pump selection and troubleshooting field guide-american water works association (awwa) (2009)

8/10/2019 Pump Selection and Troubleshooting Field Guide-American Water Works Association (AWWA) (2009)

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Pump Selection andTroubleshooting Field Guide

Richard I everly PE

A m erican Water WorksAssociation

Copyright (C) 2009 American Water Works Association All Rights Reserved


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Pump Selection a nd Troubleshooting Field Guide:Copyright 009 American Water Works Association

AWWA Publications Manager: Gay Porter De NileonSenior Technical Editor: Melissa Valen tineProduction Editor: Cheryl ArmstrongProduced by: Hop-To-It Design Works

All rights reserved. No part of this publication may be reproduced or transmitted inany form or by any means, electronic or mechanical, including photocopy, recording, orany information r retrieval system, except in t he form of brief excerpts or quotationsfor review purposes, without th e writt en permission of the publi sher.

isclaimer

The autho rs, contributors, editors, a nd publisher do not assume responsibility for thevalidity of the content or an y consequences of their use. I n no event will AWWA beliable for direct, indirect, special, incidental, or consequential damages arisi ng ut

of the use of information presented in this book. In particular, AWWA will not beresponsible for any costs, including, but not limited to those incurred as a resultof lost revenue. In no event sha ll AWWA s liability exceed th e amount paid for thepurchase of this book.

Library of Congress Cataloging-in-Publication D at a

Beverly, Richard P.Pum p selection and troubleshooting field guide / b y Richard P. Beverly.

Includes bibliographical references a nd index.ISBN 978-1-58321-727-4 (alk. paper)

1 Pumping machinery. I. American Water Works Association. 11 Title.TJ900.B485 2009628.1 44-dc22

p. cm.

2009016035

Am erican Water WorksAssociation

6666 We st Quincy AvenueDenver CO 80235 30983 0 3 . 7 9 4 . 7 7 11



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List of iguresFigure 1 1Figure 1-2Figure 1-3Figure 2-1Figure 2-2Figure 2-3aFigure 2-3bFigure 2-3cFigure 2-4Figure 2-5Figure 2-6Figure 2-7Figure 2-8Figure 2-9Figure 2-10Figure 2-1 1Figure 2 12Figure 2-13Figure 2-14

Figure 2-15Figure 2-16Figure 2-17Figure 2-18Figure 2-19Figure 2-20Figure 3 -1Figure 3-2Figure 3-3Figure 3-4

Figure 3- 5Figure 3-6Figure 3-7Figure 3 8Figure 4- 1Figure 4-2Figure 4-3Figure 4- 4Figure 5 1Figure 5-2Figure 5-3Figure 5-4Figure 5-5Figure 5-6

Typical centrifugal pump systems 5Pumping height miscellaneous pumps 6Typical centrifugal pump family of curves 9Net positive suction head NPSH) 14NPSH requirements 15 Typical close coupled centrifugal pump 17Vertical centrifugal pump 18Frame-mounted centrifugal pump 18Typical centrifugal pump family of curves 19 Impeller curves 20 Hydraulic efficiency curves 2 1 Brake horsepower curves 22 Net positive suction head curve 23 Proposed design efficiency 24Pump selection, operation, and variable-frequency drives 26 Centrifugal pump curve shape 27 Typical vertical line shaft turbine pump 29Unstable operation 3 1 Typical vertical turbine pump curve 32

Typical submersible pump cross section 3 3 Deep well submersible pump 3 4 Typical submersible pump curve 35 Typical submersible pump installation 36 Duplex unit with valve box 36Pressure gauge installation 40 Effects of throttling 43 Effects of throttling on motor size 45 Variable-frequency drive 46Variable-speed pump calculations 47

1,750-rpm pump curve 491,150-rpm pump curve 49 Comparison o 1,750- versus 1,150-rpm speed 5 1 Large potable water system operation 52 Typical close coupled centrifugal pump 58Frame-mounted centrifugal pump 62 Vertical centrifugal pump 6 4 Typical vertical line shaft turbine pump 65 New type o chemical fee pump 72 Diaphragm pumps 72 Diaphragm pump assembly 73Diaphragm pump head assembly 74Diaphragm pump suction cycle 75 Diaphragm pump discharge cycle 76

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Figure 5-7Figure 5- 8Figure 5-9

Figure 5-10Figure 5-11Figure 5-12Figure 5-13Figure 5-14Figure 5-15Figure 5-16Figure 5-17Figure 5-18Figure 5 19Figure 5-20Figure 5-21Figure 5-22Figure 5 23Figure 5-24Figure 5 25Figure 5-26Figure 5-27

Diaphragm pump viscous solutions 77 Polymer dilution/mixing system 77Peristaltic pump system 78

Progressive cavity pumps 79Polymer day tanks/mix tanks 80Chemical system assemblies 82 Automatic flushing system 84Manual flushing 8 4 Injector assembly 85Small pump and mix tank assemblies 90Pump installation 9 1 Dry feeders 92Fluoride saturator 95 Desired mixing 9 6 Direct chemical injection 97Direct injection 97 Injection quill 9 8 Mechanical flash mixer 9 9 Static mixer 99 Flow over weir chemical injectionDiaphragm pump spare parts K17 105

100

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Abbreviations De n t ons

Cavitation. Vaporization of water under low pressure conditionsusually on th e suction side of a pump), followed by implosions of the

ai r bubbles when pressurized on the discharge side.

Discharge pressure (H). The discharge pressure of a pumpusually expressed in feet of water.

Flow (Q). The discharge pressure of a pump usually expressed ingallons per minute gpm).

HorsepowerWater horsepower frictionless). The power required to lift

a weight of water t o a specific height, not including friction. It isusually calculated in terms of foot-pounds per minute, o r gallons perminute.

Brake. The actual power delivered to the water, taking intoaccount pump efficiency. Sometimes referred to a s th br king p o w rrequired t o stop the motor shaft.

Actual Motor. Brake horsepower divided by motor efficiency, andthen rounded up to the nearest commercially available motor size.

NPSH. Net positive suction head. The total suction pressure on a

pump inlet, including atmospheric.

NPSHR. The net positive suction head for a particular pump t ooperate properly a t specified flow ra te and discharge pressure.

O M. Operations and maintenance.

Trim pump. pump used t o vary the flow by a small amount t omatch changing system demand.

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Table of Contents

List of Figures vAbbreviations/Definitions vii

Acknowledgments ix

About t he Author xi

Introduction 1

Chapter 1 Pump Horsepower 3

Introduction 3Performance 3Static Head 4Friction Loss 4Horsepower Definitions and Calculations 7Example Problems 10Summary 12

Introduction 13Centrifugal Pump Types 16Centrifugal Pump Selection 22 Vertical Line Shaft Turbine Pumps 27 Submersible Pumps 33Estimating Performance 38

Chapter 3 Flow Variations 41 Introduction 41Throttling 42

Variable Speed Drive Systems 4 6 Pump Capacity Options 5 3 Summary 55

Chapter 4 Pump Troubleshooting 57Introduction 57Close Coupled Centrifugal Pumps 57 Frame Mounted Centrifugal Pumps 61 Vertical In Line Centrifugal Pumps 63Vertical Line Shaft Turbine Pumps 63Wet Well Design 66 Submersible Pumps 66Maintenance 68Recommendations 68

Chapter 2 Pump Types 13

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Chapter 5 Chemical Pumps 7 1 Introduction 71 PumpTypes 71 Accessories 79 Pump Rate Calculations 86Installation 89Dry Feeders 91Process Mixing 94Chemical Monitoring 100Process Problems and Solutions 1 1

Comments 103Recommended Maintenance 104Summary 104

On Site Records 107 Accessories 108Written Procedure 108

Chapter 6 Operation and Maintenance Manual 107

Appendix: Chemical Resistance Chart 1 9

Index 117

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ntroduction

water system consists of many components, including a powersupply, electrical wiring, switchgear, motor starters, the pumpsthemselves, a piping network o r distribution system to deliver thewater, storage reservoirs, and system controls. For these reasons,pumps should be evaluat ed according to th e needs of t he sys tem. Allt he components mus t be sized a nd selected properly for th e systemto operate correctly.

