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IS 11639 ( Part 2 ) : 1995

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I ndian St andardSTRUCTURALDESTGNOFPENSTOCKS-

CRITERIAPART 2 BURIED/EMBEDDED PENSTOCKS IN ROCK

UDC 627.844

@ BIS 1995BUREAU OF INDIAN STANDARDSMANAK BHAVAN, 9 BAHADUR SHAH ZAFAR MARC3

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Conductor Systems Sectional Committee, RVD 14

Indian Standard ( Part 2 ) was adopted by the Bureau of Indian Standards, after the draftWater Conductor Systems Sectional Committee -had been approved by the RiverDivision Council.s carry water from surge tanks or directly from a reservoir to the power house. Suchmay be laid on the surface or buried/embedded in rock or concrete. Part 1 of thiscovers the criteria for structural details of surface penstocks and this part covers thefor structural design of buried/embedded penstocks in rock or concrete.

the purpose of deciding whether a particular requirement of this standard is compliedthe final value, observed or calculated, expressing the result of a test or analysis, shall beoff in accordance with IS 2 : 1960 Rules for rounding off numerical values ( revised ) .number of significant places retained in the rounded off value should be the same as thatthe specified value in this standard.

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IS 11639 ( Part 2 ) : 1995

Indian StandardSTRUCTURALDESIGNOFPENSTOCKS-CRITERIA

PART 2 BURIED/EMBEDDED PENSTOCKS IN ROCKSCOPE

This standard ( Part 2 ) lays down therequirements and design of circulars, buried/embedded in rock or concrete,conveyance of water from a reservoir, pondsurge tank to a hydraulic turbine in hydro-

This standard does not cover the design ofmanifolds, wye-pieces, transitions,

Standards listed in Annex A arey adjuncts to this standard.HYDRAULIC DESIGN

hydraulic design of penstocks referencemade to IS 11625 : 1986.MATERIAL AND ALLOWABLE STRESSES

Steel to be used for the fabrication ofks of a hydro-electric project shouldthe following requirements:a) It should stand agai~nst maximum internalpressure including dynamic pressure,b) It should stand against frequent dynamic

changes,c) It should have required impact strengthto be able to deform plastically in thepresence of stress concentrations atnotches and bends,d) It should have good weldability withoutpreheating, ande) It should not require any stress relievingafter welding.

requirements (a) to (c) are essential while(d) and (e) are preferable.

The steel plates to be used for fabricationpenstock liners should be of fire box qualitying to IS 2002 : 1992, IS 2041 : 1982 or: 1992.

4.1.3 Nothing in the foregoing should precludethe use of material where so agreed by thepurchaser. It is recommended that, in suchcases, particular attention be given to the wel-dability and ductility of the material proposedto be used. No such material should haveelongation ( in percent ) on a gauge length of5-65,./g less than ( IOO-Rm) / 2-2 where So isthe original area of cross-section and Rm is theactual tensile strength in N/mm2 at poom tem-perature, subject to a minimum of 16 percentfor carbon and carbon manganese steels, 14percent for alloy steels other than austeniticsteels and 25 percent for austenitic steels fortest pieces obtained, prepared and tested inaccordance with appropriate Indian Standard.4.1.4 Materials used for supporting lugs,stiffeners and other similar non-pressure partswelded to penstocks should be of weldablequality and suitable in other respects for inten-ded service.4.2 Allowable Stresses4.2.1 The allowable stresses and the factor ofsafety to be adopted depend upon the yieldpoint stress and ultimate tensile strength of thesteel, loading condition and the location wheresteel lining is provided.4.2.2 Following allowable stresses should beadopted in design of steel pen&ocks:

a) In normal operating condition, the designstresses should not exceed one-third of theminimum ultimate tensile strength or60 percent of m~inimum yield point stressof steel, whichever is less.

b) In intermittent condition, the designstresses should not exceed 40 percent ofthe minimum ultimate tensile strength ortwo-thirds of minimum yield point stressCl

d)

of steel, whichever is less.In the emergency condition, the designstress should not exceed two-thirds ofminimum ultimate tensile strength or90 percent of minimum yield point stressof steel, whichever is Iss, and.In exceptional condition, the designstress should not exceed the minimumyield point stress.

