Cavitation can occur with any amount of air. They should also be used on long shaft vertical pumps to prevent excessive shaft movement that will cause problems with the packing, seals, bearings and critical dimensions. Air is entering at the pump suction. There is liquid turbulence at the pump suction. Cavitation damage on the leading edge of the impeller blade indicates internal recirculation.
Higher numbers mean that the problem is with the impeller shape or adjustment.
Cavitation damage just beyond the cut water on the casing and tip of the impeller blade indicates the impeller blade is too close to the cut water. A repaired or substituted impeller is often the cause of the problem in a non self-priming pump. Do not use packing in any pump that runs under a vacuum, as air will enter the system through the pump stuffing box. Pumps that take their suction from a condenser or evaporator. Any pump that takes its suction from a negative pressure.