valve positioner principles and maintenance guide

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Valve Positioner Principles

and Maintenance Guide

Technical ReportL

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CE

NS E D

MA T

ER

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EquipmentReliability

Plant

Maintenance

Support

ReducedCost

WARNING:

Please read the License Agreementon the back cover before removing

the Wrapping Material.

Effective December 6, 2006, this report has been made publicly available

in accordance with Section 734.3(b)(3) and published in accordance with

Section 734.7 of the U.S. Export Administration Regulations. As a result

of this publication, this report is subject to only copyright protection and

does not require any license agreement from EPRI. This notice

supersedes the export control restrictions and any proprietary licensed

material notices embedded in the document prior to publication.


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EPRI Project ManagerL. Loflin

EPRI 3412 Hillview Avenue, Palo Alto, California 94304 PO Box 10412, Palo Alto, California 94303 USA800.313.3774 650.855.2121 [email protected] www.epri.com

Valve Positioner Principlesand Maintenance Guide

1003091

Final Report, December 2001


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DISCLAIMER OF WARRANTIES AND LIMITATION OF LIABILITIES

THIS DOCUMENT WAS PREPARED BY THE ORGANIZATION(S) NAMED BELOW AS ANACCOUNT OF WORK SPONSORED OR COSPONSORED BY THE ELECTRIC POWER RESEARCHINSTITUTE, INC. (EPRI). NEITHER EPRI, ANY MEMBER OF EPRI, ANY COSPONSOR, THEORGANIZATION(S) BELOW, NOR ANY PERSON ACTING ON BEHALF OF ANY OF THEM:

(A) MAKES ANY WARRANTY OR REPRESENTATION WHATSOEVER, EXPRESS OR IMPLIED, (I)WITH RESPECT TO THE USE OF ANY INFORMATION, APPARATUS, METHOD, PROCESS, ORSIMILAR ITEM DISCLOSED IN THIS DOCUMENT, INCLUDING MERCHANTABILITY AND FITNESSFOR A PARTICULAR PURPOSE, OR (II) THAT SUCH USE DOES NOT INFRINGE ON ORINTERFERE WITH PRIVATELY OWNED RIGHTS, INCLUDING ANY PARTY'S INTELLECTUALPROPERTY, OR (III) THAT THIS DOCUMENT IS SUITABLE TO ANY PARTICULAR USER'SCIRCUMSTANCE; OR

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ORGANIZATION(S) THAT PREPARED THIS DOCUMENT

EPRI

ORDERING INFORMATION

Requests for copies of this report should be directed to EPRI Customer Fulfillment, 1355 Willow Way,Suite 278, Concord, CA 94520, (800) 313-3774, press 2.

Electric Power Research Institute and EPRI are registered service marks of the Electric PowerResearch Institute, Inc. EPRI. ELECTRIFY THE WORLD is a service mark of the Electric PowerResearch Institute, Inc.

Copyright 2001 Electric Power Research Institute, Inc. All rights reserved.


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CITATIONS

This report was prepared by

Nuclear Maintenance Applications Center (NMAC)EPRI1300 W.T. Harris BoulevardCharlotte, NC 28262

This report describes research sponsored by EPRI.

The report is a corporate document that should be cited in the literature in the following manner:

Valve Positioner Principles and Maintenance Guide, EPRI, Palo Alto, CA: 2001. 1003091.


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REPORT SUMMARY

This guide provides information for personnel involved with the maintenance of valvepositioners, including the principles of operation, applications, calibration, condition monitoring,preventive maintenance, and troubleshooting. It provides insights for experienced personnel aswell as basic information, guidance, and instructions for personnel assigned to maintain valvepositioners.

Background

A valve positioner is a device in the control loop of a flow, pressure, or level control process thatimproves valve response to changes in the demand signal from a process controller. The

positioner is used to limit control valve dead band, mitigate friction-induced nonlinearities,change valve flow characteristics, permit double-acting actuator operation, increase shutoffforces, allow for split-ranging, and add loop gain to decrease the effects of process lag and deadband.

In 1999, NMAC conducted a survey of unplanned capacity loss factors. The survey identifiedcontrol valves as the number four cause of such losses. An EPRI survey identified the existenceof information on control valves, but none for control loops and positioners. Another surveyperformed in the paper and pulp industry showed that, of 31 valve control problems, 71% wereattributable to the positioner. The next most frequent problem cause was bench set at 38%. Based

on this information, the positioner as part of the control loop was selected for further research.To provide additional information, a tutorial on control loops was also developed.

Objectives

N To help power plant maintenance personnel understand the basic principles of positionerdesigns and application

N To provide technical information for plant maintenance personnel on proper calibration,condition monitoring/preventive maintenance, and troubleshooting

N

To provide additional technical information on control loops and how the positioner actswithin the control loop

ApproachA detailed review of industry literature, product information, and standards was conducted toidentify the various designs, applications, and maintenance practices associated with valvepositioners. Utility and industry personnel were surveyed to determine specific problems andcommonly encountered failure mechanisms. Based on this information, recommendations weremade on proper calibration, condition monitoring/preventive maintenance, and troubleshooting.


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ResultsThis guide presents a thorough discussion of valve positioner design and principles of operationto provide a technical background for personnel performing valve positioner maintenance andinformation about how positioners are applied. Subsequent chapters contain the maintenanceportion of the guide and discussions on proper calibration, condition monitoring/preventive

maintenance, and troubleshooting. The emphasis of the maintenance section is not only on goodpractices, but also on how each of these maintenance areas is closely related and mutuallysupportive. The contents of this guide are intended to assist plant personnel in reducing costs andequipment unavailability and in improving equipment reliability and performance.

EPRI PerspectiveBased on industry studies, the valve positioner is the component that causes most control loopproblems. Nearly all positioner problems are the result of improper setup and maintenance. Thisguide provides maintenance personnel with details of the basic principles of positioner designand application as well as calibration, condition monitoring/preventive maintenance, andtroubleshooting. An appendix is also included that provides technical information on controlloops and the function of the positioner within the control loop.

KeywordsMaintenanceControl loopControl valvePositionerCalibration

Troubleshooting


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EPRI Licensed Material

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ABSTRACT

A valve positioner is a device in the control loop of a flow, pressure, or level control process thatimproves valve response to changes in the demand signal from a process controller. In 1999, theEPRI Nuclear Maintenance Applications Center (NMAC) conducted a survey of unplannedcapacity loss factors. The survey identified control valves as the number four cause of suchlosses. A subsequent survey identified the existence of information on control valves, but nonefor control loops and positioners. Based on these surveys and other industry information, thevalve positioner as part of the control loop was selected for further research. This guide presentsa thorough discussion of valve positioner design and principles of operation in the context of

control loop principles. It provides a technical background for personnel performing valvepositioner maintenance and information about how positioners are applied. Subsequent chapterscontain the maintenance portion of the guide with discussions on proper calibration, conditionmonitoring/preventive maintenance, and troubleshooting. The emphasis of the maintenance

section is not only on good practices, but also on how each of these maintenance areas is closelyrelated and mutually supportive. The contents of this guide are intended to assist plant personnelto reduce costs and equipment unavailability and to improve equipment reliability andperformance.


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ACKNOWLEDGMENTS

The following individuals were active members of the Positioner Maintenance Guide TaskAdvisory Group. They made significant contributions to the development of this document byattending Task Advisory Group meetings and reviewing and providing feedback on variousdrafts of the guide.

*Jim Allan Diablo CanyonSteve Ball Seabrook *Bill Bowyer Vogtle

*Harry Cole Point Beach*Randy Croxton Palo Verde*Scott Dill SalemGeorge Farley SusquehannaScott Ladd Prairie IslandChuck Linden Ft. CalhounMarie Murphy Cooper*Bill Muscia Beaver Valley*Mike Sawaya Carolina P&L*Bill Slover EPRI NMACFred Wiens South Texas Project

*Attended TAG meeting July 1819, 2001.

