pumping efficiency

Copyright © May 1997, Pacific Gas and Electric Company, all rights reserved. Revised 4/25/97

APPLICATION NOTEAn In-Depth Examination of an Energy

Efficiency Technology

AgriculturalPumping

EfficiencyImprovements

Summary

Pumping water for irrigation is the larg-est use of on-farm agricultural energy inCalifornia. Growers are fully aware ofthe need for increasing overall efficiencyof their pumping plant (pump and motorcombination), but they do not alwaysknow how to achieve it. This ApplicationNote describes some specific meas-ures.

Overall pump system efficiency de-pends on the efficiency of the motor, thepump, and the design of the piping lay-out. This Note focuses on improvementsmost directly related to the pumpingplant rather than piping. And, this Noteprimarily addresses electric motor-driven pumps, but gas and diesel en-gines are used as pump drivers as well.

The single greatest contributor to pumpinefficiency is an oversized pump. If apump is selected based on some antici-pated future condition, such as de-graded (scaled) pipe or a higher pro-jected flow to meet increased crop re-quirements, it will deliver excess fluid ata higher head than necessary whennew. A throttling valve on the pump dis-charge is often set to turn down the flowwhen the pump is oversized. Impellertrimming, pump speed changes, andparallel or series pumping are more en-ergy-efficient ways to reduce the energywaste associated with oversized pumps.

Pumping plant efficiency improvementscan also be made by directly increasingthe efficiency of the motor or pump di-rectly. Motor efficiency can often be in-creased by replacing a standard-efficiency motor with a high- and pre-mium-efficiency motor. Pump efficiency

Summary .............................................1

How This Technology Saves Energy.............................................................2

Types of Energy EfficiencyMeasures.............................................4

Applicability ........................................5

Field Observations to AssessFeasibility ............................................6

Estimation of Energy Savings ...........7

Cost and Service Life .........................8

Definitions of Key Terms ...................9

References to More Information......10

Major Manufacturers ........................10

PG&E Energy Efficiency Information© “Agricultural Pumping Efficiency Improvements”

can be improved by replacing worn ordamaged impellers.

How This TechnologySaves Energy

A pump converts mechanical energyinto pressure energy to move liquids byapplying that pressure energy, or head1,to the liquid. In a centrifugal pump, thedriver, in most cases an electric motor,rotates an impeller, which imparts en-ergy to fluid directed into the center, oreye, of the impeller. The fluid is thenacted upon by centrifugal and rotationalforces which increase its velocity. Thepump casing is designed so that theuniformly increasing area of its voluteproduces the maximum conversion ofvelocity energy of the fluid into pressureenergy. Figure 1 shows the centrifugalpump components and fluid flow pat-tern.

To better understand how to improvepump system efficiency, a brief overview

1 Bold-Italic words are defined in the sectiontitled Definition of Key Terms.

of pump operation, terminology, andpump and system curves is provided.

System Curves

Figure 2 depicts a typical pumping sys-tem and the system head curve, orsystem curve. This is the graphical rep-resentation of the head required at allflows to satisfy the system function. Thethree components of total system headare static head, design working head,and friction head. Static head is thevertical difference between the system’spoint of entry and its highest point ofdischarge. Design working head is thathead which must be available at aspecified location to satisfy design re-quirements. Friction head is the headrequired by the system to overcome theresistance to flow in pipes, valves, fit-

Suct ion

Impe l le r

ThrustBear ing

L ine Bear ings

Bear ing Hous ing

D ischarge

Shaf tS leeve

Pack ing

Volute

Inlet

Out le t

Figure 1: Components and Operationof a Centrifugal Pump (Sources:Chemical Engineering and ASHRAE)*

Design Work ing Head

Stat ic Head

Frict ion Head

Total SystemHead

Frict ionHead

Design Work ing Head

Stat ic Head

Flow

Hea

d

Figure 2: Pump System Curve


tings and mechanical equipment.

Pump Curves

Pump manufacturers provide pumphead-capacity curves, or pump curves,that predict pump performance, whichcan be shown as a single-line curve de-picting one impeller diameter (Figure 3)or as multiple curves for the perform-ance of several impeller diameters inone casing (Figure 4). To meet the widevariety of needs, manufacturers willproduce different-sized impellers to be

used inside a single casing. This keepscosts down by reducing the number ofneeded casings, but sacrifices some ef-ficiency.

