span structure with afully triangulated space frame complete with noncorrugated closurepanels. These domes can be constructed in place on the top ring of the

FIGURE 7-22 Arrangement of parts, center-supported roof.





FIGURE 7-23 Self-supported aluminum domes can be constructed in place.

tank shell sheets (Fig. 7-23) or constructed on the ground and liftedonto the top ring of shell sheets as a completed assembly. In eithercase, the mounting shoes of the dome attach to a formed angle thathas been attached to the upper horizontal bolt line of the first ringof shell sheets. The dome consists of an aluminum I-beam structure,lightweight aluminum roof panels, and flashing. Although the domeis attached to the tank shell at the perimeter roof angle, the attachmentallows necessary movement between the roof and the tank.

Tank Appurtenance and Accessory Installation

VentilatorNormally, the roof ventilator is located in the center of the roof atthe roof cap (Fig. 7-24). The ventilator capacity needs to be sufficientto pass air so that the maximum possible rate of water entering orleaving the tank will not cause excessive pressure or cause a vacuumto be developed. For potable water storage, screening to prevent birdsand bugs is required, and the ventilator must be capable of relievingpressure and vacuum if the screens become clogged.

Roof AccessoriesA hinged, lockable roof access door is normally provided near theoutside ladder to allow liquid samples to be withdrawn from the top





FIGURE 7-24 Roof ventilator usually located in center of roof at roof cap.

of the stored liquid (Fig. 7-25). The door is bolted onto an openingprovided in the roof panels for this purpose.

A roof railing assembly is installed after the roof installation iscomplete. Depending on the pitch and the type of roof, a walk-way may be provided (Fig. 7-26). The assemblies are bolted together

FIGURE 7-25 Hinged, lockable roof access door near outside ladder.





FIGURE 7-26 A walkway may be provided.

using appropriate fasteners; sealer and neoprene pads are used wheresupport brackets attach directly to the roof panels.

A caged ladder with a roof manway landing platform that meetsrequirements of the Occupational Safety and Health Administration(OSHA) is supplied with most tanks (Fig. 7-27). Depending on tankheight, some ladder/cage assemblies also include one or more step-off platforms. Individual sections of the ladder/cage assemblies areconstructed on the ground. The sections are then attached to the ringsof shell sheets during the tank erection process.

Shell PenetrationsTank manufacturers’ policies vary concerning penetrations throughthe tank sidewall for piping and other instrumentation. Some manu-facturers supply the openings in the panels shipped from the factory;others provide detailed instructions for locating and cutting the open-ings in the field during or after tank erection. Depending on the open-ing size, the tank manufacturer may require and provide the meansto reinforce the area around the opening. Depending on the tank coat-ing, the reinforcement can be a plate welded around the opening inthe factory or bolted on in the field (Fig. 7-28).





FIGURE 7-27 Cagedladder is suppliedwith most tanks.

FIGURE 7-28 Depending on coating, tank can be reinforced in factory or field.





FIGURE 7-29Brackets holdingoverflow pipeshould be correctlylocated.

After the tank sidewall has been tied into the foundation sheet, oneor more access door assemblies (manways) are mounted to the tanksidewall within the lowermost full-height ring of shell sheets (refer toFig. 7-26).

A variety of overflow piping designs can be installed. Care shouldbe taken to ensure the brackets holding the overflow pipe are correctlylocated and cushioned against the tank to prevent damage to the coat-ing (Fig. 7-29).

When level indicators and other control devices are being in-stalled, workers need to heed the tank manufacturer’s instructionsregarding penetrations of the sidewall and prevention of coatingdamage.

CompletionAfter all tank sections have been erected, all appurtenances have beeninstalled, and piping is complete, the interior is cleaned of all con-struction equipment and debris. Any damaged coating areas on thetank interior or exterior are repaired in accordance with the coatingmanufacturer’s instructions. The tank exterior is examined to ensurethat all safety decals are in place, if applicable.





FIGURE 7-30 Completed reservoir tanks.

The tank is tested for leaks by filling it to the overflow level. Anyleaks found are repaired in accordance with the tank manufacturer’srecommendations. The test liquid is usually disposed of using thetank’s drain system.

FIGURE 7-31Completedstandpipe.





Once testing is complete, the tank should be disinfected. AWWAStandard C652 and the tank manufacturer’s recommendations shouldbe followed to achieve proper tank disinfection.

All construction equipment is removed and the tank site is cleanedin accordance with the project specifications. Figures 7-30 and 7-31show the completed tanks.




C H A P T E R 8Inspecting New-Tank

Construction

Steven P. Roetter, P.E.Tank Industry Consultants

The purchaser should inspect the tank as construction proceeds to en-sure that the structure complies with specifications, both in form andin quality. This chapter discusses the specific items to be checked bythe purchaser and those that are the responsibility of the constructor.

On the job, the purchaser and the constructor should keep in mindthat a quality structure is the result of the cooperation and affirma-tive efforts of all parties. If the relationship between the purchaserand the constructor is combative or antagonistic, the project is likelyto be completed late and quality will probably be adequate at best.On the other hand, a purchaser with a sound knowledge of specifica-tions, standards, and trade practices can work with the constructor tocomplete a high-quality project on schedule.

Responsibility for QualityIn general, tank constructors are responsible for maintaining the qual-ity of their work. The American Water Works Association (AWWA)D100 Standard for Welded Carbon-Steel Tanks for Water Storage andits D103 Standard for Factory-Coated Bolted-Steel Tanks for WaterStorage give the constructor the responsibility for designing and con-ducting a welding or bolting quality assurance program. According tothe provisions of AWWA D103, the constructor is responsible for se-lecting a factory-coated bolted tank (manufactured in accordance withthat same standard) that meets the capacity and height or diameter re-quirements of the owner. This is a logical assignment of responsibility,because the designing and building of tanks is a specialized field, and

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284 C h a p t e r E i g h t

specialized equipment and knowledge are required to conduct a totalquality control program. Nonetheless, the owner or the owner’s engi-neer, as well as third-party consultants where needed, should monitorthe work for inevitable human errors and misunderstandings.

Even before construction begins, the owner or engineer shouldcheck shop drawings to ensure that the special requirements of the jobare met. The constructor is usually responsible for the structural designof the tank, but the interplay of the legally defined responsibilities ofthe engineering and construction phases is not firmly established. Theengineer reviewing the drawings should clearly specify the purposeand the extent of the review to avoid being held legally liable for amore detailed examination than was actually performed.

A quality assurance program for the tank project should take intoaccount each of the functions involved in building a tank. In gen-eral, these functions fall under the following categories: foundation,fabrication, steel delivery, tank erection, field-applied coatings, shop-applied coatings, and appurtenances.

The Foundation and Composite-Tank PedestalThe foundation for a tank and the concrete pedestal of a compositetank are the most significant sources of potential failure, and so mostowners and engineers apply greater expertise to inspecting the foun-dation than to any other function of tank construction. If neither theowner nor the owner’s engineer has expertise in this area, other ex-perts should be contacted. The reinforced-concrete foundation andthe soil on which it bears are made up of nonhomogeneous materialsover which there is little control. In addition, the foundation is fre-quently installed by a specialty constructor or under another divisionof the water project, giving the tank constructor little control over thisfunction. Figure 8-1 shows foundation construction in progress.

Soil InvestigationThe purchaser should be at the site when the borings are being takenas part of the soil investigation. This gives the purchaser better insightinto problems that might be encountered during construction of thefoundation.

Activities Before Concrete Is PlacedThe excavation should be properly shored to prevent cave-in. The soilconditions at the bottom of each excavation should be evaluated toconfirm that they are the same as those used when the foundationdesign was developed. The bottom of the excavation can be sealedwith a mud mat to prevent water from changing the soil character-istics and to provide a solid surface from which to work. If piles are



Inspecting New-Tank Construction

285I n s p e c t i n g N e w - T a n k C o n s t r u c t i o n

FIGURE 8-1 Foundation construction in progress.

being driven, the purchaser must determine if the piles are drivingas predicted. The purchaser should be on the job to verify that thepile-driving log is properly completed. Site and final concrete eleva-tions should be confirmed. Placement of forms, reinforcing steel, andanchor bolts should be verified, and photographs should be taken todocument these steps before the constructor allows the concrete to beplaced.

ConcreteIf the owner, constructor, or engineer does not have extensive previ-ous experience with the materials supplied by the concrete ready-mixplant, a design mix should be developed and tested. The consistencyof the concrete should be evaluated as it comes out of the chute, andconcrete test cylinders should be taken. The storage location for thetest cylinders should provide satisfactory moisture conditions andcontrolled temperature. The cylinders should not be transported dur-ing initial cure. Test cylinders that are taken after approximately one-eighth of the truck’s load has been discharged give a more represen-tative sample of the concrete than the initial material that is placed.Slump tests should also be performed. For large pours, it is necessaryto sequence the pouring operations so that the concrete does not setup before placing fresh concrete next to it, creating “cold joints.” Theconcrete should not be dropped into the forms from excessive heights.After the concrete is placed, it should be vibrated with a mechanical





vibrator to ensure proper distribution of the material. Mechanical vi-brators should not be used to move the concrete from one locationto another. Proper removal of the forms and of all form ties shouldbe verified. Tolerances on concrete foundations are given in AWWAD100 and AWWA D103. The purchaser’s representative should be fa-miliar with American Concrete Institute (ACI) Standards 301 (Specifi-cations for Structural Concrete for Buildings) and 318 (Building CodeRequirements for Structural Concrete).

With a good cure, quality concrete develops the necessary com-pressive strength and is less likely to experience surface deterioration.If the surface of the foundation is not cured properly to protect it fromthe elements, chipping or spalling failure of the concrete can occurlong before the steel tank deteriorates.

BackfillingIf backfilling is done improperly, the concrete is overstressed. Properbackfilling techniques are necessary to provide a structurally stablefoundation, to prevent water from ponding on moisture-weakenedsoils, and to make the site more solid for the tank-erection crew.

Proper backfilling operations require the specialized services ofa qualified soil-testing laboratory to determine the optimal moisturecontent and maximum density of the backfill material, check the mois-ture content of the material being placed, and conduct relative den-sity tests in the field after the backfill material has been placed andcompacted. This last test ensures that the specified degree of soil com-paction has been obtained. Adequate soil compaction is particularlyimportant for the foundations of ground storage tanks. Although thebottoms of these tanks are usually quite flexible, particular care is nec-essary for backfilling pipe trenches beneath the tank and the soil or fillmaterial adjacent to concrete ringwalls. Severe differential settlementin these places can rupture the underlying pipes and cause possiblefailure of the tank bottom.

The contract documents may assign the responsibility for provid-ing necessary soil-testing services to either the tank constructor or theowner. In either case, copies of all soil test reports should be furnishedpromptly to all interested parties.

FabricationIt is recommended that the owner and the engineer visit the construc-tor’s facility while the tank is being fabricated. Fabricators approachthe process differently in terms of flow of materials and the sequenceof operations, which eventually influences how the structure is evalu-ated. The owner should inspect the quality of shop fabrication, weld-ing, and fit-up; the type of surface preparation; and the shop coating





if applied. The owner should compare the mill test certifications tothe heat numbers stamped on the steel plates. These mill test certifi-cations should be filed for future reference. The material should alsobe examined for excessive corrosion or pitting, signs that the materialhas been stored outdoors for too long.

The amount of fabrication performed in the shop varies depend-ing on the contractor. One function that will be performed in the shopis rolling of the plates. Table 19 in the D100 standard outlines the con-ditions under which the shell plates must be rolled. If D100 requiresthat the shell plates be rolled, the owner should verify that they arerolled to the correct radius. The conditions for welding and the avail-able equipment make welding in the shop much easier than weldingin the field. The welds should be visually evaluated in the shop to de-termine if they meet the requirements of D100 and can be adequatelypainted. Welding repairs can be more effectively performed in theshop than in the field.

This visit also serves to open communications for the balance ofthe project.

Steel DeliveryThe owner should be on hand when the steel is delivered to the jobsite. The owner can help resolve conflicts with neighboring propertyowners, document any damage occurring in the unloading process,and protect underground utilities on the site or under the access road.

Tank ErectionErecting and welding or bolting the steel are tasks for which the exper-tise of the constructor is vital to the success of the project. Erecting steelis a dangerous operation, requiring skills acquired only through ex-perience. During this phase, the purchaser’s representative may needassistance. Independent testing laboratories are usually equipped totake radiographs of welded seams, but they know little about steelerection and fit-up and are not willing to climb to the heights usuallyassociated with water-storage facilities.

Using someone from another tank constructor’s organization asthe purchaser’s representative can lead to conflicts of interest andother problems. It is very difficult for a competitor to be unbiased inthe evaluation of another constructor’s work. Even if this competitoris fair, it is difficult for the tank constructor to accept the opinion asan unbiased one. Therefore, it is usually best to secure the services ofa consultant who specializes in this type of inspection work and hasthe expertise and climbing ability to accomplish the job.





Fit-Up Quality

Tank BottomThe levelness of the tank’s base plate(s) is critical if the rest of the tankis to be erected properly. The constructor’s steel-erection supervisorshould check the foundation(s) for differences in elevation. Any suchdifferences should be compensated for by shims underneath the baseplate(s). If this task is not done properly, or if there were fabricationproblems with the steel, the purchaser may see slivers of steel that needto be cut from seams, frequent use of a large hammer to form the steelvariations in the seam gaps, or plates not aligned in accordance withthe tolerances required in AWWA D100 Section 10.6.3. These prob-lems usually produce a tank of unacceptable aesthetic or structuralquality.

Tank ShellAWWA D100 has several fit-up requirements. Most are related to ap-pearance of the structure, but improper fit-ups can be structurallysignificant if bad enough. AWWA D100 addresses plumbness of theshells of ground storage tanks. The shell’s deviation from verticalshould be measured as the shell is erected, and variations from verti-cal should be corrected when they approach the limits set in AWWAD100. The standard also establishes the roundness of the shell. Thetank diameter should be measured in several locations 1 ft (0.3048 m)above the tank bottom corner weld. The measurements should notexceed the tolerances established in AWWA D100. AWWA D100 alsoestablishes tolerances for peaking and banding of the shell of groundstorage tanks. Peaking is the out-of-plane distortion across a verticalweld seam, and banding is the out-of-plane distortion across a hori-zontal weld seam. To measure peaking and banding, a sweep boardis useful. A sweep board can be constructed from a piece of plywood36 in. (0.9144 m) long. One side of the plywood board should be curvedto the radius of the tank, and the other side should be flat. The curvedside should be used to measure peaking, and the flat side should beused to measure banding. The offset of aligned shell courses is gov-erned by AWWA D100. During fit-up, it should be verified that theplates are aligned within the tolerances established in D100, and thesetolerances should be maintained throughout the welding process.

Double-Curved, Axisymmetrical, Conical, and Cylindrical SectionIn D100, tolerances for these sections are specifically established forstability and are structurally significant. The owner should verifythrough the formulas given in D100 that the double-curved, axisym-metrical, conical, and cylindrical sections are within the tolerancesestablished.





Welding QualityAccording to AWWA D100, the constructor is required to check thequality of the welding. It is the job of the owner to monitor the con-structor’s quality control program. First, the owner should collect thecertification papers of all welders on the site. These papers detail thetypes of welding and the steel thicknesses for which the welder iscertified. The most common method of evaluating weld quality isby means of radiography. The purchaser should participate in theselection of radiograph locations, watch for documentation of the ra-diographs, and review the radiographs with the constructor’s qualityassurance expert. The areas selected should be in strict accordancewith the AWWA D100 standard, which requires that the radiographsreflect the general quality of welding on the tank. Equal proportionsof shop, ground, and air welds should be reflected in the radiographs.The contractor should document these radiograph locations on a roll-out. AWWA D100 has radiograph inspection standards that must beused for evaluating discontinuities and defects present in radiographfilm. The penetrameter, which ensures the reviewer that the radio-graph was sensitive enough to identify the smallest defect addressedby the standard, must be visible in each radiograph. It is vital thatthe radiographs be evaluated in accordance with this criterion andthat any repairs and follow-up radiographs be conducted in accor-dance with D100. It is also important for the tank constructor andpurchaser to visually inspect all welds to ensure the removal of allweld splatter, sharp surfaces, overlaps, and unacceptable undercutsthat will be detrimental to the coating life. Welds do not need to beperfectly smooth, but sharp edges must be removed. Ground storagetanks erected under AWWA D100 Section 14 Alternative Design Basisrequire many more radiographs than standard tanks.

Bolting AssemblyBolted-steel tanks require the proper placing of steel sheets, gaskets,and sealants. Some erection methods may also require pre-tensioningthe sheets and tightening the bolts to a prescribed torque. These detailsare covered by the manufacturer’s erection instructions and drawings.The engineer or purchaser may require that a set of these instructionsbe included with the shop drawing package that is submitted.

Tank AppearanceTank appearance is of great importance to many owners. The finalappearance is known only after the tank is coated, when dents andbuckles become apparent. It is then that the owner expresses dissat-isfaction. Determining how well the tank complies with the speci-fications and applicable codes and negotiating a settlement for poorappearance are time-consuming and stressful. Usually these problems





can be avoided if the constructor checks to see that the tank is level,round, and plumb as it is being built. Incorporating dimensional tol-erances into the contract will also minimize disputes. AWWA D100specifies some of these tolerances.

Surface RegularityA smooth, regular surface provides a good base for the application of aprotective coating system, thus helping reduce maintenance costs. Tothis end, the constructor and the purchaser should ensure that the weldcontour is smooth, that unacceptable weld undercutting is eliminated,that weld spatter is ground off, that remains of welds used to attacherection and fit-up equipment are chipped and ground smooth, andthat unacceptable gouged-out places in the steel are filled in.

Representatives of the tank constructor and the purchaser shouldbe alert for sharp edges or areas that would cause premature coat-ing failure so that corrective action may be taken as the work pro-gresses.

Factory-Coated Bolted TanksA bolted-steel tank is delivered to the location with a factory-appliedcoating. If the steel has not been damaged in transit, the surfaces willbe smooth. Each panel should be carefully inspected before erection.If the panel is damaged in transit, in handling, or during erection ofthe tank, it may be necessary to repair or replace it.

Water TestingWhen welded tanks are water tested before they are coated, any leaksthat are found can be repaired without requiring any coating to beredone. If the tank is not filled until after it has been coated, small pin-holes in the welds may be plugged temporarily with coating; thesewill cause leaks later if the coating breaks loose. The owner shouldensure that water for the test is available at the time and pressure nec-essary to coincide with the constructor’s schedule. The owner shouldalso ensure that provisions are made for draining and disposing ofthe test water. If leaks are found in factory-coated bolted tanks, theconstructor should make repairs according to the manufacturer’s rec-ommendations.

Field Cleaning and CoatingThis section discusses the cleaning and coating of welded-steel tanksafter they have been erected and before they are placed in service.





Erection Scar RemovalBy the time the tank is erected, irregularities in the surfaces of thetank should have been eliminated. The erection crew has the equip-ment and scaffolding to smooth out these defects economically andeffectively. On the other hand, the coating crew is often subcontractedor made up of members of another department of the tank company.Their rigging is sometimes not as well suited for this cleanup work asthe scaffolding of the erection crew.

Occasionally, abrasive blasting reveals small laminations in thesteel plate. If these are not removed completely by blasting and remainlarge enough to produce holidays in the coating system, they should beremoved by grinding. Occasionally, deeper laminations may requirewelding or further testing.

Steel CleanlinessThe first requirement for a good coating is a clean surface. The steelshould be free from dirt and oil, both of which may accumulate duringconstruction. All weld seams, abraded areas, scratches, shop or fieldmarkings, or poorly adhering shop primer should be removed byabrasive blast cleaning. The areas cleaned by abrasive blasting shouldblend well into the adjoining undisturbed shop primer. Some shop-applied primers must be scarified or otherwise prepared before ensu-ing coats are applied.

The purchaser should also be aware that welding or cutting activ-ity on one side of a plate is likely to damage the coating bonding on theopposite side of the plate. This is especially important if shop primingis used. The areas opposite welding or cutting operations should beexamined for coating damage resulting from the heat induced by thecutting or welding process.

The manuals Good Painting Practice and Systems and Specificationsvisual standards and an inspection manual available from the Societyfor Protective Coatings give good guidelines for inspecting coatings.

Inspection InstrumentsInstruments needed to inspect coating include at a minimum a wet-film thickness gauge, a calibrated dry-film thickness gauge, equip-ment for measuring air temperature and humidity, a steel-temperaturethermometer, a surface-profile measuring device, and a wet-sponge-type holiday detector. The holiday detector is used to inspect the coat-ing for voids that will cause premature coating failure. If full-timeinspection is not conducted, destructive testing involving the use ofa Tooke Gage and/or other instruments will be necessary to evaluatethe thickness of each coat and to obtain an indication of the cleanlinessof the substrate.





Inspection PlanningThe purchaser should plan work to aid in the timely completion of thetank field coating. This will require open lines of communication withthe coating company and an understanding of the effects of weatheron coating progress. The constructor will also need to work efficientlyin good weather.

The purchaser should state requirements for the number of lo-cations to be tested according to the total surface area of the plate.Minimizing testing is unwise, but an excessive number of testing lo-cations places an unreasonable burden on the constructor and cansubstantially delay the progress of the tank coating. SSPC PA2 delin-eates procedures for measuring coating dry-film thicknesses. To avoidexcessively delaying the coating progress, large tanks may requiremore than one purchaser’s representative to conduct the required fieldtests.

Technical Aspects of CoatingsToday’s coatings require exactness in measuring and mixing com-ponents and thinners. The appropriate application equipment mustbe used, and the proper combination of humidity and dew point,air, and steel temperatures is critical during both application andcuring.

The tank interior must be ventilated to ensure the safety of work-ers and to allow for the proper curing of the field-applied coatings.Fans or air horns are usually required to move air through the tank.Even with forced-air ventilation, proper breathing equipment is nec-essary for the safety of the workers and the purchaser’s represen-tative(s).

Shop-Applied CoatingsBolted tank panels are coated at the factory under controlled condi-tions. AWWA D103 requires that the panels be grit-blasted to near-white metal (SSPC SP10) and coated within 15 minutes of cleaningto prevent rust from starting. The coating is then either baked on orfused on. If desired, the purchaser may observe these operations dur-ing shop inspection.

If specified, a preconstruction primer may be shop applied to newwelded-steel tanks. Observation of the shop painting and fabricationof the steel components of welded-steel tanks is necessary to evaluateproper fabrication techniques, shop surface preparation, and shopprimer application.





Mechanical and Electrical AppurtenancesMechanical and electrical items that need to be checked vary depend-ing on the equipment specified for a particular project. The followingshould be checked on every project:� Electrical wiring should meet applicable codes.� Conduits, fixtures, pipes, valves, or other items should not

interfere with the safety of the ladders or platforms.� Cathodic protection anodes and hand-hole covers should beproperly placed, and the purchaser’s representative shouldwitness the energizing of the cathodic protection system andthe initial potential profile being conducted as per the projectspecifications.� All hatches should be locked.� Safety belts and sleeves should be furnished for the laddersafety devices. Safety sleeves should be checked for properoperation along the full height of the rails or cables. Any coat-ing, deviations, or obstructions that prevent the free operationof the sleeve should be removed.




C H A P T E R 9Operation

Jose N. Hernandez, P.E.City of Cleveland

Sami F. Sarrouh, P.E.Brown and Caldwell

There are an estimated 400,000 potable water storage tanks in theUnited States. Worldwide, the total number of tanks is undoubtedlystaggering. Water-storage facilities have played and continue to playa pivotal role in the growth and success of water distribution systems.

Existing and new tanks come in a variety of styles, constructionmaterials, configurations, and functions, and they have a wide rangeof capacities. Tanks have been constructed from materials as diverseas steel, wood, and concrete; they can be at ground level, elevated, orburied. They can have many different inlet and outlet configurationsand can hold as little as 10,000 gal (37,854 L) or as much as 140 mil gal(529,957.6 kL) or more (Fig. 9-1).

These and other factors influence tank performance and capabil-ities. Such characteristics also play a role in how utility personneloperate the tanks, and how they affect the important tank functionssuch as emergency storage capacity and the preservation of waterquality. Concerns about the effects of tank characteristics and opera-tion on water quality present new challenges for both operators andtank design engineers.

It is the intent of this chapter to provide the reader with practicalinformation, in addition to technical and scientific data, regarding thedesign and operation of potable water storage tanks.

Although tanks are often the most visible part of a water distribu-tion system, they are just one among several major components in asystem designed to bring potable water to utility customers. Potableor drinking water can be defined as the water delivered to the con-sumer that can be safely used for drinking, cooking, and washing(De Zuane 1997).

295




296 C h a p t e r N i n e

FIGURE 9-1 Masonry and concrete reservoir, capacity 23 mil gal (87,064 kL),in Parma Heights, Ohio.

A utility’s water may begin its journey to its customers from anonpotable (raw) source or from another utility acting as a wholesaler.Raw water can be groundwater, such as in the case of wells. It can alsobe surface water from lakes, rivers, or dams (Fig. 9-2). It may also bedesalinated brine or seawater.

The raw water is treated according to rules and regulationsestablished by government regulatory agencies. Myriad treatmentstrategies are available. The applicability of each treatment strategydepends on the characteristics of the source water and the technology

FIGURE 9-2 Water intake crib in Lake Erie; Cleveland, Ohio, is in theforeground.



OperationOperation

297O p e r a t i o n

Raw watersource

Treatmentplant

Clearwell

Elevatedtank

Pumpingstation

Groundtank

Trunk and distributionwater mains

Surgetank

FIGURE 9-3 Major components of typical water distribution system.

required to make it potable. There is an extensive list of referenceson potable water treatment. AWWA’s Water Quality and Treatment,A Handbook on Drinking Water, published by McGraw-Hill (AWWA2010), is an excellent general reference on this subject. Once treated,water is usually stored in a large tank or reservoir at or near thetreatment plant. Finished water is then pumped into a water dis-tribution system serving a community or service area, as demandrequires.

Water distribution systems comprise several major components(Fig. 9-3). Pipes and piping systems include trunk and distributionmains, valves, and hydrants. Trunk mains are large-diameter pipesthat carry water away from treatment plants. Trunk mains branch offinto distribution mains. Distribution mains are smaller pipes that carrythe water through individual streets or zones. Each water customerhas at least one service tap into a distribution main. In a well-designeddistribution system, trunk and distribution mains act as a network, orweb, with many connecting nodes. The web connections are redun-dant. If a section of pipe inside the web should break, valves couldbe closed to isolate a small affected area and conduct repairs whilemaintaining service to the remaining customers in the system. Thepipe network’s design serves another function. A completely closedpipe network has no dead ends, thus eliminating the possibility ofwater stagnation.

When a distribution system’s area is very large or encompassessubstantial differences in elevation, it is often necessary to divide thesystem into two or more pressure zones, or “highs.” Booster pumpingstations are used to raise the pressure of the water as it is pumped froma low-pressure zone to a high-pressure zone.

Storage tanks are connected to the trunk-main web. Under opti-mal conditions, a distribution system is designed so that one or morestorage tanks are located at the opposite end of the system’s grid fromthe pumping station.

The main function of a water distribution system is to deliver suf-ficient quantity of potable water at a minimum established pressure



OperationOperation


to utility customers. This role has been expanded in recent years tothat of preserving the quality of the water in the system. More detailedinformation regarding the design and operation of potable water dis-tribution systems is available from many sources, including the WaterDistribution Systems Handbook (Mays 2000).

Modeling of Tanks in Water Distribution SystemsTanks are an integral element of water distribution systems. Waterdistribution system modeling has become an essential tool for distri-bution system operators, scientists, and engineers. Models represent awater distribution system as a web or network in which different hy-draulic scenarios can be simulated. The network model is representedby a collection of pipe lengths interconnected in a specific topologicalconfiguration by node points where water can enter or exit the sys-tem (Fig. 9-4). Tanks represent boundary conditions in these models.Traditionally, tanks in water distribution system models define thehydraulic grade line limits at its boundaries.

Modeling is concerned with water behavior when the water isboth moving and stationary. An important parameter in a water dis-tribution system is pressure. For instance, pressure is a function ofdepth within a tank. The pressure is the same at two points that areat the same fluid depth regardless of the shape or volume of the tank.Pascal’s law states that the pressure at any point inside a tank hasthe same magnitude in all directions. In other words, pressure is ascalar quantity, not a vector. Pressure is always perpendicular to asubmerged surface regardless of the surface’s shape or orientation.

Treatmentplant

Tank 1

Tank 2

Tank 3

FIGURE 9-4 Node representation of a distribution system network; arrowsindicate average pipe segment flow.



OperationOperation


Elevatedtank

Pump

Groundtank

Trunk and distributionwater mains

V 2/2gEnergy grade line

Hydraulic grade line

Totaldynamic

head

Hstatic

Hatmospheric

Datum

Absolute zero pressure

FIGURE 9-5 Distribution system energy lines. (Note: V2/2g = velocity head.)

The energy head at any point in the distribution system is calcu-lated using the Bernoulli equation:

H = P�

+ V2

2gc+ Z

Each point in the system has an energy level, H. The energy gradeline (EGL) is a graph of the total energy versus position in a pipelinefrom a reference line or datum. Friction losses in the pipe cause theEGL to slope downward in the direction of flow. Since friction inthe pipe is a function of fluid velocity, the faster the flow, the steeperthe EGL. The hydraulic grade line (HGL) is a graph of the sum of thepressure and potential energies versus position in a distribution sys-tem from the reference line or datum. The HGL level at any point inthe distribution system is equal to the height a water column wouldrise inside a vertical conduit open to the atmosphere. It can also rep-resent the height of the water in a tank connected to the system at thatpoint. The kinetic energy component is the difference between the EGLand the HGL. Figure 9-5 shows distribution system energy lines.

In most water distribution applications, the elevation and pressurehead terms are much greater than the velocity head term. For thisreason, velocity head is often ignored, and modelers work in terms ofhydraulic grades rather than energy grades. Therefore, given a datumelevation and an HGL, the pressure can be determined as (Walski,Chase, and Savic 2001):

P = � (HGL − Z)

Both the EGL and the HGL increase at the location of a pumpwithin the pipeline by an amount equal to the head added by pump-ing. The head added by pumping (hA) may be calculated if the flow



OperationOperation


velocity and gauge pressure are known for the suction and dischargepump nozzles.

hA =(

P�

+ V2

2gc+ Z

)discharge

−(

P�

+ V2

2gc+ Z

)suction

The head added by the pump is a function of the flow rate throughthe pump. A graph of pump head versus flow rate is called the pumpcurve. The plotted head is the head difference across the pump and isgiven the name total dynamic head (TDH).

Traditional hydraulic modeling provided an insight into how thewater level in tanks changed under simulated demand conditions.However, the water in the tank was considered of invariable quality.Hence, there was no difference in quality whether the water was en-tering the tank, inside it, or leaving it. In the past decade, increasedemphasis on water quality in storage facilities prompted more detailedinvestigations of water dynamics in tanks.

Modeling of water flow regimes and water quality in tanks hasbeen done using multiple methods:� Scaled laboratory models� Tracer studies� Semiempirical mathematical models� Computational fluid mechanics using finite-element or finite-

volume models

Scaled Laboratory ModelsFor more than a century, scaled models have been used to validatehydraulic structures, vessels, and tank designs. Geometrically similarscaled-down models are tested under laboratory conditions to sim-ulate actual operating conditions involving fluid flow phenomena,since governing equations regarding the latter do not have exact math-ematical solutions. The laws governing tank scale-model constructionare known as principles of similitude. These principles make it possi-ble to construct models that accurately represent actual tank perfor-mance.

The main dimensionless parameters used in scale models areFroude number

Fr = U√Lg

Reynolds number

Re = UL�

�



OperationOperation


Weber number

We = U2 L�

�

where U is the free stream velocity of the system; L is the characteristiclength, diameters, or depths; g is the gravitational acceleration; � iswater density; � is the viscosity of water; and � is the surface tensionof water.

A tank and a geometrically similar model are in similitude if Re ,Fr ,and We are the same. The scaling of the model, however, is based on thepredominant parameter for each particular application. The Froudenumber is generally used in the scaling of water-storage facilities. Ascale model should be as large as possible to minimize scaling errorson tank performance. Many sources quote typical scale ratios for tankmodels in the range of 1:30 to 1:50.

Chemical tracers or dyes are used to visualize streamlines in themodel. However, most sophisticated technology, such as optical tech-nology and lasers, has been used to quantitatively measure scalarvalues. The objective of these tests is to trace water streamlines fromthe inlet as they mix with ambient water in the vessel and exit the tank.The tracer is monitored at the outlet and compared with a constantinfluent concentration over time to determine how much time waterspends in the tank (residence time).

The effects of temperature on the buoyancy of influent water areof particular interest for tank modeling. Temperature distribution intanks can be measured at different locations and depths in the modelwith temperature probes.

Tracer StudiesTracer studies are conducted in water storage tanks to determine res-idence time and/or water quality distribution (Fig. 9-6). Tracers maybe chemicals or dyes that can be tracked or measured by ion-specificelectrodes, conductivity probes, colorimeters, or visual/camera ob-servation. A tracer can be introduced at a known concentration by adosing pump at the tank’s inlet piping. Typical tracers include chlorideions (such as sodium chloride, calcium chloride, and lithium chloride),fluoride ions, and the fluorescent dye Rhodamine WT. Local regula-tory agencies must approve the use of a particular tracing chemicalfor studies of actual potable water tanks. The tracer should not beconsumed or removed during treatment. Fluoride ions are not typ-ically present naturally in water. Therefore, fluoride can be used inlower concentrations than chloride tracers. Rhodamine WT must beused following certain guidelines found in Appendix D of the August1999 US Environmental Protection Agency (USEPA) Disinfection Pro-filing and Benchmarking Guidance Manual (USEPA 1999). Selection of a



OperationOperation


0 1 2 3 4 5

Time (h)

24

22

20

18

16

14

12

10

Chlo

ride ion c

oncentr

ation (

mg/L

)

FIGURE 9-6 Tracer study results of live pass-through distribution system tankwith plug flow to determine contact time.

particular chemical tracer may depend on the salt concentrationspresent in the water. Specific instructions on chemical tracers and theconditions under which they are most effective are found in AppendixD of the Guidance Manual. If a tracer study is needed to find the con-tact time (CT), a water system should consult the latest tracer studyguidance from its regulatory agency.

The tracer chemical should be added at the same injection pointsas the disinfectant to be used in the CT calculations. Tracers are com-monly added in two ways: the step-dose method and the slug-dosemethod. In the step-dose method, the tracer is injected at a con-stant dosage, and the endpoint concentration is monitored. To acquire90 percent recovery for the tracer, endpoint sampling should continueuntil the tracer concentration reaches steady-state level. In the slug-dose method, however, a large dose of tracer chemical is instanta-neously injected. The tracer concentration is then monitored at theendpoint until the entire dose (slug) has passed through the tank. Fig-ure 9-7 shows apparatus to measure CaCl tracer concentration. A massbalance is required for this method to determine whether the entiretracer dose was recovered. Contact time is then determined mathe-matically from the concentration versus time profile. A tracer study isgenerally done as follows:� Determine the flow rate or rates to be used in the study.� Select the tracer chemical and determine the raw water back-

ground concentration of the tracer chemical. This is needed



OperationOperation


FIGURE 9-7 Apparatus to measure CaCl tracer concentration at tank inlet andoutlet. Ion-specific probes can be seen at left. Data were recorded on a PC.

to determine the dosage of chemical to feed and to properlyevaluate data.� Determine the appropriate tracer dosage.� Determine tracer addition locations, sample collection logis-tics, and sampling frequency. Sampling frequency depends onthe size of the tank—the larger the tank, the less frequent sam-pling is needed, but the longer the duration of the samplingevent.� Conduct the tracer test using either the step-dose or slug-dosemethod.� Compile and analyze the data.� Calculate CT.

Semiempirical Mathematical ModelsThese models interpolate or extrapolate experimental data using sim-plistic mathematical relationships to predict tank performance onthe basis of inlet and outlet operational conditions and simplified



OperationOperation


geometric parameters. Some of these models have an extensivedatabase of experimental data so that they can produce highly accu-rate results if the operational conditions are within the data’s range.These models have been adopted by the USEPA for outfall modelingof discharges into bodies of water.

