All liquid has small micro vapour bubbles in it. As the water characteristics change, with regard to temperature and pressure, the size of these bubble is affected.
In the suction pipe, where pressures can be very low or in some cases negative, the vapour bubbles can expand in the liquid. If of sufficient volume and size, these bubbles can dramatically affect the pump's output performance and there is a risk of air blockages/starvation.
The real evidence of cavitation is a popping or crackling heard within the pump body, usually at the suction port. This is caused by the vapour bubbles imploding as they pass through a dramatic change in the water conditions.
The water (plus vapour bubbles) is pulled into the pump body. At the point where the water enters the pump impeller it goes from a state of low pressure to an area of higher pressure as the impeller induces energy (pressure) into the liquid. This change in pressure causes the bubbles to suddenly collapse, hence the noise.
Under severe cavitation situations, the performance of the pump will deteriorate and in the longer term very serious damage can be caused to the impeller - or occassionally complete destruction. Case iron and bronze impellers show evidence of this by appearing perforated - like an Aero chocolate bar! Stainless steel impellers tend to be more robust in these applications.
The exact NPSH (nett positive suction head) available can be calculated and plotted against the selected pump to ensure cavitation-free operation.
Source
ÐÇ¿Õ´«Ã½ Sustainable Design