Fluid System 13-Positive Displacement Pump

Lab. Mekanika Fluida Teknik Mesin-FTUI Dr.Ir. Harinaldi, M.Eng


Lab. Mekanika Fluida Teknik Mesin-FTUI Dr.Ir. Harinaldi, M.EngIntroductionThe reciprocating pump is a positive displacement pump as it sucks and raises the liquid by actually displacing it with a piston/plunger that executes a reciprocating motion in a closely fitting cylinderThe amount of liquid pumped is equal to the volume displaced by the pistonThe pumps designed with disk pistons create pressures upto 25 bar and the plunger pumps built up still higher pressuresDischarge from these pumps is almost wholly dependent on the pump speedThe total efficiency of a reciprocating pump is about 10 to 20% higher than a comparable cerittifugal pump


Lab. Mekanika Fluida Teknik Mesin-FTUI Dr.Ir. Harinaldi, M.EngThe reciprocating pump is generally employed for:Light oil pumpingFeeding small boilers condensate return, andhydraulics pressure systems


Lab. Mekanika Fluida Teknik Mesin-FTUI Dr.Ir. Harinaldi, M.EngClassificationReciprocating pumps are classified as:According to the water being in contact with piston:Single-acting pump: water is in contact with one side of the pistonDouble-acting pump: water is in contact with both sides of the pistonAccording to number of cylinders:Single cylinder pumpMulti cylinder pumpAccording to prime mover of the piston:A power pump is one that reciprocates the pumping element with a crankshaft or camshaftA direct-acting pump is a reciprocating pump driven by a fluid which has a differential pressure


Lab. Mekanika Fluida Teknik Mesin-FTUI Dr.Ir. Harinaldi, M.EngComponents and Working PrinciplesThe main parts of a reciprocating pump are:Cylinder PistonSuction valve Delivery valveSuction pipe Delivery pipeCrank and connecting rod mechanism operated by a power'source e.g. steam engine, internal combustion engine or an electric motor


Lab. Mekanika Fluida Teknik Mesin-FTUI Dr.Ir. Harinaldi, M.EngComponents and Working PrinciplesWorking of a single-acting reciprocating pump:initially the crank is at the inner dead centre (IDC) and crank rotates in the clockwise directionAs the crank rotates, the piston moves towards right and a vacuum is created on the left side of the piston, causing suction valve to open and consequently the liquid is forced from the sump into the left side of the piston.When the crank is at the outer dead centre (ODC) the suction stroke is completed and the left side of the cylinder is full of liquidWhen the crank further turns from ODC to IDC, the piston moves inward to the left and high pressure is built up in the cylinderThe delivery valve opens and the liquid is forced into the delivery pipe. The liquid is carried to the discharge tank through the delivery pipeAt the end of delivery stroke the crank comes to the I.D.C and the piston is at the extreme left position


Lab. Mekanika Fluida Teknik Mesin-FTUI Dr.Ir. Harinaldi, M.EngComponents and Working PrinciplesWorking of a double-acting reciprocating pump:suction and delivery strokes occur simultaneously. When the crank rotates from IDC in the clockwise direction, a vaccurn is created on the left side of piston and the liquid is sucked in from the sump through value S1At the same time, the liquid on the right side of the piston is pressed and a high pressure causes the delivery valve D2 to open and the liquid is passed on to the discharge tank. This operation continues till the crank reaches ODCWith further rotation of the crank, the liquid is sucked in from the sump through the suction valve S2 and is delivered to the discharge tank through the delivery valve D1. When the crank reaches IDC, the piston is in the extreme left position. Thus one cycle is completed and as the crank further rotates, cycles are repeated.


Lab. Mekanika Fluida Teknik Mesin-FTUI Dr.Ir. Harinaldi, M.EngComponents and Working PrinciplesVariations of discharge through delivery pipe (Qd) with crank angle ()a double-acting reciprocating pumpa single-acting reciprocating pump


Lab. Mekanika Fluida Teknik Mesin-FTUI Dr.Ir. Harinaldi, M.EngDischarge, Work done and PowerSingle Acting Reciprocating PumpD = diameter of the cylinder (m)A = cross-sectional area of the piston/cylinder (m2)r = radius of crank (m)N = speed of the crank (rpm)L = length of the stroke (= 2r) (m)hs = height of the centre of the cylinder above the liquid surface (m)hd = height to which the liquid is raised above the centre of the cylinder (m)Suction volume during suction stroke :Discharge of the pump per second :Weight of fluid delivered per second :Power required to drive the pump :


Lab. Mekanika Fluida Teknik Mesin-FTUI Dr.Ir. Harinaldi, M.EngDischarge, Work done and PowerDouble Acting Reciprocating PumpD = diameter of the cylinder (m)d = diameter of piston rod (m2)Other parameters are similar to single acting pumpVolume delivered in one revolution of the cranck :Discharge of the pump per second :Weight of fluid delivered per second :Power required to drive the pump :Area on one side of the piston:Area on other side of the piston where piston rod is connected :


