section 9 valves, servos, motors, and robotsftp.feq.ufu.br/luis_claudio/segurança/safety... ·...

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30 August 1999 0:54 MHOO9-09.tex MH009-09.SGM MH009v3(1999/07/12)

SECTION 9VALVES, SERVOS, MOTORS,AND ROBOTS∗

Mark AdamsFisher Controls International, Inc., Marshalltown, Iowa(Process Control Valves)

Len AuerRosemount Inc., Eden Prairie, Minnesota (Current-to-PressureTransducers for Control-Valve Actuation)

Allen C. FagerlundR. A. Engel Technical Center, Fisher Controls International, Inc.,Marshalltown, Iowa (Control Valve Noise)

Bill FitzgeraldFisher Controls Company International, McKinney, Texas(Control Valve Troubleshooting)

David A. KaiserCompumotor Division, Parker Hannifin Corporation, RohnertPark, California (ServomotorTechnology in Motion Control Systems)

J. J. KesterBodine Electric Company, Chicago, Illinois (Stepper andOther Servomotors)

S. LongrenLongren Parks, Chanhassan, Minnesota (Servomotors)

Richard H. OsmanAC Drives, Robicon Corporation, Pittsburgh, Pennsylvania(Solid-State Variable-Speed Drives)

Bert J. PetersonFisher Controls International, Inc., Marshalltown, Iowa(Process Control Valves)

C. PowellGMFanuc Robotics Corporation, Auburn Hills, Michigan(Robots)

* Persons who authored complete articles or subsections of articles, or otherwise cooperated in an outstanding manner infurnishing information and helpful counsel to the editorial staff.

9.1

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9.2 PROCESS/INDUSTRIAL INSTRUMENTS AND CONTROLS HANDBOOK

Marc L. RivelandApplied Research, Fisher Controls International, Inc.Marshalltown, Iowa (Control-Valve Cavitation)

PROCESS CONTROL VALVES 9.5

INTRODUCTION 9.5

CHRONOLOGY 9.5

CONTROL VALVE BODIES 9.7

General Categories of Control Valves 9.7Sliding-Stem Valves 9.7Ball Valves 9.10Eccentric-Plug Valves 9.11Butterfly Valves 9.11Special Control Valves 9.16

CONTROL VALVE PERFORMANCE [1] 9.21

Dead Band 9.22Actuator–Positioner Design 9.22Valve Response Time 9.22Valve Type and Characterization 9.22Valve Sizing 9.23Economic Results 9.23

VALVE SELECTION 9.23

General Selection Criteria 9.23Pressure Ratings 9.24Operating Temperature 9.24

MATERIAL SELECTION 9.25

Carbon-Steel Bodies and Bonnets 9.25Alloy-Steel Bodies and Bonnets 9.25Stainless-Steel Bodies and Bonnets 9.25Selection of Materials 9.25Trim Parts 9.26Valve Packing [1] 9.26

VALVE CAPABILITIES AND CAPACITIES 9.28

Flow Characteristic 9.28Rangeability 9.29Pressure Drop 9.29End Connections 9.30Shutoff Capability 9.30Flow Capacity 9.30

VALVE SIZING 9.32

Basic Sizing Procedure 9.33Choked Flow 9.33Viscous Flow 9.34Piping Considerations 9.34Gas and Steam Sizing 9.35

ACTUATORS 9.35

Power Source 9.35Failure Mode 9.36Actuator Capability 9.36Control Functions 9.36Economics 9.37Actuator Designs 9.37Actuator Sizing 9.42Summary of Actuator Selection Factors 9.43

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VALVES, SERVOS, MOTORS, AND ROBOTS 9.3

VALVE CONTROLLERS AND ACCESSORIES 9.45

Valve Positioners and Controllers 9.45Digital Positioners [1] 9.46Electropneumatic Transducers 9.46

CONTROL VALVE INSTALLATION [1] 9.48

Storage and Protection 9.48Installation Techniques 9.48

SUMMARY CHECKLIST 9.52

REFERENCES 9.54

CONTROL VALVE TROUBLESHOOTING 9.54

Common Valve Maintenance Procedures 9.59Lapping the Seats 9.60

REFERENCES 9.63

CONTROL VALVE CAVITATION: AN OVERVIEW 9.63

INTRODUCTION 9.63

CAVITATION FUNDAMENTALS 9.64

Valve Hydrodynamics 9.64Cavity Mechanics 9.65Damage Mechanisms 9.66Scale Effects 9.67

CAVITATION ABATEMENT STRATEGIES 9.67

SYSTEM-LEVEL CAVITATION CONTROL 9.68

Valve Placement 9.68Backpressure Devices 9.68Gas Injection 9.69

CONTROL-VALVE SOLUTIONS 9.69

Material Selection 9.69Special Trim Designs 9.70

CONTROL-VALVE SELECTION 9.71

Background 9.71Cavitation Parameters and Coefficients 9.72Example 9.74

CLOSURE 9.74

REFERENCES 9.74

CONTROL VALVE NOISE 9.75

INTRODUCTION 9.75

NOISE TERMINOLOGY 9.75

SOURCES OF VALVE NOISE 9.77

Mechanical Noise 9.77Hydrodynamic Noise 9.77Aerodynamic Noise 9.78

NOISE PREDICTION 9.78

Hydrodynamic Noise: IEC 534-8-4 [1] 9.78Aerodynamic Noise: IEC 534-8-3 [2] 9.78

NOISE CONTROL 9.79

Quiet Valves 9.79Path Treatment 9.79

REFERENCES 9.82

SERVOMOTOR TECHNOLOGY IN MOTION CONTROL SYSTEMS 9.82

INTRODUCTION 9.82

TYPES OF SERVO MOTORS 9.83

General Characteristics and Comparison of Servo Motors 9.85

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GENERAL CONSIDERATIONS 9.90

Motor Parameters, Definitions, and Terminology 9.90Name-Plate Ratings 9.91

ELECTRICAL CONSIDERATIONS 9.93

Regulatory Considerations 9.93Speed versus Torque Curves 9.94Thermal Ratings–Insulation Class 9.96

MECHANICAL CONSIDERATIONS 9.98

Mounting 9.98IP Classification 9.101Couplers 9.101Bearings 9.102Lubrication 9.104Seals 9.105Vibration 9.105

REFERENCES 9.106

SOLID-STATE VARIABLE SPEED DRIVES 9.107

INTRODUCTION 9.107

REASONS FOR USING A VARIABLE SPEED DRIVE 9.107

SEMICONDUCTOR SWITCHING DEVICES 9.108

DRIVE CONTROL TECHNOLOGY 9.109

SOLID-STATE DC DRIVES 9.110

SUMMARY OF THYRISTOR DC DRIVES 9.111

AC VARIABLE FREQUENCY DRIVES 9.111

INDUCTION MOTOR VARIABLE SPEED DRIVES 9.112

CURRENT-FED VS. VOLTAGE-FED CIRCUITS: THE TWO

BASIC TOPOLOGIES 9.114

MEDIUM-VOLTAGE VARIABLE FREQUENCY DRIVES 9.114

THE LOAD COMMUTATED INVERTER 9.116

FILTER COMMUTATED THYRISTOR DRIVE 9.118

CURRENT-FED GTO INVERTER 9.119

NEUTRAL-POINT-CLAMPED INVERTER 9.120

MULTILEVEL SERIES-CELL INVERTER 9.121

CYCLOCONVERTER 9.121

COMPARISON OF MEDIUM-VOLTAGE MOTOR DRIVES 9.122

BIBLIOGRAPHY 9.123

ROBOTS 9.123

BASIC FORMAT OF ROBOT 9.123

AXES OF MOTION 9.124

Degrees of Freedom 9.124Work Envelope 9.127

LOAD CAPACITY AND POWER REQUIREMENTS 9.127

DYNAMIC PROPERTIES OF ROBOTS 9.127

Stability 9.128Resolution and Repeatability 9.130Compliance 9.132

END-EFFECTORS (GRIPPERS) 9.132

WORKPLACE CONFIGURATIONS 9.134

Work Cells 9.134

ROBOT PROGRAMMING AND CONTROL 9.134

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CURRENT-TO-PRESSURE TRANSDUCERS FOR CONTROL-VALVE

ACTUATION 9.139

TRADITIONAL FLAPPER-NOZZLE DESIGN 9.139

INTRODUCTION OF NEW I/P CONCEPTS 9.141

ELECTRONIC FEEDBACK 9.143

PROCESS CONTROL VALVES

by Bert J. Peterson∗

INTRODUCTION

The control valve, or final control element, is the last device in the control loop. It takes a signalfrom the process instruments and acts directly to control the process fluid. Control valves maintainprocess variables such as pressure, flow, temperature, or level at their desired value, despite changesin process dynamics and load. Control valves must be designed to accommodate the needs andcharacteristics of the process fluid they control. Likewise, the control valve must react to the protocoland needs of the controlling devices in the process control system. The evolution of control valves isin response to the combined forces of the processes they handle and the systems that control them.Evidence of these factors exists in the design of valve bodies, actuators, valve controllers, and interfaceaccessories.

This article covers control valves in their major subsegments. Following a brief history of con-trol valves, a summary of valve bodies and of the criteria for valve applications, sizing, and se-lection is presented. These factors are strongly influenced by the needs and specifications of theprocess. The next section discusses the increasingly important consideration of control valve per-formance. A review of actuator styles, distinctions, and selection follows the performance section.This review centers on the needs of the control valve body and the needs of the process and powersource. Finally, the article covers digital valve controllers, positioners, and other valve and actuatoraccessories.

CHRONOLOGY

The modern history of control valves is rooted in the industrial revolution and began with the inventionof the industrial steam engine. Steam to power these engines built up pressures, which had to becontained and regulated. Instantly, valves (which had existed since the middle ages) took on newsignificance. The process control systems at that time were simple: human beings. Things changed

* Senior Publications Specialist, Fisher Controls International Inc., Marshalltown, Iowa.

