design note on surge shaft

YAMNE STAGE –II H.E.P (84 MW)_________________________________________________________________________

TABLE OF CONTENTS

Sr. No. DESCRIPTION

1. Design of Surge Shaft

(1.1) Location(1.2) Hydraulic Design(1.3) Structural Design(1.4) Others

2. Appendix

(2.1) Diameter of surge Shaft using Thoma Criteria(2.2) Area of Restricted Orifice using Calame & Gaden equation(2.3) Maximum Up-Surge Level in Surge Tank(2.4) Minimum Down- Surge Level in Surge Tank(2.5) Speed Rise(2.6) Pressure Rise(2.7) Structural Design of Surge Shaft

3. Drawings

4. Quantities

(4.1) Summery of quantities Estimation(4.2) Detailed quantities Estimation

5 Construction Planning

(5.1) Constructional Methodology(5.2) List of Equipments

__________________________________________________________________________________TECHNICAL MEMORANDUM ON SURGE SHAFT


(1) SURGE SHAFT DESIGN (WRITE UP)



1. SURGE SHAFT

1.1 Location

Surge shaft has been located on the ridge with flat topography with general

ground elevation of El. 536.58 m. The location shall be accessible from

existing road by taking off an additional branch road.

1.2 Hydraulic Design

Hydraulic Design of the surge tank has been carried out using IS: 7396 (Part-

1) – 1985. A restricted orifice type surge shaft has been assumed.

According to Thoma criteria with a factor of safety of 1.6, minimum area

required for the surge shaft works out to 562.8 sq.m, (Appendix 2.1), with

diameter of 26.7 m. Since the construction of such huge diameter surge shaft

in this project geology is difficult as such provision of upper and lower

expansion gallery has been envisaged with a constructible surge shaft

diameter, from the earlier experience and available running project of similar

magnitude. The surge shaft of 16 m diameter with restricted orifice adopted

for study with a provision of 5 m diameter D- shaped 122 m long lower and

210 m long upper expansion galleries with a slope of 1 in 150 m.

TO verify these dimensions and arrangement of surge shaft a study of surge

analysis and speed rise and pressure rise was done by using WHAMO (Water

Hammer and Mass Oscillation) of USACE.

The size of the orifice has been calculated to satisfy the condition given by

Calame and Gaden (Appendix 2.2). An orifice size of 7.8 sq. m. is provided.

On the perusal of results after running this model following inference has been

notified. The maximum upsurge level in the surge tank has been worked out

corresponding to the full load rejection at the highest reservoir level. Maximum

upsurge level works out to be El. 532.5 m. Considering a freeboard, the top

of the surge tank has been kept at El. 536.5 m (Appendix 2.3).

The minimum down surge has been calculated considering 100% load

rejection followed by full load acceptance at the instant of maximum negative



velocity in the head race tunnel at minimum reservoir level .This gives the

maximum down surge level of El. 503.3 m (Appendix 2.4). The invert of the

HRT (El.482.75 m) has been kept sufficiently below this level so as to ensure

sufficient water cover over the tunnel overt to avoid vortex formation.

Speed rise (Appendix 2.5),

Pressure rise (Appendix 2.6),

The height of the surge tank from the crown of HRT (El.489.75 m) to the top of the surge shaft (El. 536.58 m) works out to be 46.83 m.

1.3 Structural Design

The surge shaft with RCC lining has been envisaged to sustain the fluctuating

water column. (Appendix 2.7).

To restrict /check the loss of water i.e. impervious conditions.

To restrict the skin failure / crack of concrete.

The thickness of lining has been designed on the basis of Lame’s

theory of thick cylinder keeping in view to sustain the external pressure,

when the shaft is drained.

1.4Others

The surge shaft shall be provided with a vertical slide type gate at the

downstream end to close the penstock / pressure shaft for inspection and

maintenance.

The surge shaft details have been provided in drawing nos. ________



(2) APPENDIX



APPENDIX 2.1

WORKING SHEET FOR THE CALCULATION OF DIAMETER OF SURGE

SHAFT:

A surge tank has been envisaged at the downstream end of the HRT to

provide the open reflection column of water to limit the transmission of water

surges due to water hammer phenomenon to HRT. The surge tank would

also assist in improving the regulation and to provide water supply to turbines

in case of sudden start up of a machine.