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IS 11639 ( Part 2 ) : 199542.3 When rock participation is considered inzhedesign, the stresses in steel lining undernormal loading condition without rock partici-pation should also be checked and should notexceed 90 percent of minimum yield point stressor tcvo-thirds of minimum ultimate tensilestrength, whichever is less. In intermittent andemergency conditions of loading it should notexceed the minimum yield point stress.4.2.4 Loading conditions should be consideredas given below:

a)

b)

c)

Normal condition includes static headalong with pressure rise due to normaloperation or head at transient maximumsurge, whichever is higher.Intermittent condition includes thoseduring filling and draining of penstocksand maximum surge in combination withpressure rise during normal operation.Emergency condition includes partial gateclosure in critical time of penstock; 2L/a seconds ) at maximum rate, and thecushioning stroke being inoperative inone unit.

d) Exceptional condition includes slam shut,malfunctioning of control equipment inthe most adverse manner resulting in oddsituation of extreme loading. This shouldnot be taken as a design criteria.4.3 Joint Efficiency4.3.1 Joint efficiency or weld factor assumedfor purpose of design varies for different kindof joints and different methods of inspectionand testing. The joint efficiency also varies fordifferent type of steel.4.3.2 Joint efficiency as specified in Table 1.1 ofIS 2825 : 1969 should be adopted.4.4 For liner thickness exceeding 38 mm andspecials like manifolds, transitions, etc, stressrelieving should be done as specified inIS 2825 : 1969.5 DESIGN LOADSThe steel lining has to withstand the internalwater pressure as well as external pressureswhich may be caused either during groutingoperations or when the penstock is dewatered.5.1 Internal Pressure5.1.1 The steel lining of a penstock is designedfor maximum internal pressure which is causeddue to maximum water level in reservoir orin surge tank, as the case may be, plus the anti-cipated increase in pressure due to waterhammer effect development when arresting orreleasing the flow of water. It should be

computed for both normal as well as emergencyconditions. The plate thickness of the penstockshould be checked for both these conditions.5.1.2 Water Hammer EflectRapid opening or closing of the turbine gatesproduces a pressure wave in the penstock, ter-med as water hammer effect. Detailed waterhammer analysis for various conditions ofoperations as specified in IS 12967 (Part 1 ) :1990 should be carried out for computingwater hammer effect.5.1.3 Pressure Wave Velocit y

i ) The pressure wave velocity in a steelpenstock carrying water may be computedas follows:1 425

-__a= 1 + (d/loot)wherea = pressure wave velocity in m/s,d = diameter of penstock in m, andt =2 thickness of penstock shell in m.

ii) For a pipe concreted in solid rock, thepressure wave velocity may be taken equalto 1425 m/s ( velocity of sound in water )5.1.4 Pressure Rise GradientPressure rise due to water hammer is measuredabove static water level in reservoir or maxi-mum upsurge level in case of surge tank and itis assumed to vary uniformly along penstock,from maximum at turbine end to zero at reser-voir level or maximum upsurge level, as the casemay be, as given in IS 7396 ( Part 1 ) : 1985.5.2 External Pressure5.2.1 The steel lining should be designed forthe external water pressure head which is eitherthe difference between the ground level verti-cally above the penstock and the penstockinvert level or the maximum level from whichthe water is likely to find its way around steellining, whichever is less.5.2.2 The liner should also be checked againstgrouting pressure during construction.5.3 Longitudinal Stresses Caused by RadialStrainRadial expansion of steel caused by internalpressure tends to cause longitudinal contractionwith corresponding tensile stress equal to 0.303times the hoop tension in the circular lining.