Computational Fluid Mechanics Using Finite-Element orFinite-Volume ModelsA multitude of computational fluid dynamics (CFD) software pro-grams are commercially available. Although the mathematical algo-rithms might differ slightly, they all predict fluid behavior in vesselsand around structures by solving the Navier–Stokes equations. Thesealgorithms use a multitude of turbulence models to predict time-averaged turbulent flow characteristics. Often, the software packagesoffer a choice of turbulence models to the modelers. The model cho-sen affects the accuracy of the solution depending on the phenomenainvolved in each particular study.

Grid generation is another critical software component that candrastically affect accuracy and convergence. The modeler must de-termine the regions where finer grids are required and track conver-gence at critical locations. Models differ in their ability to handle com-plex geometries, curved surfaces, inlets/outlets, buoyancy effects, andboundary conditions such as free boundaries (e.g., changing level atthe water-to-air boundary) (Fig. 9-8). Transient flows, separation and

FIGURE 9-8 Computational fluid dynamics model of reservoir with two inletsand two outlets showing velocity contours. Light gray is highest velocity anddark gray is stagnant.



OperationOperation


reattachment, vortices, and swirl are not accurately predicted by cur-rent mathematical procedures. Specialized application software pack-ages are available to predict such phenomena, but they are limited inapplication and cannot easily be generalized due to their reliance onnumerous empirical and semiempirical equations. These limitationsallow general-purpose CFD modeling to be more useful for bafflingand plug flow optimization than mixing, for example. Some packagesinclude dispersion models or the ability to incorporate transport equa-tions programmed by the modeler. Nevertheless, the capability of thepackages to produce accurate predictions is limited by the mathemat-ical constraints of modeling curvilinear flow patterns and fluctuatingor time-dependent phenomena. Their capability is also limited by themodeler’s grasp of advanced fluid mechanics principles and his orher ability to detect an improbable or inaccurate result. Otherwise,the phrase “garbage in, garbage out” may apply. Many CFD usershave learned that validating models against real-world testing or ex-perimental data is needed to prevent faulty or inaccurate solutions.CFD is a step in the design process that minimizes trial and error, butit is not in itself a validation of design.

Water Quality Issues for Water Storage Tanks inDistribution Systems

The quality of potable water in the United States is defined by regula-tions that govern all facets of water treatment and distribution. Theseinclude, among others, means of filtration and disinfection, reduc-tion or elimination of contaminants, control of taste and odor, back-flow prevention, and distribution system minimum pressures. TheUSEPA’s Division of Drinking Water has the primary charge of es-tablishing and enforcing these regulations. Regulations set forth bythe Safe Drinking Water Act (SDWA) directly affect the operation ofwater distribution systems. “The intent of the SDWA is for each stateto accept primary enforcement responsibility (primacy) for the oper-ation of the state’s drinking water program. Under the provisions ofthe delegation, the state must establish requirements for public wa-ter systems that are at least as stringent as those set by the USEPA.In some states, the primacy agency is the health department, andin others it is the state’s environmental protection agency, depart-ment of natural resources, or pollution control agency” (Von Huben1999).

Amendments to the SDWA made in 1996 included the requirementthat water utilities create and distribute consumer confidence reports(CCRs). The yearly CCR includes information regarding compliancewith maximum contaminant levels (MCLs) and treatment techniquesfor drinking water. Utilities serving more than 10,000 people were



OperationOperation


FIGURE 9-9 Chlorine residual analyzers monitor inlet and outlet disinfectantresidual at ground storage tank.

mandated to mail the CCR directly to their customers. The reportdiscloses contaminant levels in a community’s drinking water supplyalongside MCLs and maximum contaminant level goals (MCLGs). Italso tells what type and concentration of disinfectant, such as chlorineor chloramines, is used by the utility.

Whenever a disinfectant molecule reacts with organic or inorganicmolecules in the water, it forms what are commonly known as disinfec-tion by-products (DBPs). DBPs are a collection of chemicals, includingtrihalomethanes (THMs) and haloacetic acids (HAA5), that are highlysoluble in water. Animal testing has demonstrated that some of thesecompounds have carcinogenic properties. Other disinfectants, such aschloramines, chlorine dioxide, and ozone, may yield DBPs upon react-ing with organic or inorganic molecules. The reaction products fromsuch interactions are similar to those produced by chlorine, but theytend to occur in smaller concentrations. Figure 9-9 illustrates a chlorineresidual analyzer. The USEPA announced in December 1998 the Dis-infectant/Disinfection By-Product Rule, Stage 1 (D/DBP). This rulecreated the following MCLs: total THMs, 80 �g/L; HAAs, 60 �g/L;and bromate, 10 �g/L. The rule also established MCLs for disinfec-tants: chlorine, 4 mg/L; chloramines, 4 mg/L; and chlorine dioxide,0.8 mg/L (AWWA 2003 Principles and Practices).

The USEPA published the Stage 2 Disinfectant/Disinfection By-Product Rule (Stage 2 DBPR) on January 4, 2006. The Stage 2 DBPR



OperationOperation


builds on existing regulations by requiring water systems to meetDBP MCLs at each monitoring site in the distribution system to betterprotect public health.

The Stage 2 DBPR includes a provision requiring all communitywater systems (CWSs) and only nontransient noncommunity wa-ter systems (NTNCWS) serving more than 10,000 people to conductan initial distribution system evaluation (IDSE). NTNCWSs servingfewer than 10,000 are exempted from IDSE requirements, but willneed to comply with the Stage 2 DBPR compliance monitoring re-quirements. The goal of the IDSE is to characterize the distributionsystem and identify monitoring sites where customers may be ex-posed to high levels of total trihalomethanes (TTHM) and haloaceticacids.

Compliance with the IDSE requirements includes the modelingor monitoring of distribution system site on the basis of criteria out-lined by the EPA that include all water-storage facilities. Utilities arerequired to report trended averages per site and address MCL viola-tions.

The Total Coliform Rule mandates monthly monitoring of distri-bution system samples for coliform bacteria. A fecal or Escherichia colibacteria test must be performed on any positive sample. Additionalsamples and analyses are required at the positive-sample point as wellas in its vicinity within 24 hours. Utilities must submit a sampling plan,and samples must be taken at the customers’ taps or from taps thatare representative of the distribution system. The Total Coliform Ruledoes not require the testing of water from storage facilities or theiroutlet pipes.

The Lead and Copper Rule (LCR) mandated desktop studies todetermine the need for corrective action regarding lead and copperconcentrations. Many utilities discovered that they required the ad-dition of corrosion-inhibiting agents to arrest the release of copperor lead ions from piping materials into the water. Action levels forcopper and lead were set at 1.3 mg/L and 0.015 mg/L, respectively.Pilot studies were conducted by utilities to identify the most suitableinhibitor for their system. Orthophosphate and phosphoric acid arepopular choices. Stagnant areas of distribution systems, such as deadends or poorly mixed tanks, readily lose their residual concentrationover time. Residual phosphate compounds used as inhibitors do notreadily dissipate like disinfectants do. In the absence of sufficient dis-infectant, these compounds can foster the development of biologicalagents such as algae and bacteria. Regular flushing of dead ends indistribution systems and appropriate tank design and operation areneeded to eliminate this potential problem.

The water quality in distribution systems’ water storage tanks andreservoirs is affected by several other factors. Some likely issues, theircause, and suggested solution are listed in Table 9-1.



OperationOperation

Issue

Possib

leC

ause(s

)Suggeste

dSolu

tion(s

)

Low

dis

infe

ctant

resid

ual

inw

ate

rente

ring

the

sto

rage

faci

lity

� Excessiv

ew

ate

rtr

ansport

tim

eto

tank

� High

bio

logic

alor

chem

ical

dem

and

indis

trib

ution

sys

tem

� Distr

ibution

sys

tem

back

flow

or

pip

ein

tegrity

issues

� Number

and

siz

eofta

nks

in

series

upstr

eam

offa

cilit

yand

their

wate

rqualit

yis

sues

� Investigate

low

flow

or

dem

and

for

siz

eofm

ain

s� Boo

st

dis

infe

ctant

inla

rge/m

ultiz

oned

dis

trib

ution

sys

tem

s� Find

valv

eis

sues

intr

unk

main

s� Bio

film

son

inte

rior

pip

esurf

ace

are

difficu

ltto

elim

inate

,

pre

vention

isbest

choic

e:adequate

pip

eve

loci

ty,

dis

infe

ctio

n,and

flushin

g� Imp

rove

dis

infe

ctio

nand

flushin

gaft

er

main

repair

or

insta

llation

� Elimin

ate

dead

ends

indis

trib

ution

sys

tem

� Findand

repair

main

bre

aks,le

aks,and

back

flow

issues

� Solve

issues

at

upstr

eam

tanks,if

any

� Boost

dis

infe

ctant

Nitrifica

tion

� Chlora

min

es

� Ensure

adequate

chlo

ram

ines

resid

ualth

roughout

the

sys

tem

(2.0

to2.5

mg/L)

� Reduce

wate

rage

by

avo

idin

gexc

essiv

esto

rage

resid

ence

tim

e � Usech

lorine:a

mm

onia

:nitro

gen

ratio

of4.5

:1

Inadequate

tank

turn

ove

r� Low

inflow

and/or

outfl

ow

� Lowta

nk

volu

me

change

ove

r

tim

e � Lownorm

aldem

and

vers

us

tank

siz

e � Impro

per

tank

loca

tion,ele

vation,

or

siz

e

� Hydra

ulic

analy

sis

toin

vestigate

pro

per

dis

trib

ution

pip

ing,

pum

pin

g,or

sto

rage

requirem

ents

� Revis

itta

nk

loca

tion

and

ele

vation

� Excessiv

epum

popera

tion

308



OperationOperation

Str

atifica

tion

� Tank

geom

etr

yand

loca

tion

of

inle

tsand

outlets

� Buoya

ncy

caused

by

tem

pera

ture

diffe

rence

betw

een

tank

am

bie

nt

and

influent

wate

r

� Increase

tank

turn

ove

r� Imp

rove

tank

mix

ing

� Properly

loca

tein

lets

and

outlets

Loss

ofdis

infe

ctant

resid

ualin

dis

trib

ution

sys

tem

tanks

� Accum

ula

tion

ofsedim

ents

:

chem

icaland

bio

logic

aldem

and

� Exposure

toultra

viole

t(U

V)lig

ht

(open

tanks)

� Diffusio

ndue

toam

bie

nt

air

tem

pera

ture

� Corro

sio

nofta

nk

meta

ls� Exc

essiv

ew

ate

rta

nk

resid

ence

tim

e

� Perio

dic

cleanin

gofta

nk

sedim

ents

� Elimin

ate

tank

ele

ments

that

allo

wsedim

ents

toacc

um

ula

te

or

pro

vide

habitat

for

bio

film

s� Inst

all

appro

priate

tank

cove

r� Att

ank

desig

n,decr

ease

ratio

betw

een

wate

rsurf

ace

are

a

and

tank

volu

me

� Desig

nve

nting

topre

vent

exc

essiv

eair

dra

fts

above

wate

r

surf

ace

� Conduct

periodic

tank

main

tenance

and

rehabili

tation

� SeeS

tratifica

tion

Bio

film

s� Ava

ilabili

tyofnutr

ients

inw

ate

ror

tank

surf

ace

s� Low

dis

infe

ctant

resid

uals

� Nitrifi

cation

� Warm

tem

pera

ture

s� Cor

rosio

nin

hib

itors

� Tank

coatings

� Perfo

rmperiodic

tank

cleanin

gs

� Monitor

conditio

nofve

nts

and

hatc

hes

topre

vent

bio

logic

al

pollu

tants

from

ente

ring

tank

� SeeLoss

ofD

isin

fect

ant

Resid

ual

� SeeN

itrifica

tion

� Impro

veta

nk

turn

ove

rand

mix

ing

� Repla

cew

ith

NS

F61-a

ppro

ved

coatings

that

do

not

pro

mote

bio

gro

wth

;m

inim

ize

crevi

ces,gaps,and

substr

ate

imperf

ect

ions

befo

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TA

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Suggeste

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(Continued)

309



OperationOperation

Issue

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310



OperationOperation

Conta

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9-1

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olu

tions

(Continued)

311



OperationOperation


Water Quality Monitoring in Distribution System TanksThe importance of monitoring the quality of water in storage facili-ties is fast becoming a focus of interest. Although no US regulationsrequire the monitoring of water quality parameters in storage tanks,there are several important reasons why utilities should maintain amonitoring program for tanks. Other countries, such as the UnitedKingdom, mandate frequent monitoring of tank water quality.

Tanks are an intermediate stage between the water treatmentplants and customers. Depending on the size of a distribution system,demand, and other factors, water in tanks could have left the treatmentprocess from several minutes to several days, or even longer, before itentered the tank. Testing of water quality parameters in storage pro-vides a utility with valuable information regarding what happens tothe water as it moves through the distribution system en route to thecustomer. Testing can also indicate whether detrimental occurrencesare taking place in the storage facility itself. Sampling data can alsobe used in the determination of tank mixing efficiency and turnover.

Security ConcernsThe events of September 11, 2001, raised questions about the safetyof the nation’s water supply. In general, water tanks are accessiblethrough access hatches, air vents, overflows, and access ladders. Theseentry points can be tempting targets for vandals or criminals attempt-ing to contaminate the water supply. They are also potential sourcesof accidental contamination from rainwater, birds, insects, and othernatural sources. Continuous online monitoring of water quality pa-rameters can provide an indication or alarm should contaminationoccur. The affected tank can be isolated from the system, and author-ities and the public can be notified. Figure 9-10 illustrates protectivemeasures for water tanks.

Tank-Monitoring ProgramThe tanks themselves can be a source of concern. Leaching from coat-ings, red water and bacteriological concerns from rusting wet surfacesor overhead structural elements, accumulation of sediments that maycontain biofilms, settled metal particles, or other detriments to waterquality may be present in storage. Some may engender complaintsabout the water’s taste, odor, or appearance.

A tank-monitoring program provides a utility with backgrounddata of tank parameters for different seasons and weather conditions;it helps establish schedules for tank inspection and maintenance; and itcan serve as a guide to water quality managers for planning treatmentstrategies for changing distribution system conditions.

Although no regulatory requirements in the United States man-date tank monitoring, there are regulations that require periodic water



OperationOperation


FIGURE 9-10 Tank overflow and catch basin are protected from insects orvandals by combination of stainless-steel cage and insect screen; flow switch(not shown) triggers overflow alarm.

testing in distribution systems. The National Primary Drinking WaterRegulations (CFR 141.23), the Surface Water Treatment Rule (SWTR),the Lead and Copper Rule (LCR), the Total Coliform Rule, and Stage1 of the D/DBP all establish contaminant restrictions and monitor-ing requirements for distribution systems. Table 9-2 provides waterquality parameters and related regulatory information.

Regulations cover such important contaminants as total and fecalcoliform bacteria and heterotrophic bacteria, disinfection by-productssuch as trihalomethanes (THMs) and haloacetic acids (HAAs), nitrite,nitrate, lead, and copper. They also limit the concentration in the sys-tem of residual disinfectants such as chlorine or chloramines.

The USEPA has identified several secondary contaminants(nonenforceable) under the National Secondary Drinking Water Regu-lations (NSDWR). The enforceability of this rule is up to the individualstates’ regulatory agencies. Table 9-3 lists the secondary nonenforce-able contaminants.

At a minimum, utilities should attempt to monitor disinfectantresidual, heterotrophic plate count (HPC), and coliform bacteria inwater-storage facilities. Suspected biofilms on tank surfaces shouldbe sampled and speciated to determine source or cause. An ideal wayto test a tank for biofilms is to prepare a metal or concrete coupon witha coating and finish similar to those of the tank. The coupon can be



OperationOperation

Para

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Dis

trib

ution

Sys

tem

s

314



OperationOperation


Secondary Maximum

Parameter Conditions Contaminant Level

Aluminum Colored water 0.05–0.2 mg/L

Chloride Salty taste 250 mg/L

Color Visible tint 15 color units

Copper Metallic taste, blue-green

stain

1.0 mg/L

Corrosivity Metallic taste, corrosion,

fixture staining

Noncorrosive

Fluoride Tooth discoloration 2 mg/L

Foaming

agents

Frothy, cloudy, bitter taste,

odor

0.5 mg/L

Iron Rusty color, sediment,

metallic taste, reddish or

orange staining

0.3 mg/L

Manganese Black to brown color, black

staining, bitter metallic

taste

0.05 mg/L

Odor Rotten-egg, musty, or

chemical smell

3 TON (threshold

odor number)

pH Low pH: bitter metallic

taste, corrosion

6.5–8.5

High pH: slippery feel, soda

taste, deposits

Silver Skin discoloration, graying

of the white part of the

eye

0.10 mg/L

Sulfate Salty taste 250 mg/L

Total

dissolved

solids (TDS)

Hardness, deposits,

colored water, staining,

salty taste

500 mg/L

Zinc Metallic taste 5 mg/L

Source: Adapted from Kirmeyer et al. (1999; p. 42, Table 2.2).

TABLE 9-3 Secondary Nonenforceable Contaminants in Water DistributionSystems



OperationOperation


Parameter Indication Parameter Is Present

Iron oxide Distribution system corrosion

Aluminum hydroxides Excess aluminum after flocculation

Calcium carbonates Supersaturation of minerals in hard waters

Manganese Source-water problem

Heterotrophic plate

count (HPC)

Taste and odor problems; recurring bacterial

counts

Depth of sediment Rate of accumulation; disinfectant residual

loss due to resuspension

Gross microbial

examination

System cross-connection, poor hydraulic

circulation, or failed facility vent screening

Source: Adapted from Kirmeyer et al. (1999).

TABLE 9-4 Sediment Monitoring Parameters

fastened to a wet tank surface so that its conditions mimic those of thetank walls. In this manner, the coupon can be retrieved from the tankfor analysis without interfering with tank function.

Samples from tank bottom sediment may be collected to determinechemical composition or biological activity. Kirmeyer et al. (1999) rec-ommend the sediment monitoring parameters outlined in Table 9-4.

The need to monitor other bulk water parameters varies depend-ing on the type of tank, source water, environmental conditions, andso on. Monitoring the amount of sediment accumulated in the tankbottom can give a utility an indication when the next cleaning cycleshould take place. Table 9-5 lists water quality monitoring parametersrecommended for storage facilities.

Several nitrification tests should be performed where chloraminesare used for secondary disinfection. As a minimum, tests for hetero-trophic plate counts (HPC), nitrite, nitrate, ammonia, and chlorinespecies can help to ensure that optimal conditions are maintained instorage facilities when chloramination is used.

Sampling Methods and EquipmentThe main issue regarding monitoring of water quality parameters isaccess to adequate sampling points. The tools used depend on thetype of sample being retrieved, available access to the tank, and theutility’s budget. Regardless of method, technicians must be consciousof water quality concerns regarding accidental contamination. Thematerials for the sampling equipment must be compatible with use inpotable water, if applicable.



OperationOperation


Sampling Procedure

Parameter Purpose Used

Alkalinity Indicates potential buffering

capacity (pH stability)

On-line ion-selective

electrode or grab

sample and

laboratory analysis

Aluminum Indicates potential

coagulant overfeeding


electrode or grab

sample and

laboratory analysis

Ammonia,

total and/or

free

Indicates potential for

nitrification


electrode or grab

sample and

laboratory analysis

Chlorine

residual,

total and/or

free

Indicates protection from

bacterial growth and

provides early warning

sign of water quality

deterioration; monitored

at inlet and outlet to

control rechlorination

when practiced

On-line chlorine

residual analyzer

or grab sample and

amperometric

titration laboratory

analysis

Coliform, total

and/or

fecal

Indicates the presence of

indicator bacteria

Grab sample and

laboratory analysis

Conductivity,

specific

Can quickly indicate relative

changes in total

dissolved solids (e.g.,

alkalinity)


electrode or grab

sample and

laboratory analysis

Disinfection

by-products

Represents potential for

ongoing chemical

reactions and DBP

formation

Grab sample and

laboratory analysis

Heterotrophic

bacteria

Indicates conformance to

MCL; provides early

warning sign of water

quality deterioration

Grab sample and

laboratory analysis

Iron Indicates potential

corrosion reactions


electrode or grab

sample and

laboratory analysis

TABLE 9-5 Water Quality Monitoring Parameters for Storage Facilities(Continued)



OperationOperation


Sampling Procedure

Parameter Purpose Used

Nitrate Indicates possibility of

nitrification


electrode or grab

sample and

laboratory analysis

Nitrite Indicates possibility of

nitrification


electrode or grab

sample and

laboratory analysis

pH Indicates changes from the

water source; indication

of corrosion of concrete

or an unlined new facility


electrode or grab

sample and

laboratory analysis

Taste and

odor

Evidence of water quality

problem in progress

Grab sample and

laboratory analysis

Temperature/

temperature

profile

Differences in storage

facility indicate possible

stratification and

stagnant zones—early

warning sign of potential

microbial problems

On-line sensor

Turbidity Provides early warning sign

of water quality

deterioration

On-line turbidimeter

sensor and

analyzer

Source: Adapted from Kirmeyer et al. (1999; p. 46, Table 2.4).

TABLE 9-5 Water Quality Monitoring Parameters for Storage Facilities(Continued)

Sampling is divided into two categories: grab samples and contin-uous sampling. Grab samples are small volumes of tank water man-ually collected by a technician to be analyzed either in the field orunder laboratory conditions. Continuous sampling is accomplishedby means of electronic equipment such as ion-specific probes, temper-ature probes, or on-line colorimetric chemical analyzers. The sensorsare in the tank water, or tank water is continuously or periodicallypiped to the sensor. The data are recorded locally either on paper orelectronically, or they are transmitted via telemetry to the utility’s datastorage and monitoring facility.

Kirmeyer et al. state that

a [monitoring] program that takes all necessary parameters into accountand schedules sampling when needed to provide adequate informationis a conceptual starting point. A recommended approach. . .is to further



OperationOperation


break down sampling needs into two categories of routine sampling anddiagnostic sampling. Routine sampling is defined as monitoring of param-eters on a regular (continuous, weekly, monthly) basis and may includeregulatory and operating parameters. Diagnostic sampling is defined asspecial purpose monitoring to document condition or to determine thecause of a problem. Routine monitoring is used to document general waterquality conditions, whereas diagnostic monitoring is often more facilityspecific. Diagnostic monitoring can first provide a baseline for a storagefacility and identify problems. After the problems are corrected, routinemonitoring can then be used to track tank conditions to detect or preventthe recurrence of the problems or the beginning of new ones.

The types of data being collected or the purpose for the data col-lection dictates whether grab or continuous sampling is required fora particular parameter.

Grab SamplesIn theory, technicians using grab sampling techniques can collect ev-ery type of parameter. Certain types of tank-water tests such as het-erotrophic plate count, total coliform bacteria, chemical compositionof tank sediments, biofilm analysis, and disinfection by-products canonly be performed on manually collected tank water, as they requireanalysis under laboratory conditions. A rare exception is when a tankis located next to the utility’s laboratory and sampling lines exist di-rectly between the tank and the laboratory. The laboratory technicianor chemist is then able to fill the test vial directly from the sampling tap.

In most cases, technicians must travel to several tanks in remote lo-cations as often as the monitoring program dictates the taking of grabsamples. Under the best of circumstances, the sampling program in-cludes the installation of permanently and properly located samplingtaps on the tanks. The taps can be connected through small-diameterpiping to several sampling locations inside the tank. This scheme canprovide a more or less three-dimensional view of the tank’s waterquality parameters. The more sampling taps are installed, the morecomplete the water quality picture will be. The ideal locations for sam-ple points are the inlet(s) and outlet(s), if any; locations that may besubject to short-circuiting or stagnation, such as regions not in thegeneral path between the tank’s inlet and the outlet; and at severaldepths to test for the effects or the presence of stratification.

The sampling lines can be designed such that the water will flowout of the tank by gravity or be pumped out with the use of a jet (vac-uum) or metering pump. Technicians must flush the sampling linesfor sufficient time to provide a proper sample from the tank waterat the sampling point. Twenty minutes of flushing is usually suffi-cient. Some systems allow for continuous flow through the samplinglines, which facilitates this step. Others are designed so that the sam-pling lines can be periodically disinfected and flushed. Permanently



OperationOperation


installed sampling lines are a security concern, as they may provideeasy access for intentional contamination of the tank water. Sanitationof the sampling collection area and frost proofing are also concerns.Sampling lines create an environment in which a relatively small vol-ume of tank water is enclosed by a relatively large surface area. Thiscondition is favorable to the formation of biofilms inside the sam-pling line. Technicians should monitor test results such as elevatedbacterial counts or turbidity that may show such a problem exists.In general, sampling taps should be enclosed and protected from theelements. Pump station buildings, insulated enclosures, and heatedunderground vaults are all potential locations for sampling taps. (Thevaults are not so convenient if confined space is a concern.)

Most tanks, however, are not fitted with sampling taps, and tech-nicians must use roof hatches to gain access to the tank. This impliesthat the technician must be fit, trained, and equipped to safely climbthe tank and open the hatch and that the tank meets all OccupationalSafety and Health Administration (OSHA) requirements for climb-ing safety. Several depth sample collection probes and tubes, such asweighted collection bottles with string-operated trap doors or rodswith check valves, can be used to collect samples at different depths(Fig. 9-11). Although samples can be taken at various water depthsusing these devices, the number of sampling locations is limited bythe location and number of roof hatches.

The sampling equipment, including tethers, must be adequatelydisinfected before it is introduced into the tank. Care must be observed

FIGURE 9-11 Grab sample retrieved from ground tank using calibrated depthsampling tube with check valves.



OperationOperation


so that the equipment disinfection process is compatible with the pa-rameter being tested. For instance, disinfecting a sample bottle andtether with 200 ppm of chlorine and then lowering the bottle into thetank to retrieve a chlorine residual sample may not yield a satisfactoryresult. The water must be collected in a properly prepared and labeledbottle suitable for the type of sample being prepared, and it must betransported to the laboratory in a timely and sanitary manner to pre-serve the validity of the test results. Some tests, such as colorimetricchlorine residual, may be performed by the technician on site, as thereis a chance that the residual concentration may decrease in transit.

More than one technician may perform grab samples. Proper train-ing in sample collection and the use of calibrated tethers or rods helpsmaintain consistency among technicians and samples. Both on-siteand laboratory analyses should be performed by a certified waterquality laboratory using the methods specified in Standard Methodsfor the Examination of Water and Wastewater and the USEPA’s Manual ofMethods for Chemical Analysis of Water and Wastes.

Continuous SamplingClimbing tanks to take grab samples can be dangerous or impossible,especially during rainy or cold weather. Continuous online sampling,which relies on technology rather than technicians to collect, analyze,record, and communicate the data, does not require that operatorsclimb the tank to retrieve water or that they even be present duringsampling, and its performance is independent from weather condi-tions. Online sensing probes may be located at various locations insidethe tank, or water from various locations in the tank may be broughtto an analyzer through sampling lines. The analyzer, data recorders,telemetry, and other equipment should be located in a secure, heated,and sanitary location to preserve the integrity of the readings. Figure9-12 shows a pressure transmitter, analyzer, and corrater.

Online sampling technology offers the advantage of a continu-ous data stream or data collected at relatively brief time intervals,depending on the type of test, type of instrument, and the utility’sdesired sampling rate for particular parameters. Grab samples reflectonly a momentary condition of the water in the tank, often with noaccurate time reference.

The analyzed data can be stored on site using a personal computer,data logger, printout, or pen-chart. Operators can retrieve the data ineither electronic or paper form at scheduled intervals that can varyfrom daily to yearly, depending on the purpose of the testing. Data canalso be sent to a central data collection and monitoring facility using atelephone modem connection, remote terminal unit (RTU) with radiofrequency communication capability, cellular technology, broadband(T lines), satellite, and so on. These methods of communication are



OperationOperation


FIGURE 9-12 Left to right: tank water elevation (pressure transmitter),disinfectant residual (analyzer), and water corrosivity (corrater) are monitoredfor ground water storage tank.

discussed in the section “Telemetry” of this chapter. The amount ofmonitoring data storage is limited by several factors:� Number of parameters� Total sampling rate for all the parameters� Capacity of the data storage device� Frequency of data retrieval by the utility� Size of the data packages being stored for each parameter

Online monitoring can also serve a local control function. Thisis particularly useful in cases where rechlorination is desirable(Fig. 9-13). Online chlorine residual analyzers can serve both as mon-itoring devices for chlorine residual and as controllers for chlorine-boosting feed systems. For example, when an analyzer at the inlet ofa tank detects that the chlorine residual is below a level accepted bythe utility’s water quality manager for the tank or is in violation ofminimum disinfectant concentration regulations, it can start or adjustthe on-site chlorine feed to match a target concentration. It is idealfor this process to take place at the inlet of the tank to provide somecontact time for the disinfectant and as a backup safety mechanism forpotential malfunctions of unmanned chlorine-feed systems. Ideally, acontroller for such system should be able to determine whether the



OperationOperation


FIGURE 9-13 Hypochlorite generation system makes disinfectant from brinefor rechlorination at this water-storage facility.

water is inflowing or outflowing from a single inlet/outlet tank; andthe boosting should only take place into the inflow. A second chlorineresidual analyzer monitoring the outflow from a tank with an inletand an outlet can be integrated as part of a feedback loop control forthe boosting system and/or can serve as an alarm trigger in case ofoverfeeding.

It is important to note that the effectiveness and accuracy of an on-line monitoring system is only as good as the maintenance it receives.A calibration schedule must be established for each type of analy-sis equipment being used. Most ion-specific probes have a life-spanand must be replaced as recommended by the manufacturer. Someanalyzers have moving parts that require cleaning and maintenance.Chemical solutions used by analyzers should be checked and replen-ished as needed. Sampling hoses must be kept clean and should bereplaced when fouling is suspected or on a regular schedule. Mainte-nance technicians first introduced to a monitoring program should bemade aware of these issues and should be appropriately trained.

Water Storage Tank Applications and Their OperationWater storage tanks are categorized by function. The main categoriesare clearwells, distribution system storage tanks, surge relief tanks,and hydropneumatic tanks.



OperationOperation


Clearwells

Treatment plants require tanks in which to store treated water be-fore it is pumped into the distribution system. These tanks are com-monly known as clearwells. Clearwell sizing is critical to reducingor eliminating fluctuations in use of filtered water. These tanks alsoprovide the utility with disinfectant contact time (CT) credit. TheSWTR requires that all surface water treatment facilities provide fil-tration and disinfection that achieves at least a 99.9 percent (3-log)removal-inactivation of Giardia lamblia cysts and a 99.99 percent (4-log) removal-inactivation of enteric viruses. The SWTR assumes thatfor effective filtration, a conventional treatment plant achieves 2.5-log removal of Giardia and a 2-log removal of viruses. Disinfection isrequired for the remainder of the removal-deactivation. The amountof disinfection credit to be awarded is determined with the CT con-cept, CT being defined as the residual disinfectant concentration (C ,in mg/L) multiplied by the contact time (T10, in minutes) betweenthe point of disinfectant application and the point of residual mea-surement. The SWTR Guidance Manual (USEPA 1991) provides tablesof CT values for several disinfectants that indicate the specific disin-fection or CT credit awarded for a calculated value of CT (AWWA1990).

Clearwell design must take into account CT requirements for theparticular treatment process served by the tank. It is a recommenda-tion of the Ten States Standards (1992) that intermittently operatedfiltration plants with automatic high-service pumping from the clear-well during nontreatment hours provide enough extra clearwell vol-ume to compensate for depletion of storage during nighttime hours toensure adequate disinfectant CT. It is commonplace to design clear-wells with two or more compartments. One compartment may beremoved from service for maintenance or during times of low de-mand. Methods for optimizing CT in clearwells will be discussed inthe section on plug flow in this chapter. Figure 9-14 is an aerial viewof a baffled tank optimized for contact time.

Clearwell operation must follow preset parameters for flow andelevation. It is desired that a clearwell should operate in a conditionas close to steady state as possible. The hydraulic control of raw waterpumping, filter effluent flow, and pumping out into the distributionsystem should be coordinated and interlocked, if possible, to preventpulsating flows and other transients. Interlocks should also maintainelevations and should properly shut down during power failure. Thisallows for fast recovery when power is restored. To further preventflow fluctuations, since most raw water pumps are vertical pumps,caution should be exercised so that pumps are not operated in unstableregions of their curves.



OperationOperation


FIGURE 9-14 Aerial view of baffled tank optimized for contact time.

Distribution System TanksDistribution system tanks are by far the most common category oftanks in service. This type of tank serves multiple purposes for waterutilities, including the following:� Reduce the need for pumping during high water demand

periods (peak shaving)� Act as pressure relief should a pressure transient occur in adistribution system� Provide extra water storage for emergencies such as fires orpower outages� Help optimize water treatment by allowing treatment plantsto maintain relatively constant treatment rates� Help reduce the size of trunk and distribution mains� Act as a sediment trap to settle solids from distribution systemwater� Serve as a landmark, even a source of identity and pride, fora community

The most important parameters in the design of distribution sys-tem water storage tanks are location and size. A tank serves a specificarea of a distribution system. Hydraulic engineers, designers, andmodelers must determine how much water must be stored in the tank



OperationOperation


to serve both typical and emergency capacity demand. The water mustbe stored at sufficient elevation to meet the pressure requirements ofthe service area. It is important to keep in mind the preservation ofwater quality when sizing the tank and determining its operatingcharacteristics.

Distribution system tank operation is inherently intertwined withthe pumping facilities that feed it, other parallel tanks and their eleva-tions and capacities, the size of upstream piping, and valve operation.Each of these is discussed hereafter.

Isolation valves in a distribution system may be inadvertentlyclosed after repair of a water main break or after a new installation,for example. This may cause a lower-than-expected pressure at the endof a system, where tanks are likely located. Such a condition may pre-vent adequate filling of a tank, increase water transport time betweenthe pump station and the tank (system residence time), and createcustomer dissatisfaction from low pressures. Periodic isolation valvesurveys and valve exercising should be carried out to ensure propervalve operation and position to prevent isolation valve mishaps. Hy-draulic modeling is a tool that can be used to assist in the locationof improperly positioned isolation valves, pressure zone boundaryvalves, and faulty pressure regulators. All of these may reduce sup-ply pressure to tanks.

Altitude valves (Fig. 9-15) are installed at the system connectionto tanks. There are two types of altitude valves: single acting and

FIGURE 9-15 Piston-style altitude valves shown here control flow and waterelevation for two adjoining storage tanks.



OperationOperation


double acting. Single-acting altitude valves allow water to enter thetank when it is less than full. They do not reopen if the system pres-sure drops; they remain closed until some other connection or a checkvalve bypass causes the tank to draft, dropping the water level be-low the valve’s set point. Double-acting altitude valves are used forsingle-inlet/outlet tanks, since they eliminate the need for a checkvalve bypass. This reduces installation cost and possible transientscaused by check valve slam. Double-acting valves close when the wa-ter elevation in the tank reaches a set point, and they open when thesystem pressure drops below another set point. Altitude valves canhave hydraulic or electric solenoid pilot controls that allow for throt-tling inflow and outflow, further controlling fill and draft cycle times.Altitude valves, in general, have a considerable pressure drop. Thisis why many are installed with a bypass containing a remotely con-trolled valve, such as a motor-operated butterfly valve. Some utilityoperators opt to lock the altitude valve in the full open position anduse a motor-operated valve to isolate the tank if the water level getstoo high. Some operators attempt to control the tank level by turningpumps on and off, throttling pump discharge check valves, or reduc-ing a variable-speed pump’s revolutions per minute (rpm), essentiallynegating the reason for having an altitude valve.

In most water distribution systems, a tank experiences two cyclesof fill and draft in a 24-hour period. The fill cycles usually take placeat noon or midnight, ±3 hours. To optimize power consumption, thefill and draft periods should be long enough that the fewest numberand the smallest sizes of pumps are needed to satisfy demand. Thisalso ensures that tank turnover is optimal.

The number and size of pumps operating upstream of a tank canaffect the available pressure at the base of the tank. In most cases,a pump is run continuously, and a second pump is added to helpsatisfy demand during peak hours if the tank cannot supply enoughpeak shaving. This number of pumps varies seasonally. Hydraulicmodeling is useful not only during system design, but it can also bevaluable for system operation because it can predict the amount ofpumping required under various system conditions.

