Cavitation occurs when the pressure of a liquid at a constant temperature falls below its saturated vapour pressure point (or boiling point).
The figure shows the curve for saturated water vapour pressure as a function of the temperature (in this case, the liquid is water).
The figure shows that the liquid’s saturated vapour pressure (equivalent to its boiling point) in a given situation (A), can be reached either by raising its temperature or by lowering its pressure. The dashed line shows the liquid’s boiling point at atmospheric pressure.
When cavitation occurs, air bubbles continuously form and collapse (implode) in the liquid. This generates noise and can lead to damage in the installation. In a heating system, cavitation often occurs in pumps if the pressure of the pump’s suction side is too low. To avoid cavitation in a pump, the minimal inlet pressure should be above the NPSHR of the pump.