watermeter selction
Transcript of watermeter selction
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A recent announcement from the
Chicago Department of Water prompt-
ed me to think about an aspect of
plumbing design that may be taken
for granted. The news is that the City
of Chicago will undertake the major
exp ense of installing water m eters in
all houses in the city. Evidently, there
are thousands of houses in Chicago
that have ne ver been m etered. The
residents have been charged for water
usage, but the charge was based on
an estimate rather than on actual use.
Someone now has figured out that the
initial water-meter installation cost will
be offset by the additional revenue.
The concern is that many engineers
who design w ater systems con nected
to municipal water supply may take
water-meter selection and sizing for
granted. Regularly, mu nicipal w ater
purveyors dictate the type, size, and
cost of the water meter to be used for
buildings. This practice has become
more prominent with the advent of
remote meter reading and digital con-
nectivity. It is very common for a
municipality to read a water meter
through the telephone system. This iseasy to do, even with yesterdays tech-
nology. Many designers have become
familiar with the size and type of water
meter that the water purveyor provides
for a certain building type. Therefore,
they can establish the amount of pres-
sure loss to apply to the pipe-distribu-
tion sizing calculations from the meter
man ufacturers p ressure loss curves.
What about installations where the
plumbing contractor is required to
provide the w ater meter based on thedesigners plans and specifications? A
water meter typically is located at the
water service entrance to a building.
Therefore, it is the first device calcu-
lated into a water-distribution system
design that inevitably results in having
the proper flow and pressure at the
farthest fixture in the building. Water
meters have other applications in
plumbing design, such as tenant sec-
ondary metering, control of the appli-
cation rate in irrigation systems, meas-
urement of water usage or loss for
process or boiler systems, and control
of the regeneration cycles for water
softeners, to name just a few.
Sizing Is CriticalMy last article, on temp ered water
systems, outlined the false concept of
matching the mixing valves inlet sizes
with the pipe sizes. The same princi-
ples ap ply to w ater-meter sizing,
although the systems dynamics are
Water-M eter Selection and SizingTim othy A.Smith, CPD
70 Plumbing System s & Design Jul/Aug 2003
DESIGNERS NOTEBOOK
Figure 1. Descriptive Specifications for Four Types of Water Meters
Disc MeterA. Positive displacement type AWWA C700-02 and/or C710-02 (cast bronze body C700-02/ plastic
body C710-02)1. (Bronze) or (plastic) body constructed with a nutating disc or oscillating piston; flanged or
threaded conn ectionsB. Application: Residential and small commercial buildi ngs with flush tank toilets or small quantity
of flush valves
C. Commentary: Available in sizes 1/2-in. through 2-in.; 1/4-gpm mini mum through 160-gpm
peak flow; available with analog/digital remote reading capability; accurate low flow range; pres-sure loss increases with flow somewhat proportionally to the recording inaccuracies.
Turbine MeterA. Turbine type, AWWA C701-02
1. Bronze body with rotating im peller (rotar) located within the measuring chamber mounted in
the center of the flow; vortex reducing flow vanes; flanged connections
B. Application: Commercial, process systems, large institutional, and facilities of potentially high
usage and high flow ratesC. Commentary: Available in sizes 2-in. through 20-in.; 2 1/2- gpm minimum through 19,000-gpm
continuous; remote reading is available; small fluctuation in accuracy throughout entire flow
range; inaccurate for flows less than 2 gpm.
Compound MeterA. Combination type, AWWA C702-01
1. Bronze body with dual measuring chambers separated by a pressure/velocity-sensiti vemechanism or plate; positive displacement chamber and turbine chamber combination; as
pressure loss through positive displacement (low-flow) chamber increases, the mechanism
or diverting plate shifts the flow to the turbine (high-flow) chamber; flanged connections
B. Application: Commercial, institutional, places of assembly, and facilities with fluctuating usageand flow rates
C. Commentary: Available in sizes 2-in. through 6-i n.; 1/4-gpm mi nimum through 1,200-gpm peak
flow; available with analog/digital remote reading capability; accurate low and high (peak) flowrange; progressing pressure loss until mid-flow range when low-flow chamber diverts to thehigh-flow chamber with lower pressure drop; can experience quick pressure-loss spike when
switching from low-flow to high-flow chambers.
