If a pump's inlet pressure falls too low, the liquid boils into vapour bubbles. They are swept into the high-pressure side and collapse violently against the impeller, eating away the metal.
It is the formation and violent collapse of vapour bubbles in a liquid that has been pulled to a pressure below its vapour pressure. In a pump it shows up as noise, vibration, falling performance and, eventually, a pitted, eroded impeller.
Cavitation is invisible from the outside except for noise and performance loss. A cutaway shows exactly where the bubbles are born, where they collapse against the metal, and why the damage appears on the impeller.
The impeller lowers the pressure at its eye to draw liquid in. If the inlet pressure is too low to start with, the pressure at the eye falls below the liquid's vapour pressure.
Below that pressure the liquid boils, even when cold. A cloud of vapour bubbles appears on the low-pressure side of the vanes.
The vanes fling the liquid outward, carrying the bubbles with it into a region where the pressure is rising again.
Rising pressure turns the vapour back to liquid, and the bubbles implode. Because they collapse so fast, the liquid hammers the metal surface with tiny, intense impacts.
Repeated collapse pits and eats away the impeller, especially near the vane tips. Performance falls, noise rises, and eventually the impeller must be replaced.
The local pressure in the liquid falls below its vapour pressure. In a pump this usually means the inlet pressure is too low, the liquid is too hot, or the pump is run beyond the flow it can safely draw.
Both turn liquid to vapour, but boiling is caused by heating, while cavitation is caused by a pressure drop. A cold liquid can cavitate if the pressure falls far enough.
Like gravel or marbles being pumped through the machine, a rattling, crunching noise on top of the normal hum. Vibration and falling flow usually come with it.
Yes. The collapsing bubbles erode the impeller surface, leaving a pitted, sponge-like texture. Left alone, it destroys the impeller and the pump's performance.
By keeping enough pressure at the inlet: a bigger suction pipe, a lower pump position, a cooler liquid, or reducing the flow so the pump is not starved. A proper net positive suction head keeps the pressure above the vapour pressure.