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Pump Wisdom
Pump Wisdom

Essential Centrifugal Pump Knowledge for

Operators and Specialists

Second Edition

Robert X. Perez
San Antonio, Texas
USA

Heinz P. Bloch
Montgomery, Texas
USA
Copyright © 2022 by the American Institute of Chemical Engineers, Inc. All rights reserved.

A Joint Publication of the American Institute of Chemical Engineers and John Wiley & Sons, Inc.

Edition History
1st edition: Wiley 2011

Published by John Wiley & Sons, Inc., Hoboken, New Jersey.

Published simultaneously in Canada.

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Library of Congress Cataloging-­in-­Publication Data

Names: Perez, Robert X., author. | Bloch, Heinz P., 1933– author.
Title: Pump wisdom : essential centrifugal pump knowledge for operators and
specialists / Robert X. Perez, San Antonio, Texas, USA , Heinz P. Bloch,
Montgomery, Texas USA.
Description: Second edition. | Hoboken, NJ., USA : Wiley, [2022] | Includes
index.
Identifiers: LCCN 2021046316 (print) | LCCN 2021046317 (ebook) | ISBN
9781119748182 (hardback) | ISBN 9781119748199 (adobe pdf) | ISBN
9781119748236 (epub)
Subjects: LCSH: Pumping machinery.
Classification: LCC TJ900 .B6483 2022 (print) | LCC TJ900 (ebook) | DDC
621.6/9–dc23
LC record available at https://lccn.loc.gov/2021046316
LC ebook record available at https://lccn.loc.gov/2021046317

Cover Design: Wiley

Cover Images: © Matveev Aleksandr/Shutterstock; engineer story/Shutterstock;
prabhjits/Getty Images; Grassetto/Getty Images

Set in 9.5/12.5pt STIXTwoText by Straive, Pondicherry, India

10 9 8 7 6 5 4 3 2 1
“Dedicated to the reliability technicians and engineers who will not rest until
enhanced process pump safety and reliability become the new normal.”
 vii

Contents

Preface ix

1. Principles of Centrifugal Process Pumps 1

2. Pump Selection and Industry Standards 15

3. Foundations and Baseplates 23

4. Piping, Stationary Seals, and Gasketing 33

5. Rolling Element Bearings 51

6. Lubricant Application and Cooling Considerations 71

7. Lubricant Types and Key Properties 85

8. Bearing Housing Protection and Cost Justification 93

9. Mechanical Sealing Options for Long Life 101

10. Pump Operation 117

11. Impeller Modifications and Pump Maintenance 133

12. Lubrication Management 145

13. Pump Condition Monitoring: Pump Vibration, Rotor Balance,

and Effect on Bearing Life 153
viii Contents

14. Drivers, Couplings, and Alignment 165

15. Fits, Dimensions, and Related Misunderstandings 175

16. Using Failure Statistics and Root Cause Analysis Findings to Guide
Reliability Improvement Efforts 191

17. Repair, Replace, or Modify? 213

18. Centrifugal Pump Monitoring Strategies 231

19. Final Thoughts 249

Index 251
 ix

Preface

In the 10 years since the first edition of this book was compiled, upgrading in
accordance with this text was largely practiced by a few best-­in-­class performers
(“BiCs”). The feedback from these fluid machinery users was very gratifying and
the publishers expressed interest in a second, expanded Edition. Robert Perez was
asked to join the two-­man writing team and the results are found in this updated
Second Edition text. True to the subtitle we selected, you will find this volume to
contain Essential Centrifugal Pump Knowledge for Operators and Specialists.
Next to electric motors, centrifugal pumps are the most frequent rotating
machine man has built and put to use. Centrifugal pump users have access to
hundreds of books and many thousands of articles dealing with pump subjects.
So, one might ask, why do we need this text? We co-­authors are certain that we
need this text because an unacceptably large number of centrifugal process pumps
fail catastrophically every year. An estimated 95% of these are repeat failures and
most of them are quite costly, or dangerous, or both.
Essentials concentrate on explaining the many elusive failure causes that mani-
fest themselves as repeat failures. We saw it as our mandate and task to clearly
map out permanent remedial action. Our intent was to steer clear of the usual
consultant-­conceived generalities and give you tangible, factual, and well-­defined
information throughout.
As any close review of what has been offered in the past will uncover, many
texts were written to primarily benefit one particular job function, ranging from
centrifugal pump operators to pump designers. Some books contain a hidden bias,
or they appeal to a very narrow spectrum of readers; others are perhaps influ-
enced by a particular centrifugal pump manufacturer’s agenda. Give this text a
chance, you will see that it is different. You will not find it in other texts. We gave
it the title Pump Wisdom because wisdom is defined as applied knowledge. If you
concur with this very meaningful definition you will be ready for a rather serious
challenge. That challenge is to practice wisdom by acting on the unique knowledge
this text conveys.
x Preface

