Waterhammer
Rapidly closing or opening valves - or starting stopping pumps - may cause pressure transients in pipelines known as waterhammers
A "Waterhammer" in pipe or tube is a pressure spike caused by sudden variation of flow rate.
Waterhammers can be created if
- valves opens or closes to fast
- pumps suddenly stops or starts
- parts of the pipeline bursts
and velocity energy is converted to pressure energy. Since the water flow is restricted inside a pipe, a shock wave will travel forth and back through the incompressible water in the pipeline deflecting everything in its path.
Since liquids have a very low compressibility the resulting pressure energy can be very high. If the intensity in the shock wave is high, physical damage to the system can occur.
The pressure spike in a pipeline caused by a closing or opening a valve can be estimated as
dp = 0.070 dv l / t (1)
where
dp = increase in pressure - pressure spike (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 spike (water hammer) in a 100 ft water pipe where the water flow velocity is reduced from 6 ft/s to 0 ft/s when a solenoid valve closes in 0.1 s - can be estimated 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 spike (water hammer) can be estimated 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
- use soft starters to start / stop pumps
to avoid water hammers damaging piping systems.
Note - water hammers can be more damaging in low pressure applications.
Water Hammer Calculator
Imperial Units
dv - change in velocity (ft/s)
t - time to close valve (s)
l - length of pipe (ft)
SI Units
dv - change in velocity (m/s)
t - time to close valve (s)
l - length of pipe (m)
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