Engineering Practice

Sizing, Specifying and SeiectingCentrifugal Pumps

Follow these tips todetermine preliminarypump sizing, to supportcost-estimation efforts

Asif RazaZeton Inc.

Determining the proper prelimi-nary size for centrifugal pumpsduring the initial stages of anypump-specification exercise

requires numerous calculations andassumptions. This article reviews thesteps needed to size centrifugal pumpsduring the early stages of a project, tosupport initial cost-estimation efforts.Such early pump-sizing efforts are im-portant steps toward final pump selec-tion and detailed engineering.

Sizing centrifugal pumpsNormal flowrate and rated flow-rate. To define the normal and ratedflowrates, refer to the heat and massbalance of your project. Normal fiow-rate is the flow at 100% capacity.Rated flowrate is the design marginthat is added to the normal flowrate(typically on the order of 10-30%), toaccommodate potential short-term ex-cursions in flowrate during operation.Rated flowrate is usually defined bythe user. If it is not defined, consider10% for normal-service pumps, and20-30% for critical-service pumps,such as distillation column refluxpumps, reactor feed and furnace-feedpumps and other feed pumps that playa critical role in the overall process.Pressure-drop calculations. Theintent of these calculations is to de-termine suction pressure and rateddifferential pressure. Please referto sample calculation #1 (discussedbelow) and Figure 1 for performingpump-sizing calculations, which in-

C-100

12.5 ft

F-102 , E-103

t-) Control y^ H-^ ' —-valve ' L,-.. I H-105Filter Heat

exchangers Furnace

22 ftGrade

FIGURE 1. Refer to this pump-system sketch for the pump-design calculationsdiscussed here. Note, a distance of 2 ft between the pump-iniet piping and grade and3-ft distance between pump-discharge piping and grade is assumed during initial siz-ing. These vaiues should be confirmed once pump selection is done

elude calculation of suction pressureand rated differential pressure.Net pressure suction head (NPSH)calculations. During the early stagesof the project, the plant layout is notyet firm. Hence, NPSH available forthe pump(s) cannot yet be calculatedwith confidence. However, it is okayto carry out preliminary NPSH cal-culations using information from theprehminary layout. Note that NPSHvalues can be increased by later modi-fications to the layout. NPSH plays avery important role during pump se-lection, and could significantly impactthe cost of the pump if a lower-NPSH(required) pump is specified, sincepumps with a lower NPSH require-ment tends to be more expensive.The goal is to calculate a preliminaryNPSH value and provide it to thepump vendors to get initial feedback.This allows both parties to determinewhether a pump with the specifiedNPSH can be achieved or not, can beachieved with some modification tothe plant layout, or can be achievedby selecting a pump with lower NPSHrequirements. Based on the vendor'sfeedback, modify your layout to have adesign margin between the availableand required NPSH. A design marginof 3-4 ft is widely accepted, per indus-try standards. See sample calculation#2 for calculating NPSH (available).

Horsepower and efflciency. In theearly stages of the project, you willhave to provide preliminary horse-power to the electrical engineer forload calculations. From the rated dif-ferential pressure, calculate the ratedhorsepower. Since the pump has notyet been selected, you can assume apump efficiency between 50 and 60%.Make a judgment call — 50% is typi-cally a sufficient value for calculatinghorsepower during the initial stagesof a project.Shutoffpressure. Shutoff pressure isrequired in early stages of the projectto determine the flange rating for thedischarge piping of the pump. Thereare several ways to calculate shutoffpressure. A more conservative methoduses the following formula:

Differential pressure x 1.25 -i-maximum suction pressure

Maximum suction pressure iscalculated by:

Pressure safety valve {PSV) set pres-sure X 10 - 21% accumulation -i- statichead based on high liquid level

If a user does not have informationsuch as the PSV set pressure in theearly stages of the project, a simpleapproach for calculating shutoff headis to use this formula:

Rated differential head x 1.5

CHEMICAL ENGINEERING WWW.CHE.COM FEBRUARY 2013 4 3

Engineering Practice

Impeller 1

Impeller 2

Best efficiencypoint (BEP)"

Rowrate produced

*Ttie recommended operating range is between70-120% of the BEP flowrate

* * BEP is a point on the curve, is specific to eachpump, and is provided by the pump manufacturer

FIGURE 2. Thiis pump-performancecurve shows ttie best efficiency point(BEP) and ttie recommended operatingrange, wtilcti are tielpful during pumpselection. Note the shaded region rep-resents the operating range for differentimpeller sizes

Speci:fying centrifugal pumpsThe following steps must be carriedout during the specification of centrif-ugal pumps:Gather basic process data. Fill outthe pump data sheet with the follow-ing basic information: Suction pres-sure, normal and rated dischargepressures, normal fiowrate and ratedflowrate, fluid properties, such as den-sity, viscosity and vapor pressure atoperating temperature.Determine a preliminary valuefor NPSH. Please refer to the NPSHcalculations presented below (samplecalculation #2).Specify desired materials of con-struction. This should come from theend user or from a metallurgist who isinvolved in the project and is respon-sible for specifjring the appropriatemetallurgy of the equipment.Specify sealless versus mechanicalseals. Magnetic-drive sealless pumpsare desirable for many applicationssince they eliminate the need for me-chanical seals, and thus eliminate theinherent risk of leakage and mainte-nance associated with such seals. Butsealless pumps also have drawbacks,such as an inability to handle largerparticles in the process fluids. Andin some applications, relatively highdifferential pressure requires hightorque, which may be beyond the ca-pabilities of the sealless pump.

Rely on the end-user's experience

TABLE 1 .PUMP-SIZING SPREADSHEET FOR CALCULATING SUCTIONPRESSURE AND RATED DIFFERENTIAL PRESSURE (CALCULATION #1 )

Rated flowcondition

Normal flowcondition

Suction pressure

Source pressure, psig

Static head = 10.5 ft (ft x specific gravity/2.31),psi

Sucticn line loss, psi

Pump sucticn pressure, psig

30

3.2

0.3

(30+3.2)-0.3 = 32.9

30

3.2

0.3

(30+3.2)-0.3 = 32.9

Discharge pressure

Delivery pressure, psig

Static head = 19 ft (ft x specific gravity/2.31), psi

Line loss, psig

Control-valve pressure drop, psid

Filter pressure drop, psid

Heat exchanger 1 pressure drop, psid

Heat exchanger 2 pressure drop, psid

Furnace pressure drop, psid

Orifice ficwmeter pressure drop, psid

Contingency, psig

100

5.8

33

38

14.4

14.4

14.472

2.8810

100

5.8

23

83.510

10

10

50

10

Differential pressure, psig

Discharge pressure, psig

Minus sucticn pressure, psig

Total pump differenfial pressure, psig

Pump head (psi x 2.31 /specific gravity)

Hydraulic power = gal/min x pump head (ft) xspecific gravity/3,960, hP

Efficiency at 3,600 rpm, %

Rated power = Hydraulic power/efficiency, hp

=100+5.8+33+14.4+14.4+14.4+72+2.88+38+10=305

32.9272

900

382

60

637

=100+5.8+23+10+10+10+50+2+83.5+10=305

32.9

272

900

318

60

530

regarding whether to use a sealed orsealless design pump. Consider seal-less pumps for liquids that are flam-mable, toxic and corrosive. For in-stance, many facilities use traditionalsealed pumps for pumping water, andseallesB pumps for pumping acids andalkalis and other corrosive liquids.Work closely with your vendor andseek guidance based on previousexperience in applications withsimilar service. If a sealless pump isnot available, consider using a doublemechanical seal to minimize the riskof leakage. Rely on the vendor's expe-rience, as well, in selecting the mostappropriate mechanical seal for yourservice. Provide vendors with as muchprocess data as you have, to ensureproper seal selection for your service.Classify non-ANSI, sub-ANSI andANSI-API pumps. Ultimately, theselection of specific centrifugal pumps

