How to Select the Right Centrifugal Pump: A Brief Survey of Centrifugal Pump Selection Best Practices

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© 2015 Robert X. Perez. All rights reserved.

No part of this book may be reproduced, stored in a retrieval system, or transmitted by any means without the written permission of the author.

Published by AuthorHouse  09/23/2015

ISBN: 978-1-5049-2267-8 (sc)

ISBN: 978-1-5049-2268-5 (hc)

ISBN: 978-1-5049-2266-1 (e)

Library of Congress Control Number: 2015911270

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Table of Contents

Dedication

Acknowledgement

Preface

Chapter 1 - Introduction to Centrifugal Pumps

Chapter 2 - Can I Use a Centrifugal Pump?

Chapter 3 - How Good Pumps Become Bad Actors

Chapter 4 - 10 Pump Selection Rules of Thumb

Chapter 5 - Bid Tabulations

Chapter 6 - The most important day in a pump’s lifetime

Chapter 7 - The Pump Selection Process

Appendix A - How to Calculate Pipe Friction Head Losses for Newtonian Fluids

Appendix B - Mechanical Seal Selection Primer

Appendix C - Centrifugal Pump Selection Checklist

Appendix D - Useful Conversions

Dedication

To my mother, Dolores Perez, for teaching me the importance of education and hard work.

Acknowledgement

The author would like to thank:

1) His wife, Elaine, for providing valuable feedback on the layout and format of the book and helping with editing.

2) Jacques Chaurette, Mechanical Engineer, for providing the material found iin Appendix A: How to Calculate Pipe Friction Head Losses for Newtonian Fluids

3) Julien LeBleu of Sage Technologies LLC for reviewing the manuscript and pointing out areas for improvement.

Preface

The goal of this short book is to provide those who need to select reliable and efficient centrifugal pumps for industrial applications with some practical tips and advice on how to evaluate the many pump design possibilities available on the market today.

The technical discussion presented here assumes that the reader has a basic understanding of centrifugal pump construction and terminology. My book, Operators Guide to Centrifugal Pumps is an excellent primer for those who feel the need for a basic review of centrifugal pump technology before tackling the topics in this book.

Choosing a centrifugal pump from the countless options available can be daunting, but someone has to make the decision. Many factors, such as the required flow, differential pressure, suction conditions, etc., must be weighed against the capital costs and cost of energy for the pumps considered. To determine the right pump, you must consider the overall cost of ownership, which includes capital cost, operating costs, and maintenance cost. What good is a low cost pump if it is inefficient or if is costly to maintain?

The selection methodology offered in this book focuses mainly on hydraulic design considerations, but also touches on mechanical design details. A large part of the book is devoted to key hydraulic deign parameters, such as specific speed (Ns) and suction specific speed (Nss), NSPHa, NPSHr, and NPSH margin ratio. Analyzing these basic hydraulic parameters allow you to quickly determine if a centrifugal pump makes sense for your particular application.

If you do decide a centrifugal pump will work for your application, then you need to be able to evaluate the various bids returned by pump manufacturers. Chapter 5 covers how to tabulate quotes from pump manufacturers in order to properly evaluate their bids and select the best overall option.

Using the advice in this book along with your company’s centrifugal pump guidelines, should ensure the selection of the optimum pump for your given application. Remember when in doubt, your company’s specifications along with current governing pertinent standards, such as API, PIP, ANSI, etc., should be considered the final word in the selection process.

I hope readers will find this book to be a useful addition to their technical libraries. My ultimate goal was to provide a valuable reference book that will be relevant for years to come.

Robert X. Perez

Chapter 1

Introduction to Centrifugal Pumps

Pumps are required throughout the process industry to move liquids from point A to point B, as shown in Figure 1.1 below where a pump takes suction flow from an overhead process vessel and pumps it into the process. Pump functions may involve moving liquid from a lower pressure to a higher pressure, lifting the liquid from a lower to a higher elevation or a combination of both. Liquid normally wants to move from a higher pressure to a lower pressure and from a higher level to a lower level. Getting liquid to move against its normal tendencies requires the addition of energy. That’s where pumps come in. Pumps are devices that transform mechanical energy into useful liquid energy.

Fig. 1.1 - A centrifugal pump takes suction from an overhead process vessel and pumps product into a process.

Centrifugal pumps are one of the simplest of all the pump designs. They have one moving part. The rotor has an impeller that accelerates liquid from its suction eye or inlet (see Figure 1.2) to a maximum speed at its outer diameter.

Figure 1.2- Centrifugal Pump Impeller and Volute

The liquid is then gradually decelerated to a much lower velocity in the stationary casing, called a volute casing. As it slows down, due to the increasing cross sectional area of the casing, pressure is developed, until full pressure is developed at the pump’s discharge. This simplicity of design and operation is what makes centrifugal pumps one of the most reliable of pump designs, assuming they are applied properly.

This process of converting velocity to pressure is similar to holding your hand outside of a moving automobile. As the high velocity air hits you, it slows down and pushes your hand back due to the pressure developed. Similarly, if you could insert your hand into the pump casing at the impeller exit and catch the liquid, you would feel the pressure produced by dynamic action of the impeller. When any high velocity stream slows down, pressure is created. (This effect is called Bernoulli’s Principle, which simply states that energy is always conserved in a fluid stream.) The greater that the impeller diameter or rpm is, the greater the exit velocity is and, therefore, the higher pressure developed at the pump’s discharge.

Another benefit of centrifugal pumps is that they can cover a wide range of hydraulic requirements. Thus, they can be used in a wide range of flow and pressure applications. They can easily provide flows from less than 10 gallons per minute (37.9 liters per minute) to well over 10,000 gallons per minute (gpm) (379 000 liters/minute). Centrifugal pump impellers can easily be staged, that is, arranged so that one impeller’s output is directed to a subsequent impeller; over 4500 pounds per square inch (psi) (31026 kPa) of pressure can be generated.

One key disadvantage of centrifugal pumps is that their efficiencies are usually less than that of positive displacement pumps. Whereas positive displacement pumps can deliver