Pumps

Pumping of Liquids

Chris FinchPump Selection And Design

What type of pump for what duty, centrifugal v positive displacement etc. Sizing and selection of centrifugal pumps. Includes producing system curve, pump laws, calculating NPSH, minimum flow recycle, controlling flow by variable speed or control valves. How to size control valves.NPSH Net Positive Suction Head

Useful TextsC&R Series Volume 6 Sinott & TowlerC&R Series Volume 1Sulzer Centrifugal Pump Handbook- Sulzer Engineering CompanyFlow of Fluids Through Valves, Fittings & Pipe - Crane Valve CompanyPerrys Chemical Engineering Handbook

Why Include Pumping Of Liquids?They are one of the main work horses of chemical plantsA distillation column will have a minimum of three process pump dutiesWill be more for more complicated for columns with side streams, multiple feeds etc.Will be utility pump requirements as wellIf one of the pumps is not running the column will not operate.Two Main Classes Of PumpsCentrifugal

Positive DisplacementPositive Displacement PumpsCan be classed as eitherReciprocatingPiston, DiaphragmRotaryGear or Rotary LobeProgressive CavityPeristaltic Centrifugal Pump TypesThere are many different typesOften Characterised by Impellor DesignMulti-Stage Centrifugal Pumps often used for high pressure dutiesWill not look at most mechanical aspectsWill discuss shaft sealing methods laterViscosity UnitsKinematic Viscosity = (nu) 1CSt = 10-2 St = 10-6 m2/sSt = Stoke Dynamic Viscosity = (mu)1cP=10-2 P= 10-3 kg/ms=10-3 Pa.sP = PoiseKinematic Viscosity=Dynamic Viscosity/DensityWill Deal With Mainly Newtonian Fluids

Effect of Viscosity on Centrifugal PumpsIncreasing Viscosity degrades performanceNormal Maximum Economic Viscosity is 150 to 500 CSt.Can use up to 1000 CSt Viscosity Tendency now is to push viscosity limit for centrifugal pumps as they give smooth flows, easy to control, simpler design.

Effect of Viscosity on Positive Displacement PumpsDecreasing Viscosity degrades performanceTo pump 1 CSt materials many PD pumps require space between pump body and rotors to be very small (tight clearances)PD pumps can be designed to handle viscosities of several million CSt.

Centrifugal Pump

Pump Affinity Laws Centrifugal PumpsQ = Volumetric FlowN = Impellor Speed revolution per minuteD = Impellor DiameterH= Differential Head or PressureP= Power Consumption

Assume pump geometry impellor design etc. is unchanged

VolumeQ1/Q2=(N1/N2)(D1/D2)

Head or PressureH1/H2=(N1/N2)2(D1/D2)2

PowerP1/P2=(N1/N2)3(D1/D2)3

Assumes pump and impellor design do not change! Centrifugal Pumps only.Example of Pump Laws

Example of increasing speed from 1450 to 2000 rpm for centrifugal pump curve shown earlierImpellor diameter is constantImportant to remember that you must modify all values.

Volume Q1/Q2=(N1/N2) Head H1/H2=(N1/N2)2H 1450Q 1450H 2000Q145048091.30475089.4694510085.6137.940.815077.6206.93520066.6275.92625049.5344.8Head v PressureCentrifugal Pumps Generate Head of LiquidA Given Pump With A Given Speed and Impellor Diameter Will Raise a Liquid to a Certain Height Regardless of The Density of The LiquidDenser Liquid Will Require Higher PowerPower RequiredPower Transferred to Liquid=Q*H**g - WattsQ=Volumetric Flow l/sH=Head - m=Density kg/lg=Acceleration due to Gravity 9.81 m/s2

Divide by pump efficiency to get drive power required

Typical Centrifugal Pump CurvesUnless otherwise stated curves will be for waterBesides the Q/H Curve already seen will contain additional informationQ/H curves for different impellor sizes, the largest and impellor size fitted to that pump body and possible intermediate impellor sizes.Pump efficiency lines can be contours connecting points of same efficiencysingle line efficiency V flow for given impellor sizeInformation on Minimum Flow required The lowest flow that the pump can operate at without being damagedIf the lowest flow is to prevent overheating then flow will need adjustment for different liquid specific heats.Sometimes lowest flow condition is for mechanical reasonsPower CurvesCan be shown as a curve for a given impellor diameter, if only one power curve shown this will be for the largest impellor.A series of curves of fixed power which cross the Q/H curvesThis power shown account for efficiency, basically can be used to size motor kW required

