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Water Hammers

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

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A Water Hammer is a pressure change caused by a variation of the 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 in the path.

Pressure transients in pipe lines caused by a shock wave when closing or opening a valve can be calculated as

dp = 0.070 dv l / t                                      (1)


dp = increase in pressure (psi)

dv = change in flow velocity (ft/s)

t = valve closing time (s)

l = upstream pipe length (ft)

  • 1 ft (foot) = 0.3048 m
  • 1 ft/s = 0.3048 m/s
  • 1 psi (lb/in2) = 6894.8 Pa (N/m2)

Example - Water Hammer generated when closing a Solenoid Valve

The pressure increase (water hammer) in a 100 ft water pipe where water flow velocity is reduced from 6 ft/s to 0 ft/s when a solenoid valve close in 0.1 s can be calculated as

 dp = 0.070 ((6 ft/s) - (0 ft/s)) (100 ft) / (0.1 s) 

    = 420 (psi)

With a closing time of 1 s (solenoid valve with damper) - the pressure increase (water hammer)  can be calculated as

dp = 0.070 ((6 ft/s) - (0 ft/s)) (100 ft) / (1 s) 

    = 42 (psi)

Note! - it is important to open and close valves slowly and use soft starters to start and stop pumps to avoid damaging water hammers in piping systems. 

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