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है”ह”ह

IS 7396-1 (1985): Criteria for hydraulic design of surgetanks, Part 1: Simple, restricted orifice and differentialsurge tanks [WRD 14: Water Conductor Systems]

IS : 7396 ( Part 1) - 1985 ( Reaffirmed 1995 )

Indian Standard

CRITERIA FOR HYDRAULIC DESIGN

OF SURGE TANKS

APART 1 SIMPLE RESTRICTED ORIFICE AND DIFFERENTIAL SU-RGE TANKS

( First Revision )

Gr 6

First Reprint JUNE 1998

UDC 627.846 : 624.04

0 Copyright 1986

BUREAU OF INDIAN STANDARDS MANAK BHAVAN, 9 BAHADUR SHAH ZAFAR MARG

NEW DELHI 110002

1 September 1986

IS : 7396 ( Part 1) - 1985

Indian Standard CRITERIA FOR HYDRAULIC DESIGN

OF SURGE TANKS

PART 1 SIMPLE, RESTRICTED ORIFICE AND DIFFERENTIAL SURGE TANKS

( First Revision )

Water Conductor Systems Sectional Committee, BDC 58

Chair man

SHRIP. M. MANE 39, Shivaji Co-operative Housing Society, Pune

Members Rep resenting

CHIEF ENGINEER Mukerian Hyde1 Project Design, Chandigarh DIRECTOR ( Affernate )

CHIEF ENGISEER (HP) Tamil Nadu Electricity Board, Coimbatore S[!PZI:INTI NL)I~G ENGINEER ( Alternate )

CHIEF ENGINEFR (General) Public Works Department, Madras CHIEF E%GI\EER (IRRIGATION) (Alternate )

CHIEF ENGINEER (CIVIL DESIGNS) Karnataka Power Corporation Limited, Bangalore

SHRI P. R. MALLI KARJUNA ( Alternate ) CHIEF ENGINEER Bhakra Beas Management Board, Chandigarh

SHI~I SU[)EKSHAN KUMAR ( Alternate ) CHIEF ENGINEER ( IRRIGATION Sours ) Public Works and Electricity Department, Mysore

SUPFRINTENDIFG ENGINEER ( Alternate ) SHRI C. ETTY DARWIN DJRECTOR

In personal capacity ( P.O. Mufturla, Trivandrum ) Central Soils and Materials Research Station,

New Delhi DEPUTY DIRECTOR ( Alternufe )

DIRECTOR ( HCD-I ) Central Water Commission. New Delhi DEPUTY DIRECTOR ( HCD-I.) ( Alternate )

DR A. K. DUBE Central Mining Research Station Unit, Roorkee DR J. L. JETHWA ( AIternate )

DR B. PANT WateeoR;;Trces Development Training Centre,

( Continued on page 2 )

0 Copyright 1986 b I

BUREAU OF INDIAN STANDARDS

I This publication is protected under the Indian Copyright Act ( XIV of 1957 ) and I reproduction in whole or in part by any means except with written permission of the publisher shall be deemed to be an infringement of copyright under the zaid Act. I

IS : 7396 ( Part 1) - 1985

( Continuedffompuge 1 )

Members Representing

SHRI J. P. GUPTA Power House Designs, Irrigation Department, Government of Uttar Pradesh, Roorkee

SHRI A. P. GUPTA ( Alternate ) SHRI M. V. S. TYENGAR

SHKI M. G. KHAN ( Alternate ) JOINT DIRECTOR RESEARCH ( GE-II )

The Hindustan Construction Co Ltd, New Delhi

Research, Designs and Standards Organization, Lucknow

SHRI P. N. KHAR National Hydroelectro Power Corporation Ltd, New Delhi

SHRI A. K. MEHTA National Projects Construction Corporation Limited, New Delhi

SHRI S. C. BALI ( Alternate ) MEMBER ( CIVIL ) SHRI G. PAN-I’

Kerala State Electricity Board, Trivandrum ~Geological Survey of India, New Delhi

&RI N. K. MANDWAL ( Alternate ) SHRI A. R. RAICHUIC In personal capacity ( 147, Garodiunugor, Bombay ) I SHRI Y. RAMA KRISHNA RAo Andhra Pradesh State Electricity Board, Hyderabad

SUPERINTENDING EKGINEER (DESIGN AND PLANNING ) ( AIternate )