NPSHr This stands for Net Positive Suction Head RequiredUnits will normally be in Absolute Meters of FluidOccasionally if the curve is calculated for a specific liquid may be in pressure units typically mBarA.It is the minimum net pressure required at the pump inlet nozzle to prevent the pump cavitating.Best Efficiency Point BEPThis is the best point typically for pumps to operateIt will sometimes be marked on curve for each impellor sizeOtherwise determine from efficiency curvesEnd of Curve ConditionsAvoid operating at the right end of the pump curveNPSHr increases greatly.The drive system is likely to become overloaded because of increased power consumptionOne common cause is starting pump with empty discharge line and no flow restriction.

Simplified Curve For IllustrationNo Minimum Flow Information

Min FlowBEPImpellor Diameter

CavitationThis occurs when the pressure falls to such a level that the liquid in the pump boils. NPSHaP2

P1P2P3Restriction Orifice ExampleFor the pump curve on the next slideSize an orifice to restrict the flow to the minimum flow required for the 115mm diameter impellorAssume the static head on pump suction and discharge are equalAssume the total length of recycle line is equivalent to 15 metres of 25 mm NB sched 40 pipework

Min FlowBEPFlow Through OrificeRelationship between pressure and flow for orificeq=3.48x10-6d12C(h)0.5q=flow in m3/sd1= internal diameter of orifice mmC=flow coefficient for orificeh=head loss in meters of fluid C is a function of = diameter of orifice/internal diameter of pipeProcedure Sizing Recycle OrificeIt is normal practice to ignore the pressure drop in the pump suction line.Remember pressure drop is proportional to approximately the 5th power of pipe diameter.For the example we are considering the pump inlet pipework will be 100 mm NB.So therefore pressure drop per unit length in suction pipework will be less than 0.001 that of a 25mm recycle line.

Maintaining Minimum Flowd = Destination Static HeadDatum LevelDischarge PipeworkInlet PipeworkMinimum Flow Orifice

Since the water is being returned to its original pressure status we can ignore static pressure differences. This may not always be the case.From the pump curve read flow and head at minimum flow for chosen impellor size.Say 15.5 Meter Head at flow of 1.5 m3/hNext calculate pressure drop through the recycle pipework excluding orifice plateSee next slide gives head loss of 0.47 m of fluid calculate in same way as beforeSo orifice net pressure loss needs to be 15.03 m of fluid.

Recycle Pipework Pressure dropRecycle LineViscosity1cpDensity1000kg/m3Pipe ID26.6mmFlow1500Kg/hPipe Length15mPipe X area0.000556m2Volumetric flow rate1.5m3/hAverage fluid Velocity2698.869m/h0.749686m/sCalculate Re19941.64Absolute roughness0.04572mmFlow regime is turbulentRelative roughness0.001719Read f from chart0.029Head Loss hL= fLv2/D2g0.468454M of fluidP=fLv2/2D

4595.536Pascals0.045955BarSizing Orifices is an iterative procedureIn this case we are calculating the size of an orifice to give us a known pressure drop at a known flow.The first step is to guess the answer - diameter of orificeCalculate the pipe Reynolds NumberCalculate the ratio of orifice to pipe diameterRead C coefficient discharge from chart see Crane and following slides.This is for instrument tapings so calculate net pressure drop using correction factor

Substitute known pressure and flow into equation q=3.48x10-6d12C(h)0.5 Or use nomograph in Crane to obtain d1Change initial guess and iterate until figures are in good agreementSo how do you make initial guessYou may find published data for water air and steam, once you have some experience you will get close with first guessMy gut feel is we guess 8mm orifice ID for this example!

From Pressure Recovery Chart see for =0.3Net pressure loss is 68% of calculated valueSo in calculation we will use pressure loss of 15.03/0.68=22.1 m of water across orifice instrument tapping points

It is possible to get cavitation in restriction orifices very rare as pressures are high.