The factors used to size a pump include flow Q-gpm), pressure

(H head i n feet), an d motor horsepower HP). These factors are ofmost i nteres t to operation an d maintenance (O M) personnel. Oncea pump has been designed a nd installed, th e flow and pre ssure ar ethe factors most easily verified by an operator. Other factors, suchas pum p type , impeller size, bearing selection, etc., ar e normally t heprovince of th e designer or manufacture r.

Pumps are usually the most expensive par t of a water system tooperat e. Even so, they a re easy to neglect when the y a re operatingproperly or seem to be. Pu mps can also be very expensive to repa ir orreplace after a failure. Such repairs can be tim e consuming a nd t akestaff away from other im porta nt duties. The author is aware of onefacility where th e wat er storage in t he system was nearly depletedwhile an essential pump was being replaced. No fire flow capacitywas available during th at time as well.

It is th e int en t of th is handbook to provide quick a nd easy methodsto determine whether a pump is operating properly. For example,by using a pumps f mily o u r v e s the complete s et of curves for aparticular pump), it is possible to evaluate a pumps performanceusing a pr essure gauge an d flowmeter, and by touch. Information inthis handbook describes pump operation a nd how th e factors flow,discharge pressing, and horsepower) can be easily used to evaluateperformance. O M personnel do not necessarily need to know how todesign a pump, but it would be helpful to be able to re ad th e familyof curves an d under stand thei r meaning.

The handbook is organized into six chapters , as follows:

Chapter Pum p Horsepower

Procedures for calculating horsepower required for a pump are



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Pump Selection and Troubleshooting Field Guide

described. By using the family of curves, a determination can bemade wh ethe r a pump is operating in th e proper range.

Chapter 2 Pump Types

Reading a nd unders tand ing pump curves is discussed, and severalcommon pump types a re described, along with operational guidelinesan d methods of evaluating pump performance by touch and by usinga pres sure gauge and Aowmeter.

Chapter Variable Flow

The effects of varying th e flow in a pump by thrott ling or changingspeed a re discussed, along with th e benefits of each.

Chapter Pum p Troubleshooting

Methods of identifying typical pump problems are discussed,together with common solutions.

Chapter 5 Chemical Pum ps

Feed systems for common types of chemicals are discussed, alongwith sizing criteria and O M recommendations. Typical problemsand solutions a re also presented.

Chapter 6 Operation a nd Maintenance Manu al

Reference information is recommended for on-site records.



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CHAPT

Pump Horsepower

IntroductionOnce a pump is operating correctly, it should continue t o do so i f it

is maintained properly and the system conditions remain the same.However, the available horsepower may limit the opera ting range ofa pump. For example, changes in the demand of the system may re-quire a n increase in horsepower, or even a new pump.

Calculations should be kept on site to allow a quick review of thesizing of a pump and motor. If there are changes, or if a pump isnot operating properly, the sizing crite ria can quickly be reviewed forcompliance. Although O M personnel do not normally have to makethose calculations, they have been included in operator certificationtests. The most important procedure is the ability to use horsepowerinformation to read a pump curve. This chapter and Chapter 2 dem-onstrate how this is performed. Information should be available toO M staff for th is purpose.

PerformanceThe performance factors of a pump need to be calculated during

design and used for evaluation. The factors include the design flow

(Q), usually expressed in gallons per minute, and pressure (or head)in pounds per square inch or feet of water. Q is determined by theprocess requirement, and is different for each installation. Thepressure or head required for the pump discharge includes the sumof the static head and friction loss caused by piping, valves, fittings,flowmeters, etc. A general discussion of discharge pressure (pressureboost) is included t o determine the horsepower for a pump.

It is important to understand that the discharge pressure of apump alone is usually not a n indication of the power/pressure addedby the pump. The pressure boost (pressured added) has t o take intoaccount the suction side pressure. For example, if the discharge pres-sure of a pump is 100 psi, and the suction side pressure is 20 psi(flooded suction), the pressure boost by the pump is 80 psi (100 - 20).

3



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Pump Selection and Troubleshooting Field Guide

With a suction lift of 5 psi and a discharge pressure of 100 psi, the

pressure boost would be 105 psi (100 + 5 .The design (normal) discharge pressure, the normal suction side

pressure, and the normal pressure boost should be identified for O Mpersonnel in the recommended on-site O M manual. The followingdiscussion is intended to help O M personnel understand these ter msand how they are calculated.

Static Head

The pumping height, or static head, is illustrated in Figure 1-1.The pumping height is the vertical distance from the original wa-ter surface to the finished water surface, whether the pump has asuction lift or a positive suction head (flooded suction). For a con-servative approach, it is important that the maximum level of thereceiving reservoir be used for design, along with the lowest levelof the water source. Figure 1-2 illustr ates the variation th at can oc-cu r in t he original water source. For example, in a deep well pump,it is common for a significant drawdown to occur when the pump is

st ar te d up, an d th e final pumping level anticipated should be used forcalculation purposes.

Friction LossOnce a system has been designed, the friction loss can be calcu-

lated from the length of pipe and the number of fittings, valves, flow-meters and other devices in the line. It should be noted that pumpcontrol valves or other pieces of equipment can generate a significanthead loss at the design flow. The individual manufacturers litera-tu re is used to obtain t hat information. The allowance for friction lossshould be included in th e designers calculations an d included in th erecommended on-site O M manual. The tot al pump pressure (H) isth e su m of the maximum stat ic head (or vertical lif t from water levelto water level), the to tal friction loss, and the velocity head.

Pump Pressure - Pump Boost H-ft) =(Vertical lift + friction loss + dynamic pressure)

A n adaption of Bernoullis Equation)

The static discharge head (H) that a pump must overcome isth e vertical elevation ga in (lift) from th e pump to the discharge waterlevel. The static head (lift) can be measured by a pressure gauge ad-jacent to th e pump when it is not i n operation.



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Pump Horsepower 5

Water Surface

ALTERNATIVE A

WaterSurface WaterSurface

LTERNATIVE B a

WaterSurface -

d

_ _ c

TOTAL HEAD = STATIC HEAD + FRICTION LOSSFigure 1 1 Typical centrifugal pump sys t ems

The vertical lift is the difference from the intake water level toth e discharge water level, a s shown on Figu re 1-1. With a flooded suc-tion, the vertical lift (water level to water level) can be measured a sth e difference in th e static pressure measured by pressure gauges onboth sides of a pump when it i s not i n operation (assuming the re is acheck valve on the pump discharge). In a suction lift condition, the liftfrom th e water source to the pump must be added to the static head to

calculate the vertical lift.Friction loss is t he force, or pressure loss, required to push waterto the discharge point or water level. The total amount of friction isth e sum of the loss through th e piping, valves, elbows, tees, a nd otherfittings between the pump an d the discharge point.

Dynamic pressure is th e force required to move water through



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6 Pump Selection and Troubleshooting Field Guide

Water Surface

SUBMERSIBLEPUMP VERTICAL

TURBINE ONRIVER INTAKE

StaticWater Level

-DrawdownDuringPumping

SubmersiblePump

DEEP WELLPUMP

Figure 1-2 Pump ing height miscellaneous pumps

a pipe at a specific velocity. The value for dynamic loss can be foundin a hydraulic handbook for piping. If the pipe size is adequate and

th e velocity is low, the dynamic pressure is usually not a large value.Total discharge pressure the tot al pressure t ha t a pump oper-

ate s aga inst, can be measured by a pressure gauge close to th e pumpwhile it is operating The total discharge pressure, a s measured, in-cludes the static lift, friction loss, an d dynamic pressure. It should benoted t ha t t hi s pressure only perta ins to th e flow (Q) a t the time ofthe measurement.

Note: It is not intended for th e operator to make these calculations.Each individual item should be compiled by t he designer and included

in t he recommended pump manual to be available a t the site.The pressure boost is the power input by the pump. It is the

to tal discharge pressure minus any positive suction pressure, or plusany negative suction pressure. As always, th e pressure boost is fromwater level to water level. The pressure boost is the value th at mustbe used when referring t o the pump operational curve.