If enough pumps are in operation, the tank will be filled withleftover water after system demand is satisfied. If upstream pumpingis reduced before the tank is full, several things may occur; for instance:� The tank may not fill at a rate fast enough to provide sufficient

turnover and/or mixing.� If the fill rate is low enough, the tank may not fill high enoughto help peak shave pumping demand during the next high-demand period.� If pumping is reduced excessively, the tank may begin draftingeven though it has not been filled.



OperationOperation


If the upstream pumping is larger than demand so that the tankfills at a fast rate, it may cause a different set of events:� The altitude valve will close to prevent overflow, preventing

the tank from floating on the system. This creates a “hard”system condition. In other words, the tank is no longer avail-able to dissipate pressure transients in the system, and thepressure in the system could change drastically, causing thepumps to operate at higher pressures that may result in pumpshutdowns. Pump shutdowns cause system pressure surges,which are further exaggerated by the subsequent opening ofthe altitude valve when the system pressure drops below thevalve’s set point. This operation mode is undesirable.� The high inflow into the tank may fluidize sediments at thebottom of the tank, and they may reenter the distribution sys-tem at the next draft, creating water quality concerns.� High flows and flow reversals may scour water mains; thisalso creates water quality issues, such as red water and re-duction in disinfectant residuals.

Low pressures are seen at a tank when a system outgrows thecapacity of its water mains. Some operators address this issue by in-creasing pumping at the upstream pump stations, or they add boosterpumps between stations. But this is only a temporary fix that may re-sult in large pressure variants between the pump station and the tank.Additionally, the water mains may not provide sufficient flow capac-ity to fill the tank within the required time.

Oversized water mains do not have a low-pressure problem, butexcessively low flow rates can cause increased distribution system res-idence time, biofilms, and sediment accumulation, resulting in areasof high disinfectant decay. The overall effect is a reduction of disinfec-tant residual available at the tank.

Tanks installed in parallel to serve the same pressure system arerarely of the same dimensions. Some even have different overflow andbottom elevations. Further, the ability of the utility to supply waterto each tank varies because of its location, adjacent demands, andwater main size. This results in different fill rates. Hence, the tanksmay not readily cycle in unison. Hydraulic models may help properlysize the mains and valves to new tanks, providing more uniformitywith less throttling. Modeling can also be used to devise elaborateoperational schemes, resulting in similar uniformity in tank use. Inthe absence of a hydraulic model, operators often resort to throttlingof inlet and outlet valves to equalize the fill or draft time such thatfill and draft cycles occur simultaneously, preventing the need to shutoff one tank before the others. Although hydraulically it is not critical



OperationOperation


for tanks to cycle in unison, water tank residence time will vary, andwater quality may become inconsistent between facilities in the samesystem. However, the effect of differing tank shutoff times in the samesystem may render transient travel times not constant. Hence, pumpcheck valves at pump stations may require a much longer closing oropening time to prevent pressure surges.

Distribution system water storage tanks should be periodicallytaken out of service for cleaning and inspection. Most of the waterin the tank should be allowed to flow back into the distribution sys-tem. A few feet or meters of water, typically between 3 and 5 ft (0.9and 1.5 m), are left in the tank to prevent collected sediments fromreentering the system. Debris and sediments are brought to the tankfrom the system’s piping and are collected at the bottom of tanks.This is a desirable means of removing contaminants and particulatesfrom the water. However, as discussed in the water quality section ofthis chapter, several potential biological and chemical water qualityissues are associated with tank sediment. Removal of this sediment isthe only way to eliminate these potential problems. NSF International–approved chemicals are available that remove persistent, troublesomestains , such as those caused by manganese and iron deposits. Tanks’inlets and outlets should be designed such that settled solids are notfluidized by the inflow and do not return to the distribution systemthrough the outlet.

In the authors’ experience, a good practice is to allow the wa-ter main supplying the tank to flush through the tank, using it as asediment trap. This can be achieved by emptying the tank and thenopening the tank shutoff valve while the drain is open. This createshigh-velocity scouring of the mains in the vicinity of the tank. A con-siderable amount of sediment has been removed from tank supplymains using this scheme. The flushing can be accomplished in as littleas a couple of hours. The drained water must be treated accordingto local regulations (dechlorination and solids removal, for example)prior to disposal.

The tank should be isolated with at least two valves in series withthe tank. All hatches should be open, and the tank drain should bekept open. After all water and sediment are drained from the tank,the tank is typically powerwashed prior to inspection. Regulationsrequire that the tank be disinfected before it is returned to service. Thetank should be disinfected according to the latest version of AWWAC652, Standard for Disinfection of Water-Storage Facilities. The stan-dard describes the materials needed, facility preparation, applicationof disinfectant to interior surfaces of the tank, and sampling and test-ing procedures for the detection of coliform bacteria. It also containsinstructions for disinfecting equipment used in on-line underwaterinspections.



OperationOperation


Three methods of tank disinfection are discussed in the standard.Only one method is required to be used, although it is possible tocombine two or all three methods, if necessary. The three methodsare � Full tank chlorination so that, at the end of the retention pe-

riod, a free chlorine residual equal to or greater than 10 mg/Lis achieved.� Spraying the interior tank surfaces with a solution of 200mg/L of available chlorine.� Chlorinating the bottom portion of the tank with 50 mg/Lof available chlorine, then filling the tank to overflow andmaintaining a free chlorine residual of at least 2 mg/L for24 hours.

The standard should be consulted for more detailed instructions.According to the Ten States Standards, two or more sets of sam-ples taken from the tank at 24-hour intervals shall indicate that wa-ter is microbiologically satisfactory before the facility is placed inoperation.

One of the main functions a tank serves is to dissipate pressuretransients that may take place in the distribution system. When atank is taken offline, a distribution system may become vulnerable towater hammer or other damage stemming from a potential pressuresurge. It is a good practice to plan the installation of temporary pres-sure relief valves at critical locations in the distribution system for theduration of the tank work. Insulated enclosures should be providedalong with the surge relief valves in locations subject to freezing con-ditions. Large customers in the tank’s service area should be notifiedand encouraged, if at all possible, not to run processes that demanda great amount of water while the tank is offline. This is importantbecause emergency water storage is lost when a tank is taken outof service. The risk of losing water pressure during a fire is reducedwhen large customers temporarily abstain from using large volumesof water during tank work.

The Federal Aviation Administration (FAA) determines and reg-ulates structures that may pose an obstruction hazard to air traffic.Elevated tanks, tanks located at high elevations, and those within anFAA-designated three-dimensional controlled volume of airspace atthe end of airport runways are required to carry aviation obstructionmarkings and/or equipment. The equipment may be an air traffic ob-struction light at the highest point on the tank. Utility operators andmaintenance crews must monitor the light’s operation and replacethe light when it burns out. Failure to comply with FAA requirementsmay lead to fines in the best of cases, or an airplane’s collision withthe tank in the worst case.



OperationOperation


Cold-Weather Operation of Distribution System TanksTable 9-6 describes the influence of atmospheric temperature on steeltanks. Tanks in areas where the lowest one-day mean temperature(LODMT) is less than 5◦F (−15◦C) are likely to experience tank-freezing conditions. Other factors may contribute to tank freezing evenin areas where the LODMT is above 5◦F (−15◦C):

� Elevated tanks with small-diameter (less than 36 in. [0.9 mm])uninsulated risers� Tanks with uninsulated and unheated appendages that expe-rience no flow (e.g., a drain valve at the end of a nipple)� Tanks experiencing overflow problems because of frozen orotherwise malfunctioning controls for valves or pumps� Tanks with abnormally static water conditions� Tanks with inadequate vents or overflows

The National Bureau of Fire Underwriters publishes tables giv-ing the heat loss per hour from various types of tanks. A 250,000-gal(1-ML) elevated tank located in a −10◦F (−23◦C) area losses 1 millionBtu/h (292 kJ/s) in a 12-mph (19-km/h) wind. Move the tank to a−40◦F (−40◦C) location, such as International Falls, Minnesota, andthe heat loss almost doubles (Knoy 1991a, 1991b). See the isothermalmap in Fig. 9-16.

Although it is possible to replace most of the water in a tank withwarmer water, that is seldom the way the tank is operated. Water stor-age tanks (particularly elevated tanks and standpipes), because theyare connected to the main only by the riser pipe, usually float on thesystem. As a result, it is possible that a tank can serve a system thatuses several times more water per day than the tank capacity yet stillreceive only a small percentage of fresh water daily. The operating pro-cedures discussed in the following paragraphs allow a tank to makeoptimal use of the heat available in incoming water and highlightlimitations and effects of water quality.

Most utilities in cold-weather regions experience a drop in de-mand during the winter months. If the upstream pump station to atank that is part of such a utility’s distribution system does not allowfor adequate reduction in pumping so that the tank is allowed to cycle,tank turnover may be jeopardized, water quality may suffer, and, inextreme weather, ice may form. Drafting the tank and filling it mayslow the formation of ice caps inside tanks. This is the only opera-tional tool available to slow icing. The more a utility is able to cyclea tank, the less ice buildup will occur. However, tanks should not bekept at low levels for long periods, because the amount of heat energycontained in the tank is a function of the volume of tank’s water. The



OperationOperation

Add

Btu

per

Lin

eal

Ft

(kW

-hr/

m)

Unin

sula

ted

Ste

elR

iser

3ft

4ft

(0.9

14

m)

(1.2

20

m)

Dia

.D

ia.

Heat

(Btu

)

Loss

per

Ft2

(kW

-hr/

m2)

Atm

ospheric

Tank

Tem

pera

ture

Radia

ting

(◦ F[◦ C

])Surf

ace

Tank

Capacit

ies—

Thousands

ofU

SG

allons

(Thousands

ofLit

ers

)

25

30

40

50

75

100

150

200

250

(94.6

)(1

13.6

)(1

51.4

)(1

89.3

)(2

83.9

)(3

78.5

)(5

67.8

)(7

57.1

)(9

46.4

)

Square

Feet

(Square

Mete

rs)

ofTank

Surf

ace

∗

1,2

10

1,3

25

1,5

50

1,8

00

2,3

70

2,8

45

3,7

05

4,4

70

5,2

40

(112.4

)(1

23.1

)(1

44.0

)(1

67.2

)(2

20.2

)(2

64.3

)(3

44.2

)(4

15.3

)(4

86.8

)

Btu

Lost

per

Hour,

Thousands

(kW

)

35

[1.7

]32.3

40

43

51

59

77

92

120

145

168

50

69

(0.1

02)

(0.0

12)

(0.0

13)

(0.0

15)

(0.0

17)

(0.0

23)

(0.0

27)

(0.0

35)

(0.0

42)

(0.0

49)

(0.0

48)

(0.0

66)

30

[−1.1

]46.1

56

62

72

83

110

132

171

207

242

144

192

(0.1

45)

(0.0

16)

(0.0

18)

(0.0

21)

(0.0

24)

(0.0

32)

(0.0

39)

(0.0

50)

(0.0

61)

(0.0

71)

(0.1

38)

(0.1

84)

25

[−3.9

]61.5

75

82

96

111

146

175

223

275

323

255

340

(0.1

83)

(0.0

22)

(0.0

24)

(0.0

28)

(0.0

33)

(0.0

43)

(0.0

51)

(0.0

65)

(0.0

81)

(0.0

95)

(0.2

45)

(0.3

27)

20

[−6.7

]77.2

94

103

120

139

183

220

287

346

405

380

506

(0.2

43)

(0.0

28)

(0.0

30)

(0.0

35)

(0.0

41)

(0.0

53)

(0.0

64)

(0.0

84)

(0.1

01)

(0.1

19)

(10.3

65)

(0.4

86)

15

[−9.4

]93.6

114

125

146

169

222

267

347

419

491

519

692

(0.2

95)

(0.0

33)

(0.0

37)

(0.0

43)

(0.0

49)

(0.0

65)

(0.0

78)

(0.1

02)

(0.1

23)

(0.1

44)

(0.4

99)

(0.6

65)

10

[−12.2

]110.9

135

147

172

200

263

316

411

496

582

670

893

(0.3

49)

(0.0

39)

(0.0

43)

(0.0

50)

(0.0

59)

(0.0

77)

(0.0

93)

(0.1

20)

(0.1

45)

(0.1

71)

(0.6

44)

(0.8

59)

5[−

15.0

]128.9

156

171

200

233

306

367

478

577

676

820

1,0

92

(0.4

06)

(0.0

46)

(0.0

50)

(0.0

59)

(0.0

68)

(0.0

89)

(0.1

08)

(0.1

40)

(0.1

69)

(0.1

98)

(0.7

88)

(1.0

5)

0[−

17.8

]148.5

180

197

231

268

352

423

551

664

779

982

1,3

09

(0.4

68)

(0.0

53)

(0.0

58)

(0.0

68)

(0.0

79)

(0.1

03)

(0.1

24)

(0.1

61)

(0.1

95)

(0.2

28)

(0.9

44)

(1.2

6)

−5[−

20.6

]168.7

205

224

262

304

400

480

626

755

884

1,1

52

1,5

36

(0.5

32)

(0.0

60)

(0.0

65)

(0.0

77)

(0.0

89)

(0.1

77)

(0.1

41)

(0.1

83)

(0.2

21)

(0.2

59)

(1.1

1)

(1.4

7)

332



OperationOperation

−10

[−23.3

]190.7

231

253

296

344

452

543

707

853

1,0

00

1,3

29

1,7

71

(0.6

01)

(0.0

68)

(0.0

74)

(0.0

87)

(0.1

01)

(0.1

32)

(0.1

59)

(0.2

07)

(0.2

50)

(0.2

93)

(1.2

8)

(1.7

0)

−15

[−26.1

]213.2

258

283

331

384

506

607

790

954

1,1

18

1,5

15

2,0

20

(0.6

72)

(0.0

76)

(0.0

83)

(0.0

97)

(0.1

12)

(0.1

48)

(0.1

78)

(0.2

31)

(0.2

79)

(0.3

27)

(1.4

6)

(1.9

4)

−20

[−28.9

]236.8

287

314

368

427

562

674

878

1,0

59

1,2

41

1,7

18

2,2

91

(0.7

47)

(0.0

84)

(0.0

92)

(0.1

08)

(0.1

25)

(0.1

65)

(0.1

97)

(0.2

57)

(0.3

10)

(0.3

63)

(1.6

5)

(2.2

0)

−25

[−31.7

]262.3

318

348

407

473

622

747

972

1,1

73

1,3

75

1,9

26

2,5

63

(0.8

27)

(0.0

93)

(0.1

01)

(0.1

19)

(0.1

38)

(0.1

82)

(0.2

19)

(0.2

85)

(0.3

43)

(0.4

03)

(1.8

5)

(2.4

6)

−30

[−34.4

]288.1

349

382

447

519

683

820

1,0

68

1,2

88

1,5

10

2,1

45

2,8

60

(0.9

09)

(0.1

02)

(0.1

12)

(0.1

31)

(0.1

52)

(0.1

99)

(0.2

40)

(0.3

13)

(0.3

77)

(0.4

42)

(2.0

6)

(2.7

5)

−35

[−37.2

]316.0

383

419

490

569

749

900

1,1

71

1,4

13

1,6

56

2,3

81

3,1

74

(29.3

)(0

.112)

(0.1

22)

(0.1

43)

(0.0

67)

(0.2

19)

(0.2

64)

(0.3

43)

(0.4

13)

(0.4

85)

(2.2

9)

(3.0

5)

−40

[−40.0

]344.0

417

456

534

620

816

979

1,2

75

1,5

38

1,8

05

2,6

20

3,4

94

(0.9

97)

(0.1

22)

(0.1

34)

(0.1

56)

(0.1

82)

(0.2

39)

(0.2

87)

(0.3

73)

(0.4

50)

(0.5

29)

(252)

(3.3

6)

−50

[−45.5

]405.6

491

538

629

731

962

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OperationOperation


FIGURE 9-16 Isothermal lines for lowest one-day mean temperatures andnormal daily minimum 30◦F (−1◦C) temperature line for January, UnitedStates and Southern Canada.



OperationOperation


FIGURE 9-16 (Continued)

lower the volume of water in a tank, the less heat energy is presentin the tank. It is this heat energy that minimizes ice formation. Forexample, it takes longer for a pitcher of water than a glass of water tofreeze. The authors have been successful in eliminating icing by usingtank mixing systems in combination with normal tank operation.



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FIGURE 9-17 More than 100 tons (90.72 metric tons) of “dirty” ice wasdiscovered inside this 3-mil-gal (11.35-ML) elevated tank in the spring of2004. It took weeks for repair crews to remove the ice before work on thetank could begin.

Elevated tanks are more susceptible than ground tanks to icingconditions (Fig. 9-17). Ground tanks are more likely to experienceicing than partly or fully buried reservoirs. Insulating the tanks is nota viable option; insulation only slows heat loss but does not stop it.Hence, prolonged extreme weather eventually causes icing despite theinsulation. The reason partly or fully buried tanks have less propensityto freeze is that the ground temperature is much higher than the airtemperature below freeze depth. Freeze depth is the depth of groundfrom surface (grade) below 32◦F (0◦C). Ice formation on the interiorsurfaces of the tank acts as insulation and retards further icing of thewater. However, formation of an ice cap on the surface of the water isthe most damaging result of cold weather.

Ice can have several detrimental effects on tank longevity andwater quality. The physical effects of ice have been well documented.Floating and falling ice can scrape coatings and damage or destroystructural elements of the tank such as tie rods, ladders, overflowweirs and piping, painter’s rings, and so on. The effect is cumulativeand progressive; more and more damage is done in each cold seasonif the ice problem is not addressed.

Although it is often desirable to take tanks down for inspectionor rehabilitation work during low-demand winter months, cautionmust be taken to ensure that the tank is drained before ice forms.Otherwise, substantial amounts of ice may remain in the tank after it isdrained. In some cases, so much ice may be present that it would need



OperationOperation


to be physically removed before work can proceed. This can causeconsiderable unexpected delay and added expense to tank inspectionor rehabilitation.

Thawing a frozen tank is a difficult task. There are, however,some alternatives for dealing with such situations. A common tech-nique is to lift a heavy-duty hose 3/4 in. (19 mm) in diameter fromthe ground up over the top of the tank. The end of the hose termi-nates with a 1/2-in.- (12-mm)-diameter pipe about 10 ft (3 m) long.This pipe is dropped through the tank vent or manhole located di-rectly over the inlet or riser pipe. It must be kept slightly off the iceto keep it from sticking to it. Warm water from a fire department tanktruck is pumped through the hose, parting the ice as the probe dropsdown. The water must be used judiciously. Stopping in the middle ofthis process could allow the thawing pipe to freeze in the tank, whichmay mean the equipment must be left in the tank for the rest of thewinter.

When the tank truck is empty, it must be refilled from utility mains.However, the refill water will not be as warm as the original water inthe truck, which was taken from the heated fire station. Steam gen-erators have been attached to this type of probe, but the warm-watermethod seems to work best, is cheaper, and creates fewer safety haz-ards. The same equipment can be used to thaw a frozen riser. How-ever, it may be difficult to thread the probe into the riser pipe. Thetank drawings or a recent tank inspection report should be reviewedbefore this type of riser thawing is attempted, unless the position-ing of the piping arrangement is known from experience. Artificiallythawing tanks is expensive and dangerous. In addition, a warm frontmay move through the day after the crew has thawed the tank, whichmakes a high thawing bill hard to justify.

Rust, coating debris, and damaged structural elements are nor-mally encapsulated in tank ice. Because the damage occurs as the icedevelops, the substances—which may include biofilms, metals, de-bris, and assorted particulates such as coating chips—become trappedin the ice for the duration of the cold season. Most of the accumula-tion is in the tank’s ice cap. These substances are released when the icethaws. Since they are released near the surface, they may be suspendedlong enough to be drafted out of the tank with the outflow.

Another way ice formation can affect water quality in tanks is thatif the ice cap becomes thick enough (as it often does), the water levelunderneath the ice may drop while the ice cap remains stationary.This creates a small vacuum above the surface of the water that cancause air and chlorine (if used as a secondary disinfectant) to leave thewater. This wet chlorine gas and air can cause accelerated corrosionin the areas damaged by the ice.

Several devices on the market claim to prevent ice formation insidetanks. Some involve the use of mechanical mixers such as pumps



OperationOperation


or propellers, some use air bubblers (bubbles remove heat from thewater, and there must be an influx of warmer water for the system tooperate properly), and others use turbulent jets. They are all essentiallymixing devices with varying degrees of mixing efficiency and energyconsumption. A small tank may be fitted with a recirculation pumpand a water heater. This energy-intensive option is limited by tanksize and equipment expense.

There are reasons beyond tank operation that may make a tanksusceptible to freezing problems. Isolation or altitude valves may beimproperly applied or malfunctioning, which creates conditions ofoverflow, low flow, or no flow that may cause a tank to freeze. Valves’hydraulic control piping should be protected against freezing by pro-viding heated enclosures, insulation, or electric heat tracing.

Some traditionally designed air vents for tanks can collect conden-sation, freezing rain, or snow that freezes on vent parts and hindersoperation. A common problem in such cases is the insect screen serv-ing as a substrate for ice formation. Water outflow from a tank with afrozen vent creates a vacuum condition that may result in progressivecollapse of the tank shell until structural failure allows venting or thewater outflow stops. Several manufacturers supply frostproof tankvents designed either to prevent the formation of ice on their parts orto dislodge any ice formed by using a tank vacuum condition. Utilitycrews should inspect tank vents periodically to ensure their properfunctioning.

A common cause of tank freezing is inadequate cover over the pipeleading to the tank. Sometimes soil conditions preclude installing thetank foundation deep enough to provide adequate frost cover; in suchcases, fill should be brought in for cover, or other means should bedevised to insulate the pipe.

Site grade In some tower-type elevated tanks, the riser foundation isbuilt higher than the column foundations. The site should be gradedhigher in the center to prevent the inlet/outlet piping from being ex-posed to the atmosphere between the concrete riser foundation tunneland the earth.

Compacting fill If the earth over the connecting piping is not com-pacted properly, it will settle during the first few years of operation.The ensuing lack of adequate cover, combined with moisture satu-ration of the depression, creates a potential trouble area that mayfreeze.

Supervising the covering The base of the tank is usually the locationof the interface between constructors, and it may become a no-man’s-land. Unless the construction work is properly supervised and in-spected, the piping may not even get backfilled before the first winteroperation (AWWA 2003).



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Tank overflows are subject to icing when the tank overflows at aslower rate than the freezing rate. This may occur for any of severalreasons: a utility may want to misguidedly trickle-overflow a tank toprevent it from freezing; the hydraulic controls for the altitude valvemay be set above the overflow elevation or may be damaged by ice orfrozen; an altitude or isolation valve may be leaking; telemetry maybe faulty due to weather conditions or damage; or the altitude valvemay have been locked in the open position so that the tank can floaton the system, and the system pressure may have increased beyondthe tank capacity.

When tanks overflow in freezing weather, several problems candevelop. An overflow-to-grade may freeze solid, especially wherethere is a trickling overflow, where screens are plugged, or whereflap or rubber valves are stuck on the discharge. If water continuesto be pumped into the tank after the overflow pipe is frozen solid,the tank may overflow through the roof hatch for a while and thenthrough the vent. The tank will then freeze solid, build up pressure,and burst.

When water overflows through the hatch and vent, it invariablyforms a large icicle, weighing tons, on the tank exterior. The sameproblem can be caused by normal overflow through a stub overflow(one extending only a few feet or meters from the shell of the tank).Either situation places a large eccentric load on the structure which,in the case of water towers, can exceed the structure’s design stress.The icicle usually forms on the side of the tower that is away fromthe prevailing wind, and the wind and icicle together create additiveloadings. Even if the structure is not damaged by the ice load, it maybe damaged when the ice thaws or breaks off and falls. Eccentric iceloadings or tower members damaged by falling ice have caused watertowers to fall (AWWA 2003).

Proper tank design and operation will prevent many of the freez-ing problems discussed previously and will allow operators to dealwith the problems that do occur. Special consideration should begiven to tank inside appurtenances. Tanks located in areas where theLODMT is −5◦F (−15◦C) or colder should not be equipped with insideladders or overflow pipes. As ice forms and moves up and down, itcan exert tons of force on ladders and pipes, tearing them loose fromtheir supports and possibly ripping or punching holes in the container.The resulting leak will occur at a very inopportune time. If an insideoverflow pipe is broken, the tank will rapidly lose all water down tothe break, creating a large icy area on the ground below. If the ventis plugged with ice or snow, the tank roof may collapse when waterevacuates the tank rapidly.

It is acceptable to equip a tank with inside ladders and overflowpipes if the tank is known to have a high turnover rate of warm water.A ladder and overflow can also be installed at the center of the tank



OperationOperation


and supported by the access tube, as in single-pedestal tanks andextremely large column-type tanks.

The use of interior girders, roof bracing, painters’ rails, or virtuallyany other protrusion below the high water line or within an area af-fected by floating or suspended ice is a poor design practice for areaswith an LODMT of −20◦F (−29◦C) or colder. Certain local conditionsor tank use patterns such as those discussed earlier may cause equallysevere icing problems in warmer areas.

Surge Relief TanksSurge tanks are connected to trunk mains at locations where there isa change in elevation, such as a peak or a knee. They are designed tohold a specific volume of potable water at a specific head or pressure.Special valve arrangements maintain the water level until a down-surge or column separation occurs in the nearby main. Within a fewseconds, a large amount of water is released to fill the void created.This prevents the destructive vacuum from damaging the pipe andminimizes flow reversal, which can create extremely high pressure inthe pipe. The size of the surge tank and its associated piping and thespeed of water delivery are based on transient surge analysis, whichshould be conducted by experienced professionals. Misapplied surgetanks may cause more damage than if there was none. Figure 9-18shows an insulated surge tank.

FIGURE 9-18 Insulated surge tank in metropolitan area provides protectionfrom water column separation to large pump station located approximately200 ft (60.961 m) lower than tank.



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Since these tanks do not float on the distribution system, specialattention must be given to the quality of water, disinfection residual,and turnover in the tanks. In colder climates, special consideration hasto be given to eliminate freezing, which can affect the available volumeand speed of delivery at the desired head. A circulation pump changesthe water in the tank at a sufficient rate to maintain water quality. Tankwater is returned to the system, and fresh water is supplied from thesystem upstream of the return location. In cold climates, surge tanksare typically insulated, and a recirculation pump may be used to cyclethe water in the tank through heaters. The chlorine residual shouldbe monitored in tanks whose recirculation systems do not exchangewater with the main. If needed, chlorine should be boosted using arechlorination system.

Hydropneumatic TanksHydropneumatic pressure tanks are used in very small systems to re-duce the amount of pumping required to provide water at pressure.They can also serve as temporary replacements to elevated storagein small systems during prolonged rehabilitation work. Hydropneu-matic tanks are pressure vessels typically made of steel. A portion ofthe tank is kept filled with compressed air. Once full of water, the tankprovides water in excess of the pump capacity as required. This keepsthe pump from short-cycling and provides pressured water for shortperiods during power outages. These vessels can also act as pressuresurge relief tanks.

Fluid Dynamics in TanksThe water entering the tank may be turbulent (as in the case of a pipeinlet) or laminar (as in the case of a tank with a large-diameter wetriser). The inlet geometry and the flow characteristics affect how thewater interacts with the tank’s structure and the ambient water withinit. During design, depending on the tank’s operational requirements,it is typically desired to have one of two flow modes, plug or mixed.

Plug FlowShort-circuiting, in general, occurs when influent water bypasses mostof the tank volume, having only minimal interaction with ambientwater, and flows directly to the outlet. Clarifiers, clearwells, or tankswhere contact time is needed should not be designed to have any formof short-circuiting. It is ideal if the water enters the tank and leavesthe tank in an orderly fashion like a train, with the oldest water in thetank leaving first. This is referred to as the first-in, first-out flow modeor the regime generally known as plug flow.

In such a flow mode, a decrease in tank elevation decreases theflow area. This increases the speed by which the influent water reaches



OperationOperation


the outlet. Hence, the detention time of water in the tank decreases.Also, depending on geometry, some areas inside the tank may expe-rience a change in the nature of the flow. For example, a separationarea near where water changes direction may trap more water andincrease velocities in the rest of the cross-sectional area, which maycause some mixing and/or short-circuiting.

The Disinfection Profiling and Benchmarking Guidance Manual of theUSEPA (1999) has procedures for CT calculations that list bafflingfactors for various baffling conditions. These are approximate, and theregulatory agency usually approves the baffling factor during designreviews. Computational fluid dynamic analysis can accurately predictbaffling factors by modeling a tracer flow through the tank. However,the use of CFD is limited to design validation and should not be a toolfor determining baffling factors because drastic difference in resultsare possible depending on the model and modelers’ limitations. Tracerstudies are the most accurate means of determining baffling factors.A tank’s theoretical detention time (TDT) is computed by dividingthe volume (V) of the tank by the peak hourly flow rate (Q): (TDT= V/Q). The baffling factor, T10/T , is multiplied by the TDT to yieldan estimate of the contact time (also known as the effective detentiontime), T10, as follows:

T10 = Contact Time = V/Q × T10/TBaffling factors are a function of tank design. T10/T equal to 1.0

represents pure plug flow characteristics where TDT is equal to thecontact time, T10. Figure 9-19 illustrates a CFD model of a baffledclearwell showing velocity contours.

FIGURE 9-19 Computational fluid dynamics model of baffled clearwell showingvelocity contours.



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Baffling Condition T10/T Baffling Description

Unbaffled (mixed

flow)

0.1 None, agitated basin, very low

length-to-width ratio, high inlet and

outlet flow velocities

Poor 0.3 Single or multiple unbaffled inlets and

outlets, no intrabasin baffles

Average 0.5 Baffled inlet or outlet with some

intrabasin baffles

Superior 0.7 Perforated inlet baffle, serpentine or

perforated intrabasin baffles, outlet

weir or perforated launders

Perfect (plug flow) 1.0 Very high length-to-width ratio

(pipeline flow), perforated inlet,

outlet, and intrabasin baffles

Source: USEPA (1999).

TABLE 9-7 Baffling Classifications and Factors

Baffling classifications and factors as given in the USEPA GuidanceManual are shown in Table 9-7.

A discussion of CT calculations is outside the scope of this text.However, we shall discuss the effect of baffling geometries on deten-tion times. Baffles are obstructions in a tank that can be made froma variety of materials. These obstructions contain and direct the flowin tanks to create plug flow conditions. Ideally, the objective is totransform the tank into a wound pipe, creating a first-in, first-out con-dition. Baffle configurations can be numerous; however, some bafflearrangements produce better plug flow than others. Design modifica-tions that can increase T10 may allow the same inactivation (CT) with adecreased disinfectant residual or a decrease in tank size. The follow-ing summarizes some of the design features that optimize clearwellplug flow design and operation.

Inlet TreatmentThe inlet to a tank can be modified—for example, by adding a perfo-rated wall—so that the flow entering the tank does not create an un-even stream but is distributed evenly along the flow’s cross-sectionalarea. In addition, the flow should not be directed such that it impingeson and attaches itself to a boundary; this creates a skewed velocityprofile.

Flow Area Length Versus WidthTo optimize the total water volume, proper baffling must be designedto eliminate short-circuiting and dead zones (stagnant areas). The flow



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area should be long and narrow, eliminating secondary flows andallowing a pressure gradient along the length of the flow channelonly. Ideally, the entire volume of water should be in motion in onedirection with no recirculation, stagnation, or mixing.

Number of Turns and Angle of Flow Direction ChangeThe number of direction changes in a baffled tank should be mini-mized. Turns skew the velocity profile, causing the water at the outer(concave) edge of the turn to move faster than at the near (convex)end. The increased turbulence creates some localized mixing. The an-gle of flow direction affects the amount of skew and the size of theseparation region in which recirculation occurs. Keeping the turns asmild as possible improves the plug flow. This means that curvaturesare better than corners, especially if the radius of curvature is largerthan the width of the flow area. The tank designer should take thefollowing into consideration:� Avoid any dead zone by eliminating (filling in) corners be-

tween horizontal and vertical surfaces.� Add perforated walls at each change of flux direction. To ob-tain good water distribution on the whole cross section, a cer-tain head loss must be created through holes, given by thefollowing formula:

�P = k (V2/2g)

where

k = 0.62v = velocity through hole, in feet or meters per secondg = acceleration of gravity = 32.2 ft/s2 (9.80665 m/s2)A velocity of 2 ft/s (0.6 m/s) creates sufficient head loss to ensure

good distribution.

Laminar Versus Turbulent Velocity ProfileThe perforated walls just discussed can be used at various intervalsalong the flow path to modify the shape of the velocity profile from alaminar bullet shape to a more blunt (turbulent) shape, thus producinga more uniform velocity distribution across the flow cross section.

Outlet TreatmentTo maintain water level, an outflow weir may be provided at the tank’soutlet. Water drop should be minimal to prevent aeration of water anddisinfectant loss. However, a bypass through the weir should be addedto allow drafting of the clearwell in an emergency. A perforated wall inthe vicinity of the outlet may help in eliminating preferential currentsand help ensure an even distribution into the outflow from all partsof the baffled channel’s cross-sectional area.



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FIGURE 9-20Computational fluiddynamics model of0.8 baffling factortank.

Inflow and Outflow ControlTo prevent the creation of transients and encroachment of high-velocity influent water into the lower-velocity ambient water, the flowinto and out of the tank should be controlled to maintain steady-stateconditions.

Baffling factors are directly proportional to water depth. As theflow depth decreases, the baffling factor decreases. Figure 9-20 showsa CFD model of a 0.8 baffling factor tank. In addition, the decrease inwater depth increases the average velocity across the baffled channel.This decreases the theoretical detention time, resulting in a consid-erable decrease in contact time. A way to ensure a constant waterdepth is to use variable-speed pumping with a proportional integralderivative (PID) controller using tank elevation as a feedback and thedesired water depth as a set point. Valve throttling can achieve similarconditions but is not as accurate or energy efficient.

Mixed FlowMixed flow occurs when the influent water impinges on the ambientwater, resulting in a diluted volume representative of their propor-tions. The water leaving the tank is no longer the oldest water in thetank but is of an averaged age based on the tank’s turnover ratio andmixing efficiency. Just as the baffling factor represents the scale of plugflow present in a pass-through tank, mixing efficiency or effectivenessrepresents the amount of mixing achieved. Just as baffling schemesare not equal in performance, performances of mixing schemes differ.Mixing in tanks can be achieved in many ways; we shall attempt todiscuss most of them, concentrating on turbulent jet mixing due to itsapplicability in water storage tanks as discussed hereafter.



OperationOperation


Mixing Times and DynamicsMixing is a function of time. For mixing to be complete, it must beefficient and sufficient. Whenever an attempt is made to mix multipleingredients, the amount of time during which the ingredients are sub-jected to the mixing process dictates the degree of mixing. The natureof a particular mixing process determines its mixing effectiveness orefficiency. Therefore, the mixing time needed to provide a homoge-nous tank is indirectly proportional to its mixing efficiency. Inefficientmixing may cause water in some areas of the tank not to be mixed.Water in a tank without an effective mixing system may have stratifi-cation, short-circuiting, or stagnant areas. Even a significant amountof filling and drafting would not result in mixing in the tank. Calcu-lation of a minimum theoretical mixing time is insufficient to make adecision regarding the mixing condition of a tank. Consider, for ex-ample, a standpipe that experiences stagnation due to a stratificationproblem, or a tank with stagnation issues due to short-circuiting. Ifsufficient mixing time is available, as calculated by the method dis-cussed later in this section, the stagnant water will remain there andwill not mix.

Basic Discharge Flux QuantitiesThe basic discharge (from diffuser) flux quantities are used to deter-mine mixing characteristics.� Discharge volume flux: Qj = ujAj� Discharge momentum flux: Mj = uj Qj� Discharge buoyancy flux: J j = g′

j Qj

where

j = parameters pertaining to the discharge jetA = cross-sectional areau = velocity along main flow axisg′

j = reduced gravitational acceleration of the discharge jet due tothe density difference between the effluent and ambientenvironment (AWWA 2003)

The momentum flux affects the relatively near-field mixing re-gion and has minimal affect on the far field region. Momentum fluxis directly proportional to the micromixing; however, it is not an in-dication of mixing in the entire tank. Available research specificallydictates that macromixing or large-scale folding of the interfaces iswhat affects mixing efficiency, not micromixing or small-scale wrin-kling. There is ample research on turbulent mixing, and the more ap-propriate measures to use would be the coefficient of variance, decayfunction, and/or mixing efficiency calculations based on fluid inter-faces (Hjertager et al. 2008; Nathman, Aguirre, and Catrakis 2004).