Orifice Calculation Iteration 1Internal Diameter of Pipe = 26.6 mm Orifice ID = 8mm = 8/26.6 = 0.30075Calculate pipe Reynolds Number as previously=19936Read C from charts = 0.615Solve q=3.48x10-6d12C(h)0.5 for d1d1 = 6.43

Size orifice to give pressure drop of 15.03meter waterAt Flow of1.5m3/h0.000417m3/sDensity1000kg/m3Viscosity1cPInternal Diameter of Pipe26.6mm0.0266mInitial Guess Orifice Diameter8mm ratio =0.300752Pipe cross section area0.000556m2Liquid velocity0.749481m/sPipe Reynolds Number19936.2Beware Units !!!!!Read C from Charts0.615From chart % net pressure drop68.00%Head Loss for calculation22.10294Solveq=3.48x10-6d12C(h)0.5for d1d12=q/((3.48x10-6)*C(h)0.5)d12=41.31423d1=6.427615

Read c = 0.615So our initial guess was too largeModify orifice guess need slightly smaller than initially calculated value due to C decreasing with decreasing in this regionTry new d1 of 6.5So = .244And net pressure loss is now 72% of calculated valueNew orifice diameter6.5mm ratio =0.244361From chart % net pressure drop72.00%Head Loss for calculation20.875m waterRead C from Charts0.61Solveq=3.48x10-6d12C(h)0.5for d1d12=42.86045d1=6.546789So we now have reasonable agreement for the guess of 6.5 to calculated value of 6.54 mmAn error of less than .05 mm is OK for this type of calculationWould specify orifice of 6.55 mm internal diameterThe biggest error can be reading value of C from chart depending on Reynolds NumberI chose badly for the example

Two issues with restriction orificesNeed to be wary if orifice gets below 6mmAs Unless the duty is clean there will be a risk of orifice blocking with dirt particles.You may also find small orifices will be noisy The solution to both these problems is to have two orifices in seriesNeed to have at least 20 pipe diameters between themSize each for 50% of required pressure drop.

Nomograph to solve orifice equation for d1 from CranePower SavingRequired minimum flow for centrifugal pumps are of the order 10-15% of design flow.When pump is at duty point, 5-7% power is being wasted on minimum flow recycleIn order to save energy then an interlock to close an actuated valve in the recycle line when minimum flow is exceeded is often installedThis addition is normally cost effective especially if the duty flow is being measured for other reasons.Maintaining Minimum Flowd = Destination Static HeadDatum LevelDischarge PipeworkInlet PipeworkMinimum Flow OrificeFTFSHow Flow Trip WorksWhen the flow measured by the Flow Transmitter Rises above approx. 1.5 times minimum flow the Flow Switch closes the Actuated Valve in the Pump RecycleWhen the flow measured by the Flow Transmitter falls below 1.1 times minimum flow the Flow Switch opens the Actuated Valve in the Pump RecycleThis range of values gives ample offset to prevent valve constantly opening and closing.

Net Positive Suction HeadNPSHr This stands for Net Positive Suction Head Required- Specific to PumpUnits will normally be in Absolute Meters of FluidOccasionally if the curve is calculated for a specific liquid NPSHr may be in pressure units, typically mBarA.It is the minimum net pressure required at the pump inlet nozzle to prevent the pump cavitating.

Net Positive Suction Head AvailableWill now work out NPSHAFor the pump to not cavitate NPSHA>NPSHrBoth need to be in absolute pressure unitsNPSHA=(Po+Ho)-(Suction Pipework P)- (Vapour Pressure of Fluid at operating temperature)NPSHr increases with flow so calculate at maximum flow that you are designing for.NPSHr typically 3-8 meter absolute of fluidNPSHAPo = Origin PressurePd = Destination PressureHd = Destination Static HeadHo = Origin Static HeadDatum LevelDischarge PipeworkInlet PipeworkNormally dont have any issues unless source tank is under vacuumOr liquid is viscousOr liquid is close to boiling point.Solution is to increase inlet pipework diameter, reduce fittings, increase static head.Pump manufactures often have options to decrease NPSHr but typically reduces pump efficiency

NPSH ExampleFor the pump curve on next slide at duty point marked Source pressure 50 mbaraSource static head 6 metersLength of suction pipe 10 metres, 1 reduced bore ball valve 3 x 1.5 d bendsWork out inlet pipework diameter to satisfy suction head requirementsOperating temperature 20C

