ISA CNTRL VALVE PRIMER-2009 Control Valve Primer A User-s Guide (Fourth Edition).pdf

1、CONTROL VALVE PRIMERA Users GuideFourth EditionCONTROL VALVE PRIMERA Users GuideFourth EditionHans D. BaumannNoticeThe information presented in this publication is for the general education of the reader. Because neither the author nor the publisher have any control over the use of the information b

2、y the reader, both the author and the publisher disclaim any and all liability of any kind arising out of such use. The reader is expected to exercise sound professional judgment in using any of the information presented in a particular application.Additionally, neither the author nor the publisher

3、have investigated or considered the effect of any patents on the ability of the reader to use any of the information in a particular application. The reader is responsible for reviewing any possible patents that may affect any particular use of the information presented.Any references to commercial

4、products in the work are cited as examples only. Neither the author nor the publisher endorses any referenced commercial product. Any trademarks or tradenames referenced belong to the respective owner of the mark or name. Neither the author nor the publisher makes any representation regarding the av

5、ailability of any referenced commercial product at any time. The manufacturers instructions on use of any commercial product must be followed at all times, even if in conflict with the information in this publication.Copyright 2009 ISA The Instrumentation, Systems, and Automation SocietyAll rights r

6、eserved. Printed in the United States of America. 10 9 8 7 6 5 4 3 2ISBN-13: 978-1-934394-50-2No part of this work may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior writ-ten per

7、mission of the publisher.ISA67 Alexander DriveP.O. Box 12277Research Triangle Park, NC 27709Library of Congress Cataloging-in-Publication DataBaumann, Hans D., 1930- Control valve primer : a users guide / Hans D. Baumann. - 4th ed. p. cm. Includes bibliographical references. ISBN 978-1-934394-50-2 (

8、pbk.) 1. Pneumatic control valves-Handbooks, manuals, etc. I. Title. TJ223.V3B38 2008 629.8045-dc22 2008024257DEDICATION This book is dedicated to my wife, Sigrid, for her lifetime of support and to the many friends at ISA for their help and encouragement I received throughout more than forty years.

9、 viiTable of ContentsINTRODUCTION TO THE FOURTH EDITION XIFOREWORD XIIIABOUT THE AUTHOR XV1 WHAT IS A CONTROL VALVE AND HOW DOES IT AFFECT MY CONTROL LOOP? 1What to Look for in a Good Control Valve Design, 6References, 82 WHY NOT USE A SPEED-CONTROLLED PUMP? 9References, 103 WHAT VALVE TYPE SHALL I

10、CHOOSE? 11Selecting the Right Valve Type, 11Criteria for Valve Selection: A Summary, 164 THE SELF-ACTING REGULATOR, WHY NOT? 19References, 205 VALVE SIZING MADE EASY 21Valve Sizing, 21What are the Right Flow Conditions?, 25Pipe Reducers, 26Correcting for Viscosity, 28Sizing Control Valves for Viscou

11、s Fluids - An Explanation, 37Metric Units, 39What Size Valve to Choose, 40Adjustable Travel - Adjustable Cv, 42References, 456 SIZING AND SELECTIONLET THE COMPUTER DO IT ALL! 47Partial Tabulation of Vendors Offering Computer Programs for Control Valve Sizing and Selection, 497 WHAT ABOUT FAIL-SAFE?

12、51viii CONTROL VALVE PRIMER8 WHY MOST PEOPLE CHOOSE “EQUAL PERCENTAGE” AS A FLOW CHARACTERISTIC 53How to Calculate the “Installed” Flow Characteristic, 57References, 609 VALVE POSITIONERS 61When to Use Valve Positioners, 61What to Look for When Specifying Positioners or Transducers, 63Smart Position

13、ers Smart Valves, 65Accessories and Software, 69Connecting It All to the Control System, 70References, 7110 THE MYSTERY OF LINE PRESSURE-PRODUCED VALVE STEM FORCES, OR SELECTING THE CORRECT ACTUATOR SIZE 73The Valve Does Not Close Properly, 73Control Valve Data Sheet, 74The Valve Is Unstable, 76Refe

14、rences, 8011 HOW TO INSTALL A CONTROL VALVE 81Other Things That Can Go Wrong, 82References, 8312 HOW GOOD IS THE VALVE THAT I PURCHASED? 85References, 8913 WHEN DO I NEED TO “HARD FACE” THE VALVE TRIM AND OTHER QUESTIONS CONCERNING VALVE MATERIAL 91Erosion Caused by Solid Particles in the Fluid Stre

15、am or Granular Materials such as Coal Slurry, 91Cavitation, 92Erosion by Wet Steam, 92References, 9314 CONCERN FOR THE ENVIRONMENT 95Will My Valve Be Too Noisy?, 95Methods of Aerodynamic Valve Noise Reduction, 100How About Cavitation?, 106Noise Produced by Turbulence and Cavitation of Liquids, 111Av

