Valve Cavitation & Water Hammer
Cavitation occurs when the pressure of a flowing fluid drops
below the vapor pressure of that fluid (Figure 3). In this two-step
process, the pressure first drops to the critical point, causing
of vapor to form. These are carried with the flow stream until
reach an area of higher pressure. The bubbles of vapor then suddenly
collapse or implode. This reduction in pressure occurs when
the velocity increases as the fluid passes through a valve. After
fluid passes through the valve, the velocity decreases and the
pressure increases. In many cases, cavitation manifests itself
However, if the vapor bubbles are in contact with a solid surface
when they collapse, the liquid rushing into the voids causes high
localized pressure that can erode the surface. Premature failure
the valve and adjacent piping may occur. The noise and vibration
caused by cavitation have been described as similar to those of
gravel flowing through the system.
Water hammer is a series of pressure pulsations of varying magnitude
above and below the normal pressure of water in the pipe.
The amplitude and period of the pulsation depend on the velocity
the water as well as the size, length, and material of the pipe.
Shock loading from these pulsations occurs when any moving
liquid is stopped in a short time. In general, it is important
quickly closing valves in an HVAC system to minimize the occurrence
of water hammer.
When flow stops, the pressure increase is independent of the
working pressure of the system. For example, if water is flowing
5 fps and a valve is instantly closed, the pressure increase is
same whether the normal pressure is 100 psig or 1000 psig.
Water hammer is often accompanied by a sound resembling a
pipe being struck by a hammer—hence the name. The intensity
the sound is no measure of the magnitude of the pressure. Tests
indicate that even if 15% of the shock pressure is removed by
or arresters, adequate relief is not necessarily obtained.