•    When the water is flowing through a pipe it possesses some momentum. This momentum is destroyed, if the flowing water is suddenly brought to rest by closing the valve.

•    The sudden change in momentum will cause a sudden rise in pressure.

•    The high pressure wave is transmitted along the pipe forward and backward.

•    The sudden rise in pressure has the effect of hammering action on the wall of the pipe is known as water hammer.

•    If the hammer blow is very high, the pipe can be burst.

•    To avoid the pipe bursting, the valve of pipeline or penstocks is to be closed gradually.

•    The pressure rise due to water hammer depends upon

(i)    Velocity of flow
(ii)    Length of pipe
(iii)    Time taken to close the valve
(iv)    Elastic properties of materials

Following cases of water hammer in pipes depending upon the time taken in closing the valve:
1.    Gradually closure of valve,
2.    Suddenly closure of valve

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