COMPARISON BETWEEN POSITIVE DISPLACEMENT & ROTARY PUMPThe main diference between them is the way that energy is added to thefuid to be converted to pressure increase.In dynamic pumps, energy is added to the fuid continuously throughthe rotary motion of the blades. These rotating blades raise themomentumof fuidandthemomentumthenisconvertedtopressureenergy through difuser in pump outlet. In posii!" disp#ac"m"n pumps, the energy is added periodically tothefuid. thepumphasreciprocatingmotionbypistonsfor example.When the fuid enters the pump through valves, the reciprocating pistonbegins to press the fuid resulting in going out of the pump with pressurerise.The above shown diagramis a simple illustration of the fowratedependenceonthepressureorheadintwomajortypesofpumps. Inreciprocatingpumpthelossesincreaseswithhighheadotherwisefowrateisnearlyconstant. spumpta!esaspeci"camountof fuidanddischarge it.Whileontheotherhandwecouldsimpleobserveaslopegraphforacentrifugal pump. This is because with increase speed of rotor it will drawmore fuid but at the same time for this pump pressure will decrease.#o on basis of this $" p%"&"% c"n%i&u'a# pump &o% (i'( )o$ %a"s &posii!"disp#ac"m"n pumps &o% (i'(p%"ssu%"%"*ui%"m"ns.$entrifugal pumpsarethemost commonone. Therearemanyotherfeatures which efect the selection of pump. %ecause diferent types ofpumps are suitable for diferent functions.&ere we have following propertiesviscosity is one of the majorconsideration in pump selection. Viscosiyis the internalresistance between diferent layersof fuidduringfow. Incentrifugalpump due to high viscosity therotor have to overcome the frictionlosses. While the positivedisplacement or reciprocatingpump will avoid lea!ages and thusincreases e'ciency.The other most important factor isp%imin'+ (riming is the process to"ll the casing with fuid. (umpscould be self primed or else.All of this means that if you intend to use acentrifugal pumpyou'regoingtohavetocome up with some sensible method of priming it. Your choices will include: Install a foot valve in the suction piping to insure the liquid will not drainfrom the pump casing andsuctionpipingwhenthepumpstops. Keepinmindthat thesevalves have a nasty habit ofleaking. vacuatetheair inthesystemwith a positive displacementpriming pump operatingbetween the pump and a closeddischarge valve. !ill thepumpwithliquidpriorto starting it. "onvert the application to a selfprimingpumpthat maintainsareservoir of liquid at its suction.s the pressure developed isrelated to the head by thee)uationhead = pr / sp.weight, the pressure available willbe proportional to the speci"cweight of the li)uid. This means thatthe pressure *or pressure diference+created with air will be only around,-.// times that with water*densityofwater0,///!g- m1 and dry air at #.T.( has a density of,.2 !g-m1 +. Therefore, if the pumpisnotprimed, thesuctionpressurecreatedwill notbesu'cienttoliftwater.When to use a Positive Displacement Pump#hen to use a centrifugal or a $ositive %isplacement pump &'$% $ump() is not always aclear choice. *o make a good choice between these pump types it is important to understandthat these two types of pumps behave very differently.FLOW RATE VERSUS PRESSURE+ylookingat the performance chart totheright you can see ,ust howdifferent thesepumpsare. *hecentrifugal hasvaryingflowdependingonpressureor head- whereas the$%pump has more or less constant flowregardless of pressure.FLOW RATE VERSUS VISCOSITYAnother ma,or differencebetweenthepumptypes is the effect viscosity has on the capacityof the pump. You will notice in the flow ratechart how the centrifugal pump loses flow asthe viscosity goes up but the $% pump.s flowactually increases. *his is because the higherviscosity liquids fill the clearances of thepump causing a higher volumetric efficiency./emember- this chart shows only the effect ofviscosityonthepumpflow0whenthereisaviscosity change there is also greater line lossin the system. *his means you will also haveto calculate the change in pump flow from thefirst chart for this pressure change. EFFICIENCY VERSUS PRESSURE*he pumps behave very differently when considering mechanical efficiency as well. +y looking at theefficiencychart totheright youcanseethe impact of pressure changes on the pump.s efficiency."hangesinpressurehavelittleeffect onthe $% pump but a dramatic one on the centrifugal.EFFICIENCY VERSUS VISCOSITY1iscosity also plays an important role in pump mechanical efficiency. +ecause the centrifugal pumpoperates at motor speed- efficiency goes down as viscosity increases due to increased frictional losseswithin the pump. fficiency often increases in a $%pumpwithincreasingviscosity. 