A pump operates over a range of headand flows for a given speed and impellerdiameter. Change either and a givenpump will operate on a different curve. Itis characteristic of centrifugal pumpsthat, for any given speed, as flowthrough the pump increases, its headdecreases. The pump design point isthe point on the curve where maximumefficiency is attained. Lines forming aconcentric pattern around the design

point indicate areas of equal efficiency.

A system curve can be developed andoverlaid on the pump curve. The inter-section of the pump head-capacitycurve and the system curve will be theoperating point for the pump (Figure 5).This condition represents the point atwhich pump head matches system headas defined by the system piping configu-ration.

The efficiency of agricultural pumpingplants can be improved by:

�� Replacing a standard-efficiencymotor with a high- or premium-efficiency motor

�� Improving pump efficiency by ad-justing or replacing worn impeller(s) orbowl(s)

�� Reducing total dynamic head bychanging impeller diameter or motorspeed, or by using multiple pumps

Total Head FlowEff ic iency

Brake Horsepower

Net Posi t ive Suct ion Head

Hea

d

F low0

10" Dia.

Figure 3: Single Line Pump Curve(Source: Peerless Pump)**

0

90

60

100

110

80

140

70

50

130

40

120

010

020

030

040

050

060

070

080

090

010

0011

0012

0013

00

U.S. Gallons per Minute

Tota

l Hea

d in

Fee

t

Figure 4: Multiple Pump Curves


Types of Energy EfficiencyMeasures

Pumping plants can be optimized by anumber of measures discussed in thissection. As stated earlier, this Applica-tion Note focuses on measures directlyrelated to the pumping plant and its op-eration; piping system characteristicsare beyond its scope. And, this Noteprimarily addresses electric motor-driven pumps, but gas and diesel en-gines are used as pump drivers as well.

Improve Motor or PumpEfficiency

Pump system efficiency can be im-proved directly by increasing the effi-ciency of the motor and/or pump. Thefollowing specific actions can be taken:

�� High-Efficiency Motors: Agricul-tural pumps normally run 3,000 hours ormore annually. The 3 to 5 percent in-crease in motor efficiency of a high- orpremium-efficiency motor can providequick paybacks.

�� Replacing or Repairing the PumpImpeller and/or the Pump Bowl: Aretrofit of the impeller and bowl assem-bly of a pumping plant, and the optimalplacement of the bowls relative to waterlevels, can improve pump system effi-ciency.

�� Pump Adjustment: Proper adjust-ment of the impeller relative to the bowlassembly will minimize the clearancebetween impeller and bowl and maxi-mize the quantity of water pumped.

Decrease Total Dynamic Head

Oversized pumps are the single largestsource of energy waste in pumpingsystems. The situation arises in the de-sign process. Engineers add pressurelosses to the system head to allow forscaling and fouling of piping that occursover time; margin is also added to en-sure that the pump selected will deliverthe required flow. After installation, thesystem head may be less than antici-pated, especially early on. If pump se-lection is based on some anticipatedfuture condition, such as "old" pipe or ahigher projected flow to meet increasedcrop requirements, it will deliver excessfluid at a higher head than necessary. Ifallowed to operate at an excessive flow,the pump "runs out on its head-flowcurve" until system resistance matchesthe pump head. As the pump operatesfurther to the right on the curve, opera-tion becomes less predictable and moreunstable. Motor overloading or cavita-tion may occur as a result. A valve onthe pump discharge is often set to turndown the flow. The extra head is thentaken as a pressure drop across thevalve. Figure 6 depicts this situation ona system curve.

FrictionHead Total

DynamicHead

Stat ic Head

Operat ingPoint

HeadCapacity

Curve

SystemCurve

Pre

ssur

e R

ise

P F

eet

Water Flow Rate (gpm)

Figure 5: System Curve and PumpCurve (Source: Peerless Pump)**


There are several more energy-efficientways to reduce the total dynamic headand minimize overpumping:

�� Impeller Trimming: One way is tochange the impeller size. This can bedone by trimming the existing one or re-placing it. Pump manufacturers normallybuild their pump bodies to accept arange of impeller diameters. Changingthis diameter raises or lowers the entirepump curve. Practically speaking, im-peller trimming is limited to a 20% re-duction in diameter.

