bureau of reclamation-large pumping unts-em40

A WATER RESOURCES T E C H N I C A L P U B L I C A T I O N

ENGINEERING MONOGRAPH NO. 40

Selecting Large Pumping Units

UNITED STATES DEPARTMENT OF THE INTERIOR BUREAU OF RECLAMATION

8. P E R F O R M I N G O R G A N I Z A T I O N R E P O R T NO.

Willlam Duncan, Jr. and Carlos G . Bates Engineering Monograph 40 I

9 . P E R F O R M I N G O R G A N I Z A T I O N N A M E A N D A D D R E S S 10. W O R K U N I T N O .

Engineering and Research Center Bureau of Reclamation 11. C O N T R A C T O R G R A N T N O .

l same I

Denver, Colorado 80225

12 . S P O N S O R I N G A G E N C Y N A M E A N D A D D R E S S

14. S P O N S O R I N G A G E N C Y C O D E t------ 13. T Y P E O F R E P O R T A N D P E R I O D

C O V E R E D

1 1 5 . S U P P L E M E N T A R Y N O T E S

16 . A B S T R A C T

This monograph provides guidelines for designers or estimators who plan or lay out large-capacity pumping plants. Empirical charts and curves and analytical methods are presented to facilitate selection of the best type of pump and to estimate pump performance characteristics, submergence, dimensions, and mass. An example is given starting with basic head and capacity requirements, and working through selection of pump type and estimation of speed, specific speed, submergence, dimensions, pump mass, and power requirements. Included are illustrations of typical pumping plants.

1 7 . K E Y WORDS A N D D O C U M E N T A N A L Y S I S

0 . DESCRIPTORS-/ *pumps/ impellers/ spiral cases/ mixed flow pumps l propeller pumps l centrifugal pumps/ *pumping plants/ design criteria1 cavitation index

a 1 I J N C L A S S I F I E D I

b . IDENTIFIERS-/ Dos Amigos Pumping Plant, C A I Flatiron Powerplant, CO/ Oahe Dam, S D l Senator Wash Dam, CA

c . C O S A T I F i e l d / G r o u p 13G COWRR: 131 1 2 1 . N O . O F P A G E

33 2 2 . P R I C E

18 . D I S T R I B U T I O N S T A T E M E N T A v a i l a b l e f rom t h e N a t r o n o l T e c h n i c a l I n fo rma t ion Serv rce , Opera t i ons D i v r s i o n , S p r i n g f i e l d , V i r g i n i a 2215 1.

1 9 . S E C U R I T Y C L A S S ( T H I S R E P O R T !

U N C L A S S I F I E D 2 0 . S E C U R I T Y C L A S S

( T H I S P A G E )

A WATER RESOURCES TECHNICAL PUBLICATION Engineering Monograph No. 40

Selecting Large Pumping Units

BY William Duncan, Jr. Carlos G. Bates

Engineering and Research Center Denver, Colorado 80225

United States Department of the Interior Bureau of Reclamation

As the Nation's principal conservation agency, the Department of theInterior has responsibility for most of our nationally owned publiclands and natural resources. This includes fostering the wisest use ofour land and water resources, protecting our fish and wildlife, pre-serving the environmental and cultural values of our national parksand historical places, and providing for the enjoyment of life throughoutdoor recreation. The Department assesses our energy and mineralresources and works to assure that their development is in the bestinterests of all our people. The Department also has a major responsi-bility for American Indian reservation communities and for peoplewho live in Island Territories under U.S. administration.

ENGINEERING MONOGRAPHS are published in limited editions for thetechnical staff ofthe Bureau of Reclamation and interested technical circlesin Government and private agencies. Their purpose is to record develop-ments, innovations, and progress in the engineering and scientific tech-niques and practices that are employed in the planning, design,construction, and operation of Reclamation structures and equipment.

December 1978

III81 METRIC

U.S. GOVERNMENT PRINTING OFFICEDENVER

For Sale by the Superintendent of Documents, U.S. Government Printing Office,Washington, D.C. 20402, or the Bureau of Reclamation, Attention 922 Engineering and Research Center,

P.O. Box 25007, Denver Federal Center, Denver, Colorado 80225.Stock Number 024-003-00137-6

Preface

The objective of this monograph is to provide guidelines for selecting thetype of pump to best meet large-capacity pumping requirements and forestimating the performance characteristics, required submergence,dimensions, and mass of the pump. The guidelines and data presented arebased on both Bureau of Reclamation experience and basic theory and fromrecommendations in literature cited in the bibliography. The results shouldbe sufficiently accurate for initial plant layout and cost estimation.

This monograph was prepared by William H. Duncan, Jr., mechanicalengineer, and Carlos G. Bates, Head, Hydraulic Machinery Section,Mechanical Branch, Division of Design, Engineering and Research Center,Denver. Richard N. Walters made a substantial contribution to thetechnical presentation.

iii

Symbol Quantity Metric unit U.S. customary unit

d(i) Spiral case diameter at (i) mm ftD] Discharge diameter of impeller mm ft

Da Inlet diameter of impeller mm ftf Frequency Hz Hzg Gravitation constant

(acceleration) m/s2 ftls2h Pump best efficiency head

(design head) m ftH Head produced by pump m ft

Ha Atmospheric pressure (head) m ftHs Suction head m ftHL Head loss (suction side) m ftHv Vapor pressure head of water m ftKu Speed constantK3 Experimental design constant

n Rotational speed r/min r/minn' Trial rotational speed r I min r/min

ns Pump specific speed(r/min)ym3/s (r/min)ygallmin

mO.75 (ft)o.75

n' Trial pump specific speed(r/min)ym3/s (r/minh/gallmin

s mO.75 (ft)o.75

NPSH Net positive suction head m ft

Q Capacity (discharge) m3/s fP/s orgallmin

S Suction specific speed (r/min)ym3/s (r/min)ygallminmO.75 (ft)o.75

V Velocity of water m/s ftlsp Power watt horsepower

a Speed ratio factor(1 Cavitation coefficient

(Thoma-sigma)11 Pump design efficiency percent percent

Letter Symbols and Quantities

iv

Contents

Preface. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Letter Symbols and Quantities. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. ...Capacity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Head .Pump Specific Speed. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Net Positive Suction Head...

