Reciprocating Pumps

RECIPROCATING

PUMPSPrinciples Components Performance

OPERATING PRINCIPLESbull A reciprocating positive displacement pump is one inwhich a plunger or piston displaces a given volume offluid for each strokebull All reciprocating pumps have a fluid-handling portioncommonly called the liquid end which has the followingcomponents (Figure 69)- a displacing solid called a plunger or piston- a container to hold the liquid called the liquid cylinder- a suction check valve to admit fluid from the suction pipeinto the liquid cylinder- a discharge check valve to admit flow from the liquidcylinder into the discharge pipe- packing to seal tightly the joint between the plunger andthe liquid cylinder to prevent the liquid from leaking outof the cylinder and air from leaking into the cylinderChapter 4 Reciprocating Pumps 93C H A P T E R 4FIGURE 69 Liquid End of a Reciprocating PumpDuring the Suction StrokeReproduced with permission of McGraw-Hill from Karassik I J (ed)Pump Handbook 2nd ed 1986

bull To pump liquid the following sequence of events occurs- where the plunger is moved out of the liquid cylinder thepressure of the liquid within the cylinder is reduced- when the pressure becomes less than in the suctionpipe the suction check valve opens and the liquid flowsinto the cylinder to fill the volume being vacated by withdrawalof the plunger During this phase of the operationthe discharge check valve is held closed by the higherpressure in the discharge pipe This portion of the pumpingaction is called the suction stroke- the withdrawal movement must be stopped before theend of the plunger gets to the packing The plunger movementis then reversed and the discharge stroke portion ofthe pumping action is started (Figure 70)94 Pumps Reference GuidePackingPlungerSuction1048576PipeSuction1048576Check ValveDischarge 1048576Check ValveDischarge1048576PipeMotionbull Pumping cycle of a single-acting reciprocating pump- it is called single-acting because it makes only one dischargestroke in one cycle

- movement of the plunger into the liquid cylinder causesan immediate increase in pressure of the liquid containedtherein This pressure becomes higher than suction pipepressure and causes the suction check valve to close- with further plunger movement the liquid pressure continuesto rise When the liquid pressure attains that in thedischarge pipe the discharge check valve is forced openand liquid flows into the discharge pipe The volumeforced into the discharge pipe is equal to the plungerFIGURE 70 Liquid End of a Reciprocating PumpDuring the Discharge StrokeReproduced with permission of McGraw-Hill from Karassik I J (ed)Pump Handbook 2nd ed 1986Chapter 4 Reciprocating Pumps 95Plunger PackingSuction1048576PipeSuction1048576Check ValveDischarge 1048576Check ValveDischarge1048576PipeMotiondisplacement less small losses The plunger displacementis the product of its cross-sectional area multiplied by thelength of the stroke The plunger must be stopped before itimpacts on the bottom of the liquid cylinder The motion ofthe plunger is then reversed and the plunger proceeds tothe suction strokebull Many reciprocating pumps are double-acting ie theymake two suction and two discharge strokes for one reciprocatingcycle Most double-acting pumps use a pistonwhich as a displacing solid is sealed to a bore within the liquidcylinder Figure 71 shows a double-acting liquid endbull Double-acting pumps have also two suction and twodischarge check valves one of each on either end of thepiston The piston is moved by a piston rod The piston rodseal (packing) prevents liquid from leaking out of the cylinderWhen the piston is moved in the direction shown the rightside of the piston is on a discharge stroke and the left sideof the piston is simultaneously on a suction stroke The pistonseal must seal tightly to the cylinder liner to preventleakage of liquid from the high pressure right side to thelow pressure left side The piston must be stopped before ithits the right side of the cylinder The motion of the pistonis then reversed so the left side of the piston becomes thedischarge stroke and the right begins the suction strokebull A reciprocating pump must have a driving mechanism toprovide motion and force to the piston The two mostcommon driving mechanisms are a reciprocating steamengine and a crank and throw device Pumps that use thesteam engine are called direct-acting steam pumps Pumpsthat use the crank and throw device are called power pumps96 Pumps Reference GuideFIGURE 71 Double-Acting Liquid EndReproduced with permission of McGraw-Hill from Karassik I J (ed)Pump Handbook 2nd ed 1986

