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TECHNICAL REPORT
ANSI/ISA –TR75.04.01 –1998
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Control Valve Position Stability
Approved 15 July 1999 TM
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ANSI/ISA – TR75.04.01 TR75.04.01 –1998, Control Valve Position Stability ISBN: 1-55617-678-3
Copyright © 1998 by the Instrument Society of America. All rights reserved. Printed in the United States of America. No part of this publication may be reproduced, stored in a retrieval r etrieval system, or transmitted in any form or by any means (electronic, mechanical, photocopying, recording, or otherwise), without the prior written permission of the publisher. ISA 67 Alexander Drive P.O. Box 12277 Research Triangle Park, North Carolina 27709
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Preface
This preface, as well as all footnotes and annexes, is included for information purposes and is not part of ANSI/ ISA-TR75.04.01-1998. This technical report has been prepared as part of the service is ISA, the international society for measurement and control, toward a goal of uniformity in the field of instrumentation. To be of real value, this document should not be static, but should be subject to periodic review. Toward this end, the Society welcomes all comments and criticisms, and asks that they be addressed to the Secretary, Standards and Practices Board; ISA; 67 Alexander Drive; P.O. Box 12277; Research Triangle Park, NC 27709; Telephone (919) 549-8411; Fax (919) 549-8288; Email:
[email protected]. The ISA Standards and Practices Department is aware of the growing need for attention to the metric system of units in general, and the International System of Units (SI) in particular, in the preparation of instrumentation standards, recommended practices, and technical reports. The Department is further aware of the benefits to U.S. users of ISA standards of incorporating suitable references to the SI (and the metric system) in their business and professional dealings with other countries. Toward this end, this Department will endeavor to introduce SI-acceptable metric units in all new and revised standards to the greatest extent possible. Standard for Use of the International System of Units (SI): The Modern Metric System, published by the American Society for Testing and Materials as IEEE/ASTM SI 10-97, and future revisions, will be the reference guide for definitions, symbols, abbreviations, and conversion factors. It is the policy of the ISA to encourage and welcome the participation of all concerned individuals and interests in the development of ISA standards, recommended practices, and technical reports. Participation in the ISA standards-making process by an individual in no way constitutes endorsement by the employer of that individual, of ISA, or of any of the standards, recommended practices, and technical reports that ISA develops. CAUTION—ISA ADHERES TO THE POLICY OF THE AMERICAN NATIONAL STANDARDS INSTITUTE WITH REGARD TO PATENTS. IF ISA IS INFORMED OF AN EXISTING PATENT THAT IS REQUIRED FOR USE OF THE STANDARD, IT WILL REQUIRE THE OWNER OF THE PATENT TO EITHER GRANT A ROYALTY-FREE LICENSE FOR USE OF THE PATENT BY USERS COMPLYING WITH THE STANDARD OR A LICENSE ON REASONABLE TERMS AND CONDITIONS THAT ARE FREE FROM UNFAIR DISCRIMINATION. EVEN IF ISA IS UNAWARE OF ANY PATENT COVERING THIS STANDARD, THE USER IS CAUTIONED THAT IMPLEMENTATION OF THE STANDARD MAY REQUIRE USE OF TECHNIQUES, PROCESSES, OR MATERIALS COVERED BY PATENT RIGHTS. ISA TAKES NO POSITION ON THE EXISTENCE OR VALIDITY OF ANY PATENT RIGHTS THAT MAY BE INVOLVED IN IMPLEMENTING THE STANDARD. ISA IS NOT RESPONSIBLE FOR IDENTIFYING ALL PATENTS THAT MAY REQUIRE A LICENSE BEFORE IMPLEMENTATION OF THE STANDARD OR FOR INVESTIGATING THE VALIDITY OR SCOPE OF ANY PATENTS BROUGHT TO ITS ATTENTION. THE
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USER SHOULD CAREFULLY INVESTIGATE RELEVANT PATENTS BEFORE USING THE STANDARD FOR THE USER’S INTENDED APPLICATION. HOWEVER, ISA ASKS THAT ANYONE REVIEWING THIS STANDARD WHO IS AWARE OF ANY PATENTS THAT MAY IMPACT IMPLEMENTATION OF THE STANDARD NOTIFY THE ISA STANDARDS AND PRACTICES DEPARTMENT OF THE PATENT AND ITS OWNER. ADDITIONALLY, THE USE OF THIS STANDARD MAY INVOLVE HAZARDOUS MATERIALS, OPERATIONS, OR EQUIPMENT. THE STANDARD CANNOT ANTICIPATE ALL POSSIBLE APPLICATIONS OR ADDRESS ALL POSSIBLE SAFETY ISSUES ASSOCIATED WITH USE IN HAZARDOUS CONDITIONS. THE USER OF THIS STANDARD MUST EXERCISE SOUND PROFESSIONAL JUDGMENT CONCERNING ITS USE AND APPLICABILITY UNDER THE USER’S PARTICULAR CIRCUMSTANCES. THE USER MUST ALSO CONSIDER THE APPLICABILITY OF ANY GOVERNMENTAL REGULATORY LIMITATIONS AND ESTABLISHED SAFETY AND HEALTH PRACTICES BEFORE IMPLEMENTING THIS STANDARD.