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TABLE 1 Principal Selection Criteria and Availability of Generic Valve Styles

MainTypical Typical Relative

Valve Style Characteristics Available Size RangeStd. body Std. End Typical Max. FlowMaterials Connection Pressure Capacity

Regular sliding Heavy duty, NPS 1/2–20 DN 15–500 Cast iron; Flanged, Class 2500 PN 420 Moderatestem versatility carbon, alloy, or

welded

stainless steelscrewed

Bar stock Compact NPS –3 DN 15–80 Stainless steel; Flangeless, Class 600 PN 100 Lownickel alloys screwed

Economy sliding Light duty, NPS 1/2–4 DN 15–100 Bronze, cast Screwed, Class 300 PN 50 Lowstem inexpensive

iron,carbon flangedsteel

Through-bore ball On–off NPS 1–24 DN 25–600 Carbon steel or Flangeless Class 900 PN 150 Highservice stainless steel

Partial ball Characterized NPS 1–24 DN 25–600 Carbon steel or Flangeless, Class 600 PN 100 Highfor throttling stainless steel flanged

Eccentric plug Erosion NPS 1–12 DN 25–300 Carbon steel or Flangeless, Class 600 PN 100 Moderateresistance, stainless steel flangedversatility

Swing-through No seal NPS 2–36 DN 50–900 Cast iron; Flangeless, Class 2500 PN 420 Moderatebutterfly

carbonor lugged, welded

Lined butterfly Elastomer or NPS 2–24 DN 50–600 Cast iron; Flangeless, Class 150 PN 20 HighPTFE lined carbon or lugged

stainless steelHigh-performance Offset disk, NPS 2–72 DN 50–1800 Carbon steel or Flangeless, Class 600 PN 100 High

butterfly flexible seals stainless steel luggedSpecial Custom to NPS 2–24 DN 50–600 Carbon, alloy, or Flanged, Class 4500 PN 760 High

application stainlesswelded

steel

stainless steel

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(Continues)

quickly, however. The invention of the flyball governor heralded a new era of “feedback control” andpermanent linkage of process to control valves.

The next leap of technology took place in 1875, when William Fisher invented the self-containedautomatic pump governor. This device used pump output pressure to control valves that throttle steamflow to the pump engines. It was the first process control device to achieve a set point by offsettingprocess pressure acting on a piston with the force of a mechanical spring. This combination offorces, linked to a valve body, was the basis on which control valve actuators and control valves laterevolved.

For the next 50 years, control valves consisted of a variety of self-contained governors (nowcalled regulators), float valves, and lever valves. The most common method of valve actuator was aspring-opposed piston or diaphragm motor (an actuator) that operated directly from the process fluid.

In the mid-1930s, pneumatic pressure control instruments began to emerge. Instrument companiescoaxed the valve companies to make valve actuators that reacted to standardized pneumatic signalsrather than process pressure. The new control instrumentation improved the fidelity of process controldramatically and required an upgrade of control valve components. Characterized valve plugs weredeveloped, and the valve positioner made its debut.

As process pressure increased, high-pressure valves were developed. As flow rates increased,larger-capacity valves were developed. Valves changed to cope with process changes. The late 1970switnessed a wholesale move to centralized electronic control, and control valves were modified toaccept analog electronic signals. Today, control valves exist in an environment of distributed digitalcontrol, integral digital control, integral accessories, modularity, and two-way communication. Theevolution continues. Control valves are adapting to change in the processes they control and to theinstruments that control them.

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Rel. Noise or Application PressureShutoff Cavtitation Available Control Flow Temp. Drop

Best Economic SizeCapability Trim Option Characteristic Rangeability Range Capability Range

Equal percentage, HighExcellent Yeslinear, quick

Moderate Quite low to

opening, specialvery high

NPS 1–4 DN 25–100

Excellent No Equal percentage, Moderate Moderate Moderate NPS –1 DN 15–25linear

Good Yes Equal percentage, Moderate Moderate Moderate NPS 1–2 DN 25–50linear

Excellent Yes Equal percentage Low Moderate Moderate NPS 4–8 DN 100–200

Excellent Yes Equal percentage High Quite low to Moderate NPS 4–8 DN 100–200quite high

Excellent No Linear Moderate Quite low to High NPS 4–8 DN 100–200quite high

Poor No Equal percentage Moderate Very low to Moderate NPS 6–36 DN 150–900quite high

Good No Equal percentage Low Moderate Low NPS 6–24 DN 150–600

Excellent No Linear Low Very low to Moderate NPS 6–72 DN 150–1800quite high

Excellent Yes Custom Moderate to Very low to High to very – –high quite high high

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TABLE 1 (Continued )

CONTROL VALVE BODIES

General Categories of Control Valves

Control valve here means any power-operated valve, whether used for throttling or on–off control.Varieties from which to select, as listed in Table 1, include sliding stem valves and rotary valves.Typical sliding-stem valves are straight-pattern valves (sometimes called globe valves) and angle-pattern valves. Rotary valves include ball and butterfly valves. Other varieties such as motorized gatevalves, louvers, pinch valves, plug valves, and self-operated regulators are not considered here. Thesemajor types, sliding-stem and rotary, are further divided into ten subcategories according to relativeperformance and cost. Despite variations found within each category—such as cage guiding and stemguiding—all valves within a given subcategory can be considered very much alike in the early stagesof the valve selection process. Selecting a valve involves narrowing your selection to one of thesesubcategories and then comparing specific valves in that group (Table 2).

Designations NPS and DN are used in Table 1 and throughout this section. NPS is a designationfor nominal pipe size. It comprises the letters NPS followed by a dimensionless number, whichis indirectly related to the physical size, in inches, of the end connections. DN is an internationaldesignation for nominal diameter. It comprises the letters DN followed by a dimensionless wholenumber, which is indirectly related to the physical size, in millimeters, of the end connections.

Sliding-Stem Valves

The most versatile of the control valves are the sliding-stem valves. Straight-pattern, angle-pattern,and three-way valves can be purchased in sizes ranging from NPS 1/2 to NPS 20 or from DN 15 to

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TABLE 2 Control-Valve-Characteristic Recommendations for Liquid-Level, Pressure,and Flow Control∗

Liquid-Level Systems

Control Valve Pressure Drop Best Inherent Characteristic

Constant �P Linear

Decreasing �P with increasing load, �P at Linearmaximum load >20% of minimum-load �P

Decreasing �P with increasing load, �P at Equal percentagemaximum load <20% of minimum-load �P

Increasing �P with increasing load, �P at Linearmaximum load <200% of minimum-load �P

Increasing �P with increasing load, �P at Quick openingmaximum load >200% of minimum-load �P

Pressure Control Systems

Application Best Inherent Characteristic

Liquid process Equal percentage

Gas process, small volume, less than 10 ft of pipe Equal percentagebetween control valve and load valve

Gas process, large volume (process has receiver,distribution system, or transmission line exceeding100 ft of nominal pipe volume), decreasing �Pwith increasing load, �P at maximum load >20%of minimum-load �P

Linear

Gas process, large volume, decreasing �P withincreasing load, �P at maximum load <20%of minimum load �P

Equal percentage

Flow Control Processes

Location ofBest Inherent Characteristic

Flow measurement control valve Small range of flow butsignal in relation to Wide range of flow large �P change at valve

to controller measuring element set point with increasing load

Proportional In series Linear Equal percentageto flow In bypass† Linear Equal percentage

Proportional to In series Linear Equal percentageflow squared In bypass Equal percentage Equal percentage

∗ Based on a combination of applied control theory and actual experience. (Fisher Controls International, Inc.)† When control valve closes, flow rate increases in measuring element.

DN 500. Examples of sliding-stem valves are shown in Figs. 1–5. More choices of materials, endconnections, and control characteristics are available for sliding-stem valves than for any other productfamily. Sliding-stem valves are available in cage-guided, post-guided, and stem-guided designs withflanged, screwed, or welding ends. Economical cast iron as well as carbon steel, stainless steel, andother high-performance body materials are available. Pressure ratings up to and above Class 2500 orPN 420 are available. Their precise throttling capabilities, overall performance, and general sturdiness

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FIGURE 1 Reduced trim, angle-pattern sliding-stem valve shows the capability for trim reductionin a sliding-stem valve. The valve also features anoutlet liner for resistance to erosion. The unbalancedplug provides tight shutoff but requires a largeractuator than balanced designs. (Fisher Controls In-ternational, Inc.)

FIGURE 2 Standard straight-pattern sliding-stem valves are availablein a broad range of sizes, materials, and end connections. The balancedplug shown reduces unbalance force and allows the use of smaller actua-tors. A soft seat provides tight shutoff. Valves such as this are the firstchoise for applications smaller than NPS 3 or DN 80. (Fisher ControlsInternational, Inc.)

make sliding-stem valves a bargain, despite their slight cost premium. The buyer gets a rugged,dependable valve intended for long, trouble-free service. Sliding-stem valves are built ruggedly tohandle conditions such as piping stress, vibration, and temperature changes. In sizes NPS 3 or DN 80,incremental costs over rotary valves are low in comparison to the increments in benefits received.

For many extreme applications, sliding stem valves are the only suitable choice. This includesvalves for high pressure and temperature, antinoise valves, and anticavitation valves. Because ofprocess demands, these products require the rugged construction design of sliding-stem products.

Bar stock valves are small, economical sliding-stem valves whose bodies are machined from barstock (Fig. 6). Body sizes range from fractions of an inch up to NPS 3 or DN 80; flow capacitiesgenerally are lower than those of general-purpose valves. End connections usually are flangeless (forclamping between piping flanges) or screwed. The main advantage of this type of valve is that farmore materials are readily available in bar stock form than in cast form. Consequently these valvesare often used where there are special corrosion considerations. However, their compactness andgeneral high-quality construction make them attractive for flow rates below the range of the regularsliding-stem subcategory, even when corrosion is not a consideration. Overall, they are an economicalchoice when they can be used.

The lowest-cost products among sliding-stem valves are called general-purpose or economy bodies.These valves are used for low-pressure steam, air, and water applications that are not demanding (Figs.7 and 8). Available sizes range from NPS 1/2–4 or DN 15–DN 100. Body materials include bronze,cast iron, steel, and stainless steel (SST). Pressure classes generally stop at Class 300 or PN 50.

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FIGURE 3 Severe-service capability in a large, straight-patternvalve. It features a drilled-hole cage for attenuation of flow noiseby splitting the flow into multiple passages. Hole spacing is con-trolled carefully to eliminate jet interaction and high resultantnoise levels. (Fisher Controls International, Inc.)

FIGURE 4 High-pressure globe valves are typically availablein sizes NPS 1–20 (or DN 25–500) and Classes 900, 1500, and2500 (or PN 150, PN 260, and PN 420). These valves providethrottling control of high-pressure steam and other fluids. Antinoiseor anticavitation trim is often used to handle problems caused byhigh-pressure drops. (Fisher Controls International, Inc.)

Compared to regular sliding-stem valves, these units are very simple, their actuators are smaller, andtheir cost is normally three-quarters to one-half as much. Severe service trims for noise and cavitationservice are not usually available in these products.

Ball Valves

There are two subcategories of ball valves. The through-bore or full-ball type shown in Fig. 9 is oftenused for high-pressure drop throttling and on–off applications in sizes to NPS 24 or DN 600. Full-port designs exhibit high flow capacity and low susceptibility to wear by erosive streams. However,sluggish flow throttling response in the first 20% of ball travel makes full-bore ball valves unsuitablefor throttling applications. Newer designs in full-ball, reduced-bore valves provide better response.Pressure ratings up to Class 900 or PN 150 are available, as are a variety of end connections and bodymaterials. Another popular kind of ball valve is the partial-ball style (Fig. 10). This subcategory is verymuch like the reduced-bore group, except that the edge of the ball segment has a contoured notch shapefor better throttling control and higher rangeability. Intended primarily for modulating service, notmerely for on–off control, partial-ball valves are generally higher in overall control performance thanfull-ball products. They are engineered to eliminate lost motion, which is detrimental to performance.The use of flexible or movable metallic and fluoroplastic sealing elements allows tight shutoff andwide temperature and fluid applicability. Their straight-through flow design achieves high capacity.Sizes range through NPS 24 or DN 600. Pressure ratings go to Class 600 or PN 100. Price is normallylower than that of globe valves.