Also acknowledged is George Gassman, Senior Research Specialist, Final Control Systems,Fisher Controls International, a division of Emerson Processes. Gassman provided technicalinsight in the principles of positioner operations.

NMAC was supported in this effort by Bill Slover.


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CONTENTS

1 INTRODUCTION.................................................................................................................. 1-1

1.1 Background............................................................................................................... 1-1

1.2 Electronic/Digital Positioners..................................................................................... 1-1

1.3 Approach................................................................................................................... 1-1

1.4 Highlighting Key Points ............................................................................................. 1-2

1.5 Glossary.................................................................................................................... 1-21.6 References................................................................................................................ 1-2

2INTRODUCTION TO THE CONTROL LOOP ...................................................................... 2-1

2.1 Purpose .................................................................................................................... 2-1

2.2 Overview................................................................................................................... 2-1

2.3 Description ................................................................................................................ 2-1

2.4 Summary................................................................................................................... 2-3

2.5 Additional Information................................................................................................ 2-4

2.6 Reference ................................................................................................................. 2-4

3POSITIONER DESIGN AND APPLICATION....................................................................... 3-1

3.1 Introduction................................................................................................................... 3-1

3.2 Two Black Boxes....................................................................................................... 3-1

3.2.1 Black Box One...................................................................................................... 3-1

3.2.2 Black Box Two...................................................................................................... 3-2

3.2.3 Discussion............................................................................................................ 3-2

3.3 Positioner Design ...................................................................................................... 3-33.3.1 Function ............................................................................................................... 3-3

3.3.2 Building Blocks ..................................................................................................... 3-4

3.3.3 Input Signal Conversion........................................................................................ 3-4

3.3.3.1 Nozzle-Flapper.............................................................................................. 3-5

3.3.4 Output Signal Generator....................................................................................... 3-7


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3.3.4.1 Connecting the Input Converter to Output Generator..................................... 3-7

3.3.4.2 Types of Output Signal Generators ............................................................... 3-7

3.3.5 Output Signal Correction (Feedback).................................................................. 3-11

3.3.5.1 Purpose....................................................................................................... 3-11

3.3.5.2 Interface Between Valve and Positioner ...................................................... 3-11

3.3.5.3 Positioner Input ........................................................................................... 3-11

3.3.5.4 Motion-Balance and Force-Balance (Balancing Mechanisms)..................... 3-13

3.4 Classification of Commonly Used Positioners.......................................................... 3-14

3.5 Positioner Application.............................................................................................. 3-15

3.5.1 Limiting Control Valve Dead Band ...................................................................... 3-15

3.5.2 Mitigating Stiction or Stick-Slip............................................................................ 3-15

3.5.3 Change Valve Response .................................................................................... 3-16

3.5.4 Control Double Acting Actuator........................................................................... 3-163.5.5 Increase Shutoff Forces...................................................................................... 3-16

3.5.6 Split-Ranging...................................................................................................... 3-16

3.5.7 Delays Due to Distance Between Controller or I/P Converter and Valve............. 3-17

4CALIBRATION .................................................................................................................... 4-1

4.1 Calibration and Condition Monitoring/Preventive Maintenance.................................. 4-1

4.2 Basic Calibration ....................................................................................................... 4-1

4.2.1 Bench Set Confirmation........................................................................................ 4-1

4.2.2 Feedback Alignment............................................................................................. 4-2

4.2.3 Zero and Span Adjustment ................................................................................... 4-3

4.2.3.1,Purpose......................................................................................................... 4-3

4.2.3.2,Static Band.................................................................................................... 4-3

4.2.3.3 Adjustments .................................................................................................. 4-4

4.3 Functional Check ...................................................................................................... 4-5

4.4 Data Acquisition Systems.......................................................................................... 4-6

4.4.1 Calibration Using the Data Acquisition System..................................................... 4-6

4.4.2 Hysteresis/Dead Band (Dynamic Error) ................................................................ 4-8

4.5 References................................................................................................................ 4-8

5CONDITION MONITORING/PREVENTIVE MAINTENANCE............................................... 5-1

5.1 Condition Monitoring ................................................................................................. 5-1

5.1.1 Continuation of Calibration ................................................................................... 5-1


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5.1.2 Data Acquisition Systems..................................................................................... 5-2

5.1.2.1 Other Recommended Performance Curves Using a Data AcquisitionSystem...................................................................................................................... 5-2

5.2 Preventive Maintenance............................................................................................ 5-3

5.2.1 Air Supply Check.................................................................................................. 5-35.2.2 Walkdown Inspection............................................................................................ 5-4

5.2.3 Internal Inspections (as Applicable)...................................................................... 5-4

5.3 References................................................................................................................ 5-5

6TROUBLESHOOTING......................................................................................................... 6-1

6.1 Introduction ............................................................................................................... 6-1

6.2 Approach................................................................................................................... 6-1

6.2.1 Use of Data Acquisition System............................................................................ 6-1

6.2.2 Symptom-Based Approach................................................................................... 6-2

6.2.3 Positioner ............................................................................................................. 6-3

A CONTROL LOOP DETAILS ...............................................................................................A-1

A.1 Purpose ....................................................................................................................A-1

A.2 Control Loop Block Diagram......................................................................................A-1

A.3 Loop Elements..........................................................................................................A-2

A.3.1 Sensor/Transmitter ............................................................................................... A-2

A.3.2 The Controller (Including the Comparator)............................................................ A-3A.3.2.1Proportional Control.......................................................................................A-3

A.3.2.1.1 Offset.....................................................................................................A-5

A.3.2.2Integral (Reset) Control ................................................................................. A-5

A.3.2.3Derivative (Rate) Control ............................................................................... A-5

A.3.2.4Controller Problems and Tuning .................................................................... A-5

A.3.3 The Final Control Element .................................................................................... A-6

A.4 Control Loop Problems..............................................................................................A-6

A.4.1 Dead Time............................................................................................................A-6

A.4.1.1Sources of Dead Time................................................................................... A-7

A.4.1.2Component Dead Time ................................................................................. A-7

A.4.1.3Identifying Dead Time Problems.................................................................... A-7

A.4.1.4Dealing with Dead Time ................................................................................ A-8

A.4.1.5Lag................................................................................................................ A-9


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A.4.2 Linearity................................................................................................................A-9

A.4.2.1Inherent and Installed Flow Characteristics ................................................. A-10

A.4.2.2Hysteresis/Dead Band/Stem Friction........................................................... A-11

A.4.2.3Dynamic Error ............................................................................................. A-11

A.5 Scaling Calculations................................................................................................A-11

A.6 Reference ...............................................................................................................A-12

BAOV POSITIONER CHECKLIST ........................................................................................B-1

CGLOSSARY........................................................................................................................C-1

DDIGITAL POSITIONERS.....................................................................................................D-1

D.1 Introduction ...............................................................................................................D-1

D.2 Reason for Digital......................................................................................................D-1

EPOSITIONER PROBLEM CASE HISTORIES..................................................................... E-1

E.1 Introduction ...............................................................................................................E-1

E.2 Case Histories...........................................................................................................E-1

E.3 Reference .................................................................................................................E-9

FMINIMUM INTEGRATED ABSOLUTE ERROR TUNING.................................................... F-1

GSUMMARY OF KEY POINTS .............................................................................................G-1


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LIST OF FIGURES

Figure 2-1 Example of a Control Process................................................................................ 2-2

Figure 3-1 Positioner Block Diagram....................................................................................... 3-4

Figure 3-2 Input Signal Converters.......................................................................................... 3-5