must be based on the end user's re-quirements, process conditions, andthe cost of the equipment. In general,the cost increases in this order: Non-ANSI, sub-ANSI, ANSI and API, whilenon-ANSI pumps have the lowest cost.A non-ANSI pump usually finds its ap-plication in small sizes handling lesscritical service, such as water that isbeing pumped at relatively low pres-sure and low temperature. An ANSIpump is usually used in applicationsrequiring relatively larger sizes (forinstance, more than 10 hP) in chemi-cal or hydrocarbon service.

However, some ANSI pumps may belimited to a maximum casing pressure.For higher-casing pressures, the usermay have no choice but to consider acustom-made pump or an API pump.API pumps tend to be considerablymore expensive than ANSI pumps, asthey are typically used in hydrocarbon

4 4 CHEMICAL ENGINEERING WWW.CHE.COM FEBRUARY 2013

I I I I 1 I I I I I I I M I I—Q

0 100 2Ö0 3Ô0 4Ö0 5Ó0 6Ó0 7Ó0 8Ó0 9Ö0 1,0001,100 1,2001,300Flowrate, U.S. gal/m

FIGURE 3. This figure shows a reiationship between flowrate and head, power, effi-ciency and NPSH. Note the NPSH curve is bowi-shaped. NPSH requirements increasewith flow. iVIake sure you analyze this vaiue carefully and select a NPSH (required)value that corresponds to the rated fiowrate

service involving high temperaturesand very high pressures. Such pumpsare widely used in refinery service.Specify design pressure and tem-perature. Specifying design pressureand temperature based on the designconditions of the pump suction vesselmay seem to be the easiest approach.But check for any possible upset condi-tions that might warrant an increasein design pressure and temperature,such as a new process stream enteringthe pump suction in a different pro-cess operating mode.Specify the motor requirements.The required power supply, whetherit is 460 V at 60 Hz, or 230 V at 50Hz, must be supplied by the end userLastly you have to specify the hazard-ous area classifications and tempera-ture rating of the motor Check withyour electrical engineers to identifythe required hazardous area classi-fication. Motor temperature ratingshould also come from the end user.This is based on the lowest autoigni-tion temperature of the componentsinvolved in the process. Keep in mind,most of the time, a motor rating up toT3A is available at no added cost, butthe cost increases substantially if amotor with a rating of T4 or higher isspecified.

Next, you need to specify whetheryou want a fixed-speed motor or avariable-speed drive motor If you arecontrolling the pump fiow with a speedcontroller, then you must select aninverted-duty, variable-speed motor.However, note that if there is a controlvalve on the pump discharge, then youmust use a fixed-drive motor.

Sample calculationsConsider the pump system sketchshown in Figure 1. A fiuid with a vaporpressure of 45.9 psia at operating tem-perature of 430°F with a viscosity of0.5 cP is pumped at normal fiow of2,000 gal/min. The specific gravity is0.7 and the delivery pressure is 100psig. The operating pressure of col-umn C-100 is 30 psig. The atmosphericpressure at site is 14.5 psia. Assumea rated fiowrate of 2,400 gal/min (1.2times the normal fiowrate).

Note that two sets of calculationsare done for calculating horsepower— one for normal flowrate and otherfor rated fiowrate. Refer to Figure 1for calculating static head. Pressuredrop across filters, heat exchangers,orifice meters and furnaces are takenfrom actual equipment vendor quotes.These quotes may be available fromdifferent disciplines, such as mechani-cal and instrumentation departments.