SHRI G. V. SATMAYE General Designs Organisation, Nasik DR H. R. SHARMA* General Electricity Authority, New Delhi SHRI S. C. SF.N Assam State Electricity Board, Gauhati

SHRI N. K. DAS ( Alternate ) SHRI A. K. SRIKAN?TAH Himachal Pradesh State Electricity Board,

Sundernagar SHRI RANJODH SINGH ( Alternate )

REPRESENTATIVE Central Water and Power Research Station, Pune SHRI A. V. GOPALAKRISHNA ( Alternate )

SHRI G. RAMAN Director General, IS1 ( Ex-officio Member ) Director ( Civ Engg )

Secretary SHRI HEMANT KUM~R

Assistant Director ( Civ Engg ), ISI

Panel for Surge Tanks, BDC 58 : P4

Convener

DR H. R. SHARUA

Members

Central Electricity Authority, New Delhi

CHIEF ENGINEER ( CIVIL DESIGNS > Karnataka Power Corporation Ltd, Bangalore SHRI O.-P. DAT~A Bhakra Beas Management Board, Chandigarh

SHRI J. S. KHURANA ( Alternate ) DIRPC~OR Institute of Hydraulics and Hydrology, Public

Works Department, Poondi DIRECTOR ( HCD-I ) Central Water Commission, New Delhi

DEPUTY DIRECTOR ( HCD-I ) ( Alternate )

*Chairman for the meeting. ( Continued on page 21 )

IS : 7396 ( Part 1) - 1985

Indian Standard CRITERIA FOR HYDRAULIC DESIGN

OF SURGE TANKS

PART 1 SIMPLE, RESTRICTED ORIFICE AND DIFFERENTIAL SURGE TANKS

( First Revision )

0. FOREWORD

0.1 This Indian Standard ( First Revision ) was adopted by the Indian Standards Institution on 31 December 1985, after the draft finalized by the Water Conductor Systems Sectional Committee had been approved by the Civil Engineering Division Council.

0.2 The surge tank absorbs the water hammer or elastic shock waves com- ing from the pressure pipe line or tunnel on closure or opening of the valve or turbine gates and also to supply/store additional water during load acceptance/rejection during load demand/rejection until the conduit velocity has accelerated/decelerated to the new steady state value. Surges or mass oscillations occur in the surge tanks, the hydraulic design of which is so done as to keep the surges within reasonable limits.

0.3 This standard is being prepared in parts. Other parts of this standard are as follows:

Part 2 Tail race surge tanks

Part 3 Special surge tanks ( under prepration )

Part 4 Multiple surge tanks

0.4 This standard was first published in 1974. This revision has been made in view of the experience gained during the course of these years in use of this standard. Manning’s formula has been added in this revision for calculating the friction losses.

1. SCOPE

1.1 This standard ( Part 1 ) lays down the criteria for hydraulic design of simple, restricted orifice and differential surge tanks.

3

IS : 7396 ( Part 1) - 1985

2. TERMINOLOGY 2.0 For the purpose of this standard, the definitions given in IS : 4410 ( Part 10 )-1969* and the following shall apply. 2.1 Simple Surge Tank - A simple surge tank is a shaft connected to pressure tunnel directly or by a short connection of cross-sectional area not less than the area of the head race tunnel ( Fig. 1 ).

FIG. 1 SIMPLE SURGE TANK

2.2 Restricted Orifice Surge Tank - A simple surge tank in which the inlet is throttled to improve damping of oscillations by offering greater resistance and connected to the head race tunnel with or without a connecting/ communicating shaft ( Fig. 2A and 2B ).

SURGE SHAF 1

2A 28

FIG. 2 RESTRICTED ORIFICE SURGE TANK

*Glossary of terms relating to river valley projects: Part 10 Civil works of hydro- electric generation system including water conductor system.

4

IS : 7396 ( Part I ) - 1985

2.3 Differential Surge Tank - Differential surge tank is a throttled surge tank with an addition of a riser pipe may be inside the main shaft, connect- ed to main shaft by orifice or ports as shown in Fig. 3A. The riser may also be arranged on one side of throttled shaft as shown in Fig. 3B. In case of type shown in Fig. 3A port holes are generally at the bottom of the riser at the sides.

