Physics of Cavitation

Cavitation is the phenomenon of formation of vapor bubbles in the fluid stream in the proximity of the leading edge of the impeller blades. This happens as a consequence of the fluid pressure locally falling below the vapor pressure due to the fluid being accelerated.

When these vapor bubbles move downstream along the blades, the local pressure increases again above the vapor pressure, so the bubbles collapse. The collapse causes a hammering effect on the pump, vibrations, and a high frequency noise (as if gravel was running through the pump). In the long run, this can cause a reduction of the pump’s lifetime and fluid contamination.

A pump working in cavitation will not operate at a stable working point, therefore the performance parameters might not reflect the expected performance (as charted in the Flow-Pressure curve plot).

Cavitation is affected by the density of the fluid, its viscosity, temperature, and other physical properties (e.g. C_p, density of saturated vapor, surface tension). The most important parameter is the amount of cavitation nuclei. Cavitation nuclei is the term used to described the microscopic gas bubbles that are present in the fluid and act as starting point for the local evaporation of the fluid during cavitation.

To avoid cavitation, it is important to maximize the pressure at the inlet of the pump, so that the pressure doesn’t fall below vapor point at any time.

Pump manufacturers provide the absolute pressure needed at the inlet of the pump to avoid cavitation. This value is called NPSH_r and will be examined in detail in the next topic.