If vendor quotes are not availableduring preliminary pump sizing, thenassumptions must be made based oninteractions with other disciplines.For instance, it is okay to assume apressure drop of 10 psid across heatexchangers or a pressure drop of 5-10psid across a filter. Pressure dropacross a vessel filled with catalystshould be calculated using the Ergunequation. These pressure-drop valuesare finalized when equipment designis finalized and are used for final pumpsizing, during the detailed engineeringphase, to check the rated differentialpressure and rated brake horsepower.It is helpful to perform these calcula-tions using an Excel spreadsheet.

The pump-sizing calculations alsoprovide pressure-drop data across thecontrol valve, under normal- and rated-flow circumstances. Specify control-valve pressure drop at the rated fiow,following the widely accepted rule ofthumb — that is, pressure drop is 25%of the dynamic head loss at rated fiow.In this case, djmamic head loss at therated fiow is 151 psig (the sum of pres-sure drop across filters, heat exchang-ers, furnaces, orifices and line losses.Hence the differential pressure acrossthe control valve in this scenario atrated fiow is 38 psig).

Note: If the calculated pressure dropacross the control valve is less than 10psig, use a minimum value of 10 psigfor the control valve at rated fiowrate.Now adjust the pressure drop acrossthe control valve at normal flow andtry to match the discharge pressureuntil it is equal at normal and ratedflow. Pressure drop is directly propor-tional to the square of the fiow, hencepressure drop across rated fiow is cal-culated using the following formula(Note that the rated fiow is 1.2 timesthe normal flow):

Filter pressure drop at normal flow =10 psig

Filter pressure drop at rated fiow =10 X (1.2 X 1.2) = 14.4 psig

In reality, the discharge pressure atnormal flow and rated flow may not bethe same, but the two values will bevery close. A pump is designed to oper-ate at rated flow conditions. However,a pump operates at normal flow mostof the time during normal operation.

Normal operationDuring normal operation, because theflow is lower than the rated flow, thepump will try to develop more head.During this scenario, the control valvewill start closing and will consumemore pressure drop. This will have theeffect of moving the pump back ontothe pump curve.

Here, you will notice that pressuredifference in rated flow is 38 psig(specified by the designer) and is 83.5psig in normal fiow. The difference be-tween these two values is the excessdynamic head between normal fiow-rate and rated fiowrate. The relation-

CHEMICAL ENGINEERING WWW.CHE.COM FEBRUARY 2013 4 5

Engineering Practice

ship between the pump's head-capac-ity curve and pipe-system relationshipresistance is shown in Figure 4.

Pressure drop across the controlvalve should not be included as a partof the dynamic head loss. The gap be-tween the head-capacity curve and thesystem-resistance curve is availablefor throttling (control-valve pressuredrop). Control valve pressure drop atnormal fiow is higher than the pres-sure drop at rated flow. While pipedynamic head loss increases at higherfiowrates (rated flow), control-valvepressure drop decreases. At higherfiowrates, the control valve has toopen more and pass larger flow withless resistance.

Designers should appreciate theimportance of specifying the correctpressure drop for the control valveat different fiow conditions, to en-sure a rugged system design. If a sys-tem is poorly defined, the pump willnever be able to control the fiow andit will never provide proper fiow atthe required head. The efficiency willbe low and the pump will consumemore power.

It is also advisable to install a globevalve at the pump discharge, to allowfor throttling the fiow and adjustingthe fiow and discharge pressure. How-ever, please keep in mind that the in-stallation of a globe valve will incur aconstant pressure drop, which must beaccounted for during head-losscalculations.

Ultimately, the calculated control-valve pressure drop at normal andrated flows will be given to the in-strument engineer who is responsiblefor specifying and sizing the controlvalves for your project.Sam,ple calculation #1. Table 1shows the results of a pump-sizing ex-ercise, in which suction pressure andrated differential pressure were calcu-lated.Sample calculation #2. Static pres-sure available at the pump suctioninlet = (Operating pressure of thevessel -h static head) - suction-pipinghead loss at rated fiow.NPSH (available) = Static pressure atthe pump suction inlet - vapor pres-sure at the operating temperature.The results of these sample calcula-tions are shown in Table 2.

