tail race surge tanks

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IS:7396(Part2) -1965(Reaffirmed 1995)

Indian Standard

CRITERIA FOR HYDRAUL IC DESIGN OFSURGE TANKS

PART 2 TAIL RACE SURGE TANKS

( First Revision )

First Reprint OCTOBER 1998

UDC 627.846.04

0 Copyright 1985

BUREAU OF INDIAN STANDARDSMANAK BHAVAN, 9 BAHADUR SHAH ZAFAR MARG

NEW DELHI 110002

Gr 4 October 1985

Reaffirmed 2000


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IS: 7396 Part 2 ) I 1985

I n d i an St an d a r d

CRITERIA FOR HYDRAUL IC DESIGN OF

SURGE TANKSPART 2 TAIL RACE SURGE TANKS

F i r st R evi si on

Water Conductor Systems Sectional Committee, BDC 58

ChairmanSHRI P. M. MANE

39, Shivaji Co-operative Housing Society, PuneMembers Rcpressnting

CHIEB ENGINEER Mukerian Hyde1 Project Design, Government of

DIRECTOR ( Alternale )Punjab, Chandigarh

CHI ER ENGINE ~ (HP ) Tamil Nadu Electricity Board, Coimbntore

SUPERINTENIXNQ NQINEI~R AltcrnatsCIXIEF ENOINEPL: GENERAL ) Public Works Drpartment, Government of TamilNadu, Madras

Cam,r ISNOINX~ER IRRIQATIOS 1.Afternafe )

CHIE F ENNOINEHR CIVIL DESIQNS Karnataka Power Corporation Limited, BangaloreSHRI P. R. MALL I KARJ UNA ( Alternate )

CHIEF ENQINI~ER Beas Sutlai L ink Project. SundernaaarSURI SUDERSHAN KUMAR ( Alternate )

_ ,

CHIEF ENQINEER ( IRRIQATION Public Works and Electricity Department,SOUTH ) Government of Karnataka, Mysora

SUPERINTENDINQ NQINEER ( Alternate )SHRI C. ETTY DARWIN In nersonal cauacitv ( Muttnda P. 0. Trioandrum 1DIRECTOR

DEPUTY DIRECTORDIRECTOR (HCD-I)

DEPUTY DIRE CTOR

Central Soils and Materials Research Statiod,New Delhi

( Alternate )Central Water Commission, New Delhi

( HCD-I ) ( Alternate )

( Conhmed on page 2 )

(6 Cofiyright 985BUREAU OF INDIAN STANDARDS

This publication is protected under the Indian Copyright Act XIV of 1957) andreproduction in whole or in part by any means except with written permission of thepublisher shall be deemed to be an infringement of copyright under the said Act.


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1s : 7396 Part 2 ) - 1985

( Cont i nued rom pqe 1 )

M cm hers Represent i ng

DR A. K. Dt_mts Central Mining Research Staticn Unit, RoorkceDR J . L . JETHW~ dItarnatc

SHRI J . P. GUPTA Power House Designs, I rr igation Department,Government of Uttar Pradesh, RoorkeeSHRI M. V. S. IYE NoAR The Hindustan Construction Company Limited,

New DelhiSHRI M. G. KUAN ( Alternate )

J OINT DIRECTOR RE YEASC~ Research Designs and Standards Organization,(GE-II ) Lucknow

SERI P. N. KHAR National Hydroelectro Power Corporation Limi-ted. New Delhi

SERI A. K. MEHTA National Projects Construction CorporationLimited, New Delhi

SHRS S. C. BALI ( Allerna6e )MEMBER ( CIVIL ) Kerala State Electricity Board, TrivandrumSERI G. PANT Geological Survey of India, New Delhi