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FIGURE 5 Three-way valves have three end connec-tions to allow for converging (flow mixing) or diverging(flow splitting) operation. (Fisher Controls International,Inc.)

FIGURE 6 Bar-stock valves provide economical solutions tosmall flow requirements. Pressures to 1500 psig (104 bars) andtemperatures to 450◦F (232◦C) can be handled. Compact spring-and-diaphragm actuators complement these small valve bodies. (H.D. Baumann Inc.)

Eccentric-Plug Valves

Eccentric-plug valves combine many features of sliding-stem and rotary products and use rotaryactuators. These valves are available for different types of service. The valve in Fig. 11 is used fora variety of fluids in both industrial process and utility applications. The valve in Fig. 12 featuresoversized shafts and rigid seat design for severe service and erosion resistance. Both designs haveexcellent throttling capability and combine many of the good aspects of rotary and sliding-stem valves.Sizes are available through NPS 8 or DN 200 in ratings to Class 600 or PN 100. Flanged and flangelessconstructions are usually available.

Butterfly Valves

Butterfly valves are divided into three subcategories: swing through, lined, and high performance.The most rudimentary is the swing-through design (Fig. 13). Rather like a stovepipe damper, butconsiderably more sophisticated, this kind of valve has no seals—the disk swings close to, but clearof, the body’s inner wall. Such a valve is used for throttling applications that do not require shutofftighter than ∼1% of full flow. Sizes range from NPS 2 to NPS 96 or DN 50 to DN 400. Body materials

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FIGURE 7 Screwed-end bronze valve body capable of handling manyutility applications. It is complemented by a wide variety of seat ring sizesand control characteristics. (H.D. Baumann Inc.)

FIGURE 8 Valve style typical ofgeneral-purpose valves. Availability usu-ally extends to size NPS 4 or DN 100and to Class 300 or PN 50 ratings.These valves feature compact, reversiblediaphragm actuators and inexpensive po-sitioners. (Fisher Controls International,Inc.)

FIGURE 9 High-pressure ball valve, featuring heavy shafts and full-ball design. Thevalve shown can be used for pressure drops to 2220 psi (152 bars). Class 600 and 900 orPN 100 and 150 bodies are available; sizes range to NPS 24 or DN 600. (Fisher ControlsInternational, Inc.)

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FIGURE 10 Applications to Class 600 or PN 100 can be handled by this segmented or partial ball valve. The flangelessbody incorporates many features to improve throttling performance and rangeability. Tight shutoff is achieved by eithermetal or composition seals. (Fisher Controls International, Inc.)

FIGURE 11 Rotary eccentric-plug valve for industrial process and utility appli-cations. (H. D. Baumann Inc.)

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FIGURE 12 Rotary eccentric-plug valve for severe applications and highly erosive process fluids. (Fisher ControlsInternational, Inc.)

FIGURE 13 Swing-through butterfly valve provides an economical solution tohigh-flow throttling applications. Leakage is higher than for other designs becauseno sealing mechanism is used. (Fisher Controls International, Inc.)

are cast iron, carbon steel, or stainless steel. Mounting is flangeless, lugged, or welded. Body pressureratings up to Class 2500 or PN 420 are common, and wide temperature ranges are also available.While a very broad range of designs is available in these products, they are limited by lack of tightshutoff.

Need for no or low leakage requires the lined and high-performance butterfly valves. Lined butterflyvalves feature an elastomer or polytetrafluoroethylene (PTFE) lining that contacts the disk to providetight shutoff (Fig. 14). Because this seal depends on interference between disk and liner, these designsare more limited in pressure drop. Temperature ranges are also restricted considerably because of theuse of elastomeric materials. A benefit, however, is that because of the liner, the process fluid nevertouches the metallic body. Thus these products can be used in many corrosive situations. Elastomer-lined butterfly valves are generally the lowest-priced products available as control valves in mediumto large sizes.

High-performance butterfly valves such as the one shown in Fig. 15 are characterized by heavyshafts and disks, full pressure rated bodies, and sophisticated seals that provide tight shutoff. These

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FIGURE 14 Lined butterfly valves offer tight shutoff but are limited to low temperatures. Liner mate-rial keeps process away from the metallic body, eliminating many corrosion problems. (Fisher ControlsInternational, Inc.)

FIGURE 15 High-performance butterfly valves provide excellent performance and value. High-pressure capa-bility, tight shutoff, and excellent control are standard. Designs are available in Classes 150, 300, or 600 (PN 20,PN 50, or PN 100) and sizes to NPS 72 or DN 1800. (Fisher Controls International, Inc.)

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valves provide an excellent combination of performance features, light weight, and very reasonablepricing. Eccentric shaft mounting allows the disk to swing clear of the seal to minimize wear andtorque. The offset disks used allow uninterrupted sealing and a seal ring that can be replaced withoutremoving the disk. High-performance butterfly valves come in sizes from NPS 2 to NPS 72 or DN50 to DN 1800 with flangeless or lugged connections. Bodies are carbon-steel or stainless steel,and pressure ratings are up to Class 600 or PN 100. With their very tight shutoff and heavy-dutyconstruction, these valves are suitable for many process applications. Advanced metal-to-metal sealsprovide tight shutoff in applications that are too hot for elastomer-lined valves to handle.

Special Control Valves

Standard control valves can handle a wide range of control applications, which can be defined as:

� pressure ratings to Class 2500 or DN 420� −150 to 450◦F (−101 to 232◦C)� flow coefficient values of 1.0–25,000 Cv or 22,000 Kv

� within the limits imposed by common industrial standards

Perhaps the need for careful consideration of valve selection and the need for special valves becomemore critical for applications outside the standard limits mentioned above. However, corrosiveness andviscosity of the fluid, leakage rates, and many other factors demand consideration, even for standardapplications.

Special valves can include body liners and seals to contain corrosive or toxic materials (Fig. 16)or valves with special trims for special purposes (Fig. 17). Some categories of special valves arediscussed in the following sections.

High-Capacity Control Valves. The following are often considered to be special valves:

� globe-style valves larger than size NPS 12 or DN 300� ball valves larger than size NPS 24 or DN 600� high-performance butterfly valves larger than size NPS 48 or DN 1200

As valve sizes increase arithmetically, static pressure loads at shutoff increase geometrically.Consequently, shaft strength, bearing loads, unbalance forces, and available actuator thrust all becomemore significant with an increasing valve size. Normally, the maximum allowable pressure drop isreduced on large valves to keep design and actuator requirements within reasonable limits. Even withlowered working pressure ratings, the flow capacity of some large-flow valves is tremendous.

Noise levels must be carefully considered in all large-flow installations because sound pressurelevels increase in direct proportion to flow magnitude. To keep valve-originated noise within tolerablelimits, large cast (Fig. 18) or fabricated (Fig. 19) valve body designs have been developed. Thesebodies, normally cage-style construction, use unusually long valve plug travel, a great number ofsmall flow openings through the wall of the cage, and an expanded outlet line connection to minimizenoise output and reduce fluid velocity.

Low-Flow Control Valves. Low-flow applications are commonly handled in one of two ways. Onemethod is with special trims in standard control valve bodies. The special trim is typically made upof a seat ring and valve plug that have been designed and machined to very close tolerances to allowaccurate control of very small flows. These types of constructions can often handle CV or KV valuesas low as 0.03 or 0.025. Using these special trims in standard control valves provides economy byreducing the need for spare-parts inventory for special valves and actuators. Using this approach alsomakes future flow expansions easy by simply replacing the trim components in the standard controlvalve body.

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FIGURE 16 Special sliding-stem valve used for severely corrosive ortoxic fluids. It has a full PTFE liner and PTFE trim. It also has a bellowsseal to eliminate leakage. (Fisher Controls International, Inc.)

FIGURE 17 Special sliding-stem valve for super-heater bypass service in power plants. The applica-tion requires tight shutoff and flows that range fromcold cavitating water to flashing water to superheatedsteam. (Fisher Controls International, Inc.)

Control valves specifically designed for very low-flow rates (Fig. 20) also handle these applications.These valves often handle CV or KV values as low as 0.000001. In addition to the very low flows, thesecontrol valves are compact and light weight.

Valves for Sanitary Service. Valves for sanitary service (Fig. 21) are used in the food and bever-age, pharmaceutical, biotechnical, and semiconductor industries. Sanitary valves have several designfeatures specifically for the intended service: highly polished surfaces, self-draining features, andminimum areas that can hold process fluid that is not moving in the process stream. Diaphragm-typevalves are often used for this service because this design minimizes or eliminates valve trim guidingsurfaces that are in contact with process fluid. (Instead of a valve plug, a diaphragm seats against a seatring to control the flow of process fluid.) Sanitary valves are often designed for CIP (clean in place)and SIP (sanitize in place) procedures. End connections are often weld ends or clamped connections.

High-Temperature Control Valves. Control valves for service at temperatures above 450◦F (232◦C)must be designed and specified with the temperature conditions in mind. At elevated temperatures,

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FIGURE 18 High-capacity cast globe valve for noiseattenuation service. (Fisher Controls International, Inc.)

FIGURE 19 Extremely high-flow fabricated valve. The valve can be custommade to match the required flow and piping configuration, and it features adrilled-hole noise-reduction trim. (Fisher Controls International, Inc.)

such as with boiler feedwater systems and superheat bypass systems, the standard materials of controlvalve construction might be inadequate. For instance, plastics, elastomers, and standard gaskets oftenprove unsuitable and must be replaced by more durable materials. Metal-to-metal seating materialsare always used. Semimetallic or laminated graphite packing materials are commonly used, andspiral-wound stainless steel and flexible graphite gaskets are necessary.

Chromium-molybdenum (Cr-Mo) steels are often used for valve body castings for temperaturesabove 1000◦F (538◦C). Chromium-molybdenum steel (such as WC9) is used up to 1100◦F (593◦C).For temperatures up to 1500◦F (816◦C), 316 stainless steel (such as CF8M) is often selected. Fortemperatures between 1000 and 1500◦F (538 and 816◦C), the carbon content must be controlled tothe upper end of the range, that is, 0.04–0.08%.

On high-temperature service, extension bonnets help protect packing box parts from extremelyhigh temperatures.