Although both of us had written or co-­authored other books and dozens of arti-
cles on centrifugal pump reliability improvement, some important material is too
widely dispersed to be readily accessible. Moreover, some important material has
never been published before. We again set out to assemble, rework, condense, and
explain the most valuable points in a text aimed at ever wider distribution. With
the addition of troubleshooting material, coverage of sensors, and centrifugal
pump surveillance topics, it is now a text with, we hope, even more permanence
and “staying power” than its predecessor edition. To thus satisfy the scope and
intent of this book, we stayed with our original intention to keep theoretical expla-
nations to a reasonable minimum and to limit the narrative to about 250 pages.
Putting it another way, we wanted to squeeze into these 250 pages material and
topics that will greatly enhance both centrifugal pump safety and reliability. All
that is needed is the reader’s solid determination to pay close attention and to fol-
low up diligently.
Please again realize that in years past, many centrifugal pump manufacturers
have primarily concentrated their design and improvement focus on the hydraulic
end. Indeed, over time and in the decades since 1960, much advancement has
been made in the metallurgical and power efficiency-­related performance of the
hydraulic assembly. Meanwhile, the mechanical assembly or drive-­end of cen-
trifugal process pumps was being treated with relative indifference. In essence,
there was an imbalance between the attention given to pump hydraulics and
pump mechanical issues.
Recognizing indifference as costly, this text will indeed rectify some of these
imbalances. Our narrative and illustrations are intended to do justice to both the
hydraulic assembly and the mechanical assembly of centrifugal process pumps.
That said, the book briefly lays out how centrifugal pumps function and quickly
moves to guidelines and details that must be considered by reliability-­focused
readers. A number of risky omissions or shortcuts by centrifugal pump designers,
manufacturers, and users are also described.
The co-­authors are indebted to AESSEAL. A worldwide manufacturer of seal-
ing products, the company was unselfish in providing many images and special
artwork without making us wait for the approval of layers of bureaucracy which,
in a number of other instances, has delayed or even prevented the move to more
reliable pump products. The company has demonstrated an exemplary respect for
the environment and is often considered as an example of how a business should
conduct itself for the mutual benefit of all parties.
Please take from us a strong measure of encouragement: Make good use of this
book. Read it and apply it. Today, and hopefully years from now, remember to
consult this material. Doing so will acquaint you and your successors with cen-
trifugal pump failure avoidance and the more elusive aspects of preserving pump-­
related assets. And so, while you undoubtedly have more problems than you
 Preface xi

deserve, please keep in mind that you also deserve access to more solutions than
you previously knew about or presently apply. Sound solutions are available, and
they are here, right at your fingertips. Use them wisely; they will be cost-­effective.
The solutions you can discern from this text will have a positive effect on centrifu-
gal pump safety performance and asset preservation. They have worked at Best-­of-­
Class companies and cannot possibly disappoint you.

Robert X. Perez, P.E., and Heinz P. Bloch, P.E.

