POSITIVE DISPLACEMENT PUMPSDOE-HDBK-1018/1-93PumpsPositiveDisplacementPumpProtectionPositive displacement pumps are normally fitted with relief valves on the upstream side of theirdischarge valves to protect the pump and its discharge piping from overpressurization. Positivedisplacement pumps will discharge at the pressure required by the system they are supplying.The relief valve prevents system and pump damage if the pump discharge valve is shut duringpump operation or if any other occurrence such as a clogged strainer blocks system flow.SummaryThe important information in this chapter is summarized below.Positive Displacement Pumps SummaryThe flow delivered by a centrifugal pump during one revolution of the impeller dependsupon the head against which the pump is operating. The positive displacementpump delivers a definite volume of fluid for each cycle of pump operationregardless of the head against which the pump is operating.Positive displacement pumps may be classified in the following ways:Reciprocating piston pumpGear-type rotary pumpLobe-type rotary pumpScrew-type rotary pumpMoving vane pumpDiaphragm pumpAs the viscosity of a liquid increases, the maximum speed at which a reciprocatingpositive displacement pump can properly operate decreases. Therefore, as viscosityincreases, the maximum flow rate through the pump decreases.The characteristic curve for a positive displacement pump operating at a certainspeed is a vertical line on a graph of head versus flow.Slippage is the rate at which liquid leaks from the discharge of the pump back tothe pump suction.Positive displacement pumps are protected from overpressurization by a relief valveon the upstream side of the pump discharge valve.ME-03Rev. 0Page 28
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