Cryogenic Service Valves. Cryogenic service normally involves temperatures below −150◦F(−101◦C). Plastic and elastomeric components often cease to function appropriately at tempera-tures below 0◦F (−18◦C). At these temperatures, components such as packing and plug seals requirespecial consideration. For plug seals, a standard soft seal will become very hard and less pliable, thusnot providing the shutoff required from a soft seat. Special elastomers have been applied in thesetemperatures but require special loading to achieve a tight seal.

Packing is a concern in cryogenic applications because of the frost that can form on the valves.Moisture from the atmosphere condenses on colder surfaces and can freeze into a layer of frost. Asthe stem is stroked by the actuator, the layer of frost on the stem is drawn through the packing, causing

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FIGURE 20 Control valve designed forvery low flow rates. (H. D. Baumann Inc.)

FIGURE 21 Control valve for sanitary service; it has highlypolished surfaces and clamp-type end connections. (H. D. Bau-mann Inc.)

tears and loss of seal. The solution is to use extension bonnets (Fig. 22). Extension bonnets allow thepacking box area of the control valve to be warmed by ambient temperatures, thus preventing frostfrom forming on the stem and packing box areas. The length of the extension bonnet depends onthe application temperature and insulation requirements. The colder the application, the longer theextension bonnet required.

Valves for Nuclear Service. Nuclear-service valves must meet many special requirements. Strictcompliance with government regulations is required. Manufacturers in many areas must providedocumented evidence that components were manufactured, inspected, and tested by proven techniquesperformed by qualified personnel according to documented procedures.

Valves Subject to Sulfide Stress Cracking. NACE International (National Association of CorrosionEngineers) is a technical society concerned with corrosion. NACE has a standard that provides guide-lines for the selection of materials that are resistant to sulfide stress cracking. Sulfide stress crackingis a concern with oil and gas that contains hydrogen sulfide (commonly called sour gas service).

The NACE standard lists the types of materials and their heat-treating conditions that are mostresistant to sulfide stress cracking. In some areas, conformance of portions of the standard is requiredby law.

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FIGURE 22 Typical extension bon-net to help isolate packing fromprocess temperatures. (Fisher ControlsInternational, Inc.)

FIGURE 23 Steam conditioning valve that combines steam pressure reduc-tion and desuperheating. (Fisher Controls International, Inc.)

Steam Conditioning Valves. A steam conditioning valve is used for the simultaneous reduction ofsteam pressure and temperature to the level required for a given application (Fig. 23). Frequently, theseapplications have high inlet pressures and temperatures and require significant reductions of both.Steam conditioning valves are best as forged and fabricated bodies that can better withstand steamloads at elevated pressures and temperatures. Forged materials permit higher design stresses, improvedgrain structure, and an inherent material integrity over cast valve bodies. The forged construction alsoallows pressure ratings to Class 4500 or PN 760.

Spraywater is provided to the valve to cool the steam. The spraywater nozzles are near the flow venacontracta below the main valve seat. The water is injected at a point of high velocity and turbulence,where it is distributed quickly and evenly throughout the flow stream.

The forged and fabricated design allows the manufacturer to provide different sizes and pressureclass ratings for the inlet and outlet connections to more closely match the adjacent piping.

Other advantages of combining the pressure reduction and desuperheater function include:

� improved spraywater mixing because of the optimum utilization of the turbulent expansion zonedownstream of the pressure reduction elements

� improved rangeability� increased noise abatement; there is additional attenuation of noise as a result of the spraywater

injection

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� in some designs, improved response time because of an integrated feedforward capability� ease of installing and servicing only one device

CONTROL VALVE PERFORMANCE [1]

Global competition in the process industries is putting increasing pressure on companies to providethe highest quality products and the maximum plant throughputs with fewer resources. While meetingthese demands, companies also must meet ever-changing customer needs.

A company makes a profit through the production of a quality product that conforms to a set ofspecifications. Any deviation from the established specification means lost profit because of exces-sive material use, reprocessing costs, or wasted product. Reducing process variability through betterprocess control allows optimization of the process and the production of products right the first time.

The control valve is often overlooked when process variability is reduced because its impact ondynamic performance is not realized. Extensive studies of control loops indicate that as many as80% of the loops do not do an adequate job of reducing process variability. Furthermore, the controlvalve was found to be a major contributor to this problem for a variety of reasons.

To verify performance, manufacturers must test their products under dynamic process conditions.Evaluating control valve assemblies under closed-loop conditions provides the only true measureof variability performance (Fig. 24). Closed-loop performance data prove significant reductions inprocess variability can be achieved by choosing the right control valve for the application.

For best performance, valves must be optimized or developed as a unit. Some of the most importantdesign considerations include:

� dead band� actuator–positioner design� valve response time� valve type and sizing

FIGURE 24 Testing under dynamic process conditions can demonstrate control valve per-formance. (Fisher Controls International, Inc.)

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Dead Band

Dead band is a major contributor to excess process variability, and control valve assemblies can be aprimary source of dead band in an instrumentation loop.

Dead band has many causes, but friction and backlash in the control valve, along with shaft wind-up in rotary valves, and relay dead zone are some of the more common forms. Because most controlactions for regulatory control consist of small changes (1% or less), a control valve with excessivedead band might not even respond to many of these small changes. A well-engineered valve shouldrespond to signals of 1% or smaller to provide an effective reduction in process variability.

Actuator–Positioner Design

Actuator and positioner design must be considered together. The combination of these two pieces ofequipment greatly affects the static performance (dead band), as well as the dynamic response of thecontrol valve assembly and the overall air consumption of the valve instrumentation.

The most important characteristic of a good positioner for process variability reduction is that itbe a high-gain device.

Typical two-stage positioners use pneumatic relays at the power amplifier stage. Relays are pre-ferred over spool valves for this purpose because relays can provide a high-power gain that givesexcellent dynamic performance with minimal steady-state air consumption.

Positioner designs are changing dramatically, with microprocessor devices becoming increasinglypopular. These microprocessor-based positioners provide dynamic performance equal to the bestconventional two-stage pneumatic positioners. They also provide valve monitoring and diagnosticcapabilities to help ensure that initial good performance does not degrade with use.

High-performance positioners with both high static and dynamic gain provide the best overallprocess variability performance for any given valve assembly.

Valve Response Time

For optimum control of many processes, it is important that the valve reach a specific position quickly.A quick response to small signal changes (1% or less) is one of the most important factors in providingoptimum process control. In automatic, regulatory control, the bulk of the signal changes receivedfrom the controller is for small changes in position. If a control valve assembly can quickly respondto these small changes, process variability is improved.

Valve Type and Characterization

The style of valve used and the sizing of the valve can have a great impact on the performance of thecontrol valve assembly in the system. Although a valve must be of sufficient size to pass the requiredflow under all possible contingencies, a valve that is too large for the application is a detriment toprocess optimization.

Flow capacity of the valve is also related to the style of valve through the inherent characteristic ofthe valve. The inherent characteristic is the relationship between the valve flow capacity and the valvetravel when the differential pressure drop across the valve is held constant. Also see the discussion inthe Valve Capabilities and Capacities section.

For process optimization, the installed flow characteristic of the entire process and the gain aremore important.

The best process performance occurs when the required flow characteristic is obtained throughchanges in the valve trim rather than through the use of positioner cams or other methods. Properselection of a control valve designed to produce a reasonably linear installed flow characteristic overthe operating range of the system is a critical step in ensuring optimum process performance.

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Valve Sizing

Process variability reduction efforts are limited by valves being oversized. The oversizing results fromusing line-size valves, especially with high-capacity rotary valves, as well as the conservative additionof multiple safety factors at different stages in the process design.

The second way oversized valves hurt process variability is that an oversized valve is likely tooperate more frequently at lower valve openings, where seal friction can be greater, particularly inrotary valves. Because an oversized valve produces a disproportionately large flow change for a givenincrement of valve travel, this phenomenon can greatly exaggerate the process variability associatedwith dead band that is due to friction.

When selecting a valve, consider the valve style, inherent characteristic, and valve size that willprovide the broadest possible control range for the application.

Economic Results

Consideration of the performance factors discussed can have a dramatic impact on the economic resultsof an operating plant. More and more control valve users focus on dynamic performance parameterssuch as dead band, response times, and installed gain (under actual process load conditions) as a meansto improve process-loop performance. Although it is possible to measure many of these dynamicperformance parameters in an open-loop situation, the impact these parameters have becomes clearwhen closed-loop performance is measured (Fig. 24).

Process industries have become increasingly aware that control valve assemblies play an importantrole in loop/unit/plant performance. They also have realized that traditional methods of specifyinga valve assembly are no longer adequate to ensure the benefits of process optimization. Althoughimportant, such static performance indicators as flow capacity, leakage, materials compatibility, andbench performance data are not sufficiently adequate to deal with the dynamic characteristics ofprocess control loops.

Parts of the loop cannot be treated individually to achieve coordinated loop performance. Likewise,performance of any part of the loop cannot be evaluated in isolation. Isolated tests under nonloaded,bench-type conditions will not provide performance information that is obtained from testing thehardware under actual process conditions.

VALVE SELECTION

Picking a control valve for a particular application once was simple. Usually only one general type ofvalve was considered—the sliding-stem valve. Each manufacturer offered a product suitable for thejob, and the choice depended on obvious considerations such as cost, delivery, vendor relationships,and user preference. Selection is now considerably more complicated—especially for engineers withlimited experience or for those who have not kept up with changes in the control valve industry. Formany applications, an assortment of sliding-stem, ball, and butterfly valves is available. Some aretouted as universal valves for almost any size and service, and others are claimed to be optimumsolutions for narrowly defined needs. Like most decisions, the selection of a control valve involves agreat number of variables. Presented here is an overview of the selection process.

General Selection Criteria

Most of the considerations that guide the selection of the valve type are rather basic. However, thereare some matters that might be overlooked by users whose familiarity is limited. A checklist includesthe following:

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1. Body pressure rating and limits

2. Size and flow capacity

3. Flow characteristics and rangeability

4. Temperature limits

5. Shutoff leakage

6. Pressure drop (shutoff and flowing)

7. End connection requirements

8. Material compatibility and durability

9. Lifecycle cost

Pressure Ratings

The most common pressure ratings for steel and stainless-steel valves are Classes 150, 300, and 600[2], [3] or PN 20, PN 50, or PN 100. For a given body material, each rating prescribes a profileof maximum pressure that decreases with temperature according to the strength of the material.Each material also has minimum and maximum service temperatures based on loss of ductility orloss of strength. For most applications, the required pressure rating is dictated by the application.However, because not all products are available for all ratings, it is an important consideration forselection.

Operating Temperature

Required temperature capabilities are usually also a foregone conclusion, but one that is likelyto further narrow the range of selections. Considerations here include the strength or ductility ofthe body material as well as the relative thermal expansion of the valve internal parts. Tempera-ture limits also might be imposed as a result of disintegration of soft parts at high temperaturesor loss of resiliency at low temperatures. The soft materials under consideration include variouselastomers, plastics, and PTFE. They might be found in parts such as seat rings, seal or pistonrings, packing, rotary shaft bearings, and butterfly valve liners. Typical upper temperature limitsfor elastomers are in the 200–350◦F (93–177◦C) range, and the general limit for PTFE is 450◦F(232◦C).