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IS 11639 ( Part 2 ) : 19956 STRUCTURAL DESIGN OF STEEL LINING6.1 Design for Internal PressureThe stresses in the steel liner and the surround-ing concrete/rock may be computed using theformulae given in 6.1.1 to 6.1.3.6.1.1 H oop St r ess i n Penstock Wi th out RockPart ic ipat ionIf the surrounding rock is very badly fracturedand weak or the minimum rock cover is inade-quate. the steel lining should be designed forfull internal pressure. The hoop stress in suchcase may be computed using the formula:

fst _+where

_&t = hoop tensile stress in steel liner inN/m%,P = internal pressure in N/m2,R = internal radius of penstock in m, andt = thickness of liner shell in m.

NOTE - Rack cover is considered inadequate whenit is less than 40 percent vertically and 120 percenthorizontally of the internal pressure head underFormal loading condition.

6.1.2 H oop S tr ess in Penstock in U nfi ssuredConcreteIf the penstock is embedded in well reinforced,homogeneous mass concrete without fissures orcracks, for example, penstock through a conc-rete gravity dam adequately reinforced againstcracking, the hoop stress in the steel liner andthe surrounding concrete may be computedusing the following formulae:

P,=P[ 1-& R 11 + E,t ( ;I -+,: ) . (i)C2 + R2K I = cRRB - . (ii)

. (iii)fct = (P - Ps) x :g +_z2;

whereP, = pressure shared by steel liner inp = total internal pressure in N/mz,R -_ internal radius of penstock in m,

. . . (iv)

N/m%

C = radius of external surface of concretein m,

it = thickness of steel lining in m,Es = modulus of elasticity of concrete inN/mz.Es = modulus of elasticity of steel in N/m2,CLS Poissons ratio for steel,IJC= Poissons ratio for concrete,fs t = hoop tensile stress in steel liner inN/m2, andf c t = hoop tensile stress in concrete at innersurface of concrete in N/m?

6.1.3 If the penstock is embedded in fissuredconcrete and rock, the stress in steel and rockmay be computed using the following formulae:

d2 - C2+ Cd 1 vi)Ps = PO + ( P - POc

T&5i ll t_ $ ] .! ,...... (vii)f st _+ . . . . . . . . . . . . . . . . . . (viii)f r + _ ( p - Ps R

where;zP=Ps =PO =

YO =

d . . . . . . . . . . . . . . . . . . (ix)a dimensionless parameter,hoop tensile stress in liner in N/mz,total internal pressure in N/mz,pressure shared by steel lining inWm2,

R =c =d =

t =Es =

pressure required to close the gap YObetween liner and concrete/rock,initial gap between the liner and theconcrete caused due to shrinkage andcreep of concrete and temperatureeffect in m,internal radius of penstock in m,outside radius of concrete lining in m,radius to the end of radial fissure inrock in m (where the in-s i tu com-pressive stresses in rock are justexceeded by the tensile stresses causedby internal pressure ),thickness of steel liner in m,modulus af elasticity for steel inNlm2,

3

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: 1995Er = modulus of elasticity for rockmass inN/m2,EC = modulus of elasticity for concretein N/ma,CL8= Poissons ratio for steel,pr = Poissons ratio for rockmass, andfit - tangential tensile stress in rockmassdue to internal pressure at dmetresaway from centre of penstock.

maximum hoop tensile stress in steelfSt should not exceed the product of jointand the allowable stress.Design for External PressureCri t i cal External Pressure for Unsti@ened

l external pressure for unstiffenedliner shell may be computed using the

K=E zzE, =

ratio of pipe diameter to platethickness,ES1-1*2s

modulus of elasticity of steel inN/m2,Poissons ratio of steel,critical external pressure at bucklingin N/ms,initial gap between steel lining andconcrete in m,

R = radius of steel liner in m, andFy = yield point stress in steel in N/m?Amstutzs formula

critical stresses in the liner are given byof the following two equations:

1P,,.K2. b = 0.175 $ (fr - f n

whereK 2R=-- tE = +-p%yy= fy1/1-y + /A-fY = yield stress of steel in N/ma,EB = modulus of elasticity of steel in N/m?ICE= Poissons ratio ofsteel,R =Z radius of penstock in m,Y,, = initial gap between liner and concretein m,h = allowable stress in steel in N/ms,P,, -= critical external pressure in N/m?and

