1、 AMERICAN NATIONAL STANDARD ANSI/ISA-75.25.01-2000 (R2010) Test Procedure for Control Valve Response Measurement from Step Inputs Approved 16 June 2010 ISA-75.25.01-2000 (R2010) Test Procedure for Control Valve Response Measurement from Step Inputs ISBN: 978-1-936007-49-3 Copyright 2010 by the Inter
2、national Society of Automation. 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 system, or transmitted, in any form or by any means (electronic, mechanical, photocopying, recording, or otherwise), with
3、out the prior written permission of the Publisher. ISA 67 Alexander Drive P. O. Box 12277 Research Triangle Park, North Carolina 27709 USA - 3 - ISA-75.25.01-2000 (R2010) Preface This preface, as well as all footnotes and annexes, is included for information purposes and is not part of ISA-75.25.01-
4、2000 (R2010). The standards referenced within this document may contain provisions which, through reference in this text, constitute requirements of this document. At the time of publication, the editions indicated were valid. All standards are subject to revision, and parties to agreements based on
5、 this document are encouraged to investigate the possibility of applying the most recent editions of the standards indicated within this document. Members of IEC and ISO maintain registers of currently valid International Standards. ANSI maintain registers of currently valid U.S. National Standards.
6、 This document has been prepared as part of the service of the International Society of Automation (ISA), 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 welcom
7、es 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; E-mail: standardsisa.org. The ISA Standards and Practices Departme
8、nt 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 further aware of the benefits to USA users of ISA standards of incorporating suitable r
9、eferences 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, recommended practices, and technical reports to the greatest extent p
10、ossible. Standard for Use of the International System of Units (SI): The Modern Metric System, published by the American Society for Testing Gibson, J. E. “Nonlinear Automatic Control,“ McGraw-Hill, 1963. Not the same as in ISA-51.1-1979 (R1993), linearity: the closeness to which a curve approximate
11、s a straight line. 3.13 overshoot: the amount by which a step response exceeds its final steady state value. Refer to Figure 24 of ISA-51.1-1979 (R1993). Usually expressed as a percentage of the full change in steady state value. 3.14 position Z: the position of the closure member relative to the se
12、ated position. In this standard and ISA-TR75.25.02-2000 (R2010), expressed as a percent of span. 3.15 resolution: smallest step increment of input signal in one direction for which movement of the output is observed. Resolution is expressed as percentage of input span. The term in this document mean
13、s: the tendency of a control valve to move in finite steps in responding to step changes in input signal applied in the same direction. This happens when the control valve sticks in place, having stopped moving after the previous step change. ISA-75.25.01-2000 (R2010) - 14 - 3.16 response: the time
14、history of a variable after a step change in the input. In this standard and ISA-TR75.25.02-2000 (R2010), the step response can be stem position, flow, or another process variable. 3.17 response flow coefficient vRC :* apparent flow coefficient as determined by testing in an operating type environme
15、nt. The data available in the operating environment may differ from the laboratory data required by valve sizing standards. 3.18 response gain ZG : the ratio of the steady state magnitude of the process change Z divided by the signal step s that caused the change. One special reference response gain
16、 is defined as that calculated from the 2 percent step size response time test. This is designated as Z02G . ZG = Z / s Z02G = 02Z / 02s 3.19 sampling interval st : the time increment between sampled data points. It is the inverse of the sampling rate, 0s0f/1tf = . As used in this standard and ISA-T
17、R75.25.02-2000 (R2010), since more than one variable is being sampled, it is the time between the sets of sampled data. Ideally, all variables in one set are sampled at the same time. If data is recorded using analog equipment, the time constant for the recording equipment shall be less than or equa
18、l to the maximum allowed t . 3.20 sampling rate 0f : the rate at which data samples are taken or the number of samples per unit time. See sampling interval. 3.21 shaft windup:* in rotary valve systems, the tendency of the drive shaft to twist under load while the closure member is stuck at a given p
19、osition. 3.22 sliding friction rF or rT : the force or torque required to maintain motion in either direction at a prescribed input signal ramp rate. 3.23 static: means without motion or change McGraw-Hill, “Dictionary of Scientific and Technical Terms,“ sixth edition, 2002; readings are recorded af
20、ter the device has come to rest. Static performance can be measured either without process loading (bench top tests), with simulated or active loading, or under process operating conditions. This kind of test is sometimes called a dynamic test McGraw-Hill above, which may cause confusion. The static
21、 behavior characteristics identified as important to the control valve performance are the dead band, the resolution, and the valve travel gain. 3.24 steady state: a condition of a dynamic system when it is at rest at a given value. In testing the responses of a dynamic system, step test methods are
22、 often used. The resulting system transitions from an initial steady state value to a new final steady state value. 3.25 step change: a nearly instantaneous step change made to an input signal of a dynamic system with the intention of stimulating a step response of the dynamic system. Such a test is
23、 used to characterize the step response of the dynamic system. - 15 - ISA-75.25.01-2000 (R2010) 3.26 step change time sct : the time between the start of a signal input step and when it reaches its maximum value. 3.27 step test: the application of a step change to an input signal in order to test th
24、e step response dynamics. 3.28 step response time (86T ): the interval of time between initiation of an input signal step change and the moment that the response of a dynamic reaches 86.5% of its full steady state value. The step response time includes the dead time before the dynamic response. 3.29
25、 step size s : the difference between the beginning and ending signal in a step change expressed as a percent of the signal span. 3.30 stiction (static friction):* resistance to the start of motion, usually measured as the difference between the driving values required to overcome static friction up
26、scale and downscale ISA-51.1-1979 (R1993). 3.31 stick/slip:* a term that attempts to explain jerky or “sticky” motion by postulating that static friction differs substantially from sliding friction. However, friction is rarely directly measured, and “sticky” behavior can be caused by other physical
