Fluid Mechanic: Water Hammer

Water hammer (or, more generally, fluid hammer) is a pressure surge or wave resulting when a
fluid in motion is forced to stop or change direction suddenly (momentum change). Water
hammer commonly occurs when a valve is closed suddenly at an end of a pipeline system, and a
pressure wave propagates in the pipe. Valve closure can result in pressures well over the steady
state values, while valve opening can cause seriously low pressures, possibly so low that the
flowing liquid vaporizes inside the pipe.

The excessive pressure may fracture the pipe walls or cause other damages to the pipeline
system. The possible occurrence of water hammer, its magnitude and the propagation of the
pressure wave must be carefully investigated in connection with the pipeline design.
(J.Houghtalen:2009)

The simple example where it is happen when you turn OFF a water flow suddenly (like a
dishwasher, a clothes washer, a toilet or even closing a hand faucet suddenly. Shutting off the
flow suddenly sends a pressure or shock wave down the water line through the water shocking
the pipes and creating the 'hammer' noise. These shock waves travel faster than the speed of
sound and can exert very great instantaneous pressures. Over time, water hammer can damage
pipes, valves and eventually weaken pipe joints. (R. Hancock: 2002)

The Hazard causes by water hammer are various. Firstly danger of high pressure: the
excess pressure resulting from water hammer is added to the initial pressure. If the total pressure
is greater than a maximum permissible pressure there is a risk of the pipe rupture or joint failure.
Secondly is a danger of negative pressure: negative relative pressure may result from water
hammer, if these pressure are below -10m of water column, a pocket cavitation or water vapor is
created, with possible collapse of the conduit wall or sucking in the joint seals. The pipe lining
material may also be damaged. Finally risk of the pipe fatigue resulting from rapidly alternating
positive and negative pressures with singly may not lead to actual failure of the conduit.
(Y.Labye: 1988)

Click Here to read more>>>>