OF THE : TANK

Z _I-.. Zl

*, RL t

3A 36

FIG. 3 DIFFJXRENTIAL SURGE TANK

2.4 Load Acceptance - It is a sudden increase power station.

2.5 Load Rejection - It is a sudden decrease in power station.

in electric load upon the

electric load upon the

2.6 Speed-No-Load - It is a condition where the turbine is ‘put on the line and-is synchronized but its power output is insignificant and is ready to pick up any load that may come.

3. NOTATIONS

3.1 For the purpose of this standard, the following notations shall have the meaning indicated against each:

Ao =

AP =

At =

A, =

A, =

Area of orifice or ports

Cross-sectional area of penstocks

Area of riser of differential surge tank

Net cross-sectional area of surge tank

Cross-sectional area of head race tunnel

5

IS : 7396 ( Part 1) - 1985

J&h = Thoma area of surge tank

c = Velocity of propagation of pressure wave

Cd = Coefficient of discharge of orifice or ports

CSP =

D=

E =

El =

Es =

Ew =

f=

F, =

g=

H=

Ho =

hr =

hrp =

ho, =

hod =

K =

Km =

Coefficient of discharge of riser spill

Diameter of head race tunnel

Young’s modulus of elasticity of the material of the conduit wall (steel)

Young’s modulus of elasticity of concrete

Young’s modulus of elasticity of the rock

Young’s modulus of elasticity of water

Friction factor governing head loss [to be taken from IS : 4880 ( Part 3 ) - 1976” ]

Factor of safety over Ath

Acceleration due to gravity

Gross head on turbines

Net head on turbines

Total head loss in head race tunnel system

Total head loss in penstock system

Head loss in orifice for restricted orifice surge tank

Head loss due to orifice for differential surge tank

Ratio of total power generated by the station to that of grid

A constant depending upon the efficiencies of turbine and generator

L= Length of head race tunnel

Ls, = Length of riser spill in crest

m= Reciprocal of Poisson’s ratio for rock

P= Power generated

‘Code of practice for design of tunnels conveying water: Part 3 Hydraulic design.

6

Ph =

Q=

Qd =

Qo =

R, =

R, =

t=

V’ =

yl, =

vo* =

v,* =

v,* =

y* -_

Vl *=

VP =

w=

yo =

Z=

znl =

IS:7396(Part1.)-1985

Pressure due to water hammer in the conduit upstream of surge tank

Discharge through turbines

Maximum discharge supplied by the surge tank in case of specified load acceptance

Maximum discharge through the turbines

Internal radius of the pressure conduit

Outer radius of the pressure conduit

Time, reckoned from the start of transient

Volume of water in surge tank corresponding to Z

Volume of water in the conduit in a given time interval At = V,At. At

Velocity of flow in tunnel corresponding to maximum steady flow, upstream of surge tank

Velocity of flow in tunnel at any instant, upstream of surge tank

Velocity of flow in conduit at any instant, downstream of surge tank

Velocity of flow in tunnel corresponding to change in dis- charge upstream of surge tank = vl-vr

Velocity of flow in tunnel corresponding to initial steady flow (upstream of surge tank)

Velocity of flow in tunnel corresponding to final steady flow (upsteam of surge tank)

Specific weight of water

Difference between reservoir level and the centre line of the pipe

Water level in surge tank measured positive above reservoir level

Maximum surge level above maximum reservoir level

*To be taken as positive when the flow is downstream and negative when flow is upstream.

7

4

IS : 7396 ( Part 1) - 1985

Maximum or minimum surge height measured from final steady state level in surge tank

Height of riser top measured positive above reservoir level

Surge height corresponding to change in discharge neglecting friction and orifice losses

2, =Vo L At li -F-A,

Water level in riser of differential surge tank measured positive above reservoir level

Coefficient df hydraulic losses such that hr = 8 V,

Coefficient of resistance for orifice such that

ho, = ?j (V, - VA2

Overall efficiency of the station, inclusive of pipe-line losses

Overall efficiency of the station inclusive of pipe line losses at maximum discharge through the machines

Ratio of area of surge tank to that of’conduit = -$

NOTE - Whenever units are used the equation shall be balanced.