Propeller MeterA. Propeller type, AWWA C704-02
1. Iron or fabricated steel body and thermoplastic impeller with deep impeller vanes, curved
and tapered (similar to a propeller) for reduced resistancy; mounted in open pipe with wafer
or lug connectionsB. Application: Process or industrial facilities, well-pump di scharge, pumping stations or sewage
treatment plants where flow rates are very high and constant
C. Commentary: Available in sizes 2-in. through 72-in.; 30-gpm mi nimum through 50,000-gpm
continuous; unable to accurately register low and intermittent flows; broader band of inaccuracy;exceptional for high, continuous flow applications.
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more forgiving of an improperly sized
water meter than an improperly sized
three-way m ixing valve. Rem em ber,
using the one-size, one-type-fits-all
philosophy is really asking for trouble.
Improperly sized water meters can
affect the operation of the buildings
water distribution system and plumb-
ing fixtures, which also can affect the
health of the buildings occupants.
Undersized water meters can cause
excessive pressure loss, reduced flow,
and noise. High-pressure losses
through meters at peak flow rates can
result in pressure surges, water ham-
mering, and the inability to maintain
proper residual pressure at the fixtures
or equipment. High-pressure loss
through a meter also corresponds
with a high-flow velocity and thus
results in more noise. Improperly
sized meters can cause negative pres-
sures in the piping system and lead to
contamination of the potable water
supply by backflow or
back siphonage. In addi-
tion to being unecon omi-
cal, oversized water
meters typically do not
accurately measure mini-
mal flow rates. Larger
meters are more expen-
sive than smaller meters.
Municipalities u sually
assess meter and tap fees
according to size, so
oversizing can become
expensive for a buildings
owner.
When selecting and
specifying a domestic
water meter for a build-
ings service entrance, you
must understand the
bu ildings size, fun ction ,
fixture types, usage occu-
pancy, and peak popula-
tion. These factors will
allow you to determine
the minimum and m axi-
mum flow rates and
enable you to select the
proper water meter, based
on pressure loss, record-
ing accuracy, and capabili-
ty of accommodating the
projected flow variations.The ASPE Data Books pro-
vide good resources for
calculating the buildings
water usage. Remember to
always specify tapered fit-
tings when reducing or
increasing from the meter
size to calculated pipe
sizes. The alternative
reducing/increasing
method using p ipe bushings can p ro-
duce three to ten times the pressureloss of tapered fittings, depending on
size.
M eter TypesThe American Water Works
Association (AWWA) has pro mu lgated
widely recognized and accepted stan-
dards for the construction of water
me ters. All water m eters man ufactured
Jul/Aug 2003 Plumbing Systems & Design 71
DESIGNERS NOTEBOOK
continued on page 72
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for domestic water service use in the
United States are manufactured in
acco rdan ce with AWWA stand ards.
These standards set forth construction
criteria, physical construction, internal
comp onen ts, dimensions, and testing
criteria to help ensure uniformity
across the industry. All water-meter
man ufacturers that me et the AWWA
standards can provide the test results
for their products. The designer
should maintain a water meter prod-
uct binder with pressure-loss curves
and accuracy graphs, produced in
acco rdan ce w ith AWWA stand ards for
each meter type and size.
Figure 1 lists descriptive specifica-
tions for various types ofwater meters utilized for
domestic water applications.
Table 1, Table 2, and
Table 3 are reprints from
AWWA Standard M22, as
reprinted in ASPE Data Book
Volume 2, Chapter 5. These
tables will provide additional
guidelines for selecting and
sizing water meters, but the manufac-
turers of water meters can supply test
curves of each meter type and sizethat better define actual performance
and characteristics. See Figure 2 for
typical performance and accuracy
graphs.
M eter SelectionThe first step in selecting a meter is
to establish the purpose for metering
the service. If the meter is to be used
for billing purposes, accuracy is
critical. Process applications may only
require approximate flow recording.
Positive d isplaceme nt m eters are the
best selection for small commercial or
institutional facilities or residen ces.
They have excellent accuracy at low
flows and, if sized properly, can accu-
rately measure the peak flow rates.