""""

... ...

""'"

... ... ... .. .. . ..

Suction Specific Speed. . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Thoma Cavitation Coefficient. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Affinity Laws and Hydraulic Similarity. . . . . . . . . . . . . . . . . . . . . . . . . . .

Unit CharacteristicsSample Design Problem. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7Head Range. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7Trial Pump Specific Speed. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7Capacity Requirements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8Rotational Speed and Pump Specific Speed. . . . . . . . . . . . . . . . . . . . . . . . 8Pump Submergence. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8Pump Dimensions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9Impeller and Total Pump Masses. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 11Pump Power Requirement. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 11Comments on Pump Selection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 11Bibliography. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 12

Basic Equations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. Inside Back Cover

Conversion Factors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Outside Back Cover

Page

iii

iv

1

1233344

v

Number

VI

CONTENTS

FIGURES

12

PagePump type selection guide. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 13Loss in efficiency and capacity due to wear

between overhauls. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 14Expected pump efficiency versus specific speed. . . . . . . . . . .. 15Net positive suction head 16Pump specific speed versus design head 17Recommended minimum sigma at best efficiency point.. .. 18Typical variation of critical sigma versus discharge. . . . ., 19Flatiron and Dos Amigos performance curves. . . . . . . . . . .. 20Oahe and Senator Wash performance curves. . . . . . . . . . . . ., 21Snake Creek performance curves. . . . . . . . . . . . . . . . . . . . . . . . .. 22Dos Amigos performance curves. . . . . . . . . . . . . . . . . . . . . . . . ., 23O'Neill performance curves. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 24Inlet and discharge diameter approximations. . . . . . . . . . . ., 25Approximate spiral case dimensions. . . . . . . . . . . . . . . . . . . . .. 26Impeller and total pump mass - centrifugal

vertical spiral-case pumps. . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 27Vertical column pump mass. . . . . . . . . . . . . . . . . . . . . . . . . . . . ., 28Typical 250-m (820-ft) head pumping plant. . . . . . . . . . . . . . .. 29Typical 60-m (200-ft) head pumping plant. . . . . . . . . . . . . . . ., 30Typical 30-m (100-ft) head wet-pit pumping plant. . . . . . . . .. 31Typical 17-m (55-ft) head dry-pit pumping plant. . . . . . . . . ., 32Prospective 3-m (10-ft) head pumping plant. . . . . . . . . . . . . .. 33

3456789

101112131415

161718192021

Introduction

This monograph covers pumping unit capac-ities ranging from 3 to 280 m:J/s (100 to 10 000ft3/s). Units smaller than 3 m:J/s usually can befound in manufacturers' catalogs whereindesign and estimating data are readily avail-able. Presently, the largest pumps the Bureauoperates are rated 62.3 mJ/s at 38 m (2200 ffJ/sat 125 ft) oftotal head, and it is not foreseen thatUSBR will require larger units. However, pump-turbines have been built for larger capacities;the USBR is considering pump-turbines of 710m.l/s (25 000 fP/s) capacity.

In selecting the number and size of units toperform required duties, consideration must begiven to reliability, flexibility, and cost.

Whereas it may be a wise decision to selectonly one unit for a powerplant supplying powerto an interconnected transmission system, itcould be a very poor selection to have only oneunit if a water supply was entirely dependent onuninterrupted pumping capability. Thus, moreunits would be expected in a pumping systemthan in a power system. The time scheduled formaintenance and the effects of an unscheduledoutage of the largest unit should be considered.

Standard designs and identical hydraulicunits are desirable from an engineering andmaintenance standpoint. However, the unitsshould be selected to match variations in heador capacity without causing excessive loss inefficiency and unusual wear problems. Thewater to be pumped should be analyzed andpump materials selected accordingly to resistcorrosion. Priming equipment usually is a-voided by setting the impeller inlet edge belowminimum water surface elevation and/orproviding adequate positive suction head forwater to fill the pump case.

Pumps are classified by distinguishingfeatures such as:

. Impeller characteristics (axial flow, mixedflow, radial flow, open, semiopen or en-closed, single suction or double suction,etc.),

. Pump casing design (spiral, single volute,double volute or diffuser, turbine, cir-cular, etc.),

. Orientation of pump shaft axis (vertical,inclined or horizontal),

. Intake design (wet pit, dry pit), and

. Number of stages.Figure 1 is a general guide for selecting the

type of pump best suited to meet various headand capacity requirements. However, in selectingthe type pump best suited to a particularsituation, economics of plant construction,efficiency of the units, and operation andmaintenance costs should be considered.

Pump and motor dimensions and costs can beminimized by using high rotational speeds.However, in providing optimum performance athigh rotational speeds, a pump will require deepsubmergence, possibly leading to increasedplant construction costs. Likewise, capitalexpenditures to increase unit efficiency byusing a diffusion casing, enlarging flowpassages, or other means should be comparedwith the savings in power costs during the life ofthe project.

To select a pump and prepare preliminarydesigns, operational requirements must beanalyzed and estimates made of rotationalspeed, submergence requirements, pump dim-ensions, pump mass, efficiency, and powerrequirements.