Lab. Mekanika Fluida Teknik Mesin-FTUI Dr.Ir. Harinaldi, M.EngCoefficient of DischargeCo-efficient of dischargeIn a reciprocating pump, the actual discharge (Qact) is always slightly different from the theoretical discharge (Qth) due to following reasons:Leakage through the valves, glands and piston packingImperfect operation of the valves (suction and discharge)Partial filling of cylinder by the liquidThe ratio between actual discharge and theoretical discharge is known as the co-efficient of discharge (Cd) of the pumpWhen the value of Cd is expressed in percentage, it is known as volumetric effidency of the pumpVolumetric efficiency depends upon the dimensions of the pump and its value ranges from 85-98%


Lab. Mekanika Fluida Teknik Mesin-FTUI Dr.Ir. Harinaldi, M.EngSlipSlipThe difference between the theoretical discharge and actual discharge is called the 'slip' of the pump:The slip is oftenly expressed in percentageThe percentage of slip for the pumps maintained in good condition is of the order of 2% or even lessin some cases Qact. may be more than Qth and the slip will be 'negativeThe slip will be negative when there is a direct connection between the suction and delivery sides before the end of suction strokeThis happens if the momentum of liquid in the suction pipe is large enough to open the delivery valve before the beginning of delivery strokeThe negative slip is possible in case of pumps having long suction pipe and a short delivery pipe, especially when these are operating at high speeds.


Lab. Mekanika Fluida Teknik Mesin-FTUI Dr.Ir. Harinaldi, M.EngAcceleration of PistonEffect of Acceleration of Piston on Velocity and Pressure in the Suction and Delivery PipesIf the crank rotates uniformly and the connection rod is long enough compared to the radius of crank, the piston makes simple harmonicThis causes acceleration during the first half of the stroke and deceleration during the second half of the strokeAngle turned by the crank in time t :Let :A = cross-sectional area of the piston/cylinder (m2)a = area of the pipe, suction or delivery (m2)l = length of the pipe, suction or delivery (m)r = radius of the crank (m)= angular speed of the crank (rad/s)v = velocity of fluid in the pipe (m/s)The corresponding distance (x) travelled- by the piston :


Lab. Mekanika Fluida Teknik Mesin-FTUI Dr.Ir. Harinaldi, M.EngAcceleration of PistonVelocity of the piston :Acceleration of the piston :From continuity considerations, the volume of liquid flowing from the pipe equals the volume of liquid flowing into the cylinder :Velocity of fluid in the pipe :Acceleration of fluid in the pipe :Force required to accelerate the fluid in the pipe :Intensity of pressure due to acceleration :


Lab. Mekanika Fluida Teknik Mesin-FTUI Dr.Ir. Harinaldi, M.EngAcceleration of PistonPressure head due to acceleration:In suction pipe:In delivery pipe:Pressure head due to acceleration is a function of angular displacement

haKeterangan0o(l/g)(A/a)2rAwal langkah (percepatan)90o0Pertengahan langkah180o-(l/g)(A/a)2rAkhir langkah (perlambatan)


Lab. Mekanika Fluida Teknik Mesin-FTUI Dr.Ir. Harinaldi, M.EngAcceleration of PistonIn that case the pressure head due to acceleration:In case the connecting rod is not very long as compared to crank length then it cannot be assumed that the piston has a simple harmonic motionn = ratio of the length of connecting rod to the crank length

haKeterangan0o(l/g)(A/a)2r(1+1/n)Awal langkah (percepatan)90o0Pertengahan langkah180o-(l/g)(A/a)2r(1-1/n)Akhir langkah (perlambatan)


Lab. Mekanika Fluida Teknik Mesin-FTUI Dr.Ir. Harinaldi, M.EngAcceleration of PistonEffect of Acceleration of Piston on Friction in PipesThe liquid flowing through suction and delivery pipes causes loss of head due to friction which is given by Darcy-Weisbach equation as:Due to the effect of acceleration of piston the friction varies:f = coefficient of fricitionl = length of the pipe, suction or delivery (m)d = diameter of the pipe (m)v = velocity of fluid in the pipe (m/s)

hfKeterangan0o0Awal langkah (percepatan)90o(4fl/2dg)[(A/a)r]2Pertengahan langkah180o0Akhir langkah (perlambatan)


Lab. Mekanika Fluida Teknik Mesin-FTUI Dr.Ir. Harinaldi, M.EngIndicator DiagramA diagram which shows the pressure head of the liquid in the cylinder corresponding to any position during the suction and delivery strokesPressure head is taken as ordinate and stroke length as abscissaIdeal Indicator DiagramObtained by neglecting the loss of head due to friction in the suction and delivery pipes and the effect of acceleration of pistonSingle cylinder single acting pumpThe work done by the pump per second is:since AN/60 = constant


Lab. Mekanika Fluida Teknik Mesin-FTUI Dr.Ir. Harinaldi, M.EngIndicator DiagramEffect of acceleration in pipes on indicator diagramAt the beginning of the stroke the piston moves outwardit create not only a negative pressure equal to the suction head but also accelerate the liquidSo that separation does not take place, the absolute pressure at the beginning of stroke should not fall below the vapour pressureSuction