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Mixing Times and EfficiencyThe mixing time equation currently in use for water storage tankshas been derived from experimental data on 4- and 6-ft- (1.2-and 1.8-m)-diameter cylindrical tanks under fill-and-draw operation(Grayman et al. 1999).

Mixing time (in seconds) = 10.2 V2/3/M1/2 = 9 V2/3(d/Q)

where

M = momentum (inflow velocity times inflow rate, UQ)V = volume of water in the tank at the start of the fill cycle in cubic

feet (cubic meters)Q = inflow rate in cubic feet per second (cubic meters per second)U = inflow velocity in feet per second (meters per second)d = tank inlet diameter in feet (meters)

This calculated mixing time must exceed the tank’s fill time forsufficient mixing (Grayman et al. 1999). In small tanks, such as models,the effects of mixing by molecular diffusion affect a larger proportionof the tanks’ volume within a given time than is the case with large livetanks, in which the concentration gradients may be lower or dissipatemore quickly, causing slower diffusion. Hence, the previous equationmay overestimate the mixing capabilities of a tank. Nevertheless, thisapproach is a good tool, since a tank that does not meet the criteria ofthe previous mixing time equation most probably has water mixingproblems.

On the other hand, if a tank meets the criteria, there is still agood chance that it has a problem regardless of the apparent sufficientexchange of water volume. The most definitive and accurate means ofchecking a tank for water quality is to take multiple samples at differ-ent areas and elevations and calculate the tank’s coefficient of variance(CoV), as discussed later. More research should be conducted to de-termine satisfactory CoV or mixing efficiency values. Some authorsset a target CoV of 0.05 (5 percent) for well-mixed tanks. Recent pub-lications point to a CoV of 0.1 (10 percent) as a more attainable goal.Although we agree with the higher CoV requirement change (less mix-ing), we still believe it may not always need to be this stringent. Manytanks that meet the criteria of the previous mixing time equation donot have a CoV less than 0.1, which further reinforces the statementsmade about its limitations.

Some references state that a CoV between 0.05 and 0.1 is compara-ble to achieving complete mixing. A CoV of 0.05 is typically consideredin the industry to constitute an excellent mixing condition for a widevariety of applications (such as paints), all of which are required to bemore homogeneous than ambient potable water in tanks. A CoV valueclose to zero reflects optimum homogeneity, while a value close to 1.0



OperationOperation


implies the solution is heterogeneous. High levels of homogeneity arenot needed in tanks to maintain water quality. In addition, it may bevery expensive to achieve CoV levels less than 0.1 (10 percent). A CoVof 0.15 (15 percent) is considered good for industrial applications ifmixing time is increased by approximately 25 percent. A higher CoVmay then be acceptable if we are able to increase mixing time, slightlyboost incoming disinfectant residual, or make other less expensivetank modifications that can slow disinfectant deterioration—or if it isdetermined that the higher variation’s effect on water quality is ac-ceptable. Hence, adequate mixing is subject to many interpretationsfrom many perspectives. It should be based on achieving water qualityobjectives with an efficient use of resources. Nevertheless, one cannotgo wrong by targeting a CoV lower than or equal to 10 percent if theability to do tank analysis is hindered.

The coefficient of variance is defined as

CoV = �/xave

where

n = number of samples� = standard deviation of the measured readings corrected to the

true value of the population by using n−1 in the denominatorxave = average of the measured readings

Determination of CoV is the most definitive method of establish-ing tank mixing performance. Several samples are collected at var-ious elevations and areas within a tank. A good rule of thumb isto collect at least six samples; more samples should be collected fortanks larger than 1 mil gal (3.78 ML). Any number of parameters canbe studied—disinfectant residual, disinfection by-products, turbidity,temperature, and so on. A CoV of 0.2 or less is an indication that thetank is sufficiently mixed and does not require mixing enhancements.

Another widely used parameter for quantifying mixing effective-ness is the range mixing effectiveness (ERange). The ERange is defined as

ERange = 1 −RangeOut/RangeIn.

where

RangeOut = range of concentration readings leaving the tankRangeIn = range of concentration readings entering the tank

An ERange value of 1.0 implies excellent or homogeneous mixing,whereas a value of zero implies a heterogeneous solution or poormixing. This measure may yield misleading effectiveness values ifshort-circuiting is present in a tank. However, this anomaly will berevealed when the tank is drafted and stagnant water leaves the tank.Stagnant water may cause an ERange value below 1.0.



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The advantage of this method of mixing evaluation is that it lendsitself easily to online monitoring, since only the inlet and outlet re-quire continuous sampling and testing for water quality parameters(disinfectant residual, for example).

Mixing time calculations involving inflow and outflow ratesshould be compared with tank ambient volume to determine whetheror not a mixing system would work. If tank turnover rate is insuf-ficient—that is, if the volume of water entering and leaving does notchange a certain volume in the tank—turbulent jet mixing systemsmay not result in a homogenous tank, regardless of the mixing effi-ciency. The required percentage of tank volume to be exchanged isa function of the scalar concentrations of the ambient water and theincoming water. These scalars include disinfectant residual, contam-inants, disinfection by-products, and so on. Other influential param-eters may be the temperatures of the incoming and ambient water,chemical compositions and biological contents, atmospheric condi-tions, and the tank’s physical condition.

Mixing TheoryOf the many mixing technologies, no single one can be used for all mix-ing duties. This makes choosing mixers a complex task that requiresadequate understanding of mixing processes—their application andlimitations. Many texts have been written on this subject. Hereafterwe shall attempt to discuss the subject in general and then focus onits applications in tanks.

The mixing of one or more components or materials in a fluidsystem can be described in terms of two separate but interlinked pro-cesses, macromixing and micromixing:

� In macromixing, there is no mixing on the molecular scale, butfluid elements or different components are well distributed orblended to create uniformity throughout the mixture.� Micromixing is complete mixing of species on the molecularlevel. Droplet or particle size reduction (dispersion) of one ormore components of the mixture produces increased homo-geneity of the system.

Most fluid-mixing problems may be analyzed in terms of the mis-cibility or ease of mixing of the components. The ease of distributionaffects the mixing approach to be adopted. For instance, where therate of reaction between the components is to be improved (such ascombining water with a high disinfectant concentration with waterthat contains bio-matter water or with low-residual water), the mix-ing approach is focused on maximizing distribution (macromixing).



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For mixing immiscible fluids such as chlorine gas or solids with water,the mixing approach is focused on reducing droplet or particle size tomaximize the area of contact (micromixing). Hence, micromixing hasa higher tendency to resuspend settled solids or diffuse gases.

Another point of consideration is the mode of operation involved,of which the fluid mixing is normally only a part. The most importantdistinction that affects the mixing operation is whether the tank op-eration is a batch process (distinct fill or draft cycles) or a continuousprocess (pass-through where fill and draft occur simultaneously). Ina batch process, a discrete volume of material is mixed in the tank,whereas in a continuous process, a stream of material is mixed.

The Mixing ProcessFluid mixing is the process by which a nonuniform system is madeuniform. The degree of mixing or uniformity can be analyzed by eval-uating how well the flow is macromixed or micromixed. A measureof uniformity is the coefficient of variance, which is the ratio of thestandard deviation in concentration and the average concentration.The CoV may be viewed as a measure of macromixing. Therefore, ifthe CoV is close to 1.0, the fluid is not mixed; if it is close to zero, it ishomogenous.

To analyze the degree of micromixing, another quantity, the decayfunction, is evaluated. The decay function (d) can be expressed as theratio between the cross-sectional average of concentration fluctuationsand the cross-sectional average of the concentration (Nathman et al.2004).

A recirculation zone in the area near the inlet increases the mixingeffect. Experimental results show a uniform concentration distribu-tion at that point. However, farther along the path, the concentrationbecomes less uniform as the flow becomes better macromixed thanmicromixed (Nathman et al. 2004). In other words, the turbulence inthe region very near the inlet can result in micromixing, but micromix-ing cannot be maintained as the flow gets farther from the source. Thishas been confirmed by several researchers, and the mechanisms thatcause each type of mixing are known.

Before discussing mixing mechanisms, it must be understood thatthe aforementioned research implies that even high-jet-velocity inletsources will always revert to macromixing as the flow gets fartherfrom the source, whereas micromixing is limited to areas close to thesource. The energy cost required to expand this effect to cover an en-tire tank or reservoir would be excessive. In addition, such a high levelof mixing is not necessary to achieve high mixing efficiency, as will beshown later. Micromixing is also not maintainable, because as soon asthe inflow to the tank stops, density and temperature gradients willincrease the coefficient of variance with time. Biochemical reactions



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taking place over time also contribute to the increase in the coeffi-cient of variance. In addition, the higher velocities reduce the totalamount of tank influent water because of the effect of the resultinghigher head loss on pump performance. Reducing the amount of wa-ter with high disinfectant residual entering the tank results in a loweroverall increase in tank disinfectant residual over time. Therefore, cre-ating high-velocity jets in an effort to increase micromixing is not anoptimum way of mixing tanks.

Tank Mixing SystemsVarious tank mixing systems employ several energy transfer and con-version mechanisms to create various effects for particular mixingresults. In the case of distributive or macromixing, swirl created byrotating parts or directed flow causes laminar thinning of the materialinterfaces, thereby increasing volumetric combination of the materi-als. A repeated cutting and folding action of the mixture also increasesthe distribution of various material components. The effectiveness andefficiency of a mixer in distributive mixing is therefore a function ofhow the machine interacts with the fluid in a geometric sense. Thatis, the volume enclosed by the outer interfaces (i.e., the interfaces be-tween the pure fluid and the mixed fluid) rather than the interfacialsurface area is what determines mixing efficiency.

Conversely, the effectiveness and efficiency of a mixer in disper-sive mixing (micromixing) is dependent on the means of the system’sshearing interaction with the fluid. The higher the shear stress, thesmaller the resulting particles or droplets are in the mixture. The uni-formity of the stress distribution determines the uniformity of themixing. Without uniform distribution of the shear stresses, it is im-possible to guarantee that the same level of mixing is applied to allparts of the water in the tank.

Mixing technologies are often available in either batch or contin-uous form, but rarely both. In situations where both are offered, thereare typically some performance trade-offs. Care must be taken to se-lect a system that performs effectively with the tank’s operationalscheme. In addition, energy consumption and availability must beconsidered. Since the amount of water to be mixed is huge, the en-ergy needed to mix the tank can add up to a considerable amountover time. Also, many tanks are constructed in remote areas whereelectrical power may not be readily available. Some mixing systems,due to the requirement for high inlet pressure, may require additionalpumps, pump replacement, or a change in operation points of existingpumps. This not only changes the distribution system’s performanceand efficiency during filling, but it also may affect the drafting of thetank, which could become critical in achieving fire flows from the tank.We shall next discuss some of the various mixer types on the market.



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Mixer TypesThe many types of mixers applicable to water mixing in tanks may becategorized as follows:

ImpellersThese have specially shaped blades on a rotating shaft driven by a gearmotor and/or variable-speed drive. They are used almost exclusivelyfor batch processes. Applying them in water tanks or towers may beprohibitive because of structural support requirements and energyconsumption issues. Also, these systems may not work in changingwater levels.

Static MixersThese devices require continuous fluid motion to work, so they areused in continuous processes. They comprise a set of nonmoving ob-structions or orifices in a pipeline. The obstructions or orifices areshaped and positioned in such a way as to create cutting and fold-ing effects and/or turbulence for mixing of piped fluid streams. Staticmixers are a reliable and low-cost alternative. Nevertheless, any high-pressure drop across the mixer may require larger and more expensivepumps, increase energy consumption, or alter pump operating points.

DispersersDispersers comprise a range of complex machines and systems thatdeliver relatively uniform dispersions in particular fluid applications.A valve homogenizer comprises a very high-pressure pump and acontrolled valve nozzle through which the fluid is forced at very highvelocity to rupture the droplets through extensional stressing. The jet-impinging mixer, another type of disperser, uses high-velocity fluidstreams—except that in this case, the fluid is jetted against a plate orcontra-jet to rupture the droplets or particles using impact stressing.The high level of mixing that these systems provide is localized; useof this technology in water tanks would require many nozzles or jets,making it high in installed cost and energy consumption.

Pump MixersAvailable in both batch and continuous forms, pump mixers use in-ternally generated energy to force fluid through small nozzles at veryhigh velocities while extending and shearing it. The fluid flowingthrough the nozzles at high velocity then impinges on an internalwall of the mixer. A dynamic cutting and folding action added tovigorous turbulent flow provides distributive mixing. Pump mixersare suited to a wide variety of applications because they can handle awide range of materials and viscosities with high mixing performance.In addition to energy consumption and availability issues, these sys-tems may not be as effective in large tanks. Tests conducted for the US



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Department of Energy have shown that a considerable volume of wa-ter in large tanks remains unmixed (Lee and Dimenna 2001).

Turbulent Jet MixingTurbulent mixing is involved in most mixing processes due to its abil-ity to mix and transport species, momentum, and energy much fasterthan can be done by molecular diffusion (Hjertager et al. 2008). De-tailed understanding of the mixing process and validation of proto-types are even more critical in mixing systems used in tanks that storepotable water for human consumption, because they can affect waterquality.

Turbulent jet discharge into a crossflow (i.e., a transverse jet) isa turbulent free-shear flow (Shan and Dimotakis 2001). Crossflow ina tank can be created by change in water elevation and/or outflowstreamlines (as in a pass-through tank). The discharge, in the formof a round buoyant jet into a nonturbulent stratified crossflow, maycontain five asymptotic regimes (AWWA 1986) in which self-similarflow conditions exist:

1. Pure jet

2. Pure plume

3. Pure wake

4. Advected line puff

5. Advected line thermal

Of these regimes, the first three are free turbulent flows dominatedby transverse shear (shear normal to jet axis); the latter two are dom-inated by azimuthal shear (parallel to jet axis). In the latter case, aninternal double-vortex structure is generated within the jet. No self-similar regime is possible in the presence of density stratification (Jirka1999). In that case, axial pressure forces influence and finally destroythe boundary-layer evolution of the flow and lead to strong horizontalspreading—the so-called collapse motion during the terminal-layerphase of a buoyant jet in stratified surroundings. The flow is no longerjet like. In actual discharge situations, one or more of the five regimescan occur as asymptotic regimes. Regime (1) is often the initial regime(whenever the jet velocity Uj > ambient velocity ua). Regime (5) isusually—but not always—the final regime (Jirka 1999). Figure 9-21shows a model of a positively buoyant turbulent jet.

The buoyant jet is a flow phenomenon with free turbulence. Itrepresents a gradually evolving flow along its axis and thus exhibitsboundary-layer characteristics with its possibilities for mathematicalsimplification including self-similarity techniques (Schlichting 1968).However, because of the variety of forcing elements, buoyant jet mo-tions are in general not self-similar (Jirka 1999). They are self-similar



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FIGURE 9-21 Model of positively buoyant turbulent jet.

only in five possible asymptotic regimes in which they have an in-variant internal force balance, invariant turbulence, and entrainmentproperties. In between these regimes, the buoyant jet properties arevariable and cannot be scaled uniquely by local jet parameters (Jirka1999).

The local Reynolds number (Re) in the case of a turbulent jet ina quiescent reservoir is approximately equal to the jet exit Reynoldsnumber Rej = d jU j/� (d j is the jet diameter, U j is the jet velocity, and �is the kinematic viscosity of water). If the jet momentum flux MjU j =� j�(d j/2)2U j

2 (� j is water density and Mj is the mass of water in the jet)is held constant as the jet diameter decreases and its velocity increases,the jet approaches a point source of normal momentum, which gener-ates a counterrotating vortex pair (Shan and Dimotakis 2001). At lowReynolds numbers, in particular, tertiary and sometimes quaternaryvortices are formed such that the vortices no longer have equal circu-lation, and each vortex could have a different induced velocity. In thatcase, the induced vertical velocities would be substantially smaller,and the overall mean trajectory would be shallower. At high-Re trans-verse jets, the counterrotating vortex pair is the dominant structureand the primary mechanism for entrainment of free stream fluid. Thevortex circulation is a decreasing function of downstream distance de-caying through viscous diffusion (Shan and Dimotakis 2001). In otherwords, high-Re (more turbulent) inlet sources would result in a flowwith higher vertical velocities and a vortex pair as the main sourceof water entrainment. As will be explained later, entrainment is themajor mixing mechanism. The higher vertical velocity means that theinfluent would reach the water surface faster, resulting in even lessentrainment of the ambient water in the tank. Hence, the high level



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of micromixing caused by the high Reynolds number results in morehomogenous mixing in the vicinity of the inlet, but less water is mixed.

Experimental data indicate that the scalar gradients are steeper inthe horizontal directions than in the vertical directions. Anisotropy ofthe transverse jet’s scalar field is in contrast to the far field isotropyfor axisymmetric jets discharging into a tank (Shan and Dimotakis2001). Scalar mixing in the transverse jet is enhanced by increasingthe Reynolds number. In the case of turbulent jets discharging into atank, the concentration’s probability distribution functions lose theirpeak at the highest Reynolds number (Shan and Dimotakis 2001). Thismeans that when discharging into a reservoir or tank in which thereisn’t a crossflow, mixing is decreased in the far field at high Reynoldsnumbers (or influent velocities). The minimum Reynolds number forwhich turbulent mixing can be considered as fully developed is ap-proximately 10,000 (Nathman et al. 2004).

On the basis of the previous analysis, tank mixing systems needto reduce the Reynolds number to slightly above 10,000 by reducingthe velocity through each inlet. An efficient way to accomplish thisis to have more inlets to spread the flow. Having more inlets resultsin a larger volume of water involved in near-field micromixing, andit introduces the influent at various locations in the tank, further en-hancing the distribution of the micromixing effect.

Flow-dependent mixing is explained by noting that turbulent mix-ing is essentially a three-stage process (Shan and Dimotakis 2001):

Entrainment: Engulfment of irrotational (ambient) flow into the tur-bulent flow region, or macromixing.

Stirring: Kinematic motion responsible for creating interfacial areabetween species.

Molecular mixing: Diffusive mixing on the molecular scale, or mi-cromixing.

The balance among these three stages determines the probabil-ity distribution function of the mixed water. Nevertheless, the meanconcentrations are a measure of entrainment rather than of molecu-lar mixing. Hence, the transverse jet entrains less ambient fluid thanthe ordinary turbulent jet. Transverse jets homogenize the entrainedfluid more thoroughly. This indicates that for transverse jets, there ismore stirring and molecular mixing; for ordinary jets, there is moreentrainment (Shan and Dimotakis 2001).

Increases in water elevation in tanks with mixing systems resultin little transverse motion. Transverse motion is especially negligi-ble in a large tank unless it is a pass-through tank with considerableand independent inflow and outflow rates. Therefore, the influent jetsneed to entrain larger volumes to mix the whole tank. To maximizeentrainment, the inlets should be located such that the inflow engulfs



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the maximum amount of volume possible. This is done by creatingsheet flows at different elevations that intersect and or interact in thefar field. This is further enhanced if the influent is buoyant, as willbe shown later, whether positively or negatively due to temperaturedifference between influent and ambient water.

Turbulent Jet Mixing EfficiencyTurbulent jet mixing efficiency is determined by the behavior of themixed influent/ambient water interfaces. Knowledge of the dynam-ics of the interfaces is crucial for physical descriptions, predictions,and control of the mixing efficiency (Nathman et al. 2004). Althoughit has long been recognized that large-scale entrainment is importantfor mixing, one must understand that entrainment alone sets an up-per bound only on mixing efficiency. In other words, knowledge ofthe growth rate of a turbulent shear flow is not sufficient to deducemixing efficiency (Nathman et al. 2004). The crucial point is that thevolume enclosed by the outer interfaces (i.e., the interfaces betweenthe influent water and the ambient tank water)—rather than the in-terfacial surface area—determines mixing efficiency. Hence, it is thelarge-scale dynamics of the outer interface that provide the dominantcontributions to the mixing efficiency (Nathman et al. 2004).

On the basis of high-resolution measurements in the far field offully developed round jets, it was found that large-scale folding of theinterfaces, as opposed to the small-scale wrinkling of the interfaces,provides the dominant contribution to mixing efficiency (Nathmanet al. 2004).

A discharge with no buoyancy is referred to as a nonbuoyant jet orpure jet. A release of buoyancy only (no initial momentum) is calleda pure plume. A discharge with both momentum and buoyancy iscalled a buoyant jet or forced plume. Positively buoyant flows aredefined where the buoyancy force acts vertically upward against thegravity force; negative buoyancy is defined as acting downward inthe direction of the gravity force (MixZone 2005).

Dilution in turbulent buoyant jets is caused by entrainment ofsurrounding ambient water into the influent water jet. Entrainment isa turbulent process caused by shear stress between the discharge flowand the surrounding ambient water (MixZone 2005).

As briefly stated earlier, to improve mixing efficiency, a tank mix-ing system would have to engulf or entrain as large a volume of wateras possible during inflow. To take maximum advantage of this mixingphenomenon, a tank mixing system must have inlets creating sheetsof flow at different elevations. The interfaces should then fold overeach other for optimum mixing efficiency. The directed momentum ofthe created sheets of flow would create more folding if the inlet ori-entations and design allow for the energy to be converted to directedmomentum rather than excessive stirring turbulence. This process



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requires less throttling at each inlet and multiple inlets at theproper elevations. The locations of the inlets should be such thatboundary-layer attachments occur only as part of the entrainmentscheme and do not disrupt it.

Mixing RegionsThe hydrodynamics of an influent continuously discharging can beconceptualized as a mixing process occurring in two separate areas.

In the first area, the initial jet characteristics of momentum flux,buoyancy flux, and outfall geometry influence the jet trajectory andmixing. This area, called the near field, encompasses the jet subsurfaceflow and any surface or bottom interaction (or, in the case of a stratifiedambient, terminal–layer interaction). The mixing zone is the part ofthe near-field area in which the initial dilution of a discharge occurs.Many hydrodynamic definitions of mixing zones include both near-field mixing and boundary-interaction processes (MixZone 2005). Inthis area, mixing system design can usually affect the initial mixingcharacteristics through appropriate manipulation of design variables.In particular, designs with dynamic bottom attachments should beavoided (MixZone 2005). Dynamic plume attachments occur whenthe discharge flow interacts strongly with a boundary in the nearfield. Such near-field boundary interactions present the possibilityof high influent concentrations near the discharge (MixZone 2005).Often, near-field attachments are avoidable with proper design of themixing system. This flow also exhibits a subsequent buoyant liftoffand an unstable near field (MixZone 2005).

Two types of attachment are typically found: wake attachmentforced by the crossflow and Coanda attachment forced by the entrain-ment demand of the influent jet itself. A physical description of theseprocesses is given below (MixZone 2005).

In wake attachment, the presence of the discharge structure andthe jet influx interrupts the ambient velocity field and causes a recir-culation area in the wake downstream from the discharge (MixZone2005).

A Coanda attachment is a rapid dynamic attachment that occurswhen a jet discharges close to a nearby parallel boundary. This processis referred to as a Coanda effect. It occurs because of the entrainmentdemand of the jet flow at its periphery. If a boundary limits the ap-proach flow of ambient water, then low-pressure effects cause the jetto be deflected toward that boundary, thereby forming a wall jet. Thus,the mixing process of Coanda-attached flow is governed by the dy-namics of the wall jet (MixZone 2005). Figure 9-22 shows a negativelybuoyant turbulent jet flow with wall attachment.

This implies that a tank mixing system discharging very close tothe bottom or surface may cause flow attachment to the bottom or abuoyant film at the top with reduced mixing.



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FIGURE 9-22 Negatively buoyant turbulent jet flow showing wall attachment.(Source: MixZone.)

Boundary interactions occur when the flow contacts the surface,bottom, or sides or forms a terminal layer in a density-stratifiedambient environment (Fig. 9-23). Boundary interactions also deter-mine whether mixing is controlled by stable or unstable conditions atthe discharge source (MixZone 2005). Boundary interaction generally

FIGURE 9-23 Turbulent jet flow into density-stratified tank model. (Source:MixZone.)



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provides the transition from near-field (discharge source–controlled,or micromixing region) to far-field (ambient environment–controlled,or macromixing region) mixing processes. However, boundary inter-actions in the form of dynamic plume attachments to the bottom areconsidered near-field mixing processes (MixZone 2005).

As the turbulent buoyant jet travels farther away from the source,the source characteristics become less important. Conditions existingin the ambient environment control trajectory and dilution of the flowthrough the spreading of buoyant density-current motions and pas-sive diffusion due to ambient turbulence. This region is referred to asthe far field (MixZone 2005). A counterrotating vortex pair has beennoted in the far field of a transverse jet; however, the mean flow stateis not necessarily a symmetric vortex pair but can be unsteady andasymmetric under certain conditions (Shan and Dimotakis 2001).

The rotational flow created by the vortex is what helps to makethe flow regime more homogenous. Hence, a properly designed tankmixing system would attempt to encourage or extend the creation oflateral vortices that do not reach the surface so that it enhances themixing of the largest volume of water.

The assessment of near-field stability (i.e., distinguishing stablefrom unstable conditions) is a key aspect of analyzing influent di-lution and modeling the mixing zone. It is especially important forunderstanding the behavior of the two-dimensional plumes resultingfrom multiport diffusers (MixZone 2005). Discharge plumes may beclassified as having the following characteristics:� Stable discharge conditions usually occur for a combination

of strong buoyancy and weak momentum (MixZone 2005).� Unstable discharge conditions occur when a recirculation phe-nomenon appears in the discharge vicinity. This local recircu-lation leads to re-entrainment of already mixed water backinto the buoyant jet region (MixZone 2005).

The previous discussion of stability emphasizes the importance ofeliminating recirculation at the discharge vicinity. Recirculation at theinlets can be minimized by (1) reducing influent velocity to minimizethe recirculation region size or (2) having a boundary surface veryclose to the point of discharge to prevent or minimize rotational flowin the direction opposite that of the discharge. In the second case, thismeans not using discharge nozzles.

Buoyant DischargesInformation about the density distribution in the ambient water bodyis very important for correctly predicting influent discharge plumebehavior. Density currents are buoyancy driven far-field flows thatare defined by transverse horizontal spreading while being advected



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downstream by an ambient current. These spreading processes canintrude into the ambient flow, forming a buoyant upstream wedgeand stagnation point. These flows are caused by the density differenceof the mixed flow relative to the ambient density. Density currents arepreceded by turbulent jet mixing in the near field and are followedby passive diffusion in the far field. Density currents may or may notform upstream intrusions, depending on the crossflow magnitude andinternal buoyancy at boundary interaction (MixZone 2005).

Buoyant jets discharged horizontally along the water surface froma laterally entering channel or pipe bear some similarities to the moreclassical submerged buoyant jet. For a relatively short initial distance,the effluent behaves like a momentum jet, spreading both laterally andvertically due to turbulent mixing (MixZone 2005). After this stage,vertical entrainment becomes inhibited due to buoyant damping ofthe turbulent motions, and the jet experiences strong lateral spread-ing. During stagnant ambient conditions, ultimately a reasonably thinlayer may be formed at the surface of the receiving (ambient) water;that layer can undergo transient density-current buoyant spreadingmotions (MixZone 2005).

In the presence of ambient crossflow, buoyant surface jets mayexhibit any one of following three types of flow features (MixZone2005): � They may form a weakly deflected jet that does not interact

with the bottom or surface.� When the crossflow is strong, they may attach to the down-stream boundary, forming a bottom-hugging plume.� When a high discharge buoyancy flux combines with a weakcrossflow, the buoyant spreading effects can be so strong thatan upstream intruding plume is formed that also stays closeto the surface near the inlet.

Density currents are effective transport mechanisms that canquickly spread a mixed effluent laterally over large distances in thetransverse direction, particularly in cases of strong ambient stratifi-cation. In this case, influent of considerable vertical thickness at theterminal level can collapse into a thin but very wide layer, unless thisis prevented by lateral boundaries (MixZone 2005).

If the influent water is nonbuoyant or weakly buoyant, there isno buoyant spreading area in the far field—only a passive diffusionarea. Depending on the type of near-field flow, ambient density strat-ification, and boundary interaction process, several types of densitycurrent buoyant spreading may occur:� Spreading at the water surface� Spreading at the bottom� Spreading at a sharp internal interface with a density jump



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FIGURE 9-24 Buoyant discharge from single port inlet at 45-degree angle intostagnant tank.

� Spreading at the terminal level in a continuously (e.g., lin-early) stratified ambient.

Turbulence in the ambient environment becomes the dominatingmixing mechanism in the far field at sufficiently large distances fromthe discharge point. In general, the passively diffusing flow growswider and thicker until it interacts with the vessel bottom and/orsides (MixZone 2005). The strength of the ambient diffusion mecha-nism depends on several factors that relate mainly to the geometryof the ambient shear flow and the amount of ambient stratification.From classical diffusion theory, gradient diffusion processes in thebounded flows can be described by constant diffusivities in the ver-tical and horizontal direction that depend on turbulent intensity andon channel depth or width. In the presence of a stable ambient strat-ification, the vertical diffusive mixing is generally strongly damped(MixZone 2005).

In the surface approach condition, the weakly bent flow impingeson the surface at a near-vertical angle (>45 degrees) (Fig. 9-24). Afterimpingement, the flow spreads more or less radially along the wa-ter surface as a density current. In particular, the flow spreads somedistance upstream against the ambient flow and laterally across theambient flow. The strong buoyancy of the discharge dominates thisspreading. The lateral spreading of the flow in the surface impinge-ment area is driven by both the flow momentum and the buoyancyforce (MixZone 2005).



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Since influent water is rarely at the same temperature as ambientwater in the tank, buoyant flow can be used as a free source of mixingenergy. Substantial additional mixing can be created and further op-timized if the tank mixing system is designed to use density streamsas part of the entrainment scheme. This would further negate the useof inlets at the bottom or near the surface in a tank mixing apparatus.Such inlets prevent the density streams’ interfaces from folding dueto stratification and prevent the engulfment or entrainment of largevolumes of water needed for mixing efficiency.

Flow Diffusers

InfluentA multiport diffuser is a linear structure consisting of many more orless closely spaced ports or nozzles that inject a series of turbulent jetsinto the ambient receiving water body. These ports or nozzles maybe connected to vertical risers attached to an underground pipe ortunnel or they may simply be openings in a pipe lying on the bottom(MixZone 2005).

Flow diffusers in water tanks’ mixing systems should be designedto use and accommodate all the physical phenomena associated withturbulent jet mixing. Such systems should optimize mixing efficiencyusing as little energy as possible, as follows:� Use reasonable inlet velocities. Higher velocities are not only

energy consuming, but are also detrimental to proper mixingas shown earlier. This requires a multitude of inlets to suffi-ciently divide the flow so that velocities are lower.� Do not use nozzles—not only because of higher velocities andhead loss, but also because more recirculation is associatedwith nozzles. Orifices are closer to the conduit/pipe surfaceand minimize such recirculation.� Entrain larger volumes of ambient water by having flowstreams at different levels and sides, creating three-dimensio-nal mixing effects. A three-dimensional arrangement wouldbe required for the inlets such that the influent creates undis-turbed streams or currents, which engulf the majority of am-bient volume.� Enhance folding of the interfaces in the far field (Fig. 9-25).The inlets must be positioned so that inflow streams impingeon each other and interact with boundary surfaces in such amanner that they create large-scale folding and lateral vor-tices.� Use density currents.



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FIGURE 9-25 Sheet flow from multiport diffuser into tank model showingformation of vortices and folding of interfaces in far field. Note: better mixingdistribution in far field.

EffluentThe flow diffuser discussed so far is an influent structure. However,it may also be a tank effluent structure. The filling cycle typically usesa fraction of the total time of a tank’s operating cycle. The tank mayremain idle for some time or may draft for a long time, feeding backinto the system. Reliance on influent mixing alone is not optimum, be-cause throughout the longest part of the operational cycle, no mixingis taking place. To optimize mixing, it is ideal to also mix during thedraft cycle. After the fill cycle, ambient water in the tank will stratify,lose disinfectant, or be rendered otherwise nonhomogeneous becauseof some physical or biochemical activity. As a result, water qualitymay progressively decrease.

More importantly, impurities or disinfection by-products may set-tle, stratify, or accumulate unevenly because of temperature gradientsand removal of fluid solely at one or two locations on the tank bot-tom. To prevent the possibility of high concentrations of accumulatedimpurities (such as some disinfection by-products or solids being fedback into the distribution system by excessive drafting), it is prudentto mix or blend the effluent water from various areas and elevationsof the tank as it is drafted. At a minimum, effluent mixing will accom-plish the following:� Prevent the sequential removal of stratified or accumulated

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� Minimize the number of dead spots due to drafting from theregions close to the outlet� Decrease reliance on passive diffusion which may be dis-rupted by many environmental factors� Decrease the possibility of icing in colder climates due to dis-tributed motion of the fluid and removal of fluids near thetop� Remove water close to the top, where biological activity ishigher and disinfectant residual is lower� Make effluent more uniform and provide a more homo-geneous starting point for the next fill cycle, making turbulentjet mixing more effective� Because of uniformity of mixing, achieve better and moremeaningful tracking of water age� Eliminate mixing cost, since water from the tank or reservoir ismixed by gravity and forced back into the distribution system.

When water is combined from different areas and elevationsthrough the diffusers, it creates large interface areas internally whichare then stirred by the turbulence in the diffusers and tanks’ piping,valves, and fittings. This further mixes the flow to create a more ho-mogeneous effluent and consistent water quality.

Tank VentingMost water storage tanks are nonpressurized tanks that require ad-equate venting. By allowing the removal or replenishment of air aswater enters or exits the tank, venting prevents both pressurized andvacuum conditions. Atmospheric tanks are not designed to handlepressurization; the absence of sufficient venting to handle the air out-flow generated as water enters the tank would cause the air in thetank to compress and exert pressure on the tank walls that may ex-ceed design stress limits. Likewise, tanks are not designed to handlethe vacuum conditions created when water is drafted from a tankwithout adequate venting. Buckling of tank walls takes place evenwhen differential pressure is small.

The styles of air vents most commonly found in water tanks arethe mushroom, pan, and 180-degree types. AWWA D100, Standardfor Welded Steel Tanks for Water Storage, requires that one tank vent,even if more than one is required, always be located near the center ofthe roof. A reasonable offset is allowed for tanks designed with centerdry-access tubes. Vent designs, examples of which are given in Figs.9-26 to 9-28, should meet the following requirements:



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Tank roof assembly

24 in. (5

88 m

m)

Screen

FIGURE 9-26 Double 90-degree elbow roof vent detail. (Source: AWWA ManualM42, Steel Water Storage Tanks.)� Prevent insects and animals from entering the tank (a non-

corrodible mesh is recommended)� Prevent rainwater or surface water from entering the tank� Prevent air drafts from entering the tank

AA

Plan view

Vent

diam

.

Cover diam

eter

(Outside diameter)

(Inside diameter)

(Outside diameter)

(Hole in roof)

Section A-A

Tank roof

3/16

FIGURE 9-27 Pan deck vent detail. (Source: AWWA Manual M42, Steel WaterStorage Tanks.)



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Support bars

Carbon-steel body

Roof

PTFE gaskets (typical)

Air pressure

1/2- no. 13 × 15 flattened expanded metal bird screen

Screen(brass material is normal)

Pressure palletVacuum pallet

Air vacuum

Install vent vertical 5 +

FIGURE 9-28 Typical clog-resistant vent detail. (Note: PTFE = polytetrafluoroe-thylene. Pallets should be removed during coating to prevent clogging of thescreens. Periodic inspection and maintenance are required to keep in properworking condition.) (Source: AWWA Manual M42, Steel Water Storage Tanks.)

� Exclude dust and debris, as much as possible, from enteringthe tank� Provide some level of security against accidental or inten-tional contamination� Prevent direct sunlight from entering the tank� Be frostproof in cold-weather areas� Be tall enough, or installed high enough, not to be blocked bydrifting snow or debris

It is a requirement of the Ten States Standards that overflows notbe considered as vents. Obviously, a tank using its overflow as a ventwould be left without venting during overflow conditions. It alsopoints out that vents on ground-level tanks should terminate in aninverted U shape with its opening 24 to 36 in. (609.6 to 914 mm) abovethe tank’s roof or ground. The U-shaped overflow should be coveredwith 24-mesh noncorrodible screen installed within the pipe at thelocation least susceptible to vandalism. AWWA manual M42, SteelWater Storage Tanks, recommends clog-resistant vents with pressure-and vacuum-releasing pallets.

