1、 NOTICE OF COPYRIGHT This is a copyright document and may not be copied or distributed in any form or manner without the permission of ISA. This copy of the document was made for the sole use of the person to whom ISA provided it and is subject to the restrictions stated in ISAs license to that pers
2、on. It may not be provided to any other person in print, electronic, or any other form. Violations of ISAs copyright will be prosecuted to the fullest extent of the law and may result in substantial civil and criminal penalties. TECHNICAL REPORT ISA-TR75.04.01-1998 (R2006)Control Valve Position Stab
3、ilityApproved 29 November 2006ISA-TR75.04.01-1998 (R2006) Control Valve Position Stability ISBN: 978-0-9791330-2-2 0-9791330-2-5 Copyright 2006 by ISA. All rights reserved. Not for resale. Printed in the United States of America. No part of this publication may be reproduced, stored in a retrieval s
4、ystem, 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 - 3 - ISA-TR75.04.01-1998 (R2006) Preface This p
5、reface, as well as all footnotes and annexes, is included for information purposes and is not part of ISA-TR75.04.01-1998 (R2006). This document has been prepared as part of the service of ISA towards a goal of uniformity in the field of instrumentation. To be of real value, this document should not
6、 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
7、; Fax (919) 549-8288; E-mail: standardsisa.org. 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. The Department is
8、 further aware of the benefits to USA 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
9、revised standards, recommended practices, and technical reports 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 b) the static fluid pressure acting on the stem area of sliding-stem va
10、lves; 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 ISA-TR75.04.01-1998 (R2006) - 14 - e) frictional forces caused by packing
11、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 str
12、eam enters the valve; d) fluid forces fluctuate due to slug flow of a two-phase stream, downstream flashing, or cavitation; and e) the valve trims 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
13、gradients and potential instability. At the present stage in the development of control valve technology, this document 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
14、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. F
15、orce reversals are not uncommon in pressure-balanced designs. Careful consideration should be 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 p
16、osition 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
17、 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 document. 5 Measurement of position instability 5.1 Total control valve stem position mechanical stability is the total absence of valve stem movement
18、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
19、 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
20、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
21、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: Instability Percentage = travel Rated(100)amplitude) motion unsta
22、ble (Maximum- 15 - ISA-TR75.04.01-1998 (R2006) 5.3 Example: Rated Travel = 50 mm Unstable Motion Amplitude = 2.5 mm Instability Percentage = 550)100)(5.2(= 5.4 The instability percentage from the example does not imply any acceptable level. This page intentionally left blank. - 17 - ISA-TR75.04.01-1
23、998 (R2006) 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 standar
24、d 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 o
25、f 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 Torqu
26、e 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. Fi
27、rst, 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
28、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 mu
29、st 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. “Ana
30、lytical 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 ISA, 67 Alexander Drive, P.O. Box 12277, Research Triangle Park, NC 27709, Telephone: (919
31、) 549-8411, Fax: (919) 549-8288. ISA-TR75.04.01-1998 (R2006) - 18 - 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
32、 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 act
33、uator 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
34、 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 natur
35、e 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 valves moving parts, such as the plug of a sliding-stem valve, or the ball or disc of a rotary valve. In most cases, the appro
36、priate 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 si
37、ze 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
38、) 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
39、 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.
40、Gulland and A. F. Scott, Transactions Institute of Instrument Measurement Control, Vol. 3, No. 2, April-June 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 insta
41、lled 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, inst
42、ability 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 paper presents the stability anal
43、yses 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. - 19 - ISA-TR75.04.01-1998 (R2006) Abstract Stability
44、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 tec
45、hnique 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-op
46、posed 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. Stabilit
47、y 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 p
48、erformance 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
49、. 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. This page intentionally left blank. Developing and promulgating sound consensus standards, recommended practices, and technic