4. DATA -REQUIRED

4.1 The following data are required for the hydraulic design of surge tanks:

a> b) cl

4

e) f-1 d h)

Length, diameter and maximum discharge of head race tunnel;

Length, diameter, number of penstocks and discharge of each;

Geometrical details of entry control structures, bends, transitions, etc, which contribute to hydraulic losses;

Gross head on generating units;

Operating water levels of reservoir; Power generated by the station; Total power in the grid;

Normal variation of load in the grid and the percentage shared by the station;

*To be taken as positive when the flow is downstream and negative when flow is upstream.

8

IS:7396(Partl)-1983

j) Number of machines along with their characteristics, including efficiency, time-closure/opening of gate or deflector;

k) Critical load changes;

m) ,Boundary friction factor and other coefficients for hydraulic losses for maximum, minimum and average conditions; and

n) Provision of pressure relief valves,’ if any, and their characteristics.

5. DESIGN

5.1 Design Conditions

5.1.1 The surge tank shall be designed to accommodate the maximum and minimum water levels anticipated under worst conditions ( see 6.1.3 ). The conditions given in 5.1.1.1 and 5.1.1.2 shall be considered for design.

5.1.1.1 The maximum upsurge level in the surge t&k shall be worked out correspondiilg to:

a) the fd load rejection at the highest reservoir level, land

b) where considered necessary specified load acceptance followed by full ‘load rejection at the instant of maximum velocity in the head rape tunnel and higher of the two shall be adopted.

5.1.1.2 To obtain minimum downsurge level the worst of the following two conditions shall be considered:

a)

b)

Full load rejection at minimum reservoir level followed by specified load acceptance at the instant of maximum negative velocity in head race tunnel, and

SpCcified load acceptance at load or speed-no-load condition at the minimum reservoir level.

NOTE - This presumes that the load acceptance by different turbines in the station shall be such so as not to create superposition of surges.

5.2 Coefficient of Hydraulic Losses

5.2.1 For tunnels flowing full, friction loss may be calculated either by using Ma!,-;ing’s formula or Darcy-Weisbach formula.

5.2.1 i :‘,i I) :~,v~‘s formda

.The formula is given below:

hr = V2 N2 L

&I” . . . . . . .

9

IS : 7396 ( Part 1) - 1985

where

5.2.1.2 0,014.

hr = head loss due to friction in metres,

V = velocity of water in tunnel in m/s,

N = rugosity coefficient,

L = length of the tunnel in metres, and

R = hydraulic radius.

For concrete lined tunnels, the value of N varies from 0,012 $0

5.2J.3 For unlined tunnels, the value of N depends upon the nature of rock and quality of trimming. Recommended values of N for various rock surface conditions are given below:

Surface Characteristics

Very rough

Surface trimmed

Surface trimmed and invert concreted

Value of N r--__h--_ Minimum Maximum7

o-04 0.06

o-025 0.035

0.02 o-03

5.2.1.4 The value of p shall then be determined from the following formula:

v2 N2 L gvS =-__ PI3

$ other losses in tunnel system . . . . . . (2)

5.2.2 Darcy- Weisbach Formula

The formula is given below:

jjr = $!$ . . . . . . (3)

5.2.2.1 The value of p shall then be calculated from the following f formula:

fZv2 B r2 = 2gD + other losses in the tunnel system . . . . (4)

10

IS : 7396 ( Part 1) - 1985

5.2.3 As the minimum coefficient gives the worst upsurge while the maximum gives the worst downsurge. These value; shall be used for relevant conditions. In case of combination of load variations both the values of coefficient shall be taken into account and the one which gives the worst, shall be adopted.

5.3 Area of Surge Tank

5.3.1 To ensure the hydraulic stability of surge tank, its arc ’ hs governed by Thoma criteria. Considering a station in isolation, ‘I I~~_.~.. criteria is:

Ath = L At VI”

@VI” Ho ’ 2 g . . . . . . (5)

NOTE - The minimum value of p shall be used in the above formula.

5.3.2 If the reservoir level varies significantly, Ath may vary appreciably between maximum and minimum reservoir levels and in such cases the maximum value of Ath shall be adopted.

5.3.3 If it can be ensured that the power station is to always operate in a grid, the stabilizing effect of the grid may be taken into account and the area of surge tank in that case may be calculated from the following formula:

A, = Ata [l--1*5 (Z--K)] ._. . . . . (6)

5.4 Factor of Safety - Area of surge tank calculated by equation (5) or (6) gives the minimum area of surge tank. The following minimum factor of safety on the area shall be used to secure rapid rate of damping required in practical operation:

Type of Surge Tank Factor of Safety

Simple 2.0

Restricted orifice or differential l-6

5.5 Extreme Surge Level

5.5.1 The extreme water levels in the surge tank for the conditions enumerated in 5.1 are determined by integrating the equations of mass oscillations fo,r the respective tanks. These equations are non-linear. As such they are integrated either graphically ( for example, Schoklitsh’s method, see Appendix A ) or numerically ( for example, Pressel’s method ).