LangkahHead percepatanHead vakumHead mutlakAwal(ls/g)(A/as)2rhs+ (ls/g)(A/as)2rHatm- [hs+ (ls/g)(A/as)2r]Tengah0hsHatm- hsAkhir- (ls/g)(A/as)2rhs - (ls/g)(A/as)2rHatm- [hs - (ls/g)(A/as)2r]


Lab. Mekanika Fluida Teknik Mesin-FTUI Dr.Ir. Harinaldi, M.EngIndicator DiagramEffect of acceleration in pipes on indicator diagramIn the beginning of delivery stroke the liquid in the delivery pipe is acceleratedAt the end of delivery stroke the liquid is retardedThe absolute pressure head at the end of delivery stroke should not be less than vapour pressure to avoid separation.Delivery

LangkahHead percepatanHead GaugeHead Mutlakbegin(ld/g)(A/ad)2rhd+ (ld/g)(A/ad)2rHatm+ [hd+ (ld/g)(A/ad)2r]middle0hdHatm+ hdend- (ld/g)(A/ad)2rhd - (ld/g)(A/ad)2rHatm+ [hd - (ld/g)(A/ad)2r]


Lab. Mekanika Fluida Teknik Mesin-FTUI Dr.Ir. Harinaldi, M.EngIndicator DiagramDue to acceleration in suction and delivery pipes, the indicator diagram has changed but the area of indicator diagram remains unalteredThus the total work done remains the sameThe main effect of the acceleration head is that it increases the negative head at the beginning of suction strokeIf the simple harmonic motion does not take place, the straight lines AB and CD will become slightly curved


Lab. Mekanika Fluida Teknik Mesin-FTUI Dr.Ir. Harinaldi, M.EngIndicator DiagramEffect of friction in pipes on indicator diagramVariation of hf with is parabolic; zero at the beginning and end of the strokes; maximum at the middle of the strokesSuctionWork done against friction in suction

LangkahHead GesekanHead VakumHead MutlakAwal0hsHatm- hsTengah(4fls/2dsg)[(A/as)r]2hs + (4fls/2dsg)[(A/as)r]2Hatm- hs + (4fls/2dsg)[(A/as)r]2Akhir0hsHatm- hs


Lab. Mekanika Fluida Teknik Mesin-FTUI Dr.Ir. Harinaldi, M.EngIndicator DiagramEffect of friction in pipes on indicator diagramVariation of hf with is parabolic; zero at the beginning and end of the strokes; maximum at the middle of the strokesDeliveryWork done against friction in delivery

LangkahHead GesekanHead GaugeHead MutlakAwal0hdHatm+hdTengah(4fld/2ddg)[(A/ad)r]2hd+ (4fld/2ddg)[(A/ad)r]2Hatm+ hd+ (4fld/2ddg)[(A/ad)r]2Akhir0hdHatm+ hd


Lab. Mekanika Fluida Teknik Mesin-FTUI Dr.Ir. Harinaldi, M.EngIndicator DiagramEffect of acceleration and friction in pipes on indicator diagramThe work done by the pump per second is:


Lab. Mekanika Fluida Teknik Mesin-FTUI Dr.Ir. Harinaldi, M.EngAir VesselsA closed chamber containing compressed air in the upper part and liquid being pumped in the lower partOne air vessel is fixed on the suction pipe just near the suction valve and one is flxed on the delivery pipe near the delivery valve. The air vessels are used for the following purposes:To get continuous supply of liquid at a uniform rate To save the power required to drive the pumpTo run the pump at much higher speed without any danger of separation


Lab. Mekanika Fluida Teknik Mesin-FTUI Dr.Ir. Harinaldi, M.EngAir VesselsIndicator diagrams without and with air vesselsA = area of cross-section of the cylindera = area of cross-section of suction or delivery pipeId = length of delivcry pipe beyond the air vesselId'= length of delivery pipe between cylinder and air vessells = length of suction pipe below air vesselIs'= length of suction pipe between cylinder and air vesselhad = pressure head due to acceleration in delivery pipehas= pressure head due to acceleration in suction pipehfd = loss of head due to friction in delivery pipe beyond the air vesselhfd = loss of head due to friction in delivery pipe between cylinder and air vesselhfs= loss of head due to friction in suction pipe below the air vesselhfs,= loss of head due to friction in suction pipe between cylinder and air vesselwithout air vesselswith air vessels


Lab. Mekanika Fluida Teknik Mesin-FTUI Dr.Ir. Harinaldi, M.EngAir VesselsWithout air vesselsWork done against frictionWith air vesselsThe velocity of flow in pipes may be assumed constant and equal to the average mean flow velocityMean velocity of flowFriction Head lossWork done against friction


Lab. Mekanika Fluida Teknik Mesin-FTUI Dr.Ir. Harinaldi, M.EngAir VesselsWork SavedRatio of work done again frictionPercentage of work saved in pipe friction by fitting air vesselsFor double acting reciprocating pump


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Fluid System 13-Positive Displacement Pump