Water hammer occurs in installations where sudden closure of a valve causes water to surge back and forth at supersonic speeds. Due to this phenomenon, the reservoir end of the pipe reflects negatively whereas the valve end reflects positively causing pressure to fluctuate rapidly. Immediately after valve closure, a steam column is generated and an air pocket forms behind the moving water. As the water hits the closed valve, the water reverses its direction and quickly fills the void where the air pocket had formed. The waters change in direction causes massive forces back through the pipe.
The quicker the valve closes the more the pipe deflects. A strong enough force can dislodge diaphragms and rupture valve seals. The amplitude of the transient pressures and time duration of the transient position depends on various factors in a system such as flow velocity, pipeline material and system boundary conditions. The magnitude of a pressure change under a transient condition is expressed by Joukowsky’s law for instantaneous valve closure (ASHRAE Standards)
The Cs for water is 4720 fps, although the elasticity of the pipe reduces the effective value. The above equation is valid for a pulse propagating in downstream direction. Conversely, the pressure change will be negative for a wave propagating in upstream direction
Water-hammer proof features are critical to design because they decrease the pressure spikes caused by a quickly closed valve. Decreasing the water pressure will reduce the pipes deflection and secure the diaphragms and valve seals.