IS : 7396 ( Part 1) - 1985

5.52 Simple Surge Tank - The sat&i&:

following equations of AOW should be

L dV, - -:- 2 f P VI2 + 7 dt

-‘L -3 0 (dynamic equation ) 2g

. . . . (7)

NOTE - Positive value of p y12 and $- shall be used when the flow is from

reservoir to surge tank,, negative when the fiow is towards the reservoir.

dZ At V, - A, dt + Q (continuity equation) . . . . . .

( VI” P=KmQ H+Z+ 2g __ - hf, >

(power equation) . . . . (9)

The maximum surge level for the case of complete load rejection may be determined from the following formula:

I; zm L -- 2 g+ @” YO’L - -$ V-02 2 g 9 B” voa

C e- L$ (2, A- @ Vo2) ] = 0 . . . . . . (10)

55.3 Restricted Orifice Surge Tank - The surges will depend on the resistance offered by the orifice. As such the size and shape of the orifice should be decided first.

5.5.3.1 The resistance offered by the orifice of area A, shall be calculated from the following formula:

Q02 h,, = -- cd2. A,a. 2 g

. . . . . .

The values of the coefikient of discharge Cd usually vary between 0.6 and 0.9 depending on the shape, size and number of orifices. The area of the orifice is so chosen as to satisfy the condition given by Calame and Gaden for maximum flow which is as follows:

$2: + 4 hr < k, 6 &= + + hf . . . . . . (12)

5.5.3.2 It is desirable to provide .area of orifice such that the pressure in the tunnel at orifice due to water hammer caused by total load rejection is nearly equal to the pressure due to maximum rise of water level in the surge tank at the time of worst upsurge. Fbr any area of the orifice the maxifium surge and the water hammer pressure are determined from 5.5.3.3 and 5.5.3.4. ‘5.

12

IS : 7396 ( Part 1) - 1985

5.5.3.3 The maximum surge above the reservoir level shall be derived from the equation:

L zln 2g~@-tqY~,” - (p+?) vo2

B + I-- -- C

L (Q + 9) 2 g Q (B + q)” vo2 1

~[e-y (p+ rl) Zm+PVoz)] L- 0 . . . . . . (13)

The curves given in Fig. 4 may also be used for calculation of maximum surge. These values may be checked by numerical integration of the following equations:

L dV1 g*-iig- + 2 f p v,z f ?j (VI - v2 )2 + qg = 0 . . . . . . (14)

NOTE - The values of p VI2 and i$ shall be positive or negative in accordance

with Note under 5.52 whereas (VI - and negative tor VI < V,.

VS)% shall be taken as positive for VI > V,

At VI = -4 g + Q . . . . . . (IS)

A, += -F

cd A, 42 g ho, .

P=Km Q H + Z + -;$- - hfp zk ho, . . . . . . (17)

5.5.3.4 The water hammer pressure in the conduit upstream of surge tank shall be calculated from the following equation:

A2.4 VCl XT@-- c

. . . . . . (18)

NOTE -This formula is approximate in as much as it does not consider time closure characteristics of valves [ see 4.1 ( j ) ] but is considered accurate enough for the purpose in view here.

The assumed area of the orifice should be so altered that the values of water hammer pressure and the pressure due to upsurge are nearly the same.

13

IS : 7396 ( Part 1) - 1985

f-0 2-o

1a+w2

z,

FIG. 4 MAXIMUM UPSURGE FOB TOTAL REJECTION

5.5.3.5 The velocity of propagation of a pressure wave c is computed as below:

a) For a thin pipe the thickness of which is small as compared to its diameter,

14

IS:7396(Partl)-1985

[ g/w D 111

C= Ww + E.e 1 . . .

b) For a thick walled pipe,

C= C glw 1s

l/Ew + (2/E) {(h2 + &*)/(&” - &*)} 1

. . . . . . (20)

c) For a steel and concrete lined tunnel,

g/w C

= l/E,, + 2 (1 - A) . . . . . . (21)

where

x = 2; + &+&la i (m+mz Rt . . . . . . (22) 2 1 * d

5.5.3.6 For preliminary designs of thin-walled pipes the following table gives sufficiently accurate values:

yo, m 10 30 60 100 140 300 600

c, m/set 665 717 777 837 883 1 000 1 110

5.5.3.7 Any shape of the orifice may be adopted but a circular shape is preferable. If the gates are to be provided in the surge tank, the gate slots generally function as orifices and additional area as necessary may be provided in the form of a circular hole at the top of the tunnel in the riser.