Temperature affects valve selection by excluding certain valves that do not have high- or low-temperature options, such as lined butterfly valves. It also might have some effect on valve per-formance. For instance, going from PTFE to metal seals for high temperatures generally increasesthe shutoff leakage flow. Similarly, high-temperature metal bearing sleeves in rotary valves imposemore friction load on the shaft than PTFE bearings do, so that the shaft cannot withstand as high apressure-drop load at shutoff.

Selection of valve packing is very often based on the service temperature. Two packing types,PTFE V rings and graphite, meet most packing requirements. These materials have proven reliable,inexpensive, and effective.

PTFE V-ring packing is composed of solid rings of molded PTFE. Generally, in a given packingset, there are two or more packing rings with a V cross section, a male adaptor, and a female adaptor.The packing can be used over a temperature range of −40 to 450◦F (−40 to 232◦C) and for nearlyall chemicals. PTFE packing can be used with a spring or as jam-type packing. Stem friction is low.This packing is the preferred packing material for most applications. Packing is discussed in greaterdetail in the Material Selection section.

Graphite packing systems are used mainly for temperatures above 450◦F (232◦C). They are com-posed of graphite ribbon rings, graphite filament rings, and sacrificial zinc washers. The graphite ringsperform the sealing function and the zinc washers protect the valve stem from galvanic corrosion.

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MATERIAL SELECTION

Material compatibility and durability are complex considerations. The consideration might be corro-sion by the process fluid, erosion by abrasive material, flashing, cavitation, or simply one of processpressure and temperature. The material used for piping is a good predictor of control-valve bodymaterial. However, because the velocity is higher in valves, other factors must also be considered.When these items are included, often valve and piping materials will differ. Trim materials are usuallya function of the body material, temperature range, and qualities of the fluid. When a body materialother than carbon, alloy, or stainless steel is required, the use of alternate valve types, such as linedor bar stock, should be considered.

Control valves are required to function with precision in some very extreme environments. Anumber of factors must be considered to ensure that a material will perform properly in service. Thesefactors fall primarily into two categories: suitability to function mechanically and compatibility withthe process environment. These constraints conflict in many instances, making it difficult to satisfyall considerations with a single material. In these cases, the best compromise must be identified.

Carbon-Steel Bodies and Bonnets

The most standard material for control-valve bodies is carbon steel such as WCB or WCC steel. Carbonsteel is easily cast, welded, and machined. It is used for a large majority of process applications becauseof its low cost and reliable performance. Its use is strongly recommended over any other material ifpossible because of its broad availability and low cost.

Alloy-Steel Bodies and Bonnets

When higher temperatures or pressures are involved, alloy steels are often specified. Most are steelswith chromium or molybdenum added to enhance their resistance to tempering and graphitizationat elevated temperatures. The chromium and molybdenum additions also increase their resistance toerosion in flashing applications. Among the more popular materials are WC9 and WC6 steel.

Stainless-Steel Bodies and Bonnets

The most common stainless steel used for bodies and bonnets is CF8M, which is the cast version ofS31600. With its nominal 191/2% Cr, 101/2% Ni, 211/2% Mo composition, CF8M is a relatively low-costmaterial with good high-temperature properties and excellent resistance to corrosion.

Selection of Materials

Comparing pressure-temperature ratings in ASME B16.34 is much simpler when the ratings arepresented in graphic form. The first discovery that is made is that the Class 150 ratings for WCB,WCC, WC9, and C5 are identical over their common temperature ranges, and that CF8M is onlyrated slightly lower at temperatures below 550◦F (288◦C). The second discovery is that the ratingplots for these materials in all other classes have the same shapes, and all that changes is the y-axisscale for the allowable pressures. Figure 25 is a plot of the pressure-temperature ratings, where theallowable pressure has been normalized to 100 percent. This curve is representative of the relativepressure-temperature ratings of the materials for ANSI Classes 300 through 4500. If the materialproviding the maximum allowable pressure at any temperature is determined from the plot, threematerial regimes can be established. From ambient temperature to 750◦F (399◦C), WCC has thehighest pressure-temperature ratings of this group of materials. From 750◦F to 950◦F (399–510◦C),WC9 has the highest ratings, and from 950◦F (510◦C) to higher temperatures, CF8M has the highestratings.

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FIGURE 25 Control valve pressure ratings depend on temperature and material. This chart compares normalized ratingsas a function of these factors. These are relative ASME B16.34 pressure-temperature ratings (ratings vs. WC9 steel at roomtemperature). (Fisher Controls International, Inc.)

Trim Parts

Valve trim components have much different material requirements than valve bodies and bonnets.Trim parts are not pressure retaining, so they are not directly safety related. However, because the trimcomponents provide the flow control, they are very important to valve performance. Trim materialsmust usually have excellent resistance to corrosion by the process fluid. If not, adequate flow controland mechanical stability will not be maintained. Each component must have other characteristics,depending on the valve design, process fluid, and application.

Valve Packing [1]

Most control valves use packing boxes with the packing retained and adjusted by a flange and packingnuts (Fig. 26). PTFE and graphite materials are often used as packing rings.

PTFE V-Ring Packing.

� plastic material with the ability to minimize friction� molded in V-shaped rings that are spring loaded and self-adjusting in the packing box; lubrication

is not required� resistant to most known chemicals, except molten alkali metals� requires extremely smooth stem finish to seal properly; packing will leak if the stem or packing

surface is damaged� normally used from −40◦F to +450◦F (−40 to +232◦C)� not suitable for nuclear service because PTFE is easily destroyed by radiation.

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FIGURE 26 Typical spring-loaded PTFE V-ringpacking. (Fisher Controls International, Inc.)

FIGURE 27 One example of the new packingtechnologies is the ENVIRO-SEAL packing system.(ENVIRO-SEAL is a mark owned by Fisher ControlsInternational, Inc.)

Laminated and Filament Graphite Packing.

� suitable for high-temperature nuclear service or where low chloride content is desirable (GradeGTN)

� provides excellent sealing, high thermal conductivity, and long service life, but produces high stemfriction and resultant hysteresis

� impervious to most hard-to-handle fluids and high radiation� can be used for temperatures from cryogenic to 1200◦F (649◦C)� lubrication is not required, but an extension bonnet or steel actuator yoke should be used when

packing box temperature exceeds 800◦F (427◦C)

Improved Packing Technology. Environmental concern over fugitive emissions has resulted ingovernmental regulations that restrict the amount of emissions of various fluids that can be permitted.This concern, along with the economic concern over the loss of valuable process fluids, made itnecessary to improve the sealing of valve stems and shafts. New valve packing technologies meetthese challenges. Also, the new technologies extend packing seal life and performance.

One example of the new packing technologies is the ENVIRO-SEAL packing system (Fig. 27).Improved sealing is made possible by:

� anti-extrusion rings that contain the pliable seal material� proper alignment of the valve stem or shaft within the bonnet bore� managed packing force provided through springs� minimizing the number of seal rings to reduce consolidation, friction, and thermal expansion

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Packing selection is often based on process temperature; that is, PTFE is selected for tempera-tures below 450◦F (232◦C), and graphite is selected for temperatures above 450◦F (232◦C). Con-siderations also should include the effect of packing friction on process control, seal performance(pressure/temperature/emission sealing capabilities), and service life.

When selecting a packing seal technology for fugitive emission service, it is important to answerthe following questions to help ensure long-term performance. Detailed answers based on test datashould be available from the valve manufacturer.

1. Was the packing system tested within the valve style to be used?

2. Was the packing system subjected to multiple operating cycles?

3. Was the packing system subjected to multiple thermal cycles?

4. Were packing adjustments made during the performance test?

5. Was the packing system tested at or above the service conditions of the planned application?

6. Did testing of packing systems for rotary valves include deflection of the valve shaft?

7. Was stem leakage monitored using a procedure covered by government regulations or an industry-accepted practice?

8. Were the packing components examined for wear after the completion of each test?

9. Was the compression load on the packing measured as the test progressed?

10. Are the test results documented and available for review?

VALVE CAPABILITIES AND CAPACITIES

Flow characteristic, rangeability, pressure drop capabilities, end connection style, shutoff, and ca-pacity are very important to consider when you select a valve. Valve manufacturers publish thesecharacteristics as specifications in sales literature or data sheets.

Flow Characteristic

The next selection criterion—inherent flow characteristic—refers to the pattern in which the flow atconstant pressure drop changes according to valve position. Typical characteristics are quick opening,linear, and equal percentage. The choice of characteristic has a strong influence on the stabilityor controllability of the process, because it represents the change of valve gain relative to travel.Most control valves are carefully “characterized” to exhibit a certain flow characteristic by means ofcontours on a plug, cage, or ball element. Some valves are available in a variety of characteristics tosuit the application, while others offer little or no choice.

To determine the best flow characteristic for a given application quantitatively, a dynamic analysisof the control loop can be performed. In most cases, however, this is unnecessary; reference toestablished rules will suffice. Figure 28 illustrates typical flow characteristic curves. The quick-opening flow characteristic provides for maximum change in flow rate at low valve travels with afairly linear relationship. Additional increases in valve travel give sharply reduced changes in flowrate, and when the valve plug nears the wide open position, the change in flow rate approaches zero.In a control valve, the quick-opening valve plug is used primarily for on–off service, but it is alsosuitable for many applications where a linear valve plug would normally be specified.

The linear flow characteristic curve shows that the flow rate is directly proportional to the valvetravel. This proportional relationship produces a characteristic with a constant slope so that withconstant pressure drop, the valve gain will be the same at all flows. The linear valve plug is commonlyspecified for liquid-level control and for certain flow control applications requiring constant gain.

In the equal-percentage flow characteristic, equal increments of valve travel produce equal per-centage changes in the existing flow. The change in flow rate is always proportional to the flow rate

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FIGURE 28 Many control valves offer a choice of control characteristics. Theselection to match process requirements is guided by simple rules. Adherence tothe guidelines will help ensure a stable process operation. (Fisher Controls Inter-national, Inc.)

just before the change in valve plug, disk, or ball position is made. When the valve plug, disk, or ballis near its seat and the flow is small, the change in flow rate will be small. With a large flow, the changein flow rate will be large. Valves with an equal-percentage flow characteristic are generally used onpressure control applications, and on other applications where a large percentage of the pressure dropis normally absorbed by the system itself, with only a relatively small percentage available at thecontrol valve. Valves with an equal-percentage characteristic should also be considered where highlyvarying pressure drop conditions can be expected. Table 2 lists characteristic recommendations byprocess type.

