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 [...]... 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... differential Pumps are also used to move liquids from a low elevation into a higher elevation, and to move liquids from one place to another Pumps are also used to accelerate liquids through pipes How do pumps work? The fluid arrives a t the pump 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... pathway increases, the rotary velocity decreases and even more energy Know and Understand Centrifugal Pumps and pressure is added to the liquid (again Bernoulli’s Principle) The liquid leaves the pump at discharge pressure, prepared to overcome the resistance in the system The flow from a centrihgal pump is mostly governed by the speed of the driver and the height of the impeller blades The pressure... 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 t o suck-up... the limits of the mechanical seal This 7 Know and Understand Centrifugal Pumps P - - I sp.gr = 1.25 Figure 1-4 _- sp.gr = 1.OO sp.gr = 0.75 ~~~ pressure is very important with double mechanical seals, because it governs the 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... the speed of the motor and the diameter of the impeller Other factors play a lesser role in the pump’s flow and pressure, like the number, pitch, and thickness of the impeller blades, the internal clearances, and the presence and condition of the wear bands In simple terms, we could say that PD pumps perform work by manipulating the available space inside the pump Centrihgal pumps perform work by manipulating... suction lift test is performed on the pump and the pressure in the suction vessel is lowered to the point where the pump suffers a 3% loss in total head This point is called the NPSHr of the pump Some pump manufacturers perform a similar test by closing a suction valve on a test pump and other manufacturers lower the suction elevation Know and Understand Centrifugal Pumps The definition of NPSHr may change... 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 to a shaft The shaft spins and is powered by the motor or driver We use the term driver because some pumps are attached to pulleys or transmissions The fluid enters into the eye of the impeller and is trapped... elevation; the pressure head (Hp) or the pressures to be overcome; the friction head (Hf) and velocity head (Hf), which are frictions and other resistances in the piping system These heads are discussed in Chapter 8 The head formula is the following: 5 Know and Understand Centrifugal Pumps Where: H = head P = psi d = density Pressure can be converted into head with the following equation: Head@ = 2.31 x Pressure... the Ha, we add the Hs 2 The Hs, static head, is the static height in feet observed from the level in the vessel to be drained to the centerline of the pump If the 15 Know and Understand Centrifugal Pumps - Properties o f water I Atmospheric and barometric pressure readinqs at different altitudes Altitude Barometric pressure Atmospheric pressure Feet Meters In Hg mm Hg Psia Feet water -1000 -500 -304.8 . 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. 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. displacement pumps Centrifugal pumps Conceptual difference Centrifugal volute pumps Types of centrifugal pumps Overhung impeller Impeller between the bearings Turbine pumps Specific duty pumps