SII HI N. K . MAN DWAL ( Alter e)SHRI A. R. RAI~HI J RSKRI K . SUN~I RI ~~DY

Innn~r~alp~a~eciiy (J t4a:), Garodi anag r,Electrmtty

Bo& ayd),

Hyderabad

SUPERI NTENDI~Q ENQINEER( DESIGN AND PL ANNI NG ) ( Alternals 1

SmmG. V. SATHAYE General Designs Organization, NasikDn H. R. SRAILMA General Electricity Authority, New DelhiSrcnr s. c. SEN Assam State Electricitv Board. Gauhati

SHRI N.K. DAS ( Allernnfr )SHRI A. K. SHIICANTIAH Himachal Pradesh State Electricity BoardSundernagar

SHRI RAXJ ~D~ S~son ( nltertzale )Sttnr G. RADIAN, Director General, IS1 ( Ex-oficio M ember )

Director ( Civ Engg )Secretary

SHRI HEYANT KTJMARAssistant Director ( Civ Engg ), IS1

Panel for Surge Tanks, BDC 58 _: P4

ConvenerSnnx P. C. SAXENA

M embers

Central Water and Power Research Station, Pune

SURI A. V. GOPALAKBIBHNA ( Al ternate toShri P. C. Saxena )

SEMI 0. P. DATTA Bhakra Beas Management Board, ChandigarhSIIRI J . S. KH URANA ( Alternate )

CHIEF ENOINEEII, CSVIL DIWIQNS ) Karnataka Power Corporation Limited,, BangaloreDIRXCI.OIL Institute of Hydraulics Hydrology, TrtuvelloreDIRECTOR ( HCD-I 1 Central Water Commission, New Delhi

Dt?,PtjTY DIRECTOR HCD-I ) ( Akernate )( Confinued ONpage 14 )

2


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ES : 7396 ( Part 2 ) - 1985

I ndiun St andard

CRITERIA FOR HYDRAUL ICSURGE TANKS

PART 2 TAIL RACE SURGE

DESIGN OF

TANKS

( Fi rst Rev i si on

0. FO-REWORD

0.1 This Indian Standard ( First Revision ) was adopted by the IndianStandards Institution on 31 January 1985, after the draft finalized by theWater Conductor Systems Sectional Committee had bee.11 approved by theCivil Engineering Division Council.

0.2 In an underground development, tail race surge tanks are usuallyprovided to protect tail race tunnel from water hammer effect due to flue-tuation in load. These are located downstream of turbines which dischargeinto long tail race tunnels under pressure. The ncccssity of tail race surgetank may be eliminated by ensuring free-flow conditions in the tunnel butin case of long tunnels this may become uneconomical than a surge tank.

0.3 This standard was first published in 1975. This revision has been madein view of the experience gained during the use of this standard. Thefollowing additions have been made in this revision:

a) Worst conditions for the design of tail race surge tanks,

b) Mannings formula for calculating the friction losses, and

c) Requirements for providing systemdrainage.

0.4 Depending upon the type of development,a single surge tank downstream of turbine or

tank system.with head race surge tank.

of drifts and galleries with

tail race surge tank may bemay form a multiple surge

0.5 Various types of development with tail race tunnel are classified intothe following from the point of view of hydrodynamics of tail race surgetank ( see Fig. 1 ):


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IS : 7396 Part 2 ) - 1985

Type A - Free-flow tail race tunnel without tail race surge tank,

Type B - Tail race pressure tunnel with tail race surge tank,

Type C - Tail race tunnel flowing either free or under pressuredepending on tail water level condition, and

Type D - Systems of head race and tail race surge tanks hydrauli-cally connected.

0.5.1 Arrangement of Type A is usual with development with almostconstant tailwater level. The tunnel is designed to accommodate surges, ifany, which are covered in IS : 7916-1975*. In case of long tunnels this maybe uneconomical.

0.5.2 Arrangement of Types B and C have only a single tail race surgetank.

0.5.3 Details of Type D of surge tank system are shown in Fig. 2.

0.6 This standard is being prepared in parts. Other parts of this standardare as follows:

Part 1 Simple, restricted orifice and differential surge tanks,

Part 3 Special surge tanks ( under preparation ), and

Part 4 Multiple surge tank systems.