Capacity

A plant serving a distribution canal orpipeline obviously requires more regulatingcapability than a plant pumping from onereservoir to another or to a feeder canal. Theformer may require a number of units or eventwo or more sizes of units to meet demand. Forsmall plants, using catalog-size pumps, acommon selection is;

. Two units at one-third plant capacity,

. One unit at one-sixth, and

. Two units at one-twelfth plant capacity.This combination provides flow increments of

one-twelfth plant capacity while only one-thirdcapacity is lost when the largest unit is out ofservice. For large plants with specially designedunits, variable-pitch pumps in axial and mixed-flow designs may be economical. Such units candeliver from 50 to 100 percent of maximum

PumpHead range flow type Customary name

meters feet

3-9 10-30 Axial These are referred to as propeller pumps from thedesign in the impeller and the lifting action of theblades on the liquid.

9-18 30-60 Mixed Francis-style double-curvature vanes usually areused in the impeller design ofthese pumps commonlycalled Francis pumps.

18-300 60-1000 Radial Since centrifugal forces define the principal actionof these pumps, they often are referred to as centri-fugal pumps.

2 SELECTING LARGE PUMPING UNITS

capacity at either a variable or constant headand operate with good efficiency. Thus, a plantcontaining four fixed-pitch units and twovariable-pitch units can deliver any capacityfrom one-twelfth to maximum capacity withgood efficiency. The cost of variable-pitchpumps is about 30 percent higher than fixed-pitch pumps. Offsetting this cost, a fixed-pitchpump requires a larger-consequently moreexpensive-motor to accommodate overcapac-ity in the pump design and variation in head.

Other methods for obtaining flexibility indischarge rate are: multispeed, variable speed,

throttling, and bypassing. These methods havenot proved economical for high capacityirrigation pumping.

Head

This monograph discusses best efficiencyheads ranging from 3 to 300 m (10 to 1000 ft). Asillustrated in figure 1, the approximate headrange of a single stage for three types of pumps(classified by impeller design) is:

Note that all three types of pumps considered are often classified in the category of centrifugal pumps because of therotary action of the impellers.

Mixed-flow, variable-pitch pumps have beenused for heads up to 76 m (250 ft). Single-stagepumps are desirable for reasons of lower costand simplicity. However, multistaging isapplicable to improve efficiency, to obtain asteeper head-discharge curve, or to reduce re-quired net positive suction head. Single-stagepump-turbines are being built for 610 m (2000 ft)of head or more. At the A. D. EdmonstonPumping Plant on the California Aqueduct [1]1,the pumps are four-stage, for a total design headof600 m (1970 ft), resulting in an optimum pumpspecific speed considering efficiency and sub-mergence requirements.

The head range that a pump must operatewithin is an important consideration. For acanal relift plant, the head may be nearly

'Numbers in brackets refer to items In the bibliography.

constant when individual discharge lines areused. The head will vary more if several pumpsare manifolded to a single discharge line. TheSan Luis Pumping/Generating Plant, Cali-fornia [2], is notable as it has a head range from30 to 100 m (98 to 330 ft) for filling a largereservoir. Two speeds are used to satisfy thehead range. At Snake Creek Pumping Plant,North Dakota, the requirement is to pump from0 to 23 m (0 to 75 ft) of static head.Interchangeable bowls and impellers fulfill thespecified performance.

Variation in head requires a deeper sub-mergence, especially when the variation fromthe design head is to lower heads. When a unit isrequired to pump from a storage reservoir to acanal, the head will vary; but the minimumhead will occur at maximum suction head (fullreservoir)-which is helpful.

INTRODUCTION 3

USBR practice is to add a small percentage topump design capacity to allow for wear prior toscheduled overhauls (fig. 2).

Pump Specific Speed

The specific speed ns of a pump is:

n - nVQs-- ho.7;,

where:

n = rotational speed, r/min,h = best efficiency head developed, m (ft),

andQ

= best efficiency discharge, m:J/s (gal/min).

Pump specific speed is defined as therotational speed at which a given pump orgeometrically and hydraulically similar pumpdischarges 1 m:J/s of discharge under 1 m ofhead (1 gal/min at 1 ft of head) while operatingat the best (peak) efficiency point. The pumpspecific speed characterizes the type and shapeof the impeller and is used to predict otherimportant pump characteristics, dimensions,and mass. To obtain an approximate value of nsin units of r/min, mIls, and m, multiply U.S.customary ns (r/min, gal/min, and ft) times0.01936.

For double suction pumps, it is USBR practiceto use one-half the capacity (QI2) ofthe pump tocalculate ns and S. Thus, an identical specificspeed versus head graph applies to both singleand double-suction pumps.

The range of pump specific speed can becategorized:

. High specific speeds greater than 155(8000) usually indicate an axial-flow-typeimpeller.

. Low specific speeds of 87 (4500) or lessindicate the radial-flow-type impeller.

. Medium values of specific speed generallyindicate a mixed-flow-type impeller.

There are no definite limits to define theoperating regimes for the three types ofimpellers. However, experience has shown thatan axial-flow-type impeller cannot be usedefficiently for high heads and is seldom used for

heads greater than 9 m (30 ft) per stage. Theradial-flow type is the most efficient at highheads and has been used for heads up to 610 m(2000 ft) per stage. Figure 1 shows the usualoperating regimes for the different types ofpumps. Figure 3 presents curves of expectedpump efficiency versus specific speed forvarious pump capacities.

