Cavitation, and how to prevent it.

A symptom of cavitation is typically a significantly increased noise level (sounds like marbles) and concurrently decreased flow rate of the pump.

• The shock waves that arise from the bubbles collapsing when the fluid reaches a higher pressure area make too much noise, decrease the pump's performance, and may even harm the impeller, seal, and/or bearings.

Some of the fluid may evaporate, creating tiny vapor bubbles that are eventually carried by the fluid and implode when they reach areas of higher pressure. The input of a pump impeller has the lowest pressure.

• The fluid's vapor pressure is temperature-dependent and rises with rising temperatures. Calculating the plant's "net positive suction head" should be done using the maximum vapour pressure if the fluid is pumped at various temperatures (NPSH).

The two most important conditions for cavitation are:

• Required Net Positive Suction Head Exceeds Available Net Positive Suction Head (NPSHa). Due to low pressure, water vaporizes (boils), and higher pressure implodes into the vapor bubbles as they pass through the pump, resulting in degraded performance and the possibility of severe damage.
• Recirculation of suction or discharge water. Pumps are constructed for a particular flow range. In instances where there is insufficient flow through the pump, the turbulence and vortices that result can reduce performance and cause damage to the pump.

To prevent cavitation, the fluid pressure must be greater than the vapor pressure at all points and operating temperatures of the pump.

By Luis Gerardo Aragon Islas (Pump Systems Academy trainer)