16、oidance of Leaky Valve Stem Packings, 111Bellows Seals, 114References, 11515 SEAT LEAKAGE AND SEAT MATERIALS 117Table of Contents ix16 VALVES FOR SANITARY OR ASEPTIC SERVICE 121References, 12717 FIFTEEN COMMANDMENTS: WHAT YOU SHALL NOT DO! 12918 ELECTRIC VERSUS PNEUMATIC ACTUATORS 131References, 133

17、19 SAVING ENERGY 135References, 13620 THE BUS SYSTEM TO THE RESCUE, OR WHAT THE FUTURE MAY BRING 137The Coming Wireless Ethernet, 141Thinking “Green” and Saving Energy, 142References, 143Bibliography, 143APPENDIX A: REFERENCESTABLES AND FIGURES 145APPENDIX B: CONTROL VALVE STANDARDS AND RECOMMENDED

18、PRACTICES 155INDEX 159xiINTRODUCTION TO THE FOURTH EDITIONKeeping track of the ever evolving changes in industry, the fourth edition of theControl Valve Primer has been updated and significantly enlarged by adding thefollowing new or revised materials:enlarged chapter on digital positioners,increase

19、d coverage of DCS Systems and smart positioners,a new hydrodynamic and cavitation noise estimate,updated standards to reflect latest IEC versions,a new chapter on seat leakage and seat materials,additional illustrations, updated standards listing, andupdated Aerodynamic Noise equations.The Editorsxi

20、iiFOREWORD This book is intended primarily for those novice instrument engineers who, eventhough they have been crammed full of feedback theory from their college controlcourses and consider themselves experts in computer programing, are suddenlyfaced with the challenge of designing a control loop i

21、n a process plant. Here, theydiscover that, in addition to all the vaguely familiar electronic hardware such ascontrollers and transmitters, they also need a final control element. This turns outto be a cross between a piece of plumbing hardware and a pneumatic logic sys-tem. Such people are usually

22、 ill-equipped to answer the 30-plus entry questionson a typical control valve specification sheet in the allotted time of, say, 10 min-utes. Therefore, having a reference book at their disposal might make their jobeasier, and it might even help them to distinguish what is relevant when bom-barded wi

23、th conflicting claims from competing control valve vendors. At the veryleast, it may keep them from wasting valuable time by not having to wadethrough many hundreds of pages of scientifically worded handbook trivia. While it is true that any personal computer can handle complex sizing equa-tions, I

24、still feel it is useful to have some simple formulas that can be handled by apocket calculator if the need arises. This led me to propose short cuts in the sizingand noise equations. The reader is hereby cautioned that the opinions expressed herein are strictlythose of the writer, and no guarantee i

25、s given, nor implied, as to the accuracy ofthe books contents. As a matter of fact, some of the statements are considereddownright controversial. I am seldom accused of being unbiased. It all started when, as president of one of the smaller control valve companies,I visited a major tobacco company i

26、n Richmond, Virginia in order to do somesales promotion. However, after having carried about half a dozen valve samplesup to the second floor conference room, the leader of the group of young instru-ment engineers stated flatly, “Please no sales talk. Instead, answer some of myquestions.” As it turn

27、ed out, he liked the answers, and at the end of the sessionsuggested I write a book. Well, here it is. xivxvABOUT THE AUTHOR Hans D. Baumann received an industrial engineering education in his nativeGermany and studied under U.S. Government sponsorship at Western ReserveUniversity and later at North

28、eastern University, culminating in a Ph.D. inMechanical Engineering from Columbia Pacific University. In addition, he isregistered as a Professional Engineer in four states. During his professional career,he personally designed or directed the development of over 28 valve lines. One ofthem, the famo

29、us “CAMFLEX” valve and its derivations, is produced in eightcountries where over three million units have been sold. He is credited with over150 U.S. and worldwide patents and has published 115 papers and articles inaddition to co-authoring seven handbooks on valves, instrumentation, andacoustic. He

30、 also was named by InTech magazine one of fifty most importantinnovators and wrote the acclaimed business book The Ideal Enterprise.Prior to founding his own control valve manufacturing company in 1977, hewas an International Consultant, Corporate Vice-President of Masoneilan-Inter-national, Inc., a

31、nd Manager of R he was named Honorary Mem-ber of the Spanish Chemical Engineering Society, and Honorary Life Member ofthe Fluid Controls Institute. His many valve designs have been honored with agold medal from Germany, prizes from France and Japan, and seven U.S. “Vaaler”awards. He is a member of S

32、igma Xi, the scientific research society.11WHAT IS A CONTROL VALVE AND HOW DOES IT AFFECT MY CONTROL LOOP? Control valves may be the most important, but sometimes the most neglected,part of a control loop. The reason is usually the instrument engineers unfamiliar-ity with the many facets, terminolog