Net Positive Suction Head

NPSH (net positive suction head) is defined asthe total suction head above vapor pressure atthe highest point ofthe impeller inlet edge. TheNPSH available for the pump, at a given site, iscalculated from the equation:

NPSH =Ha + Hs - Hu - HL

where:

Ha = atmospheric pressure head,Hs = suction head,Hu = water vapor pressure head, andH L = suction side head losses.

Figure 4 illustrates NPSH. A particular pumpdesign requires a certain minimum NPSH head(required) to prevent cavitation. The availableNPSH at the plant site must be equal to orgreater than the required NPSH. Operationwith less than the required NPSH will cause thehead and efficiency to drop, and destructivecavitation will occur on the impeller blades.

Figure 5 illustrates the upper limit of pumpspecific speed versus design head for variousconditions of Hs. The figure is based on datafrom pumps and pump-turbines which areoperational. A higher speed and a wide range inhead generally necessitate a higher value ofavailable NPSH.

Suction Specific Speed

Suction specific speed S is defined as:

nVQ ( h )0,7,')S = (NPSH)o.7.') = ns NPSH


where:

h = best efficiency head developed,n = rotational speed,Q

= best efficiency discharge, andNPSH = net positive suction head at the site or

absolute suction head less vapor p;es-sure head.

The parameter S is used for pumps to describethe suction characteristics of an impeller. An Svalue within 153 to 155 (7900 to 8000) has beenfound to produce the best performance. If animpeller is designed for a higher suction specificspeed to reduce the required NPSH, the bladeentrance-angle must be flattened. This resultsin lower efficiency and a larger impeller-eyediameter for a given capacity [3]. Except forvery special cases, higher suction specificspeeds should not be considered.

Thoma Cavitation Coefficient

The parameter used for defining the oper-ating condition, with respect to cavitation, iscommonly known as the Thoma cavitationcoefficient and is represented by the Greek lettersigma u. It is the ratio of NPSH to total pumphead or:

u = NPSHH

For large pumping units, USBR definescritical sigma as the sigma value at whichcavitation causes a I-percent loss in head.Critical sigma is determined by model tests foreach impeller design. With the critical sigmavalues known, the required pump submergencecan be determined with respect to suction waterlevel for any environment. Critical sigma andrequired NPSH increase rapidly with dischargeabove the best efficiency point. Bureau exper-ience has shown that if a pump is operated at ahead considerably below best efficiency head(high discharge), no amount of available NPSHwill prevent cavitation.

Figure 6 shows the recommended sigma val ueversus pump specific speed. Figure 7 illustratestypical variations of critical sigma versusdischarge for different pump specific speeds.Note that the curves on figures 5 and 6 are basedon a constant suction specific speed S of

approximately 154 (7950). A similar curve tofigure 5 in the Hydraulic Institute Standards [4]e~hibits a varying suction specific speed.FIgure 5 generally indicates a lower pumpspecific speed limit per given design head andsuction head up to a specific speed ofapproximately 70 (3700), and higher limitsabove 70 than recommended in the HydraulicInstitute Standards.

Affinity Laws and Hydraulic Similarity

For a given centrifugal pump of constantimpeller discharge diameter D, the performancevariables of capacity Q, head H, power P, androtational speed n, at points of equal efficiency'1, vary according to the relations:

. Capacity is directly proportional to speed,

. Head is proportional to the square of thespeed, and

. Power is proportional to the cube of thespeed.

For mixed-flow and radial-flow pumps, whenspeed is held constant and the impellerdischarge diameter is varied slightly, therelations between points of equal efficiency canbe expressed as:

. Capacity is directly proportional to diameter,

. Head is proportional to the square of thediameter, and

. Power is proportional to the cube of thediameter.

If both nand D are varied, both relationsapply simultaneously.

The affinity laws which follow from theserelations can be applied to calculate changes inpump performance due to varied rotationalspeed or impeller discharge diameter of a givenpump.

For constant diameter: For constant speed:

Qa=

na

Qb nbQa

=Da

- -Qb Db

z: =(~:r z: =(~:r

~: =(::YP (D ):1~ = --.!!:Pb Db

INTRODUCTION 5

where:a and b denote thesame pump run atdifferent speeds.

where:a and b denote thesame pump withslightly different im-peller diameters.

For geometrically and hydraulically similarmachines (equal specific speeds) the perform-ance data obtained from one unit can be used toestimate the performance of another unit (i.e.,model test data used to estimate prototypeperformance) using the following laws of pumpscaling [5]:

~:= (::) (z:r

z: = (::r(~:r;: = (::r (z: r

and a modified Moody equation to determineefficiency:

~~ ::

= (z: f14

where:

a and b denote two geometrically andhydraulically similar pumps.

Unit Characteristics

Sample Design Problem

There is a requirement for three equal-sizecanal relift pumping units discharging into acommon line where the unit discharge center-line is at approximately 1000 m. The pumps areto be submerged to avoid the need for vacuumpriming equipment.

. Minimum capacity at maximumhead (per unit) .. .. .. ., .. .. .. ., .14.2 m:J/s

. Static lift. . . . . . . . . . . . . . . . . . . . . . 61 m

. Average water temperature. . .. 35C

Figure 1 shows that consideration should begiven to the radial-flow, vertical-shaft, spiral-case-type pump.

. Canal water surface may varyfrom normal. . . . . . . . . . . . . . . . . . .

. Total suction side head loss perunit (including trashrack loss) .

. Discharge side head loss per unitthrough the manifold. . . . . . . . . .

:to.9m

0.3m

1.5 m

With three units in operation at full capacity,the head loss in the discharge line (includingvelocity head loss at the discharge structure) is5.5 m. The hydraulic units are to be designed forbest efficiency with two units operating. Thepumps' will be driven by synchronous electricmotors.

