POSITIVE DISPLACEMENT PUMPSDOE-HDBK-1018/1-93PumpsPOSITIVE DISPLACEMENT PUMPSPositive displacement pumps operate on a different principle than centrifugalpumps. Positive displacement pumps physically entrap a quantity of liquid at thesuction of the pump and push that quantity out the discharge of the pump.EO 2.1STATE the difference between the flow characteristics ofcentrifugal and positive displacement pumps.EO 2.2Given a simplified drawing of a positive displacement pump,CLASSIFY the pump as one of the following:a.Reciprocating piston pumpb.Gear-type rotary pumpc.Screw-type rotary pumpd.Lobe-type rotary pumpe.Moving vane pumpf.Diaphragm pumpEO 2.3EXPLAIN the importance of viscosity as it relates to theoperation of a reciprocating positive displacement pump.EO 2.4DESCRIBE the characteristic curve for a positivedisplacement pump.EO 2.5DEFINE the term slippage.EO 2.6STATE how positive displacement pumps are protectedagainst overpressurization.IntroductionA positive displacement pump is one in which a definite volume of liquid is delivered for eachcycle of pump operation. This volume is constant regardless of the resistance to flow offeredby the system the pump is in, provided the capacity of the power unit driving the pump or pumpcomponent strength limits are not exceeded. The positive displacement pump delivers liquid inseparate volumes with no delivery in between, although a pump having several chambers mayhave an overlapping delivery among individual chambers, which minimizes this effect. Thepositive displacement pump differs from centrifugal pumps, which deliver a continuous flow forany given pump speed and discharge resistance. Positive displacement pumps can be grouped into three basic categories based on their designand operation. The three groups are reciprocating pumps, rotary pumps, and diaphragm pumps.ME-03Rev. 0Page 18
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