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 xi xlll 1 1 1 4 4 4 5 5 5 6 6 7 12 12 13 14 24 24 24 25 25 26 35 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 37 37 39 39 39 42 42 44 44 46 48 48 50 51 51 51 54 55 56 57 57 57 58 58 58 59 60 61 62 63 64 65 66 66 70 71 71 72 73 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 76 76 77 77 78 80 81 82 85 85 92 92 92 96 96 97 110 110 113 113 116 116 117 120 122 123 124 125 128 128 133 136 137 140 141 vi i [...]... This periodic servicing is actually a production function to return the pump to its best or original efficiency Centrifugal pumps also require that the fluid be available to the pump’s suction nozzle with sufficient energy Centrifugal pumps cannot suck or draw the liquid into the pump housing The principal pumping unit of a centrifugal pump is the volute and impeller (See Figure 1-3) The impeller is attached... Basic Pump Principles ATMOSPHERIC DISCHARGE HEAD I I I N 1 C M Y - - .^ Figure 1-7 push the liquid up into the discharge vessel This pump (Figure 1-7) is said to be in suction lift In this case, the pump must aspirate or lift the liquid up from the suction vessel into the pump and then push the liquid up into the discharge vessel In this case the total head is the discharge head plus the suction lift... curve It varies by design, size, and the operating conditions It is determined by a lift test, producing a negative pressure in inches of mercury and converted into feet of required NPSH An easy way to understand NPSHr is t o call it the minimum suction pressure necessary t o keep the pumped fluid in a liquid state L I According to the Standards of the Hydraulic Institute, a suction lift test is performed... atmospheric pressure) and the term TORR This conglomeration o f values and conversion rates causes confusion In order t o understand pumps, it‘s best t o think o f vacuum as a positive number less than 14.7 psi In our experience, we’ve found that considering vacuum in this form aids the understanding o f net positive suction head (NPSH), cavitation, suction specific speed (Nss), and the ability o f pumps. .. 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... suction nozzle as it flows through the Know and Understand Centrifugal Pumps / I \\ r suction piping The fluid must be available to the pump with sufficient energy so that the pump can work with the fluid’s energy The pump cannot suck on or draw the liquid into the pump The concept of the fluid being available to the pump is discussed in detail in Chapter 2 of this book Positive displacement (I D) pumps. .. Pump and Motor Alignment Introduction Types of misalignment Alignment techniques Equipment alignment sequence Coupling alignment 142 142 144 145 153 153 11 Bearings Introduction Bearing lubrication Bearing failure Bearing maintenance Bearing seals 155 12 Pump Shaft Packing History Vegetable fibers Reciprocating action Packing Rotary action Synthetic fibers Compression packing The lantern ring The packing... Analysis Pump maintenance files Failure analysis on centrifugal pumps Why is this pump in the shop? 226 226 227 228 16 Avoiding Wear in Centrifugal Pumps Introduction Erosion Corrosion Wear rings Fluid velocity accelerates wear Turbulence Throttling 232 232 232 233 233 235 235 236 17 Pump Piping Introduction Piping design to drain tanks and sumps The Submergence Laws 238 238 238 243 248 Index ix Prologue... pressure setting of the barrier fluid Head versus pressure Figures 1 4 and 1-5 show the relationship between head and pressure in a centrifugal pump moving liquids with different specific gravities There is more on this in Chapter 7 The above graphic shows three identical pumps, each designed to develop 92.4 feet of head When they pump liquids of different specific gravities, the heads remain the same,... to inventions, tools and devices used in the chemical process industry H e is an active member of American Society of Mechanical Engineers (ASME) and writes a column called ‘The Pump Guy’ in Energy Tech Magazine About the Authors Angel Custodio I c Angel specializes in the installation and implementation of Preventive Maintenance Systems through his consulting engineering company formed in 1987 in . Cataloguing in Publication Data Bachus, Larry Know and understand centrifugal pumps 1 .Centrifugal pumps I. Title II.Custodio, Angel 62 1.6’7 ISBN. 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