Chapter 11 - Causes of intermittent and chronic cavitation

Abstract

This chapter discusses the causes of cavitation. Net positive suction head required (NPSHR) is a pump characteristic which defines the net positive suction head required at the pump suction flange to avoid cavitation. It varies with pump flow rate; the higher the flow rate, the greater is the NPSHR. NPSHR increases dramatically at extremely low-flow rates, very close to shut-off (zero flow), due to the increased thermal energy absorbed by the fluid. NPSHR is independent of the system characteristics, and remains the same regardless of which system the pump is installed in. There are three prime causes of low NPSHA induced cavitation. The first is the high flow, which causes a greater velocity increase into the impeller eye, and an increased pressure drop in the pump inlet passage to the impeller and in the suction piping and vessels. Second cause is low flow, which creates an abnormal increase in the temperature of the fluid, as it gains energy from the impeller, and the recirculation flow through wear rings or front impeller clearance allows a high percentage of flow to gain additional energy. The third cause is the loss of suction pressure due to changed conditions in the suction system. Further, where the NPSH margin is small, proper vigilance of suction vessel level and periodic assessment of pump performance is required to assess the degree of opening of running clearances.