Water Hammer

Whenever a valve is closed in a pipe, a positive pressure wave is created upstream of the valve and travels up the pipe at the speed of sound. In this context a positive pressure wave is defined as one for which the pressure is greater than the steady state pressure. This pressure wave may be great enough to cause pipe failure. This phenomena is called as Water Hammer

Critical time (t_c) of closure of a valve is equal to 2L/c, where L is the length of the pipe in the upstream of the valve up to the reservoir, and c is the velocity of sound in fluid.

If the closure time of a valve is less than t_c the maximum pressure difference developed in the downstream end is given by r vc. Where v is the velocity in the pipeline.

Water hammer pressures are quite large. Therefore, engineers must desgin piping systems to keep the pressure within acceptable limits. This is done by installing an accumulator near the valve and/or operating the valve in such a way that rapid closure is prevented. Accumulators may be in the form of air chambers for relatively small systems, or surge tanks. Another way to eliminate excessive water hammer pressures is to install pressure-relief valves at critical points in the pipe system.