Figure 3-3 Nozzle/Flapper Operation ...................................................................................... 3-6

Figure 3-4 Balance Beam........................................................................................................ 3-7

Figure 3-5 Spool Valve (Pneumatic)........................................................................................ 3-8

Figure 3-6Double Three-Way Poppet Directional Control Valve ............................................. 3-9

Figure 3-7Throttle Directional Control Valve......................................................................... 3-10

Figure 3-8Pressure Control Directional Control Valve .......................................................... 3-11

Figure 3-9 Feedback Cam Characterization.......................................................................... 3-12

Figure 3-10 Pneumatic Transmission Lag: Time to Reach 63.2% Final Value (Time

Constant 9 ) .................................................................................................................... 3-17

Figure 4-1 Calibration of Valve Positioner ............................................................................... 4-7

Figure A-1 Level Control Block Diagram ................................................................................ A-1

Figure A-2 Proportional Band................................................................................................. A-3

Figure A-3 Controller Output Response to Square Pulse Showing Gain ................................ A-4

Figure A-4 Stability.................................................................................................................A-4

Figure A-5 Process Variable Versus Controller Output Showing Stiction (Limit Cycle)........... A-8

Figure A-6 Robustness Plot ................................................................................................... A-9

Figure A-7 Valve Flow Characteristics.................................................................................. A-10

Figure E-1 Severely Deformed Positioner Bellows (Input Signal Converter)TravelVersus Positioner Input Pressure ....................................................................................E-1

Figure E-2 Double Acting CylinderStability Problem due to Insufficient Air Pressure(I/P Input Signal/Actuator Position Versus Time) ............................................................. E-2

Figure E-3 Double Acting CylinderStability Problem due to Insufficient Air Pressure(Cylinders Differential Pressure [Low Cyl]/Actuator Position Versus Time) ...................... E-3

Figure E-4 Double Acting CylinderCorrected Stability Problem due to Insufficient AirPressure (Cylinders Differential Pressure [Low Cyl]/Actuator Position Versus Time) .......E-4

Figure E-5 Double Acting CylinderStability Problem due to Insufficient Air Pressure(As-Found Versus As-Left Actuator Position Versus Control Signal Input).......................E-5

Figure E-6 Positioner with No Air LeakSupply Pressure/Actuator Pressure/ActuatorPosition Versus Time Plot ...............................................................................................E-6


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Figure E-7 Positioner with Internal Air LeakSupply Pressure/ActuatorPressure/Position Versus Time Plot ................................................................................ E-7

Figure E-8 Positioner with Internal Air LeakSupply Pressure Versus ActuatorPressure Plot...................................................................................................................E-7

Figure E-9 Air Leak Downstream of PositionerSupply Pressure/Actuator

Pressure/Position Versus Time Plot ................................................................................ E-8Figure E-10 Air Leak Downstream of PositionerIncreasing/Decreasing Supply

Pressure Versus Time Plot ............................................................................................. E-9

Figure F-1 Area Representation of Integrated Absolute Error................................................. F-1


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LIST OF TABLES

Table 2-1 The Positioner in the Control Loop ......................................................................... 2-3

Table 3-1 Classification of Commonly Used Positioners....................................................... 3-14

Table 4-1 Suggested Static Band Pressure Values................................................................ 4-5

Table 4-2 Suggested Intermediate Median Values for Selected Pressure Inputs ................... 4-6

Table 5-1 Condition Monitoring Performance Curves............................................................. 5-2

Table 6-1 Final Control Element Problem Symptoms/Causes................................................ 6-2

Table 6-2 Positioner Problem Symptoms/Causes .................................................................. 6-3

Table F-1 Post-Test Settings to Achieve Minimum Integrated Absolute Error Tuning.............. F-2


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1INTRODUCTION

1.1 Background

In 1999, the EPRI Nuclear Maintenance Application Center (NMAC) conducted a survey ofunplanned capacity loss factors. The survey identified control valves as the number four cause ofsuch losses. To address this issue, an additional survey was completed to find out which areashad not been previously addressed by earlier EPRI documents. It was determined that ControlValve Guidelines [1] provided a detailed treatment of the control valve proper and, to some

extent, addressed some of the accessories. What appeared to be missing was information oncontrol loops. Further study determined that to provide a document meaningful to experiencedtechnicians and their supervision, the document should focus on components that havehistorically received little attention. In addition, it was beneficial to survey other industries todetermine the leading causes of control loop problems. One of the most useful surveys wasperformed in the paper and pulp industry. This survey showed that of 31 valve control problems,71% were attributable to the positioner. The next most frequentproblem cause was bench set at38%. Based on this information, the positioner as part of the control loop was selected for furtherresearch.

1.2 Electronic/Digital Positioners

This guide gives details only for pneumatic positioners. The population of electronic and digitalpositioners used in the nuclear industry is still small, and experience is limited. Appendix Dcontains a brief summary of digital positioners.

1.3 Approach

In many cases, the positioner is mounted by the control valve manufacturer and has beenadjusted as required to give satisfactory results. However, when positioner troubleshootingbegins in earnest, a routine maintenance action canturn into a frustrating learning experience.This occurs partly because the positioner is part of a complex group of components called a

control loop. When a problem is encountered, the focus tends to be upon the controller or thevalve. In addition, fixing these components may result in satisfactory operation even though itseems that something else is wrong.

Therefore, this guide provides the basis for a systematic approach to troubleshooting thatdistinguishes between the component that causes a problem and the component that is affectedby that problem. The approach is to briefly discuss the control loop and then focus on thefunction of the positioner in the control loop. Subsequent sections address positioner design and


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application, calibration, condition monitoring/preventive maintenance, and troubleshooting. Theappendices contain control loop details, checklists for positioner checkout, a glossary,information about digital positioners, and positioner problem case histories.

1.4 Highlighting Key Points

Throughout this report, key information is summarized in key points. Key points are bold-lettered boxes that succinctly restate information addressed in detail in the surrounding text,making the key point easier to locate.

The primary intent of a key point is to emphasize information that enables individuals to takeaction for the benefit of their plant. The information included in these key points was selected byNMAC personnel, consultants and utility personnel who prepared and reviewed this report.

The key points are organized according to the three categories: O&M costs, technical, andhuman performance. Each category has an identifying icon, as shown below, to draw attention to

the specific category when quickly reviewing the guide.

Key O&M Cost Point

Emphasizes information that will result in reduced purchase, operating,

or maintenance costs.

Key Technical Point

Targets information that will lead to improved equipment reliability.

Key Human Performance Point

Denotes information that requires personnel action or consideration in

order to prevent injury or damage or ease completion of the task.

Appendix G contains a listing of all key information in each category. The listing restates eachkey point and provides reference to its location in the report. A review of this listing can helpusers of this guide determine if they have taken advantage of key information that the authorsbelieve would benefit the users plants.

1.5 Glossary

A glossary of terms used in this guideline is contained in Appendix C.

1.6 References

1. Control Valve Guidelines, EPRI, Palo Alto, CA: 1994. TR-102051-R1.


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2INTRODUCTION TO THE CONTROL LOOP

2.1 Purpose

Positioners are part of a control loop. If not specified properly or if not maintained properly,positioners can have an unacceptable effect on process control, costing both maintenance timeand materials. These effects may result in valve hunting, in a condition called limit cycle, orthey can result in plant shutdown due to internal leakage. In addition, positioners are controlloops themselves and subject to control loop problems. Therefore, when dealing with positioner

problems, you are really dealing with control loop problems at both system and componentlevels. This section provides information about control loop basics.