The following people served as members of ISA Subcommittee SP75.04: NAME
COMPANY
J. Reed, Chairman W. Weidman, Managing Director G. Baenteli G. Barb K. Black J. Borge W. Caudill *R. Lytle J. McCaskill *P. Schafbuch A. Shea
Norriseal Parsons Energy & Chemicals Group, Inc. Bechtel Corporation Consultant Cashco, Inc. Neles Controls, Inc. Arco Products Company Fisher Controls International, Inc. TAPCO International Fisher Controls International, Inc. Copes-Vulcan, Inc.
____________________________ *One vote per company
The following people served as members of ISA Committee SP75: NAME
COMPANY
*D. Buchanan, Chairman W. Weidman, Managing Director T. Abromaitis J. Addington H. Backinger G. Baenteli G. Barb H. Baumann
Union Carbide Corporation Parsons Energy & Chemicals Group, Inc. Red Valve Company, Inc. Fluid Controls Institute J. F. Kraus & Company Bechtel Corporation Consultant H. D. Baumann, Inc.
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K. Black H. Boger G. Borden, Jr. S. Boyle *R. Brodin F. Cain C. Corson *C. Crawford L. Driskell J. Duhamel A. Engels H. Fuller *J. George M. Glavin L. Griffith F. Harthun B. Hatton J. Jamison R. Jeanes J. Kersh C. Koloboff G. Kovecses C. Langford A. Libke R. Louviere O. Lovette, Jr. L. Mariam J. McCaskill A. McCauley, Jr. R. McEver H. Miller T. Molloy L. Ormanoski J. Ozol W. Rahmeyer J. Reed *G. Richards *M. Riveland K. Schoonover *A. Shea E. Skovgaard H. Sonderegger R. Terhune *R. Tubbs D. Wolfe ____________________________
Cashco, Inc. Masoneilan/Dresser Consultant Neles Controls, Inc. Fisher Controls International, Inc. Flowserve-FCD Fluor Daniel, Inc. Union Carbide Corporation Consultant R-K-L Praxair, Inc. Valvcon Corporation Richards Industries, Inc. Grinnel Corporation Four G Group Consultant Honeywell, Inc. Bantrel, Inc. TU Electric M. W. Kellogg Company Consultant Yarway Corporation Cullen G. Langford, Inc. DeZurik Valve Company Creole Engineering Sales Company Consultant FlowSoft, Inc. TAPCO International Chagrin Valley Controls, Inc. Bettis Corporation Control Components, Inc. CMES, Inc. Frick Company Commonwealth Edison Utah State University Norriseal Richards Industries, Inc. Fisher Controls International, Inc. Con-Tek Valves, Inc. Copes-Vulcan, Inc. Leslie Controls, Inc. Grinnell Corporation Cranmoor Copes-Vulcan, Inc. Agren-Ascher Company, Inc.
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*One vote per company
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This technical report was approved for publication by the ISA Standards and Practices Board on July 15, 1998. NAME
COMPANY
R. Webb, Vice-President H. Baumann D. Bishop P. Brett W. Calder III M. Cohen H. Dammeyer W. Holland H. Hopkins A. Iverson K. Lindner V. Maggioli T. McAvinew A. McCauley, Jr. G. McFarland E. Montgomery D. Rapley R. Reimer J. Rennie W. Weidman J. Weiss J. Whetstone M. Widmeyer R. Wiegle C. Williams G. Wood M. Zielinksi
Altran Corporation H. D. Baumann, Inc. Chevron Production Technology Honeywell, Inc. Calder Enterprises Senior Flexonics The Ohio State University Southern Company Services, Inc. Consultant Ivy Optiks Endress + Hauser GmbH + Company Feltronics Corporation Instrumentation & Control Engineering LLC Chagrin Valley Controls, Inc. Honeywell, Inc. Fluor Daniel, Inc. VECO Rapley, Inc. Rockwell Automation A-B Factory Mutual Research Corporation Parsons Energy & Chemicals Group, Inc. Electric Power Research Institute National Inst. of Standards & Technology Consultant CANUS Corporation Eastman Kodak Company Graeme Wood Consulting Fisher •Rosemount Systems, Inc.