Fall 2020
 1

Principles of Centrifugal Process Pumps

Pumps, of course, are simple machines that lift, transfer, or otherwise move fluid
from one place to another. They are usually configured to use the rotational
(kinetic) energy from an impeller to impart motion to a fluid. The impeller is
located on a shaft; together, shaft and impeller(s) make up the rotor. This rotor is
surrounded by a casing; located in this casing (or pump case) are one or more
stationary passageways that direct the fluid to a discharge nozzle. Impeller and
casing are the main components of the hydraulic assembly; the region or envelope
containing bearings and seals is called the mechanical assembly or power end
(Figure 1.1).
Many process pumps are designed and constructed to facilitate field repair. On
these so-­called “back pull-­out” pumps, shop maintenance can be performed,
while the casing and its associated suction and discharge piping (Figure 1.2) are
left undisturbed. Although operating in the hydraulic end, the impeller remains
with the power end during removal from the field.
The rotating impeller (Figure 1.3) is usually constructed with swept-­back vanes,
and the fluid is accelerated from the rotating impeller to the stationary passages
into the surrounding casing.
In this manner, kinetic energy is added to the fluid stream (also called pump-
age) as it enters the impeller’s suction eye (A on Figure 1.3), travels through the
impeller, and is then flung outward toward the impeller’s periphery. After
the fluid exits the impeller, it gradually decelerates to a much lower velocity in the
stationary casing, called a volute casing, where the fluid stream’s kinetic energy is
converted into pressure energy (also called pressure head). The combination of
the pump suction (inlet) pressure and the additional pressure head generated by
the impeller creates a final pump discharge pressure that is higher than the suc-
tion pressure [3].

Pump Wisdom: Essential Centrifugal Pump Knowledge for Operators and Specialists,
Second Edition. Robert X. Perez and Heinz P. Bloch.
© 2022 The American Institute of Chemical Engineers, Inc. Published 2022 by John Wiley & Sons, Inc.
2 1 Principles of Centrifugal Process Pumps

Mechanical Hydraulic
assembly assembly

Hydraulic assembly
Impeller/propeller
Suction inlet
Volute
Seal rings

Mechanical assembly
Shaft seal
Labyrinth
Mechanical
Packing
Shaft
Bearings
Housing/frame
Drive coupling/sheave

Figure 1.1 Principal components of an elementary process pump. Source: SKF USA,
Inc. [1].

­Pump Performance: Head and Flow

Pump performance is always described in terms of head H produced at a given

flow capability Q, and hydraulic efficiency η attained at any particular intersec-
tion of H and Q. Head is customarily plotted on the vertical scale or vertical axis
(the left of the two y-­axes) of Figure 1.4; it is expressed in feet (or meters).
Hydraulic efficiency is often plotted on another vertical scale, the right of the two
vertical scales, i.e. the y-­axis in this generalized plot.
 ­Pump Performance: Head and Flo 3

Figure 1.2 Typical process pump with suction flow entering horizontally and
vertically oriented discharge pipe leaving the casing tangentially. Source: Emile
Egger & Cie. [2].

Head is related to the difference between discharge pressure and suction pres-
sure at the respective pump nozzles. Head is a simple concept, but this is where
consideration of the impeller tip speed is important. The higher the shaft rpm and
the larger the impeller diameter, the higher will be the impeller tip speed – actually
its peripheral velocity.

Figure 1.3 A semiopen impeller with five vanes. As shown, the impeller is configured for
counterclockwise rotation about a centerline “A.”
4 1 Principles of Centrifugal Process Pumps

SH r

NPSHr
NP
H.Q. 90
80
70

Efficiency %
Head

60
f%

50
Ef

40
. I.O 30
SP. GR

Power
Power 20
10
0
0 Flow rate BEP

Figure 1.4 Typical “H–Q” performance curves are sloped as shown here. The best
efficiency point (BEP) is marked with a small triangle; power and other parameters are
often displayed on the same plot.

