51691375 Design Note on Surge Shaft

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

TABLE OF CONTENTS Sr. No. DESCRIPTION 1.

Design of Surge Shaft

2.

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

3.

(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 Drawings

4.

Quantities

5

(4.1) Summery of quantities Estimation (4.2) Detailed quantities Estimation Construction Planning (5.1) (5.2)

Constructional Methodology 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 (Part1) – 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 __________________________________________________________________________________ TECHNICAL MEMORANDUM ON SURGE SHAFT


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.

1.4 Others 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 Asth =

L × At 2g × β × H

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, V =

116 = 2.8546 m / sec 40 .6357

R = Hydraulic Mean Radius = 1.78 m β = 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 β shall be determined from the following formula: V 2n2 L + other losses in tunnel system R4/3

β v2 =

Considering other losses is tunnel system is within 10% of friction loss in tunnel. V 2 n 2 L  ∴βv 2 = 1.1 4 / 3   R 

Cross-sectional area of surge tank required Asf =

L × Af

β ×v2 × H

×

v2 2g

β should be calculated for the value of n are n = 0.012 n = 0.018 As per Thoma criteria minimum value of β

shall be used in the above

formula. •

When ‘n’ is to be considered as equal to 0.012  2.8546 2 × 0.012 2 × 8400  9.8567  β v = 1.1   = 1.1  4/3  2.1531   (1.78 )   2

1.1 [4.5779] = 5.03569 β = 0.6179 sec2/m



•

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

 2 2 2.8 5 ×4 0.06 1 × 8  β v = 1.1  ( 1.7 ) 48/ 3  2

 4  0 0 22 .178  = 1.1    2.1531   

1.1 [10.3] = 11.330 β = 1.390 sec2/m

As per Thoma criteria for calculation of diameter of surge shaft tank minimum value of β is to be taken. The cross sectional area of Surge Tank required, for n=0.012 ,β=0.6179 Asth =

8400 × 40 .6357 341339 .88 = 2 × 9.81 × 0.6179 ×80 .23 972 .644

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 =

4 Asth *   π 

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:

Z* 1 Z* 3 + h f ≤ ho r ≤ + h f 2 4 2 4 Where hor

= Head loss offered by orifice

Z*

= Surge height corresponding to change in discharge neglecting friction and orifice loss

hf

= Head losses in head race tunnel = β 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 Z* = v

LAt gAsth

Z * = 2.8546 ×

8400 × 40 .6357 = 37 .56 m 9.81 × 200 .96



Z * hf + = 27.82 2 4

And,

Z* 3 + h f = 30.33 2 4 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. hor =

Q2 2 2 C d * Ao * 2 g

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 hor hor =

116 2 = 29 .41m 0.62 2 * 7.79 2 * 2 * 9.81

27.82 m < 29.41 m < 30.33 m Thus, satisfying the Calame and Gaden condition Hence, envisaged the area of orifice is A0 = 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-2.3.1 RESERVOIR LEVEL (FRL) LOADING CONDITION ft 1712.5776

m 522

1712.5776

UP-SURGE

100-0

ft 1746.90

m 532.461

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 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.2.4.1

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  b2 − a2   σ Pc = c  2 2  b + a 

Where, Pc = External (circumferential) pressure, Kgf/ cm2. σc = Permissible comp. stress in concrete, Kgf/ cm2 σc = 0.446 fck N/mm2 (Refer IS: 456 – 1978)

For M25 grade concrete, fck, Characteristic strength of concrete = 25 N/mm2 σc = 11.15 X 10.1971 Kgf/cm2. σc = 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 4.66 b 2 − a 2 b 2 − 82 ∴ = 2 = 113 .70 b + a 2 b 2 + 82

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


51691375 Design Note on Surge Shaft

Recommend Documents