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Foreword
This Technical Report discusses control valve stem position mechanical stability, establishes a measurement criterion for position instability and provides a bibliography of published papers.
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Abstract
This document is intended to help the user recognize, measure, and diagnose the unstable stem motion of a valve.
Key Words
Control valve stem position mechanical stability, unstable motion, maximum amplitude, design of the valve closure member, pressure-balancing, deadband, hysteresis, position instability, fluid forces, actuator forces, control signal, force gradient, pressure balanced.
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Contents
1 Scope ........................................................................................................................ 11 2 Purpose..................................................................................................................... 11 3 Definitions ................................................................................................................ 11 4 Discussion ................................................................................................................ 11 5 Measurement of position instability ...................................................................... 13 Annex A — References .............................................................................................. 15
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1 Scope
This Technical Report discusses control valve stem position mechanical stability and establishes a measurement criterion for position instability of the valve. Other forms of instability associated with control valves and control systems are not covered.
2 Purpose
This document is intended to help the user recognize, measure, and diagnose the unstable motion of a valve. A reference section (Annex A) with abstracts provides further references.
3 Definitions
3.1 position instability: Is evidenced by uncontrolled fluctuating valve travel. It is caused by the fluid forces interacting with the actuator forces. It is a persistent cyclic motion inconsistent with control signal to the valve. It is not a static deviation caused by dead band or hysteresis.
3.2 control loop instability: Is a regular oscillation of a feedback control system caused by excessive loop gain. It is independent of external disturbances.
3.3 flow rate instability (bistable flow): Is an abrupt change in the control valve flow rate that occurs independent of changes in valve position. It may be caused by variable wall attachment of the fluid stream at the valve orifice, by flashing, or by cavitation.
3.4 hunting: Is a continuing cyclic motion caused by friction, with the positioner or controller attempting to find the set position.
4 Discussion
4.1 Position instability, as defined in paragraph 3.1, may occur when the immediate force-totravel gradient associated with the action of the flowing fluid on the moveable valve trim ANSI/ISA-TR75.04.01-1998
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overcomes the stiffness of the actuator, particularly on flow-to-close valves. Electromechanical and hydraulic actuators, because they are inherently stiff, are rarely subject to position instability unless there is mechanical backlash. Pneumatic actuators that depend upon a compressible fluid are more susceptible to position instability. However, the stiffness of pneumatic actuators varies greatly according to actuator design and application. Mechanical spring rate, actuator gas density (pressure), and actuator tare (clearance) volume all contribute to pneumatic actuator stiffness.
4.2 An analysis of forces includes the following: a) the differential fluid pressure acting across the effective unbalanced area of the valve closure member; b) the static fluid pressure acting on the stem area of sliding-stem valves; c) buffeting forces associated with the fluid velocity, such as vortex shedding, impact, turbulence, cavitation, and flashing; d) the actuator spring(s), mechanical or pneumatic, and the opposing pneumatic pressure; and ` ` ` , ` ` ` ` , , ` , , ` , ` , , ` -
e) frictional forces caused by packing and other mechanical interfaces.
4.3 The fluid forces tend to promote instability when a) the pressure differential fluctuates or changes in a manner to overcome or reinforce the actuator force; b) the effective unbalanced area of the valve trim changes abruptly; c) a variable density multiphase stream enters the valve; d) fluid forces fluctuate due to slug flow of a two-phase stream, downstream flashing, or cavitation; and e) the valve trim’s pressure balancing port senses a pressure spike inconsistent with the average pressure on the trim.
4.4 Several methods can be used to analyze the force gradients and potential instability. At the present stage in the development of control valve technology, this Technical Report endorses no single method of stability analysis but includes references and abstracts that may be used as guides.
4.5 The design of the valve closure member and pressure-balancing flow passages can influence its vulnerability to unstable operation. Closure members designed for full or partial pressure balancing can be especially susceptible to instability, due to the amount of fluid force variance being a high fraction of the low normal force from the pressure differential. Force reversals are not uncommon in pressure-balanced designs. Careful consideration should be 12
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given to ensure that pressure-balanced closure members have well-averaged pressure distribution on the effective surfaces.