The concept of head can be visualized by thinking of a vertical pipe bolted to the
outlet (the discharge nozzle) of a pump. In this imaginary pipe, a column of fluid
would rise to a height “H”. If the vertical pipe would be attached to the discharge
nozzle of a pump with higher impeller tip speed, the fluid would rise to a greater
height “H+”. It is important to note that the height of a column of liquid, H or H+,
is a function only of the impeller tip speed. The specific gravity of the liquid affects
power demand but does not influence either H or H+. However, the resulting
discharge pressure does depend on the liquid density (specific gravity or Sp.G.).
For water (with an Sp.G. of 1.0), an H of 2.31 ft equals 1 psi (pound-­per-­square-­
inch), while for alcohol, which might have a Sp.G. of 0.5, a column height or head
H of 4.62 ft equals 1 psi. So if a certain fluid had an Sp.G. of 1.28, a column height
(head H) of 2.31/1.28 = 1.8 ft would equal a pressure of 1 psi.
For reasons of material strength and reasonably priced metallurgy, one usually
limits the head per stage to about 700 ft. This is a fairly important rule-­of-­thumb
limit to remember. When too many similar rule-­of-­thumb limits combine, one
cannot expect pump reliability to be at its highest. As an example, say a particular
impeller-­to-­shaft fit is to have 0.0002–0.0015 in. clearance on average size impeller
hubs. With a clearance fit of 0.0015 in., one might anticipate a somewhat greater
failure risk if this upper limit were found on an impeller operating with maximum
allowable diameter.
 ­Operation at Zero Flo 5

On Figure 1.4, the point of zero flow (where the curve intersects the y-­axis) is
called the shut-­off point. The point at which operating efficiency is at a peak is
called the best efficiency point, or BEP. Head rise from BEP to shut-­off is often
chosen around 10–15% of differential head. This choice makes it easy to modulate
pump flow by adjusting control valve open area based on monitoring pressure.
Pumps “operate on their curves” and knowledge of what pressure relates to what
flow allows technicians to program control loops.
The generalized depictions in Figure 1.4 also contain a curve labeled NPSHr,
which stands for Net Positive Suction Head required. This is the head of liquid
that must exist at the edge of the inlet vanes of an impeller to allow liquid trans-
port without causing undue vaporization. It is a function of impeller geometry
and size and is determined by factory testing. NPSHr can range from a few feet to
a three-­digit number. At all times, the head of liquid available at the impeller inlet
(NPSHa) must exceed the required NPSHr.

­Operation at Zero Flow

The rate of flow through a pump is labeled Q (gpm) and is plotted on the hori-
zontal axis (the x-­scale). Note that for a given speed and for every value of head
H we read off on the y-­axis, there is a corresponding value of Q on the x-­axis.
This plotted relationship is expressed as “the pump is running on its curve.”
Pump H–Q curves are plotted to commence at zero flow and highest head.
Process pumps need a continually rising curve inclination and a curve with a
hump somewhere along its inclined line will not serve the reliability-­focused
user. Operation at zero flow is not allowed and, if over perhaps a minute’s dura-
tion, could cause temperature rise and internal recirculation effects that might
destroy most pumps.
But remember that this curve is valid only for this particular impeller pat-
tern, geometry, size, and operation at the speed indicated by the manufacturer
or entity that produced the curve. Curve steepness or inclination has to do
with the number of vanes in that impeller; curve steepness is also affected by
the angle each vane makes relative to the impeller hub. In general, curve
shape is verified by physical testing at the manufacturer’s facility. Once the
entire pump is installed in the field, it can be re-­tested periodically by the
owner–purchaser for degradation and wear progression. Power draw may
have been affected by seals and couplings that differ from the ones used on
the manufacturer’s test stand. Occasionally, high efficiencies are alluded to in
a manufacturer’s literature when bearing, seal, and coupling losses are not
included in the vendor’s test reports.
6 1 Principles of Centrifugal Process Pumps

