In many control valves, the pressure at the vena contracta
will drop below the vapor pressure of the liquid. When
this occurs, small bubbles of gas will form as the liquid
vaporizes. As the pressure then rises above the vapor
pressure again, these small bubbles collapse or implode
as the vapor turns back into liquid. The damage is inflicted
as the bubbles implode. The implosion of the vapor
bubbles is very energetic and forms jets of fluid which can
tear small pits into the metal. This is called cavitation.
Cavitation damage destroys both piping and control
valves, often resulting in catastrophic failure. It causes
valves to leak by eroding seat surfaces. It can drill holes
through pressure vessel walls. Even low levels of cavitation
will cause cumulative damage, steadily eroding parts
until the part is either repaired, or it fails.
This Slideshare explains valve cavitation and provides solutions to minimize or eliminate its effects.