Risk Factors

Pump operation:
The most dangerous valve operation is the closing of a check-valve after pump-stop, especially at a power failure. If the discharge pipe is filled at pump start, water hammer at pump start is negligible.

Pipe profile:
As the minimum pressure line depends on such factors as the wave speed and the pump's moment of inertia, it will have the same shape irrespectively of the pipe profile (if no vaporisation occurs). The magnitude of sub-pressure the pipe will experience will therefore depend on the pipe profile i.e. the distance between the minimum pressure line and the pipe level, see figure.

Pipe length:

The shorter the pipe is, the sooner the reflecting pressure wave comes back and raises the pressure line.

Moment of inertia:
The higher the moment of inertia, the longer time the pump will rotate after shut-off and the less the pressure line will drop before the reflecting wave raises it again.

Wave propagation velocity:
The wave propagation velocity depends on the elasticity of the pipe walls and the compressibility of the liquid. Typical values for pipes containing water are: PVC a=300 m/s, steel a=1100 m/s