­Impellers and Rotors

Regardless of flow classification centrifugal pumps range in size from tiny

pumps to very big pumps. The tiny ones might be used in medical or laboratory
applications; the extremely large pumps may move many thousands of liters or
even gallons per second from flooded lowlands to the open sea.
All six of the impellers in Figure 1.5 are shown with a hub fastening the impel-
ler to the shaft, and each of the first five impellers is shown as a hub-­and-­disc
version with an impeller cover. The cover (or “shroud”) identifies the first five as
“closed” impellers; recall that Figure 1.3 had depicted a semiopen impeller.
Semiopen impellers are designed and fabricated without the cover. Finally, open
impellers come with free-­standing vanes welded to or integrally cast into the hub.
Since the latter incorporate neither disc nor cover, they are often used in viscous
or fibrous paper stock applications.
To properly function, a semiopen impeller must operate in close proximity to a
casing internal surface, which is why axial adjustment features are needed with
these impellers. Axial location is a bit less critical with closed impellers. Except on
axial flow pumps, fluid exits the impeller in the radial direction. Radial and mixed
flow pumps are either single or double suction designs; both will be shown later.
Once the impellers are fastened to a shaft, the resulting assembly is called a rotor.
In radial and mixed flow pumps, the number of impellers following each other,
typically called “stages,” can range from one to as many as will make such multi-
stage pumps practical and economical to manufacture. Horizontal shaft pumps
with up to 12 stages are not uncommon; using more than 12 stages on a horizontal
shaft risks causing the rotor to resonate or vibrate at a so-­called “critical speed.”
Vertical shaft pumps have been designed with 48 or more stages. In vertical

Specific speed (Ns)

10 20 40 60 120 200 300 SI

US

500 1 000 2 000 3 000 6 000 10 000 15 000

Radial-vane Francis-vane Mixed flow Axial flow

Figure 1.5 General flow classifications of process pump impellers.

 ­The Meaning of Specific Spee 7

pumps, shaft support bushings are relatively lightly loaded; they are spaced so as
to minimize vibration risk.

­The Meaning of Specific Speed

Pump impeller flow classifications and the general meaning of specific speed
deserve further discussion. Moving from left to right in Figure 1.5, the various
impeller geometries reflect selections that start with high differential pressure
capabilities and end with progressively lower differential pressure capabilities.
Differential pressure is simply discharge pressure minus suction pressure.
Specific speed calculations are a function of several impeller parameters; the
mathematical expression includes exponents and is found later, in Figure 1.6.
Staying with Figure 1.5 and again moving from left to right, we can reason that
larger throughputs (flows) are more likely achieved by the configurations at right,
whereas larger pressure ratios (discharge pressure divided by suction pressure)
are usually achieved by the impeller geometries closer to the left of the illustration.
Impellers toward the right are more efficient than those near the left, and pump
designers use the parameter specific speed (Ns) to bracket pump hydraulic effi-
ciency attainment and other expected attributes of a particular impeller configu-
rations and size. Please be sure not to confuse a very similar sounding parameter,
pump suction specific speed (Nss or Nsss), with the specific speed (Ns). For now, we
are strictly addressing specific speed (Ns).
As an example, observe the customary use, whereby with N and Q – the typical
given parameters that define centrifugal process pumps – one determines a pivot
point. Next, with pivot point and head H, one can easily determine Ns. In
Figure 1.5, Ns is somewhere between 500 and 15 000 on the US scale. Whenever
we find ourselves in that range, we know such a pump exists, and we can even
observe the general impeller shape. Keep in mind that thousands of impeller com-
binations and geometries exist. Impellers with covers are the most prominent in
hydrocarbon processes, and an uneven number of impeller vanes is favored over
even numbers of vanes for reasons of vibration suppression.
Pump specific speed, Figure 1.6, might be of primary interest to pump design-
ers, but average users will also find it useful. On the lower right, the illustration
gives the equation for Ns; it will be easy to see how Ns is related to the shaft speed
N (rpm), flow Q (gpm or gallons/minute), and head H (expressed in feet). This
mathematical expression also has two strange-­looking exponents in it, but the Ns
nomogram conveys more than meets the eye and can be quite helpful.
If now, we had the same or some other Q and would want to see what hap-
pens at some other speed, we would again draw a straight line to establish the
pivot point. Drawing a line from whatever H is specified through the pivot
8 1 Principles of Centrifugal Process Pumps

1 500

1 000

800
700 Pivot line
600
Q N Ns
500
10 000 10 000 10 000
400

300 5 000 5 000 5 000

4 000 4 000 4 000
200 3 000 3 000 3 000

2 000 2 000 2 000

150
rpm

1500 2 1500 1500

100 1000 1000 1000 Ns = 850

800
Flow rate (gpm)
Head/stage (ft)

80
70 600
60 500 500 500
400 1 400
50
300 300
40
200 200
30
150

100 100
20
80
15 60
50
40
10 30
1 With Q&N,
20 determine pivot point
15 2 With pivot point & H,
5 determine Ns
10
1/2
Ns = N Q
H 3/4