5.5.3.8 The orifice should be kept as near to the head race tunnel periphery as possible. Suitable shapes and streamlinings shall be provided at the top and bottom of orifice to obtain the desired coefficients for the up and down surges.

5.5.3.9 It is recommended that model tests should be conducted to determine the required streamlinings on the top and bottom of the orifice and the coefficient of discharge for entry and exit. Model experiments should test the behaviour for same range possible in these values between model and prototype.

5.5.4 Diflerential Surge Tank - The surges that occur in a differential surge tank will depend on the-design of riser and orifice.

15

IS : 7396 ( Part 1) - 1985

5.5.4.1 Riser - The area of the riser shall not be less than three- fourths of the area of the conduit to avoid rapid changes in the water level and facilitate proper governing of machines. The top of the riser should be so chosen that the maximum level of the water spilling over the riser is less than or equal to the maximum surge level in the main tank. The rapidity of change in water levels should be related to characteristics of turbine governor and minimum area acceptable to turbine governor shall be provided.

5.5.4.2 OriJce - The orifice is primarily designed to supply water in case of specified load demand. In case of rejection the area of the orifice may be kept as small as possible because the upsurge is restricted by the riser. The area of the orifice can be determined from the following formula:

A, = -___Qd___ Cd &z2g

. . . . . . (23)

5.5.4.3 The equations of flow to be satisfied are as follows:

L dV, - dt + Z1 f pV12+ -F$ = 0 (dynamic equation) . . . . (24) s

VI2 NOTE - Positive values of BV,’ and - 2g

shall be used when the flow is from

reservoir to surge tank, negative when flow is towards the reservoir.

P = K,Q (

H+Zr+T- 2g

hf, >

(power equation) . . . . (25)

Continuity equations for the various conditions of flow in surge tank are as follows_ -

a) For Zr greater than Z,, and Z

At VI = A, $r + CS, Lsp ( Zl - Zqp

+ CdAodWZI-Z+Q

b) For Zr equal to Z and greater than ZSp

dz At V, =-’ A, %L + A, -;li- -t Q

c) For Zr and Z less than ZSp

4 2 si.3 --I +Q

. . . . (26a)

. . . 1 WW

. . . . (26~)

16

IS : 7396 ( Part 1) - 1985

NOTE - Positive values of cd A0 2/ 2 g/z, _ 2, shall be taken when 2, is greater than Z and negative when Z1 is smaller than 2.

d) For Z1 less than Z,, and 2 greater than Zrp

da At vt = A, d t - - cs, Lap (Z -zsP)3’3

- cd A, %b(Z---d + Q . . . . . . . . . (2W

6. HEIGHT OF SURGE TANK

6.1 The top of the surge tank may be kept 1.5 m above the highest water level while the bottom of the surge tank may be kept 2.0 m below the lowest surge level determined by calculation or by model experiments ~for the worst conditions. The extra provision at the top may be in the form bf a solid retaining wall wherever surge tank is brought to the surface and is covered all around by such a wall. It should be ensured that there is a sufficient water cover over the tunnel to avoid vortex formation even at the lowest water level in the surge tank. If likelihood of vortex formation is established by model experiments, vortex-suppression devices may be con- sidered. Model and prototype behaviour in respect of vortex formation can differ substantially. Therefore adequate allowance should be given while fixing the minimum water cover.

6.1.1 If the topography near the surge shaft area permits, suitabe arrange- ments for spillover may be considered so as to restrict the surge height [ see IS : 7396 ( Part 3 )* 1.

7. GENERAL REQUIREMENTS

7.1 In orifices and ports where the velocities are so high as to cause erosion and cavitation damage, steel lining should be provided at these locations.