2.2 Overview

The control loop is fundamentally nothing more than a group of components, normally in aseries, each of which responds to input from a previous component by supplying output to thenext component. The goal of the loop is to work together to control/maintain a process as desiredwhen challenged by some disturbance. To operate as desired means that there is a processvariablefor example, flow or temperaturethat will be maintained at a value. To do thisautomatically without operator action, some type of feedback is necessary to provide this

maintenance. In the simplest case, we may only want to control a flow, and we do so bymeasuring the flow and causing a valve to close or open based on whether the flow value is highor low when compared to the desired value. In another case, we may want to control tank levelby controlling how much flow is going into or out of the tank. Alternatively, we can control thetemperature of a fluid exiting a heating unit by varying the voltage applied to the heating elementused as the source of heat.

2.3 Description

Each process control loop may be described in terms of process variables and control elements.The control elements include a sensor, transmitter (usually lumped with the sensor), controller,

and final control element. In the following example of a control process (see Figure 2-1), we aretrying to maintain a process variable, the tank level.


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Figure 2-1Example of a Control Process

The tank level is measured by the level sensor, and the level transmitter sends a correspondingsignal to the controlling device. In most cases, this signal is electrical and has been scaled to

correspond to a maximum and minimum level of interest. In the controlling device, the levelsignal is compared to a signal that corresponds to the desired tank level (the setpoint). In somecases, this comparison may take place before the controlling device provides a signal. The resultis an error signal that has a direction (high/low) or polarity (plus/minus) that is desired to indicatewhich way the tank level is from the desired level. The controlling device has been programmed(adjusted) to provide a signal to the positioner of the final control element (FCE), an air operatedflow control valve (AO FCV) that causes it to open or close so that the error signal becomeszero. If everything functions correctly, the system is in equilibrium with the flow-in and theflow-out essentially the same.

However, the flow-in is subject to disturbances, and as a result, the level changes. As anexample, suppose that the flow-in becomes less. The tank level begins to drop because the flow-

out has become greater than the flow-in. The level sensor detects this, and the correspondingsignal is sent by the transmitter and immediately compared to the desired value. The controllergenerates a signal to the positioner, which will cause the AO FCV to move in the closeddirection.The positioner compares the valve position with the signal and causes the valve tomove as required to a new position. As the flow-out is reduced to below the flow-in, the tanklevel finally begins to recover. Depending on the controlling device, the level either returns tothe desired level or to one that is slightly lower (with an offset). In most power plant controllingdevices, the processreturns to the desired level without offset.

The preceding example shows how a control loop functions. As with every technology,understanding the language is essential to understanding the concepts. Information about control

loops includes terms likefeedbackand manipulated variable. Table 1 contains some terms andhow they apply to the example. These terms are also defined in Appendix C and in ProcessInstrumentation Terminology [1].


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2-3

Table 2-1The Positioner in the Control Loop

Terminology Physical Representation

Controlled variable Tank level

Manipulated variable Flow-out

Setpoint The (electrical equivalent of the) desired tank level

Summing point Where (the electrical equivalents of) the actual tank level (fromthe transmitter) and the set point are compared. A comparator.

Feedback element The level transmitter

Disturbance Flow-in

Error Output of the summing point

The process above is what is called a closed loop because a feedback occurs through the leveltransmitter. On a control panel, closed-loop operation is known as auto control. If the feedbackpath is broken (for example, the transmitter fails as-is), then the control loop becomes an openloop and the system is in manual control. An operator then has to directly manipulate the flow-out in response to level changes.

The enemies of all closed loops are dead time and non-linearity. Dead time is the delayassociated with a control loop response.Non-linearity is the failure of a control loop to respondto an error with the same magnitude of response over the range of control (for a detaileddiscussion of these topics, see Appendix A). Positioners can help to mitigate these problems.

One of the positioners functions is to help combat dead time by supplying additional gain, oramplification, to the control loop. Gain increases response time and, therefore, improves thecontrol loops ability to deal with disturbances. However, too much gain can also be a problem,causing instability in the loop.

To mitigate process non-linearity, positioners can be adjusted or characterized to produce anoutput that algebraically subtracts the non-linearity over the range of control. This is done bycharacterizing a cam. Characterization is discussed in Chapter 3.

2.4 Summary

Positioners are part of a control loop. If not specified properly or if not maintained properly, theycan have an unacceptable effect on process control, costing both maintenance time and materials.The control loop is fundamentally nothing more than a group of individual components in aseries, each of which responds to input from the previous component by supplying output to thenext component The goal of the loop is to work together to control a process as desired whenchallenged by some disturbance.


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2-4

To operate control loops, use feedback to monitor a controlled variable, and send signals to amanipulated variable to keep the controlled variable at some setpoint. Control loops usingfeedback are called closed loops. All closed loops experience time delay and non-linearity inprocessing changes to the controlled variables. The delay of the most concern is dead time,which positioners can improve by supplying gain. To mitigate process non-linearity, positionerscan be adjusted or characterized, using a cam, to produce an output that subtracts the deviationsover the range of control.

2.5 Additional Information

For additional information on control loops, see Appendix A, Control Loop Details.

2.6 Reference

1. ANSI/ISA S51.1-1979 (R1993), Process Instrumentation Terminology,Instrument Society

of America, Research Triangle Park, NC: 1993.


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3-1

3POSITIONER DESIGN AND APPLICATION

3.1 Introduction

Every manufacturer supplies a technically accurate service manual that describes theconstruction, operation, and calibration of the positioner at various levels of detail. Therefore, anunderstanding of the basic designs and applications is useful for interpreting the manual duringcalibration, maintenance and troubleshooting. This section will review basic positioneroperation, break a positioner up into discrete components, describe various design principles for

these components and provide a table of commonly used positioners classifying them accordingto the principles. Using these principles of operation, the section will describe how positionersare applied to solve many problems that occur in control valve applications, including theproblems of dead band and linearity discussed in the previous section.

3.2 Two Black Boxes

Before discussing positioner design, it may be helpful to think about the positioner as a blackbox. This will help to understand positioner capabilities and function. In fact, two black boxesare discussed in this section, and both look the same on the outside. Each has two pneumaticinputs and one pneumatic output. The inputs come from a supply of air, for example, at 20 psig

(137.9 kPa), and a pressure regulator that we can vary from 0 psig to 30 psig (206.8 kPa). Theinputs are called supply and signal, respectively. The output of each is routed to the diaphragm ofan air actuator. The actuators begin stroking at about 3 psig (20.7 kPa) and completely strokewith about a 12 psi (82.7 kPa) change of air pressure. Three pressure gages measure the inputfrom the regulator and the output to each actuator. A mechanical linkage from the actuator stemto the black box corresponds to the position of the stem.

3.2.1 Black Box One

The signal to black box one (BB1) is increased. The actuator stem begins to move when thepressure is just over 3 psig (20.7 kPa). It continues to move until it reaches some limited

positionfor example, the backseat in the attached valveat just under 15 psig (103.4 kPa).The distance traveled is proportional to the amount of pressure at the input. In addition, theoutput pressure closely follows the signal pressure, but it may not be the same. As the signalpressure varies up and down, the position and the output pressure respond up and down. As thesignal is increased to just below 15 psig (103.4 kPa), the valve stops moving, and the pressuregoes to 20 psig (137.9 kPa) and stops.


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3-2

The signal is set to 9 psig (62.1 kPa), and the actuator stem is gagged at about the mid-positionof travel. The signal is increased to 9.1 psig (62.7 kPa). However, instead of going to 9.1 psig(62.7 kPa), the output pressure increases to 20 psig (137.9 kPa), or the supply pressure.Correspondingly, when the signal is decreased by 0.1 psig (0.7 kPa)that is, to 8.9 psig (61.4kPa), the output goes to 0 psig. Repeating the change from 9.0 psig (62.1 kPa) but going to 9.2psig (63.4 kPa) only makes the pressure change faster.