Figure 1.6 Pump specific speed nomogram allowing quick estimations. Shown in this
illustration is a hypothetical pump application with a flow of 100 gpm operating at
3600 rpm (line 1). To develop 150 ft of head with a single stage, an impeller with a
specific speed of 850 (line 2) would be required.
 ­Process Pump Type  9

point and to Ns we would not like to select pumps with an Ns outside the rule-­
of-­thumb range from 500 to 15 000. In another example, we might, after estab-
lishing the pivot point, wish to determine what happens if we select an
impeller with the maximum head capability of 700 ft and draw a line through
the pivot point. If the resulting Ns is too low, we would try a higher speed N
and see what happens.
While there are always fringe applications in terms of size and flow rate, this
book deals with centrifugal pumps in process plants. These pumps are related to
the generic illustrations of Figures 1.1 and 1.2 and others in this chapter. All
would somewhat typically – but by no means exclusively – range from 3 to per-
haps 300 hp (2–225 kW).

­Process Pump Types

The elementary process pumps illustrated in Figures 1.1 and 1.2 probably incor-
porate one of the radial vane impellers shown in Figure 1.5. If a certain differen-
tial pressure is to be achieved together with higher flows, such a pump is often
designed with a double-­flow impeller (Figure 1.7). One of the side benefits of
double-­flow impellers is very good axial thrust equalization (axial balance). A
small thrust bearing will often suffice; it is shown here in the left bearing housing.

Figure 1.7 Double-­flow impellers are used for higher flows and relatively equalized
(balanced) axial thrust. Source: Mitsubishi Heavy Industries, Ltd. [4].
10 1 Principles of Centrifugal Process Pumps

Note that the two radial bearings are plain, or sleeve-­type. Certain sleeve bearings
have relatively high speed capability.
If elevated pressures are needed, several impellers are lined up in series on the
same pump rotor. Of course, this would then turn the pump into a multistage
model Figure 1.8.

­Process Pump Mechanical Response to Flow Changes

After the pumped fluid (also labeled pumpage, or flow) leaves at the impeller tip,
it must be channeled into a stationary passageway that merges into the discharge
nozzle. Many different types of passageway designs (single or multiple volutes,
vaned diffusers, etc.) are available. Their respective geometry interacts with the
flow and creates radial force action of different magnitude around the periphery
of an impeller (Figure 1.9). These forces tend to deflect the pump shaft; they are
greater at part-­flow than at full flow.

­Recirculation and Cavitation

Recirculation is a flow reversal near either the inlet or discharge of a centrifugal

pump. This flow reversal produces cavitation-­erosion damage that starts on the
high-­pressure side of an impeller vane and proceeds through the metal to the low-­
pressure side [5].
Pump-­internal recirculation can cause surging and cavitation even when the
available net positive suction head (NPSHa) exceeds the manufacture’s published

Figure 1.8 A multistage centrifugal process pump.

 ­Recirculation and Cavitatio 11

Figure 1.9 Direction and magnitude of fluid forces change at different flows.
Source: World Pumps, February 2010, pp. 19.

NPSH required (NPSHr) by considerable margins. Also, extensive damage to the

pressure side of impeller vanes has been observed in pumps operating at reduced
flow rates. These are the obvious results of recirculation; however, more subtle
symptoms and operational difficulties have been identified in pumps operating in
the recirculation zone.
Symptoms of discharge recirculation are the following:

●● Cavitation damage to the pressure side of the vane at the discharge

●● Axial movement of the shaft, sometimes accompanied by damage to the
thrust bearing
●● Cracking or failure of the impeller shrouds at the discharge
●● Shaft failure on the outboard end of double-­suction and multistage pumps
●● Cavitation damage to the casing tongue (see Chapter 11) or diffuser vanes
Symptoms of suction recirculation are the following:
●● Cavitation damage to the pressure side of the vanes at the inlet
●● Cavitation damage to the stationary vanes in the suction
●● Random crackling noise in the suction; this contrasts with the steady crackling
noise caused by inadequate net positive suction head
●● Surging of the suction flow