4.6 Several factors unrelated to fluid flow may cause inconsistency between valve position and the command to the actuator. Dead band, for example, can be created by backlash or friction in the valve or actuator. Hysteresis affects valve position according to the direction of travel. Both dead band and hysteresis cause the valve position to lag the signal. Hysteresis and dead band have not been found to cause position instability but can cause control loop instability. This type of loop instability is beyond the scope of this Technical Report.
5 Measurement of position instability
5.1 Total control valve stem position mechanical stability is the total absence of valve stem movement when the signal to the actuator is constant. Position instability, that is, valve stem movement, is not an absolute phenomenon. It occurs in many control valves to some degree. In most applications where it exists, it is not noticeable or does not exceed the acceptable limit for the application. The acceptable level of instability is a subjective quantity and varies with the application. There is a need for a quantitative method to describe instability as it exists or to specify an acceptable performance level. Though unstable motion can be described in terms of amplitude, frequency, and wave form, the following rating system applies only to amplitude. Frequency and wave form are not considered relevant to this measurement.
5.2 Measure the maximum amplitude of the unstable motion when the signal to the actuator is constant. The amplitude measurement technique may be that which is deemed appropriate for the application, such as a linear scale, dial-indicator, or motion transducer. To determine the instability percentage, use the following equation to calculate the percentage of the rated valve travel that is unstable: (Maximum unstable motion amplitude)(100) Instability Percentage = ------------------------------------------------------------------------------------------------------------------Rated travel
5.3 EXAMPLE: Rated Travel
= 50 mm
Unstable Motion Amplitude
= 2.5 mm
Instability Percentage
( 2.5 ) ( 100 ) = ---------------------------- = 5 50
5.4 The instability percentage from the example does not imply any acceptable level.
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Annex A — References
The following references contain abstracts (listed by date of publication) * to serve as a guide to specific areas of interest: “Selecting Spring Spans for Control Valve Actuators” by J. T. Muller, Fluid Controls Institute, 1965. Abstract The problem of specifying standard 3-15 and 3-27 (sometimes referred to as 6-30) spring ranges for control valves, between user and manufacturer of control valves, has been the cause of much confusion and discussion. The confusion is caused by the lack of proper understanding of the difference in variable stem thrust requirements of unbalanced and so-called semi-balanced valves. The following, prepared for the Engineering Standards Committee of the Control Valve Section of the Fluid Controls Institute, Inc., is an attempt to give a simple understanding of the problem and the solution. “Effect of Fluid Compressibility on Torque in Butterfly Valves” by Floyd P. Harthun, ISA Transactions, Vol. 8, No. 4 (1969), pp. 281-286. Abstract A technique is presented by which the shaft torque resulting from fluid flow through butterfly valves can be determined with reasonable accuracy for both compressible and incompressible flow. First, the general torque relationship for incompressible flow is established. Then, an effective pressure differential is defined to extend this relationship to include the effect of fluid compressibility. The application of this technique showed very good agreement with experimental results. “Valve Plug Force Effects on Pneumatic Actuator Stability” by Richard F. Lytle, Advances in Instrumentation , Vol. 25, Part 3 (1970), paper no. 70-765. Abstract A study of valve plug forces and the effects of these forces on the dynamic stability of pneumatic actuators shows that actuator sizing criteria must include total dynamic stiffness of the installed valve-actuator system along with static thrust requirements. Buffeting forces and negative plug force gradients are described. Frequency response techniques are used to develop actuator stability criteria based on installed actuator stiffness. “Analytical Predication of Valve Stability” by Gareth A. Keith, Advances in Instrumentation , Vol. 25, Part 4 (1970), paper no. 70-838.