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Pump Horsepower 7

Pu mp an d motor efficiencies a re factors that also affect t he pumphorsepower an d will be discussed fur ther on in this handbook.

Horsepower Definitions and Calcu lationsHorsepower. By definition, horsepower is a measure of the rat e

at which work is done.

One horsepower = 33,000 ft-lb/min= 550 ft-lb/sec

= 746 wa tt s (or kw)

Water horsepower. Water horsepower is the work required tolift a weight of water to a defined height per unit of time (usually asecond or a minute), For the purpose of this field guide, friction isneglected.

Work = weight x stat ic heighttime

For example: The work requi red t o lift 10 lb of water one foot inone minute is as follows:

Work = 10 lb x 1 f t = 10 ft-lb/min1 min

Water horsepower equals actual work per minute divided by

33,000 ft-lb per minute.

Water H P = worklmin3 3.00 0 ft-lb/min

For pumps:

Water HP = Q (gal/min) X 8.34 lb/gal x H (ft)

33,OO ft/lbmin= Q x 8.34 x H (ft)

33,000 ft-lbmin



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= Q x 8.34 x H33,000

= QH3,957

Rounded off:

Water horsepower = Q x H3 , 9 6 0

It should be noted that this equation is th e one often used in op-erator certification tests.

Brake horsepower. Brake horsepower equals wate r horsepowerdivided by pump efficiency.

Pumps have inefficiencies as a result of water slippage, axialth ru st i n the volute, a nd routing the discharge wate r out. The act ualhorsepower required by the pump must take that inefficiency intoaccount.

Bra ke horsepower = water horsepowerpump efficiency

Pu mp efficiency an d brake horsepower ar e shown on th e manufac-turers pump curves. Figur e 1- 3 llustrates a typical family of curvesfor a centrifugal pump with that information. Pump efficiency is al-

ready ta ken into account on th e bra ke horsepower curves shown onthis graph.

Actual horsepower. Actua l horsepower equa ls brake horsepowerdivided by motor efficiency. New motors should have a n electrical ef-ficiency of 92 percent or greater . Older ones may be substan tial ly lessand should be considered for replacement. The electrical efficiencyshould be shown on the motor nameplate.

Motor Horsepower Actual)

Motors do not operate at 100 percent efficiency. Th e ac tual motorhorsepower required has to t ake motor efficiency in to account (some-times referred to as electrical efficiency).



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Pump Horsepower

Ln

W

1,750 RPM)

Figure 1-3 Typical cen t r i fuga l pu mp fami ly of curves

Motor horsepower (actual) = brake horsepowermotor efficiency

water horsepower

pump efficiency x motor efficiency

-

Metric horsepower. Metric horsepower is not normally used inthe United States . It is given for reference only.

P = 1 min X H3.9 0 5.74

Where:P = water horsepower

l/min = liters per minuteH = head, in m eters3,905.74 = 3,960 x 0.9863

* 1 cheval = 0.9863 or French horsepower



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Example ProblemsIn the following section, a number of example problems a re given

for calculating horsepower and power usage. These problems a re sim-ilar to those that might occur on an operators certification test anddo not include friction loss.

Example A:

A pump raises a flow of 60 gpm of water from level A, with anelevation of 100 feet, to level B, with an elevation of 210 feet. What

is the horsepower being used?

Answer:

H P = QxH,960

Q = 60gpm

H = (210 f t - 100 ft)

= 60 X (210-100)3,960

P = 1.67

Is the answer in water horsepower, brake horsepower, or actualhorsepower?

Answer:

Because no efficiencies a re given, the answer is assumed to be interms of water horsepower.

Example B:

If the pump and motor have a combined efficiency of 60 percent,how much actua l horsepower would be used in th e previous question?

Answer:H P = water horseDower

motor efficiency x pump efficiency



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Pump Horsepower 11

H P = 1.67 = 2.78 H P.60

Wha t motor size would be selected?

Answer:The requ ired motor size would be rounded up to 3 HP. Depend-

ing on the actual pump operating range, the motor used might be th enext size up. (To be discussed i n the next section.)

Example C:Calculate th e cost of power of the previous example.

Pumping Energy Costs:Watts = H P x 746kw = watts / 1,000hr = hours of operationkWh = kilowatts x h r

= 7 4 6 H P x h r1,000

TPC = kWh x unit cost of power

Calculate kilowatts.2.78 H P x 746 wat ts/HP = 2.07 kw

1.000

Assume the cost of power to be $0.025 kWhOperational Power Cost = 2.07 kw x $0.025 kWh

= $0.052/hr

Example D:

If electricity costs 2.5 cents per kWh, per above, what is th e month-ly cost if th e pump in th e previous two questions r uns continuously?

Answer:Calculate kilowatts.Power usedCost of Power

= 2.07 kw (from previous)= $0.052/hr (from previous) X 30 d X 24 hr/d= $37.44/mo



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Example E:

Calculate the pump efficiency using t he power draw.The power delivered by a motor i s usual ly expressed i n horsepow-

er. Because one 1) horsepower equals 746 wa tt s (or about Yi of a kilo-watt), the formula for efficiency becomes:

efficiency = 746 x P x 100watts input

For Example: If a motor uses 1,000 watts of power and deliversone 1) horsepower to a water pump, its efficiency is:

Motor efficiency = 746 watts x HP x 1001,000

Motor efficiency = 74.6%Note: Does not include pump efficiency.

SummaryFor on-site evaluation, it should not be necessary for O&M staff

to calculate th e horsepower of a n existing facility. If problems occurduring sta rt -u p, he designer should be consulted. However, th e horse-power calculations should be available i n a n on-site manu al for reviewif needed. The information contained in this section, plus th at i n thefollowing sections, should help th e O&M staff make their own evalu-ation. The evaluation by

O Mstaff will be especially important over

time, as th e pump ages. Also, any older motors should be included i nth e capital improvements plan and replaced a s soon a s practicable.

Another potential problem that could occur over time is the pos-sibility of changed conditions in a system. Typical examples a re in-creased demand and res tru ctur ing of a pressure zone. Should prob-lems of any sort occur, design verification may be required. A sum-mary of the design conditions should be available a t each facility forth at purpose.

Additional pump information and evaluation criter ia a re present-ed in the following sections of this handbook.



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CHAPT

Pump Types

IntroductionThere are many types of pumps used in water and wastewater

utility systems, treatmen t plants, and in commercialhndustrial sys-tems. Applications range from sewage pumps to ultra-clean indus-trial systems.

The key word is systems. A water system is a combination ofpumps, valves, fittings, and piping designed to deliver water for manyuses. Although there are portable pumps for specific purposes, mostpumps a re installed in permanent facilities. The function of pumps,a s used i n th is discussion, is to increase flow and pressure to satisfyth e needs of th e system.

It is not the purpose of this handbook t o discuss the operation ofall th e many types of pumps in use. Rather, the in tent is to discuss thedesign and O M requirements of several of the most common pumptypes. The principles involved in operation, design, and evaluation ofthese pumps can then be applied to others. Although O M personnelmay not be required to design a pump, th e information contained inth is handbook is intended to help personnel unders tan d how a pumpworks and help in troubleshooting.

The goal for thi s chapter i s to discuss sizing and to ill us trate howto read an d use the complete family of curves for a pump. Using thisinformation, O M staff should have th e information necessary to de-termine the flow of a pump with a pressure gauge if it is operatingproperly an d to also determine if it is not.

System RequirementsWhen sizing or selecting a pump, it is first necessary to determine

the system requirements. As a minimum, a simple flow diagram

should be developed, a s shown in Figure 1-1. n th is figure, Alterna-tive A illustrates a pump with a suction lift, whereas Alternative Bshows a flooded suction. I n either case, the required pump pressureor head is calculated as the total static head from water surface to

13



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water surface, plus friction of the piping system, including valves,

fittings, an d flowmeters. Head is the pressure measured in feet orpounds per squ are inch psi). The flow in gallons per minute gpm)is determined by th e needs of the system. Also the total static headmay vary, depending on the level in the receiving reservoir, and th elevel in th e source water. The pump should be designed to operate a tthe maximum.