Rangeability

One aspect of flow characteristic is its rangeability, which is the ratio of maximum and minimumcontrollable flow rates. Exceptionally wide rangeability might be required for certain applications tohandle wide load swings or a combination of start-up, normal, and maximum working conditions.Rotary valves, especially partial ball valves, normally have greater rangeability than sliding-stemvarieties.

Pressure Drop

The maximum pressure drop the valve can tolerate at shutoff and when partly or fully open is animportant selection criterion. Sliding-stem valves are generally superior in both regards because ofthe rugged, well-supported design of their moving parts. Unlike most sliding-stem valves, many rotaryvalves are limited to pressure drops well below the body pressure rating, especially under flowingconditions, because of dynamic stresses imposed on the disk or ball segment by high-velocity flow.

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Noise and cavitation are two considerations that, although unrelated, are often grouped togetherbecause they both usually accompany high pressure drops and flow rates. They are handled by spe-cial modifications of more or less standard valves. Cavitation is the noisy and potentially damagingimplosion of bubbles formed when the pressure of a liquid momentarily dips below its vapor pres-sure through a constriction at high velocity. In controlling gases and vapors, noise results from theturbulence associated with high-velocity streams. When cavitation or noise is judged likely to bea problem, its severity must be predicted from the valve’s specifications according to well-knowntechniques, and valves with better specifications must be sought if necessary. Cavitation-control andnoise-control trims for various degrees of severity are widely available in regular sliding-stem valves—at a progressive penalty in terms of cost and flow capacity. Rotary valves have more limited noise-and cavitation-control options and are also much more susceptible to cavitation and noise at a givenpressure drop. Please refer to subsequent articles in this handbook section concerning control-valvenoise and cavitation.

End Connections

At some point in the selection process, the valve end connections must be considered. The questionto be answered is simply whether the desired connection style is available in the valve style beingconsidered. In some situations, end connections can quickly limit the selection or dramatically affectthe price. For instance, if a piping specification calls for welded connections only, the choice mightbe limited to sliding-stem valves. The few weld-end butterfly and ball valves that are available arerather expensive.

Shutoff Capability

Some consideration usually must be given to a valve’s shutoff capability, which ordinarily is ratedin terms of classes specified in ANSI/FCI 70-2 [4] or IEC 534–4. In actual service, shutoff leakagedepends on many factors, including pressure drop, temperature, the condition of the sealing surfaces,and—very importantly for sliding- stem valves—the force load on the seat. Because shutoff ratingsare based on standard test conditions (Table 3), which might be very different from service conditions,service leakage cannot be predicted very well. However, the shutoff classes provide a good basis forcomparisons among valves of similar configuration.

Tight shutoff is particularly important in high-pressure valves because leakage can cause seat dam-age, leading to ultimate destruction of the trim. Special precautions in seat materials, seat preparation,and seat load are necessary to ensure success. Valve users tend to overspecify shutoff requirements,incurring unnecessary cost. Actually, very few throttling valves really need to perform double dutyas tight block valves. Since tight shutoff valves generally cost more initially and to maintain, seriousconsideration is warranted.

Flow Capacity

The criterion of capacity or size can be an overriding constraint on selection. For very large lines,sliding-stem valves are much more expensive than rotary types. On the other hand, for very small flows,a suitable rotary valve might not be available. If the same valve is desired to handle a significantlylarger flow at a future time, a sliding-stem valve with replaceable, restricted trim might be indicated.Rotaries generally have much higher maximum capacity than sliding-stem valves for a given bodysize. This fact makes rotaries attractive in applications where the pressure drop available is rathersmall. But it is of little or no advantage in high pressure drop applications such as pressure regulationor letdown.

At the risk of overgeneralizing, you can simplify the process of selection roughly as follows.For most general applications it makes sense, both economically and technically, to use sliding-stem

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TABLE 3 Maximum Leakage and Test Conditions for Control-Valve Leakage Classes

ANSIB16.104-1976∗ Maximum Leakage∗ Test Medium Pressure and Temperature

Class II 0.5% valve capacity at full travel Air Service �P or 50 psid (3.4-bar differential),whichever is lower, at 50–125◦ (10 – 52◦C)

Class III 0.1% valve capacity at full travel Air Service �P or 50 psid (3.4-bar differential),whichever is lower, at 50–125◦ (10–52◦C)

Class IV 0.01% valve capacity at full travel Air Service �P or 50 psid (3.4-bar differential),whichever is lower, at 50–125◦ (10–52◦C)

Class V 5 ×104 mL min psid in. port dia (5 ×1012 m3 s bar differential mm port dia) Water Service �P at 50 to 125◦ F (10 –52◦C)

Nominal port diameter

in. mm Bubbles min mL min

1 25 1 0.15 Air Service �P or 50 psid (3.4-bar differential),whichever is lower, at 50–125◦ (10– 52◦C)11/2 38 2 0.30

Class VI 2 51 3 0.4521/2 64 4 0.603 76 6 0.904 102 11 1.706 152 27 4.008 203 45 6.75

∗ Copyright 1976 Fluid Controls Institute, Inc. Reprinted with permission.

/ / / / / /

/ /

F

F

F

F

9.3

1

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valves for the lower ranges, ball valves for intermediate capacities, and high-performance butterflyvalves for the very largest sizes. For the very least demanding services, in which price is the dominantconsideration, one might consider economy sliding-stem valves for the small-size applications andbutterfly valves for the largest.

For sizes of NPS 1/2 to NPS 3 or DN 15 to DN 80, general-purpose sliding-stem valves providean exceptional value. For a minimal price premium over rotary products, they offer unparalleledperformance, flexibility, and service life. The premium for these devices over rotary products iswarranted. For severe service applications, the most frequently used, and often the only availableproduct is the sliding-stem valve.

Applications ranging from NPS 4 to NPS 6 or DN 100 to DN 150 are best served by such transitionalvalve styles as the eccentric plug valve or the ball valve. These products have excellent performanceand lower cost. They also offer higher capacity levels than globe designs.

In sizes NPS 8 or DN 200 and larger, pressures and pressure drops are much lower. This givesrise to the possibility of using high-performance butterfly valves for most situations. These valves areeconomical, offer tight shutoff, and provide good control capability. They provide cost and capacitybenefits well beyond those of globe and ball valves.

Special considerations require special valve solutions. There are valve designs and special trimsavailable to handle high-noise applications, cavitation, high pressure, high temperature, and combi-nations of these conditions.

The obvious point here is different types of valves are appropriate for use in different size ranges,because they provide the most cost-effective solution in each given instance. If you stick with thesame type of valve over a wide size range, you sacrifice either performance at the low end or economyat the high end, or both.

After going through all the other criteria for a given application, people who specify valves oftenfind that they can use several types of valves. From there on, selection is a matter of price versuscapability as discussed here—coupled with the inevitable personal and institutional preferences.Because no single control-valve package is cost effective over the full range of applications that arenormally encountered, it is important to keep an open mind for alternative choices.

VALVE SIZING

It used to be common practice in the industry to select valve size strictly as a function of pipe size.Soon it became apparent that this practice contributed to very poor control and resulting processproblems. The wide range of flow, pressure, and fluid conditions required a more in-depth selectionmethodology. With time, methods were developed and the days of selecting a valve based on pipesize are gone forever.

Selecting the correct valve size for a given application requires a knowledge of the flow and processconditions the valve will actually see in service as well as information on valve function and style.Sizing valves is based on a combination of theory and empirical data. The results are predictable,accurate, and consistent.

Early efforts in the development of valve sizing centered around liquid flow. Daniel Bernoulliwas one of the early experimenters who applied theory to liquid flow. Subsequent experimentalmodifications to this theory produced a useful liquid-flow equation.

Q = Cv

√P1 − P2

G

where Q = flow rateCv = valve sizing coefficient, determined by testingP1 = upstream pressureP2 = downstream pressureG = liquid specific gravity

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This equation rapidly became widely accepted for sizing valves on liquid service, and manufac-turers of valves began testing and publishing Cv data in their catalogs.

It was inevitable that the good results obtained from the Cv equation would strongly tempt its useto predict the flow of gas. The results, however, were inaccurate. Modifications were made to theequations over time, with consequent improvement of results. Various companies used techniquesthey developed, but there was no common formulation until the Instrument Society of America (ISA)put forth its standardized guidelines.

In order to assure uniformity and accuracy, the procedures for measuring flow parameters andfor valve sizing are addressed by ISA standards. Measurement of Cv and related flow parameters iscovered extensively in ANSI/ISA S75.02, 1981 [5]. The basic test system and hardware installation areoutlined so that coefficients can be tested to an accuracy of ±5 percent. Water is circulated through thetest valve at specified pressure differentials and inlet pressures. Flow rate, fluid temperature, inlet anddifferential pressure, valve travel, and barometric pressure are all measured and recorded. This yieldssufficient information to calculate necessary sizing parameters. Numerous tests must be performedto arrive at the values published by the valve manufacturer for use in sizing. It is important, also, thatthese factors be based on tests, not estimates, because the results are not always predictable.

Basic Sizing Procedure

The procedure by which valves are sized for liquid flow is straightforward. Again, to ensure uniformityand consistency, a standard exists that delineates the equations and correction factors to be used for agiven application (ANSI/ISA S75.01-1985 [6]).

The simplest case of liquid-flow application involves the basic equation developed earlier. Rear-ranging the equation so that all of the fluid and process-related variables are on the right-hand side,the expression for the valve Cv required for the particular application is:

Cv = Q√(P1 − P2)/G

Based on a given flow rate and pressure drop, a required Cv value can be calculated. This Cv canthen be compared to Cv values for a particular valve size and valve design. Generally, the required Cv

should fall in a range of between 70% and 90% of the selected valve’s Cv capability. Allowance forminimum and maximum flow pressure conditions should also be considered.

Once a valve has been selected and Cv is known, the flow rate for a given pressure drop, or thepressure drop for a given flow rate, can be predicted by substituting and solving for the appropriatequantities in the equation.

This basic liquid equation covers conditions governed by the test assumptions. Unfortunatelymany applications fall outside the bounds of these standards and therefore outside of the basic liquid-flow equation. Rather than develop special flow equations for all of the possible deviations, it ispossible to account for different behavior with the use of simple correction factors. These factors,when incorporated, change the form of the equation to the following:

Cv = Q

NFp FR√

P1 − P2/G

where N = numerical coefficient for unit conversionFp, FR = correction factors

Choked Flow

A plot of the basic equation (Fig. 29) implies that flow can be increased continually by simplyincreasing the pressure differential across the valve. In reality the relationship given by this equation

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FIGURE 29 Sizing equations suggest that as the pressure drop is increased, flow willincrease proportionally—forever. In reality, this relationship holds only for certain conditions.As the pressure drop is increased, choked flow caused by the formation of vapor bubbles inthe flow stream imposes a limit on liquid flow. A similar limitation on flow of gases is realizedwhen velocity at the valve vena contracta reaches sonic velocity. These choked-flow conditionsmust be considered in valve sizing. (Fisher Controls International, Inc.)

holds for only a limited range. As the pressure differential is increased, a point is reached wherethe realized flow increase is less than expected. This phenomenon continues until no additional flowincrease occurs in spite of increasing the pressure differential. This condition of limited maximumflow is known as choked flow. This phenomenon occurs on both liquids and gases. It is necessary toaccount for the occurrence of choked flow during the sizing process to ensure against undersizing avalve. Predictions must be made using a valve recovery coefficient FL for liquids and XT for gases.