Power pumps must be connected to an external prime

mover such as an electric motor steam turbine or internalcombustion engine

COMPONENTS OF A POWER PUMPbull The subsequent data is a description of the fluid and powerend components of a power pumpLIQUID END

bull The liquid end comprises the cylinder plunger or pistonstuffing box check valves and manifolds (Figure 72)Chapter 4 Reciprocating Pumps 97Suction1048576PipeDischarge1048576PipeMotionValveValveValveValvePiston 1048576PackingPistonPiston1048576RodPiston1048576Rod1048576PackingDischarge ManifoldSuction ManifoldLiquid CylinderFIGURE 72 Liquid End of a Horizontal Power PumpReproduced with permission of McGraw-Hill from Karassik I J (ed)Pump Handbook 2nd ed 1986

Cylinderbull The cylinder is the container where the pressure is developedCylinders on many horizontal pumps have the suctionand discharge manifolds made integral with the cylinderVertical pumps usually have separate manifoldsPlunger and Pistonbull The plunger or piston transmits the force that developsthe pressure Pistons are used for liquid pressures up to1000 psi For higher pressures a plunger is usually used(typical range 1000 to 30000 psi)98 Pumps Reference Guide1048576

Stuffing BoxSuction Manifold1048576104857610485761048576_ ___1048576 1048576_ _ _ _ _1048576 1048576_ _ __ _104857610485761048576104857610485761048576__ _1048576 10485761048576____1048576 _ _

_Discharge ManifoldDischarge1048576ValveCylinderSuction1048576ValvePlungerStuffing Boxbull The stuffing box prevents the plunger or the piston rodfrom leaking liquid to atmosphere or allowing air to enterthe liquid end of the pump It consists of a casing upper andlower bushing packing and glandCheck Valvesbull These valves dependent on the stroke of the plunger or pistoneither allow liquid to flow through or halt the entering

or leaving the liquid end of the pump There are many typesof valves their use is dependent on the applicationbull The main parts of the valves are the seat and the plate Theplate movement is controlled by a spring or retainer Theseat normally utilizes a taper where it fits into the cylinderor manifold The taper not only gives a positive fit but alsoallows easy interchangeability of the seatbull Figure 73 shows various check valves with their applicationManifoldsbull Manifolds are the chambers where liquid is dispersed or collectedfor distribution before or after passing through thecylinder On horizontal pumps the suction and dischargemanifold is usually made integral with the cylinder Mostvertical pumps have the suction and discharge manifold separatefrom the cylinderPOWER END

bull The power end comprises the crankshaft connecting rodcrosshead pony rod bearings and frame (Figure 74)Chapter 4 Reciprocating Pumps 99FIGURE 73 Types of Check ValvesReproduced with permission of McGraw-Hill from Karassik I J (ed)Pump Handbook 2nd ed 1986100 Pumps Reference Guide1048576___1048576____1048576 _1048576_1048576_1048576_1048576___104857610485761048576_1048576___1048576____ 1048576_1048576___1048576__1048576_

SKETCHBABABAABBATYPE PRESSURE APPLICATIONPlateWingBallPlugSlurry50001000030000600025000Clean fluid Plate1048576is metal or plasticClean fluidsChemicalsMud slurry Pot1048576dimensions to API-121048576Polyurethane or1048576Buna-N insertChemicalsFluids with particles1048576Clear clean fluid1048576at high pressure1048576Ball is chrome platedA=Seat Area10485761048576B=Spill Area

Crankshaftbull The crankshaft provides the method of obtaining oscillatingmotion on the plunger An eccentric offset equivalent to onehalf the required stroke is cast into this component Theconnecting rod is affixed to this offset and transfers thepower

Connecting Rodbull The connecting rod transfers the rotating force of the crankshaftto an oscillating force on the wrist pin Connectingrods are split pendicular to their center line at the crankpinend for assembly of the rod onto the crankshaftFIGURE 74 Power End of a Horizontal Power PumpReproduced with permission of McGraw-Hill from Karassik I J (ed)Pump Handbook 2nd ed 1986Chapter 4 Reciprocating Pumps 101_______ ___

_

_1048576____ __

Oil Scoop_1048576CrankshaftFrameCrosswayCrosshead VentWrist PinPony RodLoad on 1048576Connecting RodLoad on FrameCrossheadbull The crosshead moves in a reciprocating motion and transfersthe plunger load to the wrist pin The crosshead isdesigned to absorb the radial load from the plunger as itmoves linearly on the crosswayBearingsbull Both sleeve and anti-friction bearings are used in powerpumps Some frames use all sleeve others use all antifrictionand still others use a combination of both typesof bearingsFramebull The frame absorbs the plunger load and torque On verticalpumps with an outboard stuffing box the frame is in compressionWith horizontal single-acting pumps the frame isin tension