A quick-­reference illustration was provided by Warren Fraser Figure 1.10. We

should note that recirculation and the attendant failure risks are low in pumps
delivering 2500 gpm or less at heads up to 150 ft. In those pumps, energy levels
may not be high enough even if the pump operates in the recirculation zone. As
a general rule for such pumps, minimum flow can be set at 50% of recirculation
12 1 Principles of Centrifugal Process Pumps

Suction
recirculation

Cavitation damage from low

net positive suction head
Suction recirculation
cavitation damage
Discharge recirculation
Discharge recirculation
cavitation damage

Figure 1.10 Where and why impeller vanes get damaged. Source: Fraser [5].

flow for continuous operation and 25% of recirculation flow for intermittent
operation [6].

­The Importance of Suction Specific Speed

Note that pump suction specific speed (Nss or Nsss) differs from the pump specific
speed Ns discussed earlier. For installations delivering over 2500 gpm and with
suction specific speeds over 9000, greater care is needed. Suction specific speed
(Nsss or Nss) is calculated by the straightforward mathematical expression:
1/ 2
 r / min   gal / min  / eye 
N ss
 NPSH r 
3/ 4

wherein both the flow rate and NPSHr pertain to conditions published by the
manufacturer. In each case, these conditions (flow in gpm) and NPSHr are
observed on the maximum available impeller diameter for that particular pump.
The higher the design suction specific speed, the closer the point of suction
recirculation to what is commonly described as rated capacity. Similarly, the
closer the discharge recirculation capacity is to rated capacity, the higher the effi-
ciency. Pump system designers are tempted to aim for highest possible efficiency
 ­The Importance of Suction Specific Spee 13

and suction specific speed. However, such designs might result in systems with
either limited pump operating range or, if operated inside the recirculation range,
disappointing reliability and frequent failures.
Although more precise calculations are available, trend curves of probable
NPSHr for minimum recirculation and zero cavitation-­erosion in water,
Figure 1.11, are sufficiently accurate to warrant our attention [7]. The NPSHr
needed for zero damage to impellers and other pump components may be many
times that published in the manufacturer’s literature. The manufacturers’ NPSHr
plot (lowermost curve in Figure 1.11) is based on observing a 3% drop in discharge
head or pressure; at Q = 100%, we note NPSHr = 100% of the manufacturer’s
claim. Unfortunately, whenever this 3% fluctuation occurs, a measure of damage
may already be in progress. Assume the true NPSHr is as shown in Figure 1.11
and aim to provide an NPSHa in excess of this true NPSHr.
In Ref. [7], Irving Taylor compiled his general observations and alerted us to this
fact. He cautioned against considering his curves totally accurate and mentioned
the demarcation line between low and high suction specific speeds somewhere
between 8000 and 12 000. Many data points taken after 1980 point to 8500 or 9000
as numbers of concern. If pumps with Nss numbers higher than 9000 are being
operated at flows much higher or lower than BEP, their life expectancy or repair-­
free operating time will be reduced.
In the decades after Taylor’s presentation, controlled testing has been done in
many industrialized countries. The various findings have been reduced to rela-
tively accurate calculations that were later published by HI, the Hydraulic

Trend of probable NPSHr for zero cavitation-erosion

Various pumps (high head) = > 650 ft. (~200 m), first stage
H-Q

100 400

75 300
% NPSHr
%H

High head
50 200
high suct. specified speed
High head
low S.S.S
25 100 Low-moderate head
high S.S.S
Low-moderate head
0 0
0 20 40 60 80 100 120 low S.S.S
NPSHr for 3% head drop
%Q

Figure 1.11 Pump manufacturers usually plot only the NPSHr trend associated with the
lowermost curve. At that time a head drop or pressure fluctuation of 3% exists and
cavitation damage is often experienced. Source: Taylor [7].
14 1 Principles of Centrifugal Process Pumps

Institute [8]. Relevant summaries can also be found in Ref. [9]. Calculations
based on Refs. [8, 9] determine minimum allowable flow as a percentage of BEP.
Note, again, that recirculation differs from cavitation, a term which essentially
describes vapor bubbles that collapse. Cavitation damage is often caused by low
net positive suction head available (NPSHa). Such cavitation-­related damage starts
on the low-­pressure side and proceeds to the high-­pressure side. An impeller
requires a certain net positive suction head; this NPSHr is simply the pressure
needed at the impeller inlet (or eye) for relatively vapor-­free flow.