Head Range

Begin by calculating the required total headrange. In this case, the best efficiency (design)head is to occur with two units operating and astatic lift of 61.0 m.

Design head (100 percent head) =61 + 0.3 + 1.5+ 5.5 (2/3F = 65.2 m.

Note that, for a number of units using acommon discharge line-during operation atless than full capacity-the discharge line headloss is roughly equal to:

~

(Operating capacitY) j 2* (d ' hX ISC argefull capacityline loss at full capacity)

The minimum head will occur with one unitoperating and a static lift of 61.0 - 0.9 =60.1 m.

Minimum head =60.1 + 0.3 + 1.5 + 5.5 (1/3)2 =62.5 m =96 percent of design head.

The maximum head will occur with threeunits operating and a static lift of 61.0 + 0.9 =61.9 m.

Maximum head =61.9 + 0.3 + 1.5 + 5.5 =69.2 m =106 percent of design head.Trial Pump Specific Speed

It is assumed the pumps are to be submerged1 m (Hs = 1 m). Using figure 5, (pump specificspeed versus design head at various suctionheads) the upper limit of pump specific speedcan be estimated and used as the trial pumpspecific speed n~ However, it is noted that figure5 is based on sea level with a water temperatureof 29C, and does not include suction side headlosses He Therefore, prior to using figure 5 thedesired suction head Hs-at plantsite el~va-tion and plantsite water temperature-shouldbe corrected to sea level and a water temper-ature of 29C.

For better accuracy the trial pump specificspeed can be calculated considering that suc-tion specific speed S should approximate 154and using the equations:

NPSH =Ha + Hs - Hv - HL

S - ,( h )0.75 S- ns NPSH'

or n~

(N;S HY7;)

From figure 6, at the plantsite elevation of1000 m and a water temperature of 35C:

Ha = 9.18 m atmospheric pressure, and Hv =0.57 m vapor pressure

'When the di"ha'ge hne h,a" In" in "',.e;"a" "a

--lexponent of ~.85 for the pipe friction portion.

oft.

he 10

..

88' and2.0 for any fIttIng 1088 and velocity head 10~~N {2002. 7

~'.""'---'-""""---

./ ,.;eciamation'x'i_~;ervice Center


Suction side losses HL were given as 0.3 mand suction head Hs has been assumed to be1.0 m.

Thus, the available NPSH for the pumpingplant with I-m suction head Hs is estimated to be:

NPSH== 9.18 + 1.0 - 0.57 - 0.3 == 9.31 m

and

n' - 154s - ( 65.2 )0.759.31 == 35.8Capacity Requirements

For estimating the capacity to be specified atdesign head h, reference is made to theperformance curves of existing units of similarpump specific speeds. The curve on figure 8shows a typical plant having a specific speed of39. At 106 percent design head (maximum headfor the example), capacity should be about 95percent of that at best efficiency (design) headh. Therefore, to deliver 14.2 m:J/sin the exampleplant (at maximum head), the capacity atdesign head (design capacity) should beincreased to:

Q==

14.2== 14.9 m3/s0.95

The drop in efficiency and capacity caused bywear (between overhaul periods) also must beconsidered. Figure 2 presents percent loss inefficiency versus .,!h;..J?l for different waterenvironments. Generally, canals are subject tocontamination from windblown sand and siltequivalent to condition "B" (fig. 2);

thus:

_fh-

...L65.2== 21

"Q - "14.9 .

From figure 2, a 2-percent loss in capacity canbe predicted between 3-year overhaul periods.Therefore, the design capacity must be in-creased:

hence:

Q== (14.9) 1.02 == 15.2 m3/s

Rotational Speed and Pump Specific Speed

Calculate the trial rotational speed n' fromthe trial pump specific speed n~;

where:

n~ hO.75 35.8 (65.2)0.75n' == -- ==210 r/minQ 15.2

However, since the pumps are to be driven bysynchronous electric motors, the pump rota-tional speed should equal a synchronous speed.Consideration should be given to the fact thatfor extremely large motors a multiple of fourpoles is preferred [6]; however, standard motorsare available in most multiples of two poles.

Determine speed as follows:

Rotational speed, n ==120 (frequency)number of poles

n==

7200, at 60 Hz

number of poles

Using a multiple of two poles, the closest60 Hz synchronous speed to 210 r/min is:

-

60 (120)n

- 34 == 212 r/min

Therefore, the pump specific speed for thegiven condition is:

n==

nVQ==

212 Vi52== 36.0S hO.75 (65.2).75

Pump Submergence

The submergence of the units can beestimated using ns == 36.0. From figure 6, at bestefficiency head, the recommended minimumsigma C1is:

UNIT CHARACTERISTICS 9

0" =1212 (ns)I.:J:J

=1212 (36.0)I.:J1

=0.142106 106

From the performance curves on figure 8, atminimum head (96 percent design head),capacity will be about 105 percent of designcapacity. Figure 7 (typical variation of criticalsigma versus discharge) shows that at 105percent capacity the expected critical sigma willbe approximately 110 percent of critical sigmaat best efficiency point. Therefore, at minimumhead, critical sigma should be approximately:

0" = 0.142 (1.10) =0.156

The same procedure for maximum head (106percent design head) results in a critical sigmavalue of 0.132;

whereas:

~ -

NPSH, or NPSH = O"H

v - H

At maximum head, NPSH =0.132 (69.2) =9.13 mAt design head, NPSH =0.142 (65.2)= 9.26mAt minimum head, NPSH =0.156 (62.5) = 9.75m

For a plant at an elevation of 1000 m andaverage water temperature of 35C, figure 6shows:

. Atmospheric pressure head, Ha.. ..9.18 m

. Water vapor pressure head, Hv'"

. 0.57 m. Total suction side losses, HL . . . . . . .0.3 m

Therefore, the highest point of the inlet edgeof the impeller should be at least (fig. 4):

9.75 - 9.18 + 0.57 + 0.3 =1.44 m below the inletcanal water surfaceelevation which pro-duces minimum totalhead,

9.26 - 9.18 + 0.57 + 0.3 = 0.95 m below the inletcanal water surfaceelevation which pro-duces design head,and

9.13 - 9.18 + 0.57 + 0.3 =0.82 m below the inletcanal water surfaceelevation which pro-duces maximum totalhead.