1 = thickness of steel liner in m.NOTE - Lower of the two values of critical externalpressure calculated by above formulae should beadopted.6.2.2 The maximum external pressure on thepenstock should not exceed two-thirds times thecritical external pressure calculated accordingto 6.2.1. If the maximum external pressureexceeds the value equal to two-thirds timesthe critical external pressure for unstiffenedshell, stiffeners should be provided to preventbuckling.6.2.3 Cri t i cal External Pressure *for St i ffened Shel l6.2.3.1 Critical external pressure for steel linerwith stiffening rings should be computed byTimoshenko equation. The critical externalpressure should not be less than l-5 times themaximum external pressure.

Timoshenko equation:

where

E= ---!?!_l-G%z = thickness of liner plate in m.

4

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IS 11639 Part 2 ) : 1995R = radius of pcnstock in m.L = spacing of stiffener rings in m.n ;- number of full waves on bucklingchosen to make PCr a minimum. Forn values varying from 1 to 18, the

values of PCr should be calculatedand a graph plotted to obtain mini-mum PCr.P r = critical external pressure in kN/m*.Es = modulus of elasticity for steel inkN/ma.ps ==Poissons ratio of steel.6.2.3.2 Critical external pressure may also bedetermined directly from Fig. 1 based on theformula given in 6.2.3.1.

6.2.4 Spacing of St i ffener Ri ngThe centre to centre spacing of stiffener ringsshould not be more than 240 times and not lessthan 60 times the thickness of steel liner.6.2.5 Size of StifSener RingsThe size of stiffener rings may be worked outusing Vaughans formulae by trial and errormethod so that the ~external critical pressure isnot less than 1.5 times the maximum externalpressure.

KI PZ,, + K,Pacr - I& PC, + K , = 0w-here R4Kl = E, t,

K4 = 12 Csy2 s4A2 V2 RR = radius of penstock in m,A - total area of the composite section inme,A, = sectional area of the stiffener ringin my,t Z.ZZquivalent thickness in m,

=I =

b :z =v =

t++-associated width in m,b + 1.56, R. tcontact zone width of stiffener onferrule in m,moment of inertia of the combinedsection about neutral axis in m*,distance between the neutral axis ofthe combined section and the outerextreme edge of the stiffener in m,

pCF = external critical pressure in N/me,t = thickness of liner in m,=Y = permissible yield stress of liner inN/m%, andY0 = initial freedom of the shell to become

distorted which is algebraic sum ofinitial out of roundness, thermalshrinkage or expansion after installa-tion but before loading expansion dueto shrinkage, due to prestress, etc,lit m.7 THICKNESS OF LINING7.1 The minimum thickness of the lining shouldnot be less than:

a>b) minimum handling thickness,thickness required for internal pressure orexternal pressure ( t ) plus 1.5 mm corro-sion allowance. If the inside surface ofthe penstock is painted with some anticor-rosive paint like epoxy paint, corrosionallowance may be neglected.7.2 Regardless of pressure conditions, a mini-mum handling thickness given by followingformula is recommended to provide rigidityrequired during fabrication:

t0 = R - i- 0.25-- 200where

to = minimum handlingR = radius of penstock

thickness in m, andin m.

8 CHANGE IN THE THICKNESS OF STEELLINING8.1 The thickness of steel lining at differentlocations depends upon various factors. Whenthe penstock passes through different layers ofrock, the steel thickness provided in bad ormedium layer of rock is extended into nextbetter layer for a length equal to at least onediameter of the penstock.8.2 The difference in steel plate thicknesses inadjoining ferrules should not be more than5 mm.8.3 When lining emerges out of a tunnel, itshould be designed for full internal pressureand due care should be taken cf stress concen-trations occurring in the surrounding rock,9 CONCRETE LINING AROUND THE LINERThe concrete lining around the penstock linerand the rock supports, if required, should bedesigned according to provisions of IS 4880( Part 4 ) : 1971 and IS 4880 ( Part 5 ) ; 1972.