2ote howrapidlyefficiencydropsoffforthecentrifugalpump as viscosity increases.NET POSITIVESUCTIONHEAD REQUIREMENTSAnother consideration is 2$34/. Ina centrifugal the 2$34/varies as a functionof flow- whichisdetermined by pressure and viscosity as discussed above. In a $% pump- 2$34/ varies as afunction of flow which is determined by speed. *he lower the speed of a $% pump- the lowerthe 2$34/.NET POSITIVE SUCTION HEAD AVAILABLE:OPERATING AT DIFFERENT POINTS ON THE CURVEAnother thing to keep in mind when comparing the two types of pumps is that a centrifugalpump does best in the center of the curve. As you move either to the left or right- additionalconsiderations come into play. If you move far enough to the left or right- pump life is reduceddue to either shaft deflection or increased cavitation. #ith a $% pump you can operate thepumponanypoint of thecurve. Infact thevolumetricefficiencyas apercent actuallyimprovesat thehighspeedpart of thecurve. *hisisduetothefact that thevolumetricefficiency is affected by slip- which is essentially constant. At low speed the percentage of slipis higher than at high speed. *he data presented in these charts is the actual data for a specificapplication. *he centrifugal was picked at its +est fficiency $oint &+$) and the $% pump&Internal 5ear) was selected to match the flow- viscosity- and pressure. %ifferent applicationswill have different curves and efficiencies. *hese curves are presented as an e6ample of theperformance behavior differences of the two pump principles.PUMP SELECTION SCENARIOS2ow that you have a clearer understanding of the performance differences between these two pumpprinciples- whenwouldyouchoosetousea$%pump7*hefollowingchart listsseveral suchscenarios.4I54 1I3"83I*YAsillustratedbythegraphsabove- evenmodest viscositiesdramaticallyaffect theflowrateandefficiencyof acentrifugal pump. #hilemanycentrifugals are cataloged to 9-::: c3t and higher- $% pumps are clearly thebetter choice when considering the high energy costs resulting from this lostefficiency.8$/A*I25A#AY !/8;*4 ;I%%< 8!*4 "=/1"entrifugals do not operate well when being run too far off the middle ofthe curve. At best- this results in reduced efficiency which would requirelarger motors and higher energy costs. At worst- this can result in cavitationdamage- shaft deflection- andprematurepumpfailure. $%pumpsontheotherhandcanberunatanypointontheircurvewithoutdamagingthepump or greatly affecting the efficiency.1A/IA*I823 I2$/33=/*he first graph above clearly illustrates the effect that even modest changesinpressurecanhaveontheflowrateofacentrifugal pump.Additionalrestrictions such as debris in a filter- corroded > rough piping- or a valve lefttoo far closed &or too far open) can have a dramatic effect on a centrifugalpump.sflowrateandefficiency.$%pumpsmaintaintheirflowrateandefficiency even with significant changes in pressure.1A/IA*I823 I21I3"83I*Y;any liquids vary in viscosity depending on temperature or due to chemicalreaction. Ariseinviscositywill independentlyalter the flowrate andefficiency. Add to that the rise in pressure due to the increase in frictionallinelosses and$%pumpsbecome theclearchoiceforvariable viscosityapplications.4I54$/33=/3#hile some centrifugals can be run in series to boost their pressures- nonecan compete with $% pumps for high pressure applications. $ressure limitswill depend on the design of each pump- but pressures of ?@: $3I &@A: feet)are not unusual for a $% pump- with some models going over B-::: $3I&C-::: feet). *he capability for a $% pump to produce pressure is so greatthat some type of system overpressure protection is required.34A/323I*I1