Additional submergence may be consideredto provide a factor of safety against cavitationand loss of efficiency.

Pump Dimensions

After estimating the rotational speed andpump specific speed with the given design head,the curves and equations of figure 13 are usedto estimate the impeller inlet diameter D:J andthe impeller discharge diameter Dl.

For estimating Db determine the speedconstant Ku [7] from either the curve (fig. 13) orthe polynomial approximation:

6.4 nsKu = 0.82 + 103 -

3.3 (nsF106

Dl is calculated from the equation:

Dl =84 600 Ku v'h

n

With the values from the example, n = 212r/min, h = 65.2 m, and ns = 36,

calculate:

K = 0 82 + 6.4 (36)-

u. 1033.3 (36)2

= 1.05106

and

Dl = 84 600 (1.05) 65.2212 =3383 m

D3 is determined by using figure 13:

Select the speed ratio factor a for the given nsfrom either the curve or from the equation:


a = 810 (1~~Or7()7

For the given design head, calculate D;Jfrom theequation:

D:I = 550aVhn

whereupon:

( n )11.7117a =810 10~0 =77.226and

D., = 550 (77.226) ~= 1618 mm, 212

It is noted that, with the approximateimpeller inlet diameter DI known, the suctiontube dimensions also can be estimated by usingUSBR Report No. HM-2 [8].

U sing the estimated impeller dischargediameter Dl and pump specific speed ns, figure14 is used to estimate the following spiral casedimensions:

RlR~R;JR4

A~JEFG

= 0.180 = (3383) = 609 mm= .155 = (3383) = 524 mm

.125 = (3383) = 423 mm= .090 = (3383) = 304 mm= .835 = (3383) = 2825 mm= 1.065 = (~j383) = 3603 mm

.980 = (3383) = 3315 mm

.890 = (3383) = 3011 mm

Spiral case dimensions also can be approx-imated analytically by using Stepanoffs [7]volute velocity equation:

V=K3V2gh

where:

v = velocity in the spiral case, andK3 = an experimental design constant:

K3 = 1.15 (ns) .11.1;1

whence K3 may be calculated.

Assuming V is constant and Q increases indirect proportion to the angular distance fromthe cut-water ("The wall dividing the initialsection and the discharge nozzle portion of thecasing *

* * "[9]) or can be otherwise predicted,

the spiral case diameter d can be approximatedat various locations (i). From the equation ofcontinuity:

d(i) = Q(j) 1060.7854 V

To find the radial length from the unitcenterline to the outside of the spiral caseat any location (i), add one-half the impeller

discharge diameter D1, plus the spiral casediameter d(i), plus 0.1 times the impellerdischarge diameter to allow for the diffuserring. Thence, at any location (i) around the case,the radial length from the unit centerline to theoutside of the spiral case can be approximatedby:

Radial length =Q(i) 106

+ 0.6 Dl0.7854 V

In the example, dimension F at location 2 (fig.14) is calculated,where:

h = 65.2 m, Q = 15.2 m3/s, and ns = 36.0K3 = (ns)-O.33 = 1.15 (36.0)-o.:J,3 = 0.35

V = K3 V2gh :0 0.35 V2 (9.82) 65.2 = 12.5 m/s

Assuming location (2) is nearly 2700 from thecutwater:

Q(2) = 270 (Qd . ) = 0.75 (15.2) = 11.4 m3/s360 eSlgnd(2) = (11.4) 106 = 1078 mm0.7854 (12.5)

Therefore, R(2) = 538 mm as compared to R(2) =524 mm from experience curves, and

F = d(2) + 0.6 Dl = 1078 + 0.6 (3383) = 3108 mm

In this case, F (as calculated) is less than thedimension F of 3315 mm previously predicted

Empirical AnalyticalDimension curves method

millimeters millimetersE 3603 3274G 3011 2909A ~J 2825 2652

UNIT CHARACTERISTICS 11

by the experience curve of figure 14. Thismethod, though perhaps less reliable, has theadvantage of being easily programmed on ahand calculator to quickly calculate estimates.From figure 14:

Impeller and Total Pump Masses

In considering a spiral-case pump, for aparticular design head h and impeller dischargediameter Db figure 15 is used to estimate theimpeller mass and the total pump mass. Thecurves are based on data from existing pumpdesigns and the equations shown (fig. 15) arepolynomial approximations of the curves. Notethat the total pump mass is expressed by twoseparate curves. One curve is used when thedesign head is less than 30 m and the otherwhen the design head is greater than 45 m.Intermediate design heads require interpol-ation.

For the example, where the design head h wasgreater than 45 m and the discharge diameter is3383 mm, the total pump mass is about 94metric tons. The impeller mass is approx-imately 16 t. A similar computation can bemade to estimate the mass of a vertical-column pump with the experience curves shownon figure 16.

Pump Power Requirement

When calculating the pump power require-ment P, for subsequent motor sizing, perform-ance curves for a pump of similar specific speedshould be used to predict the operatingconditions that will demand maximum power.In the sample problem, from the shape of thecurves (fig. 8, Flatiron), the power requirementis expected to be greatest at maximum capacity.

