Cavitation is the name given to the phenomenon of a liquid forming vapor bubbles when it’s vapor pressure equals the external pressure on the liquid. Boiling water is the most common example of this process. When cavitation occurs in a pump, the change of phase has harmful effects and can lead to loss of flow or cause physical damage to the pump. Pumps running in a cavitating condition make a loud rumbling noise and sound like gravel is going through them. This rumbling noise is caused by the vapor bubbles collapsing and returning to a liquid as they move into higher pressure areas of the impeller. Pumps should not be operated for any length of time if cavitation is suspected, and steps must be taken to correct the problem.
Liquids have a property called vapor pressure which is the absolute pressure that the liquid exerts on its surroundings. The vapor pressure is related to the specific liquid and the temperature of the liquid. Increasing the temperature of the liquid will increase the vapor pressure exerted by the liquid. Water is the most commonly encountered fluid and has a vapor pressure of .023 bar at 20° C. When the temperature is increase to 100° C, the vapor pressure increase to 1.013 bar and water begins to vaporize or boil because the vapor pressure equals the atmospheric pressure on the liquid. If the pressure on the liquid is increased, the boiling stops and the temperature can increase until the vapor pressure again equals the pressure on the liquid. This boiling action can take place at lower temperatures if the pressure is reduced below atmospheric; this is what happens at the suction inlet of a pump.
Centrifugal pumps rely upon the pressure exerted on the surface of the suction supply to force the liquid into the eye of the impeller. These pumps create a low pressure zone in the suction eye that is below the pressure on the free surface which makes the liquid flow into the impeller eye. This low pressure zone is also what starts the cavitation process. If the vapor pressure of the liquid is greater than the pressure of the impeller eye, the liquid will turn to vapor and expand in volume about 43,000 times at 20° C. As the fluid continues through the impeller, it soon encounters a higher pressure area just past the vane inlets and the vapor returns to a liquid with a tremendous release of energy. This release of energy can break the small pieces from the impeller which results in a pitted appearance. The head developed by the pump also drops rapidly as cavitation progresses. In order to avoid these problems, the minimum absolute suction pressure should always be greater than the vapor pressure of the pumped liquid. Pmin > Pvp