0.6.1 Surges in open channels are covered in IS : 7916-1975*.

1. SCOPE

1.1 This standard ( Part 2 ) lays down the criteria for hydraulic design oftail race surge tank, hydraulically connected with a head race surge tankor without it ( that is, Types B, C and D ). However this does not covertail race surge tanks for pumped storage power houses.

2. NOTATIONS2.1 For the purpose of this standard, the following notations shall have themeaning indicated against each:

A8, = Cross-sectional area of head race surge tanks;AsI min = minimum As,;

A, = cross-sectional area of tail race surge tank;Aa rnii = minimvm A,,;

AtI = equivalent areas of head race tunnel;

Code of practice for open cllannek.

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IS : 7396 Part 2 ) . 1985

--IIL---_ -

TURBINE

/

TYPE C

TYPE DFIG. 1 DIFFERENT TYPES OF TAIL RACE DEVELOPMENT


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IS : 7396 Part 2 ) - 1985

NOTE When the tunnel cross-secti on comprisrs w nls lengths Li

of area ~~~ Ali may be taken as equivalent area of hcd race fwmd

CL1

4

-c( )1

At, = equivalent area of rail race tunnel;

NOTE _ When the twnrl cross-section comprises various lengths Liof area ~11, i t may be taken as equivalent area of tail race tunnrl

=__zLi_L1

C( 4 ti

Ath, = thoma area of head race surge tank;A& = thoma area of tail race surge tank;

s -= acceleration due to gravity;H = gross head on turbine, that is ( reservoir water level-

turbines nozzle level ), in the case of pelton wheel turbi-nes and ( reservoir water level - downstream tailwaterlevel ) in the case of reaction turbines;

Ho = net head on turbine;hfp = head loss in penstock;L1 = length of head race tunnel;

L2 = length of tail race tunnel;P = power generated;

Q = discharge through turbine ac any instant;

Q. = discharge through turbine under steady state condition;t = time reckoned fromstart of transient;

7-1 = period of oscillation for head race surge tank;

1s = period of oscillation for tail race surge tank;VI - velocity of flow in head race tunnel at any instant;Vs = velocity of flow in tail race tunnel at any instant;

VI0 = velocity of flow in head race tunnel corresponding tosteady state flow;Js0 = velocity of flow in tail race tunnel corresponding to

steady state flow;

Ztflo= total head loss in the head race tunnel up to surge tankin steady state condition;

Irf,o = total head loss in the tail race tunnel beyond surge tankin steady state condition;

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IS : 7396 ( Part 2 ) - 1985

Hfr = friction loss in head race surge tank in steady state condi-tion;

Hf2 = friction loss in tail race surge tank in steady state condi-tion;

5: = surge height in head race surge tank from reservoir levelmeasured positive upwards;

52 = surge height in tail race surge tank from tail water levelmeasured positive upwards;

PI = coefficient of hydraulic losses for head race tunnel up tosurge tank; and

l . = coefficient of hydraulic losses for tail race beyond tailrace surge tank.

3. TERMINOLOGY

3.1 For the purpose of this standard, the definitions given in IS : 7396( Part I )-1974* shall apply.

4. DATA REQUIRE~D

4.1 In addition to the data enumerated in IS : 7396 ( Part 1 )-1974*, thefollowing data are required for the design:

a) Length, cross-section and maximum discharge through tail racetunnel; and

b) Maximum and minimum tail water levels at the exit of tail racetunnel.

5. DESIGN

5.1 Design Conditions - The tail race surge tank shall be designed toaccommodate the maximum and minimum water level anticipated underworst condition as enumerated under 5.1.1 and 5.1.2.

5.1.1 The maximum upsurge level in the surge tank shall be worked outcorresponding to:

a) the full load acceptance at the highest downstream tail waterlevel; -and

b) where considered necessary, load rejection followed by specifiedload acceptance at the instant of maximum negative velocity inthe tail race tunnel, the downstream tail water level being at its

highest and higher of the two shall be adopted.Criteria for hydraulic design of surge tanks : Part 1 Simple, restricted orifice and

differential surge tanks.