Duty PointDuty Point Flow = 29.4 m3/h NPSHr= 2m From tables at 20C, density of water 998.21 kg/m3, viscosity of 1.002 cP, Vapour Pressure = 2.3393 kPaFirst guess at 80 mm NB Schedule 40 pipe Internal Diameter 77.9 mm3 bends at L/D of 14 equivalent pipe length = 3*14*0.0779 = 3.3 Meter1 ball valve at L/D of 14 = 14*0.0779= 1.1 meterTotal length of pipework =10+3.3+1.1=14.4Must also include inlet loss to pipe = 0.5v2/2g

Convert Vapour Pressure to meter of water = 2.3393*0.102=0.0239 m of waterConvert 50 mbara to meter of water =50*.0102 = 0.51Calculate frictional pressure drop = 0.64 meter of water same method as previously usedFor 80 mm NB then NPSHA=Origin Press+Static Press-Vapour Pressure-Frictional Loss0.51+6-0.0239-0.64=5.85 meterThis is greater than NPSHR of 2 m so design is OKIf we repeated calculation for 50 mm nb pipe then NPSHA = 2.3 mThis is to close to NPSHr so should stick with 80 mm diameterSuction Line80mmEquivalent Length bends3.2718MetreEquivalent Length Ball Valve1.0906MeterViscosity1.002cpDensity998.21kg/m3Pipe ID77.9mmFlow29347.37Kg/hPipe Length14.3624mPipe X area0.004767m2Volumetric flow rate29.4m3/hAverage fluid Velocity6167.742m/h1.713262m/sCalculate Re132958.3Absolute roughness0.04572mmFlow regime is turbulentRelative roughness0.000587Read f from chart0.0204Head Loss hL= fLv2/D2g0.562689M of fluidInlet head loss =0.5v2/2g0.074803m of FluidTotal head loss =0.637492M of fluidPump Drives Power SourceElectric MotorsDiesel EnginesSteam TurbinesGas TurbinesThese are the primary drives often gear boxes fixed or variable may be used between motor and pumpThe vast majority of drives are by electric motors will consider in more detail later

Diesel EnginesUseful when no electrical power supply or require pump to work in case of complete electrical failureWidely used for Fire water PumpsNormally run at fixed speed but can varyNoisy, require cooling system, need lot of space.Need fuel storageDrives up to 2MWNormally have battery powered starter motor but can be by compressed air injection. Require 5-10% of engine power to start.

Steam TurbinesTraditionally used in industries that produced waste heat steam.High capital cost especially if large distance from steam supplyNormally Require Superheated SteamOnly of use on large pumpsCan be variable speedGas TurbinesBecoming More popular for large pumps in remote locationse.g. Oil transfer pipelinesControl systems mean can be run remotelyRelatively Low Installation CostsVariable speed Can be supplied as skid mounted UnitsAvailable from 200 kW to 50 MWElectric MotorsElectric Motors are available from fractions of a kW, motors up to 50 MW are now common on pumps and compressorsThey can come in a number of fixed speeds.The speed of an electric motor depends on the frequency of the electrical supply and number of poles the motor has.In Europe Electricity supply frquency is 50 hertz in US it is 60 HertzCentrifugal pump speeds are normally given in rpm revolutions per minuteElectrically driven pumps often connect directly from motor and run at motor speed.Gear boxes sometimes usedSpeed of motor is = electrical supply frequency divided by number of pairs of pole in motor less a factor based on slippageSo for UK two pole motor speed in rpm = 50*60/1 = 3000 rpmIn practice they will run between 2850 and 2900 rpm.Electric motors typically have between one and four pairs of poles

In recent years the development of Variable Frequency Drives has greatly improved the use of electrically driven pumpsThey modify the frequency of supply to pump motorThe can either increase or decrease the frequency of supplySome power is lost in the VFD, they are commonly called Inverters however this is more than made up by hydraulic efficiency improvement The Inverter output frequency is controlled by a 4-20 mA signal typically from the control system.