Next, the stem is disconnected from the box, and the linkage is left in the same position as whenthe stem was locked. The signal is increased to 9.1 (62.7 kPa), and the output pressure respondsas it did with the locked stem, that is, going to 20 psig (137.9 kPa). The actuator, free to move,shifts to a fully open position. Similar locked-stem responses are obtained at greater pressures orsmaller pressures. In effect, the response of the box is no different than when the stem waslocked.

3.2.2 Black Box Two

With the valve stem free to move, black box two (BB2) responds the same as BB1. However,when the stem is gagged as before, the response is somewhat different. When the signal isincreased to 9.1 psig (62.7 kPa), the output pressure goes quickly to 14 psig (96.5 kPa) a 5 psig(34.5 kPa) increaseand stops. Increasing the signal to 9.2 psig (63.4 kPa) results in 19 psig(131 kPa) at the output. At 9.3 psig (64.1 kPa) signal input, the output is 20 psig (137.9 kPa).Correspondingly, by decreasing the signal by 0.1 psig (0.7 kPa)that is, to 8.9 psig (61.4 kPa),the output goes to 4 psig (27.6 kPa). Decreasing to 8.8 psig (60.7 kPa) results in 0 psig output.The same response is obtained when the stem is disconnected and the linkage fixed as before.

3.2.3 Discussion

Both of these black boxes have responded as positioners. When they are hooked up to the stem,the stem position corresponds to the signal received. As long as the stem is free to move, there isvirtually no difference between having a positioner or having the signal connected directly to thevalve actuator diaphragm/piston. The response is the same.

The action of a positioner is very clear when the stem is prevented from moving. Thiscorresponds to factors such as friction and inertia. The positioner responds by immediatelysupplying gain to the signal. The process does not have to drift further from the setpoint to obtainan error signal large enough to cause movement. This gain quickly overcomes the resistance tomovement.

Because the black box attempts to drive the stem to some desired position (as observed by anincreased/decreased output pressure to the actuator) when the stem is prevented from moving (adisturbance), the black box must contain components that are acting together as a control loop.

The black boxes illustrate two types of gain. In the case of BB1, the gain is called flow gainbecause the flow increases with increased signal pressure and output continues to rise as long asan input signal is present. In the case of BB2, the gain is called locked-stem pressure gain


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(sometimes called open-loop gain) because output pressure has a fixed relationship to the inlet orsignal pressure.

In reality, the difference between the black boxes (pressure versus flow gain) is not seen. Whenpressure changes are made to a pressure-gain type of unit, the changes are so large (that is, 1 psig[6.9 kPa]) or more) that when multiplied by the gain, the output pressure exceeds the remainingmargin between the initial pressure and either zero pressure or the supply pressure. As a result,pressure changes appear the same as seen in flow amplification. In normal maintenanceactivities, the values or type of gain is unimportant, but knowledge of their existence mayprovide understanding of a problem or the result of some test.

The locked-stem result and disconnected fixed-linkage-position response are the same. Thismeans that it is the linkage that is providing feedback to the positioner to null or correct the inputsignal. Stem position is the desired response; therefore, the linkage provides feedback to thepositioner. Because there is feedback, the positioner is a closed control loop.

The boxes have a maximum or minimum output pressure equal to the supply pressure (20 psig

[137.9 kPa]) or zero. This means that at the extremes of travel, a positioner can provideadditional force to the stem because it is not limited to signal range, for example, 3 15 psig(20.7103.4 kPa), but to the larger range of zero to the supply pressure.

3.3 Positioner Design

3.3.1 Function

Based on the capabilities described in the black box discussion, a positioner can have thefollowing functions:

N

Provide an output pressure that tracks the input signal closely. This does not necessarilymean that the input and output pressures are the same. The actuator characteristics or eventhe positioner characteristics may introduce some difference. What this does mean is that forany input signal, there is a corresponding actuator position that is always the same for anygiven pressure.

N Provide an output pressure that increases (or decreases) rapidly whenever there is adifference between the desired position and the corresponding input pressure. In other words,provide a gain. This may be either a flow gain or a pressure gain. When the position iscorrect, the positioner reduces the gain rapidly.

N Provide for tracking in which the output pressure increases with the input pressure increase(direct acting), or in which the output pressure decreases with the input pressure increase(reverse acting).

N

An additional function not described in the black box discussion is the outputcharacterization that mitigates process non-linearity (see Appendix A, Section A.4.2Linearity).


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3.3.2 Building Blocks

Within any positioner is a set of discrete components that function together as described above.In all cases, a motion of one of these components causes a series of events that result in acounter-motion. This motion may be the simple movement of components or a result from the

application of force. There are a limited number of ways that these component can be designedand assembled to make this happen.

The block diagram in Figure 3-1 illustrates this assembly. These components perform thefollowing functions within the positioner:

1. Input signal conversion: The pneumatic input is converted to a mechanical motion.Positioner gain may be developed here using a pneumatic amplifier. In such cases, thepositioner is referred to as a two-stage device.

2. Output signal generation: A mechanical motion causes a directional control valve to changeposition and supply air to (or exhaust air from) the actuator. Positioner gain will normally be

developed here, usually through the use of spring or spring-like devices.

3. Output signal correction: The gain developed earlier is reduced to zero.

Figure 3-1Positioner Block Diagram

3.3.3 Input Signal Conversion

The input signal can be sent to the positioner is one of two ways:

N A pneumatic signal (315 psig [20.7103.4 kPa], 630 psig [41.4206.8 kPa] 327 psig[20.7186.2 kPa], etc) directly from the controller

N An electrical signal (420ma, 1050ma, 010v) from the controller that is converted to apneumatic signal by a current to pneumatic (I/P) converter or a voltage to pneumatic (E/P)converter that is either external or internal to the positioner.


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The pneumatic signal must be converted to a mechanical motion. This conversion process alwaysbegins with a diaphragm or bellows chamber. The changing pressure is transformed to a linearmotion. Simplified sketches of these components are shown in Figure 3-2.

Figure 3-2Input Signal Converters

The linear motion is now used directly to position a directional control valve within thepositioner, or it is used to modulate the flapper of a nozzle-flapper.

3.3.3.1 Nozzle-Flapper

Input signal conversion may employ a preamplifier to enhance positioner response. Thenozzle-flapperis a device that produces an amplified pneumatic signal. The flapper is sometimescalled a vane.

Figure 3-3 shows the basic layout of the nozzle-flapper. In this example, the bellowsexpands/contracts, moving the flapper toward or away from the nozzle. During a steady stateoperation, supply air passes through two restrictions. The first restriction is a fixed orifice andsized to permit adequate flow for nozzle-flapper operation without affecting the supply pressure.The second restriction is caused by the flapper moving toward/away from the nozzle. Moving theflappertoward the nozzle increases the pressure in the nozzle chamber and vice versa. In otherwords, the orifice restriction size is smaller than the nozzle restriction size, which allows thesupply pressure to bleed to atmosphere faster than it enters the unit through the fixed restrictionwhen the flapper is away from the nozzle.


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Figure 3-3Nozzle/Flapper Operation

Amplification or gain comes about because it takes very little motion of the flapper and hencevery little pressure change in the input signal converter to give wide variations in the controlpressure. The input bellows (or diaphragm in some cases) is designed to produce relatively largemovement to the flapper relative to its pressure changes. Thus, a small pressure change in theinput will produce a large change in the nozzle chamber pressure. The nozzle chamber pressureis then piped to a diaphragm-type signal converter that is connected to the output signalgenerator. A nozzle-flapper/diaphragm-like signal converter combination is commonly called atwo-stage device and is characterized by high gain and rapid response. In some processes, this

may be desirable, such as feedwater control.