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IS I 7396 Part 2 ) - 1985

5.1.2 The minimum down surge level in the surge tank shall be workedout corresponding to:

a) the full load rejection at the lowest downstream tail water level;and

b) where considered necessary specified load acceptance followed byfull load rejection at the instant or maximum positive velocity inthe tail race tunnel the downstream tail water level being at itslowest and the lower of the two shall be adopted.

NOTE This presumes that the load acceptance or rejection by different turbi-nes in the stations shall be such as not to create super position of surge.

5.1.3 For tunnels flowing full, friction loss may be calculated either byusing Mannings formula or Darcy-Weisbach formula:

5.1.3.1 M annings formula:

The formula is given below:h = vs.Ns_L

f R 4 3

where

hf = head loss due to friction in metres;V = velocity of wzter in the tunnel, in m/s;N = rugosity co-efficient;1: = length of the tunnel, in metres; andR = hydraulic radius

AreaWeYtIGiEGG

, in metres.

5.1.3.2 For concrete lined tunnels, the value of N varies from 0.012to o-014.

5.1.3.3 For unlined tunnels, the value of .N depends upon the natureof rock and the -quality of trimming. Recommended values of N for variousrock surface conditions are given below:

Surface characteristics Val ue of .N

M & & i

*-_-c---~

Maximum

Very roughSurface trimmedSurface trimmed and invert

concreted

o-04 O-060.025 o-0350.02 0.03

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18~ 7396 ( Part 2 ) - 1985

5.1.3.4 The values of p shall then be determined from the followingformula:

p vs =V N L

____- + Other losses in tunnel system.R419

5.1.3.5 Darcy- Weisbach formulaThe formula is given below:

5.1.3.6 The value of @ shall then be calculated from the followingformula:

p va a 4%;. + Other losses in tunnel system.

5.2 Criteria for stability shall be ensured for the tail race surge tankfunctioning as single surge tank or functioning in unison with head racesurge tank.

5.2.1 Tail surge tank functioning as single surge tank for the arrange-ment in Types B and C shall satisfy the Thoma criteria for stability andthe dimension of the surge tank shall be fixed as for a simple surge tarikgiven in IS : 7396 ( Part 1 )-1974*.

5.2.2 Where the tail race surge tank is functioning in unison with head

race surge tank ( Type D ) the minimum cross-section of head race andtail race surge tanks shall satiscy the criteria given below to ensurestability:

At,& = _At i...L12 g PI HoAt, * La

Ath, - -.2gk4Ho

. ..(I)

...wAs, min = ( 1 + Ctr ) Ath, . . . (3)

As, min = ( 1 +$-) Aths

where Y is given by equation (5), and Ct is a constant and funclion ofoscillation periods of two tanks. Value varies over a very wide limit ( from0.2 to 1-o ).

For preliminary design a value of Ct = 0.7 may be assumed

r = AsL_ +As, min . ..(5)

Also, AsI min = ( r + C, ) Ath, . . . (6)_I_-_---

*Criteria for hydraulic design of surge tanks: Part 1 Simple, restricted orifice anddifferential surge tanks.

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IS : 7396 ( Part 2 ) - 1985

5.2.3 From equations given in 5.2.2 stability curve shown in Fig. 3 isobtained to separate the stable and unstable regions. The greater of ~thevalues of ASI min obtained from equation (1) and (6) shall be taken toobtain stability curve. The values of As, and As, adopted shall fall withinstable zone.

5.2.4 The cross-sectional arra adopted shall be such as not to create:resonancebetween the oscillation in two t:utl;s, that is, the ratio of oscilla-tion periods, r,/T,, shall be outside the range 0.8 to 1.2. The ratio ofoscilIation periods is expressed as:

or

As, = -++ (+- ) x As, .*12 2

The straight lines plotted for Y-,/ T2 - 0.8 and 7,/ T2 = I .2 formthe limit lines for range of possible resonance. The values of .4sl and As,shall not fall in this zone as shown in Fig. 3 to avoid resonance.