27、effects (e.g., positioner behavior, at small amplitudes). 3.32 stick/slip cycle:* a term that attempts to describe a limit cycle caused when the control valve “sticks” and suddenly “slips” during a change in input signal. It is the result of static friction combined with a positioner and actuator sy
28、stem that does not provide enough force to overcome friction at low positioner error values. 3.33 time constant : for first order dynamic systems, the interval of time between initiation of an input signal step change and the moment that a first order dynamic system reaches 63.2% of the full steady
29、state change. The term is used in this standard and ISA-TR75.25.02-2000 (R2010) to describe the dynamic characteristics of the analog measuring instruments. 3.34 valve travel gain:* the change in closure member position divided by the change in input signal, both expressed in percentage of full span
30、. s / XGX= 3.35 valve system approximate time constant ():* the time constant of a first order response without dead time, which may fit the actual control valve step response reasonably well. The approximate time constant is defined to provide a basis for comparison of the valve with other time con
31、stants, such as the closed loop time constant for the control loop. A first order system reaches 86.5% of its final step response value in two time constants; the approximate time constant is considered to be one half of the step response time, 86T . The use of the approximate time constant in no wa
32、y implies that the response of the control valve is first order. The step response of the control valve is typically complex, having dead time initially, followed by potentially complex dynamics before the steady state is achieved. 86T includes the dead time in the initial part of the response, as w
33、ell as the possibility of slower settling in the last portion of the response. Some valve positioner designs attempt to achieve a slow-down in the final part of the response in order to limit overshoot. attempts to produce a simple linear time constant approximation of the control valve dynamic resp
34、onse, which can be ISA-75.25.01-2000 (R2010) - 16 - compared to the closed loop time constant of the control loop on the same basis in time constant units. Note that as the portion of 86T that is dead time increases, this approximation becomes less ideal. 3.36 velocity limiting:* the maximum rate of
35、 change that a system can achieve due to its inherent physical limitations. 3.37 wait time wt :* the time spent after a step input change waiting for the response to come to the new steady state value. 3.38 X-Y plot:* a plot of the output excursions plotted against input excursions. Input-output plo
36、ts are useful for defining the steady state characteristics of nonlinearities. 4 General test procedures 4.1 Test valve conditions The test valve shall be set to its desired test configuration. For in-process testing, the valve may already be configured properly or further adjustment may not be poss
37、ible. This includes configuring the valve assembly with the desired packing type and condition, positioner if applicable, and actuator configuration. The positioner configuration shall include any applicable adjustments such as gain setting. In some cases, preliminary tests may be performed such as
38、testing to assure there is no excessive overshoot. (Excessive overshoot is not defined here and the amount allowed may vary according to the application but is to be reported.) All applicable characteristics of the valve configuration that would affect test results shall be reported. 4.2 Test system
39、 Testing to determine the response of a control valve requires a signal generator or source and instruments to measure the input signal, the position of the closure member and, for laboratory testing or in-process testing, the desired response variable. The response variable could be derived from ot
40、her variables that may need to be measured as well. The tests can be performed manually with appropriate instrumentation but computers are recommended for all or at least part of the testing and analyses. When measuring response time, data shall be collected fast enough to give good time resolution
41、using the requirements below for st . Measurement of static behavior (dead band, gain, and resolution) generally does not depend on sample interval and can be performed using existing field instrumentation, with the sample interval reported. For a control valve with a pneumatic input signal, the inp
42、ut signal shall be measured as close as possible to the device input port to avoid input distortion caused by the piping. The total time for the complete input signal step change, sct , shall meet the requirements given below. Valve position should be measured as close as possible to the closure mem
43、ber or at least at a location that closely approximates the closure member position. Care should be taken to avoid measurement errors due to excessive elastic deformation, clearances, linkages, etc. In all cases, the location of measurement points shall be reported. 4.3 Measuring instrument requirem
44、ents The measurement of each output variable, which includes the combined effects of transducers, any signal conditioning equipment, and recording equipment shall meet the following minimum requirements. - 17 - ISA-75.25.01-2000 (R2010) Time constant 20T86* Resolution 3resolutionvalve, 10resolutionv
45、alvepreferably Uncertainty 5 percent of full scale value (where the full scale value is the range of the measured variable known or estimated as the control valve goes from 0 to 100 percent open, preferably 2 percent of full scale value) Sampling interval, 20Torsec0.5oflessert86s* Input step change
46、time, 20Tt86sc * NOTE *Instrumentation used to measure the static parameters dead band, gain, and resolution need not meet these requirements but time constants, st , andsct , must be reported. For in-process tests, the flowmeter time constant would not have to be 20T86 , unless it is used to measur
47、e86T . But if installed in-process instrumentation used to measure 86T does not meet these requirements, an external position transducer and recording equipment which meet the above requirements are recommended. 4.4 Process variable For laboratory and in-process dead band and resolution testing, a p
48、rocess variable shall be measured, if possible, in addition to the input signal and the position. ISA-TR75.25.02-2000 (R2010) provides guidance for choosing the best process variable out of those that may be available at a specific plant or laboratory. The response flow coefficient, vRC , shown belo
49、w, is a simplified flow coefficient recommended for use as the process variable, if measurement of the variables necessary to calculate it is possible. It is used here because an accurate determination of vC is outside the scope of this standard and may not be feasible in many plant and in some laboratory environments. Measurements of dead band and resolution using vRC would equal those using vC since changes would be equal. This assumes the flow through the control valve is fully turbulent and not choked. If that i
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