Surge Tank - positive and negative water hammer, causes & prevention

Surge Tank - positive and negative water hammer, causes & prevention

 Surge Tank

The major role of the surge tank is to protect the penstock pipe from the serious effect of the negative pressure effect.

Control of discharge

At the steady state operation of the power plant, the discharge through the penstock is smooth and is sufficient to provide the load demand. But, when the load demand fluctuates from normal value, the discharge Q should also be maintained accordingly in order to supply the necessary demand. The control of water delivery to turbine is done by the closed loop control system associated with speed governor automatically in modern power plants.

positive water hammering

When the electrical load decreases from steady value, the speed of the generator shaft increases and hence frequency of the system also increases and the speed sensor senses the change in speed and sends error signal to the governor and governor gives command signal to the valve to gets closed by some extent to reduce discharge in order to save the turbine from high speed. The closure of the valve is so sudden that it generates the extensive positive pressure that reflects back from valve to the surge tank along the penstock with velocity of sound. This is known as positive water hammer effect. 

Negative water hammering

In case of increase in electrical load, the speed of the turbine shaft decreases and need to be increased up to pre-defined value by sudden opening of gate/valve. This sudden opening causes the air bubbles that travel backward. This is known as negative water hammering effect. 

Effects of water hammering

In both cases, if the surge tank is absent, then the extensive pressure due to water hammer tends to cause serious damage to the penstock pipe. In case the pipe is not strongly fixed upon the blocks, according to conservation of liner momentum principle, the momentum developed by the pressure may cause deformation of penstock pipe and in case of strongly fixed, the water hammer pressure causes damages pipe. 

pressure rise due to water hammering

Prevention - By surge tank

By the use of surge tank, the water hammer effect on the penstock is reduced. During the positive water hammer, the water level in penstock rises in order to balance the pressure. During negative water hammer, the water level in the penstock goes down. In this way surge tank prevents the penstock from water hammer effect.