1、 ANSI Technical Report prepared by ISA ANSI/ISA-TR75.25.02-2000 (R2010) Control Valve Response Measurement from Step Inputs Approved 18 July 2010 ANSI/ISA-TR75.25.02-2000 (R2010) Control Valves Response Measurement from Step Inputs ISBN: 978-1-936007-50-9 Copyright 2010 by ISA. All rights reserved.
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3、A 67 Alexander Drive P. O. Box 12277 Research Triangle Park, North Carolina 27709 - 3 - ANSI/ISA-TR75.25.02-2000 (R2010) Preface This preface, as well as all footnotes and annexes, is included for information purposes and is not part of ANSI/ISA-TR75.25.022000 (R2010). The standards referenced withi
4、n 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 this document are encouraged to investigate
5、 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 maintains registers of currently valid U.S. National Standards. This document has been prepared as part of
6、 the service of 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 welcomes all comments and criticisms and asks that they be addressed to the Secretary, Stand
7、ards 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 Department is aware of the growing need for attention to the metric system of units in general
8、, 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 references to the SI (and the metric system) in their business and professional dealing
9、s 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 possible. Standard for Use of the International System of Units (SI): The Modern Metric
10、 System, published by the American Society for Testing ISA; 67 Alexander Drive; P. O. Box 12277; Research Triangle Park, NC 27709. This technical report applies to throttling control valves in closed loop control applications. The concept has some application to open loop control applications but do
11、es not address control valves used in on-off control service. In the context of this document, the control valve includes these components a valve, an actuator, a motion conversion mechanism, and accessories such as a positioner, transducer, signal booster relay, air set, snubber, etc. Abstract This
12、 technical report describes the characteristic response of a control valve to step input signal changes, considering the factors that affect this response, the impact of the response on the quality of process control, and the appropriate control valve specifications. In this document, a control valv
13、e is the complete control valve body, with actuator and any accessories required for normal operation assembled and ready for use. This document identifies and defines four regions of control valve response to step input changes of varying sizes and provides guidance that can be used to relate the c
14、ontrol valve performance to process control. Key Words Accessories, actuator, amplitude, backlash, bench test, closed loop, closed loop time constant, control valve, dead band, dead time, dynamic response, flow coefficient, hunting, in-process test, laboratory test, limit cycle, motion conversion me
15、chanism, nonlinear, open loop, overshoot, process control, process response, resolution, response, static, steady state, stem position, step change, step input signal changes, step response time, step size, step test, throttling control valve, time constant, valve, velocity limiting. This page inten
16、tionally left blank. - 13 - ANSI/ISA-TR75.25.02-2000 (R2010) 1 Purpose This technical report describes the characteristic response of a control valve to step input signal changes. It considers the factors that affect this response, the impact of the response on the quality of process control, and th
17、e appropriate control valve specifications. In this document, a control valve is the complete control valve body, with actuator and any accessories required for normal operation assembled and ready for use. This document supports standard ANSI/ISA-75.25.01-2000 (R2010), “Test Procedure for Control V
18、alve Response Measurement from Step Inputs.“ See the standard for the test procedures. Users and manufacturers have developed a better understanding of the effects of control valve response characteristics on process control. This document identifies and defines four regions of control valve respons
19、e to step input changes of varying sizes. Existing standards do not include the definitions and methods to measure certain valve characteristics now understood to be important. This technical report provides guidance that can be used to relate the control valve performance to process control. 2 Scop
20、e This technical report applies to throttling control valves in closed loop control applications. The concept has some application to open loop control applications. It does not address control valves used in on-off control service. The “control valve” in the context of this document includes the fo
21、llowing components: Valve: A valve is a device used for the control of fluid flow. It consists of a fluid containing valve body assembly, one or more ports between connection openings and a moveable closure member, which opens, restricts or closes the port(s) (see ANSI/ISA-75.05.01-2000 (R2005), “Co
22、ntrol Valve Terminology“). Actuator: An actuator is a device that supplies the force and causes the movement of the valve closure member. Commonly these are fluid or electrically powered (see ANSI/ISA-75.05.01-2000 (R2005). Actuators often use air but other types use electric, hydraulic and electro-
23、hydraulic power. Motion conversion mechanism: A mechanism installed between the valve and the power unit of the actuator to convert between linear and rotary motion where required. The conversion may be from linear actuator action to rotary valve operation or from rotary actuator action to linear va
24、lve operation. Accessories: Additional devices used in the operation of the control valve. As described in ANSI/ISA-75.05.01-2000 (R2005), typical examples include a positioner, transducer, signal booster relay, air set, snubber, etc. 3 Definitions This document and ANSI/ISA-75.25.01-2000 (R2010) ma
25、ke use of terms as defined in ISA-51.1-1979 (R1993) “Process Instrumentation Terminology,“ and some of the essential terms are repeated here for convenience. In the specific area of nonlinear dynamics, it was determined that some terms defined in ISA-51.1-1979 (R1993) lacked the precision desired fo
26、r these documents. Others were inconsistent with the terminology used in the nonlinear control literature. A common set of definitions is used in ANSI/ISA-75.25.01-2000 (R2010) and this document. Those used only in this document are marked with an asterisk (*). 3.1 backlash:* in process instrumentat
27、ion, a relative movement between connected mechanical parts, resulting from looseness when motion is reversed ISA-51.1-1979 (R1993). Sometimes also referred to as slop, lost motion, or free play. ANSI/ISA-TR75.25.02-2000 (R2010) - 14 - Figure 1 Dead band and resolution 3.2 closed loop time constant:
