CENTRIFUGAL PUMPS Fluid FlowOperatingCharacteristicsofaCentrifugalPumpNormally, a centrifugal pump produces a relatively low pressure increase in the fluid. Thispressure increase can be anywhere from several dozen to several hundred psid across acentrifugal pump with a single stage impeller. The term PSID (Pounds Force Per Square InchDifferential) is equivalent to DP. In this context, it is the pressure difference between the suctionand discharge of a pump. PSID can also be used to describe a pressure drop across a systemcomponent (strainers, filters, heat exchangers, valves, demineralizers, etc.). When a centrifugalpump is operating at a constant speed, an increase in the system back pressure on the flowingstream causes a reduction in the magnitude of volumetric flow rate that the centrifugal pump canmaintain.Figure 7 Typical Centrifugal PumpCharacteristic CurveAnalysis of the relationship between thevolumetric flow rate ( ) that a centrifugalVpump can maintain and the pressuredifferential across the pump (DPpump) isbased on various physical characteristics ofthe pump and the system fluid. Variablesevaluated by design engineers to determinethis relationship include the pump efficiency,the power supplied to the pump, therotational speed, the diameter of the impellerand blading, the fluid density, and the fluidviscosity. The result of this complicatedanalysis for a typical centrifugal pumpoperating at one particular speed isillustrated by the graph in Figure 7.Pump head, on the vertical axis, is thedifference between system back pressure and the inlet pressure of the pump (DPpump). Volumetricflow rate ( ), on the horizontal axis, is the rate at which fluid is flowing through the pump. TheVgraph assumes one particular speed (N) for the pump impeller.CavitationWhen the liquid being pumped enters the eye of a centrifugal pump, the pressure is significantlyreduced. The greater the flow velocity through the pump the greater this pressure drop. If thepressure drop is great enough, or if the temperature of the liquid is high enough, the pressuredrop may be sufficient to cause the liquid to flash to steam when the local pressure falls belowthe saturation pressure for the fluid that is being pumped. These vapor bubbles are swept alongthe pump impeller with the fluid. As the flow velocity decreases the fluid pressure increases.This causes the vapor bubbles to suddenly collapse on the outer portions of the impeller. Theformation of these vapor bubbles and their subsequent collapse is cavitation.HT-03 Page 48 Rev. 0
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