Water Hammer

The water hammer is a liquid shock wave resultant from the sudden beginning or stopping of flow. It is exaggerated by the density of the fluid, initial system pressure, speed of sound in the fluid, elasticity of the fluid and pipe, change in velocity of the fluid, thickness of the pipe, and to the valve operating time.

During the closing of a valve, kinetic energy of the moving fluid is transformed into potential energy. Elasticity of the fluid and pipe wall generates a wave of positive pressure back to the fluid's source. Whenever this wave reaches the source, the mass of fluid will be at rest, though under incredible pressure. The compressed liquid and stretched pipe walls will now begin to discharge the liquid in the pipe back to the source and return to the static pressure of the source. This discharge of energy will form another pressure wave back to the valve. Whenever this shockwave arrives at the valve, due to the momentum of the fluid, the pipe wall will start to contract. This contraction is transmitted back to the source that places the pressure in the piping below which of the static pressure of the source. Such pressure waves will travel back and forth numerous times until the fluid friction dampens the alternating pressure waves to the static pressure of the source. Generally, the whole hammer procedure occurs in less than one second.

The preliminary shock of unexpectedly stopped flow can induce transient pressure changes which exceed the static pressure. When the valve is closed gradually, the loss of kinetic energy is slow. When it is closed rapidly, the loss of kinetic energy is very quick. Shock wave outcomes since of this quick loss of kinetic energy. Shock wave caused by the water hammer can be of adequate magnitude to cause physical damage to equipment, piping, and personnel. The water hammer in pipes has been recognized to pull pipe supports from their mounts, rupture piping, and cause the pipe whip.