33、ies, and areas of engineering disciplines suchas fluid mechanics, metallurgy, noise control, and piping and vessel design thatcan be involved depending on the severity of service conditions. Any control loop usually consists of a sensor of the process condition, a trans-mitter, and a controller that

34、 compares the “process variable” received from thetransmitter with the “set point,” i.e., the desired process condition. The controller,in turn, sends a corrective signal to the “final control element,” the last part of theloop and the “muscle” of the process control system. While the “sensors” of t

35、heprocess variables are the eyes, the “controller” the brain, then the “final controlelement” is the hands of the control loop. This makes it the most important, alassometimes the least understood, part of an automatic control system. This comesabout, in part, due to our strong attachment to electro

36、nic systems and computerscausing some neglect in the proper understanding and proper use of the allimportant hardware.Control valves are the most common type of final control elements; however,there are other types such as: devices that regulate (throttle) electric energy such as silicon-controlled

37、rectifiers, variable speed drives, feeders, pumps, and belt drives, and dampers. Some of these devices perform functions similar to control valves and couldbe used as an alternative. For example, in order to control the pH level, a variable2 CONTROL VALVE PRIMERstroke-type metering pump may be used

38、to inject acid into wastewater (instead ofusing a control valve lined with polytetrafluorethylene PTFE). What then is a control valve? This is a difficult question since there is consid-erable overlap with other types of valves. For example, a valve operating strictlyin the on-off mode (such as the

39、hydronic solenoid valve in your home heating sys-tem) could be replaced by a simple ball valve operated by a pneumatic cylinder, atype usually referred to as an “automated valve.” The distinction between “automated” and “control” valves is usually consid-ered to be the ability of the latter to “modu

40、late,” i.e., to assume an infinite numberof “throttling” travel positions during normal control service. Physically, there are three basic components of a control valve: The valve body subassembly. This is the working part and, initself, a pressure vessel. The actuator. This is the device that posit

41、ions the throttling ele-ment inside the valve body. Accessories. These are positioners, I/P transducers, limitswitches, handwheels, air sets, position sensors, solenoidvalves, and travel stops. A more detailed breakdown of the various types of valves, actuators, andpositioners is shown in Figure 1-1

42、. Now lets discuss what the control valve should do. Referring to Figure 1-2,which shows a somewhat simplified process control loop diagram, we see threeimportant function blocks above the control valve symbol. The first one is control valve gain, Kv. This is determined by the “installedflow charact

43、eristic” of the valve (quite different from the characteristic shown inthe vendors catalog). Kv tells you how much the flow through the valve is chang-ing per a given signal change. For input see Chapter 8. The second block shows the control valve dead time, TDv. This is the time ittakes before a va

44、lve moves following a controller signal change. This is usuallydetermined by the valve and actuator friction but may include time lags due tolong pneumatic signal transmission lines and the time to build the pressure up ina diaphragm case, for example. Finally, the third block shows the time constan

45、t of the valve, TCv. This is sim-ply related to the stroking speed of the actuator or actuator/positionercombination (see Chapter 9), i.e., how fast the valve is responding to an upset inyour controlled variable. All these function blocks interact, and each one shouldbe considered in evaluating a co

46、ntrol valve application. The “ideal” valve should have a constant gain throughout the flow range, i.e.,a linear “installed” flow characteristic, no dead time with packing tightened, anda time constant that is different from that of the process by at least a factor ofthree. Need I tell you that there

47、 is no such thing as the “ideal” control valve? So letsattempt to develop a workable compromise. WHAT IS A CONTROL VALVE AND HOW DOES IT AFFECT MY CONTROL LOOP? 3Figure 1-1. Basic control valve terminology from ANSI/ISA-75.05.01-2000 (R2005). 4 CONTROL VALVE PRIMERFigure 1-1. (continued)WHAT IS A CO

48、NTROL VALVE AND HOW DOES IT AFFECT MY CONTROL LOOP? 5Figure 1-2. Schematic block diagram of controller, control valve, and process in a control loop. 6 CONTROL VALVE PRIMERWHAT TO LOOK FOR IN A GOOD CONTROL VALVE DESIGNBesides the obvious, such as good quality workmanship, correct selection of mate-

49、rials, noise emission, etc., special attention should be paid to two areas: Low dead band of the actuator/valve combination (with tightpacking). Tight shutoff, in cases of single-seated globe valves and somerotary valves (if required). The prime concern of an operator of a process control loop is to have a loopthat is stable. (Nothing makes people more nervous than a lot of red ink and scat-tered lines on a strip of paper from a recorder.) The final control element willinfluence the stability of a loop more than all the other control elements com-bined. The biggest culpr