TECHNICAL REPORT
ISA-TR75.04.01-1998 (R2006)
Control Valve Position Stability
Approved 29 November 2006
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ISA-TR75.04.01-1998 (R2006)
Control Valve Position Stability
ISBN: 978-0-9791330-2-2
0-9791330-2-5
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ISA-TR75.04.01-1998 (R2006)
Preface
This preface, as well as all footnotes and annexes, is included for information purposes and is not part of
ISA-TR75.04.01-1998 (R2006).
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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 be static but should be subject to periodic
review. Toward this end, the Society welcomes all comments and criticisms and asks that they be
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Research Triangle Park, NC 27709; Telephone (919) 549-8411; Fax (919) 549-8288; E-mail:
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ISA-TR75.04.01-1998 (R2006)
- 4 -`
PROFESSIONAL JUDGMENT CONCERNING ITS USE AND APPLICABILITY UNDER THE USER’S
PARTICULAR CIRCUMSTANCES. THE USER MUST ALSO CONSIDER THE APPLICABILITY OF
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THE USER OF THIS DOCUMENT SHOULD BE AWARE THAT THIS DOCUMENT MAY BE IMPACTED
BY ELECTRONIC SECURITY ISSUES. THE COMMITTEE HAS NOT YET ADDRESSED THE
POTENTIAL ISSUES IN THIS VERSION.
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
______
*One vote per company
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.
The following people served as members of ISA Committee SP75 and approved
ANSI/ISA-TR75.04.01-1998:
NAME
COMPANY
D. Buchanan*, Chairman
W. Weidman, Managing Director
T. Abromaitis
J. Addington
H. Backinger
G. Baenteli
G. Barb
H. Baumann
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
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.
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
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The following people served as members of ISA Subcommittee SP75.04 and approved
ANSI/ISA-TR75.04.01-1998:
-5-
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
______
ISA-TR75.04.01-1998 (R2006)
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.
*One vote per company
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
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.
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This document was approved for publication by the ISA Standards and Practices Board on
15 July 1998.
ISA-TR75.04.01-1998 (R2006)
D. Rapley
R. Reimer
J. Rennie
W. Weidman
J. Weiss
J. Whetstone
M. Widmeyer
R. Wiegle
C. Williams
G. Wood
M. Zielinksi
- 6 -`
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.
The following people served as members of ISA Subcommittee SP75.04 and reaffirmed
ISA-TR75.04.01-1998 (R2006):
NAME
COMPANY
J. Reed, Chairman
W. Weidman, Managing Director
G. Baenteli
W. Black
J. Borge
W. Caudill
P. Schafbuch
J. Young
Consultant
Worley Parsons
Consultant
Curtiss-Wright Flow Control Corporation
Neles Automation Inc.
Arco Products Company
Emerson Process Management
Dow Chemical Company
NAME
COMPANY
J. Young, Chairman
W. Weidman, Managing Director
H. Backinger
H. Baumann
J. Beall
W. Black
H. Boger
G. Borden
S. Boyle
J. Broyles
F. Cain
W. Cohen
R. Duimstra
J. Faramarzi
J. George
H. Hoffmann
J. Jamison
R. Jeanes
C. Langford
G. Liu
J. McCaskill
A. McCauley
R. McEver
V. Mezzano
Dow Chemical Company
Worley Parsons
Consultant
Consultant
Emerson Process Management
Curtiss-Wright Flow Control Corporation
Masoneilan Dresser
Consultant
Metso Automation USA Inc.
Enbridge Pipelines Inc.
Flowserve Corporation
KBR
Fisher Controls International Inc.
Control Components Inc.
Richards Industries
Samson AG
OPTI Canada Inc.
TXU Electric
Cullen G Langford Inc.
Syncrude Canada Ltd.
Power Chokes
Chagrin Valley Controls Inc.
Consultant
Fluor Corporation
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The following people served as members of ISA Committee SP75 and reaffirmed
ISA-TR75.04.01-1998 (R2006):
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-7-
T. Molloy
L. Ormanoski
J. Ozol
W. Rahmeyer
J. Reed
E. Skovgaard
ISA-TR75.04.01-1998 (R2006)
CMES Inc.
York Process Systems
NMC Prairie Island Nuclear Plant
Utah State University
Consultant
Control Valve Solutions
This document was approved for reaffirmation by the ISA Standards and Practices Board on
29 November 2006.
NAME
COMPANY
I. Verhappen, Vice President
M. Coppler
B. Dumortier
D. Dunn
W. Holland
E. Icayan
J. Jamison
R. Jones
K. Lindner
V. Maggioli
T. McAvinew
A. McCauley
G. McFarland
R. Reimer
N. Sands
H. Sasajima
T. Schnaare
J. Tatera
R. Webb
W. Weidman
J. Weiss
M. Widmeyer
M. Zielinski
MTL Instrument Group
Ametek Inc.
Schneider Electric
Aramco Services Company
Consultant
ACES Inc.
OPTI Canada Inc.
Consultant
Endress+Hauser Process Solutions AG
Feltronics Corporation
Jacobs Engineering Group
Chagrin Valley Controls Inc.
Emerson Process Mgmt. Power & Water Solutions
Rockwell Automation
E I du Pont
Yamatake Corporation
Rosemount Inc.
Tatera & Associates Inc.
Robert C Webb PE
Worley Parsons
KEMA Inc.
Stanford Linear Accelerator Center
Emerson Process Management
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-9-
ISA-TR75.04.01-1998 (R2006)
Foreword
This document discusses control valve stem position mechanical stability, establishes
a measurement criterion for position instability and provides a bibliography of published papers.
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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- 11 -
ISA-TR75.04.01-1998 (R2006)
Contents
1
Scope ................................................................................................................................................. 13
2
Purpose .............................................................................................................................................. 13
3
Definitions........................................................................................................................................... 13
4
Discussion .......................................................................................................................................... 13
5
Measurement of position instability .................................................................................................... 14
Annex A — References............................................................................................................................... 17
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- 13 -
1
ISA-TR75.04.01-1998 (R2006)
Scope
This document 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.
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:
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:
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):
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:
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 3.1, may occur when the immediate force-to-travel gradient
associated with the action of the flowing fluid on the moveable valve trim 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
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2
ISA-TR75.04.01-1998 (R2006)
- 14 -
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 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 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 pressurebalanced 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 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 document.
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:
Instability Percentage =
(Maximum unstable motion amplitude) (100)
Rated travel
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- 15 -
ISA-TR75.04.01-1998 (R2006)
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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ISA-TR75.04.01-1998 (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 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
“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 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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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 valveactuator 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.
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 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 slidingstem 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.
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“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, 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 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 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.
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- 19 -
ISA-TR75.04.01-1998 (R2006)
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. 84780, 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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Copyright International Society of Automation
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Not for Resale, 01/03/2017 21:41:12 MST
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Copyright International Society of Automation
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ISA is an American National Standards Institute (ANSI) accredited organization. ISA administers United
States Technical Advisory Groups (USTAGs) and provides secretariat support for International
Electrotechnical Commission (IEC) and International Organization for Standardization (ISO) committees
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ISA
Attn: Standards Department
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ISBN: 978-0-9791330-2-2
0-9791330-2-5
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Developing and promulgating sound consensus standards, recommended practices, and technical
reports is one of ISA’s primary goals. To achieve this goal the Standards and Practices Department
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