Large tanks should be provided with more than one vent. Oneshould be installed near the center of the roof, and the other(s) closer



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to the tank’s walls. This facilitates crossflow ventilation through thetank.

Vent sizing is of special concern in the case of tanks in systemsthat have experienced demand growth. Increased tank inflow andoutflow rates must be handled by tank venting. Undersized ventsmust be replaced with properly sized ones to prevent problems relatedto increased maximum flow rates.

TelemetryMost storage facilities for potable water are located in unmanned sites.Some tanks are located in sites manned by a handful of operatorswhose main responsibilities are to monitor a water treatment process.In either case, it is desirable to have automated systems that moni-tor hydraulic and water quality parameters of tanks. These devicescan store data in electronic form or on paper. They can also transmitinformation collected to a central location or manned facility wherean operator can keep track of and control multiple facilities through-out a plant and/or a distribution system. Telemetry is the science andtechnology of automatic measurement and transmission of data bywire, radio, or other means from remote sources, pumping stations,distribution system tanks, or other facilities or processes to receiv-ing stations for recording and analysis. Most telemetry systems usedby water utilities are commonly known as SCADA (for supervisorycontrol and data acquisition) systems.

Tank Water ElevationWater utilities that operate a SCADA system have a central monitor-ing facility where one or more operators are able to remotely controlthe fill or draft of tanks, the opening and closing of motor-operatedvalves, and chemical feed processes such as disinfectant boosting. Inaddition, the SCADA system monitors, records, analyzes, and identi-fies trends regarding myriad parameters from online sensors, analyz-ers, and transmitters at each facility in the communication network.Although some of the systems monitored by SCADA may be auto-matic (e.g., the closing of an altitude valve to prevent tank overflow,the sounding of alarms, and so on), some may be entirely controlledby the SCADA operator (e.g., starting and stopping pumps). Tank ele-vation information lets SCADA operators know when pumps shouldbe turned on or off as part of normal distribution system operation.In many cases, tank elevations are the only source of information tooperators regarding distribution system pressures.

Trended elevation data over time paints a picture of a tank’s dailyfill and draw cycles. Parameters such as rates of inflow and outflowthroughout the fill and draw cycles can be indirectly determined from



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elevation data if the tank geometry is known, and changes in tank vol-ume can be calculated over time. This trended flow and volume datacan help water quality personnel monitor mixing patterns in tanksover the course of the year and provide operators with strategies toenhance tank mixing for each tank’s operating characteristics. A bal-ance must be maintained between the need for optimum water qualityand emergency storage. Allowable low levels in tanks should leavesufficient water in storage to satisfy potential emergency demandsfrom fires, power outages, water main breaks, and so on.

Excessively high water elevation can trigger an overflow alarmto alert the SCADA operator that an altitude valve may be malfunc-tioning and that tank overflow is probable. The operator can respondby turning pumps off at upstream pumping stations and/or closing aremotely controlled motor-operated isolation valve, if one is presentat the tank. Depending on the particular circumstances, the operatorcan dispatch a road crew to the tank. Skillful operation and knowl-edge of the distribution system are required to appropriately addresshigh-pressure conditions.

Isolating a tank from the distribution system without taking othermeasures, such as shutting pumps off, may create abnormally highpressure and leave a system vulnerable to water main breaks andother catastrophic failures. Tanks that are left open to a distributionsystem provide surge relief should pressure transients be generated.Such systems are deemed soft systems. A system operated at highpressures with its tank(s) offline loses this surge protection and issaid to be a hard system. It should be noted that some utilities havechosen to forego altitude valves and rely entirely on telemetry andmotor-operated isolation valves to control water level in the tank.

Many strategies are available for sensing water level. A few ofthe most common technologies are listed here, divided into two cate-gories: contact sensors and noncontact sensors.

Contact-Level Sensing TechnologyBubbler systems use a source of compressed air to push bubbles out ofa conduit at the bottom of the tank. The higher the pressure required topush the bubbles, the higher the water level. Bubblers provide contin-uous level sensing relatively accurately, but they require an externalsource of compressed air. The air pressure is transmitted as an analogvoltage or current signal.

Radio-frequency (RF) capacitance sensors, tuning-fork sensors,and floats are switches that are engaged when submerged in waterand disengaged when water levels drop below them. Several of theseswitches can be installed on a track or some other means of support atseveral tank depths. An elevation signal is generated for each particu-lar depth where switches are located. A transmitter unit is commonly



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used to monitor each switch in the tank and transmit digital outputscorresponding to tank level. Icing is a concern with this technology.

Noncontact Level Sensing TechnologiesUltrasonic level sensors and radar level sensors are noncontact de-vices that are suspended above the water surface. They measure therate of travel of energy through air to measure the distance betweenthe sensor and the surface of the water. Although some sensors aresusceptible to condensation, many styles are available with featuresthat prevent condensation from affecting the measurement. Icing mayor may not be a concern, depending on how the sensor has been in-stalled. Another concern is the proximity to the sensor’s energy beamof tank walls or other tank appurtenances. Some sensors are able to“calibrate out” such obstacles. Under certain conditions, a stilling wellcan be used to focus the sensor’s beam through a pipe dipped into thewater. This method has the added advantage of reducing water sur-face turbulence that may affect accuracy, but it increases the chancefor icing damage.

Pressure transmitters measure the pressure, or head, of the tank atsome point along the tank’s piping or wet riser. These devices use thecompression of a pressure-sensing element, typically a strain gauge ora capacitor, to generate a continuous-voltage or current-analog signalcorresponding to tank elevation. A tap is required in the tank’s pipingor riser where small-diameter piping (copper that is 0.25 to 0.75 in. [6.3to 19.0 mm] in diameter is common) connects to the transmitter. Thissmall-diameter pipe is susceptible to freezing and should be installedin a heated or insulated enclosure in cold-weather regions. Pressuretransmitters are the most common type of level sensor used in SCADAsystems for distribution system storage tanks.

Level-sensing methods require accurate information regardingtank elevation and dimensions. A maintenance and calibration sched-ule should be followed, and good records should be kept.

Street PressureStreet pressure is measured on the street side of the altitude or tank iso-lation valve. The most common technology used is the pressure trans-mitter. A continuous-voltage or current-analog signal correspondingto the pipe pressure at the sensor elevation can be continuously sentto the SCADA operator. (The pressure transmitter need not be atthe same elevation as the pipe centerline, but this discrepancy mustbe accounted for in the determination of street pressure.) If the alti-tude valve is not locally controlled, a decrease in street pressure signalsthe operator that the altitude valve or tank isolation valve should beopened and the tank drafted to meet demand. Data on street pressureenable the operator to monitor distribution system pressures even if



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the tank has been isolated. For instance, a sharp decrease in streetpressure may be an indication of system failure such as a water mainbreak.

Disinfectant ResidualWater quality managers are installing monitors for disinfectant residu-als at representative distribution system locations. Some installationsare made because of regulatory requirements, others as a voluntarymeasure. Tanks are ideal for such systems since they are usually insecured locations, electrical power is available, and utility personnelperiodically visit the tank site.

Various technologies are commercially available to measure andtransmit concentrations of disinfectant residual. The most commonsecondary disinfection chemical in the United States is chlorine. Thefollowing methods are described for measuring concentrations ofchlorine residual at water storage facilities.

N,N-Diethyl-p-phenylenediamine (DPD) Colorimetric MethodDPD is oxidized by chlorine in solution. This results in two oxidationproducts. The contrast between the colors of these two compounds, asmeasured by a colorimeter or a spectrophotometer, reveals the amountof free or total chlorine in the water. A voltage or current-analog sig-nal corresponding to the calibrated concentration of residual is thentransmitted to SCADA.

Iodometric MethodPotassium iodide reacts with free chlorine in the sample water to pro-duce iodide. The iodide concentration is measured by the instrumentto yield total chlorine. Free chlorine is not measurable by this method.

Polarographic Membrane Sensor TechnologyA pair of electrodes is immersed in a conductive electrolyte and sepa-rated from the sample water by a chlorine-permeable membrane. Freechlorine travels through the membrane and is reduced to chloride atthe electrode’s surface. The reduction of free chlorine generates anelectric current between the electrodes that is proportional to the freechlorine concentration.

Amperometric ElectrodesCombinations of probes consisting of a silver anode and a platinumcathode measure free chlorine concentration, pH, and temperature.A current proportional to the free chlorine concentration is producedwithin the electrodes. The amperometric electrodes require replace-ment after a manufacturer-specified lifetime (Pollack et al. 1999). Al-though automated, these systems may require chemical replenish-ment and periodic maintenance and calibration to sustain accuracy.



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Disinfectant-residual sensors can also act as feedback for boosterdisinfection at storage facilities. The SCADA operator can remotelycontrol the feeding system. System parameters such as start, stop, feedrate, remaining disinfectant, and set point may be controlled and/ormonitored.

TemperatureTemperature transmitters and thermocouples can be used to measureand transmit temperatures. Low air temperature inside equipment en-closures can alert the SCADA operator to potential freezing conditionsthat are detrimental to tank-monitoring devices. A water temperatureprobe inserted either in the tank or in the pipeline leading to the tank,or in both, can provide the utility with information regarding tankstratification conditions. A complete temperature profile of a tank canbe obtained by using a weighted line of thermocouples designed tomeasure temperature at various water depths. The information maybe transmitted to SCADA or stored locally and downloaded manu-ally. Trending of such temperature profiles over time can help waterquality managers and operators to determine operational parametersfor seasonal or changing conditions.

FlowMany different types of meters can be used to measure water flow.SCADA monitoring of flow into and out of a tank can indicate prob-lems in the distribution system, assist water quality managers in de-termining optimum tank operation, determine water depletion timeduring emergencies, and so on. Some of the systems used to mea-sure flow at tanks are differential-type flowmeters such as venturis,insertion meters (V-Cone, Annubar, etc.), and orifice plates andelectronic-type meters such as ultrasonic, temperature, and magneticflowmeters. Detection of flow direction is inherent to the operation ofsome of the meters, such as magmeters. Other meters, such as ven-turis, require additional devices to determine flow direction. Eachmeter named, whether as a primary or a secondary device, makesuse of a transmitter to calculate and convert the flow into an analogcurrent or voltage signal.

SecurityAs discussed earlier in this chapter, security at water storage facilitiesis a concern to utility officials and law enforcement. SCADA systemscan also transmit data from security sensors and video from cam-eras either to the SCADA operator or directly to a separate securitySCADA monitoring center or to law enforcement monitoring officials.A variety of sensors are available to detect intrusion to a tank site or



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tampering with tank appurtenances such as hatches and vents. Each ofthese sensors is capable of sending a digital signal to SCADA that in-dicates the appropriate security-related reaction. Pan/tilt/zoom cam-eras can be operated remotely to track an intruder’s activities. Avail-able camera technology is able to register images even at very low lightlevels. Video recording devices at each site may be accessed remotelythrough SCADA to download video images of the intrusion.

Local Monitoring and ControlThe section of this chapter on water quality monitoring in distributionsystem tanks outlined several parameters that a utility may chooseto monitor. Water corrosivity, pH, conductivity, total organic carbon(TOC), and turbidity are among the most commonly encountered. Alland any of these parameters, as well as the ones described in detailpreviously, can be relayed by telemetry to the utility’s SCADA center.

Data about all parameters measured at each tank are routedthrough a common device for processing and communication. Thereare essentially two devices found throughout the industry—theremote terminal unit (RTU) and the programmable logic controller(PLC). RTUs are generally associated with remote monitoring of fielddevices. PLCs have been traditionally used for the automatic and/orremote control of processes. These differences have become cloudy inrecent years; each device can now serve both monitoring and controlduties. Debate rages on over the reliability of each system, their con-trol and sampling rates, and their capability or lack thereof to handlelarge numbers of data points, store data during power outages, andso on.

RTUs and PLCs require a protective splash-proof/weatherproofenclosure. Each data point is wired to the device’s input/output cards.The data are analyzed according to the terminal or the controller’s pro-gramming and are stored or transmitted. It is possible for either deviceto be connected to a local personal computer. Operators and mainte-nance personnel can use these local computers to monitor or trou-bleshoot data and device performance locally without the assistanceof, or feedback from, the SCADA center operator. The local computer,which is called a human/machine interface (HMI), can be a laptopcomputer brought from site to site or a desktop computer stationedat the site.

Technology options available for remote telemetry communica-tion can be categorized as follows: telephone, cellular, radio frequency,fiber, and satellite. Telephone communication technology requires ahard line be installed. The information is transmitted and receivedin analog form by means of modems at the site and at the SCADAcenter. Several communication rates are available depending on theutility’s budget and the need for fast transmission of large volumes



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of information. Available services range from basic telephone modemcommunication (2,400 bits per second [bps]) to leased duplex lineswith high broadband capability (>2 million bits per second [Mbps]).Because telephone lines are already available near most water stor-age facilities and the needed equipment is relative inexpensive andcommonly available, telephone communication is the most populartechnology used for SCADA systems. Telephone lines are suscepti-ble to weather such as lightning or windstorms, which may disableSCADA access to the facility. Careful consideration should be givenduring design of any SCADA system to default operation of controlleddevices if there is a communication failure.

Cellular communication technology can be used if the water stor-age facility and the SCADA center lie within the coverage range ofa cellular communication company. A cellular modem is installed atthe tank and at the SCADA center, but no hard wire exists. Instead,data are transmitted via a wireless cellular network of communica-tion towers and cellular antennas between the site and the controlcenter. Cellular communication is a good choice for locations wherehard phone lines do not reach, as the cost is relatively higher than for ahard telephone line and fewer broadband options are available. Manyelectronic devices, such as pressure transmitters, small process con-trollers, security sensors, and so on are now available with integratedcellular modems capable of sending a small number of monitoringand/or control signals to a cellular modem at the SCADA center.

Radio-frequency communication systems use a radio modem anda low-powered transceiver at the tank location, and a transceiver isconnected to an RF base station at the SCADA center. Several tanksites or other remote facilities can be polled over a single ultrahigh-frequency (UHF) or very-high-frequency (VHF) system. Any stationcan serve as a repeater to extend the line-of-sight transmission ofthe SCADA center (Pollack et al. 1999). In a typical application, theSCADA base requests data from a remote location, such as a tank, bytransmitting a wake-up signal to send data. When the remote beginstransmitting, the base reverts to the receive mode and collects the datapackage. After transmitting the data, the remote goes back to the re-ceive mode and awaits instructions from the base. The output of thesensors at the remote site has usually been converted to digital data bythe RTU or PLC. This signal (typically in the range of 300 to 3,000 Hz)is delivered to a modem that converts it to an analog form that canbe frequency-modulated to the RF carrier. When the base receivesthe analog data, the base modem converts it back to digital data. TheFederal Communications Commission (FCC) has allocated certain fre-quencies that can be used for fixed operation. Certain frequencies areavailable for RF transmission in the low band (25 to 50 MHz), mid-band (72 to 76 MHz), VHF band (150 to 174 MHz), UHF band (450 to512 MHz), and 900 MHz (928 to 960 MHz). The low band provides the



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best communication range because the path loss is less than at higherfrequencies. However, it is susceptible to interference from electri-cal noise. The UHF band is the most frequently used because of thelarge number of available channels and its relative freedom from elec-trical noise.

Line-of-sight studies are always required to test the feasibility forRF systems. Obtaining a license from the FCC for the exclusive useof a frequency can be expensive and time-consuming. Start-up costis high, and although maintenance cost is low, the utility becomesthe sole owner and operator of the RF communication system and isresponsible for its maintenance and upgrades (Pollack et al. 1999).

Fiber SCADA communication systems require the installation ofexpensive fiber-optic cable (often several miles or kilometers) to a fiberutility cable. Fiber provides the best broadband of any communicationmethod, often surpassing 100 Mbps. This expensive option shouldonly be considered when a utility requires the fast transmission ofvery large volumes of data, including real-time video. The fiber linescan be leased or owned, depending on availability and agreementwith the provider. When the water utility owns the line, it becomesresponsible for its maintenance and upgrades.

When a tank is located where telephone lines, cellular commu-nications, or RF systems are impractical, satellite communicationsis an option if a satellite covering the distribution system area is inspace. In this case, the satellite acts as a relay station between the tankand the SCADA center. Transmitters and receivers are required atboth ends to communicate through the satellite. This option is moreexpensive than hard phone lines or cellular technology, but it maybe well worth the cost when no other communication alternative isviable.

SCADA SystemsThe sophistication of the SCADA system depends on the utility’s bud-get, the equipment supplier, and the programmer/system integrator.Often a single operator is in charge of remotely controlling and mon-itoring thousands of data points throughout the distribution system.A master station at the SCADA center is usually a single device (cen-tralized system), a master with submasters (hierarchical system), or aparallel group of processors (distributed system). For the purpose ofthis discussion, each will be referred to as the master station.

The functions of the master station include scanning PLCs andRTUs throughout the distribution system. This is accomplished bymonitoring the proper operation of remote control devices, ensur-ing that messages from these devices are error free, retrying whenmessages are incorrect, and reporting PLC or RTU failures. A mas-ter station also processes data received from RTUs and PLCs. The



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master station does this by checking for alarm conditions, averagingand trending data, storing event changes, and entering data into adatabase.

The transmittal of operator commands is another important func-tion of the master station. The transmittal of commands involvesseveral steps: interrupting the scan and arming the proper remote,encoding and transmitting the command, verifying that the propercommand has been received, permitting execution of the command,and verifying command execution.

Master stations must also maintain a database for historical data.To accomplish this, data received from the remote location are typi-cally condensed into hourly and daily averages, peak values are de-lineated, and various data-compression techniques are used to min-imize storage. Additionally, the historical data might include statusinformation such as valve positions, water level elevations, and sim-ilar items to allow later correlation with flows and pressures. Thehistorical database must also provide very flexible data retrievalcapabilities.

The master station is also responsible for driving the human/machine interface. This is done by presenting data on video displayscreens, map boards, printers, or similar mediums; providing the abil-ity to define screen formats, including graphics; and providing theability to define report formats. Master stations provide the impor-tant function of providing failover to a backup when necessary. Thisinvolves maintenance of duplicate data files in a backup processorand monitoring of the primary processor (by the backup) and switchto the backup (i.e., failover) on detection of a stall or error. Master sta-tions may also perform advanced functions such as supply prediction,demand prediction, optimal pumping, and leak detection.

The human/machine interface is the point at which the operatorinteracts with the SCADA system. Current SCADA systems offer in-teractive HMI modules. These allow building of display screens bythose with no programming knowledge. This permits operations per-sonnel who will be using the system to design and build graphic andtabular displays that precisely meet their needs. These displays maybe interactive—that is, the symbol for a pump may change color de-pending on pump status, or a reservoir icon may “fill” as the reservoirlevel increases.

Inputs to a SCADA system occur either as real-time events au-tomatically sensed and reported by the remote control device or asmanual inputs through an HMI. Inputs from RTUs or PLCs includestatus, flow, pressure, and level. Inputs from HMIs include commandsfor open/close, run/stop, and set point.

SCADA outputs are either for driving the HMI or for executingthe commands at the remote location. HMI outputs include periodicreports, alarms, alarm summary reports, graphic pictures, displays of



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real-time data, displays of averaged or trended data, and historicalreports. Control outputs include set point and on/off or start/stop.

SCADA technology should permit building of reports by personswith no programming skills. An operator or engineer, for instance,can readily define a report format. The resulting report can then beproduced for a special study or scheduled to be automated output asa routine operational report (Gotoh et al. 1993).

HMI screens should be organized and labeled in a way that theyare easily identified and uploaded to the screen. The interface with theoperator should be clear and simple (feedback, on-screen help, andoption menus are useful tools). A sufficient number of control andmonitoring screens should be provided to keep screen cluttering to aminimum. Alarms should be in a scroll window that does not overtakethe main window. Symbols, colors, and terms should be consistent andself-descriptive. The overall screen environment should be intuitivelysimple and allow operation with minimal computer programmingskills.

Energy Conservation in the Distribution SystemIn many communities, water utilities are the largest consumers ofelectric power. Pumping is the largest consumer of electric power forutilities and is their largest operational cost. Pumping is a continuousprocess that is typically rarely interrupted. It is used to meet highdemand in the early morning and late afternoon (and/or evenings)and to fill the tanks between those times. Utilities can take advantageof elevated water storage to reduce these power expenses.

Electrical energy is converted to pressure and velocity headthrough the pumping process. When the water reaches an elevatedstorage facility, the water rises to an elevation equal to the remain-ing energy in the pipe. Hence, energy is stored in tanks in the formof potential energy, or head. Tanks can be filled during low-demandhours to take advantage of reduced power rates. The volume of storedwater and its specific head reduces the need for pumping (additionalelectric energy) during peak-demand hours. This process is known aspeak shaving. Peak shaving not only reduces power consumption, itreduces the size of pump stations and trunk mains to satisfy the samedemand. This capital cost savings is in addition to the operational en-ergy cost reduction. However, this must be weighed against the cost ofadditional storage to satisfy demand. An optimum design is one thatachieves the lowest life-cycle cost including maintenance and cost ofdemolition and replacement.

Tanks help save energy in other ways. Drafting of tanks in con-junction with pumping during hours of peak demand reduces waterflow in trunk mains. Lower pipe velocities result in less head loss



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(energy loss) due to friction. Maintaining water elevation in tankswithin a specific range can help set limits on the minimum and max-imum system curves. Coordinating that with the pump curves forcespumps to operate at or close to their highest possible efficiencies. Thesize and number of pumps installed must be optimized in keepingwith the demand conditions and storage available. Pumps operatingefficiently can reduce electric consumption drastically. In many cases,pumps with best efficiency points (the combination of flow and pres-sure at which a pump operates most efficiently) that are greater than90 percent provided wire-to-water efficiencies of less than 25 percentdue to inappropriate operation. Pump wire-to-water efficiencies dur-ing operation are commonly less than 50 percent. To improve pumpefficiency, elevated tanks should be used to satisfy the marginal de-mand, and pumps should be used only when there is enough demandin the system for them to operate near their optimum efficiency. Inaddition, allowing tanks to float on the system eliminates the needto start pumps to meet marginal demand increases, which eliminatesmaximum electric demand surcharges.

Most water utility power rates are based on demand or capacitysurcharge. That is, the water utility pays based on its peak power con-sumption during the billing period, for the entire billing period. Ratesare also higher for power consumed during peak hours. Therefore,water administrators and operators should look for ways to decreaseoverall and peak-hour power consumption.

The action of starting pumps, in particular, draws large and in-stantaneous amounts of inrush current from power grids. Electricaldistribution systems may experience serious problems if a power com-pany does not have enough standby power to meet this instantaneousdemand. In fact, utilities that require large pumps for distributionpumping often must get clearance from the power company beforestarting a pump. Power companies may also require water utilities toinstall soft-starter technology to reduce starting motor current.

Some small utilities with sufficient storage are able to shift pump-ing to periods of low electrical demand and pay a reduced rate forpower. This strategy, however, is difficult to implement in large sys-tems because of the excessive volume of storage that would be re-quired.

Many utilities have resorted to variable-speed pumping to meetvariable demand. Variable-speed drives allow pumps to operate be-low normal speeds to reduce flow and pressure output. Althoughwire-to-water efficiencies may be low at lower speeds, the amountof energy used is less, reducing energy consumption. Hence, peakpower consumption is reduced, because only a fraction of the totalpotential pump power is used. The efficiencies achieved by variable-speed pumping can be exceeded by properly designed and operatedconstant-speed pumping systems and tanks.



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Ideally, the most efficient means of conveying water is to haveno more than two pumps operating continuously at constant speednear or at optimum efficiency. The number of pumps may increaseor decrease on a seasonal basis if demand patterns change, but it re-mains constant otherwise. Tanks are filled when daily demand dropsand drafted when it increases; the draft and fill limits are set by theboundaries of the pumps’ high-efficiency region. Of course, not allsystems have pump stations or tanks that are set up to do that. Inaddition to the peak-shaving volume for energy considerations, tanksize should provide sufficient reserve storage for fire flow and emer-gencies while maintaining tank turnover for water quality. It is noteasy to optimize the tank’s diameter versus height for the requiredstorage volume to satisfy all or most of these considerations.

BibliographyAmerican Water Works Association (AWWA). 2003. Principles and Practices of

Water Supply Operations—Water Treatment. 3rd ed., p. 210. Denver, CO:AWWA.

AWWA. 1986. Maintaining Distribution-System Water Quality. Denver, CO:AWWA.

AWWA. 1990. Water Quality and Treatment, A Handbook of Community WaterSupplies. 4th ed., p. 14.4. New York: McGraw-Hill.

AWWA. 2010. Water Quality and Treatment, A Handbook on Drinking Water.6th ed. New York: McGraw-Hill.

Clark, R. M., and W. M. Grayman. 1998. Modeling Water Quality in DrinkingWater Distribution Systems. Denver, CO: AWWA.

Clesceri, L. S. (ed.), A. E. Greenberg, and A. D. Eaton. 1998. Standard Methodsfor the Examination of Water and Wastewater. 20th ed. Washington, D.C.:American Public Health Association, AWWA, and Water EnvironmentFederation.

Code of Federal Regulations. 2004. Title 14—Aeronautics and Space, Chap-ter 1, Subchapter E Airspace, Part 77. Objects Affecting NavigableAirspace. Washington, D.C.: Federal Aviation Administration, Depart-ment of Transportation.

Code of Federal Regulations. 2004. Title 40—Protection of Environment, Chap-ter 1, Part 141. National Primary Drinking Water Regulations. Washington,D.C.: US Environmental Protection Agency.

Connell, G. F. 1996. The Chlorination/Chloramination Handbook. Denver, CO:AWWA.

Crozes, G. F., et al. 1999. Improving Clearwell Design for CT Compliance. Denver,CO: American Water Works Association Research Foundation (Awwarf).

De Zuane, J. 1997. Handbook of Drinking Water Quality. 2nd ed. New York: JohnWiley & Sons.

Gotoh, K. (ed.), J. K. Jacobs, S. Hosoda, and R. L. Gerstberger. 1993. Instru-mentation and Computer Integration of Water Utility Operations, pp. 113–4.Denver, CO: Awwarf and Japan Water Works Association.



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Grayman, W. M., L. A. Rossman, C. Arnold, R. A. Deininger, C. Smith, J. F.Smith, and R. Schnipke. 1999. Water Quality Modeling of Distribution SystemStorage Facilities. Denver, CO: Awwarf and AWWA.

Guidance Manual for Maintaining Distribution System Water Quality. 2002.Denver, CO: Awwarf and AWWA.

Hjertager L. K., B. H. Hjertager, N. G. Deen, and T. Solberg. 2008. Ex-perimental and Computational Studies of Turbulent Mass Transfer ina Mixing Channel. International Journal of Chemical Reactor Engineering6:A105.

Jirka, G. H. 1999. Five Asymptotic Regimes of a Round Buoyant Jet in StratifiedCrossflow. 28th International Association of Hydraulic Engineering andResearch (IAHR) Biennial Congress, Graz (Austria).

Kirmeyer, G. J., L. Kirby, B. M. Murphy, P. F. Noran, K. Martel, T. W. Lund,J. L. Anderson, and R. Medhurst. 1999. Maintaining Water Quality in Fin-ished Water Storage Facilities. Denver, CO: Awwarf and AWWA.

Knoy, E. C. 1991a. Good Design Eliminates Frozen Storage Tanks. Opflow17(2):1.

Knoy, E. C. 1991b. Solving Cold Weather Problems for Storage Tanks. Opflow17(1):2.

Lee, S. Y., and R. A. Dimenna. 2001. Performance Analysis for Mixing Pumps inTank 18. Report WSRC-TR-2001-00391 prepared for US Department of En-ergy, contract DE-AC09-96SR18500. Aiken, SC: Westinghouse SavannahRiver Co.

Lindeburg, M. R. 2001. Mechanical Engineering Reference Manual. Belmont, CA:Professional Publications.

Mays, L. W., ed. 1999. Hydraulic Design Handbook. American Water WorksAssociation. New York: McGraw-Hill.

Mays, L. W., ed. 2000. Water Distribution Systems Handbook. American WaterWorks Association. New York: McGraw-Hill.

MixZone, Inc. 2005. www.cormix.com.Moegling, S. D. 1992. Modeling the Effects of Reservoir Mixing on Water Quality in

Water Distribution Systems. Graduate thesis, University of Akron, Akron,OH.

Nathman, J. C., R. C. Aguirre, and H. J. Catrakis. 2004. Far-Field Turbulent Mix-ing Efficiency and Large-Scale Outer-Fluid-Interface Dynamics. 42nd Ameri-can Institute of Aeronautics and Astronautics (AIAA) Conference, Sacra-mento, CA.

Pollack, A., A. S. C. Chen, R. C. Haught, and J. A. Goodrich. 1999. Options forRemote Monitoring and Control of Small Drinking Water Facilities, pp. 52–120.Columbus, OH: Battelle Press.

Recommended Standards for Water Works [Ten States Standards]. 1992. GreatLakes–Upper Mississippi River Board of State Public Health and Envi-ronmental Managers. Albany, NY: Health Research.

Roberts, P. J. W., X. Tian, S. Lee, F. Sotirepoulos, and M. Duer. 2004.Physical and Numerical Modeling of Mixing in Water Storage Tanks:Progress Report. Denver, CO: Georgia Institute of Technology andAwwarf.

Sanks, R. L., ed. 1989. Pumping Station Design. Stoneham, MA: Butterworth-Heinemann.

Schlichting, H. 1968. Boundary Layer Theory. 6th ed. New York: McGraw-Hill.



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Shan, J. W., and P. E. Dimotakis. 2001. Turbulent Mixing in Transverse Jets.Report CaltechGalcitFM:2001.006. Pasadena, CA: Graduate AeronauticalLaboratories, California Institute of Technology.

Ten States Standards. 1992. See Recommended Standards for Water Works. GreatLakes–Upper Mississippi River Board of State Public Health and Environ-mental Managers. Albany, NY: Health Research.

US Environmental Protection Agency (USEPA). 1974. Manual of Methods forChemical Analysis of Water and Wastes. Washington, D.C.: USEPA.

US Environmental Protection Agency (USEPA). 1991. Guidance Manual for Com-pliance with the Filtration and Disinfection Requirements for Public Water Sys-tems Using Surface Water Sources [Surface Water Treatment Rule GuidanceManual]. EPA 570391001. Washington, D.C.: USEPA.

US Environmental Protection Agency (USEPA). 1999. Disinfection Profiling andBenchmarking Guidance Manual. Appendix D, Determination of ContactTime. EPA-815-R-99-013. Washington, D.C.: USEPA.

US Environmental Protection Agency (USEPA). 2006. Stage 2 Disinfectant andDisinfection Byproducts Rule (Stage 2 DBP rule). EPA 815-F-05-003. Wash-ington, D.C.: USEPA. Compliance Help: http://www.epa.gov/safewater/disinfection/stage2/compliance.html#quickguides. Accessed February2008.

von Huben, H. 1999. Water Distribution Operator Training Handbook. 2nd ed.,p. 16. Denver, CO: AWWA.

Walski, T. M., D. V. Chase, and D. A. Savic. 2001. Water Distribution Modeling,p. 31. Waterbury, CT: Haestad Press.

Walski, T. M., J. Gessler, and J. W. Sjostrom. 1990. Water Distribution Systems:Simulation and Sizing. Chelsea, MI: Lewis Publishers.



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C H A P T E R 10Maintenance,

Inspection, andRepair

Jennifer Coon, C.H.M.M., C.E.T.Tank Industry Consultants

Why have a maintenance program? The answer is simple: Preventivemaintenance has been, and always will be, less expensive than cri-sis maintenance. Preventive maintenance allows owners to identifypotential problems and develop solutions before the problems reachcrisis proportion. For example, it can be much cheaper to identifyand arrest coating failure and corrosion before they turn into metalloss requiring more extensive repair. Additionally, tank painting, ifdone properly, is typically required at intervals of 15 to 20 years. If thecoating adhesion is monitored regularly during inspections, topcoatscan be applied to the exterior to restore the aesthetics and extend lifeof the original or underlying coating system beyond the anticipated15 to 20 years. Topcoating can cost only a fraction of the cost of fullrepainting.

Tank Evaluations and ResourcesThree types of evaluations are recommended during the life of thetank: (1) initial or baseline tank evaluations, (2) update evaluations,and (3) operator evaluations. Several organizations have establishedstandards by which water storage tanks are evaluated and maintained.AWWA publishes standards dealing with specific aspects of tanks.Additionally, all water storage tanks should be compliant with anylocal building codes.

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382 C h a p t e r T e n

Initial EvaluationAn initial evaluation is a thorough evaluation performed to deter-mine the tank’s structural, sanitary, safety, and coating condition. TheAWWA manual M42, Steel Water Storage Tanks, recommends that a pro-fessional engineer familiar with the design and maintenance of waterstorage tanks perform this type of evaluation. The initial evaluationis the most detailed and intensive evaluation.

Update EvaluationAn update evaluation should be performed approximately every 3 to5 years following the initial evaluation. The update evaluation is per-formed to monitor changes in the coating condition and rate of corro-sion and to verify that tank conditions have not changed significantlysince the previous evaluation or rehabilitation. The same person orfirm that performed the original initial evaluation should perform theupdate evaluation. The update evaluation is not as time-consumingand detailed as the initial evaluation.

The advantages of having an initial evaluation and subsequentupdate evaluations performed by a professional are that these evalu-ations will identify the optimum time for tank repainting and repairs,and the owner can better plan for and budget for proper tank mainte-nance or rehabilitation. The evaluations will identify all of the workthat is required to properly maintain the tank. This eliminates sur-prises and change orders during a repair or repainting project.

Operator EvaluationTank owners should perform a cursory evaluation of the tank’s con-dition at least annually. The purpose of this evaluation is to identifyitems that can be easily remedied by maintenance personnel and toindicate if any issues exist that require professional evaluation. If anysignificant deterioration is found, or if the tank has been damagedin some way, a professional should be called in to evaluate the prob-lem. Items requiring basic maintenance can be remedied by operat-ing personnel at this time. If the owner’s personnel are not properlyequipped or qualified to climb the tank, the professional person or firmthat performs the initial and update evaluations could perform thisfunction.

The advantages of operator evaluations are that any significant orserious changes that may require further evaluation by a professional,such as a potential leak or metal loss on the anchor bolts, can be iden-tified by the operator. Also, routine maintenance can be performed bywater department personnel, thus saving the costs of replacing itemsor repairing items at the next rehabilitation.



Maintenance, Inspection, and Repair

383M a i n t e n a n c e , I n s p e c t i o n , a n d R e p a i r

Resources for Tank Owners� The AWWA Standard D100 for steel water storage tanks wasoriginally published in 1935 and has undergone continual up-grading and modification.� The National Fire Protection Association (NFPA) initiallyadopted NFPA 22 Standard for Water Tanks for Private Fire Pro-tection in 1914.� AWWA M42 Steel Water Storage Tanks Manual. This manualincorporates and updates much of the information containedin AWWA D101 Standard for Inspecting and Repairing SteelWater Tanks, Standpipes, Reservoirs, and Elevated Tanks forWater Storage. The publication of D101 has been discontin-ued.

Additional Steel Tank References� AWWA D100-05—AWWA Standard for Welded Carbon SteelTanks for Water Storage� AWWA D102-06—AWWA Standard for Coating Steel Water-Storage Tanks� AWWA D103-97—AWWA Standard for Factory-CoatedBolted Steel Tanks for Water Storage� AWWA D104-04—AWWA Standard for Automatically Con-trolled, Impressed-Current Cathodic Protection for the Inte-rior of Steel Water Tanks� API Standard, ANSI/API 65-1992—Welded Steel Tanks forOil Storage� API Standard, ANSI/API 653-1995—Tank Inspection, Repair,Alteration, and Reconstruction� API Standard, ANSI/API 620-1992—Design and Construc-tion of Large, Welded, Low-Pressure Storage Tanks� NFPA 22—Standard for Water Tanks for Private Fire Protec-tion

Composite-Tank References� ACI 371R-98—Guide for the Analysis, Design, and Construc-tion of Concrete-Pedestal Water Towers� AWWA D107—AWWA Standard for Composite ElevatedTanks for Water Storage

Inspection and Repair by OperatorThe first step of any preventive maintenance program is inspectionto identify the items requiring maintenance. Following are items that





should be inspected periodically by the operator and instructions re-garding repair.