Pump power requirement P in kilowatts is:

P = 9.8 QH'r/

The pump efficiency 17, at design head andcapacity (best efficiency), is estimated fromfigure 3. Using the design parameters pre-viously calculated, the pump best efficiency willbe approximately 91 percent.

At design conditions:

P = 9.8 (15.2) 65.2 = 10 670 kW0.91However, from figure 8, at maximum capacity

(105 percent design capacity) the power re-quirement will be 102 percent of the require-ment at design capacity. Therefore, the maxi-mum pump power requirement is:

P =10 670 (102 percent) =10880 kW

A driver with a net output approximately 10percent over the maximum pump requirementusually would be required to allow for over-capacity which the pump manufacturer mayprovide to assure his guarantee is fulfilled, andto provide for operation under conditions otherthan those anticipated.

Comments on Pump Selections

After determining the principal dimensionsand mass of the pump, a layout can be made.With the aid of electrical and structuralengineers, a cost estimate can be prepared.Consideration should be given to alternatives ofrotational speed and submergence, number ofstages, and style of pump relative to con-struction cost, operation and maintenanceexpense, and replacement life.

SELECTING LARGE PUMPING UNITS

BIBLIOGRAPHY

[1] Jansen, Robert B., "Edmonston PumpingPlant: Nation's Mightiest," Civ. Eng., vol.42, No. 10, pp. 67-71, October 1972.

[2] Stroh, Raymond G., Seitz, Bradley G.,Lloyd, L. W., "Rapid Reversal of San LuisPumping-Generating Units," IEEE Trans.,Power Appar Syst., vol. P AS-89, No.6, pp.1106-1111, July/August 1970.

[3] Bates, Carlos G., "Pumping Unit StudiesFinal Evaluation Report," Prepared forOffice of Saline Water, 39 p. Bureau ofReclamation, Denver, December 1965.

[4] Hydraulic Institute, "Standards for Cen-trifugal, Rotary, and ReciprocatingPumps," 13th ed., New York, 1975.

12

[5] Stelzer, R. S., Walters, R. N., "EstimatingReversible Pump-Turbine Characteristics,"Engineering Monograph No. 39, 46 p.,Bureau of Reclamation, Denver, 1977.

[6] Walters, Richard N., Bates, Carlos G.,"Selecting Hydraulic Reaction Turbines,"Engineering Monograph No. 20, rev. ed.,55 p., Bureau of Reclamation, Denver,1976.

[7] Stepanoff, Alexey J., "Centrifugal andAxial Flow Pumps," 2d ed., J. Wiley, NewYork, 1957.

[8] Bureau of Reclamation, "Proportions ofElbow-Type Suction Tubes for LargePumps," Report No. HM-2, 10 p., Denver,June 1964.

[9] Karassik, 1. J., Krutzsch, W. C., Fraser,W. H., Messina, J. P., "Pump Handbook,"McGraw-Hill, New York, 1976.

--~-

.

METERS- f- 300

f-250i

-200 I-

1

-150

I

-

-100 I I--

R \rltJ -FlOW. VER--'CD ;PIR. CASI

-80-

- -60 !-50 ,

I i-40 1-

-1- -- - - - - - --

- W ,,-

-30 (')

,I /.I / / /

// 1/ ./II' /,I

.I /1->- [/11 / 1/f- ./ 1./1--UI--

,,- -

-

".-

----

~"-...........::

~m% (ft3/:- -', 'f" ,/ -..--..... ' :::.... -.J .-- - I

/' ,-

-~"" ' ~57 200

-

.....

""""""'......'

.....

~5.7 20-

-

.....

-......... """'.........."'"~0.85 3

---

~............~0.28 Ii'o..""

....... 0.11

~~.......

~0.060.03 I

f-ZWUa::w 95a...I

0~

90IZS? 85(f)w0

~80

>-U~

75

UI.J..b 70a...

::!:::::>a...

0wf-UWa...xw

FIGURES 15

s)

000042

10(r/min)(m%f5

mO.75(r/min)(gal/min)0.5

(ft)0.75

15 20I I

700 1000

30 40 50 60 80 100I I I I I

1500 2000 3000 4000 5000

150 200I

10000

300 400I I

15000 20000

PUMP SPEC I FIC SPEED- ns

FIGURE 3.-Expected pump efficiency versus specific speed. I06-D-882.


~

~LrHo

Minimum inletwater surface

NPSH=Ho + Hs - Hv -Hl

Hs Highest point ofinlet edge

~

~

Atmospheric pressure'"'" I

~

'""', Ho

)!(\ It~

Vapor pressure

reI

NPSH= Ho +Hs -Hv -HlInlet water surface~ 'V

~~

Hs Highest poi nt ofimpeller inlet edge

Hl = Suction side head lossesHo = Atmospheric pressure headHv = Water vapor pressure headHs = Suction head

NPSH = Net positive suction head

FIGURE 4.-Net positive suction head. lO6-D-383.

(rim i n)( gal Imin)0.5ho.75

(r/mi n)(m3/S ).5ho.75

50 000 1000

40 000

30 000 en500 c:1

020 000 400 wwCl..(/)

300ul.L.U

200 w10000 Cl..(/)150 Cl..::E

::>Cl..l.L.

5000 100 af-

4000 80 ::E-.J

3000 60 a::w50 Cl..Cl..