Net Positive Suction Head NPSH)Even in a suction lift system where the source water is lower th an

th e pump, the pressure t o th e pump must be positive. However, suc-tion pressure also includes atmospheric pressure, which must be con-sidered in t he design process. Therefore, atmospheric pressure a t th esite and potential variations must be take n into account. NPSH is de-scribed in F igu re 2-1. A se t of typical calculations for NPSH is shownin Figure 2-2. Although suction lift is allowed, it is recommended that

NPSH)

NPSHA) THE TOTAL SUCTION PRESSURE AVAILABLE

NPSHR)- THE MINIMUM NPSH REQUIRED FOR THE

- THE ATMOSPHERIC PRESSURE AVAILABLEAT THE SUCTION WATER SURFACE.

AT THE PUMP.

PUMP TO FUNCTION

WATER LEVEL

SCREENED INTAKEv RAVEL COVERNOTES

1. THE NET POSITIVE SUCTION HEAD REQUIRED (NPSHR) IS AFUNCTION OF THE PUMP AND ITS OPERATING POINT. NPSHRMUST BE LESS THAN NPSHA FOR THE PUMP TO OPERATE. IF

NPSHR BECOMES GREATER, THE WW WILL CAVITATE ANDSTOP PUMPING.

2. NOT NORMALLY A PROBLEM WITH POSITIVE SUCTION HEADAPPLICATIONS.

Figure 2-1 Net positive suction head NPSH)



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PumpTypes 15

Net Positive Suction Head NPSH)

The NPSH is the atmospheric pressure available at the water surface. For thepurposes of this discussion, NPSHA is the available pressure at the pump suction.For a pump to function properly, there is a minimum NPSHR required) whichmust be available. NPSHA is calculated s shown below.

NPSHA = A Vp + Vi + Hr)

Wltera:

A = Atmospheric pressure @V, = Vapor pressure of water ll

f i = Maxinium velocit), head 0 n pipc .. ze shm be as low aspossible)Hi = Friction losses n intake pipeline (should be as low as possible)

A typical NPSH calculation is shown below for the suction l i f t

Assume: 1 OOOf televation7 Y F = Ambient temperatureA = 32 8ft

V, = Assume negligibleHI=

v = 0 84fr

Assume 5 f t o fp ip ing and intake headloss and l o t of suction lift in the

intuke screen and support gravel.

NPSH = 32.8 - @ . 8 4 4 4 Sf) Hp (lo )= 16.963

Figure 2 2 NPSH requirements

it be kept to a minimum. Even then, any fluctuation in suction sidewater level mus t be taken into account. If the suction side water levelis reduced significantly, the pump flow rate may also need to be re-duced. Information will be presented to demonstrate how to deter-mine t he operational lim its of a pump, compared to its capability.

Suction side pressure. Pump systems are designed in manyways. Some pumps operate off of line pressure to boost the wate r to adifferent level. Other s operate in wet wells or river intakes. A mini-mum suction side pressure of 20 psi is normally required for pumpsoperating off of line pressure . With pumps operating in river intakes

or wet wellslsumps, the designer also has to take into account thephysical sizing of the wet well or sump, which will be discussed fu r-ther in this chapter.

Pump efficiency factors. In addition to satisfying system de-mands, it is also important to select a pump that is operating at ornear its peak efficiency range if possible. The designer should be



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aware t ha t impeller design and efficiency have changed in th e las t 20

years, so that newer designs are 5 to 10 percent more efficient. If thepump is that old, consideration may be given to replacing it.

Motor efficiency. There ha s also been a 5 to 10 percent increasein motor efficiency over the las t 20 years. Motors with a n efficiency of94 percent or greater are available for centrifugal pumps. Submers-ible motors may have a n efficiency in the range of 85 percent.

Replacement evaluation. An economic evaluation may be nec-essary to determine if motor should be replaced or if the entirepump and motor assembly should be replaced. Initial capital costs,maintenance costs, and electrical power costs should be consideredin a life cycle analysis to determine t he most efficient and economicalmethod of operating a system. Several economic models are availablefor a designers use.

All of the items previously mentioned must be considered in thedesign and selection of a pump. The required flow an d pressure bothsuction and discharge) must be calculated to select a specific pump.Each of these subjects will be discussed in more detail in t hi s hand-

book. Again, much of th is information i s presented as background tohelp O M personnel better und ers tand how to use pump curves.

Centri fugal Pump TypesClose coupled. Figure 2-3a illustrates a typical close coupled

centrifugal pump. Close coupled means that the motor bolts directlyto the pump volute, with no intermediate coupling, and the motorsh af t connects to t he impeller. A cutaway of th e volute of a centrifugalpump is shown in t he lower pa rt of F igure 2-3a. The inlet flow comesin to the eye center) of th e impeller, and a s the impeller spins, it forcesth e water to th e outside an d increases both flow an d pressure.

Vertical. Centrifugal pumps can be close coupled, as shown inFigure 2-3a, or i n a vertical configuration, as shown in Fi gure 2-3b.In addition, larger pumps are often frame mounted, as shown onFigure 2-3c.

Frame mounted. separate motor and pump configurationwill allow either t o be removed for mai nten ance without d istu rbin gthe other. However, the separation requires a coupling to connectthe motor sh aft to the pump shaf t. Misalignment of the two can re-su lt i n motor or pump fai lu re , or both. worn coupling could causeth e same problems. Therefore, it is recommended th at t he pump an dmotor be properly aligned by a certified pump/motor technician.



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PumpTypes 17

FLANGED PIPECONNECTIONS

COUNTERINGIMPELLER

OUNTING BASE

CROSSSECTION OFVOLUTE A N DIMPELLER

Figure 2-3a Typical c lose coupled centr i fu gal pu m p

Family of curves. The operation of a specific centrifugal pumpis illustrated by its family of curves, which is shown in Figure 1-3

repeated in Chapter 4 as Figure 4-1 . The typical family of curvesincludes impeller diameter, pump efficiency, brake horsepower, andnet positive suction head required (NPSHR). For simplicity, thi s fam-ily of curves will be separated into its individual components anddiscussed further. It should be noted that each different pump willhave its own individual family of curves. If one pump does not satisfy

th e requirements, another should be used.

1. Impeller curves. The impeller curves from Figure 2-4 ar e high-lighted in Figure 2-5. The other sets of curves are included butare faded in the background so that the relationship betweenthem can still be observed.



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OR MECHANICAL

Figure 2-3b Vertical centrifugal pump

Wa

PACKING ORMECHANICAL SEALS

SHAFT COUPLING

PUMP SUCTION

FRAME MOUNTING BOLTS

Figure 2-3c Frame-mounted cent ri fugal pump



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PumpTypes 19

Referring again to Figure 2-3a, the impeller is illustrated in thecutaway drawing of the volute. In this case, the maximum impellerdiameter is shown, which has the minimum clearance to the body ofth e volute. The maximum impeller diameter for this pump is shown as12 in. i n Figure 2-4, and the minimum is 9.0 in. Th e impeller can betrimmed anywhere between th e two, to more closely match t he desiredoperating conditions. Smaller-diameter mpellers result in lower flowand pressure. Once an impeller diame ter is selected, the pump willoperate on that impeller curve without exception. Chang ing conditionswill simply move the operating point left or righ t of its design point.If the system requires flow or pressure a t a point below the impellercurve, a separa te flow or pres sure control valve, a ft er th e pump, mustbe used. Operat ing above the curve would require a larger impelleror a completely new pump.

Selecting a pump will be discussed in more detai l later, but it is im-portan t to note that th e small er-diam eter mpellers create lower pumpefficiencies because of increased slippage in the volute. If th e efficiencybecomes too low, it may be desirable to consider a different pump.

