Cavitation is a condition in which the pressure within valve drops below its vapor pressure following with the pressure recovers and rises above its vapor pressure afterwards.
When the liquid pressure drops below its vapor pressure, the some part of liquid vaporizes and form bubbles. If the pressure recovers and rises above the vapor pressure, the bubbles will start to collapse.
The bubble implosion will produce high-energy that could results structural damage i.e erroding the valve and adjacent piping.
Cavitation can be eliminated or avoided by the following methods:
Use anti-cavitation trim. This make the pressure drop within a valve is staggered, hence the pressure never falls below its vapor pressure yet the downstream pressure target could be attained.
Change the process system such as adding an orifice plate in some distance downstream of control valve, considering the use of two control valve in series (not recommeded), or reducing process temperature which subsequently reduce the value of vapor pressure.
If cavitation is ignored, the valve may only successfully control for a short period of time before requiring maintenance or replacement.