::>

2000 40

1500 30

1000 20

15

600

FIGURES 17

Clear water at 29 e(85 OF) at sea leve Isuction specific speed, 5=154 (7950)

Single-suction overhung Impeller

HEAD( FIRST STAGE)- h10

300I

900

200 150 100 80 60 50 40I I I I

500 400 300 200

30I

100

20 15 10 8I I I

50 40 30

6 5 4 mI I ft '20 15

FIGURE 5.-Pump specific speed versus design head. 106-D-384.

/..L /

--~

I~--

ITI ~I

,~

'~1c: 0

t""'

....

'"'::"1 I

"'7 '" I""


a:: 160w~

a:140FLATIRON POWER a PUMPING PLANT,

HEAD COLORADOns =39(2000)0

z 120-U 100ZW~ 80l.L..l.L..W

-

600

(/)

0

c::w:!= 1400a..

~120

FIGURES 23

160

20

140

120

0;5 100I

~80

wu

ffi 60a..

40

010 20 30 40 50 60 70 80 90

PERCENT CAPACITY100 110 120 130 140

"'~

I

-J--1---+---f--ONEILL PUMPING PLANT,

CALIFORNIA~" ~ns=89

(4600)Variable-pitch

l\ ~'"'"." ~"

'"""-..

...........

IGb.%'",IRffi. 85%~~q5%

" " ~~"-~/-

-"""" ..........-~:/ 7...;;...", 100% ..... .... %

,,~~~~I""""'" l ~~./,/< >

I I _I I o. ~07 ./0

",~

'2 I -, I I I I / /

J f / V I /JII)

, / / / / /c:-I

0 V / / / /w Jo-~ (f) / / 1/ / IJ Iu-- I.J...

Ji / / /u Iwo-~ (f) /' / / ' I~J-~

~I, '/ / I IJ /III / / / /II

J /11 VI / '/

PE,RCEN,T I MfELLER DI?CHA~GE 9IAM~TER~ D,


((r/min)(gal/min )0.5)(ft)0.75(rim in)( m3/s)0. 5

mO.75

1307000

30

R R R R A-J G F E

1206000

110

1005000

9

400080

7

3000 60

50

2000 4

1000 2

I

10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160

Size expander foreconomic dischargepipe and valve sizes.

2

Curves are derived from both USSRdimensional data and theoreticaldimensionsbased.on constants Kuand K3 from Stepanoff [7].

FIGURE 14.-Approximate spiral case dimensions. 106-D-393.

POUNDS METRICX 1000 TONS

9040

80

7030

60 Mass in metric tons(f)2 3(f)

= 242 -4.450,

+ 3.1401- 0.190

POUNDS METRICX 1000 TONS

:r:01-We>

5OZOW0::;:;;;a.. -w 50-I(/)Z

FIGURES 29

~i'

Cl'.0'

PLAN

~Concrete

-

first stage

r.>~;:;J Concrete-

second stage

:

.---'-----

MOJ(- W. s.::"-

MIn. W5.:-=",,-

EXPLANATION

TRANSVERSE SECTION

TYPICAL 250 m (820ft) HEADPUMPING PLANT

FIGURE 17.-Typical 250-m (820-ft) head pumping plant. I06-D-396.


PLAN PUMP FLOOR

--

Mo-:_~S.

Min W.S.

XPLANA TION

~Concret, - first stag.

~Concrete - second slage

TRANSVRS SCTION

TYPICAL 60 m (200ft) HEADPUMPING PLANT

FIGURE IS.-Typical 60-m (200-ft) head pumping plant. lO6-D-397.

Veloclty=06m3/s

FIGURES


Q"-'

: il'i,1I

III

0

IIII

PLAN

Max WS-~~

,

~1

"~,~

'~

~~,

''''"

EXPLANATION "~-

CJ Concrete - first stageM>/~ Concrete - second stage

0 TYPICAL 17 m (55ft) HEADPUMPING PLANT

TRANSVERSE SECTION

FIGURE 20.-Typical17-m (55-ft) head dry-pit pumping plant. lO6-D-399.

FIGURES 33

-- -- n--- --- -~-----I I

1111",J'1 J~__~dllt_r1

J--{~]-=-""~':-III~ I --

-,

--

LJ1111

l.J 0

"I

-,1L1L---~~

PLAN

Normol W.S.~

Mln w.S ~~

TRANSVERSE SECTION

PROSPECTIVE 3m (10 ft) HEADPUMPING PLANT

FIGURE 21.-Prospectiue 3-m (lOft) head pumping plant. 106-D-400.

BASIC EQUATIONS

Using SI units Using U.S. customary units

Ku = 0.82 + 6.4 (n,) - 3.3 (n,)2 lo-" K , = 0.82 + 1.2 (n,) - 1.2 (nJ2

NPSH = H a + H, - H, - H, 9.8 Q h Power input, kilowatts = --- 'I

Power input, horsepower = - Qh , Q fti3/s 8.82v

[ l kW = 101.971 (m.kg)/s] [ l hp = 550 (ft* lb)/s]

Synchronous speed = 120 (frequency) number of poles

Velocity in spiral case, V = K3 v = K ~ ~

NPSH a = - H

Min. recommended a = 1.2 (n,)'.":' lo-" Min. recommended a = 6.3 (n,)l..j:j 10-6

CONVERSlON FACTORS

To convert from To Multiply by

foot (International) (ft) meter (m) cubic foot per second (ft3/s) cubic meter per second (m3/s) gallons per minute (gal/min) cubic meter per second (m3/s) gallons per minute (gal/min) cubic feet per second (ft"/s) horsepower, electric (hp) watt (w) horsepower, metric (hp) watt (w) pound (Ib) kilogram (kg)

To convert from customary specific speed

To the index Multiply by

*Indicates exact conversion. E-05 represents l r5 .

bureau of reclamation-large pumping unts-em40

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