1,750 RPM)

FLOW GPM)

Figure 2-4 Typical centrifugal p um p family of curves



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1,750 RPM)

-FLOW GPM) -

Figure 2-5 Impeller curves

2. Hydraulic efficiency. The hydraulic efficiency curves are highlightedin Figure 2-6. When selecting a pump, it is most desirable, butnot always possible, for the operating point to be within the areaof highest efficiency. It should be noted that the area of highestefficiency also has th e largest impeller diameter an d th e highesthorsepower requirements of a particular pump. Also, the designoperating point and any secondary points of operation should al l fallwithin th e efficiency lines shown on this graph. Operating outs idethe lowest efficiency curves may result in unstable operation, aswell as lower efficiency. If that is th e case, a different pump, thatcan operate closer to its highest efficiency point may be required.

3. Brake horsepower. Water horsepower is th at which is required toactually lift the water. Brake horsepower is th e power requi redto rotate the impeller at a given flow rate and pressure, takingin to account hydraulic inefficiencies of t he pump. The te rm itselfrefers to the power required to brake or stop the impeller. It isthe water horsepower divided by th e pump efficiency. The pump



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PurnpTypes 2 1

1,750 RPM)

FLOW GPM)

Figure 2-6 Hydraulic efficien cy curves

efficiency has already been considered in F igure 2 - 4 to show thepower requirements.

As shown on Figure 2-4 an d highlighted on Fig ure 2-7, th e brake

horsepower curves a re not parallel w ith t he various impeller curves.In fact, brake horsepower curves cross over the impeller curves. Asshown in these figures, the horsepower requirements increase asthe flow increases and a s the operating point travels to t he ri ght onthe impeller curve. The motor horsepower should be adequate forthe higher flow rates, if a pump has more than one operating pointwhich is common). In fact, it is recommended th at th e motor be large

enough for any point on the impeller curve nonoverloading across therange).

4 Net positive suctionheadrequired NPSHR). Thecurve for NPSHRis highlighted on Figure 2-8. Although the term suction head isused, a pump always needs positive suction pressure includingatmospheric), as discussed previously. It is important to note th atthe N P S H R increases significantly as the flow increases.



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22 P u m pSelection and TroubleshootingField Guide

1,750 RPM)

Figure 2-7 Brake horsepower curves

It is not recommended for a pump to operate far to th e right on itsimpeller curve. The horsepower required can increase a nd the NPSHRcan more th an double, a s shown on Figure 2-8. Therefore, it is recom-mended that flow controls be provided for any pump operating with asuction lift to prevent it from exceeding its operating limits.

To actually operate to th e right of the impeller curve, a larger mo-tor may be required, along with more positive suction pressure. Tothat end, every effort should be made to reduce head loss on the suc-tion side, including th e use of large pipe with no restrictions . Also, thedesign conditions in th e recommended on-site O M man ual shouldbe reviewed to e nsu re tha t th e pump is operating within its limits.

Centrifugal Pump SelectionWhen selecting a pump, the pr imary factors used ar e the required

flow ra te , the discharge pressure, an d th e type of service. The flow isdetermined from the system requirements, and the pressure is thecombination of static head and pressure loss head loss) caused by



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PumpTypes 23

1,750 RPM)

Figure 2-8 Net posi t ive suct ion h ead curve

piping, valves, fit tings, etc. The type of service includes the location ofthe pump with regard to the source wate r flooded suction, mountedover a sump, etc.) an d the type of source wate r clean, di rty raw wa-ter, or high-solids wastewater). Once these factors are known, thepump can be selected using the family of curves for various pumps,such as has been shown in t he previous figures.

Efficiency

The most desirable operating point would be within that part ofthe curve containing the highest efficiency. If the pump can operatewithin or ne ar that high-efficiency zone, that particular pump wouldbe a good selection. Figure 2-9.)

Motor Size

Brake horsepower. Th e design point will often fall in betweentwo impeller curves, as shown on Fi gure 2-9. For those cases, the im-peller can be trimmed from th e larger size to coincide with the design



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1,750 RPM)

FLOW GPM) t

Figure 2-9 Proposed design efficiency

point. The impeller curve resulting from the changed diameter willbe parallel to an d in between the smaller and larger impeller sizes oneither side of th e design point.

Using the design point and the new impeller size, a new impellercurve can be extended left and right of the design point to determine t herequired motor size. Once an impeller is selected, the pump must oper-ate on th at curve because of the physical constraints of the pump. If thepump is thrott led back, the operating point will travel to the left on theimpeller curve. If the pump is allowed to run faster, the operating pointwill travel to the right of the design point on the impeller curve.

It should be noted that th e horsepower shown on the previous fig-

ures refers to brake horsepower. Refer to Chapter 1. Brake horse-power already tak es into account the efficiency of the pump, a s shownon the g raph, but does not t ak e into account th e electrical motor ef-ficiency. Tr ue motor horsepower may be 6 to 8 percent higher thanshown in these figures, or more, depending on its age. Even th en, th erequired motor horsepower will usually have to be rounded to the



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PumpTypes 25

next higher size to account for a performance factor conservative de-

sign factor) and to reach a value th at is commercially available.Nonoverloading across the range. The configuration of many

pump systems often results in variations in flow from the designpoint. Moving th e pump to th e right increases flow reduces pressure,increases the NPDES, and increases the horsepower required. A sstate d previously, the impeller an d horsepower curves a re not para l-lel Figure 2-7). Moving th e operating point to the rig ht often resul tsi n the impeller curve crossing over a horsepower curve. In such cases,a smaller motor would overheat and likely fail. To avoid that condi-tion, the motor can be sized to be adequate for any reasonable condi-tion on the impeller curve. The term reasonable refers to the areawithin the family of curves. Sizing the motor using this method isreferred to by the author as nonoverloading across the range, and isrecommended method.

Using the nonoverloading method of selecting a motor is only validwhen the operating point is still within its family of curves. Oper-ating a centrifugal pump too far to the right or in an unrestrictedcondition wide open) will likely result in damage to the pump an d/or the motor regardless. It is then too fa r out of th e normal operatingrange to be considered i n design or motor selection. Flow or pressurecontrols, or both, a re recommended to stop th e pump if it begins tooperate too far off th e design point.

Pump Curve ShapeOnce a pump h as been selected, it will operate along its impeller

curve, as discussed previously. Figur e 2-10 represents a fixed impeller

size. The pressure produced is H l) with a flow of GPM,,,. For discus-sion purposes, the pump motor is assumed to operate a t 1,750 rpm.

Having selected a pump based on the design information discussedpreviously does not mean t ha t it is t he proper pump for a specific appli-cation. Pumps are often used to operate away from the design point byeither thro ttl ing back to a lower flow or reducing pressure and allowingthem t o run at a higher flow rate . In tha t case, the shape of the impellercurve may have a significant impact on the suitability of the pump tooperate within the requirements of a particu lar system. For example, in

Figure 2-11, curves are shown tha t a re both shallower and steeper thanth e typical pump curve selected previously. At th is point, it is for the de-signer to determine the effect on the system of the shape of the curve.

Normally, pump selection is relatively easy. However, therehave been many cases where th e author has had to give extensive



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I

I

1,750 RPM ,)

GPM I )

FLOW GP M)

TYPICAL PUMP CURVEN.T.S.

N-?E

THE TYPICAL PUMP CURVE REPRESENTS THESELECTION OF A SPECIFIC IMPELLER

Figure 2-10 P u m p select ion, operat ion, and v ariablefrequ ency drives

consideration t o the shape of the curve. For example, if the pump is

producing a high pressure and h as a steep curve, changing the flowrate would have the impact of changing the pressure dramatically.On the other hand, changing the pressure on a pump with a shallowcurve would have a similar impact on the flow. Raising the pressuresignificantly in a potable water distribution system could causeproblems with water heaters in individual residences with a steepimpeller curve. Therefore, the shape of the curve may dictate the typeof pump being used and is a n important consideration for the designer.I t is recommended that the O M personnel evaluate the system effectson the pump and determine if the pump is a n acceptable selection.

Pressure/Flow Relationship

Once a n impeller size has been selected for a specific pump, thepump must operate on its impeller curve as discussed previously.

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CHAPT

FlowVariations

Introduction

Pump OperationPumps are intended to function according to the system needs. Many

pumps a re configured to simply turn on and off based on level control ina receiving reservoir, as shown in Figure 1-1. On the other hand, therear e many pumping systems that require that the pump discharge ra tebe regulated or modified to meet the changing demands of the system.To satisfy such a demand, the pump discharge ra te can be modulatedusing a rate control valve, or a variable-speed-drive motor can be pro-vided to change the pump speed and therefore the flow rate.