The nozzle-flapper mechanism is used as a preamplifier and not as a primary signal generator forthe following reasons:

N If the restrictions are made large enough to make it possible to rapidly change the back-pressure, excessive amounts of air are required.

N

Large restrictions result in an increase of the blast effect (momentum of the air streamthrough the nozzle). This requires that the flapper be more robust and makes it necessary touse larger forces to overcome the blast. This robustness results in feedback mechanisms thatare less sensitive to changes in control pressure.

N If the restrictions are kept small to reduce the air consumption, it takes too long to change theback-pressure. As above, the sensitivity is reduced. Therefore, it is necessary to route thecontrol pressure to a second device, generically called a relay, which then controls the supplypressure to generate the output signal.


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The fixed flow restriction can become fouled, reducing flow rate and causing loss of sensitivity,especially when movement is demanded in one direction. In the extreme, this may result in novalve movement from one end of the stroke. All positioners using nozzle/flapper mechanismshave a means to clean this fixed flow restriction.

3.3.4 Output Signal Generator

3.3.4.1 Connecting the Input Converter to Output Generator

The input signal converter sends a linear motion that is used by the output signal generator in oneof two ways:

N The linear motion is used directly to cause the output signal generator to move and totransmit an output signal. For example, an input diaphragm linear motion is connecteddirectly to the stem of a sliding spool directional control valve.

N The linear motion is used to move a balance beam that causes the output signal generatorto move and to generate an output signal. An example of a balance beam is shown inFigure 3-4.

3.3.4.2 Types of Output Signal Generators

The term output signal generatordescribes the function of several different types of mechanismsthat are used to provide the output signal. The manufacturers terms used to describe the outputsignal generator function vary widely and include pilot valve, pneumatic relay, pneumaticamplifier, and relay. In other words, there is no common functional nomenclature for these or,for that matter, any of the other parts of the positioner. Therefore, the names of these devices are

meant to describe the particular make-up of the part by using nomenclature commonlyencountered in various industries.

Figure 3-4Balance Beam


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Sliding Spool Directional Control Valve

The sliding spool valve (see Figure 3-5) finds use in many pneumatic and hydraulic applications.It is very simple in construction and usually more rugged but slower than the two-stage device. Atypical sliding spool valve is shown in Figure 3-5. Neutral spool position is shown. If the spool ismoved to the right, then supply is ported to Cylinder A; if it is moved to the left, then supply isported to Cylinder B. Simultaneously, Cylinder B (A) is vented through a groove (identified asexhaust in Figure 3-5) or other such passage and hence to the atmosphere. The input to the spoolis usually supplied by the mechanical motion of a diaphragm. The spool may be configured toprovide two outputs for use in double-acting actuator applications or with one output plugged forspring-operated (single acting) actuators.

Figure 3-5Spool Valve (Pneumatic)

Double Three-Way Poppet Directional Control Valve

The double three-way poppet directional control valve (Figure 3-6) describes a device thatprovides the same output capability as the spool valve. For example, it can be used for double-acting or single-acting actuator applications. However, the internal design varies considerably. Itusually combines the input conversion and output signal generation in a single unit. Referring toFigure 3-6, control pressure is contained between two diaphragmsan isolation diaphragm(shown to the right of the control chamber in the figure) and a force balance diaphragm (shownto the left of the control chamber). On the other side of the force balance diaphragm is a supplypressure chamber (shown to the left of the supply pressure chamber) also with an isolationdiaphragm. The control pressure is varied by using a nozzle/flapper device as previouslydescribed. Variation in the control pressure allows the hollow center structure to shift right or

left, thus opening poppet valves at both ends. The figure shows that when the input signal islowered, it causes the flapper to move away from the nozzle and thus lowers the control pressure,allowing movement to the right. This causes supply air to be ported to Cylinder A and allowsCylinder B to exhaust through the hollow central structure. As the central structure begins tomove, however, immediate feedback to the flapper controls the positioner pressure gain. As theactuator stem begins to move, the feedback is received (output signal correction), and the flapperis brought back to its original position, thus returning the central structure to a neutral positionwith all poppet valves closed.


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Figure 3-6Double Three-Way Poppet Directional Control Valve

Throttle Valve

The throttle valve is a simple device (see Figure 3-7) that allows the air supply to go in twodirections. As the throttle moves toward the output end, more air is bled off and the outputpressure is reduced. Movement of the throttle is usually through a balance beam device asshown.


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Figure 3-7Throttle Directional Control Valve

Pressure Control Directional Control Valve

The last commonly used output signal generator is sometimes called a relay orpneumatic relay.It operates like a pressure control valve, except the adjustable coil spring is replaced by apressure (control) chamber (see Figure 3-8). The pressure in the chamber is the control pressurecreated by a nozzle/flapper device. When in equilibrium, the pressure on the other side of the

diaphragm is the output pressure. When the control pressure increases, it pushes down the upperdiaphragm, which is connected to the lower diaphragm, thus pushing the poppet down. Supplypressure is then admitted through the lower end of the poppet to the actuator until the outputpressure value provides enough force on the diaphragm to counter the control pressure and closethe poppet. Similarly, when the upper diaphragm moves up, the seat at the upper end of thepoppet moves up and allows venting from the actuator diaphragm to the exhaust. The lower seatprevents the poppet from moving up. Again, the output pressure decays sufficiently to allow theupper diaphragm to move down.


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Figure 3-8Pressure Control Directional Control Valve

3.3.5 Output Signal Correction (Feedback)

3.3.5.1 Purpose

The output signal correction is to use the feedback to nullify or balance the output when thedesired position is achieved. This relationship has been identified in Figures 3-3, 3-4, 3-6, and3-7. The output signal correctionis also the point in the positioner loop in which the output canbe modified, or characterized to mitigate process non-linearity.

3.3.5.2 Interface Between Valve and Positioner

To begin the balancing process, a rod, bracket, or other suitable device is attached to the stem.The device is mechanically routed to the positioner (for example, using linkages) to provide theappropriate motion used by the positioner: rotation or linear push-pull. If the motion is rotation,the input to the positioner is a lever.

3.3.5.3 Positioner Input

A linear feedback motion always results in a proportional feedback to provide a balance force tonullify the output. In other words, no matter where the stem is, any amount of movement of thestem always results in the same but proportional amount of movement at the positioner. There

may be an interface that allows the input range to be adjusted, but the motion remains linear.Sometimes it is necessary to modify this input. This is where the rotary motion comes in.

Rotary motion is used in the positioner to modify the feedback in a non-linear way if desired.The reason for making it non-linear is to compensate for some non-linearity in the control loop.The universal method for doing this is by using a cam. Input from the stem causes the input leverto rotate the cam (see Figure 3-9). The cam follower rides on the edge of the cam and follows the


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cam contour. This allows the original input lever motion to be changed, or characterized, beforeit is used to correct the output through the balance beam.

Figure 3-9

Feedback Cam Characterization

Normally, three cams are available as standardsquare root or quick opening, linear, and squareor equal percentagebecause these cams handle some of the more common non-linearities. Alsoavailable are blank cams that can be modified by the user to obtain other characterizations.


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3.3.5.4 Motion-Balance and Force-Balance (Balancing Mechanisms)

Output signal correction uses feedback to nullify or balance the output when the desired positionis achieved. Balancing involves one of two principles. The motion that initiated the action must

be met with a counter-motion, or the force that initiated the action must be met with a counter-force. These counter-actions are referred to as motion balance and force balance.

The motion-balance positioners use nozzle-flapper devices (however, this does not mean thatusing a nozzle flapper makes it a motion balance positioner). These positioners use a beam,commonly called a balance beam, that moves about a pivot. One part of the beam is usuallymoved by a bellows in response to the input signal. This motion moves the flapper toward/awayfrom the nozzle. The feedback from the valve position is also applied as a motion to the beam.This motion counters the original input in such a way that a different output pressure is seen atthe relay. In Figure 3-3, the flapper acts as a motion-balance device.

