Cavitation

Cause and Effect Cavitation is the formation of vapor bubbles in any flow that is subjected to an ambient pressure equal to or less than the vapor pressure of the liquid being pumped. Cavitation damage is the loss of material produced by the collapse of the vapor bubbles against the surfaces of the impeller or casing. Formation of these bubbles cannot occur if the net positive suction head supplied, or NPSHA, exceeds the NPSH required for cavitation inception. This NPSH inception value is usually significantly higher than the usually mentioned NPSH required, or NPSHR, which is based on a certain amount of cavitation being present to create a prescribed deviation in pump performance.

Diagnosis from Pump Operation Pump operation in the presence of sufficient cavitation activity will reduce both the total head and the output capacity. A steady crackling noise in and around the pump suction indicates cavitation. A random crackling noise with high-intensity knocks indicates suction recirculation but does not indicate a degradation of performance if NPSHA is greater than NPSHR. The random crackling is the unsteady occurrence of cavitation that generally accompanies suction recirculation, which, in turn, is an unsteady phenomenon.

Diagnosis from the Visual Examination of Surface Damage Cavitation damage from inadequate NPSH (that is, NPSHA < NPSHR) occurs on the low-pressure or the visible surface of the impeller inlet vane.

Instrumentation A suction gage or manometer in the pump suction can be used to determine whether the NPSH available is equal to or greater than the NPSH required from the manufacturer's rating curve (NPSHR).

Corrective Procedures If additional NPSH cannot be supplied, the capacity of the pump should be reduced until the required NPSH is equal to or less than the available NPSH. If this is not possible, impeller improvement may be necessary. This may be accomplished on some impeller designs by reworking the geometry or impeller surface finish to reduce losses, improve flow characteristics, or increase the flow inlet area (thus lowering impeller inlet velocity). If this is not possible, consideration should be given to replacing the impeller (first stage or suction impeller on multistage pumps) with one of improved suction performance. The pump manufacturer should be consulted to determine the most satisfactory course of action.

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