I T

Jiead capacity curve

A

AP1-« Í

Total head, ft

Systemresistance

Static and ^pressure head __^ AH

differentials |

^Operating points control valvepressure drop

Dynamichead lossthrough

dischargepiping

Capacity (Q), gal/min

FIGURE 4. This figure shows the relationship between a pump's head-capacitycurve and pipe system resistance (dynamic head loss). It also shows pressure dropacross a control valve at normal and rated flowrate. Note: Pressure drop across thecontrol valve at rated flow is less than the pressure drop at normal flow

Pump selectionDuring this stage of the project, youshould be getting quotes back fromthe vendor, based on the pump speci-fications you have provided. Whilevarious quotes will vary in their dol-lar value, keep in mind that a more-expensive pump does not necessarilymean that it is the best pump for thejob, and the least-expensive pump isnot worth further consideration. To as-sess competing quotes fairly, develop aspreadsheet to gather the following se-lection criteria. Assign points to eachitem that meets your specifications.NPSH. Check for NPSH (required)from the pump data sheet provided bythe vendor. How close it is to your esti-mated value of NPSH (available)? Askyourself — Can you make this pumpwork by increasing NPSH (available)?It cannot be stressed enough thatNPSH is a key parameter during pumpselection, and insufficient NPSH oftenresults in pump cavitation.

Cavitation occurs when vapor bub-bles that have formed in areas of lowstatic pressure move along the impel-ler vanes into high-pressure areas,where they rapidly collapse. The forcesproduced by these bubbles as they im-plode erode the impeller vane, result-ing in progressive pitting to the impel-lers. As a rule of thumb, an acceptablemargin between NPSH (available) andNPSH (required) is required to ensurepump reliability. A minimum marginof 3 ^ ft is a widely practiced rule ofthumb. Since the NPSH requirementincreases with increasing flow, it isimportant to consider the maximumexpected flow when specifjang an ac-ceptable NPSH margin.Rated flow and differential pres-

sure. Analyze all quotes to see whetherthey meet your specified value of ratedfiow and differential pressure.Material of construction. Does thismeet your specified material of con-struction?Analyzing pump curve and effi-ciency. Pump efficiency is a very im-portant value to be considered. Somevendors may quote a bigger pumpthan what is required. In such a case,the pump efficiency will be reduced.Designers should note that a pumpwith even 10% higher efficiency willsave thousands of dollars in powercosts over the service life of the pump.

It is good practice to examine sev-eral performance charts at differentspeeds to see if one model satisfies therequirements more efficiently than an-other. Whenever possible, the lowestpump speed should be selected, as thiswill save wear and tear on the rotat-ing parts. Efficiency can be found onthe pump curve provided by the pumpvendor. Refer to Figure 3, which showsthe relationship between efficiency andflowrate. This figure also shows the re-lationship between volumetric fiowrate,head, NPSH and brake horsepower.

Every pump has a best efficiency point(BEP), which is the flow/head combina-tion that corresponds to the highest ef-ficiency. The preferred operating regionis between 70 and 120% of the BEPfiowrate value [1], although most usersrequire the rated flow to fall between80% and 110% of BEP. The allowableoperating region varies from pump topump, and is defined as the flow rangewithin which vibrations do not exceedthe limits established by the AmericanPetroleum Institute (API) [1].

