CHAPTER 4PUMPSPumps are used for some essential services inthe Navy. Pumps supply water to the boilers, drawcondensation from the condensers, supply seawater to the firemain, circulate cooling water forcoolers and condensers, pump out bilges, transferfuel, supply water to the distilling plants, andserve many other purposes. Although the pumpsdiscussed in this chapter are used primarily inhydraulic systems, the principles of operationapply as well to the pumps used in other systems.PURPOSEThe purpose of a hydraulic pump is to supplya flow of fluid to a hydraulic system. The pumpdoes not create system pressure, since pressure canbe created only by a resistance to the flow. As thepump provides flow, it transmits a force to thefluid. As the fluid flow encounters resistance, thisforce is changed into a pressure. Resistance toflow is the result of a restriction or obstructionin the path of the flow. This restriction is normallythe work accomplished by the hydraulic system,but can also be restrictions of lines, fittings, andvalves within the system. Thus, the pressure iscontrolled by the load imposed on the system orthe action of a pressure-regulating device.OPERATIONA pump must have a continuous supply offluid available to the inlet port to supply fluid tothe system. As the pump forces fluid through theoutlet port, a partial vacuum or low-pressure areais created at the inlet port. When the pressure atthe inlet port of the pump is lower than the localatmospheric pressure, atmospheric pressure actingon the fluid in the reservoir forces the fluid intothe pump’s inlet. If the pump is located ata level lower than the reservoir, the force ofgravity supplements atmospheric pressure on thereservoir. Aircraft and missiles that operate athigh altitudes are equipped with pressurizedhydraulic reservoirs to compensate for lowatmospheric pressure encountered at highaltitudes.PERFORMANCEPumps are normally rated by their volumetricoutput and pressure. Volumetric output is theamount of fluid a pump can deliver to its outletport in a certain period of time at a given speed.Volumetric output is usually expressed in gallonsper minute (gpm). Since changes in pump speedaffect volumetric output, some pumps are ratedby their displacement. Pump displacement is theamount of fluid the pump can deliver per cycle.Since most pumps use a rotary drive, displacementis usually expressed in terms of cubic inches perrevolution.As we stated previously, a pump does notcreate pressure. However, the pressure developedby the restrictions in the system is a factor thataffects the volumetric output of the pump. As thesystem pressure increases, the volumetric outputdecreases. This drop in volumetric output is theresult of an increase in the amount of internalleakage from the outlet side to the inlet side ofthe pump. This leakage is referred to as pumpslippage and is a factor that must be consideredin all pumps. This explains why most pumps arerated in terms of volumetric output at a givenpressure.CLASSIFICATION OF PUMPSMany different methods are used to classifypumps. Terms such as nonpositive displacement,positive displacement, fixed displacement,variable displacement, fixed delivery, variabledelivery, constant volume, and others are used todescribe pumps. The first two of these termsdescribe the fundamental division of pumps; that4-1
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