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Know and Understand
CUSTODIO
c
Know and
Understand
Cen tr if ug a I
Pumps
Know and
Understand
Cent r if u g a I
Pumps
by Larry Bachus and
Angel Custodio
ELSEVIER
UK
USA
JAPAN
Elsevier Ltd, The Boulevard, Langford Lane, Cdlington, Oxford
OX5 lGB, UK
Elsevier Inc, 360 Park Avenue South, New York, NY 1001 0- 171 0,
USA
Elsevier Japan, Tsunashima Building Annex, 3-20- 12 Yushima,
Bunkyo-ku, Tokyo 11 3, Japan
Copyright 0 2003 Bachus Company, Inc.
Published by Elsevier Ltd
All rights reserved.
All rights reserved. No part of this publication may be reproduced, stored in a
retrieval system or transmitted in any form or by any means: electronic,
electrostatic, magnetic tape, mechanical, photocopying, recording or otherwise,
without permission in writing from the publishers.
British Library Cataloguing in Publication Data
Bachus, Larry
Know and understand centrifugal pumps
1 .Centrifugal pumps
I.Title II.Custodio, Angel
62 1.6’7
ISBN 1856174093
No responsibility is assumed by the Publisher for any injury and/or damage to
persons or property as a matter of products liability, negligence or otherwise, or
from any use or operation of any methods, products, instructions or ideas
contained in the material herein.
Published by
Elsevier Advanced Technology,
The Boulevard, Langford Lane, Kidlington, Oxford OX5 lGB, UK
Tel: +44(0) 1865 843000
Fax: +44(0) 1865 843971
Typeset by Land & Unwin (Data Sciences) Ltd, Bugbrooke
Printed and bound in Great Britain by Biddles Ltd, Guildford and King’s Lynn
Contents
Prologue
About the Authors
1 Basic Pump Principles
Introduction
How do pumps work?
Pressure measurement
Atmospheric pressure (ATM)
Absolute pressure (psia)
Gauge pressure (psig)
Vacuum
Pump head
Specific gravity
Pressure measurement
Pressures inside the pump
2 NPSH, Net Positive Suction Head
Introduction
Definition of NPSHr (required)
Definition NPSHa (available)
3 Cavitation
Introduction
Vapor pressure
Cavitation
The effects of vapor pressure on pump performance
Cavitation: A practical discussion
Review for preventing cavitation
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Contents
Cavitation review
Do something about cavitation!
4 The Affinity Laws
Introduction
The Laws
The Affinity Laws and the impeller diameter
What’s the practical application of these laws?
5 Useful Work and Pump Efficiency
Useful work from a pump
Flow determination
Pump efficiency
Factors that affect the efficiency
Calculating pump efficiency
6 Pump Classification
Introduction
Positive displacement pumps
Centrifugal pumps
Conceptual difference
Centrifugal volute pumps
Types of centrifugal pumps
Overhung impeller
Impeller between the bearings
Turbine pumps
Specific duty pumps
The typical ANSI pump
API (American Petroleum Institute) pumps
Vertical turbine pumps
Non-metallic pumps
Magnetic drive pumps
Canned motor pumps
Pump impellers
Turbine impellers
Conventional impellers
Suction specific speed, Nss
Open impellers
Semi open impeller
Totally enclosed impeller
Wear bands
Specific speed, Ns
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Contents
7 Understanding Pump Curves
Pump performance curves
History
Head versus pressure
H-Q
Pump efficiency
The energy (BHp) curve
The pump’s minimum requirements (NPSH)
Special design pumps
Family curves
8 The System Curve
The system controls the pump
The elements of the Total Dynamic Head (TDH)
Determining the Hs
Determining the Hp
Calculating the Hf and Hv
The dynamic system
Variable elevations
The happy zone
Dynamic pressures
Variable resistances
Short term resistance changes
Long term resistance changes
Pumps in parallel and pumps in series
Pumps in parallel
Three tips
Pumps running in series
Combined parallel and series pump operation
9 Shaft Deflection
Introduction
Operation, design and maintenance
Signs of shaft deflection
Interpreting the evidence
The sweet zone
The dual volute pump
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vi i
Contents
10 Pump and Motor Alignment
Introduction
Types of misalignment
Alignment techniques
Equipment alignment sequence
Coupling alignment
11 Bearings
Introduction
Bearing lubrication
Bearing failure
Bearing maintenance
Bearing seals
12 Pump Shaft Packing
History
Vegetable fibers
Reciprocating action
Packing
Rotary action
Synthetic fibers
Compression packing
The lantern ring
The packing lubricant
Stages in the life of packing
13 Mechanical Seals
Pump packing
History
The mechanical seal
The single, unbalanced, inside mounted mechanical seal
The single, outside-mounted, unbalanced seal
The single, balanced, internal mechanical seal
The single, balanced, external mechanical seal
Advantages of O-rings
The balance effect
Advantages of balance
Balance explained by math
Cartridge mechanical seals
Double seals
The tandem dual seal
The back-to-back double seal
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The face-to-face dual seal
Support systems for dual seals
The thermal convection tank
The turbo tank
The pumping unit
14 Failure Analysis of Mechanical Seals
Causes of premature seal failure
O-ring (the elastomer) failure
The elastomer sticks to the shaft
The springs clog and jam
The shaft frets under the shaft seal
Incorrect installation dimension
Environmental controls for difficult sealing applications
Difficult pumping applications for mechanical seals
Environmental controls
Proper pump repair alignment methods
15 Common Sense Failure Analysis
Pump maintenance files
Failure analysis on centrifugal pumps
Why is this pump in the shop?
16 Avoiding Wear in Centrifugal Pumps
Introduction
Erosion
Corrosion
Wear rings
Fluid velocity accelerates wear
Turbulence
Throttling
17 Pump Piping
Introduction
Piping design to drain tanks and sumps
The Submergence Laws
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Index
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Prologue
Very few industrial pumps come out of service and go into the
maintenance shop because the volute casing or impeller split down the
middle, or because the shaft fractured into four pieces. The majority of
pumps go into the shop because the bearings or the mechanical seal
failed.
Most mechanics spend their time at work time greasing and changing
bearings, changing pump packing, and mechanical seals. The
mechanical engineers spend their time comparing the various claims of
the pump manufacturers, trying desperately to relate the theory learned
at the University with the reality of the industrial plant. Purchasing
agents have to make costly decisions with inadequate information at
their disposal. Process engineers and operators are charged with
maintaining and increasing production.
The focus of industrial plant maintenance has always been that the
design is correct, and that the operation of the pumps in the system is
as it should be. In this book, you will see that in the majority of
occasions, this is not true. Most of us in maintenance spend our
valuable time, just changing parts, and in the best of cases, performing
preventive maintenance, trying to diminish the time required to change
those parts.
We almost never stop to consider what is causing the continual failure
of this equipment. This book will help you to step away from the
fireman approach, of putting out fires and chasing emergencies.
This book is directed toward the understanding of industrial pumps and
their systems. It won’t be a guide on how to correctly design pumps,
nor how to rebuild and repair pumps. There are existing books and
courses directed toward those themes. By understanding the real
reasons for pump failure, analyzing those failures, and diagnosing pump
behavior through interpretation of pressure gauges, you can achieve
Prologue
productive pump operation and contain maintenance costs. This book
will serve as a guide to STOP repairing industrial pumps.