Dr. M.N. Shesha Prakash, Professor, JNNCE, Shimoga 1

Surge Tank

Surge is the instantaneous rise in pressure due to sudden partial or

complete closure of valve on the downstream end of a long pipeline. Surge

tanks are generally built as a part of hydroelectric plant. In a long pipeline

(Penstock), conveying water from a reservoir to the turbines, there will be

sudden fluctuations in the discharge at the outlet of the pipeline with the

varying load on the generator coupled to the turbine. There will be need for

the generator speed to be cut down suddenly due to decrease in load which

in turn decreases the discharge. This affects over a long pipeline

instantaneously increasing the pressure at the outlet, thereby bursting the

pipe. If there is an open tank whose level is kept well above the supply

reservoir, located closer to the outlet, it can temporarily accommodate the

additional supply of water coming from both the reservoir and the

backwater from the control valve.

Similarly, there will be need for the generator speed to be increased

suddenly due to increase in the load which in turn increases the discharge.

This additional supply of discharge which has to be obtained from the

reservoir in turn immediately increases the flow velocity in the pipeline

thereby decreasing the pressure. This results in crushing of the pipe as the

external pressure is far more than the internal pressure. The surge tank if

provided can augment the supply of water due to sudden increase in

discharge temporarily and prevent the damage to the pipeline.

The surge tank is an open topped large chamber provided so as to

communicate freely with the pipe line bringing water from the reservoir.

The upper lip of the surge tank is situated at a suitable height above the

maximum water level in the reservoir. When the turbine is working under

steady load and the flow through the pipe is uniform there will be a normal

pressure gradient OE. The water level in the surge tank will be lower than

that in the reservoir by GE which represents the loss of head in the pipe

line due to friction. If now the rate of flow in the pipe line is suddenly

decreased, there will be a sudden pressure rise and this will result in a

Dr. M.N. Shesha Prakash, Professor, JNNCE, Shimoga 2

sudden rise in the water level in the surge tank so that the hydraulic

gradient is now along OF. In this situation, the water level in the surge

tank will be higher than that in the reservoir. This condition prevails only

for a short duration. The surge tank acts as an auxiliary storage reservoir

to collect the flow down the pipe when the flow through the pipe is

reduced or stopped. The excess water is accumulated in the surge tank.

This arrangement eliminates the instantaneous expansion of the pipe line

and thus prevents pipe bursting. Similarly, due to increased flow condition

during excess of water requirement will result in a sudden decrease in the

water level in the surge tank so that the hydraulic gradient is now along

OD.

Other Types of Surge Tanks, Besides the simple cylindrical surge

tank, other types are also adopted.

(i) Conical surge tank (ii) Surge tank with internal bell -mouthed spillway

(iii) Differential surge tank

Fig. a shows a conical surge tank which is similar to the simple surge tank

described earlier, except in this case the tank has a conical shape.

SurgeTankHGL for

increasedflow

OF

D

BA

C

HGL forrapidly

decreasedflow

NormalHGL

SURGE TANK

Penstock

G

E

Dr. M.N. Shesha Prakash, Professor, JNNCE, Shimoga 3

Fig. b shows a surge tank provided with internal bell -mouthed spill way.

This arrangement allows the overflow to be conveniently disposed of.

Fig. c shows a differential surge tank. This has the advantage that for the

same stabilising effect its size can be very much less than that of the

ordinary surge tank. Inside the surge tank there is a riser pipe provided

with ports at its bottom. When there is an increase in pressure in the pipe,

some small quantity of water enters the surge tank through these ports but

the major bulk of the incoming flow mounts to the top of the riser and then

spills over into the tank. Thus this provides a substantial retarding head

while in the ordinary surge tank the head only builds up gradually as the

tank gets filled- It may further be realized that the water is not allowed to

waste in the differential tank

(a) Conical Surge tank(b) Surge tank with internal

bell-mouthed spillway

Riser

Ports

(c) Differential Surge tank