Water hammer


A destructive force that takes place in piping systems when the rate of flow changes suddenly. There are multiple causes for this change in rate of flow that includes:

  • The power to the pump is lost for some reason, the pump slows down faster than the fluid flowing in the lines and liquid separation takes place.
  • Small pipe sizes can cause rapid velocity changes.

Surge tanks, air chambers vacuum valves and vacuum breakers can protect some piping systems from the affects of water hammer.

A common system layout that can cause water hammer problems is when the pump is installed above an open source and is discharging through piping to a higher level. When the pump is stopped the water in the discharge may drain back to the source through a leaking check valve.  When the pump is started the rush of water hits the first obstruction and converted to pressure energy. Vacuum relief valves allow air to enter the piping, cushioning the flow.

There are multiple explanations for the destructive forces being generated.

  • The one I like, explains that when the fluid separates, it reconnects at the speed of sound, in the medium you are pumping.
  • The speed of sound in water is 4800 feet (1500 meters) per second. This works out to about 3000 miles per hour (5400 kilometers per hour)