7.2 For all important works, it is recommended that the results are verified by model studies.

7.3 With the availability of high capacity computer analysis of surges is now easier by mathematical model and should be adopted as far as practicable.

7.4 Surge tank water level indicators-or recorders should be provided in the controlyoom for controlling the-load acceptance.

*Criteria for hydraulic design of surge tanks : Part 3 preparation ).

17

Special surge tank3 (under

IS: 7396 ( Part 1 ) -1985

APPENDIX A

( Clause 5.5.1 )

SURGE ANALYSIS BY SCHOKLITSCH GRAPHICAL METHOD

The main graph consists of a curve relating Z to VI; the time t istreated as a variable parameter so that the curve relating Z to t followseasily ( Fig. 5 ).

The origin 01 of VI — Z plane of coordinates is fixed at the staticwater surface ( reservoir level ). The Z scale is the same as that of surgetank.

The parabola of hydraulic losses ( hf = -& ~V1z ) is plotted taking hfalong the Z-axis and VI as abscissa, Izf is plotted downwards if ‘VI > 0( curve 1 ).

Assuming a constant value of time interval At, the straight line re-presenting the equation:

Avl=— j+. z . . . ...(i)

is plotted which also passes through the origin 01. A V1 is plotted alongVi-axis and has the same scale ( curve 2).

Another straight line is drawn connecting Vi as abscissa with Vt’ asordinate according to the relation:

Vt’ = vi At At . . .. (ii)

This also passes through the origin O1. It is convenient to plot Vi posi-tive downwards for VI > 0 ( curve 3).

A curve is drawn, with Oz as origin connecting the volume of surgetank, V’ to the elevation, Z. V’ is plotted as abscissa and Z as ordinate( curve 4 ). The origin of the cuxve which represents the equation

A~ = A,. AZ ( iii)

is selected at a level below the lowest water level anticipated in the surgetank.

With origin as 03, curve ( curve 5 ) is drawn with Z as ordinate andQ. bt as abscissa from the relatlon ( governing equation ):

18

. . . . .. (iv)

.,

*

s0 a“-L-------

N●

●

✼

N

IS : 7396 ( Part 1) - 1985

The starting point of calculation is at the steady state condition so that Via and Za (---_PV”I~) are known. A straight line al b, drawn through point al on curve 1 parallel to curve 2. At the initial instant &,, the quantity of water flowing through the turbine is Qa. At and is known from curve 5. The value of Vt’ at the initial stage is known from curve 3. Graphical subtraction yields:

AV’, = V’ta - Qa.nt I VI

By plotting AVla on curve 4, point b, is obtained. Point b, is obtained by intersection of the horizontal straight line through b, and the sloping line through al. Point bl lies vertically below b, on curve 1. This point bl represents the starting point for a similar process to obtain points c4 on curve 4, and c,, cl, etc.

Point b, lies on the curve of Z plotted against ~1, with its coordinate Vlb == VI, + AVl and Zb = Za + AZ. The curve representing the oscillation may be drawn through points al, b,, c, and d,. The correspond- ing value oft is always known, and curve 7 may be plotted which relates Z and t.

NOTE - In Schoklitsch method the coordinates 2 and V, of a point a,, b,, c,. . . . relate to the beginning of the time interval n t and are not mean values as are used in Pressel’s analytical method. This fact is responsible for the simplicity of the Schoklitsch method, but it demands that small time intervals At be employed to yield accurate results.

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rs:73%(Fartl)-1985

(Continaedfrompage 2 )

Members Representing

DIRECTOR(T&P) Irrigation Department, Government of Punjab, Chandigarh

SENIOR DESIGN ENGINEER ( PENSTOCKS ) ( Alternate ) SHRI A. V. GOPALAKRISHNA Central Water and Power Research Station.

Pune SHRI J. P. GIJPTA Irrigation Department, Government of Uttar

SHRI KAMAL KUMAR JAM ( Alternate ) Pradesh, Roorkee

SHRI J. L. KHOSA SHRI R. P. GOEL ( Alternate )

Bharat Heavy Electricals Ltd, Bhopal

SHRI K. RAMABHADRAN NAIR Kerala State Electricity Board, Trivandrum SUPERINTENDING ENGINEER ( PHE ) Irrigation Department, Government of Maha-

rashtra, Nagpur EXECUTIVE ENGINEER ( HDD ) 1 Alternate )

21

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