In order to determine the dimensions of surge tank, following assumptions has

been adopted:

To ensure the hydraulic stability of surge tank, its area has been calculated

according to Thomas criteria. According to Thomas, the limit cross sectional

area of surge tank is

Where,

L = Length of HRT = 8400 m

Diameter of HRT = 7.0 m (Horse shoe Shaped tunnel)

At = Cross sectional area of HRT = 0.8293*7.02 = 40.6357 m2

H = Net head = 80.23 m

Q = Design Discharge = 116 m3/sec.

Flow velocity in HRT,

R = Hydraulic Mean Radius = 1.78 m

= Resistance factor of tunnel,



The value of resistance factor is function of total head loss in water conductor

system excluding the Head Loss in pressure shaft and tail race tunnel.

As per IS: 7396 – (Part-I) – 1985

The value of shall be determined from the following formula:

v2 = + other losses in tunnel system

Considering other losses is tunnel system is within 10% of friction loss in

tunnel.

Cross-sectional area of surge tank required

should be calculated for the value of n are n = 0.012 n = 0.018

As per Thoma criteria minimum value of shall be used in the above formula.

When ‘n’ is to be considered as equal to 0.012

v2 = 1.1 = 1.1

1.1 [4.5779] = 5.03569

= 0.6179 sec2/m

When ‘n’ is to be considered as equal to 0.018

v2 = 1.1 = 1.1

1.1 [10.3] = 11.330

= 1.390 sec2/m



As per Thoma criteria for calculation of diameter of surge shaft tank minimum

value of is to be taken.

The cross sectional area of Surge Tank required, for n=0.012 ,β=0.6179

=

Asth = 351.75 sq.m.

Factor of safety for restricted orifice surge shaft, 'n' = 1.6

(IS: 7396 (Part 1)-1985, Clause 5.4)

Required area Asth = 1.6 x 351.75 = 562.8 sq.m.

Required diameter of surge tank, D =

D =26.76 m

Provide diameter of surge tank = 16 m, with area = 200.96 m2

\APPENDIX 2.2



AREA OF RESTRICTED ORIFICE USING CALAME AND GADEN

EQUATION:

Surge column shall depend upon the restriction provided by the orifice in the

bottom of surge shaft. According to IS: 7396 (Part 1)-1985 clause-5.5.3 &

5.5.3.1 the area of orifice is so chosen as to satisfy the condition given by

Calame and Gaden for maximum flow which is as fallow:

Where

hor = Head loss offered by orifice

Z* = Surge height corresponding to change in discharge neglecting

friction and orifice loss

= Head losses in head race tunnel = v2 = 5.02 m

At the time of instantaneous closure from maximum discharge, the amplitude

of maximum surge in case of undammed mass oscillation is given by

And,

The Surge will depend on resistance offered by an orifice of area A0 as = 7.79

m2. Thus size and shape of orifice should be decided first and resistance shall

be calculated by equation.



Where,

Cd is the coefficient of discharge.

Choosing A0 as = 7.79 m2 and Cd = 0.62 for rectangular gate slot.

Resistance offered by orifice

27.82 m < 29.41 m < 30.33 m

Thus, satisfying the Calame and Gaden condition

Hence, envisaged the area of orifice is = 7.8 m2.



APPENDIX 2.3

MAXIMUM UPSURGE LEVEL IN SURGE TANK

According to IS: 7396 (Part 1) – 1985, the surge tank shall be designed to accommodate the maximum and minimum water levels anticipated under worst condition.

The maximum upsurge level in the surge tank shall be worked out corresponding to:

a) The full load rejection at the highest reservoir level and,

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

The surge analysis was done in WHAMO and checked according to IS: 7396. It was observed that the maximum upsurge did not occur in the ‘a’ condition i.e. for full load rejection at highest reservoir level.

The surge levels were also calculated for other cases like 100% load acceptance followed by full load rejection at highest reservoir level, 66% load acceptance followed by full load rejection at highest reservoir level and 33% load acceptance followed by full load rejection at highest reservoir level .

It was observed from the result (shown in Table 2.3.1 and Fig.2.3.1 , Fig.2.3.2, Fig.2.3.3 & Fig.2.3.4) for restricted orifice type surge tank that the level of worst case of upsurge in surge tank, El.532.461 m occurs when full load rejection at highest reservoir level using WHAMO.