28、* the time constant of the closed loop response of a control loop, used in tuning methods such as Internal Model Control (IMC) and Lambda Tuning. The closed loop time constant is a measure of the performance of a control loop. 3.3 dead band: the range through which an input signal may be varied, wit
29、h reversal of direction, without initiating an observable change in output signal ISA-51.1-1979 (R1993). In this technical report and in standard ANSI/ISA-75.25.01-2000 (R2010) it is defined in percent of input span. Note that in some other literature this definition is used for dead zone. 3.4 dead
30、time (dT ): the time after the initiation of an input change and before the start of the resulting observable response. 3.5 dead zone:* a zone of input for which no value of the output exists ISA-51.1-1979 (R1993). 3.6 dynamic response: the time-dependent output signal change resulting from a define
31、d time-dependent input signal change. Commonly used input signal changes include impulse, pulse, step, ramp, and sinusoid McGraw-Hill “Dictionary of Scientific and Technical Terms“, sixth edition, 2002. Dynamic means that the control valve is moving. Dynamic response can be measured without process
32、loading in bench top tests with simulated or active loading in a flow laboratory or under normal process operating conditions. Dynamics are not shown b a c d Input Time Amplitude Output c dead band 10%) signal changes. 5.7.2 Actuator volume The volume of air that the positioner must supply and exhau
33、st limits the speed of response. A greater actuator volume requires a larger change in the mass of air in the actuator and may delay the response and increase the dead time. Three volumes may be described. The stroke volume is the change in volume during the stroke. The dead volume is the total volu
34、me minus the stroke volume. The total volume is fixed by the design. The stroke volume will vary with stem position and pressure creating a dynamic non-linearity. In any calculations, consider that it is actually the mass of air in the actuator that creates the pressure. 5.7.3 Supply pressure and ca
35、pacity Inadequate air supply capacity and pressure will limit the dynamic performance of the control valve. Undersized and limiting piping or tubing, air filters and supply regulators limit the air capacity. Dirty filters and partially closed block valves will slow or prevent response. The dead time
36、 and the time constant will be increased. The times for opening and for closing the valve will probably differ from each other. See ISA-7.0.01-1996, “Quality Standard for Instrument Air.“ 5.7.4 Valve pneumatic accessories Accessories such as volume boosters, quick-release valves, and solenoid valves
37、 will all affect the performance. A volume booster can improve speed of response. 5.8 Actuator size/type Selection of the actuator size requires accurate information on friction, and process pressures, temperatures, and fluid characteristics. Safety factors in sizing actuators must consider safety,
38、and the quality of the available information. Actuators are sized based on the minimum air supply pressure available but the actuator design must also withstand the maximum air supply pressure. To reduce dead time and to minimize the dead band, available stroking power must be greater than the minim
39、um force ANSI/ISA-TR75.25.02-2000 (R2010) - 24 - required to move the valve stem. An inadequate or undersized actuator or a positioner with poor performance will result in poor response in both response time and magnitude. The selection of the type of actuator involves considerations of valve size,
40、and the design details, air supply pressure, and manufacturer offerings. 5.9 Electric and hydraulic actuators The actuator discussion above applies to all types of actuators, pneumatic, hydraulic and electric, but especially to pneumatic. Hydraulic actuators are expected to provide very good perform
41、ance and are typically used for larger valves and the more difficult applications. Some types of electric motor actuators will have good resolution and may have a very small dead band, but they may lack the required speed when used with larger valves. These statements are only broad generalities. Th
42、e user must investigate the data and claims from the manufacturers. 5.10 Flow effects Dynamic imbalance from the effects of flow on the control valve closure member can degrade repeatability and dynamic linearity. Choking will limit flow capacity and vibration can affect the positioner performance.
43、A valve operated in the flow-to-close mode may show stem instability as the plug approaches the seat and the hydrodynamic plug forces increase rapidly. Some butterfly designs have a reversal in flow induced shaft torque depending on position. These forces will vary with flow rate and pressure drop.
44、5.11 Valve sizing and selection The installed flow characteristic of the valve may not be the same as the inherent flow characteristic of the valve. See further discussion in 6.3. 6 Process and control design issues 6.1 Control loop process gain range and variability A fluid process is much easier t
45、o control if the control dynamics remain nearly constant over the full range of operating conditions. The key dynamic parameters include: process gain, process time constant, dead time, controller dynamics, sensor dynamics, and control valve static and dynamic properties. The values of these paramet
46、ers and their changes over the operating range will determine how well the process can be controlled to achieve the following: a) Acceptable level of process performance during process operation b) Low process variability c) Final product of acceptable uniformity d) Low manufacturing cost e) Ability
47、 to meet manufacturing demand f) High level of plant safety g) High level of environmental compliance - 25 - ANSI/ISA-TR75.25.02-2000 (R2010) h) Successful startups, product grade transitions, and shutdowns i) Recovery from process upsets The control loop is subject to setpoint changes and to upsets
48、. It establishes new operating conditions to recover from load disturbances or to meet the new setpoint. Parameters such as the required time constant and minimum dead time are set by the process design and instrument selection. The process gain is central to the valve selection, process dynamics, a
49、nd fluid transport system. The process gain is determined by the process dynamics, the control scheme and the fluid transport system. The fluid transport system characteristics are determined by the pump/compressor, piping, equipment, and the control valve. In all cases, the valve capacity and characteristics influence the process dynamic control. Control design strategy can compensate to a certain extent for physical equipment and piping design limitations. The task of the design process is to select the right sized control valve and suitable characteristics. This wil