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( Part 2 ) : 1995

K = 2R/t i X = L/IE' = E, / l l - I i ,Es= Young' s Hodul us = 200, 345, 000 kN/ dcc= Pol ssm s Rati o = 0.25t = Thl ckness of Ltner Pl ateR = Radius of PenstockL = 5paclng of Stlffner Ringsn = Number of Full Waves on Buckling

Chosen to Make Pcrc.MlnlmumPC,= Crltlcal External Pressure kN/w?

= Yield Stress of Steel kN/m2

- 250 IY\iw

It i 96 I7

.I5 .l .25 .3 .4 .5 .E .7.6 9 I 1.5 22.35 4 5 6 71910 15 20 25 56 u 31ut71c L/ R

PIG. 1 DIAGRAMFORCRITICALEXTERNALPRESSURE

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IS -11639 ( Part 2 ) : 1995

IS No. T i t l e2002 : 1992 Steel plates for pressurevessels for intermediate andhigh temperature service includ-ing boilers ( second r evi sion )2041 : 1982 Steel plate for pressure vesselsused at moderate and lowtemperature (f ir st revision )2825 : 1969 Code for unfired pressure vessels4880 ( Part 4 ) Code of practice for design of1971 tunnels conveying water :Part 4 Structural design ofconcrete lining in rock4880 ( Part 5 ) : Code of practice for design of1972 tunnels conveying water :Part 5 Structural design ofconcrete lining in soft strataand soils

ANNEX A( Clause 2 )

LIST OF REFERRED INDIAN STANDARDSIS No. T i t l e

W$ ( Part 1 ) : Criteria for hydraulic designof surge tanks : Part 1 Simple,restricted orifice and differen-tial surge tanks (first r evi si on )8500 : 1992 Structural steel - Microallo-yed ( medium and highstrength qualities ) ( f i rs tr evi si on )11625 :1986 Criteria for hydraulic design

of penstocks12967 (Part 1) :Code of practice for analysis1990 ofhy draulic transients inhydro-electric and pumpingplants : Part 1 Criteria foranalysis

7

http://../link/15to30/2002.Bishttp://../link/15to30/2041.Bishttp://../link/15to30/2825.Bishttp://../link/31to60/4880_4.Bishttp://../link/31to60/4880_4.Bishttp://../link/31to60/4880_5.Bishttp://../link/61to88/7396_1.Bishttp://../link/61to88/7396_1.Bishttp://../link/61to88/8500.Bishttp://../link/11/11625.Bishttp://../link/12/12967_1.Bishttp://../link/12/12967_1.Bishttp://../link/12/12967_1.Bishttp://../link/11/11625.Bishttp://../link/61to88/8500.Bishttp://../link/61to88/7396_1.Bishttp://../link/31to60/4880_5.Bishttp://../link/31to60/4880_4.Bishttp://../link/15to30/2825.Bishttp://../link/15to30/2041.Bishttp://../link/15to30/2002.Bis

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Bureau of Indian Standard8BIS is a statutory institution established under the Bureau of Indian Stanahrdr Act, l986 topromote harmonious development of the activities of standardization, marking and qualitycertification of goods and attending to connected matters in the country.CopyrightBIS has the copyright of all its publications. No part of these publications may be reproducedin any form without the prior permission in writing of BIS. This does not preclude the free use,in the course of implementing the standard., of necessary details, such as symbols and sizes, typeor grade designations. Enquiries relatrng to copyright be addressed to the Director( Publications ), BIS.Revision of Indian StandardsAmendments are~issued to standards as the need arises on the basis of comments. Standardsdare also reviewed periodically; a standard along with amendments is reaffirmed w~hen such reviewindicates that no changes are needed; if the review indicates that changes are needed, it is takenup for revision. Users of Indian Standards should ascertain that they are in possession of thelatest amendments or editon by referring to the latest issue of l IS Handbook and *StandardsMonthly Addrtion.This Indian Standard has been developed from Dot : No. RVD 14 ( 17 ).

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Amend No. Date of Issue Text Affected

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