PartI:OptimizingPumpingSystemsPump Fundamentals for an Energy‐Efficient World

By Gregg Romanyshyn

The following is the first in a series ofarticles based on select chapters fromthe book Optimizing Pumping Systems,A Guide to Improved Energy Efficiency,Reliability, and Profitability, written bypump systems experts and published byPump Systems Matter and theHydraulic Institute.

The U.S. Department of Energy hasdetermined that 25 percent of industrialmotor systems energy is currentlyconsumed by pumping systems.Interest in energy efficiency is not a fad,as industrial production economics,global energy supply limitations, andenvironmental conservation realitiesfigure to be an enduring theme fordecades, if not indefinitely.

As energy costs continue to increase,pump manufacturers understand thatmaking equipment more efficient willcontribute to saving energy. Whiletraditional methods of specifying andpurchasing piping, valves, fittings,pumps, and drivers often result inlowest first cost, these methods oftenproduce systems with unnecessary,expensive energy consumption andhigher maintenance costs. A businessentity that incorporates the energy,reliability, and economic benefits ofoptimized pumping systems canenhance profits, gain productionefficiency improvement opportunities,and initiate necessary capital upgradesfor long-term business survival.

Pump FundamentalsThe pumps used in pumping systemsfall into two general categories –rotodynamic (mixed flow, centrifugaland axial flow) and positivedisplacement.

Because the majority of pumps andenergy usage in industrial andcommercial applications are in therotodynamic pump category, this articlefocuses exclusively on rotodynamicpumps.

How a Rotodynamic Pump WorksA rotodynamic pump converts kineticenergy to potential or pressure energy.The pumping unit’s energy conversioncomponents have three major parts: thedriver that turns the rotating element;the impeller and shaft (the rotatingelement); and the stationary diffusingelement.

Typically connected to the pump’srotating element by a coupling, thedriver provides the energy to rotate theshaft and impeller. With the pumpcasing and the intake system primed,the liquid enters the rotating impellereye, located along the axis of theimpeller. The liquid is accelerated intothe impeller’s vaned passageways,where the continuous transfer ofmomentum and energy conversionoccurs. As liquid flows through theimpeller passageways, velocityincreases.

When the liquid leaves the impeller,liquid velocity is greatest at the tip of thevanes. The rapidly moving liquid leavesthe pump impeller, and then enters thediffusing element of the pump. Anincrease in cross-sectional area of theflow passage occurs and the liquidslows down. The deceleration of theliquid in the diffusing section convertsthe kinetic energy of the liquid topotential or pressure energy. Thediffusing section of the pump can beeither a diffuser or a volute dependingon the pump’s configuration.

The shape, size, speed, and design ofthe impeller and diffusing sectionestablish the pump’s head and flowcharacteristics. The pump impeller anddiffusing section designs are based onthe intended application, the user’sspecifications for the pump, and thepump manufacturer’s experience. Oncea pump is selected, the casing designenvelope cannot readily be changed,but the user can often change the pumpimpeller diameter and/or adjust thespeed to better meet pumpingrequirements. For certain pumps, themanufacturer may have an alternateimpeller, designed for a higher or lowercapacity.

Pump Selection ConsiderationsSelecting a rotodynamic pump requirescareful analysis of the system headversus flow requirements; the pumpperformance characteristics; the pumpapplication; the footprint available forthe pump and driver; applicablespecifications, codes, regulations andreliability; maintainability and energycost considerations. The specifyingengineer may need to work closely withthe pump manufacturer or distributor toselect the optimal pump and its size,speed and power requirements, type ofdrive, mechanical seal, and ancillaryequipment.

Understanding the PumpPerformance CurveAll pump selections must includematching the operating characteristicsof the pump with the systemrequirements over the expected rangeof flows.

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Types of Curves: There are threebasic types of pump curves supplied bythe pump manufacturer: the selectionchart (also known as the range chart orthe family of curves), the publishedcurve and the certified curve.

Selection Chart: A selection chartshows the performance map for asimilar pump family. Figure 1 shows aselection chart for a line of general-purpose end suction pumps. The headand flow scales on the hydrauliccoverage range chart are oftenformatted on semi-log or log-log scalesto display a wider range of flow andhead values on a single chart.

The selection chart shows the variouspump sizes available for a givenmanufacturer’s pump type and speed.The required head and flowrates areplotted on the curve, and themanufacturer evaluates the pumps witha best efficiency point near the specifiedoperating points.

The selection chart is useful indeveloping a shortlist of pumps forconsideration. For example, if you werelooking for a pump running at a nominal1,800 RPM that could develop 100 ft. ofhead at 1,000 GPM; from Figure 1 it canbe seen that the 5x6x11, 5x6x13.5,6x8x11, 6x8x13.5, and possibly the8x10x13.5 size pumps overlap on theselection chart.

Published Curves: Once a shortlist ofacceptable pumps is developed, themanufacturer’s published curves can bereferenced to help determine the bestpump for the application. Figure 2 is anexample of a published curve for a5x6x11 pump running at 1,770 RPM.

Useful operating information can bederived from the manufacturer’s pumpcurve for this application, including thefollowing:

• The impeller diameter falls between10 in. and 10.5 in.

• The pump is 85 percent efficient atthe design point.