Most positioners use the force-balance concept. In the force-balance device, the input signal

creates a force that is resisted by an opposing force created by the feedback mechanism. Forcebalance devices are commonly associated with spool valves in which the spool is moved backand forth in response to the equilibrium position of the opposing forces. One of the easiest waysto identify a force-balance device is the existence of stem feedback being applied through aspring, which is called a range spring.


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3.4 Classification of Commonly Used Positioners

Table 3-1 classifies commonly used positioners according to the principles described in thischapter. While the actual designs may vary somewhat in appearance when compared to the

chapter figures, the underlying principles of operation remain the same.Table 3-1Classification of Commonly Used Positioners

ManufacturerModel

Input SignalConversion

Output SignalGenerator

Output SignalCorrection

Type ofGain

Bailey AP Diaphragm, nozzleflapper

Double three-waypoppet directionalcontrol valve

Force-balance,camcharacterization

Pressure

Bailey AV,5311450,5321030

Bellows, balancebeam

Sliding spooldirectional controlvalve


Flow

Conoflow GP 50 Bellows, nozzleflapper, diaphragm


Motion-balance,camcharacterization

Pressure

Fisher 3582 Bellows, nozzleflapper, diaphragm (inpressure controlvalve)

Pressure controlvalve

Motion-balance,camcharacterization

Pressure

Masoneilan 4600 Diaphragm Sliding spooldirectional controlvalve


Pressure

Masoneilan 7000 Bellows, balance

beam

Throttle valve Force-balance Pressure

Moore 74 Diaphragm, nozzleflapper, diaphragm(two-stage)


Force-balance Pressure

Moore 750P Diaphragm, nozzleflapper, diaphragm(two-stage)



Pressure

Valtek Beta Diaphragm, balancebeam



Flow

Valtek System 80and XL Series

Diaphragm, nozzleflapper

Double three-waypoppet directional

control valve

Force-balance,cam

characterization

Pressure


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3.5 Positioner Application

Positioners are used to solve many problems that occur in control valve applications. Thefollowing sections describe the major uses.

3.5.1 Limiting Control Valve Dead Band

Dead band is a characteristic of any mechanical device, and the control valve is no exception.Dead band refers to the delay in motion of a control valve that occurs after a reversal in demand.In a sense, it is free play. It can involve part looseness (backlash), inertia, friction, or friction.To overcome dead band, the signal to the valve is sufficiently amplified (gain is added) by thepositioner so that its effect can be minimized. Dead band adds to dead time, the enemy of controlloop stability. Dead band as much as 5% is not uncommon for a control valve without apositioner. With a positioner, dead bands can be reduced to less than 1%. There are limitationson the amount of gain that can be supplied to limit dead band. For additional details, see the

discussion on dead band in Appendix A.

3.5.2 Mitigating Stiction or Stick-Slip

Stiction (from sticking and friction) is a problem caused by the existence of two types of friction:static and dynamic. Static friction is nearly always greater than dynamic friction, which becomesa problem with packing friction because it is the most dominant form of friction found in mostcontrol valves. It can also be the result of cage style control valves that use piston rings forsealing and guiding.

Stiction is recognized quickly if valve position and controller demand are plotted on the samegraph (see Appendix A, Figure A-7). The following scenario is observed: The controller sends apneumatic signal to the valve to correct a process deficiency. The actuator pressure changes, butthe control valve does not move because of the static packing friction. Therefore, the processcontinues to drift away from the set point causing the controller to continue to change the inputto the control valve. Finally, the actuator pressure is sufficient to apply enough force for the stemto overcome the static friction. It overshoots the desired position, however, because the smallerdynamic friction dominates. Of course, the over-correction becomes a process problem; thecontroller sends the opposite signal; and an overshoot occurs again. What is seen on the graph is

a square wave valve position with a saw-tooth controller output. The two plots will be 180G

outof phase. This situation can go on forever. Another name for this kind of plot is limit cycle.Because some processes are very sensitive to striction, it can result in excessive instability and

equipment trips. A positioner is one of the most effective ways to deal with it. The packingfriction effects are not eliminated, but controlled for two reasons:

N A control valve with a positioner moves to the position demanded by the controller. Rapidand accurate positioning is assured by immediate stem feedback at the positioner. A controlvalve without a positioner receives only a pressure and relies on the process feedback loop todetect the change. Due to integral control action, the signal continues to increase untilmovement is detected through the feedback loop. For many processes, this delay in detectionresults in overshoot.


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N

Some sources refer to the positioner as a proportional controller. This means that it willprovide an output pressure to the actuator greater than the demand. This input pressure willcontinue to increase until either the proper movement is obtained or the pressure reachessupply pressure. This gain quickly overcomes the friction and, together with the feedback,mitigates the amount of jump in valve position associated with stiction.

3.5.3 Change Valve Response

The positioner cam may be used as follows:

N

To change the installed valve flow characteristic (for example, from linear to equalpercentage) in order to optimize the process

N

To linearize the valve flow characteristics instead of changing the valve trim

N To redistribute the gain of the valve (ratio of valve output to controller input) to achieve abetter controllability in certain operating ranges.

3.5.4 Control Double Acting Actuator

A controller has only one output, but the double acting actuator requires two inputs (and exhaustpaths). Some positioners are designed (or can be) to provide two output pressures and twoexhaust paths.

3.5.5 Increase Shutoff Forces

The positioner can be used to drive the output pressure to zero or to full supply pressure (forexample, 20 psig [137.9 kPa]) at the end of the valve travel, rather than just the signal pressure.In other words, without the positioner, only the signal pressure (for example, 315 psig [20.7103.4 kPa]) would be available. If the valve uses a direct acting actuator (fail open), then the fullsupply pressure is available to shut the valve. In the case of reverse acting (fail close), thepressure is completely removed, allowing full spring force to load the seat at closure.

3.5.6 Split-Ranging

Sometimes, it is desirable to control a process using two or more valves in parallel instead ofone. The advantage of this is to reduce the range of flow controlled by any one valve and, thus,gain better control. To accomplish this method of control requires that the valves be opened in

sequence usingone control signal. A positioner can be adjusted to open and stroke a valve usingany range of input signal pressures. Therefore, for a two-valve arrangement, the first valve fullyopens after the input signal has changed only half of the full range. The second valve then opensand is fully opened at the full range. It is obvious that this can be extended to a three- or four-valve arrangement if it is desirable.


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3.5.7 Delays Due to Distance Between Controller or I/P Converter and Valve

A pneumatic signal can take too long to get to the valve that it is controlling. This delay isreferred to as lag, because the response begins immediately but takes time to reach the necessarypressure. For example, if a step change in pressure is sent down a 200-foot (60.96-m), 0.375-inch

(0.9525-cm) inside diameter line, it takes 0.43 seconds for the value at the other end to reach63.2% of the input value (see Figure 3-10). In some applications, this is a problem. Adding apositioner to the valve will greatly improve the situation.

Figure 3-10

Pneumatic Transmission Lag: Time to Reach 63.2% Final Value (Time Constant 99

)


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4-1

4CALIBRATION

4.1 Calibration and Condition Monitoring/Preventive Maintenance

The goal of calibration, or alignment, is to ensure that the positioner is functioning correctlywithin design tolerances, for example, start and stop points and travel. It ensures that the fidelitybetween the input signal and the position of the valve is established. Calibration also starts thecondition monitoring/preventive maintenance (CM/PM) process (see Section 5). Finally, it setsthe stage for more efficient and rapid troubleshooting. Basic calibration is an alignment process.