Viscous Flow

One of the assumptions implicit in the sizing procedures presented up to this point is that of fullydeveloped, turbulent flow. In laminar flow, all fluid particles move parallel with one another in anorderly fashion with no mixing of the fluid. Conversely, turbulent flow is highly random in localvelocity direction and magnitude. While there is certainly net flow in a particular direction, instanta-neous velocity components in all directions exist within this net flow. Significant fluid mixing occursin turbulent flow. The factor FR is a function of the Reynolds number and describes the degree ofturbulent flow. It can be determined by a simple nomograph procedure.

Piping Considerations

When a valve is installed in a field piping configuration which is different than the standard testsection, it is necessary to account for the effect of the altered piping on flow through the valve. Recallthat the standard test section consists of a prescribed length of straight pipe up- and downstream of

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the valve. Field installation might require elbows, reducers, and tees, which will induce additionalpressure losses adjacent to the valve. To correct for this, the factor Fp is introduced.

Gas and Steam Sizing

Although most comments so far pertain to liquid sizing, they closely parallel the procedures usedfor air, gas, and steam valve sizing. The only additional steps involve correction for the physicalproperties of the particular gas and pressure ratio factors which determine the degree of compressionand predict choked flow. The general form of the sizing equation for compressible fluids is:

Cv = Q

N Fp P1Y√

X/GT1 Z

where Y = expansion factorX = �P/P1

T1 = temperatureZ = compressibility factor

For additional information on valve sizing, consult the referenced ISA publications or the manufac-turer’s literature. Computer sizing programs are available, which alleviate the need to solve complexequations manually and which provide exceptional accuracy.

ACTUATORS

Actuators are the distinguishing elements between valves and control valves. The actuator industryhas evolved to answer a wide variety of process needs and user desires. Actuators are available withmany designs, power sources, and capabilities. Proper selection involves process knowledge, valveknowledge, and actuator knowledge. A control valve can perform its function only as well as theactuator can handle the static and dynamic loads placed on it by the valve. Proper selection and sizingare, therefore, very important. The actuator represents a significant portion of the total control-valvepackage price, and careful selection can minimize costs.

The range of actuator types and sizes on the market is so great that it seems the selection processmight be highly complex. It is not. With a few rules in mind and knowledge of your fundamentalneeds, the selection process can be very simple.

The following parameters must be known at the beginning of the selection process. They areimportant because they quickly narrow the selection process.

1. Power source availability

2. Failure-mode requirements

3. Torque or thrust requirements (actuator capability)

4. Control functions

5. Economics

6. Size, modular construction, easy maintenance

Power Source

The power source available at the location of a valve can often determine what type of actuator tochoose. Typically, valve actuators are powered either by compressed air or by electricity. However,

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in some cases water pressure, hydraulic fluid, or even pipeline pressure can be used. The majority ofactuators sold today use compressed air for operation. They operate at supply pressures from as lowas 15 psig (1.0 bar) to a maximum of about 150 psig (10.4 bars).

Since most plants have ready availability of both electricity and compressed air, the selectiondepends on the ease and cost of furnishing either power source to the actuator location. One must alsoconsider reliability and maintenance requirements of the power system and their effect on subsequentvalve operation. Consideration should be given to providing backup operating power to critical plantloops.

Failure Mode

The overall reliability of power sources is quite high. However, many loops demand specific valveaction should the power source ever fail. Desired action on signal failure might be required forsafety reasons or for the protection of equipment. Fail systems store energy, either mechanicallyin springs or pneumatically in volume tanks or hydraulic accumulators. When power fails, the failsystems are triggered to drive the valves to the required position and then maintain this position untilresumption of normal operation. In many cases the process pressure is used to ensure or enhance thisaction.

Actuator designs are available that allow a choice of failure mode between failing open, failingclosed, or holding in the last position. Many actuator systems incorporate failure modes at no extracost. Spring and diaphragm types are inherently fail open or closed. Electric actuators nearly alwayshold in their last position.

Actuator Capability

An actuator must have sufficient thrust or torque for the application. In some cases this requirementcan dictate actuator type as well as power-supply requirements. For instance, large valves requiringa high thrust might be limited to only electric or electrohydraulic actuators because of a lack ofpneumatic actuators with sufficient torque capability. Conversely, electrohydraulic actuators wouldbe a poor choice for valves with very low thrust requirements. The matching of actuator capabilitywith valve-body requirement is best left to the control valve manufacturer, as there is considerablevariation in frictional and fluid forces from valve to valve.

Control Functions

Knowledge of the required actuator functions will most clearly define the options available for se-lection. These functions include the actuator signal (such as pneumatic, electric, analog, frequency),signal range, ambient temperatures, vibration levels, operating speed, cycle frequency, and quality ofcontrol required.

Generally, signal types are grouped as being either two position (on–off) or analog (throttling).On–off actuators are controlled by two-position electric, electropneumatic, or pneumatic switches.This is the simplest type of automatic control and the least restrictive in terms of selection.

Throttling actuators have considerably higher demands put on them for both compatibility andperformance. A throttling actuator receives its input from an electronic or pneumatic instrument thatmeasures the controlled process variable. The actuator must then move the final control element (valve)in response to the instrument signal in an accurate and timely fashion to ensure effective control. Thetwo primary additional requirements for throttling actuators are compatibility with instrument signaland better static and dynamic performance to ensure loop stability.

Compatibility with instrument signals is inherent in many actuator types, or it can be obtained withadd-on equipment. But the high-performance characteristics required of a good throttling actuatorcannot be bolted on. Low hysteresis and minimal dead band must be designed into actuators.

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Stroking speed, vibration, and temperature resistance must also be considered if critical to theapplication. Stroking speed is generally not critical; however, flexibility to adjust it is desirable. Withliquid service, fast stroking speeds can be detrimental because of the possibility of water hammer.

Vibration or mounting position can cause problems as the actuator weight, combined with theweight of the valve, might require bracing. If extremes of temperature or humidity are to be experiencedby the control valve, this information is essential to the selection process. Many actuators contain eitherelastomeric or electronic components, which might be subject to degradation from high humidity ortemperature.

Economics

An evaluation of the economics in actuator selection requires combining initial cost, installation,maintenance, and reliability factors. A simple actuator, such as a spring-and-diaphragm actuator,has few moving parts, is easy to service, and will normally cause fewer problems. Initial cost islow. Maintenance people understand and are comfortable working with them. An actuator madespecifically for a control valve eliminates the chance for a costly performance mismatch. An actuatormanufactured by the valve manufacturer and shipped with the valves eliminates separate mountingexpenses and ensures easier coordination of spare parts purchases. Interchangeable parts also areimportant to minimize spare parts inventory.

Savings of installation and maintenance costs are available from packages that combine the valve,actuator, and accessories in a modular unit. The components are designed to work together, externalpiping is reduced, and complicated exposed linkages are eliminated.

Actuator Designs

There are many types of actuators for rotary and sliding-stem valves. There are five major categories:

1. Spring-and-diaphragm actuators (Figs. 30 and 31).

2. High-pressure spring-and-diaphragm actuators (Fig. 32).

3. Pneumatic piston actuators (Figs. 33–36).

4. Electric motor actuators (Fig. 37).

5. Electrohydraulic actuators (Figs. 38 and 39).

Each actuator has weaknesses, strong points, and optimum uses. Most actuator designs are availablefor either sliding stem or rotary valve bodies. They differ only by linkage or motion translators. Thebasic power sources are identical (Table 4).

Most rotary actuators (Figs. 31, 35, and 36) are similar to sliding-stem actuators. Rotary actuatorsuse linkages, gears, or crank arms to convert direct linear motion of a diaphragm or piston into the 90◦

output rotation required by rotary valves. The most important consideration for control valve actuatorsis the requirement for a design that limits the amount of lost motion in the internal linkage and valvecoupling. Rotary actuators are available that use tilting pistons or diaphragms. These designs eliminatemost linkage points (and the resultant lost motion) and provide a safe, accurate, and enclosed package.

When considering an actuator design, consider the method by which it is coupled to the drive shaftof the control valve. On rotary valves, slotted connections mated to milled shaft flats generally arenot satisfactory if throttling is required. Pinned connections, if constructed solidly, are suitable fornominal torque applications. The best connectors are clamped, splined shapes. This type of connectioneliminates all lost motion, is easy to disassemble, and is capable of high torques.

Sliding-stem actuators are rigidly fixed to valve stems by threaded-and-clamped connections.Sliding stem actuators are very simple in design. Because they do not have any linkage points andtheir connections are rigid, they exhibit no lost motion and excellent inherent control characteristics.

Because rotary and sliding-stem actuators are so similar in concept and characteristics, they willnot be further differentiated in this section unless necessary.

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FIGURE 30 Spring-and-diaphragm actuators offer an excellent choice for most control valves. They are inexpensive and simple, andthey have an ever-present, reliable spring fail action. Shown are two styles. On the left, air (operating pressure). opens the valve and thespring closes it (air to open; spring closes). On the right, air to close, spring opens. (Fisher Controls International, Inc.)

Diaphragm Actuators. The most popular and widely used control-valve actuator is the pneumaticspring-and-diaphragm style (Figs. 30 and 31). Diaphragm actuators are extremely simple and offerlow cost and high reliability. Diaphragm actuators normally operate over the standard signal rangesof 3 to 15 psig (0.2–1 bar) or 6 to 30 psig (0.4–2 bars). Therefore they are often suitable for throttlingservice using instrument signals directly. Many designs offer either adjustable springs or wide springselections to allow the actuator to be tailored to the particular application. Because diaphragm actuatorshave few moving parts that might contribute to failure, they are extremely reliable. Should they everfail, maintenance is extremely simple. Improved designs include mechanisms to control the releaseof spring compression, reducing the possibility of injury to personnel during actuator disassembly.

The overwhelming advantage of the spring and diaphragm actuator is the ever-present provisionfor fail action. As pressure is loaded on the actuator casing, the diaphragm moves the valve andcompresses the spring. The stored energy in the spring acts to move the valve back to its originalposition as pressure is released from the casing. Should there be a loss of signal pressure to theinstrument or the actuator, the spring can move the valve to its initial (fail) position. Actuators areavailable for either fail-open or fail-closed action.