• The pump requires approximately 30hp at the operating point.

• The net positive suction headrequired is approximately 10 ft.

Contacting the pump manufacturer orsales office to review the suitability of agiven pump model for the specifiedservice conditions is recommendedwhen specifying a pump.

Certified Curve: After a pump has beenordered and released for construction,the manufacturer builds it, and if testingis specified, the pump is tested and acertified performance curve is supplied.For reliable, consistent test results, it isrecommended that the test beconducted in accordance with therequirements of ANSI/HI 1.6 or 2.6.Unlike the published curve, which is ageneral curve for a given pump modeltype and size, the certified curve reflectsthe actual test results for the specificpump supplied for the purchase order

Optimizing pumping systems

Pump Suction Intake ConsiderationsPump Location in the System: Thepump’s location in the system has amajor effect on the net positive suctionhead available (NPSHA). A change inthe elevation of the pump or suctionsource directly corresponds to anincrease or a decrease in the NPSHA.In a new system, placing the pump atthe lowest possible point or elevatingthe suction source can often beaccomplished with minimal cost impact.After the system is built, increasing theNPSHA — except for changing level setpoints — is often cost-prohibitive.

Pump Suction Piping: The head losscomponent of the NPSHA is based onthe friction losses in the pump suctionpiping. These losses can be significantand increase with the square of theincreased ratio of the rate of flow. Pumpperformance can be limited by theNPSHA. Reducing the piping frictionlosses may be possible by increasingthe diameter of the suction piping,reducing the number of elbows orfittings, or selecting valves with lowerlosses, i.e., by replacing a globe valvewith a gate valve.

Liquid Properties: The temperature-dependent properties of the processliquid can significantly affect NPSHA,head, rate of flow and powerrequirements. Water at 68 F has avapor pressure head of 0.78 ft., but hasa vapor pressure head of 33.9 ft. at 203F. The increased water temperaturerepresents a 33 ft. reduction in NPSHa,if no other changes are made. Changesin liquid temperature affect the liquidviscosity. For Newtonian liquids, raisingthe temperature tends to reduceviscosity, and lowering the temperaturetends to increase viscosity.

Supply Tank and AtmosphericPressure: The pressure acting on theliquid surface of a supply tank directlyaffects the NPSHA. It may be possibleto increase the NPSHa by increasingthe suction tank pressure, but thisoption shall not be selected withoutverifying the supply tank pressure ratingand related process factors.

Pump Affinity Rules: The pump affinityrules describe how changing theimpeller diameter (up to 5 percentchange only) and rotational speed affectpump performance. The pump curve isderived from a series of test pointsconnected together forming a smoothline. The discrete flow and head testvalues can be thought of as belongingto a coordinate point. When using thepump Affinity Rules, it is important toadjust both the head and flow values forthe same coordinate point.

Changes in Rotational Speed: When therotational speed of a pump is changed,the rate of flow (capacity), head, andpower for a point on the pump curvevary according to the pump AffinityRules.

Flow Q2 = Q1 x [N2/ N1]Head H2 = H1 x [N2/ N1]2

Power P2 = P1 x [N2/ N1]3

Where:Q = rate of flowH = headP = powerN = speed

subscript 1 indicates existing valuesubscript 2 indicates changed value

Optimizing pumping systems

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Figure 3 shows a pump performancecurve at the manufacturer’s test speedof 1,770 RPM and a speed of 1,500RPM. As the speed is reduced, thepump curve moves down and shifts tothe left.

The pump affinity rules do notrecommend what should be done to thepump efficiency at the new speed.However, pump efficiency usuallyfollows with the affinity rule adjustmentof flow.

The values of efficiency do not typicallychange much with modest speedchanges.

The pump affinity rules provide anaccurate representation of pumpperformance change over a range ofspeeds.

Changes in Impeller Diameter: Whenthe diameter of a pump impeller istrimmed (up to 5 percent change only),the rate of flow, head, and power for apoint on the pump curve varyapproximately with the pump affinityrules.

Flow Q2 = Q1 x [D2/ D1]Head H2 = H1 x [D2/ D1]2

Power P2 = P1 x [D2/ D1]3

Where:Q = rate of flowH = headP = powerD = impeller diameter

subscript 1 indicates existing valuesubscript 2 indicates changed value

Pump Operation: A pump must beoperated using established proceduresto minimize repairs and unexpecteddowntime. A checklist should bedeveloped to verify that all safetyprecautions, ancillary equipment andvalve settings, manufacturerrecommendations, instrumentationhook-ups, etc. are in order beforestarting a pump.

When shutting down the pump it isimportant to follow an establishedshutdown sequence for safety and toprevent hydraulic transient flow-relatedproblems, water hammer, reverserotation of the pump, unexpectedtripping of other equipment in thesystem and other problems.

Gregg Romanyshyn is the technicaldirector at the Hydraulic Institute. In thisposition, he oversees the technicalaspects related to the HydraulicInstitute. Mr. Romanyshyn has over 30years experience involved with pumprelated businesses and has been at theHydraulic Institute for 10 years TheHydraulic Institute is the largestassociation of pump industrymanufacturers in North America andserves the pump community byproviding product standards, guidelines,and references, and is a forum for theexchange of industry information.

www.pumps.org

Optimizing pumping systems