Redundancy

Water systems are often designed and constructed based on awell-defined budget. In those cases, it may be difficult to provide ahigh level of redundancy. If that is the case, it is recommended thatspace be left for at lea st one other pump that could be provided in th efut ure . Whether for satisfying system demand or providing backup,multiple pumps ar e desirable to main tai n system capability.

Sizing Strategies

There are a number of strategies th at can be used to size a pump basedon percent demand, the size of the system, redundancy required, and eco-nomics. A variable demand can also be satisfied by multiple pumps.

1. One pump at 100 percent capacity could be provided, especially i na small system. However, one pump has no redundancy.

2. Two pumps at 100 percent could be provided, which would giveredundancy. One pump could be operated at a time, and the twopumps al te rnat ed for increased life.

41



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42

3 .

4.

5.

P u m pSelection and Troubleshooting Field Guide

Two pumps at 50 percent capacity would provide a level of redun-dancy if one were to fail.

Thre e pumps at 50 percent capacity would provide one completeunit for redundancy.

Three pumps at one-th ird capacity would provide two- thi rdsredundancy if one were to fail. Three pumps also provide anamount of flow varia tion of one -th ird , two-th irds, or 100 percentcapacity.

If thr ee pumps a re provided, t he authors preference is to size oneat 50 percent capacity and the other two at 25 percent capacity. Inthis way, there is at least 50 percent redundancy if the large unitfails, whereas four different operating conditions a r e provided at 25,50, 75, and 100 percent.

MultiplePumps

A larger system may have multiple pumps of different sizes. However,the number and sizing of the pumps is always dependent on the systemneeds and any variable demand tha t ha s t o be met by the pumps. There-fore, when expanding a system or adding pumps, it is recommended thata computer model be generated to predict the best sizing strategy. Withmultiple pumps, one may have a flow control valve to allow it to be throt -tled up or down slightly. Variable-speed drives can be used for the samepurpose. In either case the intent should e to limit the frequency ofpumps turning on and off The operator at one 20-MGD facility told th eauthor tha t tu rn ing on all his pumps at the same time would result ina 10,000 peak demand charge from the electric company. The electri-cal demand charges ar e an important factor in the operation of pumps.If the system allows, there may be off-peak demand periods with lowerelectrical charges, during which pumping can take place. In terms ofoperational costs, sizing the pumps to operate properly and efficiently isone of the more important aspects of providing an economical design. Tohelp designers, the US Department of Energy mainta ins sizing guide-lines for pumping systems, which a re readily accessible.

ThrottlingAs an alternative to multiple pumps, it may be possible to sat-

isfy small changes in demand by thr ott lin g a pump. Throttling is ac-complished by increa sing or decreasing the pump discharge p res sure



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FlowVariations 43

t

8

s

1,750 KYM)k

GPM I )FLOW (GPM)

GPM 2)

TYPICAL FLOW VARIATIONSNotes:1. Throttling reduces flow, increases pressure,

and may reduce horsepower.2. Reduced head and increased low raises NPSH

requirements and can cause cavitation.

Figure 3-1 Effects of thrott l ing

by way of a modulating valve on the pump discharge. The effects ofthr ot tl ing a pump by increasing or decreasing the discharge pressurear e illustrated on Figure 3-1.

T he suc t ion s ide o a p u m p should never be throt t led, because i tincreases the net posit ive suction head required NPSHR) a n d c a ncause the p u m p to lose pr i m e a nd cav ita te .

Closing a valve on the pump discharge can increase pressure anddecrease flow up to the shutoff or no-flow condition. On the otherhand, changing the operating point by opening a discharge valve maybe limited by the normal backpressure of the system.



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44 P u m pSelection n d Troubleshooting Field Guide

I n either case, it is recommended th at th e operating point not bemoved outside the family of curves shown in Figure 2-4. Several is-sues related to thro ttl ing ar e discussed in th e following sections.

Increasing Pressure

Incre asing th e disc harge pressure of a pump moves its operat-ing point to t he left on th e impeller curve , as shown in Fig ure 3-1.The pressure may increase slightly because of changing systemdemands such as a higher reservoir level. However, significant

changes are usua lly accomplished by par tl y closing a valve on th epump discharge.Increasing the head loss by an amount of H, - H, on Figure

3-1 moves the operating point to the left, and the flow is reducedfrom GPM, to GPM,. At the same time, th e NPSHR is also reducedslightly.

Referring to the typical curves in Chapter 2 (Figures 2-4 to 2-9),moving to the left on the impeller curve may also result in lowerpump efficiency and lower horsepower requirements. Some verticalline sh af t turbine pumps may be an exception, along with other types.In Figure 2-13, th e efficiency is reduced toward the left, but th e horse-power requirements r ema in essentially constant for the vertical lineshaf t turbine pump in thi s case.

In sizing a pump for different flow conditions (or pressure), it isimportant that it meet the design flow a t th e minimum system pres-sure, as well a s th e maximum. I n addition, if a pump is to be throt-tled back, a n allowance for pressure modulation, above the minimum,

should be included in th e design calculations.

Effects of Reduced Pressure

Figure 3-2 illustrates the effect of reducing the pump pressure,causing it to operate far ther to the right on its impeller curve. Chang-in g the discharge conditions will move the operating point either t othe left or to the right, but it will always be on the impeller curve. Ifthe discharge pressure is reduced, the pump will operate farther toth e right, and th e impeller curve may cross over a horsepower curve,as shown in Figure 3-2. If the pump is allowed to operate to th e right,as shown i n t hi s figure, then it should be provided with a motor largeenough so th at it is nonoverloading across the range



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Flow Variations 45

t

8

I

i i

HORSEPOWER

GPM 1) GPM 3)

FLOW (GPM) *TYPICALHORSEPOWER LIMITS

Notes:1. Increased low due to reduced head can also exceed

the available horsepower and burn up the motor.2. Choose a motor size that is nonoverloading over the

entire range of the pump.

Figure 3 2 Effects of throttling on motor siz

Increased NPSHRIn addition to increasing horsepower, operating to the right of the de-

sign point will also increase the net positive suction head requirement.Assuming th at the pump operates properly at the design point, it

may not run properly if allowed to r un too far to th e right. For thatreason, the motor could overheat and cavitate. Therefore, the allow-

able operating conditions should be well defined so as not to exceedth e capability of the pump.

Season al VariationsAssuming a raw water int ake, th e pump might be able to operate

to th e right of its design point i n th e winter, when t he river or suction-



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FlowVariations 47

POINT

REDUCED SPEED

VARIABLE-SPEED FORMU LAS

3

GPM2 - RPM2GPM1 R P M l

Notes:1. H 2) annot be lower than the static head of the system.2 Providing the ability to reduce speed in a high static

head system may result in a larger pump and motor.

Figure 3 4 Variable-speed pump calculations

original opera ting point is shown as H, (pressure) an d GPM, (flow).When th e speed is reduced, a new pressure, H,, and flow, GPM,, a r eproduced. The resu lts of changing pump a nd motor speed ar e definedby a set of formulas as shown on the bottom of Figure 3 4. The flowvarie s directly with th e pump speed such tha t a reduction of 10 per-cent in speed also resul ts in a reduction of 10 percent of flow.

The change in pressure from H, to H, varies according to thesquare of the ratio of speed. For example, if t he flow is reduced by 10percent, the pressure is reduced by 19 percent. T he horsepower va rie saccording to t he cube of the change in speed. If the flow is reduced by10 percent, th e horsepower is reduced by 27 percent.

VFD DesignFor design purposes, as sum e that a pump is operating in a typical

system, as shown in Fi gure 1-1 wherein the pump is lifting wa ter to a



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receiving reservoir at a defined static head. I n th at case, there will bea maximum and a minimum water level condition. When designing aVFD i n th is set of circumstances, th e pump should be designed at thereduced speed condition t ha t satisfies the minimum system pressurerequirement. I n other words, regardless of percent speed, the pumpmust still produce enough pressure to lift the water the minimumdistance at the lowest operat ing condition.