�� Pump Speed Changes: Anotherway to reduce a pump's output is to re-duce its speed. This may not be as sim-ple as changing the impeller diameter.Because most pumps are driven directlyoff the shaft of their motor, to changethe speed of the pump you must changethe speed of the motor.

Induction motors , the most common,operate at synchronous speeds whichare multiples of the frequency of the 60Hz AC power delivered, perhaps 1800or 3600 RPM. To alter the speed of anAC induction motor it would be neces-

sary to either:

1) Change to a motor with a differentspeed.

2) Change to a two-speed motor.

3) Install a Variable Speed Drive(VSD) to control the frequency of thepower delivered to the motor. VSDsare useful for controlling pumps withvariable demands.

�� Parallel and Series Pumping:Parallel pumping can reduce energy re-quirements when flow requirementschange in distinct steps. Series pumpingalso can improve energy efficiency.Both are popular choices because theymeet additional criteria such as high re-liability, backup capability, low mainte-nance, and low first cost.

The following example illustrates thebenefits of series pumping. Assume 10units of water have to be pumped up aconstant slope. The required total dy-namic head is 100 feet to move water tothe highest part of the system, but thatpart only uses 3 units of water. Usingonly one pump station requires that all10 units of water be pumped with ahead of 100 feet. It may be economicalto use two pumps, one to pump all thewater at 60 feet of head and another toadd 40 feet for the three units requiredat the highest point.

Applicability

The nature of system flow requirementscan significantly impact the applicabilityof different measures to reduce the totaldynamic head. Table 1 indicates theapplicability of the different measures.

ValveThrott l ing

Valve Ful lyOpen

Flow100%

Hea

d

1 00%

Figure 6: Effect of Pump Throttling(Source: Peerless Pump)*


Field Observations toAssess Feasibility

This section discusses steps to take inthe field to identify situations where en-ergy efficiency improvements can bemade.

Related to Applicability

The following information is needed toevaluate a pump:

�� Record pump nameplate data suchas design head and flow, impeller size(if listed), speed, make and model num-ber.

�� Record the motor nameplate data.

�� Record the pressure gauge read-ing(s). Gauges are often installed withthe system for balancing.

�� Walk the system to identify prob-lems in the areas in pump noise, layoutconfiguration, valve type and throttling,and required static head. Possibleproblems with any pump or system

change should be identified.

�� Obtain the pump curves (if avail-able). By using these curves a prelimi-nary evaluation can generally be donewithout measuring the actual gpm. If thepressure (head) is known, simply readthe flow off the pump curve. Keep inmind that the pump curves are only rep-resentative of the model. Large pumpsshould be tested because small differ-ences in performance can add up tolarge energy costs. Worn pumps willoperate at a lower efficiency than thecurves would suggest; they should befully tested, or simply replaced with anefficient, appropriately sized model.

�� Consider and evaluate forchanges in the motor, the pump, andthe system.

Related to Energy Savings

The applicability of motor and pump ef-ficiency improvements and the potentialfor energy savings depend on the effi-ciency of the existing motor and pump.Table 2 provides typical Overallpumping Plant Efficiency (OPE) val-ues for various size motors. The OPEaccounts for both motor and pump effi-ciency and can be obtained with a stan-dard pump test.

Related to Implementation Cost

�� High-speed pumps are often moreefficient in small sizes. The potential forcavitation increases, however, andhigher speed may mean more frequentmaintenance.

�� A multiple-stage pump , because ofthe reduced head per stage, may be

System Flow Requirements

Eff. Measure Constant DistinctSteps

HighlyVariable

Impeller sizing X

Motor speed X

2-Speed motors X

Parallel pumping X

Series pumping X

VSDs X

Table 1: Measure Applicability Matrix


more efficient than a single-stage unit,though at higher initial cost.

�� Inefficient, low-cost pumps are in-stalled by some manufacturers to keepcosts down. Rapid paybacks are oftenpossible with their replacements.

�� A slightly over-sized pump with atrimmed impeller can be installedwhere added load or scaling is antici-pated. When system demands exceedthe capacity of the pump, only the im-peller will need to be changed.