Site Access

InspectionThe tank and the site should be monitored for signs of unautho-rized access and vandalism, which are a potential liability for the tankowner. Signs of unauthorized access include damage to the tank orsite, graffiti on the tank or site appurtenances, paint chipping causedby rocks being thrown at the tank, and bullet holes or indentions in thesteel caused by from guns being shot at the tank. Personnel shouldlook for damage or loose wiring in the site fence and barbed-wirestrands as well as gaps between the fence and the ground. The properoperation of the gate locking mechanism, site motion detectors, sitelighting, and surveillance cameras should also be verified. If the tankis equipped with an exterior ladder, the proper operation of its vandaldeterrent and locking mechanism should also be confirmed.

RepairIf the site is not already enclosed by a fence, a fence at least 6 ft(1.8 m) tall and topped with barbed wire should be installed aroundthe entire tank site. Barbed-wire strands should be added to the topof the fence if they are not already present. Personnel should thenregularly maintain the fence and barbed wire so that they are in goodcondition. Any holes, broken wire, or bent sections should be repaired.The fence should be close enough to grade to prevent intruder passageunder it. All barbed-wire strands should be taut. The fence should beequipped with a gate or gates that can be locked whenever the siteis unattended. Vegetation should be regularly trimmed back from thefence so that it does not damage or restrict view of the fence. Alllight fixtures, motion detectors, and surveillance cameras should beregularly maintained. If the tank has an exterior ladder, it should beequipped with a locked vandal deterrent.

Site Maintenance

InspectionThe operator’s personnel should evaluate the condition of the tanksite not just for appearance purposes, but also to help protect thetank from damage and corrosion. The presence of any trees, bushes,or other vegetation touching the foundation, bottom plate, or tankshould be noted. Foliage traps moisture against the steel and createsa damp atmosphere that can accelerate corrosion. These areas shouldalso be checked to see if grass clippings or other debris have accumu-lated there. If saturated or eroded soil not caused by precipitation or





overflow effluent is noted around the base of the tank, a professionalengineer familiar with water storage tank issues should be contacted,as this may signify a structural problem.

RepairTrees and bushes should be trimmed back to prevent the limbs and fo-liage from touching the tank. Vegetation should be trimmed so it doesnot grow up on the foundation, base plate(s), and tank. Personnelshould remove any debris found on the foundation and base plate(s).

When the tank site is mowed or other maintenance operations areperformed with similar equipment, the discharge should be directedaway from the base of the tank to prevent any rocks discharged fromhitting the tank and damaging its coating. This will also prevent grassclippings from accumulating on the foundations and base plates andaccelerating corrosion. Care should be taken that maintenance equip-ment, trucks, and so on do not come into contact with the tank orfoundation.

If necessary, personnel should regrade the site so that the founda-tion projects 6 to 12 in. (152 to 304 mm) above grade and adequatedrainage away from the foundation occurs. Rainwater runoff andoverflow discharge should be directed away from the foundation.

Foundation(s)

InspectionThe foundation should be checked to verify that it projects between6 and 12 in. (152 and 304 mm) above grade and that there is properdrainage away from the foundation. The foundation(s) should be ex-amined for signs of settlement and the concrete inspected for evidenceof cracking, spalling, or exposed reinforcing steel. Deep cracks or ex-tensive crumbling of the foundation signal, a potentially serious issue,and a professional evaluation should be conducted. Also, if the foun-dation tops are not approximately level with each other, this may beevidence of differential settlement of the tank foundation, and a pro-fessional evaluation should be conducted.

RepairPersonnel can apply a bonding agent and vinyl emollient concrete-patching mortar to any deteriorated areas or voids found in the con-crete foundation to build up the surface to its original contour. Thecondition of this repair should then be monitored.

Grout, Fiberboard, and Sealant

InspectionThe condition of any grout, fiberboard, or sealant located at the in-terface of the foundation and the bottom plate should be evaluated





for any cracks, voids, or deterioration. These can allow moisture tobuild up between the tank foundation and bottom plate and causeunderbottom corrosion.

RepairA flexible sealant can be applied to any deteriorated areas or voidsfound in these materials to restrict the ingress of moisture through thevoids and under the tank bottom plate. The condition of these repairsshould then be monitored.

Anchor Bolts and Chairs

InspectionThe anchor bolts and chairs should be inspected regularly by the op-erator’s personnel for any signs of corrosion and metal loss. Metal lossis most critical at or below the nut. If metal loss is observed, the areaof metal loss should be measured and compared with the diameter ofthe bolt where no corrosion has occurred. Typically, if the anchor bolthas deteriorated by more than the thread depth (to the root diameteror less), if any of the anchor bolts are bent or otherwise damaged, or ifany of the nuts are not completely threaded, a structural engineer fa-miliar with the design and maintenance of water storage tanks shouldbe contacted to evaluate the anchor bolts and chairs further.

RepairPersonnel should keep the chairs free of debris, vegetation, and grassclippings.

Manholes and Access Doors

InspectionManholes and doors should be checked periodically to confirm thatthey are secured against unauthorized entrance. Unlocked manholesand doors are a potential liability for the tank owner.

RepairPersonnel should install locks on manholes and access doors. Theyshould also replace any manhole gaskets that do not create a positiveseal. If any of the bolts have corroded, they should be replaced withstainless-steel bolts to prevent rust staining from streaking onto thetank surfaces.

Exterior Overflow Pipes

InspectionThe overflow pipe should be checked to verify that no potential existsfor cross connection between the potable water stored in the tank and





the water in the storm or sanitary sewer. The proper operation of aflap gate or elastomeric check valve should be confirmed, and theinspector should verify that no gaps exist between the flap gate andthe pipe. The condition of the screen should be assessed to verify thatit is adequate to prevent the ingress of insects and small animals. Thebrackets and associated attachments should be evaluated for corrosionand metal loss.

RepairIf gaps are noted, personnel should reposition the flap gate or elas-tomeric check valve on the overflow pipe to eliminate them. Any dam-aged screening on the overflow pipe discharge should be replaced toprevent the ingress of insects and small animals.

Venting

InspectionThe proper operation of the clog-resistant vent and its pallets needs tobe checked before and after freezing weather. The condition of the ventscreening needs to be assessed to verify it is adequate to prevent theingress of insects into the tank. Shielding over any vertically orientedscreening also needs to be assessed so that wind-driven dust anddebris do not enter the tank.

RepairPersonnel should replace any damaged vent screens to prevent theingress of insects into the tank. Any damaged shielding over thescreening should also be replaced so wind-driven dust and debrisdo not enter the tank.

Ladders

InspectionAll ladders should be carefully checked for deteriorated membersthat might pose a danger to climbers. The ladder brackets and theirattachments to the tank and the ladder itself should be checked formissing or deteriorated bolts and/or cracked welds. The rungs shouldbe inspected for metal loss, especially where they join the side rails.

RepairIf deteriorated members are noted on a ladder or its associated brack-ets, repairs should be made. If the ladder is equipped with a safe-climbing device, the device should be shielded from any paint or sol-vent being used to ensure its continued proper operation.





Balcony and/or Platform

InspectionAny balcony and/or platform should be evaluated to verify that thesurface does not retain water. If the surfaces allow water to pond, a pro-fessional engineer should be contacted to discuss options for drainage.The floor or safety railing access openings should be assessed. Clos-able covers should be located over all floor openings, and protectivechains or bars should be located at all railing access openings.

RepairPersonnel should replace any missing covers from floor openingsand/or protective chains from safety railing.

Interior Lighting

InspectionPersonnel should check to make sure all interior lighting fixtures op-erate properly. The condition and presence of the protective cages andglobes on the fixtures should be verified. The conduits associated withthe lighting should be assessed to confirm that they enclose all wiringand are adequately supported. If they do not appear to be adequatelysupported, a professional engineer should be contacted.

RepairPersonnel should maintain any interior light fixtures so that they oper-ate properly. Burned-out bulbs should be replaced, as should damagedprotective cages and globes. If the fixtures or associated conduits ex-pose wiring, it should be covered in accordance with National ElectricCode (NEC) guidelines.

Obstruction Lighting

InspectionIf obstruction lighting is required on a tank, personnel should makesure it is operational and lit. The condition of any globes and bulbsshould be verified. The lighting should be evaluated to confirm thatit is adequately braced and that it and the associated conduits do nothave exposed wiring. The condition of the photoelectric cell shouldbe checked. The proper operation of the lighting should be verifiedboth at night and during the day to ensure that the lights are on whenrequired and off during daylight hours (unless otherwise requiredby the Federal Aviation Administration) to reduce electricity use andthe frequency of replacing bulbs. If the fixture and its conduits do notappear to be adequately supported, a professional engineer should becontacted.





RepairPersonnel should replace damaged bulbs or globes. If wiring is ex-posed, it should be covered in accordance with NEC guidelines.

Inspection of the Tank Exterior

Exterior Steel Welded Tanks and LeaksThe general condition of the exterior coating and any evidence of cor-rosion should be monitored. If the exterior of the tank is in poor con-dition, the condition of the interior coating could be as poor or worse.The tank should be observed for signs of leakage or rust streaking thata leak could have caused. Leaks can develop in flat steel plates, butthe most common sites are at seams and joints. Dark rust stains areusually evidence of leakage. Small leaks in the seams may rust closedover time, so water may not actually be running down the tank at thetime of the inspection. Also, if the tank is empty, a leak may not bevisible. If a leak or excessive corrosion is noted, a professional engi-neer should be consulted regarding repair, as the leak may indicate amore serious issue.

Exterior Steel Riveted Tanks and LeaksThe general condition of the exterior coating and any evidence of cor-rosion should be monitored. If the exterior of the tank is in poor con-dition, the condition of the interior coating could be as poor or worse.The tank should be observed for signs of leakage or rust streaking thata leak may have caused. Leaks can develop in flat steel plates, but themost common sites for leaks are seams and joints. Dark rust stains areusually evidence of leakage. Small leaks in the seams may rust closedover time, so no water will actually be running down the tank at thetime of the inspection. Also, if the tank is empty, a leak may not bevisible. Rivet heads should be closely evaluated as extreme metal losson these items may indicate a structural issue. If a leak or excessivecorrosion is noted or severe corrosion observed on rivet heads, a pro-fessional engineer should be consulted regarding repair, as the leakmay indicate a more serious issue.

Exterior Steel Bolted Tanks and LeaksThe general condition of the exterior coating and any evidence of cor-rosion should be monitored. If the exterior of the tank is in poor con-dition, the condition of the interior coating could be as poor or worse.The tank should be observed for signs of leakage or rust streakingthat a leak may have caused. Leaks can develop in flat steel plates,but the most common sites for leaks are seams and joints. Dark ruststains are usually evidence of leakage. Small leaks in the seams mayrust closed over time, so water may not actually be running down thetank at the time of the inspection. Also, if the tank is empty, a leak





may not be visible. Additionally, the gaskets or sealants between thebolted joints should be evaluated to confirm they create a watertightseal. Glass-lined coatings should also be checked for damage aroundthe bolts from over-tightening. If a leak or excessive corrosion is noted,a professional engineer should be consulted regarding repair, as theleak may indicate a more serious issue.

Professional EvaluationAWWA recommends that water storage tanks be professionally evalu-ated at least every 5 years and otherwise whenever conditions warrantevaluation. A thorough professional evaluation will enable the tankowner to accurately schedule required maintenance, prolonging thestructure’s useful life.

A professional evaluation should consist of a careful study of thetank’s interior, exterior, foundation, and accessories. All necessary sur-faces on the tank should be accessed by rigging and rappelling theinterior and exterior as required by the condition and design of eachtank.

Selecting a Professional Inspection CompanyWhen retaining a firm or person to perform a professional tank evalu-ation, the owner should refer to the AWWA manual M42 Steel Water-Storage Tanks, which states: “The tank maintenance engineer shouldhave knowledge of the traditional engineering disciplines and havespecialized training and practical experience in the design, fabrica-tion, erection, inspection, sanitary integrity, coating, and maintenanceof steel water-storage tanks.”

A tank owner who invests in a professional tank evaluation shouldexpect the evaluation to be thorough, professional, and complete. Inaddition to supplying the usual components of a professional evalua-tion, the evaluation team should identify any peculiarities associatedwith the tank.

The Evaluation ReportA certified engineering report should be issued concerning the con-dition of the tank. The evaluation report should describe the obser-vations of the technicians and engineers and their recommendationsfor optimum rehabilitation. Color photographs of the tank interiorand exterior provide aid to the tank owner in analyzing the datapresented. The report should address the condition of the tank—structural, coating (including heavy-metal content analysis), corro-sion control (including cathodic protection), safety (OSHA compli-ance), operational and sanitary conditions, and compliance with otherapplicable standards—and environmental considerations such as





containment and proper disposal of abrasive blast residue. Thoseitems not in compliance with current OSHA regulations concerningsafety, sanitation, and operation should be identified so that the tankowner can make informed decisions regarding compliance with theseimportant issues.

A registered professional engineer familiar with the design, con-struction, and maintenance of water storage tanks should certify thereport, which should serve as a decision-making document forthe tank owner. The report should also include budget estimates forthe recommended work, anticipated life of the coatings and the struc-ture, and estimated replacement cost of the tank. Recommendationsshould address what rehabilitation work needs to be performed tomeet the short-term and long-term needs of the water system.

Inspection of the Tank Interior

Underwater Evaluation Using a DiverAlthough this method does not require the tank to be drained, it shouldbe taken off line and isolated from the system the entire time the diveris in the tank. When performing an underwater evaluation, the divermust wear a full dry suit and full-face diving mask to prevent contactbetween the diver’s body and the potable water. Before entering thetank, the diver and all related equipment must be thoroughly disin-fected in accordance with the latest revision of AWWA Standard forDisinfection of Water-Storage Facilities C652.

Underwater Evaluations Using a Remotely Operated Vehicle (ROV)Remotely operated vehicles can be used to evaluate the interiors ofwater tanks without interrupting service and isolating the tank fromthe system by operating valves. These ROVs provide closed-circuitvideo to an on-site technician who operates the unit. As with divers,the ROV must always be disinfected before use in potable water tanksin accordance with AWWA C652. The vehicles are typically made ofnonporous materials, and the bearing seals must be filled with a food-grade glycerin.

Float-Down EvaluationThe interior of the tank is full of water as a float-down evaluationbegins. A field technician in a small raft evaluates the interior surfacesas the owner drains the tank. The interior wet riser of elevated tanksis typically evaluated by rigging after the float-down evaluation hasconcluded. The duration of this evaluation is determined by the rateat which the tank is drained.

Drained (Dry) EvaluationDuring a dry evaluation, the tank is drained before the evaluationand dewatered. The remaining water and sediment are removed from





the tank to access the bottom plates. Technicians get to the necessarysurfaces of the tank by rigging and rappelling the interior (includingthe interior of riser pipes 36 in. [0.9 mm] in diameter and larger) asrequired by the condition and design of each tank.

Both dry and wet evaluations have limitations. First, with a divingevaluation, the diver is able to access all of the steel surfaces. Duringa dry evaluation, the tank evaluation crew can only access, via simplerigging from roof manholes and vents, surfaces that are adjacent toladders. However, the diver must evaluate the steel surfaces up close,as the limited light does not allow him or her to move away from thetank shell and evaluate the overall corrosion patterns inside the tank.Also, because there is usually silt in the bottom of the tank, the tankbottom cannot be as thoroughly evaluated by diving. When a diverstirs up this sediment, visibility is impaired, diminishing the qualityof the evaluation of the tank bottom and the lower portion of the tankshell. In addition, the diver is working with a limited supply of air,which typically causes him or her to accelerate the evaluation. Someof the physical tests that are normally performed, including adhesiontests and dry film thickness readings, cannot be accomplished on thesubmerged surfaces. Perhaps most importantly, only one diver seesthe tank interior, whereas it is ideal for several members of the tankevaluation crew to visually assess the tank interior and evaluate theproblems found. This provides for greater accuracy in the evaluation.

Structural EvaluationStructural evaluations are normally only performed if the owner or theengineer believes a tank does not meet current structural standards orif the structural integrity of the tank is suspect. Structural evaluationsshould evaluate metal loss compared to the apparent or observed orig-inal metal loss obtained by ultrasonic testing. A structural analysis isnot commonly required for properly maintained existing tanks unlessthe tank has been modified (if, for example, equipment or antennashave been added to the structure) or has experienced an extreme eventsuch as high winds or an earthquake.

The first step of a structural analysis is an engineering evaluationof the tank to determine its condition. A structural engineer shouldreview deterioration of the foundation to determine its effects on thetank’s structural integrity. A level should be used to determine if dif-ferential settlement has occurred since construction of the tank.

The original design drawings should be reviewed for compliance.Measurements should be taken in the field to analyze the tank andanchorage for compliance with current structural codes and require-ments that may have changed or come into effect since the tank wasoriginally designed and constructed. The latest AWWA standards andlocal building codes should be used. Careful attention should be given





to the tank’s compliance with current design requirements for wind,snow, and seismic loadings.

The original weld quality on welded steel tanks should be ver-ified by ultrasonic or radiographic testing. The testing is normallyperformed at locations as required by current AWWA standards andevaluated in accordance with American Welding Society (AWS) stan-dards for weld quality. If the owner has the original radiographs usedto verify the original weld quality and has confidence in their accuracy,this additional weld testing may be redundant.

Specialized Inspections

Ultrasonic Thickness MeasurementsUltrasonic thickness measurements of the steel should be taken, andareas of metal loss and deterioration should be analyzed for structuraldeficiencies.

Coating EvaluationThe coating survey should include laboratory analysis of coating sam-ples to determine the total lead content by weight. Similar tests shouldbe conducted for other regulated heavy metals such as arsenic, barium,cadmium, chromium, mercury, selenium, and silver. Additionally, thecoating type, thickness, condition, and adhesion should be tested toassess the ease of applying a topcoat to the existing coating. If suchtests indicate that topcoating is an option, recoating costs could besignificantly reduced.

Cathodic Protection EvaluationAnnual inspection of the cathodic protection system by the manufac-turer or other qualified person is recommended. At a minimum, thisshould include overall inspection of the entire cathodic protection sys-tem (including removal of expended or damaged anodes, if required),replacement of all defective parts, complete potential profile survey, aphysical check of the anode placement and wiring continuity, obser-vation for corrosion at areas of exposed steel, and a written report.

Inspections Following Extreme EventsIn the 1970s, national design standards first began to include proce-dures for designing liquid storage tanks that resist earthquakes. Thebasic design standards and codes for tanks, which focused on provid-ing better details and structural resistance, were based on observedbehavior and problems. Design standards have evolved since thattime with regard not just to seismic design, but to wind-load design.Older tanks may not meet these current standards. Therefore, inspec-tion and upgrading, and then maintaining retrofits, may reduce theeffect of a seismic or other natural event on the tank. Additionally,





inspections can identify problems that potentially could arise duringfreezing weather. Many of these problems could be easily repairedand maintained before they cause hazardous conditions or before thetank fails during freezing weather.

Owner-Performed InspectionImmediately following a tornado, hurricane, major windstorm, orearthquake, or during freezing weather, tanks should be evaluated forany possible damage. If damage is evident, a professional structuralengineer familiar with water tank design and maintenance should becontacted as quickly as possible to evaluate the structural conditionof the tank.

Professional Seismic EvaluationsIn-depth seismic evaluations (as with structural evaluations) are typ-ically above and beyond the standard initial evaluation. Seismic eval-uations are typically performed only if the owner or the engineerbelieves a tank does not meet current seismic requirements. Becauseof the ever-changing seismic regulations and prescribed design loads,some owners are compelled to do a seismic evaluation of their struc-tures, especially those built before the advent of seismic design.

In addition to a professional engineer–performed initial field eval-uation of the tank, the engineer should obtain and research all avail-able original tank erection drawings, design calculations, specifica-tions, as-built drawings, and other historical data. Based on the fieldevaluation and the historical documentation review, a certified en-gineering report should be submitted outlining the observations andrecommendations for replacement or retrofit and maintenance to meetthe objectives of the owner. The analysis should determine not onlywhether the original design complies with the current seismic stan-dards, but also whether the tank in its current condition complies withthese standards.

Because there have been significant changes in the AWWA designand construction standards (especially in the approaches to design forseismic loadings) and design philosophies, the owner may desire amore complete structural evaluation. Accordingly, the entire tank andanchorage system should be analyzed for compliance with presentstructural codes and requirements, which may have changed or comeinto effect since this tank was originally designed and constructed.Careful attention should be given to each tank’s compliance with thepresent design requirements for wind and seismic loadings. Addition-ally, the original foundation design drawings and soil report shouldbe reviewed for compliance. This design review of the foundationsand the amount of reinforcing steel actually inside the concrete willbe based solely on the drawings, as there is no economical methodof verifying the amount and the location of reinforcing steel and/or





concrete construction practices. Without the original foundation de-sign drawings, this review could not be reasonably performed, andseveral assumptions would have to be made. The engineer should al-ways provide a recommendation after reviewing the foundation de-sign regarding the necessity of additional physical foundation evalu-ation. If original drawings of the foundation(s) are not available, thenportions of the existing foundation may have to be excavated.

During the seismic evaluation, the original weld quality on weldedsteel tanks should be verified by ultrasonic or radiographic testing.The testing is typically performed at locations as required by presentAWWA standards and evaluated in accordance with AWS standardsfor weld quality. If the owner has the original radiographs used toverify the original weld quality and has confidence in the accuracy oftheir radiographs, this additional weld testing may be redundant.

If one has not been recently performed, a soil investigation may bepart of the seismic evaluation of the tank. The additional informationgained from a site-specific soil investigation is important in assessingthe overall tank behavior. The soil information may also be useful indetermining the design load to comply with the building code require-ments and to identify potential soil abnormalities that may affect theperformance of the tank. However, a soil investigation is not always anecessary expense that adds value. It is typically in the owner’s bestinterest to require soil investigations only when the information gath-ered may have a significant influence on the outcome of the seismicevaluation. It is recommended that a soil investigation be conductedonly for the following conditions:� Tanks where the foundation is extremely sensitive to the in-

tegrity of the tank� Sites believed to have a potential or known soil problems, or� Sites where the potential properties suggest that the defaultbuilding code soil factors used in determining the seismic loadare not credible.

For example, when tanks are located in areas subject to soil lique-faction or gross slip failures, additional site investigation and remedi-ation may be required. The size, location, and type of tank influencethe relative value of a soil investigation. A tank of low height withan unanchored flat bottom typically imposes less load on the soil andmay not be susceptible to soil and foundation problems during anearthquake. Conversely, a large standpipe or elevated tank with sub-stantial anchorage requirements may be significantly affected by thesoil behavior.

The existing building codes and national tank design standardsall specify factors to adjust the seismic design load for the site soilclassification. All of these documents also have a default value when





sufficient detail is not available. For many sites, the default site clas-sifications are conservative for determining the design load.

Another important factor in assessing the need for a site-specificsoil investigation is the history of tank failures during seismic events.The types of tank failures most often encountered are related to pip-ing flexibility, damage to the shell anchorage, shell buckling, or slosh-ing damage to the roof and roof support structure. Few foundationproblems resulting from earthquakes are reported. When foundationproblems are reported, they are often related to gross soil failures (e.g.,a tank sliding down the hill) that may not be addressed by the typicalsoil report or may be a consequence of inadequate anchorage design.

Tank Inspection Issues

Confined Space and Other Safety IssuesPersonnel accessing the interior of a tank should be trained in properprocedures for entering confined spaces. This includes training inmeasuring atmospheric conditions for oxygen levels and lower explo-sive limits, emergency response procedures, and roles of each of theworkers on the site crew. Worker training programs are available froma variety of sources including OSHA, which offers outreach trainingcourses through its Outreach Training Program.

Before climbing a tank, the operator’s personnel should be trainedto work at heights and should be comfortable doing so. The workershould use all appropriate safety equipment and follow all safety pro-cedures. Whenever someone enters the tank, at least one additionalperson should act as a ground person who is available to get help, ifneeded. Emergency response procedures should be established andreviewed with all crew members at the start of the tank access. Train-ing may be available through an OSHA Outreach Training Program,through your local fire department, or from recommendations by dis-tributors of fall-protection equipment.

Tank Cleaning/WashoutsAs water is held in the tank, suspended solids begin to settle out ofthe water and onto the tank bottom. Without regular washouts, largeamounts of sediment may accumulate in the tanks. In addition, properevaluation of the interior surfaces of the tank cannot be conductedwith sediment covering the bottom of the tank.

Draining the TankBefore scheduling work crews to wash out a tank, it is a good ideato determine if the tank is equipped with a drain. If so, its locationshould be noted.





Washing Out the TankThe tank can be washed out with low-volume, moderate-pressure(2,500 psi [17,237 kPa]) pumps, firefighting equipment, or othermeans. Water should be sprayed on all interior surfaces to removeas much residue as possible. In areas where sedimentation is a prob-lem or where strict local environmental regulations apply, it may benecessary to separate the sediment from the washout water and prop-erly dispose of it by some means other than allowing it to enter stormsewers or nearby streams. Also, care should be taken so that largeamounts of sediment are not allowed to enter the tank piping; thiscould clog pipes or damage valves.

If the tank has been equipped with aluminum cathodic protectionanodes, many of them may have fallen since the previous washout.Because these anodes may damage the interior coating, they shouldbe removed from the tank during the washout.

Operating Without the TankOperating without a tank may require notification of local businessesand residents so that temporary large uses of water such as lawnwatering or equipment washing can be scheduled for other days,thereby leaving the operator with adequate fire protection capacity.It may be necessary to provide pressure relief valves for the one-tankpressure planes when the single tank is out of service.

Refilling the Tank and DisinfectionThe disinfection of water storage facilities should be done in accor-dance with the latest revision of AWWA C652. This standard offersthree chlorination methods by which disinfection can be accom-plished.� Method 1. This method requires that the tank be filled with

chlorinated water (10 mg/L chlorine) for the sole purposeof disinfecting the tank. After the required retention period,the disinfection water is drained and the tank is filled withpotable water. After the potable water has satisfied bacte-riological tests and is determined to be of acceptable aes-thetic quality, the water may be delivered to the distributionsystem.� Method 2. This method requires that a chlorine solution(200 mg/L) be applied with brush or spray equipment to allparts of the tank that would be in contact with water whenthe tank is full to the overflow elevation. After rinsing, thetank is then filled with potable water. After the potable waterhas satisfied bacteriological tests and is determined to be ofacceptable aesthetic quality, the water may be delivered to thedistribution system.





� Method 3. This method requires that the tank be filled toapproximately 5 percent of the total storage volume with amixture of potable water and chlorine containing 50 mg/Lof available chlorine. After a retention time of not less than6 hours, the tank is filled to the overflow level with potablewater. After a 24-hour retention period, the water should betested. Once the water has been tested for bacteria and aes-thetic quality, the water may be delivered to the distributionsystem.

Of the three disinfection methods listed in AWWA C652-02,Method 1 is the least popular because an entire tank full of watermust be wasted to accomplish disinfection. In addition to wasting thewater, discharging large volumes of highly chlorinated water is notenvironmentally acceptable. The primary drawback to Method 2 isthat personnel disinfecting a tank must be equipped with proper res-pirators and protective clothing to help protect them from the vaporsreleased into the air when chlorine is applied.




C H A P T E R 11Potable Water

Security

John McLaughlin, P.E.Jordan, Jones and Goulding

The use of water as a leveraging tool in conflict is not new, or is the con-cept of water security. Besides the air we breathe, water is the singlemost critical element to human survival. In Water Conflict Chronology(Gleick 2008), more than 100 incidents are documented in which waterwas the cause of, or was integral to, a major conflict or event. Theseevents or types of conflicts are grouped into one or more of the fol-lowing categories: control of water resources, water as a political ormilitary tool, terrorism, water as a military target, and disputes re-lated to development of water resources. As early as 2,500 bc, waterwas used as a military tool to help defeat an enemy. With humans’reliance on safe and sustainable potable water, its use as a tool of warand conflict should be no surprise.

Threats to Water SystemsAny lack of attention to or understanding of the critical importanceof potable water changed dramatically after September 11, 2001. Sud-denly, the concepts of unconventional threats and how they could usecritical infrastructures against a population became real. An imme-diate reaction to the events of September 11 was the introduction offederal legislation to require US water systems to complete vulnera-bility assessments (VAs) and emergency response plans (ERPs). Thisfederal legislation became Public Law 107-188, and it required everypublic water system in the United States serving more than 3,300 peo-ple to complete a VA and an ERP, on a staggered schedule, before

399




400 C h a p t e r E l e v e n

December 31, 2004. The actual schedule for completion of the VAswas the following:� March 31, 2003, for systems serving a population of 100,000

or more� December 31, 2003, for systems serving a population of 50,000or more but less than 100,000� June 30, 2004, for systems serving a population greater than3,300 but less than 50,000

In each case, the system was to complete an ERP as soon as pos-sible, but no later than 6 months after completion of the VA. The ERPwas to incorporate the results of the VA.

Water systems were not fundamentally insecure before Septem-ber 11; most had fences, locks, and other systems to detect and delayintruders. Larger water systems sometimes had guards and more in-tricate electronic security systems. What fundamentally shifted wasthe focus—away from protecting water systems against natural dis-ruption and contamination and toward reducing the risk from anintentional malevolent human attack (and not necessarily from aninternational terrorist organization). Some of the most prevalent, bestdocumented, and least appreciated threats to water systems comefrom disgruntled current or former employees, a lone vandal or agroup of vandals, and common criminals.

Almost monthly since September 11, news stories have docu-mented break-ins at water facilities. These types of events almostcertainly occurred as often before 9/11, but they received little pub-licity or attention. The main difference between the pre- and post-9/11 incidents is that the Federal Bureau of Investigation (FBI) andother law enforcement agencies paid the former—usually unorga-nized attempts at vandalism—little attention. Nevertheless, the in-cidents directly pointed to the need for better risk reduction at watersystems, which quickly began to improve formerly minimal securitypractices.

DefinitionsPeople tend to think of “providing security” at water systems, and thischapter uses that terminology, but the real goal is to reduce risk byeliminating vulnerabilities. This process of risk reduction ultimatelyleads to the security that water system operators and the public seek. Inthat connection, the following definitions are provided (Sandia 2002).� Risk—Measure of the potential damage to or loss of an asset

based on the probability of an undesirable occurrence.� Risk assessment—Process of analyzing threats to and vul-nerability of a facility, determining the potential for losses,



Potable Water Security

401P o t a b l e W a t e r S e c u r i t y

and identifying cost-effective corrective measures and resid-ual risk.� Vulnerability—An exploitable security weakness or defi-ciency at a facility.� Physical protection system—Integration of people, proce-dures, and equipment for the protection of assets or facilitiesagainst theft, sabotage, or other malevolent human attacks.

The goal of any security system is to prevent an attack if possible. Itis generally not cost-effective, though, to stop any and all attacks. Cana water system afford the level of physical protection used at a nuclearfacility or strategic military facility (armed and trained guards; “killzones,” clear areas outside the perimeter where deadly force is autho-rized; and so on)? The answer to this question is almost always no,and so water system management must be willing to develop strate-gies to mitigate the consequences of an attack. This ensures that eventhough a water system may not be able to stop the attack from happen-ing, it can still cost-effectively reduce the overall level of risk. Conse-quence mitigation, in addition to reducing the risk from an intentionalmalevolent human act, also helps reduce the risk to a system from anatural disaster. By providing a double benefit, consequence mitiga-tion measures may be the most cost-effective risk reduction measuresof all.

Certain aspects of risk reduction at a potable water storage systemalso benefit the system during natural disasters. The focus of this chap-ter, however, is still on reducing risk from an intentional, malevolenthuman act.

Types of ThreatsA water-storage facility can be intentionally attacked in three ba-sic ways: physical disruption, contamination (radiological, chemi-cal, or biological), and interference with supervisory control anddata acquisition (SCADA), computer, and information technology (IT)systems.

Physical DisruptionMuch has been written about contamination being the worst-case sce-nario for a water-storage facility. This is valid and worthy of discus-sion, but perhaps the simplest and the most effective way of havingan impact on potable water storage is through physical disruption.

The amount of water that humans actually consume is only a frac-tion of a percentage of the total potable water produced. In Milwaukee,Wisconsin, and Albuquerque, New Mexico, for example, the percent-age of potable water actually consumed is one-half to one-quarter of1 percent of the total produced (Danneels 2001). Having storage, and





therefore supply, of nonpotable water still allows fires to be fought,industry to operate, contamination to be contained, and basic sani-tation to continue. In cases where potable water storage is compro-mised, potable water can be temporarily provided using bottled water,mobile treatment systems, and bulk water that has been hauled in.

The purpose of this chapter is not to identify specific vulnerabili-ties of a water-storage facility or to provide direction for adversaries;therefore, the discussion will remain general. Physical disruption ofstorage facilities generally requires some knowledge of the specificwater system to be truly effective. However, almost every water sys-tem relies on critical storage facilities that, if eliminated, would criti-cally disrupt its ability to supply water to the distribution system orto critically important customers.

It is easy to disable or eliminate a storage facility without sophisti-cated chemical or biological knowledge and equipment. Imagine thedamage that can be done to electrical systems with basic tools. Sugar inthe fuel tanks of emergency generators can create substantial damage.Valves can be broken and extensively damaged without explosives.Any simple Internet search reveals recipes for various homemade ex-plosives capable of doing substantial damage.

ContaminationThree types of contaminants are of concern in water systems. Theseare, in order of concern, biological, chemical, and radiological. Tra-ditional water treatment has focused on removal or inactivation ofnaturally occurring contaminants and contaminants unintentionallyintroduced by humans. Each case of intentional or malevolent contam-ination can cause unique problems. Besides the obvious—customersgetting sick or dying—one of the most likely overall problems is thewidespread public perception and panic that water is not safe to drink(Burrows, Valcik, and Seitzinger 1997). Additionally, there is the prob-lem of timely determination of what agent (or agents) has been intro-duced.

Charlotte-Mecklenburg Utilities (in North Carolina) has dealt withthis issue twice. The first event was unintentional and involved Foam-gate (Krouse 2001); the other occurred after 9/11 and was intentional.In each case, even with rapid detection of the contamination, the test-ing necessary to determine its exact nature and potential harmful ef-fects was one of the most difficult parts of the entire response effort.

A chemical agent might be easily detected through the taste, odor,or appearance of the water, especially if enough of the agent is presentto do physical harm to a person. The problem with radiological orbiological agents is that they are much more difficult to detect anddeal with. The first means of detecting these agents in water, evenlarge quantities of agents, might be through symptoms that do notappear in an affected population until days or weeks later. Moreover,





symptoms could still be difficult to trace back to the water systemwithout good coordination, cooperation, and relationships betweenthe water system and public health personnel. Add to these difficultiesthe fact that most potable water storage facilities are still not wellprotected and thus contamination is relatively easy to accomplish. Ifthe threat has knowledge of the system and chooses a storage facilitythat serves a critical part of the distribution system, the situation caneasily be made worse.

It would be difficult for a terrorist or other threat to have a broad,long-term impact on a water system through use of a contaminant. Ata minimum, an adversary would need each of the following to createwidespread consequences: (1) specific knowledge of which storage fa-cilities are in the most critical parts of the distribution system, (2) accessto agents and knowledge of which agent(s) might be most effectiveand difficult to detect or inactivate, and (3) access to the equipmentto distribute the agent. Many agents can be introduced into a watersupply system. Any one of them can cause panic among the public(Deininger 2000). This means that the contamination threat, thoughdifficult to carry out, cannot be minimized.

SCADA/IT InterferenceA third method of disabling a water-storage facility is through cyberattacks against a SCADA system. Fortunately, many water systemsstill practice manual operation and allow their SCADA systems toperform very little, if any, control. Those that do not practice manualoperation or that allow maximum control by their SCADA systemsrun the very real risk of losing control through hackers entering theirsystem. These hackers can be current insiders or employees, disgrun-tled former employees, lone thrill-seeking hackers, or a group of orga-nized and highly capable hackers bent on significant and coordinateddestruction.

General Site Considerations

LocationPossible locations for existing storage facilities are as varied as eachfacility’s vulnerabilities. It is difficult to conclude what would be anideal location from a security standpoint. A facility in a heavily popu-lated area might be less vulnerable because it would be harder to attackwith so many people potentially watching; it is more critical, though,because it serves more customers. In a remote setting, there are fewerpeople to observe and possibly detect an intrusion, but the criticalityof the service area is probably lower. This section will only review themore common security issues for remote and urban locations.