If the flow is to be reduced by 10 percent, 90 percent of capacity willbe pumped at the lower static head. On th e other hand, to increasethe flow from 90 percent to 100 percent, the pressure is increased by19 percent, and the horsepower required is 27 percent greater thanat the lower static head. I n other words, changing th e speed h as adramatic impact on the design of a pump an d the system.

Actual Pump Curves

To ill ust ra te the effect of varying th e pump speed, two se ts of ac-tu al curves a re provided for th e same pump a t different motor speeds.Figure 3-5 illu stra tes a pump operating at 1,750 rpm. The operat-

ing point is shown a t Q, = 1,700 gpm, th e pressure H, is 218 f t , andth e horsepower required , HP,, is 125 horsepower. The second curveis shown in Figure 3-6, wherein the motor speed is 1,150 rpm. Theflow Q2 is 1,125 gpm. T he pressure H, is 95 f t , and th e horsepowerrequired, HP,, is 30 horsepower.

l o w Speed Operating Poin tThe flow at 1,150 rpm is approximately two thi rd s of that a t 1,750

rpm. The change in operating conditions in this case is substantial.

The horsepower required was reduced from 125 to 30, and th e pres-su re from 218 f t to 95 f t of water. While the reduction in horsepow-er looks good, it is important to remember that the designer has toconsider things from the bottom up. To operate at the reduced flow,the pump pressure is 95 ft. If the ac tual static head is greater th anth at , the pump cannot operate at the reduced speed of 1,150 rpm. I naddition, only a 30-horsepower motor is needed at the lower speed.To increase th e speed to 1,750 rpm requires a n additional 95 horse-power. In other words, to reduce th e speed according to thi s example

will result in having a 125-horsepower motor th at is only producing30 horsepower a t low speed. When t he speed is increased to th e full1,750 rpm, the pump pressure increases from 90 f t to 218 ft .



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FlowVariations 49

,

a

z

FLOW GPM)

Note: Scale = 100 gpm p er square.

H,)80

70

6

50

40

30

Figure 3-5 1,750-rpm pump curve

Figure 3-6 1,150-rpm pump curve



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High-speed Operating PointIncreasing the flow from the low-speed design to the high-speed

design point can have several effects, depending on th e configurationof the system.

System configuration. For th e purposes of this discussion, it isassumed tha t th e pump discharges into a pipeline, which leadsto a gravity water storage reservoir a s shown in Figure 1-1.Increased system head loss. When the pump is large withrespect to the pipeline, increasing the flow rate may result

in increased system head loss. Depending on the design ofthe pump, the increased system head loss an d pump pressuremay offset each other. However, in many cases t he pump pres-su re may stil l be excessive.

System Head Loss Remains the Same or Nearly So)

Referring to Figure 3-4, if the discharge head H 1 at the high-speed design point is greater than the system pressure, the pumpoperating point will move to the right on the curve. For example if thesystem pressure is sti ll H,, th e operating point will move to point 3),assuming no substan tia l change in pipe friction.

When increasing speed to move from point (2) to point (l), thevariable-speed formulas on Figure 3 4 are always in effect. However,when moving from there to point 3), the pump impeller curverequirements are in control. The horsepower required may then behigher at point 3) th an a t point (1). Depending on the shape of th especific pump curve, the new operating point may be outside thepumps capability. For example, referring to the example shown inFig ure 3-7, extending t he low-speed operating point horizontally toth e right would be far outside the pumps capability. In th at case, anew pump should be selected. Once again, the shape of the pumpsimpeller curve is critical. The pump must be capable of operating atall t hr ee points, with suitable horsepower for each. If the pump cansatisfy these conditions, it will be a good choice.

Assuming the theoretical curve on Figure 3-4, extending theoperating point would work, as long as the horsepower and NPSHRconditions are met. The O M personnel should compare the actualoperating conditions against the impeller curve for each pump toverify t ha t th e pump is capable of operating a s required.



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lowVariations 5 1

z I700

FLOW (GPM)

Note: Scale = 100 gpm per square.

Figure 3-7 Comparison of 1,750- versus 1,150-rpm speed

Comparison of Operating ConditionsA comparison of the two operating conditions is shown in Fig ure

3-7. It should be noted that varying the speed this much is a n extremeexample and is not recommended. For one thing, th e horsepower re -quire ments vary too much.

In the case previously described, throttling from the high flow(GPM,) to the low flow (GPM,) only resu lts i n a reduction of 25 horse-power. In thi s case, throt tling might be more economical tha n providinga variable-speed drive. It is recommended th at a n economic evaluationbe made to determine the most efficient method of varying th e flow.

Benefits of a VFD System

Variable-frequency-drive (VFD) pump systems are often usedto match small variations in demand. Many large water treatmentplants discharge to a receiving reservoir that only holds a portion of aday's capacity. I n t hat case, it may be necessary t o match productionwith demand fairly closely, to prevent the receiving reservoir fromrun nin g dry. (See Figure 3-8.)



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52 Pump Selection and Troubleshooting FieldGuide