When the process is completed, the actuator should operate in accordance with themanufacturers specifications. This is also the opportune time to obtain baseline data that will beused for CM/PM and troubleshooting.

4.2 Basic Calibration

The basic calibration process consists of three steps. These steps may be performed together, butthe method used must be able to discriminate problems in each step. The steps may be completedmanually or by using data acquisition systems. A discussion of the latter is included at the end ofthis chapter.

The three steps are:

N Bench set confirmation

N

Feedback linkage alignment, including cam alignment

N

Zero and span adjustment

4.2.1 Bench Set Confirmation

Technically, the bench set is not part of the positioner alignment, but an improper bench set canaffect a valve operation that might be associated with the positioner operation. Bench set is the

nameplate specification used to verify a proper actuator operation. Bench set is expressed as apressure range from the start of the actuator stroke to the valves rated travel. Because actuatorspring rates are not consistent, it is reasonable to assume that only one of the bench set points canbe met. Therefore, spring adjustments must be made to meet the most important set point. Onair-to-open valves, the startpressure is important for a valve that requires positive shutoff by thespring (the spring provides the seat load). On air-to-close, the endpressure value is important tohave enough force to overcome the spring force and the valve friction and to seat the valve.


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When conducting the bench set test, the configuration of the actuator and valve is defined by thevalve manufacturer. The most common configuration is to have the actuator disconnected fromthe valve stem. Some manufacturers require that the packing load be removed but that the valveplug and stem remain attached to the actuator. This method accounts for the weight of the stemand plug in the proper operation and sizing of the valve and actuator assembly. Because thevalve forces are not present in many cases, the bench set pressure range is not the same as thepressure required to stroke the valve in actual service. Making the adjustment while the valve isin service may result in failure of the valve to fully stroke.

Key Technical Point

The bench set pressure range is not the same as the pressure required to

stroke the valve in actual service. Making the adjustment while the valve is

in service may result in unsatisfactory performance and/or make the valve

inoperable.

4.2.2 Feedback Alignment

The purpose of this alignment is to ensure that the zero and span of the feedback mechanismposition correspond to the fully open (closed) and fully closed (open) valve travel positions. Thepositioner is not in service during the performance of this alignment. Either pressure is supplieddirectly to the actuator, or the hand-wheel is used to position the valve. Failure to perform thisalignment may result in calibration problems, positioner performance issues, or componentdamage.

Feedback alignment consists of the following:

1. Verifying that the installation is in accordance with appropriate vendor information and plant

documents to determine specific requirements

2. Verifying that the positioner is mounted rigidly to the valve

3. Inspecting to determine that linkage is tight and that appropriate washers are installed

4. Checking and adjusting feedback linkage so that it is within allowable limits

5. Making sure there is freedom of movement throughout the range of travel so that thepositioner is not in the stops at the ends of travel

6. Verifying that the appropriate cam is installed or, if the cam is a multiple type, that the

feedback mechanism is on the correct range, for example, linear, square, or square root

7. Setting the feedback mechanism to the zero position on the cam

8. If applicable, setting the stroke lever parallel to the spring lever at midstroke

9. Stroking the valve and adjusting the appropriate linkages to ensure that the cam rotates in thecorrect direction, stops at the 100% position, and returns to zero as appropriate


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Key O&M Cost Point

Failure to perform feedback alignment may result in calibration problems,

positioner performance issues, and/or component damage.

4.2.3 Zero and Span Adjustment

4.2.3.1 Purpose

The purpose of the zero and the span adjustments is to synchronize the valve position topositioner demand. At first, this seems to be just varying the input pressure from the bottom tothe top of the control range (for example, 315 psig [20.7103.4 kPa]) and making sure that thevalve strokes appropriately. However, a positioner can provide one additional advantage over anoperation without a positioner. A positioner can ensure that at the extremes of the control range,the maximum force of the actuator is available. In other words, the positioner can be calibrated togive zero or full supply pressure rather than 3 psig or 15 psig (20.7 kPa or 103.4 kPa). This can

be a distinct advantage in providing tight closure of a control valve to prevent seat damage.

4.2.3.2 Static Band

To obtain the maximum actuator force, pressure static bands are established at both ends of thepressure range. In reality, they are intentional dead bands, but the term dead band is not usedhere to avoid confusion with the dead band associated with control loops. The magnitude ofthese static bands does not have to be large. A static band of about 0.3 psi (2.1 kPa) allows forboth instrument inaccuracies and positioner gain. This means that instead of expecting the valveto start opening at 3 psig (20.7 kPa) (or 15 psig [103.4 kPa ]), it should start opening at about 3.3psig (22.8 kPa) (or 14.7 psig [101.4 kPa])

The result of this is as follows when the valve is in service. The valve is given a close signal bythe controller. Assume that this is 3 psig (20.7 kPa). As the pressure reaches 3.3 psig (22.8 kPa)during the reduction to 3 psig (20.9 kPa), the valve is just shut. As the pressure goes to 3.2 psig(22.1 kPa) and below, the valve is not moving. In a sense, the stem is locked from going anyfurther closed. Recall the black box discussion in Section 3. The error of 3.3 minus 3.0 psig = 0.3psig (22.8 minus 20.7 kPa = 2.1 kPa) is multiplied by the gain of the positioner. The result is thatthe pressure to the actuator drops to 0 psig trying to make the valve respond to the lower signal.Hence, the valve is shut with maximum actuator force. For direct acting valves, the positioner iscalibrated to provide seat closure at about 0.3 psig (2.06843 kPa) below maximum, for example,14.7 psig (101.353 kPa). The same action occurs; only the actuator reaches the supply pressure

value, for example, 20 psig (137.895 kPa).

Key O&M Cost Point

Establishing pressure static bands at the ends of the normal control range of

valve operation results in tighter valve shutoff. Depending on application,

this will lessen or eliminate seat damage and/or reduce megawatt losses.


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Calibration

4-4

4.2.3.3 Adjustments

Before beginning the zero and span adjustment, determine if the positioner is a motion balancetype, for example, Fisher 3582 or Conoflow GP 50. If the motion beam has been repaired orotherwise affected by maintenance, then beam or flapper leveling is required before the zero and

span adjustments. In addition, some models require an adjustment if the unit is changed to asplit-range operation. The primary purpose is to ensure that the nozzle flapper approaches thenozzle to provide uniform variation of the control pressure with the flapper movement. Failure todo so results in the inability to properly calibrate the unit. Follow the manufacturers technicalmanual for this leveling process.

Key Human Performance Point

Before zero and span adjustments, motion balance positioners require beam

or flapper leveling if the motion beam has been affected by maintenance or if

the unit is changed to split-range operation. Failure to do so will result in the

inability to properly calibrate the unit.

Zero adjustments are always done in conjunction with span adjustments. In other words, after thespan has been adjusted, the zero adjustment must be re-verified to ensure that the zeroadjustment has not changed. The purpose of this guideline is not to provide zero and spanadjustment instructions. These must be performed in accordance with the manufacturerstechnical manual. However, because not all manufacturers provide for the establishment of staticband, the steps below are considered supplementary guidance to be used in conjunction with thetechnical manual. If a cam is installed, it is assumed that the cam has been adjusted as describedin the feedback alignment section.

Key Technical Point

Zero adjustments are always done in conjunction with span adjustments. In

other words, after the span has been adjusted, the zero adjustment must be

re-verified to ensure that the zero adjustment has not changed.

To obtain the static pressure bands, the following stepsare recommended. It is assumed that inall other respects, the valve is ready for service. For example, the packing has been adjusted tothe specified value; the stem/actuator adjustments have been made; the valve is free to stroke;and the travel is correct.

1. Verif

valve positioner principles and maintenance guide

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