Water Hammers

Rapidly closing or opening valves may cause pressure transients in pipelines known as water hammers

A Water Hammer is a pressure change caused by a variation of flow rate in a pipe or tube. Waterhammers are created by suddenly starts or stops of liquid flows. Since water flows are restricted inside pipes, shock waves of incompressible water will travel back down the pipes deflecting everything on their paths.

The pressure transients in pipes caused by shock waves when closing or opening valves can be calculated as

dp = 0.070 v l / t  (1)

where

dp = increase in pressure (psi)

v = flow velocity (ft/s)

t = valve closing time (s)

l = upstream pipe length (ft)

Example - Water Hammer when Closing Solenoid Valve

The pressure increase in a 100 ft water pipe where the velocity is 6 ft/s when a solenoid valve closes in 0.1 s can be calculated as

 dp = 0.070 5 (ft/s) 100 (ft) / 0.1 (s) 

    = 350 (psi)

If the solenoid closes in 1 s (solenoid valve with damper) - the pressure increase is reduced to 35 psi.

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