Figure 1. Mild cavitation can be seen at this impeller’s inlet area. Image courtesy of Motion.
As a liquid passes through a static pressure—which is lower than the liquid-vapor pressure—bubbles are formed. These bubbles can be up to 50,000 times larger than the original liquid size. As the bubbles move back to a static pressure higher than their liquid’s vapor pressure, they rapidly collapse or implode back to a liquid state.
This rapid collapse takes just a microsecond to happen, and the implosion creates a microjet and a shock wave during the collapse. If this implosion happens near a metal’s surface, the shock wave can loosen or damage the metal. Debris can then be washed away; this is called internal cavitation.
If these implosions and cavitation are allowed to continue, there will be additional loose areas and more metal loss. This can form pitting in the metal, or in some cases, it can resemble cheese with complete surfaces damaged.
These issues decrease pump efficiency—resulting in higher energy use, additional repair costs, and reduced pump lifespan.
Why does this happen so often in centrifugal pumps?
As the liquid enters the pump’s suction port, the impeller’s rotation creates a change in speed. Bernoulli’s principle states that as a liquid’s velocity increases, the static pressure decreases. In the case of cavitation, the liquid changes to bubbles. Now at some point, in the pump’s interior, the static pressure will go back up. This is when the implosion happens from the bubble collapsing back to liquid: cavitation.
How do I know if I have cavitation occurring in my pump?
In most cases, you can hear it. Listen for the sound of rocks or marbles inside the pump’s casing; it may help to use a stethoscope. Once you hear and identify cavitation, you will remember it. You can also see cavitation damage to your pump’s internal surfaces. In many cases, pitting can be seen on the impeller either at its eye (suction cavitation) or its tips (discharge cavitation).