3 - Pump characteristics—positive displacement vs centrifugal (kinetic)

Abstract

This chapter presents characteristics of positive displacement pumps and centrifugal pumps. The effects of viscosity, specific gravity, and system resistance on performance are described. Flow from a positive displacement (PD) pump can be produced either through rotary or reciprocating action. Rotary action discharges smooth flow, whereas reciprocating pumps produces pulsating flow characteristics. Adding multiple cylinders and timing the pumping strokes for small overlaps, will significantly reduce the pulsations in reciprocating pumps. The actual flow from a PD pump is less than the theoretical quantity by the amount of losses associated with the specific type of PD pump. As the viscosity of a fluid handled by a reciprocating pump is increased, valve action is affected. Higher valve leakage occurs at constant pump speed resulting in lower volumetric efficiency. To maintain the same volumetric efficiency, pump speed will have to be reduced as viscosity increases. Regardless of the type of PD pump, changes in specific gravity do not affect performance. It is found that the effect of increased viscosity on centrifugal pump performance can be significant.