What is the difference between flashing and cavitation in a valve?

Flashing and cavitation phenomenon can occur in piping systems carrying liquids where the flow is restricted either due to throttling of a valve or other restrictions such as restriction orifice. As the liquid passes through the restriction, its velocity increases and pressure decreases. When the pressure at the restriction (vena contracta) reaches the vapor pressure of the liquid, vapor bubbles begin to form. This process is called flashing.

The vaporisation of liquid results in a large increase in volume and high fluid velocity. We can imagine that the bubbles created by flashing get in the way of the liquid, reducing the flow area thus increasing the flow rate. Liquid droplets suspended in high-velocity vapor flow impinge on the valve plug and downstream piping causing severe damage in the form of erosion. The most severe damage occurs at the point of highest velocity near the plug and seat ring. Flashing can be identified by a high-pitched hissing sound in the vicinity of the valve.

As the liquid travels downstream and pressure recovers to a point above the vapor pressure of the fluid, the vapor bubbles begin to collapse. As the vapor bubbles collapse they produce large spherical shock waves often called microjets. The collapse of vapor bubbles in the vicinity of the pipe wall and valve surfaces causes the metal to chip away causing severe physical damage to the valve and piping. Cavitation can be identified by popping sound accompanied by vibration as the bubbles collapse.  

Due to the nature of damage caused by flashing and cavitation, it is important to prevent these phenomenon from occuring and protect the valves and piping systems.