In pumping systems, cavitation occurs when the pressure in the system or pump drops below the liquid’s vapor pressure, resulting in the formation of vapor cavities. The formation of vapor cavities can occur at many locations in the pumping system, such as valves, heat exchangers and inside the pump.
Specific to the pump, cavitation is known for causing reduced performance, noise and erosion damage. When significant, the vapor cavities that form in the impeller eye result in reduced performance, and when the vapor cavities reach a location of higher pressure (above the liquid’s vapor pressure) in the pump, they implode back to liquid, causing noise and erosion damage. The erosion damage can occur because when the vapor cavity implodes, it shoots a microjet of liquid that impacts the metal surface. To limit the effect of cavitation in pumps, the system’s net positive suction head available (NPSHa) needs to exceed the pump’s net positive suction head required (NPSHr) by a certain margin (NPSH margin).
For more information, recommendations and guidance for NPSH margin, refer to ANSI/HI 9.6.1 Rotodynamic Pumps Guideline for NPSH Margin at pumps.org.