Control of FlowWe will consider the following forms of control either alone or in combination. There are other methods but we will not consider.Throttling typically control valveSpeed controlStopping and starting multiple pumpsBy Pass Control

No ControlNot all pump installations need controlFor instance pumping between storage tanks does not require controlIn these cases it is important to have pumps correctly sizedRequire good operating procedures for starting and stopping pump so as to not damage pumps.Throttling ControlClosing a throttling valve in the pump discharge line increases frictional pressure drop and reduces flowThrottling in pump suction lines is not used as NPSHa is reduced and is likely to damage pump.Throttling increases pump power requirementsIt is typically used where small deviations in flow are required or where there are multiple destinations for flowIt typically requires less capital than VFD and is more suitable for remote locationsThrottling may be by hand valves or more likely instrument control valve.For good control you require a significant pressure drop through control valveNormal recommendation is need at least 25%of frictional pressure drop across control valve to get good controlYou can get cavitation effects in control valves to avoid this locate in high pressure end of pipework if possible.

For simplicity we will also ignore pressure recovery and the effects of reducers close to control valvesControl Valves are almost always smaller diameter than pipeWe will use following formulaFor Turbulent Flow Cv=1.16Q(SG/P)0.5 For Laminar Flow you need to apply a correction factor and the equation becomesCv=1.16FrQ(SG/P)0.5You will not be expected to calculate Fr in this moduleQ Flow m3/h, P is pressure drop bar

Control Valve ExampleFor this we will develop the example we used earlier when we plotted system curveWe will simplify by assuming that there is no minimum flow orifice for the design cases.It is required to control flow between 50 and 125 m3/h normal flow 100 m3/hControl Valve examplePo = Origin PressurePd = Destination PressureHd = Destination Static HeadHo = Origin Static HeadDatum LevelDischarge PipeworkInlet PipeworkAll manual valves reduced bore ball valvesFlow TransmitterFlow Control ValveExample Control ValveSimple exampleInlet Pipe 10 meters, 150 mm NB Sched 40, 2 1.5 D bendsDischarge Pipe 80 meters, 100 mm NB Sched. 40 Pipe, 5 1.5 D bendsOrigin Pressure Po = 1 BaraOrigin Static Head = 10 mDestination Pressure = 2.0 BaraDestination Static Head = 15 mAssume no pressure drop in Flow TransmitterFluid is water Viscosity 1 cP, density 1000kg/m3Calculation MethodFirst plot the system curve without the control valve.It needs to cross the pump curve to the right of the points at which you wish to controlRead of the pressure difference between system curve and pump curve at the points you wish to control

System Curve and Pump Curve Without Control ValveFlow m3/hHead MetreBelow are Readings From Previous SlideThis information can be used to size a control valveThis shows that when you use throttling to control by centrifugal pumps over a wide range you waste a great deal of energy at low flowsControl will be good as a high percentage of P is across control valveFlow m3/hDynamic PControl Valve P M%Control Valve P Bar502.4519.25785.41%1.891009.4310.27108.91%1.0112514.614.128.06%0.40Choosing The Control ValveNext step is to select the correct valve, this will normally done in conjunction with instrument engineer and valve suppliers We will look at selecting valve of correct cv and control characteristics.Will use the equation from earlierFor Turbulent Flow Cv=1.16Q(SG/P)0.5 Q Flow m3/h, P is pressure drop bar First calculate cv for normal control point

That is flow of 100 m3/h and P=1.01 barSubstitute in equation Cv=1.16Q(SG/P)0.5 Cv = 1.16x100*(1/1.101)0.5=115.4Repeat calculation for minimum and maximum flows givesFor 50m3/h required Cv = 42.19For 125 m3/h required Cv= 229.3Choose valve so that its normal control point is around 30% open

You will now see what effect choosing different characteristics has.To control at 100 m3/h at 30% open then fully open Cv will be 384The required travel of valve at other control points will depend on valve characteristics