Estimation of EnergySavings

The potential for improving the effi-ciency of a pumping plant is highly de-pendent on the age and design of theequipment and its operation. An old,worn pump and standard-efficiencymotor may be operating in the vicinity of50 percent efficiency. A new pump andpremium-efficiency motor could improveoverall pump efficiency to over 60 per-

cent, a 20 percent improvement.

The following equations can be used todetermine pumping power:

BHPGPM H SG3960 PEFF

=× ××

KWBHP 0.746

MEFF= ×

where:

BHP = Brake horsepower at pumpdriveshaft

H = Total dynamic head

PEFF = Pump efficiency

GPM = Flow in gallons per minute

SG = Specific gravity of the liquid(1.0 for water at 60�F)

MEFF = Motor efficiency

In addition to these equations, the Affin-ity Laws can be used to predict changesin flow, head and horsepower as pumpspeed is changed. These laws may bestated as follows:

The flow rate from a pump will vary di-rectly according to the ratio of the motorspeed:

Q2 = Q1 x ( N2 / N1)

where:

Q2 = flow rate at the new speed

Q1 = flow rate at the startingspeed

N2 = pump speed for the newcondition

Motor HP Low Fair Good Excellent

3-7.5 <44.0 44-49.9 50-54.9 >54.9

10 <46.0 46-52.9 53-57.9 >57.9

15 <47.1 48-53.9 54-59.9 >59.9

20-25 <48.0 50-56.9 57-60.9 >60.9

30-50 <52.1 52.1-58.9 59-61.9 >61.9

60-75 <56.0 56-60.9 61-65.9 >65.9

100 <57.3 57.3-62.9 63-66.9 >66.9

150 <58.1 58.1-63.4 63.5-68.9 >68.9

200 <59.1 59.1-63.8 63.9-69.4 >69.4

250 <59.1 59.1-63.8 63.9-69.4 >69.4300 <60.0 60-64.0 64.1-69.9 >69.9

Table 2: Typical Overall PumpingPlant Efficiency Classifications


N1 = pump speed for the startingcondition.

Head (pressure) will vary as the squareof the ratio of the pump speed:

H2 = H1 x (N2/ N1 )2

where:

H2 = pressure at the new speed

H1 = pressure at the startingcondition.

Required horsepower (BHP) will vary asthe cube of the ratio of the pump speedor impeller diameter:

BHP2 = BHP1 x (N2 / N1 ) 3

where:

BHP2 = brake horsepower requiredat the new speed or impel-ler diameter

BHP1 = brake horsepower requiredat the starting condition.

Standard Savings Calculation

When improvements are made in motoror pump efficiency, the following stan-dard calculations can be used to esti-mate energy savings:

Motor efficiency improvement -

KWH BHP 0.746

1MEFF1

1MEFF2

annual operating hours

savings = ×

× −

×

where:

MEFF1 = old motor efficiency

MEFF2 = new motor efficiency

Pump efficiency improvement -

KWH BHP1 0.746 1PEFF1PEFF2

annual operating hours

savings = × × −

×

where,

PEFF1 = old pump efficiency

PEFF2 = new pump efficiency

Cost and Service Life

Factors That Influence ServiceLife and First Cost

An important factor to be aware of is thevariability of the system design pointover time. If the system piping and/orflowrates vary with different seasons orcrops, the service life of a measure tooptimize the pumping system for a par-ticular configuration and flow rate maybe quite short.

Typical Service Life

Following are the PG&E CEE programassumptions for the service life of sev-eral measures:

�� Pump retrofit - 8.7 years

�� Pump adjustment - 3 years

�� Variable speed drives - 16 years

�� High efficiency motors - 15 years


�� Base-mounted pumps - 20 years

�� Pipe-mounted, sump and wellpumps - 15 years

Operation and MaintenanceRequirements

PG&E has for years offered free tests todetermine the performance of agricul-tural pumping plants under field condi-tions. Tests measure gallons per min-ute, input horsepower and water levels(pumping, static and discharge). Fromthese data a Pump Test Report is pre-pared. If potential savings are indicated,a Pumping Plant Efficiency ComparisonReport is also prepared, detailing en-ergy and dollar savings.

