What is Cavitation?
Cavitation occurs at the inlet of a pump impeller when small bubbles rapidly and repeatedly form and then almost immediately collapse sending a shock wave into the surface of the impeller.
The point of lowest pressure in a pump is found at the eye of the impeller. At this point, liquid is being drawn into the impeller but has not yet been acted upon by the rotation of the impeller. If the pressure acting on the liquid is drawn down too low, small pockets of vapor – bubbles – will form in the liquid. As the liquid is then accelerated by the rotation of the impeller, pressure increases rapidly, and the vapor bubbles collapse.
When the bubbles collapse they send out a powerful shock wave which can cause damage to the surface of the impeller over time. If cavitation is allowed to continue unchecked for an extended period of time, the impeller will sustain significant wear, pump performance will degrade, and pump life will be reduced.
What Causes Cavitation?
Cavitation is produced when the NPSH available (NPSHa) to a pump is inadequate. Pump manufacturers provide pump users with NPSH required (NPSHr) values to help pump users avoid scenarios where NPSHa is less than NPSHr. In most applications, a margin of 5 Ft or more between NPSHa and NPSHr is recommended to avoid the possibility of cavitation.
Cavitation produces a very noticeable sound. When a pump is cavitating it sounds as if small rocks are rattling around inside the pump casing. When pump operators notice the sound of cavitation within a pump they should act swiftly to either increase NPSHa to the pump or decrease pump NPSHr.