TABLE-2.3.1RESERVOIR LEVEL (FRL) LOADING CONDITION UP-SURGE

ft m ft m1712.5776 522 100-0 1746.90 532.461

1712.5776 522 0-100-0 1734.10 528.560

1712.5776 522 0-66-0 1725.60 525.969

1712.5776 522 0-33-0 1722.50 525.024



Fig.2.3.1

Fig. 2.3.2



Fig. 2.3.3

Fig. 2.3.4



APPENDIX 2.4

MINIMUM DOWNSURGE LEVEL IN SURGE TANK

According to IS: 7396 (Part 1) – 1985, the surge tank shall be designed to

accommodate the maximum and minimum water levels anticipated under worst

condition.

To obtain minimum down surge level the worst of the following two conditions

shall be considered:

a) The full load rejection at minimum reservoir level followed by specified

load acceptance at the instant of maximum negative velocity in the head

race tunnel, and

b) Specified load acceptance at load or speed-no-load condition at the

minimum reservoir level.

The surge analysis was done in WHAMO and checked according to IS: 7396. It

was observed that the minimum downsurge did not occur in the ‘b’ condition i.e.

for 100% load acceptance at no-load condition at the minimum reservoir level.

The surge levels were also calculated for other cases like 100% load rejection

followed by full load acceptance at the instant of maximum negative velocity in

the head race tunnel, 100% load rejection followed by 66% load acceptance at

the instant of maximum negative velocity in the head race tunnel and 100%

load rejection is followed by 33% load acceptance at the instant of maximum

negative velocity in the head race tunnel.

It was observed from the result (shown in Table 2.4.1 and Fig.2.4.1,Fig.2.4.2 ,

Fig.2.4.3 & Fig.2.4.4) for restricted orifice type surge tank that the level of

worst case of downsurge in surge tank, El.503.291 m occurs when 100% load

rejection is followed by 100% load acceptance at the instant of maximum

negative velocity in the head race tunnel using WHAMO.



TABLE – 2.4.1

Fig.2.4.1


RESERVOIR LEVEL (MDDL)

LOADING CONDITION

DOWN-SURGE

ft M ft m

1692.8928 516 100-0-100 MDDL 1651.2 503.291

1692.8928 516 0-100 MDDL 1656.8 504.998

1692.8928 516 100-0-66 1669.6 508.900

1692.8928 516 100-0-33 1669.6 508.900


Fig.2.4.2

Fig.2.4.3



Fig.2.4.4



APPENDIX 2.5

STRUCTURAL DESIGN OF SURGE SHAFT

According to IS: 7357 – 1974, the thickness of lining for surge shaft shall be

designed on the basis of Lame’s thick cylinder theory. For designing purpose,

the worst condition occurs from outside the lining. The lining shall be

adequate to withstand this anticipated external pressure. Indian standards

suggest providing a minimum thickness of 0.3m or thickness required to resist

maximum external pressure, whichever is higher.

Circumferential stress in lining is determined by

Pc =

Where,

Pc = External (circumferential) pressure, Kgf/ cm2.

= Permissible comp. stress in concrete, Kgf/ cm2

= 0.446 fck N/mm2 (Refer IS: 456 – 1978)

For M25 grade concrete,

fck, Characteristic strength of concrete = 25 N/mm2

= 11.15 X 10.1971 Kgf/cm2.

= 113.70 kg/cm2.

a is the internal radius of shaft = 8 m

b is the external radius of shaft (including thickness of lining)

a = 800 cm

Pc = External circumferential stress (Pressure)

Pc = External water Pressure corresponds to 46.69 m height of water.



46.69 m of water = 4.66 kgf/cm2

4.66xb2 + (64 x 4.66) = 113.70b2 – 113.70 x 64

64 [4.66 + 113.70] = 113.70b2 – 4.66b2

64 x 118.36 = 109.04x b2

7575.04 = 109.04x b2

b2 = 69.4702

b = 8.33

Thickness of lining = (b-a) = (8.33 – 8) m

= 0.33 m

Provide 500 mm thick RCC lining for surge shaft.



(3) DRAWINGS

__________________________________________________________________________________ TECHNICAL MEMORANDUM ON SURGE SHAFT

design note on surge shaft

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