Definitions of Key Terms

�� Cavitation: The collapse of vaporpockets formed in the impeller passagesbecause the absolute pressure at thepump suction nozzle has approachedthe vapor pressure of the liquid.

�� Head: A quantity used to expressthe energy content of the liquid per unitweight of the liquid, referred to any arbi-trary datum. In terms of foot-pounds ofenergy per pound of liquid pumped, allhead quantities have the dimension offeet of liquid.

�� Impeller: The heart of the centrifu-gal pump, it is the rotating piece housedin the casing, or volute, and driven by amotor. The impeller has spiral-shapedvanes whose diameter increases in thedirection of the flow. The spiral acceler-ates the velocity of the water, develop-ing pressure. Examples are shown be-

low.

�� Induction Motor: The most com-mon type of AC motor, in which a pri-mary winding on the stator is connectedto the power source, while the secon-dary winding on the rotor carries in-duced current.

�� Net Positive Suction Head Re-quired (NPSHR): The amount of pres-sure in excess of the liquid’s vaporpressure required to prevent vaporpockets from forming and bursting,leading to noisy and destructive cavita-tion. NPSHR is a characteristic of agiven pump and varies with speed andflow. It is determined by the manufac-turer and included on the pump per-

formance curve.

�� Overall Pumping Plant Efficiency(OPE): The ratio of pump output tomotor input, in percent. Equivalent tothe product of motor efficiency andpump efficiency.

�� Vapor Pressure: The pressure atwhich a pure liquid can exist in equilib-rium with its vapor at a specified tem-perature.

�� Variable Speed Drives: Suchdrives coordinate induction motor speedto the needs of the job, using semicon-ductor devices and switching circuits to

O P E N S E M I - O P E N C L O S E D


change the frequency of power deliv-ered to the motor.

References to MoreInformation

1. American Society of Heating, Refrig-erating and Air-Conditioning Engi-neers, Inc., "HVAC Systems andEquipment Handbook," 1996.

2. Chemical Engineering McGraw-HillPublications Co., "Fluid Movers:Pumps, Compressors, Fans andBlowers," 1979.

3. Luhm, G., "Multi-Stage Pumping,"PGandE Company Application NoteNo. 75, July 1986.

4. Luhm, G., "Centrifugal Pumps andPump Systems," PGandE CompanyApplication Note No. 47-56-89, May1984.

5. Luhm, G., "Parallel and SeriesPumping," PGandE Company Appli-cation Note No. 74, October 1986.

6. Pacific Gas & Electric Company,"Adjustable Speed Drives: What’s init for you," 1989.

7. Pacific Gas & Electric Company,"Agricultural Resource Guide," April1996.

8. Peerless Pump, "System analysis forpumping equipment selection," Bro-chure B-4003, 1979.

9. Peerless Pump, "Handbook of PumpEngineering Data," Brochure-EM77.

Major Manufacturers

Pumps

PACO Pumps, Inc.800 Koomey RoadBrookshire, TXTel (800) 955-5847Fax (713) 934-6090

Peerless Pump Co.1441 Peerless WayMontebello, CA 90640Tel (213) 726-1232Fax (213) 726-0814

VSDs

ABB Industrial Systems, Inc.16250 W. Glendale DriveNew Berlin, WI 53151Tel (800) 752-0696Fax (800) 648-2072

There are many other manufacturers ofrelevant equipment. A good source ofmanufacturers and products aregrouped by type of product in HeatingPiping and Air Conditioning’s annual"Info-dex" issue (Penton Publishing,Chicago, IL; (312) 861-0880).

Further information may be obtainedfrom: University of California IrrigationProgram - Dr. Blaine Hanson, (916)752-1130; Cal Poly Irrigation Trainingand Research Center - Dr. Charles Burt,(805) 756-2434.


*Reprinted with permission from “Fluid Movers:Pumps Compressors, Fans and Blowers” by S.Yedidiah. Published in Chemical Engineering.Copyright, 1979 by McGraw-Hill Companies. Allrights reserved.

Reprinted with permission. Copyright 1996, byASHRAE. All rights reserved.

**Reprinted with permission. Copyright 1979, bySterling Fluid Systems, Inc. All rights reserved.

pumping efficiency

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