Abstract
This chapter discusses the concept of net positive suction head (NPSH) and gives various mathematical formulations for its calculation. The value of NPSH calculated by the given equation is by definition called “available” NPSH (NPSHA), as it relates to the system condition available at the pump inlet. Further, this chapter explains the concept of NPSH required (NPSHR). Because of the energy conversion and the internal losses, the pressure inside the pump varies. A pressure drop between the pump inlet and the blade tip results from hydraulic losses and energy conversion, and a further pressure drop occurs after the blade tip as the fluid is accelerated again. Together, these effects make up the pressure drop between the pump inlet and the point of lowest pressure. This pressure drop is designated as ∆psuction. It can be seen that ∆psuction has a direct impact on the onset of cavitation: the greater ∆psuction, the more likely is the occurrence of cavitation. Therefore, a larger value of NPSHA will be needed to avoid cavitation. The main geometrical parameters of the inlet part of the impeller governing the value of ∆psuction are: impeller inlet eye diameter Dg, number of blades, thickness of blades at inlet, and inlet blade angle.
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