Refer to Figure 2, for the recom-

4 6 CHEMICAL ENGINEERING WWW.CHE.COM FEBRUARY 2013

TABLE 2. NPSH CALCULATIONS (CALCULATION #2)Atmospheric pressure, psiaSpecific gravity (SG), dimensionlessOriginai pressure (vessei pressure), psigStatic lieod = 10.5 ft (Note 1)Suction iine ioss, psigSuction pressure at pump inlet flange, psia

Vapor pressure, psiaNet pressure suction head (NPSH) = Suc-tion pressure - vapor pressure, psiaNPSH available, ft

14.50.730(10.5/2.31) X 0.7 = 3.18 psig0.3=30+ 3.18-(0.3)= 32.88+ 14.5 = 47.38 psia45.947.38 - 45.9 = 1.48 psia

= 1.48x2.31/0.7= 4.89Note 1: Static head is measured from the low liquid level in the vessel to the center line of thepump-suction flange, or from the vessel bottom nozzle to the center line of the pump-suctionflange. The latter is more conservative approach.

mended operating range. The shadedregion represents the operating range,that is discussed in the above para-graph. Note in this figure, there arethree curves for three different impel-ler sizes provided by the pump vendor.While selecting an impeller, it is goodpractice to select a pump with an im-peller that can be increased in size, asthis will allow for future increases inhead and capacity.Mechanical seal arrangements.When evaluating competing vendorquotes, be sure you are comparing"apples to apples." For instance, somevendors may have quoted a double me-chanical seal, while your requirementwas for a single seal. If this happens,ask the vendor to revise the quote.Motors. Check for the motor sizing,and whether it has been sized for fullrun-out case. Full run-out means thatthe motor should be sized for the max-imum flowrate the pump can deliver.The stated motor temperature ratingand specified electrical area classifica-tion must meet your requirements.Physical size of the pump. Checkfor the dimension of the pump fromthe quotes received. If space is tight,you may have to consider an inlinepump, or a high-speed, single-stagepump over a multistage pump.Design conditions. Check for designtemperature and pressure from thevendor quotes and make sure thatthey meet your requirements.Design codes. Does the quoted pumpmeet your specified design codes, suchas API, ANSI and so on.Warranty. When evaluating compet-ing pump options, check for the manu-facturer's performance warranties andfor the availability of onsite startupassistance from the vendor, if specifiedin the Request for Quotation (RFQ).Scope of supply for auxiliaryequipment. Make sure the vendor'squote includes the supply of all theaccessories required, such as lubrica-

tion-oil coolers, interconnecting pipingbetween the coolers, instruments, suchas flow switches on the cooling waterlines and so on. Be sure to compare allthe quotes on the same basis.Price. Before selecting a particularpump, make sure that you are com-paring "apples to apples," as differentvendors may have quoted differentpump options in different styles, withdifferent seal arrangements, using dif-ferent assumptions and so on.

Final thoughtsClose coordination with the pumpvendor and developing a solid under-standing of the process requirementsare essential steps during pump de-sign and selection. By understandingthe concepts of rated flow, head, suc-tion pressure and NPSH, and by un-derstanding pump curves, you will beon the right track to design and selectthe most appropriate pump that meetsall of the process requirements. •

Edited by Suzanne Shelley

Reference1. American Petroleum Inst. (API), "Centrifugal

Pumps for Petroleum, Petrochemicals andNatural Gas Industries," 10th Ed., API Stan-dards 610 (ISO 13709), Washington, D.C.

AuthorAsif Raza is a lead processengineer at Zeton Inc. (740Oval Court, Burlington, ONL7L 6A9, Canada; Phone:905-632-3123; Email: asi-fraza„us@yahoo.com). Hiswork involves the design andmanufacture of pilot anddemonstration units for re-search and development, inthe areas of petroleum refin-ing, petrochemicals, gasifica-

tion and unconventional sources of energy, suchas biomass, gas-to-liquids and coal-to-liquids.He has more than 15 years of experience in pro-cess design. His areas of interest include majorequipment sizing and selection, P&ID develop-ment, selection of control logics and processsimulation. Before joining Zeton, Raza workedas a lead design engineer with engineering, pro-curement and construction (EPC) companies, in-cluding Bantrel Inc. and SNC Lavalin Engineersand Constructors. He holds a B.Tech degree inchemical engineering from Amravati University(India). Raza is a registered professional engi-neer in the province of Ontario, and is a memberof Ontario Society of Professional Engineers.

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