Remote LocationWhen a storage facility is remotely located, its primary vulnerabil-ity is that few people are around to detect an intrusion. Unless it isa manned facility, such as a clearwell at a water treatment plant, theonly reliable means of detecting an intrusion would be through anaccurate, automated detection system, which many remote facilitiesdo not have. Even when a system has the capability of accurately de-tecting an intrusion attempt at a remote site, response would normallytake too long because of the distance from a regular patrol area.

Urban LocationAn urban or heavily populated location does not have the same vul-nerabilities as a remote location, but several inherent vulnerabilitiesstill exist. Location in a congested area means that many more peoplehave close access to the site and are potentially aware of the facil-ity’s importance. In general, in many urban areas, a lot of criminalactivity goes unnoticed and unreported. One thing common to virtu-ally all water system facilities is the presence of graffiti, especially ontanks. Most water systems have not worried about this in the past,but the presence of graffiti points to the ease of access by and thepoor detection of intruders. In addition, because these storage facil-ities are so close to large population centers and because they tendto serve more critical customers, they are usually much more vitalassets.

In both remote and urban settings, the key is good detection. Ob-viously, until a system accurately detects an intrusion attempt in thefirst place, delay of the intruder will not be possible. A response force,no matter how close or aware, will have not have any impact, and nofacility location will be safer than any other.

AccessibilityAccessibility, as discussed here, has to do with the number of peo-ple allowed to access the facility. Almost all potable water storagesystems allow nonutility personnel to have unmonitored access tostorage tanks. These are most often employees of telecommunicationcompanies, electrical utilities, and other city departments. An unsci-entific survey of results of many vulnerability assessments shows thatalmost all facilities allow this access without maintaining any directcontrol over who accessed the facility or when.

An equally critical vulnerability is the common practice by manywater systems of allowing too many of their own personnel tohave keys to facilities. Maintaining access control over the waterdepartment’s own personnel is a more difficult problem to solve than





controlling access by employees of other agencies. Many tank person-nel legitimately need access to the site, but many others have no realneed for keys or key cards.

Both cases require policies that call for monitoring of all personnel,utility or nonutility, who might access a site. Access should be limitedto those who legitimately require it. Background checks should beconducted on anyone who has access privileges.

Visibility, Perimeter, and SizeThe visibility, perimeter, and size of any site are difficult to control.Most sites are selected on the basis of hydraulic considerations andthe ability to acquire suitable property without any forethought tovulnerabilities and security, a practice that must change. Even then,cost will be a primary concern, and creative means of eliminatingvulnerabilities will be required.

A tank’s visibility is a given. Large ground storage tanks and el-evated tanks of any size are obvious. What often works in the tankowner’s favor is that most people take water tanks for granted andforget they are there. As long as police, fire, and the tank owner’spersonnel do not do this, the visibility issue can be minimized.

What should always be avoided is taking a potentially bad visibil-ity issue and making it worse. Neighborhood aesthetics may dictatesome screening, but hiding a tank too well makes it more difficultto detect intrusions. Site perimeters should not be camouflaged orscreened unnecessarily. A tank owner should enlist the public rela-tions staff to help explain this to the community. Whether the site islarge or small, the tank and related critical facilities should not beplaced near the perimeter. A small site may be dictated by economicsor location (tight, congested area), but as long as good detection ofpotential intruders is maintained, additional layers of delay can beadded without huge cost, especially at a new site.

General Tank ConsiderationsWater storage tanks tend to be fairly standard in how they are designedand accessed. The biggest differences are elevated versus ground stor-age tanks and, in the realm of elevated tanks, leg supports versusenclosed pedestal supports. There are differences in construction ma-terial (steel, concrete, or a steel/concrete composite) and variationswithin each category of tank (standpipes, clearwells). These specificdifferences tend to have less impact on tank security. For this chapter,only design elements that are pertinent to security of storage facilitieswill be discussed.





Elevated Storage TanksElevated tanks generally offer a security advantage over ground stor-age tanks in that they do not usually require integral, on-site boosterpumping. That is not to say that pumping is not part of the design ofan elevated tank system, but maintaining pumps on site is usually un-necessary. Where pumping is not integral to the tank site, the numberof vulnerabilities is reduced accordingly.

Enclosed-Base Elevated TanksThe two main types of enclosed-base elevated tanks are the fluted-column tank and the pedestal/spheroid tank. Figure 11-1 shows atypical fluted-column type of enclosed-base elevated tank; Fig. 11-2shows a typical pedestal/spheroid type of elevated tank. Both typesusually contain a single pedestrian access door with an integral lock.The fluted-column tank, with its (usually) larger-diameter base, canoften accommodate a vehicle protected by a lockable door similar toan automatic garage door. Both styles of tanks almost always containin their bases tank-specific piping (Fig. 11-3), including the supply and

FIGURE 11-1 Typical fluted-column type of enclosed-base elevated tank.





FIGURE 11-2 Typical pedestal spheroid type of elevated tank.

FIGURE 11-3 Tank-specific piping for enclosed-base and pedestal spheroidelevated tanks.





FIGURE 11-4 SCADA components stored in enclosed tank base.

discharge piping, sampling ports, overflow piping, shutoff valve(s),and an altitude control valve (if used).

An integral pumping system is commonly provided in the baseof a fluted-column tank, but usually not in a pedestal/spheroid tank.In one sense, the large size of the enclosed base is one of the pluses ofa fluted-column tank, because more assets can be stored in the space.The other side of the coin is that storing all critical assets in one placecreates potential vulnerabilities, because only a single door serves todelay an intruder.

Within the base of each style are usually found SCADA compo-nents such as tank pressure gauges, residual chlorine analyzers, re-mote terminal units (RTUs), and radio/dialer equipment (Fig. 11-4).

Internal ladders providing access to the top of the tank bowl arealmost always located in the base of both styles of tanks. These laddersallow direct access to the water storage portion of the tank by way ofdirect hatch access or through the water-storage vent.

Multicolumn TanksMulticolumn tanks have many of the same features as an enclosed-base tank, but without the same level of protection. Figure 11-5 showsthe base of a typical multicolumn elevated tank with a ladder guard.Usually, multicolumn tanks have detached underground vaults to





FIGURE 11-5 Typical base of multicolumn style tank with ladder guard.

house critical piping and shutoff and altitude control valves (seeFig. 11-6 for an example of this arrangement). SCADA componentsand other related instrumentation are sometimes housed in the samevault, but more often they are located in the open on the tank leg orpossibly in an unprotected shed detached from the tank.

FIGURE 11-6 Detached underground vault for piping and valves, multicolumntank.





In the past, ladder access was available at ground level in manysystems, but that practice began to change even before September 11.The practice of cutting off ladders 20 ft (6 m) or so above groundlevel and adding locked access gates began as a way of controllingvandalism, and it has now become an even more accepted means oflimiting access.

Ground Storage Tanks/StandpipesIn both ground storage tanks and multicolumn tanks, the piping,valves, SCADA, and so on. are usually located in underground vaultsor separate sheds, or they are mounted outside on the tank itself. Lad-ders are now being cut off above ground level, and lockable accessgates are being installed. Figures 11-5 and 11-7 show examples of howthis is accomplished on both multicolumn and ground storage facili-ties.

As noted previously, ground tanks often differ from elevated tanksbecause a booster pump station is often integral to ground tanks’operation. Usually, both the tank and the pump station are locatedon the perimeter of the same site. Often the pump station is a morecritical and easily accessed asset and becomes more of an issue to

FIGURE 11-7 Ladder cutoff and guard on ground storage tank.





secure. The principles of detection, delay, and response (Sandia 2002),along with consequence mitigation, apply to the pump station if it islocated on site.

Construction MaterialsMaterials of construction play only a minor part in the securityof a tank. Almost all tanks are constructed of concrete, steel, or asteel/concrete composite. A study of explosives, tank characteristics,and materials of construction would be needed to determine whichof the three would be most susceptible to destruction. Suffice it tosay that a steel, concrete, or composite tank of proper structural de-sign will withstand about the same level of explosive force, all otherfactors being equal.

Water-Storage VulnerabilitiesThis section is general and avoids describing specific methods andmeans of contaminating or disrupting a water system through inten-tional acts at a potable water–storage facility.

Most key elements of water system vulnerability have been cov-ered previously. Specific locations exist on most storage facilities thatare the most vulnerable points. These include vents, sampling ports,fiberglass hatches, and local chemical feed stations. Many utilitieshave hatches that are lightly screened or not screened at all becauseof wear and tear. Fiberglass hatches are common on ground storagetanks and present a minimal barrier to a determined adversary. Thelocks usually provided for metal hatch covers are of the type foundat the hardware store and are easily cut with large bolt cutters. Read-ily accessible sampling ports, fire-hose connections, or local chemicalfeed systems (for maintaining residual chlorine levels, for example)are simple points of access for possible contamination.

Disruption of a water system through physical destruction at awater-storage facility is a bit more difficult, but it is possible just thesame. It would take a large amount of explosive placed strategicallyclose to a storage facility to ensure complete destruction. Because ofthis, we tend to focus on the possibility that an adversary would at-tempt the same level of disruption through focused destruction ofcritical piping, valves, booster pumping, or other on-site components.As with a tank’s access hatches and vents, most enclosed tank basedoors or exterior vaults are only secured with a minimal hasp-and-lock system.

SCADA/IT vulnerabilities are not currently severe or common,because not many water systems rely on SCADA/IT to control func-tions. Many utilities use SCADA only to monitor a few key parametersand are alerted either when the signal is lost or when values are out





of range. This does not mean, though, that these vulnerabilities canbe ignored. Reliance on SCADA signals without verification can bedangerous, and often SCADA systems alarm over many minor occur-rences, leaving operators to filter these alarms and potentially misssomething of real importance.

SCADA-related vulnerabilities will probably increase as securitysystems (closed-circuit television [CCTV], perimeter alarms, and soon) begin running signals through the same SCADA system used foroperational data. This also opens up a new avenue to be concernedabout: A disgruntled employee who controls not only the operation ofa system but the security system as well is known as a super insider.

Effective Security/Risk-Reduction PracticesAll security or risk-reduction measures can be placed in one of sev-eral categories. The major categories are physical protection systems(PPS), operational security (OS), and consequence mitigation (CM).Within the PPS are three subcategories: detection, delay, and response(Sandia 2002). The basic concept is to try to prevent an attack fromoccurring through PPS and OS (and, as a result, through effectivedetection, delay, and response). The CM piece of risk reduction auto-matically presumes that the attack has occurred and was successful.Through good CM, a system can effectively respond to an event andminimize the damage. Water systems have an inherent ability to mit-igate consequences, because they face similar issues every day whenlines break, power goes out, spills occur, and storms move in. In someinstances, it is probably more cost-effective for the same risk reductionto focus energy not on preventing the attack but on mitigating its con-sequences. (The cost and physical difficulties of protecting every partof a water distribution system, or even the most critical parts, wouldbe extreme. However, most systems incorporate beneficial elementssuch as redundant facilities, system loops, and interconnects. A rapidresponse by personnel trained in these matters will almost certainlyreduce the attack’s effectiveness.) This does not mean to ignore theeffort to prevent an attack; it just acknowledges that no water systemcan truly afford to prevent every attack from all possible threats.

Physical SecurityPhysical protection systems are security measures such as CCTV (cam-era) systems, motion sensors, alarms, fences, locks, and guards. Thebasic concept of PPS is to detect an adversary as early as possible.Detection means not just having a camera system record an intruder,but having a person assess the alarm or image and react quickly andeffectively to alert whatever response mechanism is planned. Delay isthe combination of measures that will slow an adversary who is on the





path to the water-storage facility. As noted, early detection followedby effective delay is the ideal sequence for PPS. Response comprisesthe time and process involved to intervene with the adversary. If thedelay is inadequate, the adversary will succeed in carrying out themalevolent act before the response arrives, and therefore the responseis ineffective. A response that arrives on scene and reaches the adver-sary in time, but fails to intercept the adversary, is equally ineffective.An example of this would be having an unarmed guard trying to stopa group of heavily armed adversaries. Although the unarmed guardmay arrive in time, he or she can do little to stop the adversary.

Detection, delay, and response comprise a three-legged stool.Without all three legs in place and of equal strength, the stool willnot stand.

In addition to the information provided here, the reader shouldreview information provided in “Guidelines for the Physical Secu-rity of Water Utilities,” a Water Infrastructure Security Enhance-ment guidance document produced by the US Environmental Pro-tection Agency (USEPA) and funded by the American Society of CivilEngineers (ASCE), American Water Works Association (AWWA), andthe Water Environment Federation (WEF). Also see the USEPA’s Waterand Wastewater Security Product Guide at http://cfpub.epa.gov/safewater/watersecurity/guide/tableofcontents.cfm.

Detection Practices

Digital CCTVMany utilities installed CCTV capability before September 11. Someof these provided digital image storage. The majority used tape andrelied on an operator to see an event in real time or to forensicallyview what happened. After 9/11, digital CCTV systems became moreprevalent. These systems store images in digital format and providean alarm if the viewed image deviates from a stored baseline image.In such a case, in addition to providing the alarm, they pull up thecorrect segment of video image, including the moments immediatelypreceding and following the event.

With any camera system, lighting conditions and clear lines ofsight are critical. An uninterrupted fence line and clear areas at least15 ft (4.5 m) outside the fence line are essential to successful early de-tection. Adequate lighting, properly designed with the camera systemto provide optimum contrast, is also essential. Lights should be thequick-strike type so that after a power outage has been resolved, itdoes not take several minutes for the lights to warm up. (Quick-strikelights come up to full candlepower almost instantly after power isrestored. They do not operate without power. The best means of pow-ering lights and other critical functions during a power outage is toprovide a generator.)





Hand in hand with these systems is the ongoing maintenance ofeach part of the tank and the tank site. Areas inside and outside offences must be kept clear, lights that burn out must be immediatelyreplaced, and any camera system must be designed to work with thelight level available. These practices fall somewhat into the operationsystems category because they are policy-level practices that humanstake care of operationally. They are listed here, though, because theyare also integral to physical security, and they clearly demonstrate theneed for all protection systems to be just that—systems. Table 11-1shows some basic comparisons of CCTV technologies, with pros andcons for each.

Perimeter Detection SystemsNumerous types of perimeter detection systems are available. Thehighest levels of these involve multiple integrated systems includingcombinations of microwave, infrared, capacitance, taut wire, and fiberoptic. Fiber-optic technology can be cost-effective and can be adjustedor tuned to minimize nuisance alarms. All physical alarm systemsmust still rely on a human to assess the alarm and react properly. Table11-2 shows some basic comparisons of detection technologies, withpros and cons for each. As with the CCTV systems, the training andpolicies necessary for this are discussed in the section “OperationalSecurity.”

Guard Dogs or GeeseDepending on the criticality of the facility and whether it is operatorattended or not, trained guard dogs may be an option. This optionobviously carries certain maintenance and liability issues, but it maybe a valid option where human monitoring is difficult, requires aug-menting, or is impractical. Similar to guard dogs, but less of a liabilityconcern, are geese. The mess and maintenance for geese may be a prob-lem, but they are very good at sounding an alarm. Once the alarm issounded, a human must intervene effectively, or the alarm has notbeen fully assessed. An alarm without human assessment is not analarm at all.

Access ControlControlling access is another key component of both detection anddelay. Access control can be as simple as basic door and windowlocks or it can comprise state-of-the-art biometrics. Basic lock-and-keysystems can be effective against many adversaries, but they requirestrict key-control policies that are practiced and enforced. If everyonehas a key to all facilities and assets, locks cease to be effective. Goodkey control can detect and delay both insider and outsider adver-saries. If padlocks are used at remote storage facilities to which other





Photographic

Technology Comments

Night-vision camera � Good for day and night viewing� Will not have to redo or add lights� Expensive

Black and white (B&W)

camera

(recommended)

� Good for day and lower-light vision� Inexpensive� Not good for dark conditions� Not as easy to distinguish during the day� Will have to redo site lighting to have

effective monitoring

Color camera

(not recommended)

� Good for day viewing� Not good for low-light or dark conditions� Will have to redo site lighting to have

effective monitoring� Expensive

Day/night (color/B&W) � Color is good for day viewing� B&W is better for night viewing� More expensive than B&W or color� Will have to redo site lighting to have

effective monitoring

Recording Technology

No recording � Must monitor at all times to be functional� Nothing is available that can be used for

prosecution

Tape recording � Used for backup validation of alarms� Hard to find previously recorded moments� Cannot record while viewing a previously

recorded moment

Digital recording � Used for backup validation of alarms� Begins recording based on motion in the

field of view� All recordings are date/time stamped for

ease in finding a particular moment when

viewing� Accessible from a remote location� Images are in PC-friendly format and can

be stored electronically indefinitely

TABLE 11-1 CCTV Summary




Types

of

Dete

cti

on

Technolo

gy

Pro

sC

ons

Fence

sensors

� Vibration

� Most

eco

nom

icaland

easie

st

to

insta

llofth

efe

nce

sensors

� High

pro

babili

tyofdete

ctio

n

� Must

have

pro

perly

insta

lled

and

main

tain

ed

fence

lines

� Prone

toall

types

ofvi

bra

tions,w

hic

hca

nbe

min

imiz

ed

with

enhance

ments

:� Wea

ther

sensor

sta

tion—

feeds

weath

er

info

rmation

to

field

pro

cessor,

whic

hth

en

adju

sts

its

vibra

tion

ala

rm

sensitiv

ity

� Pulse

count

acc

um

ula

tor—

sensitiv

ity

isdete

rmin

ed

by

choosin

gnum

ber

ofpuls

es

needed

tocr

eate

an

ala

rm� Nui

sance

ala

rms

can

be

caused

by

shru

bbery

,tr

ees,

anim

als

,and

seve

rew

eath

er

that

causes

fence

to

vibra

te� Tau

tw

ire

� Notove

rly

sensitiv

eto

win

d� Ver

yre

liable

� Lowfa

lse-a

larm

rate

and

low

nuis

ance

-ala

rmra

te

� Regula

rte

nsio

nin

gm

ain

tenance

isre

quired

� Oneofth

em

ost

exp

ensiv

efe

nce

sensor

sys

tem

s

beca

use

ofla

borious

insta

llation

and

main

tenance

tim

e

� Fiber

optic

� Immune

toele

ctrica

lor

ele

ctro

magnetic

inte

rfere

nce

(EM

I)dis

ruption

� Intrin

sic

ally

safe

and

uses

very

sta

ble

equip

ment,

resultin

gin

hig

hre

liabili

ty� Adju

sta

ble

sensitiv

ity.

� Them

ore

act

ivity

there

isat

fence

,th

elo

wer

the

sensitiv

ity

sett

ing

� Sensitiv

eto

ext

rem

ete

mpera

ture

changes

and

blo

win

g

debris

� Could

be

sensitiv

eto

larg

e-a

nim

alact

ivity

� Thefe

nce

must

be

sta

ble

,fr

ee

ofvi

bra

tion,and

ingood

conditio

n

416




� Strain

-sensitiv

e

cable

s

� Capable

of“h

earing”

what

may

be

causin

gan

ala

rm(s

imila

rto

pre

ssin

gear

again

st

the

wall)

� Very

sensitiv

eto

hig

h-E

MIsourc

es

(for

exa

mple

,

substa

tions)and

radio

frequency

inte

rfere

nce

� Sensitiv

eto

poor

fence

constr

uct

ion

or

main

tenance

� E-field

� Self-adju

sting

circ

uit

reje

cts

win

d

and

am

bie

nt

nois

e� Extr

em

ely

low

nuis

ance

-ala

rm

rate

� Advers

ew

eath

er

such

as

rain

,snow,and

lightn

ing

can

create

pro

ble

ms

� Vegeta

tion

and

anim

alm

ove

ment

can

cause

sensors

to

react

� Capaci

tance

� Weath

er

and

EM

I/ra

dio

frequency

inte

rfere

nce

(RFI

)have

no

effect

on

sensors

’abili

ty.

� Genera

llym

ounte

don

top

offe

nce

,so

use

inco

nju

nct

ion

with

anoth

er

type

ofsensor

on

low

er

part

offe

nce

fabric

� Anyth

ing

makin

gphys

icalco

nta

ctth

at

changes

fence

chara

cteristics

may

cause

an

ala

rm

In-g

round

sensors

� Balance

d

pre

ssure

line

� Mostly

imm

une

tow

eath

er

and

envi

ronm

enta

lnois

e

� Should

use

additio

nalsurv

eill

ance

/dete

ctio

nw

hen

work

ing

with

larg

eexp

anses

ofco

ncr

ete

� Treero

ots

may

cause

pro

ble

ms

when

tree

blo

ws

inw

ind

� Sensitiv

ew

hen

incl

ose

pro

xim

ity

toro

ads/ra

ilsdue

to

mach

inery

.� Por

ted

coaxi

al

buried

line

� Mostly

imm

une

tow

eath

er

and

envi

ronm

enta

lnois

e

� Avoid

insta

lling

under

chain

-link

fence

s;in

sta

llat

least

3ft

(0.9

m)above

buried

meta

llic

pip

es

� Susce

ptible

toburied

meta

l� Affe

cted

by

hig

h-E

MIsourc

es

such

as

larg

eele

ctrica

l

equip

ment

or

substa

tions

(should

not

be

used

incl

ose

pro

xim

ity

toth

ese

are

as)

TA

BLE

11-2

Perim

ete

rD

ete

ctio

nTe

chnolo

gie

s(C

ontinued)

417




Types

of

Dete

cti

on

Technolo

gy

Pro

sC

ons

� Burie

dfiber

optic

� Mostly

imm

une

tow

eath

er

and

envi

ronm

enta

lnois

e� Imm

une

toele

ctrica

lor

EM

I

dis

ruption

� Adjusta

ble

sensitiv

ity

� Must

be

insta

lled

aw

ay

from

pole

sand

trees

at

a

dis

tance

equalto

at

least

the

heig

ht

ofth

epole

or

tree)

� Should

not

be

insta

lled

inor

under

concr

ete

or

asphalt

� Susce

ptible

toero

sio

nw

here

either

more

exp

osure

or

deeper

burialaffect

sth

esensitiv

itie

s� Sen

sitiv

eto

tree

roots

as

the

tree

blo

ws

inw

ind

� Burie

d

geophone

� Mostly

imm

une

tow

eath

er

and

envi

ronm

enta

lnois

e

� Sensitiv

eto

mediu

min

whic

hgeophones

are

buried

� Sensitiv

eto

trees,fe

nce

s,lig

ht

pole

s,and

tele

phone

pole

s,w

hic

hca

ntr

igger

the

ala

rms

when

blo

win

gin

win

d

Volu

metr

ic

sensors

� Active

infr

are

d� Sen

dm

ultip

le-b

eam

patt

ern

,

incr

easin

gco

vera

ge

� Good

pro

babili

tyofdete

ctio

n� Ava

ilable

inport

able

vers

ions

� Narro

wdete

ctio

nzo

ne

good

for

monitoring

perim

ete

rsect

ors

� Preci

se

alig

nm

ent

ofsensors

iscr

itic

al

� Notgood

with

hill

yte

rrain

� Sensitiv

eto

snow

and

gra

ss

aro

und

the

sensors

� Sensitiv

eto

fog,heavy

rain

,and

dust

� Sensitiv

eto

vegeta

tion

ove

rgro

wth

� Micro

wave

� Canbe

used

tom

onitor

an

are

a

or

adefinitiv

eperim

ete

rlin

e� Use

monosta

tic

sensors

where

well-

defined

are

aofco

vera

ge

is

needed

(400

ft[1

22

m]co

vera

ge)

� Bistatic

sensors

can

be

used

up

to1,5

00

ft(4

57

m)

� Sensitiv

eto

hig

h-fre

quency

spect

rum

� Sensitiv

eto

are

as

that

conta

instr

ong

em

itte

rsof

ele

ctric

field

s(r

adio

transm

itte

rs)or

magnetic

field

s

(larg

eele

ctric

moto

rsor

genera

tors

)� Can

inte

rpre

tio

niz

ation

cycl

ecr

eate

dby

fluore

sce

nt

bulb

sas

motion

� Potentialhealth

haza

rds

418




� Passiv

e

infr

are

d

� Send

multip

le-b

eam

patt

ern

,

incr

easin

gco

vera

ge

� Good

pro

babili

tyofdete

ctio

n

� Asam

bie

nt

tem

pera

ture

appro

ach

es

tem

pera

ture

of

intr

uder,

sensor

isle

ss

likely

tore

spond

� Sensitiv

eto

all

heat

sourc

es

(heate

rs,anim

als

,and

so

on)

� Preci

se

alig

nm

ent

ofsensors

iscr

itic

al

� Notgood

with

hill

yte

rrain

� Sensitiv

eto

snow

and

gra

ss

aro

und

the

sensors

� Sensitiv

eto

fog,heavy

rain

,and

dust

� Sensitiv

eto

vegeta

tion

ove

rgro

wth

� Passiv

e

infr

are

d/

mic

row

ave

� Greatly

reduce

sfa

lse-a

larm

rate

ifused

inpre

dic

table

and/or

contr

olle

denvi

ronm

ent

� Cost

effect

ive

(cheaper

than

purc

hasin

gtw

oin

div

idual

sensors

)

� Reduce

spro

babili

tyofdete

ctio

nsin

ceboth

sensors

must

positiv

ely

dete

ctbefo

resendin

gan

ala

rm� Has

all

the

cons

ofeach

tech

nolo

gy

� Potentialm

icro

wave

health

haza

rds

� Radar

� Good

for

dete

ctin

ghelic

opte

ror

pla

ne

intr

usio

ns

� Susce

ptible

touneve

nte

rrain

� High

main

tenance

� Potentialra

dio

-fre

quency

health

haza

rds

Vid

eo

sensors

� Motio

n

dete

ctio

n

� Canhelp

tolim

itfa

lse

ala

rms

� Provi

des

reco

rdofeve

nts

during

an

intr

usio

n� Mon

itoring

field

can

be

manip

ula

ted

� Typica

llyused

inco

nju

nct

ion

with

oth

er

monitoring

tech

nolo

gie

s

� Needs

lighting

� Needs

unobstr

uct

ed

view

ing

TA

BLE

11-2

Perim

ete

rD

ete

ctio

nTe

chnolo

gie

s(C

ontinued)

419





utilities and/or agencies may need access, the tank owner should avoiddaisy-chain systems (several interlocking padlocks); all that is neces-sary for an intruder to do is to break the weakest lock.

Swipe cards and/or personal identification number (PIN) accesscontrol can be more secure and allow easier “key” control. The personmust remember his or her code and remember to carry his or her key.An advantage of these systems is that they allow logging of who entersthe facility—or at least of whose card and PIN were used to enter. Thismay not stop the adversary (the wrong person with the right key oraccess code can enter), but it will dissuade those who want to escapeundetected.

Biometric systems control access by using characteristics and traitsthat are unique to an individual. Among the most common are finger-print and retina/eye scanners. These systems are virtually impossibleto trick, and they do not involve having to carry a key. Their cost mayprevent widespread use, but they can be especially effective againstan insider or as a second layer to a perimeter detection system for anespecially critical facility. See Table 11-3 for a further breakdown ofvarious access control systems.

Glass-Break SensorsDelay and response are most effective when there is early detection. Ifan adversary gets through a fence or other outer perimeter undetected,the time available to a response force for intervention is greatly dimin-ished. However, using glass-break sensors on building windows maybe necessary if perimeter detection at a fence line is not available orpractical. Certainly, it is preferable to have the extra distance and de-lay, but short of moving entire facilities, that may not be possible. Thistype of sensor may also be considered a layer in a detection systemfor a highly critical storage facility or where threat by an insider is themain concern.

Door AlarmsDoor alarms, too, are more appropriate when the adversary is an in-sider or as an extra layer in a detection system. The use of alarms forstorage facilities within the property’s perimeter can detect an insiderwho, although legitimately within the perimeter of the facility as awhole, may need to be restricted from entering key buildings thathouse specific assets.

Contaminant Detection TechnologyContamination is less likely to occur than physical disruption andmay not have the same impact. Contamination may be more difficultfor an adversary to accomplish, and detecting such an attack is alsomuch more difficult. Current technologies generally detect contam-ination by looking at the effect the contaminant has on certain key




Technolo

gy

Entr

yM

eth

od

Com

ments

Photo

ID� Veri

fica

tion

ofa

pers

onalpic

ture

on

abadge

toact

ualpers

on

wearing

badge

� Proce

dura

l—re

lies

on

identifica

tion

ofpers

on

by

guard

� Relie

son

guard

for

all

acc

ess

contr

ol

Sto

red-im

age

badge

� Verifica

tion

ofpic

ture

on

badge

to

sto

red

pic

ture

ofsam

epers

on

to

act

ualpers

on

wearing

badge

� Proce

dura

l—re

lies

on

identifica

tion

ofpers

on

by

guard

� Relie

son

guard

for

all

acc

ess

contr

ol

Pers

onalid

entifica

tion

num

ber

(PIN

)

� Corre

ctco

mbin

ation

ofnum

bers

ente

red

on

keyp

ad

for

entr

yin

to

restr

icte

dare

a

� PINentr

yca

nbe

coded

per

entr

ypoin

t,but

not

per

pers

on

� Autom

ate

dpro

cess

Key

card

entr

y� Aut

om

ate

dve

rifica

tion

ofca

rd

with

pre

dete

rmin

ed

criteria

for

acc

ess

tore

str

icte

dare

a

� Keyca

rds

can

be

coded

per

entr

ypoin

tand

per

card

� Access

can

be

denie

dto

som

eca

rdhold

ers

and

allo

wed

to

oth

ers

,or

acc

ess

can

be

only

during

cert

ain

tim

es

ofday

� Access

privi

leges

can

be

modifi

ed

� Autom

ate

dpro

cess

Key

card

/PIN

entr

y� Veri

fica

tion

by

matc

hin

gPIN

to

badge

num

ber

for

entr

y

� Must

have

both

the

card

and

PIN

for

acc

ess

� Access

can

be

denie

dto

som

eca

rdhold

ers

and

allo

wed

to

oth

ers

,or

acc

ess

can

be

only

during

cert

ain

tim

es

ofday

� Access

privi

leges

can

be

modifi

ed

� Autom

ate

dpro

cess

Bio

metr

ics

� Verifica

tion

ofa

pers

onal

chara

cteristic

toauth

orize

acc

ess

toa

restr

icte

dare

a

� Chara

cteristics

incl

ude

fingerp

rints

,re

tina

or

voic

e

reco

gnitio

n,or

face

sca

nnin

g� Eac

hch

ara

cteristic

isuniq

ue

toth

ein

div

idual

� Access

can

be

denie

dto

som

ein

div

iduals

and

allo

wed

to

oth

ers

,or

acc

ess

can

be

only

during

cert

ain

tim

es

ofday

� Access

privi

leges

can

be

modifi

ed

� Autom

ate

dpro

cess

TA

BLE

11-3

Entr

yC

ontr

olS

um

mary

421





indicators—among the most common residual chlorine and oxida-tion–reduction potential (ORP). The theory behind residual chlorineanalysis is that a biological contaminant exerts a chlorine demand andtherefore creates a drop in the residual. This unusual or unexpecteddrop would raise an alarm, but there would be no specific informationabout what caused the drop and whether it was intentional or natu-ral. The same is true for ORP detection. This indicator may react tomore contaminants, including chemical and biological contaminants,but there is no way of identifying a specific agent or of determiningwhether contamination occurred naturally or intentionally.

Several criteria should be considered when deciding whether toimplement an early warning system for water system contamination.In no particular order, they are:� Provides warning sufficiently ahead of time to allow for

proper action� Is economically affordable� Requires little skill or training� Is flexible enough to cover all possible threats� Is able to identify the source� Is sensitive to changes at regulatory levels� Provides minimal false-positive and/or negative results� Is durable and robust� Provides results that are reproducible and verifiable� Can be operated remotely� Has year-round all-climate functionality

Any decision to choose an early warning system must be madelocally, and the relative costs (monetary, physical, social, and organi-zational) must be weighed against the relative benefits. It is also im-portant to keep in mind the relative infancy of this technology. Littleis known about which contaminants the technology might mostaccurately detect, and any early warning system currently consid-ered would not likely be able to score high on all the criteria justlisted.

The following sections detail types of systems and tools that willlikely be used when early warning systems become more effective andprevalent. For much more specific information on planning, design-ing, implementing, and operating an early warning system, pleaserefer to “Early Warning Monitoring to Detect Hazardous Events inWater Supplies,” from which much of this contaminant monitoringinformation is taken (Brosnan 1999).





Contaminant AnalyzersCurrent technologies for detecting a contaminant look at its effect oncertain key indicators. Among the most common are residual chlorineand ORP, described previously. The very nature of these types of de-tection means the contaminant is already present in the system andits consequences must be mitigated.

Another technique is to use biological analyzers—organisms thatreact in certain ways to any of several toxic agents. Their reaction istied to an electronic signal that creates the alarm. The problems hereare the lack of any specificity as to the cause of the alarm and thepotential for false positives or negatives. A few examples follow:� In the dynamic fish test, golden ides are exposed to an artifi-

cial water flow/current, which they normally swim against. Ifthey detect an upset condition, they turn to avoid it, and thisaction would be detected and registered. Similar techniqueshave been used in Europe since the 1970s.� In the dynamic daphnia test, water fleas are placed in a con-trolled column of raw water and exposed to several infraredlight beams, which they regularly interrupt and which indi-cates a regular level of activity. If a contaminant is introducedinto the water, the activity level initially increases and thensharply declines because of the death or incapacitation of thedaphnia.� Recently, some locations have used the mussel as a monitoringindicator. The theory is that when mussels are subjected to acontaminant, their shells close at low contaminant levels andthen open wide at severe levels. The monitor takes severalmussels and glues one half of the shell to a wall. The otherhalf of the shell has a magnet attached that contacts a reedswitch to indicate an open or closed position. Electromagneticsensing between the two shell halves can indicate their interimpositions between fully opened and fully closed.� Delayed algal fluorescence and luminescent bacteria monitorsuse the principle that the presence of a contaminant dimin-ishes the luminescent/fluorescent level of either the algae orthe bacteria.

While these methods may not be desired for use at this stage, theygive an indication of the body of knowledge available to enhancesecurity at all your facilities.

Technology to monitor and analyze contaminants is constantlybeing developed and perfected with a goal of providing accurate,real-time capability. Already the Sandia National Laboratories staff





have developed the �ChemLab (microChemLab), a palm-sized an-alytical laboratory that can virtually instantly detect any number ofchemical or biological contaminants (Sandia 2002). This type of tech-nology, which can accurately detect and identify an agent in real time,appears to represent the future of contaminant monitoring.

Placement of Detection DevicesAccurately detecting a contaminant is only the first step. Where do youput these analytical devices to do the most good? How do you knowwhere the contaminant originated and where it may be headed in thedistribution system? Several hydraulic models on the market providea level of water quality prediction. The most widely used for con-taminant transport are Haestad’s WaterCad, MWSoft, and MIKENETfrom the Danish Hydraulic Institute. MIKENET is already being usedin Europe. The model takes analytical measurements from a series ofparameters to detect a contaminant and then applies its algorithm topredict the fate and transport of the contaminant from start to fin-ish. Critical to this or any other model is its calibration to real-worldconditions, the number and locations of analytical devices, and theoperator’s knowledge of the system. As is the case for any detectioneffort, a contamination event cannot be considered truly detected untilthe alarm has been accurately assessed.

For water-storage facilities identified as critical through pairwisecomparisons, fault tree analysis, or accurate hydraulic modeling, real-time contaminant analyzers should probably be located on site.

Delay PracticesDelay measures generally are the most cost-effective part of a riskreduction system that comprises detection, delay, response, and con-sequence mitigation. There is a multitude of number and types ofmeasures; the only limits are the constraints of the particular site.

Whether it is an operator-staffed facility such as a water treatmentplant clearwell or a remote, unmanned facility such as an elevatedwater storage tank, the most common delay features are fencing andgates. As with any protection system, fencing and gates are uselesswithout proper maintenance and training of the staff on how to max-imize their effectiveness.

