Cavitation is a common problem in pumps and control valves - causing serious wear and tear and damage. Under the wrong conditions, cavitation reduces the component life time dramatically.
Cavitation may occur when the local static pressure in a fluid reach a level below the vapor pressure of the liquid at the actual temperature.
According to the Bernoulli Equation this may happen when the fluid accelerates in a control valve or around a pump impeller.
The vaporization itself does not cause the damage - the damage happens when the vapor almost immediately collapses after evaporation when the velocity is decreased and pressure increased.
Cavitation can in general be avoided by
This can be done by:
Cavitation and damage can be avoided by using special components designed for the actual rough conditions.
By increasing the total or local pressure in the system the distance between the static pressure and the vaporization pressure is increased and vaporization and cavitation can be avoided.
The ratio between static pressure and the vaporization pressure - an indication of the possibility of vaporization, is often expressed by the Cavitation Number.
Unfortunately it may not always be possible to increase total static pressure due to systems classifications or other limitations. Local static pressure in components may be increased by lowering the component in the system. Control valves and pumps should in general be positioned in the lowest part of the systems to maximize static head.
This is common solution for boiler feeding pumps receiving hot condensate (water close to 100 oC) from condensate receivers.
The vaporization pressure depends of fluid temperature. Vapor pressure of Water, our most common fluid, is indicated below:
Note! - the possibility of evaporation and cavitation increases dramatically with the water temperature.
Cavitation can also be avoided by locating components to the coldest part of a system. It is common to locate pumps in heating systems in the "cold" return lines.
This is the same for control valves. If it is possible control valves should be located on the cold sides of heat exchangers.