Flow50m3/h100m3/h125m3/hLinear11% 30%60%Equal %40%67.5%

85%Quick Opening7%17%32%Using Variable SpeedWill now look at same example but using variable speed to control.Method is to start with the same plot of pump curve v system curveThen recalculate pump curves using pump affinity lawsWe will refer to the original pump curve as 100%System Curve and Pump Curve Without ControlFlow m3/hHead MetrePump affinity laws we will require, impellor diameter is not changed so is omitted from expression.VolumeQ1/Q2=(N1/N2)Head or PressureH1/H2=(N1/N2)2Values shown on next slideFlow M3/HSystem Curve M WaterPump Head 100%Flow 76% SpeedHead 76% SpeedFlow 87% SpeedHead 87% SpeedFlow 95% speedHead 95% Speed025.30480.0027.720.0036.330.0043.325027.754738.0027.1543.5035.5747.5042.4210034.734576.0025.9987.0034.0695.0040.6115046.0340.8114.0023.57130.5030.88142.5036.8220061.7735152.0020.22174.0026.49190.0031.5925081.7626190.0015.02217.5019.68237.5023.47Plot of Different SpeedsShows speed needs to be variable between 76and 93% of original pump speedNormally pump suppliers will provide curves at different speeds and they will show other information such as power requirements, minimum flow and NPSH required.In the UK it is common to run above 100% speeds as most pumps will be designed to work in North America where frequency of electricity supply is 20% higherBy Pass ControlRarely usedTypically most energy inefficientCan be useful for high speed and axial flow pumps due to shape of power curveMethod is to simply control flow to destination by controlling recycle flow to source vessel

By Pass Controld = Destination Static HeadDatum LevelDischarge PipeworkInlet PipeworkFTFICStopping and Starting Multiple PumpsThis is useful to employ where the required duty flow varies significantlyPumps may be operated in series or parallelTypically when static head pressure drop is greater than frictional pressure drop parallel operation may be bestConversely when frictional head loss is higher than static head loss series operation is likely to be best.Multiple PumpsFor the pump curve on next slide which will give most flowTwo Pumps In ParallelTwo Pumps In SeriesThree Pumps in Parallel

Static Head Difference is 10 M WaterSingle Pump 53.9 m3/hTwo in Parallel62.1 m3/hThree in Parallel63.9 m3/hTwo In Series76.4 m3/hPumps with steep curves are better used in parallel operationPumps with flat curves are better used in series operationA disadvantage of series operation is that the down stream pumps may be pressurised and may need internal recirculation to protect seals which leads to inefficiency

One use of multiple pumps is for duties that depend on rain water ie waste water treatment plantsIt is now common to use a combination of variable speed pumps with stopping and startingIf using parallel pumps then pumps must always be ran at the same speedParallel pumps also need some protection to prevent recirculation through non operating pumps. Non return valves can be used but will tend to pass, actuated on/off valves are better.Combining Pump CurvesFor pumps in series it is simpleAt a given flow add the pump heads togetherOften pumps in series are operated with a long length of pipe between the two pumps.In this case it better to think of the two pumps independently as the head at which the pumps operate may be different.Series CurveFlow M3/HSeries 2 Pump CurveSingle Pump Curve09648509447100904515081.640.820070352505226Parallel OperationTo get pump curve for pumps in parallel at a given head you add the flowsPumps in parallel are used oftenFrequency of use has increased in recent years due to availability of simpler variable speed controlWhen operate identical variable speed pumps in parallel you need to have them running at same speed

Parallel PumpsSingle Pump Flow Parrallel Pump FlowSingle Pump Curve Head004850100471002004515030040.82004003525050026Multi Stage PumpsMulti stage pumps are a special case of pumps in seriesThey are made as one unit with multiple impellors in series in one bodyThey give a distinctly different curvesOften used for boiler feed water pump duty

Specifying a PumpWhen it is required to obtain a new pump then most organisations will follow a fairly standard procedure.

Step 1 Process Engineer completes data sheets containing all process related in formationThere are data sheets available in blackboard for mechanically sealed pump. The process engineer will fill in all sections of the Process Data Sheet and the sections of the Mechanical data sheet highlighted in green

This is the minimum required information often there would be additional design cases- varying viscosity, multiple duties.

Step 2- The rotating equipment engineer would complete the majority of other applicable fields in the data sheet rotating equipment engineer would normally be a specialist mechanical engineer

Some fields will be filled in with phrase like Vendor to Complete

Step 3 The data sheets would be send to potential vendors for quotationStep 4 - Vendors would be returned with their offer of pump type cost etc.Step 5 -The process and rotating equipment engineer would assess the quotations for technical aspects and likely iterate the best two quotations until they are completely happy with technical specificationsStep 6 Purchasing would negotiate priceStep 7 Place Order