Compound meters (see Figure 3) are
an excellent choice for large commer-
cial or institution al facilities b ecau se
they have the ability to accurately
accommodate low flows and high
flow s through their multiple-measur-
ing chamber design. Refer to Figure 2
for performance curves for a typical
comp ound -type meter. Turbine and
propeller meters arebetter suited for contin-
uou s, higher-flow app li-
cations and are inaccu-
rate at low flows.
Turbine and propeller
meters are not recom-
mended for commer-
cial, institutional, or res-
idential buildings
because the flow rates are constantly
fluctuating and minimum flow rates
may be as low as 1/22 gpm.After the meter type is selected, a
size needs to be determined. The size
is established by comparing the mini-
mum and peak flow rates of the facili-
ty against the minimum and maximum
flow capabilities of the meter size.
The flow rate will correspond to a
pressure loss, which factors into the
pipe-sizing calculations. The greater
the pressure loss at the meter, the less
pressure is available for the systems
friction loss. Thus, larger pipe sizes
are required downstream of the
undersized meter. If a backflow pre-
venter is required o n the service (typi-
cally consuming 510 psi), it is neces-
sary to be more conservative with the
water-meter pressure-loss selection.
It is recommended that the calculat-
ed peak flow rate for the facility not
exceed 80% of the maximum capacity
of the meter. This recommendation
needs to be evaluated closely when
low incoming residual pressures are
prevalent or backflow preventers are
required. It is not uncommon to
design for a maximum of 23 psipressure loss through the water meter
where municipal water pressures are
weak.
StrainersWater meters have internal mechani-
cal components that move with the
flow of water, so it stands to reason
that debris and sediment will have an
adverse effect on the meters opera-
tion. It is recommended that an in-line
strainer on the meters inlet be speci-
fied to collect debris and sediment
and prevent them from entering the
meter body. Strainers are available
that are designed specifically for use
in conjunction with water meters.
These strainers are designed and engi-
neered to provide low pressure losses.
They also have removable screens
and accessible covers to facilitate
cleaning and maintenance. Strainer
72 Plumbing System s & Design Jul/Aug 2003
DESIGNERS NOTEBOOK
Figure 2. Typical Accuracy and Performance Graphs
Courtesy of Badger Meter, Inc.
If the meter is
to be used for
bill ing purposes,
accuracy
is critical.
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maintenan ce is critical becau se a
clogged or dirty strainer impedes flow
and increases pressure losses.
InstallationThe installer needs to strictly follow
the ma nu facturers installation instruc-
tions. Improper installation may affect
the meters operation, capability, and
accuracy. Meters should be located in
an accessible location and protected
from damage. Meters are mechanical
devices that need pe riodic testingand repair. Make sure there is amp le
room to repair or remove the unit .
Envision what i t would take to work
on the unit. Refrain from installing
meters in crawl spaces, under stairs,
or in confined spaces.
Some areas still allow the installa-
tion of water meters in exterior pits
or vaults . I do not recommend this
installation. It makes the meter hard
to m aintain and service, and exterior
pits and vaults can flood. In some
cases, submerging the water meters
causes rapid de terioration and corro-
sion to the unit. Also, there is a
remote possibility that the contami-
nated water in the pit can leech into
the potable water supply.
It is recommended that a floor drain
be installed adjacent to the watermeter, especially for larger meters. The
floor drain can reduce or eliminate
water damage to the building caused
by meter repairs, replacement, or test-
ing. Larger meters are tested in place,
so it is imperative to have somewhere
to discharge the test water.
The bottom line is meter accuracy
and pressure loss. Accuracy is critical
to the purveyor of water because
meters that inaccurately measure the
supp ly result in lost revenue . Pressure
loss is critical to designers because
they need to ensure that their engi-
neered systems operate properly.
Jul/Aug 2003 Plumbing Systems & Design 73
DESIGNERS NOTEBOOK
Figure 3. Three Types of Water Meters
Compound M eter
Disc M eter
Turbine Meter
Courtesy of Badger Meter, Inc.
Tim Smith is vice presi-dent, partner, and found-
ing member of Metro
Design Associates, Inc., in
Schaumburg, IL. He has
more than 23 years expe-
rience covering all aspects of consulting
engineering, specializing in plumbing, fire
protection, and civil engineering. His e-
mail address is [email protected].
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