RECIPROCATING PUMP PERFORMANCEbull The following data will outline the main terms involved indetermining the performance of a reciprocating pumpFigure 75 shows a typical performance curveMAIN TERMS

Brake Horsepower (BHP)bull Brake horsepower is the actual power required at the pumpinput shaft in order to achieve the desired pressure andflow It is defined as the following formulaBHPQ Pd1714 Em102 Pumps Reference GuidewhereBHP = brake horsepowerQ = delivered capacity (gpm US)Pd = developed pressure (psi)Em = mechanical efficiency ( as a decimal)Capacity (Q)bull The capacity is the total volume of liquid delivered per unitof time This liquid includes entrained gases and solids atspecified conditionsFIGURE 75 Reciprocating Pump Performance CurveReproduced with permission of McGraw-Hill from Karassik I J (ed)

Pump Handbook 2nd ed 1986Chapter 4 Reciprocating Pumps 103Displacement gpm00 307060504030201010 20 40 50 60Crankshaft (rpm)100 200 300 400 500Pump Brake HorsepowerPlunger 1048576DiaRated1048576lbin22 18 12652 14251 34 18651 12 25001 14 36501 18 45101 5700Pressure (Pd)bull The pressure used to determine brake horsepower is the differentialdeveloped pressure Because the suction pressure isusually small relative to the discharge pressure dischargepressure is used in lieu of differential pressureMechanical Efficiency (Em)bull The mechanical efficiency of a power pump at full loadpressure and speed is 90 to 95 depending on the sizespeed and constructionDisplacement (D)bull Displacement (gpm) is the calculated capacity of the pumpwith no slip losses For single-acting plunger or pistonpumps it is defined as the followingwhereD = displacement (gpm US)A = cross-sectional area of plunger or piston (in2)M = number of plungers or pistonsn = speed of pump (rpm)s = stroke of pump (in) (half the linear distance theplunger or piston moves linearly in one revolution)231 = constant (in3gal)bull For double-acting piston pumps the above is modified asfollowsD 2AaMn s231

D AMn s231104 Pumps Reference Guidewherea = cross-sectional area of the piston rod (in2)Slip(s)bull Slip is the capacity loss as a fraction or percentage of thesuction capacity It consists of stuffing box loss BL plus valveloss VL However stuffing box loss is usually considered

negligibleValve Loss (VL)bull Valve loss is the flow of liquid going back through the valvewhile it is closing andor seated This is a 2 to 10 lossdepending on the valve design or conditionSpeed (n)bull Design speed of a power pump is usually between 300 to800 rpm depending on the capacity size and horsepowerTo maintain good packing life speed is limited to a plungervelocity of 140 to 150 ftminute Pump speed is also limitedby valve life and allowable suction conditionsNumber of Plungers or Pistons (M)bull Table 1 shows the industryrsquos terminology for the numberof plungers or pistons on the crankshaft The terminologyis the same for single- or double-acting pumpsPulsationsbull The pulsating characteristics of the output of a powerpump are extremely important in pump application Themagnitude of the discharge pulsation is mostly affectedby the number of plungers or pistons on the crankshaftChapter 4 Reciprocating Pumps 105Table 2 illustrates the variations in capacity related to thenumber of plungers or pistons in the pumpTABLE 1 Terminology for the Number ofPlungersPistons on the CrankshaftNumber ofPlungersPistons Term1 Simplex2 Duplex3 Triplex4 Quadruplex5 Quintuplex6 Sextuplex7 Septuplex9 NonuplexTABLE 2 Effect of Number of Plungers onVariations from the MeanType Number of Above Below Total PlungerPlungers Mean Mean PhaseDuplex (double) 2 24 22 46 180degTriplex 3 6 17 23 120degQuadruplex 4 11 22 33 90degQuintuplex 5 2 5 7 72degSextuplex 6 5 9 14 60degSeptuplex 7 1 3 4 515degNonuplex 9 1 2 3 40deg106 Pumps Reference GuideNet Positive Suction Head Required (NPSHR)bull The NPSHR is the head of clean clear liquid required at thesuction centerline to ensure proper pump suction operatingconditions For any given plunger size rotating speedpumping capacity and pressure there is a specific value ofNPSHR A change in one or more of these variableschanges the NPSHRbull It is a good practice to have the NPSHA (available) 3 to 5 psigreater than the NPSHR This will prevent release of vaporand entrained gases into the suction system which willcause cavitation damage in the internal passagesbull Figure 76 illustrates the NPSHR for a triplex pump as a function