5.2.5 The size of surge tanks to k~e adopted finally is also governed bytopographical and geological conditions at surge tank sites.

5.3 Factor of Safety - The minimum size ( area ) of surge tank obtai-ned from 5.2 be increased to -ensure proper functioning of the system[ refer IS : 7396 ( Part 1 )-1974*].

5.4 Surge Analysis

5.4.1 The distance between the tail race surge tank and draft tube shall

be as short as possible to reduce the negative water hammer wavesdeveloping in the draft tube.

5.4.2 The extreme water levels in the t.wo surge tanks for the conditionsenumerated in 5.1 are determined by integrating numerically the equationsof mass oscillation for the two surge tanks along with governing equations.The dynamic and continuity equations shall be modified suitably for anyparticular type of surge tanks, that is, simple, differential or restrictedorifice surge tanks.

5.4.3 The basic equations for double simple surge tank system are: asfollows ( see Fig. 3 ):

*Criteria for hydraulic design of surge tanks: Part 1 Simple, restricted. orifice arIddifferential surge-tanks.

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IS-: 7396 ( Part 2 ) - 1385

STABLE

0 30 is 60 75

As2 ,m2---+

FIG. 3 CRITICAL G-ABILITY CUIWE FOR A SURGE TANK SYSTEM

a) Dynamic equation

. . . (9)

. ..( 10)

b) Continuity equation

At,. V, = A+f ~ t Q . ..(I 1)

0_ - 1h.J. ~--atG + At, v, . ..( 12)


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IS : 7396 Part 2 ) - 1985

c) Governing equation at constant output

NOTE - Positive value of p1,V14 or ~.Va~ shall he used when the flow isdownstream and negative when it is upstream.

6. HEIGHT OF SURGE TANK

6.1 Height *of surge tank shall be fixed as specified in 6 of IS : 7396( Part 1 )-1974*.

7. GENERAL REQUIREMENTS

7.1 In order to obtain as much geological information as possible beforehand and to suggest suitable design it is advised to provide an exploratorydrift. The drift should be located as close to the proposed surge shaft aspossible. In most of the cases, it would be ideal to locate the drift in sucha manner that it could be later enlarged and used for ventilation and accesscum expansion chamber. The size and shape of the expansion chambercould be designed suitably based upon the geological information obtainedfrom the drift. The extent of seepage water likely to be met could also be

gauged from the data obtained in exploratory drift and suitable drainagesystem could be suggested. Depending upon the quanti.ty of seepage watermet with, either a set of drainage holes or a system of collecting gallerieslocated around the surge shaft may be provided .to tap the water andprevent the build up of excessive water pressure behind the surge shaftlining.7.2 For all important works the hydraulic aspects may be studied onhydraulic models.

7.3 Water level indicators and or recorders for both the surge tanks shouldbe provided in control room for controlling the load acceptance.

Criteria for hydraulic design of surge tanks : Part 1 Simple, restricted orifice anddifferential surge tanks.

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IS t 7396 Part 2 ) - 1985

Continued from page 2 )

M embers Reprssent in g

DIRECTOR T 8z P ) Thein Dam Design Organization, ChandigarhSENIOR D~SIQN EWXNEER PENSTOCKS )

( Alternate )SKI J . P. GUPTA Irrigation Department, Government of Uttar

Pradesh, RoorkeeSHRI KAMAL KUMAR J AIN Alfsrnufc )

SHRI J . L. KHOSA Bharat Heavy Electricals Limited, BhopaIDR ZAPAR MEHDI Alternate )

SHRI K. RAYABHADRAN NAIR Kerala State Electricity Board. TrivandrumDR H. R. SEARMA Central Electricity Authority, New DelhiSTJ PERINTENDIN~ ENGINEER Pench Hydro-Electric Project, Nagpur

EXECXJ TIVE ENGINEER Altsrnats )

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tail race surge tanks

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