Cavitation may occur in fluid flow systems where local static pressure is below the vapor pressure
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.
What is Cavitation?
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
- increasing the distance between the actual local static pressure in the fluid - and the vapor pressure of the fluid at the actual temperature
This can be done by:
- reengineering components initiating high speed velocities and low static pressures
- increasing the total or local static pressure in the system
- reducing the temperature of the fluid
Reengineering of Components Initiating High Speed Velocity and Low Static Pressure
Cavitation and damage can be avoided by using special components designed for the actual rough conditions.
- conditions as huge pressure drops can - with limitations - be handled by Multi Stage Control Valves
- challenging pumping conditions - with fluid temperatures close to the vaporization temperature - can be handled with special pumps - working after other principles than centrifugal pumps
Increasing the Total or Local Pressure in the System
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.
Reducing the Temperature of the Fluid
The vaporization pressure depends of fluid temperature. Vapor pressure of Water, our most common fluid, is indicated below:
|Vapor Pressure |
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.