Water

Surface WaterSurface

FlowultipleStorage

Reservoirs

-

Water

Wholesale Users- - - - -Other W aterDistributors

Surface

~~~ tlow

TOTAL HEAD = STATIC HEAD FRICTION LOSS

PUMP SELECTION

Figure 3-8 Large potable water sy stem operat ion

A major benefit of a VFD system is to keep large pumps from re -peatedly starting and stopping. If multiple pumps are being used,

one t r im pump w i t h a VFD may be used to match the desired flowwithout starting or stopping any of the others. I n thi s way, only onepump needs to have a VFD, even when ther e a re multiple pumps i na water system.

VFD Design ConsiderationsIn providing a VFD in a new installation, al l the components can

be designed accordingly. However, in a retrofit, installing a VFD is nota s simple a s changing the motor sta rter . All the components need to becarefully addressed. The system design needs to be analyzed, as dis-cussed previously, and a n electrical engineer should be consulted forthe actu al installation. Several issues a re noted in the following list:

1. A VFD normally requires a larger motor and heavier wiring fromth e motor s ta rt er . Special motors may be used t ha t a re providedwith reinforced insulation .

2 . A VFD is only about 96 or 97 percent efficient. The difference inefficiency resul ts in t he generation of su bst antia l heat i n th e mo-tor starter, which must be dissipated. As a result, the electricalroom containing the VFD may require additional ventilation orai r conditioning. The author h as witnessed more th an one inst al-lation with inadequate ventilation that had a house fan coolinga n open VFD cabinet.



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lowVariations 53

3 . VFDs a re not recommended for high static head applications. Be-cause of the relationships between speed and horsepower, a muchlarger motor is typically needed for high static head applicationsand may not be th e most efficient insta llation.

4. Other design issues to be addressed by the electrical engineerinclude larger wiring required, potential resonance with otherequipment, long cable ru ns , an d potential induced voltages.

Pump Efficiency Factors

A s mentioned previously, it is important to select a pump that isoperating a t or nea r its peak efficiency range. The designer should beawa re th at impeller design a nd efficiency have changed in the l ast 20years, so th at newer designs ar e 5 to 10 percent more efficient. If thepump is that old, consideration should be given to replacing it.

Motor Efficiency

There has also been a 5 to 10 percent increase in motor efficien-

cy over the last 20 years. Motors with an efficiency of 94 percent orgreat er a re available, while submersible motors a re in t he ra nge of 85percent efficient.

Replacement Evaluation

The designer and operations personnel should be aware t hat sim-ply installing a VFD on a n existing pump may not work, unless thepump has excess capacity. The V F D will make the pump operate atlower flows and pressure than before. An economic evaluation may benecessary to determine if a motor should be replaced, or if the entirepump and motor assembly should be replaced. Initial capital costs,maintenance costs, an d electrical power costs should be considered ina complete life-cycle ana lysis.

Pump Capacity OptionsWhen sizing the pump in t erms of percent demand, ther e a re a

number of variables t o consider, including system size, present versus

future demand, variations in the existing demand, and economics.There are many designs for pump systems. Some pumps operate offof line pressure to boost the water to a different level. Others oper-at e in wet wells or river intakes. A minimum suction-side pressure of20 psi is normally required for pumps operating off of line pressure.



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With pumps operating in river intakes or wet wells/sumps, the de-

signer also must take into account th e physical sizing and configura-tion of the wet well or sump.

System SizeIn a small system, there may be only one pump to satisfy 100

percent demand. The overall system may be small, or a small pumpmay feed a small upper-level pressure zone off a larger system. Theliability in having only one pump is tha t th e system will be totally offline if th at pump fails.

Present Versus Future Demand

A water master plan for a system should include predictions ofth e fut ure demand as well as a review of present operation. Only onepump may be needed presently, whereas a second or th ir d pump maybe required i n the futur e to meet th e anticipated increase in demand.Even if only one pump is provided, space, valves, and connectionsmay be provided to allow easy installation of a future set of pumps.

All th is information should be made available to O M personnel.

Variations i n System Demand

The demand of many water systems typically varies during anygiven day and also seasonally. Holidays may also generate a changein system demand. Variations in system demand are often takeninto account by using a storage reservoir that feeds water back toth e users. In th at case, a pump can r un all day at an average flow,allowing the reservoir to be depleted during the day and refilled atnight. There are occasions, with reservoirs also, where the pumpsare required to provide a variable flow. Small receiving reservoirs orconstant-pressure pumping systems ar e a n example.

In a constant-pressure pumping application, th e author h as oftenprovided one small pump to operate all the time to maintain sys-tem pressure. When the demand increases, the pressure will beginto drop, and a second or th ir d pump can then come on to satisfy thedemand. It is desirable even i n small systems to prevent th e pumpsfrom turni ng on and off frequently. I n addition to higher electrical de-mand, it causes wear an d tea r on th e motor and motor sta rt er s. Theterm jockeypump is used by the author to describe the small pump i nth is system. Other names may be used, but th e inten t is th e same.



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FlowVariations 55

Water System Opt imization

A capital improvements plan (CIP) is recommended a s th e firststep in ensuring the highest-efficiency operation of a water system.A CIP may call for increasing th e size of small piping to lower th epumping pressure required. A CIP may include the replacement ofmotors and old, inefficient pumps, an d la rger wet wells. It may spec-ify control modifications to reduce pump s ta r t s and stops, the use ofmultiple pump sizes, or V F D rim pumps.

SummaryThe most efficient pump is usually a fixed-speed unit operating

a t its maximum available efficiency. However, the system may needa variable-speed trim pump to provide proper service. Whatever theindividual needs, a complete systems approach, including a n economicevaluation, is recommended prior to modifying or adding to a system.If the addition of a pump with a variable-speed drive can keep otherpumps from cycling on and off frequently, the V F D will be a highlyeconomical choice. All planning and design information should be madeavailable to the O M personnel and contained in a manual to be main-tained on site with other information recommended in th is handbook.



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CHAPT

Pump Troubleshooting

IntroductionThe purpose of pumps is to lift a fluid to a higher elevation. They

a re a n essenti al component of most utility systems, including po-table water, indust rial kom mer cial applications, an d sewage collec-tion a nd treatm ent. P um ps usually perform a function as par t of asystem an d m ust be designed t o operat e within th e requi rement s ofth at system. Troubleshooting a pump often involves a n ana lys is of

th e system as well.Although the re a re many pump types, th is discussion will be lim-

ited t o typical operational and design problems on some of the morecommon types, such a s centrifugal, line shaft turbine, and solids-han-dling pumps. If th e discharge pressure an d flow ra te of a n operatingpump a re close to th e design point, th e pump is fulfilling its function.On the other han d, if th e pump is not producing as designed, or if itis vibrating or exhibiting any other problems, a n investigation shouldbe made to resolve and re pa ir a ny deficiencies.

The following are some of the troubleshooting procedures usedand recommended by the author.

Close Coupled Centrifugal PumpsPreliminary Evaluation

For a close coupled centrifugal pump (Figure 4-1 , the first andsimplest evaluation to be performed is to determine if the pump isperforming as designed and if it is vibra ting excessively.

Vibration The vibration should be minimal, and t he motor shouldfeel like its runn ing smooth or very even. In addition, th e motor tem-perature should be normal. If the pump is vibrating, there ar e severalpotential causes, such a s worn bearings or seals, or cavitation. Bear-ings and seals should be replaced per the manufacturers recommen-dations. If the problems reoccur, the piping and other devices on thesuction side should be evaluated for cavitation or other inlet problems.

57



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58 Pump Selection and Troubleshooting ield Guide

IoToR PRIMING?\ CONNECTIONSMOTOR CONNECTIONS

FLANGED PPECONNECTIONS

/ l OLUTEMOUNTING BASE COUNTERING

IMPELLER

- DISCHARGE

-CROSSSECTION OF

VOLUTE ANDIMPELLER

Figure 4 1 ypical close coupled centrifugal pump

Refer also to the discussion on sump design for pumps using them. Avery warm o r hot motor may indicate simi la r problems a s previouslymentioned. The allowable tempera ture rise (above ambient) is usuallylisted on th e motor nameplate. If the temperatur e appears to be higherthan it should be, the motor andlor pump may be in danger of failurean d should be replaced or repaired a s soon as possible.

Performance The second test is to determine if the pump isproducing t he design pre ssure a nd flow. The pump should be in rea-sonable condition if it is ru nn in g smoothly and performing properly.

However, it is sti ll necessary to do the recommended maintenance.Flowmeter

If a flowmeter indicates loss of capacity in a pump, the entireinstallation should be inspected for proper operation. If all else ap-pears normal, th e met er should be calibrated. Old flowmeters an d



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Pump Troubleshooting 59

those with rotatin g or moving pa rt s ar e especially suspect, and may

need main tenance or replacement themselves.

Pressure Boost

In the absence of a meter, the flow can be approximated if theupstream an d downstream pressures around th e pump ar e known.Using t he difference in pressures (pressure boost), refer to th e act ualpump impeller curve, if available, as demonstrated i n Figu re 2-10.The flow can be approximated using the pressure reading. Determin-ing t he flow and pressur e i n this way is one of the simplest and easies tways to evaluate the operating condition of a pump.

When there is a suction lift, a vacuum gauge must be used on thesuction side of the pump or a n actual measurement taken. The vacuumreading can be compared aga inst the norm, from normal conditions and1or the value from the recommended maintenanceldesign manual, andadded to the discharge pressure to calculate the to ta l pressure boost.

For a vertical line shaft pump, th e dis tance down to th e sump orwell water level mus t be added to the discharge pressu re to calculate

th e t otal pressure boost.

Design Conditions

If the discharge pressure and flow rat e are close t o the design point,the pump is fulfilling its function. It may still have mechanical problems,such as vibration, suction-side restrictions, or hydraulic sump design,which need to be identified and addressed before production is affected.

Low Flow/PressureA lower than normal flow o r discharge pressure obviously indi-

cates that there a r e most likely problems that need t o be identifiedand repair ed. Lower than normal design flow could be th e resul t of anumber of factors, including th e following:

Cavitation Cavitation can be caused by unstable operation,which vaporizes some of t he water, creat ing bubbles that pass throughthe pump. P res sur e created by th e pump can cause these bubbles tocollapse, creating a knocking or tapping sound on the discharge pipe.Formation and collapse of

the airbubbles can, in tur n, cause pitting

an d excessive wear on the inlet s ide of the pump an d impeller. C ausesof cavitation include too high of a flow rate, which m eans t he pump isoperating too far to the right on its curve, or restrictions in the inletpiping, causing the pump to exceed its N P S H requir ements , whichcan also result in overheating an d pump failure.



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60 Pump Selection and Troubleshooting ield Guide

Restriction in inletpiping Fouling or restriction of th e inlet pip-ing can raise the NPSH to a value th at may be higher th an th at avail-able. Leaks i n the suction piping, especially in a suction lif t condition,can also cause simila r problems, including loss of prime an

pump selection and troubleshooting field guide-american water works association (awwa) (2009)

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