When a liquid flowing through a long pipe is suddenly brought to rest by closing the valve at the end of a pipe, then a pressure wave of high intensity is produced behind the valve. This pressure wave of high intensity has the effect of hammering action on the walls of the pipe. This phenomenon is known as water hammer or hammer blow.

The magnitude of water hammer depends upon

(a) The length of pipe line,

(b) The elastic properties of the pipe material,

(c) The elastic properties of the liquid flowing through the pipe, and

(d) The speed at which the valve is closed.