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ISRO 2013 Mechanical Engineering Questions, Answers and Explanation

ISRO 2013 Mechanical Engineering Questions, Answers and Explanation

14. The purpose of surge tank in a pipeline is to 
a) smoothen the flow of water
b) minimize friction loses in pipe
c) prevent occurrence of hydraulic jump
d) relieve pressure due to water hammer

Answer
d) relieve pressure due to water hammer

Explanation [Fluid Mechanics]
Surge tank is a small storage tank or reservoir required in the hydro power plants for regulating the water flow during load reduction and sudden increase in the load on the hydro generator (water flow transients in penstock) and thus reducing the pressure on the penstock.
When the load on the generator is reduced suddenly (load throw-off), governor closes the turbine gates and thus create an increase in  pressure on the penstock. This may result in water " hammering effect" and may require pipe of extra ordinary strength this pressure, otherwise the penstock may burst. To avoid this positive water hammering pressure some means is provided to take the rejected flow of water. This can be achieved by providing small storage reservoir or tank to accommodate this rejected flow. This small storage reservoir is surge tank and located close to the power station. When the turbine gates suddenly opens due to requirement of more water due to increase in the load demand on the generator, water has to rush through the pipe (penstock) and there is a tendency to cause a vacuum in the pipe supplying the water.
The pipe supplying the water must withstand the high pressures caused by the sudden closing of the turbine gates (known as positive water hammer) and there should not be any vacuum in the pipe line when the gate suddenly opens.
The water hammering is defined as the change in the pressure rapidly above or below normal pressure caused by sudden changes in the rate of water flow through the pipe according to the demand of the prime-mover. The water hammer occurs at all the points in the penstock between the forebay or surge tank and the turbine because of the sudden changes in the demand of the water during the load fluctuations.
Surge tank is an open tank which is often used with the pressure conduit of considerable length. The main purpose of providing surge tank is to reduce the distance between the free water surface and turbine thereby reducing the water hammer effect on the penstock and also and also protecting upstream tunnel from high pressure rises. Surge tank also serves as a supply tank to the turbine when the water in the pipe is accelerating during increased load conditions and as storage tank when the water is decelerating during the reduced load conditions. Surge tanks are generally built high enough so that the water cannot overflow even with a full load on the turbine. 
When load demand on the generator decreases, it leads to rise in the water level in the surge tank. This produces a retarding head and reduces water velocity in the penstock. The reduction in the velocity to the desired levels, makes the water in the tank to rise and fall until oscillations are damped out.
When load demand on the generator increases, governor opens the turbine gates in order to allow more water flow through the penstock to supply the increased load demand thereby creating a negative pressure or vacuum in the penstock. This negative pressure in the penstock creates necessary acceleration force and is objectionable for very long conduits due to difficult turbine regulation. Under this condition additional water flows from the surge tank. As a result the water level in the surge tank falls, an acceleration head is created and flow of water in the penstock increases.
Thus surge tank helps in stabilizing the velocity and pressure in the penstock and protects penstock from water hammering and negative pressure or vacuum. 


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