A simple way of making a regular chain-link fence more secureis to use razor wire at the top of the fence instead of three strands ofbarbed wire. Traditional fences consist of 6-ft to 8-ft (1.8-m to 2.4-m)chain link with three strands of barbed wire on outward-facing out-riggers. Where necessary and practical, the fence can be made moresecure by replacing the three-strand barbed wire with at least one coilof concertina or razor wire (Fig. 11-8). Even more delay can be builtin by using two layers of fencing. This system is prevalent at critical





FIGURE 11-8 Chain link fence with razor wire at top.

military or nuclear sites and may be appropriate for certain water-storage facilities.

If the facility is highly visible and is located in a neighborhood,ornamental or architectural-type security fences may be necessary.Figure 11-9 shows an example of an ornamental fence that can alsoprovide security benefits. There are numerous varieties of this typeof fence (the example in Fig. 11-9 is from Delgard) that can help withboth security and public acceptance.

Regardless of the type of fencing used, both fence and clear areasmust be properly maintained. This can be assured by establishing andcomplying with a policy to regularly check the entire fence line—thatis, to perform a touch test on the entire perimeter.

Finally, tamperproof nuts and bolts for gates and fences shouldalways be used. When reviewing the effectiveness of a perimeter fence,one of the first checks is to see if the nuts and bolts can be loosenedby finger pressure only. This is frequently the case, and it negates theeffectiveness of hardened locks, razor wire, and the like.

At perimeter entrance points for personnel, gates with effectivelocks, swipe card, or biometric access control are effective. Because ve-hicle access is commonly needed at water-storage facilities, the samelocking systems as used for personnel access should be used. Figures11-10 and 11-11 show examples of vehicle gate entrances at remotesites. Usually, the gates are only of the vehicle-access type; because





FIGURE 11-9 Architectural security fence.

the sites are unmanned, they are almost always accessed by vehicle. Ifgates exist but vehicle access is no longer allowed, Jersey-type barri-ers are very effective. These come in various forms, including plasticbarriers that can be filled with liquid to add weight. If they are to beeffective, they must remain filled with liquid.

FIGURE 11-10 Typical chain link entrance gate.





FIGURE 11-11 Typical architectural gate and multilock system.

A special caution needs to be sounded against using daisy-chainlocks if at all possible. These systems are allowed to exist because ofthe normal practice of permitting those other than the tank owner andpersonnel (e.g., people from the phone company, emergency services,police, and so on) to have unrestricted access to a site because theyhave equipment there. As noted elsewhere in this chapter, this practiceshould be stopped or at least severely restricted.

In addition to preventing unknown personnel from having unre-stricted access to a site, it is important to have an effective lock. Figure11-12 illustrates the concept of the weak link in the chain, in which asimple lock of the type available at a hardware store is all that standsin the way of an adversary.� An array of delay features can be placed between the site

perimeter and the storage tank itself. Vehicle barriers in zigzagpatterns are very effective. Additional layers of fencing willdelay an adversary who is on foot. Use reinforced glass for allexterior windows.� Depending on the nature of the operations at the site, cer-tain assets may be contained within a building on site. Thebuilding presents several opportunities to delay an adver-sary. Heavy exterior metal doors should be installed. Locksand hinges on all exterior doors should be covered with steelplates using tamperproof screws. Reinforced glass should be





FIGURE 11-12 Daisy-chained locks with weakest link.

used for all windows (if applicable). Figure 11-13 shows a typ-ical entrance configuration for a fluted-column enclosed-baseelevated tank. If this configuration is not practical, bars, cages,fence, or mesh can be installed on the inside of the windowframe. Of course, these measures are worth nothing if doorsare not kept locked and if there is no key control.

Presuming the adversary gets to the asset before a response ar-rives, further delay measures can still be used. The most common andsimplest is to build a steel cage around the actual asset. The design ofthis barrier must allow for adequate normal maintenance, but it canbe very cost-effective.

Response PracticesThe response component of physical security should focus on provid-ing the water tank owner and staff with backup communications sys-tems for all possibilities. This should include landline phones, cellular-phone backup, and even radio systems tied into emergency frequen-cies. Local law enforcement should be made fully aware of all facilitylocations and should train on these sites. Water system staff shouldalso be familiar with local law enforcement agencies and should haveall of their emergency phone numbers up-to-date and readily avail-able at all times.





FIGURE 11-13 Pedestrian and vehicle access to fluted-column base.

In conjunction with good detection system practice, nuisancealarms must be minimized to prevent the “cry wolf” problem, in whichalarms are ignored because there are too many false alarms.

If private guards are to be used, many issues must be addressed.Will they be employed 24/ 7/365? Will they be armed? What level ofauthority will they have? Will they be used at all facilities? Do theyregularly train with local law enforcement? Generally, the cost of hir-ing private guards is prohibitive. An adequate response can be madeby local law enforcement if you have worked to improve relationshipswith those agencies.

Operational SecurityOperational security (OS) can also provide security, detect and delayan adversary, and enhance response capabilities.

Detection, Delay, and Response Practices for Operational SecurityThe categories of detection, delay, and response still apply in OS, butthey are different from the physical security or PPS functions. WithOS, policies, procedures, and training—not physical features—have amuch greater role:

� From an operational perspective, one of the best ways to im-prove detection is to have a well-trained and aware staff.Preparing and fully implementing a set of security policies





and procedures along with emergency operations and re-sponse plans is absolutely the most important element of anyrisk reduction program.� If guards are to be provided, they should be on site 24/7/365and should be well trained.� Complete background checks should be conducted for all em-ployees. Focus more thorough and more frequent checks onemployees who have critical access.� Limit access and key availability to only those employees whoneed them.� Plan in advance what deliveries are expected, and record whatcompany and driver are expected when. Allow no deviationsfrom this schedule. Require that all delivery personnel be es-corted at all times on site. Institute a policy to perform basicassay tests for all chemicals arriving on site.� Establish Water Watch neighborhoods throughout your sys-tem, but begin by focusing on areas near your critical facilities.Train people in these groups in the basics of water system op-eration and, especially, what security problems to look for andwhom to call. Whoever is tasked with receiving the calls mustbe prepared to handle the situation and initiate a response.� Have local law enforcement stop and check identification onany person working in or around water system facilities (suchas hydrants, valve boxes, tanks, booster pumps, and so on).Do not assume that the worker is an authorized employee ofthe water company.� Consider splitting SCADA system monitoring into twocategories—normal monitoring and monitoring for intrusiondetection. Establish two-person control over SCADA and se-curity access to critical assets.� Strengthen existing backflow prevention policy or establish anew policy. Begin requiring backflow prevention on all con-nections and change-outs.� As already stated, probably the most effective way to delayan adversary is by establishing and implementing security-and emergency-related policies and procedures. These shouldspecifically address such areas as key control for all facilitylocks. If biometrics or other types of access control are used,proper policies and procedures still must be followed.

The same type of key control policy should apply to vehicles andat all other points where access needs to be restricted. All employ-ees should be subject to strict sign-in/sign-out procedures around allcritical facilities and when using any water system vehicle.





If an employee is alone at a facility, a regular passive and activecall-in procedure should be strictly observed. The passive programmeans that a call is regularly placed to the lone operator, either froma base facility or from local law enforcement. The calls can be placedrandomly or at regular intervals. The active method requires the loneoperator to place the regular or random calls. (Having at least twooperators on duty at all times is the ideal situation, thus eliminating orminimizing the need for active or passive call-in procedures. However,if having two people available at all times is impractical, the regularcall-in procedure is the next best thing.)

These types of policies are especially effective at reducing the riskfrom an insider adversary, but they are also applicable in defendingagainst the outsider. They are generally very cost-effective to imple-ment. The biggest obstacle is to change the ways in which a tankowner’s staff thinks and functions. Even now, not all operators andstaff members consider security to be an important part of utility op-erations. Thus, it can be difficult to achieve full acceptance of policiessuch as these.

Consequence MitigationIf detection, delay, and response have failed and a successful attackhas occurred, you are left to mitigate the consequences of that attack.For water systems, conducting mitigation may be one of the most cost-effective means of reducing the risk of future attacks and ultimatelyimproving the level of security for the system. Consequence mitigationprovides benefits after an intentional human act and after naturaldisasters. As with some high-level adversaries (e.g., international ordomestic terrorists, organized criminal enterprises, and saboteurs), anatural disaster cannot be prevented from “attacking” a water system;in either case you must be able to mitigate the consequences. Thesegeneral mitigation techniques are applicable to all sites:� Provide and maintain an inventory of replacement equip-

ment, focused on the most critical assets as determined from aSandia-based RAM-WTM (Sandia 2002) or other vulnerabilityassessment.� Do not store replacement or redundant components in thesame location or structure as the primary item.� Provide generators or other backup power at all critical facil-ities. They should be capable of powering the critical assets,at a minimum.� For utilities that use gaseous chlorine, store less total chlorineon site, assuming delivery is on time and reliable.� If you must store large quantities of gaseous chlorine on site,store it in two or more geographically distant locations tolessen the amount available at any single place.





� Provide additional gas venting and storage locations for allgaseous chlorine, and regularly provide emergency training.� In lieu of gaseous chlorine, switch to safer means of disinfec-tion such as hypochlorite delivered in bulk form or createdthrough on-site generation.� Provide tamperproof, lockable fire hydrants. Schedule regulartraining and communication with fire department personnel.� Work toward establishing system interconnects with neigh-boring utilities whenever possible. Modeling and testing ofthe feasibility of an interconnect must happen before anyphysical connection is made.

BibliographyBrosnan, T. M., ed. 1999. Early Warning Monitoring to Detect Hazardous Events

in Water Supplies. International Life Sciences Institute (ILSI) Risk ScienceInstitute Workshop Report. Washington, D.C.: ILSI.

Burrows, W. D., J. A. Valcik, and A. Seitzinger. 1997. Natural and TerroristThreats to Drinking Water Supplies. US Army Center for Health Promo-tion and Preventive Medicine. In Proc. 23rd Environmental Symposium andExhibition, American Defense Preparedness Association, Arlington, VA.

Danneels, J. J. 2001. Department Manager, Sandia National Laboratories. State-ment to US House of Representatives Committee on Science, hearing onH.R. 3178 and the Development of Anti-Terrorism Tools for Water Infras-tructure, Nov. 14, 2001.

Deininger, R. 2000. The Threat of Chemical and Biological Agents to Public Wa-ter Supply Systems. Water Pipeline Database, Science Application Interna-tional Corporation (SAIC), Hazard Assessment and Simulation Division.McLean, VA.: SAIC.

Gleick, P. H. 2008. Water Conflict Chronology (revised). Oakland, Calif.: PacificInstitute for Studies in Development, Environment, and Security.

Krouse, M. 2001. Backflow Incident Sparks Improvements. Opflow 27:2.Public Health Security and Bioterrorism Preparedness and Response Act of

2002. Public Law 107-188, 42 U.S.C. Washington, D.C.: 2002.Security Systems and Technology Center, Systems Analysis and Development

Department, Sandia National Laboratories. May 2002. Risk AssessmentMethodology for Water (RAM-WSM). Notebook Volume I. Copyright 2002Sandia Corporation. Contract DE-AC04-94AL85000. Export Control Clas-sification Number (ECCN) EAR99.

US Environmental Protection Agency. 2006. Guidelines for the Physical Secu-rity of Water Utilities. ASCE/AWWA Draft American National Standardfor Trial Use. American Society of Civil Engineers (ASCE), American WaterWorks Association (AWWA), and Water Environment Federation (WEF).Washington, D.C.: USEPA.




C H A P T E R 12Tank Rehabilitation

Gregory R. “Chip” Stein, P.E.Tank Industry Consultants

Maintaining water-storage facilities is becoming increasingly impor-tant because of rising replacement costs and the difficulty of obtainingrate increases and funding for large-scale construction operations. Al-though the cost of maintenance is also increasing, these smaller out-lays can substantially delay or even eliminate the need to replace autility’s large capital investment in tanks. This chapter is a guide tothe proactive rehabilitation of existing water tanks as well as a guideto planning short- and long-range maintenance operations on a newtank.

When renovation of an existing tank is being considered, an eval-uation must be made to determine the scope of work to be included.The costs of renovation versus replacement must be compared andamortized over the life of a new tank to determine if repair is econom-ically justifiable.

Developing SpecificationsIf an evaluation of the tank’s condition, components, and appurte-nances has determined that repair is required—and if repair is eco-nomically feasible—it is necessary to generate a set of detailed tech-nical specifications and bonding requirements.

The scope of work must be determined by evaluating the rec-ommendations and cost estimates from the inspection report andcomparing these to the availability of funds and to the tank owner’slong- and short-term plans for the tank. Often, there are multiple po-tential solutions to an observed deficiency. To determine the repair thatbest fits the utility’s needs, these solutions and their associated costsshould be evaluated in terms of the level of risk the utility is willing toaccept.

433




434 C h a p t e r T w e l v e

Standards ReferencedFor the specification writer to be effective, he or she must have aworking knowledge of and have access to the following material:� Applicable American Water Works Association (AWWA)

standards� National Sanitation Foundation (NSF) standards� SSPC Painting Manual Volume 2: Systems and Specifications.Society for Protective Coatings� Local regulations regarding volatile organic compounds(VOCs)� National Fire Protection Association regulations� All pertinent regulations from the Occupational Safety andHealth Administration (OSHA) and American National Stan-dards Institute (ANSI)

In addition to these, the specification writer must have a workingknowledge of any state or local regulations that apply to water tankrehabilitation. The writer should also be familiar with the capabilitiesand availability of qualified contractors to perform work of the natureand magnitude required.

Seismic Design StandardsAWWA D100-05 has changed the way tanks in seismic zones are de-signed. This latest AWWA D100 revision eliminates seismic “zones”altogether—instead, the coordinates of the tank are entered into acomputer program, and site-specific seismic design criteria are de-termined. The change in seismic design standards resulted from adramatic change in the way engineers view the risk of a seismic occur-rence, its potential magnitude, and its effect on a structure. Existingtanks in high-risk areas should be evaluated to determine whetherthey meet the current seismic criteria. It may be prudent to reeval-uate the seismic criteria and the original tank design criteria whenplanning future structural upgrades or modifications.

Owner’s Standard RequirementsAlso included in the project specifications should be any special re-quirements the tank owner or local regulatory agencies might have.Potential contractors must be alerted to special bonding, wage ratescales, taxes, and licenses that may be required. Local ordinancesmay have stipulations concerning hours of work, acceptable noise lev-els, requirements for air monitoring, and other construction activities.



Tank Rehabilitation

435T a n k R e h a b i l i t a t i o n

Open communication among the specification writer, the tank owner,and local officials is imperative when preparing specifications.

Environmental/Worker Safety

Lead RegulationsRegulations regarding removal of paint that contains lead and otherregulated heavy metals were changed in the early 1990s. Methods ofcompliance and the interpretation and enforcement of these regula-tions to protect the environment and workers have changed dramat-ically. Many areas of the United States now enforce a policy of noemissions into the atmosphere or past the property line. Add to thisthe concern for the safety of workers while they are removing thecoatings, and it is obvious why the cost of water tank rehabilitationhas doubled.

The largest problem has been collection of the dust and debris gen-erated by the removal of the tank’s coating while keeping workers’ ex-posure levels to heavy metals within the permissible range prescribedby OSHA in its regulation 1926.62. One solution is to shroud the entirestructure with impervious tarps and conduct open blasting within thiscontainment system. Dust collectors are then used to negate the pres-surization effect of the compressed-air abrasive blasting, producinga negative air pressure in the containment enclosure. Workers mustbe adequately equipped with respiratory protection while they are inthis hazardous environment. This relatively expensive method of con-tainment has been very successful and widely used. Other methods ofsurface preparation include the use of vacuum shrouding around blastnozzles and power tools and the use of ultra-high-pressure (35,000+psi [241.32 MPa]) water jetting.

The most promising technology currently in use and undergo-ing further development is the robotic blasting system. This systemincludes a self-contained centrifugal blasting apparatus that sealsagainst the tank surface. The unit is raised and lowered by a winch andcable. There is no compressed air, so there is no pressure to dispersethe debris that is generated. The abrasive media is typically recyclable,so the amount of debris is minimized. Additionally, because workersare outside the blasting assembly, they are not exposed to the concen-trated dust.

VOC RegulationsVolatile organic compounds, the solvents that traditionally have givencoatings their liquidity and workability, are being heavily regulatednationwide. To complicate matters, different areas of the country are



Tank Rehabilitation


adapting different acceptable levels of VOCs in industrial coatings.As the solvents in the coatings are released, the coating dries. To re-duce VOCs in their products, coating manufacturers are producingmore high-solids coatings and more water-based coatings. These newcoatings will greatly affect coating selection for topcoating operations,recoating, and construction of new tanks.

Water CirculationShort-circuiting and stagnation of water in tanks is a concern for tankowners, who have installed baffle walls and piping systems to forcecirculation and water turnover. Baffle walls should be carefully de-signed to account for their effect on the tank structure. Additionally,these walls present challenges to future tank maintenance. Piping sys-tems should be evaluated for use of dissimilar metals, increased costof interior repainting, and degree of head range loss required to runthe system.

Description of Repair WorkThe accurate and thorough description of needed repair work is oneof the most important roles of the specifying engineer. The engineer’sgoal should be to adequately describe the work so that change ordersare minimized or eliminated and bidders are all on the same page withrespect to what is required. Sometimes it is easy to accurately estimateand describe the work (e.g, installation of a safe-climbing device on aladder). However, in other situations, the repair work is not as easilyestimated (e.g., the amount of pit welding required or the length ofa crack in the concrete). For these situations, the specification shouldstipulate the method of repair while allowing bidders to submit aunit price for it (e.g., the price per foot to repair the concrete crack).This allows the specifier to minimize the possibility of a change orderand solicit prices for the unknown quantity of work in a competitivebidding atmosphere.

Surface PreparationA successful coating application depends largely on the quality of sur-face preparation. Regardless of the substrate (be it steel, concrete, ora coated surface), the area to be coated must be clean, relatively freeof contaminants, and properly abraded to receive a coating. Surfacepreparation should be specified to conform to the applicable SSPCstandards for cleanliness and the coating manufacturer’s surface pro-file requirements. Depending on the location of the water tank (incoastal or heavy-industry areas, for example), specific requirementsregarding the degree of cleanliness and additional testing require-ments may be required for surface contaminants.



Tank Rehabilitation


Coating SystemsThere is no longer any such thing as a standard coating system. Tech-nology in the painting industry, especially in the water storage tankindustry, is in a period of rapid change. There is no longer a “standardspec”—not if you want a coating system that will truly protect yourtank.

Gone are the days when conventional paints were applied overminimally cleaned surfaces by everyday laborers. Now, in a period ofincreasingly stringent environmental regulations, highly skilled tech-nicians apply sophisticated coatings onto surfaces cleaned by ever-evolving surface preparation methods. We must now “design” a coat-ing system for each tank, taking into consideration all the specificconditions that may affect the system’s performance.

Coating System SelectionFirst, we need to realize that in the past, common industry shortcom-ings caused specifiers to use improper or inadequate coating systemsfor water tanks. Engineering education was lacking with regard to con-trolling corrosion by using coatings, and so specifiers relied mainly oncoating suppliers for guidance. Thus, a trend developed among speci-fying engineers of using suppliers’ “canned” specifications rather thandeveloping a specification and system that fit the exact needs of thetank owner. Additionally, the welded-steel tank specifiers, designers,and fabricators frequently failed to recognize the need to incorporateproper design details that extend the coating life. Just because “it’sby the specification” does not mean it is the best coating design forlong-term corrosion protection.

The first step in designing a proper coating system is to deter-mine the owner’s needs and research specific operating conditions byasking questions about the tank itself.� In what environment is this tank located?� What are the constraints of the tank site?� What is the design of this tank?� What is the current condition of its coating?� What are the types of coating failures observed on this tank?� Why did these coating failures occur?� What can be done to correct these coating failures?� Where are the existing corrosion problems on this tank?� What time of year and for how long can the tank be taken out

of service for painting?� What is the level of community acceptance of this tank?� What are the owner’s short- and long-term plans for this tank?



Tank Rehabilitation


After these questions have been answered, review possible coatingalternatives. Weigh the advantages and disadvantages of each systemso that the best system can be provided for a specific tank and owner.Research of coating alternatives should begin with a thorough reviewof applicable standards. This review should include AWWA D102Standard for Painting Steel Water-Storage Tanks and the applicablestandards of the SSPC. The coatings must comply with and be testedin accordance with the requirements of the NSF for coatings in contactwith drinking water. Finally, the coating manufacturer’s performancetest data and real-life case histories should be investigated, as well asany independent laboratory testing or documented service history.

Coating System ObjectivesThe specifier should set objectives that will be compatible with theneeds of the tank painter, engineers, and, most importantly, the tankowner, as follows:� Reduce initial cost.� Provide the optimum coating life for the tank environment.� Minimize release of VOCs or other harmful materials into the

atmosphere.� Provide a coating system that will be easily maintained bytouch-up and maintenance topcoating, thus minimizing theneed for abrasive blasting to bare steel until the tank has beentopcoated several times.� Eliminate unsealed or uncoated interfaces of steel surfaces.� Provide excellent resistance to abrasion and be self-healingwhen subjected to minor abrasions and scratching.� Provide excellent resistance to ultraviolet (UV) light, moisture,oil, soil, and chemicals.� Provide a recoat window varying from a few days to as longas years for new tank projects.� Meet all NSF standards and US Environmental ProtectionAgency (USEPA) regulations.� Describe the system in generic or performance terms that donot rule out qualified coating manufacturers yet that upholdthe standards of quality and performance necessary to pro-vide the tank owner with the best possible system.

Interior Coating SystemsInterior coating systems should offer long life; ease of application;abrasion resistance; and (in the case of open-top tanks) resistance toUV light, oil, dirt, chemicals (chlorine), and other contaminants.



Tank Rehabilitation


Prior to the effective date of NSF Standard 61, Drinking WaterSystem Components–Health Effects addressing direct and indirectwater additives, there were many types of coatings:� Vinyls� Zinc dust/zinc oxide� Chlorinated rubber� Bituminous� Coal tar� Red lead� Wax grease� Phenolic aluminum

In the past, these coatings have worked with varying degrees ofsuccess. However, because strict environmental guidelines for toxins,heavy metals, VOC emissions, and other health threats have sincebeen established, significantly fewer of these coatings are likely tomeet new criteria set forth for use on tank interiors.

The success of the two-component catalyzed epoxy appears tomake it the frontrunner at this time. Epoxies can be and have beenformulated with very high solids (low VOCs) and with many chem-ical varieties available. Two-component catalyzed epoxy is a highlyversatile tank lining and coating.

Another product for consideration is the solventless 100 percentsolids polyurethanes. These products are not mixed, as conventionalepoxies and urethanes are; they are sprayed with a dual pump ar-rangement that mixes the polyurethane at the spray gun tip. The ad-vantage is less waste and a coating that cures for immersion within 48hours. Manufacturers claim that these coatings offer from 20 percent to38 percent longer life than epoxies, but in this author’s opinion, exten-sive field testing and evaluation are required to substantiate this claim.

One last coating for consideration for water immersion is not reallya coating at all. Spray metalizing using zinc, aluminum, or a combina-tion of both has been used successfully for many years. Only recentlyhas technology made this a viable option when considering costs.

Zinc coatings can also be used for direct application to the watertank. While there is a lack of extensive service history in our industry,inorganic zinc coatings could be used on surfaces intended for immer-sion in potable water if NSF certified. However, these coatings shouldnot be topcoated unless they are fully cured and hydrolyzed.

Exterior Coating SystemsLike the interior coating systems, the exterior systems should also offerlong life; abrasion resistance; ease of recoating; ease of application;and resistance to ultraviolet light, oil, dirt, salts, chemicals, and other



Tank Rehabilitation


contaminants. Because of potential community resistance, coatingsrequiring minimal to no abrasive blasting are attractive alternativesfor future repainting needs.

With ever-tightening regulations to control VOC emissions, thealternatives for exterior systems have been reduced significantly.Solvent-based aluminums, acrylics, vinyls, and chlorinated-rubberpaints do not meet most VOC restrictions and have lately been usedvery little. Although we are never quite sure what direction the regula-tory bodies will take when it comes to environmental issues, the trendappears to be to reduce emissions even further, resulting in loweringthe VOCs of all coatings. Generic coating systems that currently meetmost areas’ restrictions are high-solids alkyds, water-based acrylics,epoxies, polyurethanes, and inorganic and organic zinc-rich coatings.More stringent regulations will probably eliminate alkyds and all butthe high-solids epoxies and polyurethanes in the future.

The most widely used exterior system today is the epoxy–urethanesystem, sometimes with a zinc-rich primer and sometimes with anadditional clear urethane topcoat. Water-based acrylics are becomingmore popular—especially for overcoating, due to the minimal stressthey have on existing coatings during cure and for congested siteswhere their “dry fall” characteristics are important. (Dry fall coatingoverspray releases all of its solvents as it falls through the air. Theoverspray is dry when it contacts the surface below.)

A more recent technology is the use of solvent or new water-basedfluorourethanes. Previously, these coatings were only available as abaked-coil coating material from which more than 25 years of colorand gloss retention was normally expected. Time will tell if the newerair-dried versions will perform as well. Another category of exteriorcoatings comprises inorganic-based siloxane hybrids that claim toweather as well as, if not better than, conventional polyurethanes,but apply like high-build epoxies.

Finally, there are varieties of coatings of several generic typesthat are formulated so that they can be applied over very minimallycleaned surfaces. They can be applied over rust, rust stain, old chalkedand cracked paint, and other existing coating defects.

Environmental issues, especially for lead paint removal and abra-sive dust generation, have caused specifiers to strongly consider alter-natives to conventional cleaning methods. To avoid open-air blasting,the coating industry has developed various methods to achieve thesame degree of cleaning. Among these methods are containment of thestructure with tarp material and the use of dust collectors to producenegative air pressure inside this containment.

Risks and Benefits of RepaintingAfter considering coating systems and environmental issues, the spec-ifier and the owner must decide the risks and benefits for the var-ious options of repainting. Table 12-1 is a way of reviewing this



Tank Rehabilitation

Ris

ks

Earl

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oati

ng

Environm

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Alt

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ati

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Benefit

Cost

Failure

Rele

ase

Conta

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ati

on

Publicit

y

Do

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None/negative

0N

/A

N/A

N/A

Hig

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Spot

repair/spot

repain

tVery

low

Low

Hig

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Low

Hig

h

Zone

pain

ting

Low

Low

Modera

teLow

Low

Modera

te

Spot

repair/ove

rcoat

Modera

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odera

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igh

Low

Low

Low

Com

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tere

mova

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coat

Hig

hH

igh

Low

Hig

hH

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Hig

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Repla

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Hig

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Low

Low

Low

Low

Not

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/A

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TA

BLE

12-1

Com

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Ris

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enefits

,and

Costs

441



Tank Rehabilitation


information. Costs may be inserted using an engineering estimate oreven from the actual bids.

Another consideration is when the tank can be drained and avail-able for painting. If this cannot be done in normal warm-weatherconditions, the coating system design must specify either coatingsthat can be applied and cured at colder temperatures or coatings thatare applied while using heaters and dehumidification to control theenvironment. If the schedule is tight and downtime must be limited,multiple shifts using environmental control might overcome scheduleconstraints.

Also, the coating system must meet the owner’s aesthetic require-ments. Color availability and color and gloss retention are importantaspects of aesthetic appeal. To the public, aesthetic appeal is often themost important aspect of the coating system.

One final consideration when designing coating systems—thespecifier must keep in mind the knowledge and abilities of the poten-tial low bidder. The specifier must realize that if no independent fieldinspection of the work will occur, there is greater risk in specifying acoating system that is very sensitive to the quality of workmanship—as are nearly all of the new long-life systems. So, if little or no inspec-tion is to be performed or if prequalification of the bidders is impos-sible, it is not recommended to specify coatings that are difficult toproperly apply.

Likewise, the specifier must be prepared to address problems thatwill undoubtedly occur in the field. This may even include demon-strating to the contractor how the specified coatings are to be mixed,applied, and cured. As new systems are developed, adequate prod-uct knowledge and practical field experience with these new coatingsare lacking. Beware of being the guinea pig for new coating systems.Know what you are specifying! If you lack previous experience withthe new product, observe application procedures and gain informa-tion from knowledgeable colleagues. It is important that specifierscontinually learn about new coatings, equipment, procedures, regu-lations, and other important aspects of the coating industry throughpertinent professional organizations and societies so they can providethe best service possible.

Overseeing Painting and MaintenanceMany water tank owners seek autonomous verification that recoatingand repairs are being performed in accordance with project specifi-cations and generally accepted industry practice. By having a quali-fied and experienced professional be the on-site project representativewhile the work is being performed, the owner has independent assur-ance that the coatings will remain in good condition for their intendedservice life.



Tank Rehabilitation


Role and Qualifications of the Project RepresentativeLong before any abrasive blasting and painting are performed on thewater tank, the tank owner needs to determine what the role of theproject representative will be and what qualifications that person mustpossess. Many water utilities require registration and licensing fromone or more of the various industry associations (e.g., SSPC, NACE).However, possession of a specific industry’s license should not takethe place of numerous years of experience in administering water tankrehabilitation projects.

The role of the project representative must be determined andagreed on before the project begins. It is critical, then, that the de-scription of the project representative’s duties, including the limita-tion of authority and responsibility, be communicated and made clearto all parties, including the painting contractor. For example, whenan independent project representative is on-site, his or her responsi-bility (or lack thereof) for the safe work practices of the contractor’spersonnel should be understood. Generally this is the case, since theproject representative does not have direct control or supervision ofthe means, methods, techniques, sequences, or procedures of the con-tractor’s personnel, nor would the project representative generally beasked to issue direction regarding or assume control over the contrac-tor’s compliance with environmental regulations.

In most cases, the role of the project representative is to conducton-site observation of the work in process and help the owner deter-mine whether the work is in compliance with the specifications andwith generally accepted industry practice. The project representativeshould also be expected to document and report to the tank owner anywork that appears unsatisfactory or defective and advise the ownerwhen additional testing appears necessary.

The project representative should document his or her observa-tions daily on an observation form. Topics might include number ofcontractor’s personnel on-site, surface profile measurements, paintbatch numbers, area(s) of tank worked on, and ambient weather con-ditions. The written narrative should be supplemented with photo-graphic documentation as determined necessary by the project rep-resentative. It is critical that this documentation be distributed to theowner and to the contractor’s foreman on a regular (daily) basis sothat all parties are on the same page.

Role of the Water Tank OwnerAs previously discussed, the water tank owner is responsible forselecting the project representative and communicating the author-ity and limitations of that person’s duties to all parties. Next, theowner should designate someone else to act as the utility’s represen-tative, a person who has the authority to transmit instructions, receive



Tank Rehabilitation


information, and interpret and define the owner’s decisions. The util-ity’s representative should also be responsible for arranging for accessonto both public and private properties as necessary and for reviewingand providing input on all documentation submitted by the contractorand the engineer.

Observation of the Work in ProcessAs mentioned previously, many utility owners view independentevaluation of the coating application and repairs as an essential partof a successful tank rehabilitation project. The capability of a coatingto achieve its anticipated service life is directly related to the qualityof workmanship during application. Verification of the workmanshipassures the owner that the money spent on tank maintenance will bemaximized.

As applicable, the following should be verified daily by the on-siteproject representative:� Temperature of steel� Weather conditions (temperature, wind velocity and direc-

tion, relative humidity, and dew point)� Paint batch numbers used on the day of the observation� Location of work performed� Quality of work being performed and compliance with theproject documents� Wet and dry film thickness readings� Calibration record of dry mil thickness gauge� Measure of the paint cure� Number of workers on the job� Equipment on the job� Recommendations made� Estimated completion date� Photographs of significant details� Other pertinent data as required or requested

It is necessary that the on-site project representative, in addi-tion to having the previously discussed qualifications, be trained andqualified to competently use the equipment necessary to verify thequality of the work. The project representative generally uses the fol-lowing equipment and resources to observe the work when requiredto do so:� SSPC-Vis 1 visual blasting standards� NACE Visual Standard TM-01-70/75 (available)



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� AWWA standards D100-96 and D102-97� Testex Press-O-Film Profile Measurement System� Surface contamination detection device (soluble salts)� Wet film thickness gauge� Dry film thickness gauge� Certified thickness calibration standards� Steel temperature gauges� Sling psychrometer and psychrometric tables� Wet sponge holiday detector (low voltage)� Tooke Gage (if required)� Adhesive force measurement device (if required)

Contract Document and Specification OptionsIt is important that the project specifications include specific tank up-grades as well as the tank repainting design. Repair specifications canbe worded according to the precise methods to be used or the desiredend result. The specification writer should review the strengths andweaknesses of each approach.

Precise Methods of RepairFor a specification that outlines precise methods of repair, the spec-ification writer (and therefore, ultimately, the tank owner) exercisesa good deal of control over the contractor’s activities. This type ofspecification can result in fewer bidders who are willing to modifytheir standard procedures to comply with the specifications, and bidprices may be higher. Strictly adhered to, this method of specificationpreparation is a bit of overkill and places more liability on the specifier.

End Result OnlyThis type of specification defines the repairs to be made but leaves it tothe contractor to determine how to accomplish the repairs. The resultis more bidders and possibly lower costs. It allows the contractor to usehis or her standard methods of repair and can lead to the developmentof innovative procedures. However, the specifier and the owner havelittle or no control over methods used.

A prudent specifying engineer uses the best of each method andwrites a repair specification that results in the best bidders offeringthe most competitive bids that result in a long-lasting, high-qualityrepair. Quantity does not necessarily mean quality.

The contract documents used in a tank rehabilitation project gen-erally spell out such necessary requirements as insurance limits,



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bonding, length of contract time, and bidding information. Sometimesa utility has a complete set of contract documents; sometimes it justprovides certain documents and/or input on minimum requirementlevels. If the utility lacks a complete set of contract documents, thereare organizations that provide boilerplate documents (e.g., the En-gineering Joint Contract Documents Committee) that can easily bemodified for tank rehabilitation. Regardless of how the contract doc-uments are assembled, the utility must provide precisely exact inputin numerous areas, including minimum insurance level requirements,liquidated damages amounts, on-site availability of water and elec-tricity, and bid opening dates.

Contract AdministrationThe specifier and the tank owner should collaborate to administer theproject to make sure that the owner’s needs are being satisfied. Theowner may prefer that some activities be performed on-site during arehabilitation project; some activities are best overseen by either thespecifying engineer or the on-site project representative. The projectengineer should verify compliance with the project specifications andcontract documents to ensure that both the letter and the intent ofthe documents are being followed. The go-ahead for work to pro-ceed should not be given until all submittals have been reviewed andaccepted.

After all of the submittals have been reviewed and accepted, nu-merous other administration activities need to be done, includingthese: � Pre-job conference attendance� Consultation on adequacy of and compliance with the project

specifications� Specification interpretation� Attendance at the preconstruction meeting and all subsequentmeetings, and provision of meeting notes� Review of all contractors’ submittals and shop drawings� Review of construction schedule� Review and approval of materials� Preparation for negotiations of change orders and assistancewith supplemental agreements� Review and approval of payment requests� Dispute settlement� Public relations



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First Anniversary EvaluationA first anniversary evaluation, as recommended in AWWA D102,should be called for in the project specifications and should be sched-uled prior to the end of the one-year bonded guarantee. The purposeof this evaluation is to identify and repair defective work before thebonding period ends. The water tank owner should be responsiblefor making the tank available and coordinating the date of the evalua-tion with the contractor and the project representative. The contractorshould be required to complete the tank washout the day before theevaluation and should also be responsible for meeting the followingrequirements:� Have an experienced foreman present.� Be prepared to perform minor touch-up work.� Bring all rigging necessary to performance of the touch-up

work.� Bring at least 1 gal (3.79 L) each of the exterior primer, inter-mediate coating, and finish coating.� Bring at least 1 gal (3.79 L) each of an interior coating that canbe placed in immersion service immediately for minor spotrepairs.� Bring Scotch-BriteTM abrasive disks with power tools andsandpaper to clean the steel surface.� Supply equipment with which to apply coating repairs.� Supply equipment with which to wash out the tank and chlo-rine to disinfect it following the evaluation and any requiredtouch-up work.

The project representative should prepare and submit to the watertank owner a brief report with color photographs of the conditionsfound during the first anniversary evaluation and of the touch-upwork.



Tank Rehabilitation



Tank Rehabilitation

Steel Water Storage Tanks Design, Construction, Maintenance, And Repair

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