Occurs only in liquid service. In its simplest terms cavitation is the two stage process of vaporisation is simply the boiling of a liquid which is also known as flashing. In a control valve this vaporisation takes place because the pressure of the liquid is lowered. Instead of the more common occurrence where the temperature is raised.
As fluid passes through a valve just downstream of the orifice area, there is an increase in velocity or kind of energy that is accompanied by a substantial decrease in pressure or potential energy. This occurs in an area called the vena contracta.
If the pressure in this area falls below that of the vapour pressure of the following fluid, vaporisation (boiling) occurs. Vapour bubbles then continue downstream where the velocity of the fluid begins to slow and the pressure in the fluid recovers. The vapour bubbles then collapse or implode. Cavitation can cause a Choked Flow condition to occur and can cause mechanical damage to valves and piping.