of rotating speed and plunger diameterFIGURE 76 NPSHR for a Triplex PumpReproduced with permission of McGraw-Hill from Karassik I J (ed)Pump Handbook 2nd ed 1986Chapter 4 Reciprocating Pumps 107NPSH (ft)0100 2002015105300 400rpmPlunger10485762 12 Dia2 Dia1 12 Dia1 Dia0Net Positive Suction Head Available (NPSHA)bull The NPSHA is the static head plus the atmospheric headminus lift loss frictional loss vapor pressure velocity headand acceleration loss in feet available at the suction centerline

RECIPROCATING PUMP APPLICATIONSbull Some reciprocating power pump applications are- ammonia service- carbamate service- chemicals- crude-oil pipeline- cryogenic service- fertilizer plants- high-pressure water cutting- hydro forming- hydrostatic testing- liquid petroleum gas- liquid pipeline- power oil- power press- soft-water injection for water flood- slurry pipeline (70 by weight)- slush ash service- steel-mill descaling- water-blast servicebull The following applications should be reviewed with the108 Pumps Reference Guidepump supplier prior to implementation- cryogenic service- highly compressible liquids- liquids over 250degF- liquids with high percentage of entrained gas- low-speed operation- slurry pipeline- special fluid end materials- viscosity over 250 SSU

- movement of the plunger into the liquid cylinder causesan immediate increase in pressure of the liquid containedtherein This pressure becomes higher than suction pipepressure and causes the suction check valve to close- with further plunger movement the liquid pressure continuesto rise When the liquid pressure attains that in thedischarge pipe the discharge check valve is forced openand liquid flows into the discharge pipe The volumeforced into the discharge pipe is equal to the plungerFIGURE 70 Liquid End of a Reciprocating PumpDuring the Discharge StrokeReproduced with permission of McGraw-Hill from Karassik I J (ed)Pump Handbook 2nd ed 1986Chapter 4 Reciprocating Pumps 95Plunger PackingSuction1048576PipeSuction1048576Check ValveDischarge 1048576Check ValveDischarge1048576PipeMotiondisplacement less small losses The plunger displacementis the product of its cross-sectional area multiplied by thelength of the stroke The plunger must be stopped before itimpacts on the bottom of the liquid cylinder The motion ofthe plunger is then reversed and the plunger proceeds tothe suction strokebull Many reciprocating pumps are double-acting ie theymake two suction and two discharge strokes for one reciprocatingcycle Most double-acting pumps use a pistonwhich as a displacing solid is sealed to a bore within the liquidcylinder Figure 71 shows a double-acting liquid endbull Double-acting pumps have also two suction and twodischarge check valves one of each on either end of thepiston The piston is moved by a piston rod The piston rodseal (packing) prevents liquid from leaking out of the cylinderWhen the piston is moved in the direction shown the rightside of the piston is on a discharge stroke and the left sideof the piston is simultaneously on a suction stroke The pistonseal must seal tightly to the cylinder liner to preventleakage of liquid from the high pressure right side to thelow pressure left side The piston must be stopped before ithits the right side of the cylinder The motion of the pistonis then reversed so the left side of the piston becomes thedischarge stroke and the right begins the suction strokebull A reciprocating pump must have a driving mechanism toprovide motion and force to the piston The two mostcommon driving mechanisms are a reciprocating steamengine and a crank and throw device Pumps that use thesteam engine are called direct-acting steam pumps Pumpsthat use the crank and throw device are called power pumps96 Pumps Reference GuideFIGURE 71 Double-Acting Liquid EndReproduced with permission of McGraw-Hill from Karassik I J (ed)Pump Handbook 2nd ed 1986

Power pumps must be connected to an external prime












_

_1048576____ __




















_

_1048576____ __














_

_1048576____ __










_

_1048576____ __



















Documents

Reciprocating Pumps