* Complete copies of the ISA copyrighted papers listed here are available from the ISA, 67 Alexander Drive, P.O. Box 12277, Research Triangle Park NC 27709, Telephone: (919) 549-8411, Fax: (919) 549-8288. --```,``-`-`,,`,,`,`,,`---
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Abstract Valve stability under widely varying operating conditions is one of the many concerns of control valve application. The mathematical analysis developed to determine the unbalanced forces includes the influences of the ratio of valve pressure drop to total system pressure drop in addition to valve unbalanced area, flow characteristic, and varying plug position. The valve rate of change of unbalanced forces is then determined and compared to the rate of change of actuator forces. Valve stability is achieved when the actuator rate of change of force exceeds the rate of change of forces acting on the valve plug. The mathematical analysis is confirmed by laboratory test data. This approach has resulted in a practical analytical method to determine valve stability when controlling gas or liquid during subcritical flow conditions. “Understanding Fluid Forces in Control Valves” by Charles B. Schuder, Instrumentation Technology: Journal of the Instrumentation Society of America, Vol. 18, No. 5 (May 1971), pp. 48-52. Abstract To minimize field problems arising from fluid forces, it is necessary to identify the nature of these forces and then to relate them to valve service conditions. Eleven types of fluid reaction forces have been identified and described here. These forces act on the valve’s moving parts, such as the plug of a sliding-stem valve, or the ball or disc of a rotary valve. In most cases, the appropriate service limitation is differential pressure and not fluid velocity or hydraulic horsepower. “Problems of Undersized Actuators” by C. E. Wood and A. R. Nenn, presented at the ISA/72 Conference, October 9-12, 1972, New York City. Abstract (prepared by Committee) The selection of the type and size of the actuating device is to be determined by the control valve manufacturer. In order for the manufacturer to determine the power requirements of the individual control valve actuators, the following data are supplied on the project specification: (1) flowing quantity, (2) upstream pressure, (3) downstream pressure, (4) specific gravity of fluid, (5) flowing temperature, and (6) control valve size. These data are calculated data and not measured data such as would be available from an operating unit. They are, however, close enough to the final operating numbers to allow a manufacturer to make a reasonable estimation of power requirements. A conclusion drawn from mathematical evaluation was that the rate of change of spring force should be at least twice as large as the rate of change of stem force. “Hammering Control Valves - Diagnosis and Solution of a Stability Problem” by W. G. Gulland and A. F. Scott, Transactions Institute of Instrument Measurement Control, Vol. 3, No. 2, AprilJune 1981. (This abstract is reproduced with the permission of the Institute of Measurement and Control, 87 Gower St., London, WC1E 6AA, England.) Abstract Plug-type control valves are often installed in the flow-to-close close-on-air failure configuration. In this configuration, it is possible for the valve to become unstable even though the actuator can generate sufficient force, in the steady state, to overcome the forces opposing it. For a valve that is not fitted with a positioner, instability will occur if the curve-of-equilibrium valve-actuator pressure against lift is not monotonically increasing. If a positioner is fitted, instability will occur if the curve-of-equilibrium mass of air in the valve actuator is not monotonically increasing. This
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paper presents the stability analyses for both cases. It outlines areas where instability may occur and suggests a variety of solutions. “Control Valve and Process Stability” by Gayle E. Barb, Advances in Instrumentation , Vol. 37, Part 3 (1982), paper no. 82-901, pp. 1277-1298. Abstract Stability is defined and a technique is presented for determining stable operation of a spring opposed pneumatic-actuated control valve in a relationship with the process that is being controlled. All the information required to test for stability is not available to the valve industry. The novelty of the technique lies in the use of a programmable calculator to “crunch” all the data into two values and make a simple stability test. “Actuator Selection” by Gayle E. Barb, Advances in Instrumentation , Vol. 39, Part 2 (1984), paper no. 84-780, pp. 1319-1332. Abstract Actuator selection when using spring-opposed pneumatic diaphragm and piston actuators involves the unique combination of many variables. Analyses of many combinations are made showing the resultant direction of force action from the variables considered. Principal forces related to the process, valve, and actuator are developed. Stability criteria are also presented. “Fluid Inertia Effects on Unbalanced Valve Stability,” by Paul J. Schafbuch, Final Control Elements , proceedings of the ISA Final Control Elements Symposium held April 9-11, 1985, New Orleans, Louisiana, paper no. 85-207, pp. 31-48. Abstract Stability is an important performance consideration for control valves. One requirement for stability is that actuator stiffness should exceed the magnitude of negative plug force gradients. This study shows fluid inertia to greatly affect dynamic gradients for certain valves and at buffeting (high) frequencies, in particular. A rigorous mathematical expression for unbalanced, stem-guided valves is derived from the Joukowsky water hammer relation. This expression explains why high-stiffness piston actuators are usually necessary for unbalanced flowdown valves in liquid service. Previous works do not explain this observation except in a loose, qualitative fashion. The full expression is simplified to a practical actuator sizing guideline. Experimental results are also cited.
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