What We Have Learned

Understanding the concepts of Ns and Nss will assist in specifying better pumps. In
addition to fluid properties pump life is influenced by throughput. Just as an auto-
mobile transmission is designed to work best at particular speeds or optimum
gear ratios, pumps have desirable flow ranges. Deviating from optimum flow will
influence failure risk and life expectancy.

­References

1 SKF USA, Inc.; “Bearings in Centrifugal Pumps”, Kulpsville, PA, Publication

100-­955, Version 4/2008; excerpted or adapted by permission of the
copyright holder.
2 Emile Egger & Cie.; “Operating Manual”, Salt Lake City, Utah, and Cressier, NE,
Switzerland, 2008.
3 ITT/Goulds Pump Corporation; “Installation and Maintenance Manual for Model
3196 ANSI Pump”, Seneca Falls, NY, 1990.
4 Mitsubishi Heavy Industries, Ltd.; “Publication HD30-­04060”, Tokyo, Japan, and
New York, NY.
5 Fraser, Warren H.; “Flow recirculation in centrifugal pumps,” Proceedings of the
Texas A&M University Turbomachinery Symposium, Houston, TX, 1981, pp. 95–100.
6 Ingram, James H.; “Pump reliability–where do you start", presented at ASME
Petroleum Mechanical Engineering Workshop and Conference, Dallas, TX,
September 13–15, 1981.
7 Taylor, Irving; “The Most Persistent Pump-­Application Problems for Petroleum
and Power Engineers”, ASME Publication 77-­Pet-­5 (Energy Technology Conference
and Exhibit, Houston, Texas, September 18–22, 1977).
8 ANSI/HI9.6.3-­1997; “Allowable Operating Region”, Hydraulic Institute, Parsippany,
New Jersey, 2008.
9 Bloch, Heinz P., and Alan Budris; “Pump User’s Handbook”, 3rd Edition, Fairmont
Press, Lilburn, GA 30047, 2010 (ISBN 0-­88173-­517-­5).
 15

Pump Selection and Industry Standards

Anybody can buy a cheap pump, but you want to buy a better pump. The term
“better pumps” describes fluid movers that are well designed beyond just hydrau-
lic efficiency and modern metallurgy. Better pumps are ones that avoid risk areas
in the mechanical portion commonly called the drive-­end. That is the part of pro-
cess pumps that has been neglected most often and where cost cutting should
cause the greatest concern.
Deviations from best available technology increase the failure risk. As three or
four or more deviations combine, a failure is very likely to occur. An analogy could
be drawn from an incident involving two automobiles, with one driving behind
the other. When the trailing vehicle travelled at (i) an excessive speed, with
(ii) worn tires, on (iii) a wet road, and (iv) followed the leading car too closely, a
rear-­end collision resulted. Had there just been any three of the four violations,
the event might be recalled as one of the many “near miss” incidents. Had there
been any two of the four, it would serve no purpose to tell the story in the first place.
Too much cost-­cutting by pump manufacturers and purchasers will negatively
affect the drive-­ends of process pumps. Flawed drive-­end components are there-
fore among the main contributors to elusive repeat failures that often plague
pumps – essentially very simple machines. Drive-­end flaws deserve to be addressed
with urgency, and this short chapter will introduce the reader to more details that
follow later in Chapters 5, 8 and 10.

­Why Insist on Better Pumps

Well-­informed reliability professionals will be reluctant to accept pumps that

incorporate the drive-­end shown in Figure 2.1. The short overview of reasons is
that reliability-­focused professionals take seriously their obligation to consider the

Pump Wisdom: Essential Centrifugal Pump Knowledge for Operators and Specialists,
Second Edition. Robert X. Perez and Heinz P. Bloch.
© 2022 The American Institute of Chemical Engineers, Inc. Published 2022 by John Wiley & Sons, Inc.
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