The only real drawback to the spring-and-diaphragm actuator is a relatively limited capability.Much of the thrust created by the diaphragm is taken up by the spring and thus does not resultin output to the valve. Therefore the spring-and-diaphragm actuator is seldom used for high forcerequirements. It is not economical to build and use very large diaphragm actuators because the size,weight, and cost grow out of proportion to capability. This limitation is mitigated, however, by thefact that most valves are small and have low force requirements.

High-Pressure Spring-and-Diaphragm Actuators. High-pressure spring-and-diaphragm actuators(Fig. 32) share many of the advantages of standard spring-and-diaphragm actuators and offer additionaladvantages. The use of higher supply pressure allows the actuator to be smaller and lighter than typical

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FIGURE 31 Spring-and-diaphragm actuators offer manyfeatures that provide precise control. The splined actuatorconnection, clamped lever, and single-joint linkage all con-tribute to low lost motion. (Fisher Controls International,Inc.)

FIGURE 32 High-pressure spring-and-diaphragm actuator, featur-ing integral control and accessories and modular construction. Thespring and diaphragm are contained in the power module assembly.Tubing, linkage, and mounting brackets are either eliminated or en-closed. (Fisher Controls International, Inc.)

diaphragm actuators. The smaller size makes modular construction easier to provide. Modularitymakes maintenance easier and allows complete integration of instruments and accessories.

Piston Actuators. Piston actuators, such as those shown in Figs. 33–36, are the second most popularcontrol-valve actuator style. They are generally more compact and provide higher torque or forceoutputs than spring-and-diaphragm actuators. Piston styles normally work with supply pressures ofbetween 50 and 150 psig (3.5 and 10.4 bars). Although piston actuators can be equipped with springreturns, this construction has limits similar to those of the spring and diaphragm style.

Piston actuators used for throttling service must be furnished with double-acting positioners, whichsimultaneously load and unload opposite sides of the piston. The pressure differential created acrossthe piston causes travel toward the lower pressure side. The positioner senses the motion of the output,and when the required position is reached, the positioner equalizes the pressure on both sides of thepiston.

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FIGURE 33 Double-acting piston actuatorsare a good choice if thrust requirements ex-ceed the capability of diaphragm actuators. Pis-ton actuators require a higher supply pressure,but they have benefits such as high stiffness anda more compact size. (Fisher Controls Interna-tional, Inc.)

FIGURE 34 Spring fail action is available inthis spring-biased piston actuator. Process pres-sure acting on the valve plug can aid fail ac-tion, or the actuator can be configured so thatthe spring alone closes or opens the valve onfailure of operating pressure. (Fisher ControlsInternational, Inc.)

The pneumatic piston actuator is an excellent choice when a compact high-power unit is required.It is also easily adapted to services where high ambient temperatures are involved.

The main disadvantages of piston actuators are the high supply pressures required, the requirementfor positioners when used for throttling service, and the lack of inherent failure-mode systems. Twotypes of spring-return piston actuators are available. The variations are subtle, but significant. It ispossible to add a spring to a piston actuator and operate it much like a spring and diaphragm. Thesedesigns use a single-acting positioner, which loads the piston chamber to move the actuator andcompress the spring. As pressure is unloaded, the spring moves the piston back. These designs uselarge high-output springs, which are capable of overcoming the fluid forces in the valve.

The alternative design uses a much smaller spring and relies on valve fluid forces to help providethe fail action. In normal operation they act like a double-acting piston. In a fail situation the springinitiates movement and is helped by unbalance forces on the plug.

The only failure-mode alternative to springs are pressurized air volume tank pneumatic trip sys-tems to move the piston actuator to its fail position. Although these systems are quite reliable, theyadd to overall system complexity, maintenance difficulty, and cost. Therefore for any failure-moderequirement prime consideration should be given to spring-return actuators if they are feasible.

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FIGURE 35 This piston actuator controls a ro-tary valve. The valve linkage and clamped connectoreliminate lost motion and provide throttling accuracy.(Fisher Controls International, Inc.)

FIGURE 36 For on–off service and some throttlingapplications, requirements for accuracy and minimumlost motion are not necessary, and a simple design such asthis can save money. The actuator shown features spring-return action. (Fisher Controls International, Inc.)

Use special care during the selection of throttling piston actuators to get one that has minimalhysteresis and dead band. As the number of linkage points in the actuator increases, so does the deadband. As the number of sliding parts increases, so does the hysteresis. An actuator with high hysteresisand dead band can be quite suitable for on-off service. However, caution is necessary when attemptingto adapt this actuator to throttling service by simply bolting on a positioner.

The cost of a diaphragm actuator is generally less than that of a comparable-quality piston actuator.Part of this cost savings is in the ability to use instrument output air directly, thereby eliminating theneed for a positioner. The inherent provision for fail action in the diaphragm actuator is also aconsideration.

Electric Actuators. Electric actuators can be successfully applied in many situations. Most electricoperators consist of motors and gear trains (Fig. 37). They are available in a wide range of torqueoutputs, travels, and capabilities. They are suited for remote mounting where no other power source isavailable or for use where there are specialized thrust or stiffness requirements. Electric actuators areeconomical, compared with pneumatic ones, for applications in small size ranges only. Larger unitsoperate slowly, weigh considerably more than pneumatic equivalents, and are more costly. Precisionthrottling versions of electric motor actuators are quite limited in availability. One very importantconsideration in choosing an electric actuator is its capability for continuous closed-loop control. Inapplications where frequent changes are made in control valve position, the electric actuator musthave a suitable duty cycle.

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FIGURE 37 Technical improvements have made electric actuators practicablefor control purposes. They offer high thrust or torque and high stiffness. (El-O-Matic International.)

While having many disadvantages, the electric actuator will generally provide the highest outputavailable within a given package size. In addition electric actuators are very stiff, that is, resistant tovalve forces. This makes them an excellent choice for good throttling control of large high-pressurevalves.

Electrohydraulic Actuators. Electrohydraulic actuators, like those in Figs. 38 and 39, are electricactuators in which motors pump oil at high pressure to a piston, which in turn creates the outputforce. The electrohydraulic actuator is an excellent choice for throttling because of its high stiff-ness, compatibility with analog signals, excellent frequency response, and positioning accuracy. Mostelectrohydraulic actuators are capable of very high outputs, but they are limited by high initial cost,complexity, and difficult maintenance. Failure-mode action on electrohydraulic actuators can be ac-complished by the use of springs or hydraulic accumulators and shutdown systems.

Actuator Sizing

The last step in the selection process is the specification of the actuator size. The process of sizingis to match the actuator capabilities as closely as possible to the valve requirements. In practice,the mating of actuator and valve requires the consideration of many factors. Valve forces must beevaluated at the critical positions of valve travel (usually open and closed) and compared to actuatoroutput. Valve force calculation varies considerably between valve styles and manufacturers. In mostcases it is necessary to consider a complex summation of forces, including the following:

� Static fluid forces� Dynamic fluid forces and force gradients

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FIGURE 38 Self-contained electrohydraulic actu-ator. This single unit contains the electric motor, hy-draulic pump, reservoir, hydraulic positioner, and ac-tuator cylinder. (Fisher Controls International, Inc.)

FIGURE 39 Electrohydraulic actuators provide the ultimate in thrust,speed, frequency of response, and stiffness. The type shown is operated byan external hydraulic power supply. (Fisher Controls International, Inc.)

� Friction of seals, bearings, and packing� Seat loading

Although actuator sizing is not difficult, the great variety of designs on the market and the readyavailability of vendor expertise (normally at no cost) make detailed knowledge of the proceduresunnecessary.

Summary of Actuator Selection Factors

In choosing an actuator type, the fundamental requirement is to know your application. Control signal,operating mode, power source available, torque required, and fail position can make many decisions

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TABLE 4 Comparison of Valve Actuator Features

Advantages Disadvantages

Spring and diaphragm

Lowest cost Limited output capabilityAbility to throttle without positioner Large size and weightSimplicityInherent failure-mode actionLow supply-pressure requirementAdjustability to varying conditionsEase of maintenance

High-pressure spring and diaphragm

Compact, light weight Requires high supply pressure—40No spring adjustment needed psig (2.8 bars) or higherCostly cast components not needed Positioner required for throttingInherent fall-safe actionNo dynamic stem seals or traditional

stem connector block neededDesign can include integral

accessories

Pneumatic piston

High force or torque capability Fall-safe requires accessories orCompact, light weight addition of a springAdaptable to high ambient Positioner required for throttling

temperatures Higher costFast stroking speed High supply-pressure requirementRelatively high actuator stiffness

Electric motor

Compact High costVery high stiffness Lack of fail-safe actionHigh output capability Limited duty cycleSupply pressure piping not required Slow stroking speed

Electrohydraulic

High output capability High costHigh actuator stiffness Complexity and maintenanceExcellent throtting ability difficultyFast stroking speed Fail-safe action only with

accessories

for you. Keep in mind simplicity, maintainability, and lifetime costs. Safety is another considerationthat must never be overlooked. Enclosed linkages and controlled compression springs available insome designs are very important for safety reasons. The pros and cons of the various actuator stylesare listed under “Summary Checklist.”

The spring-and-diaphragm actuator is the most popular, versatile, and economical type. Try itfirst. If the limitations of available diaphragm actuators eliminate them, consider pistons or electricactuators, bearing in mind the capabilities and limitations of each.

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VALVE CONTROLLERS AND ACCESSORIES

No study of control valves would be complete without a look at devices that augment the valvefunction and interface it to control systems. Included in this category are devices such as digital valvecontrollers as well as traditional valve positioners, electropneumatic transducers, limit switches, andmanual actuator overrides. These devices assure controllability, provide information about valveoperation, and also allow for operation or shutdown in emergency situations.

Valve Positioners and Controllers

Positioners are instruments that help improve control by accurately positioning a control valve ac-tuator in response to a control signal. Positioners receive an input signal either pneumatically orelectronically and provide output power, generally pneumatically, to an actuator to assure valve posi-tioning. A feedback linkage between valve stem and positioner is established so that the stem positioncan be noted by the instrument and compared with the position dictated by the controller signal(Fig. 40).

Use of positioners is generally desirable to linearize the control valve plug position with a controlsignal. Positioners will often improve the performance of control valve systems. There are situations,however, where process dynamics eliminate the use of positioners. On very fast loops it has beenfound that the use of positioners will degrade performance because the response of the positionermight not be able to keep up with the system in which it is installed.

Positioners operate with a pneumatic input and output signal or with an electronic input signaland pneumatic output. Some of the electronic versions accept an analog input signal, and othersaccept a digital input signal. Digital positioners and digital valve controllers are discussed in the nextsection.

FIGURE 40 Electropneumatic positioners combine the functionof a current-to-pressure transducer with those of a positioner. Itreceives an electronic input signal and ensures valve position byadjusting output pressure. (Fisher Controls International, Inc.)

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Digital Positioners [1]

Digital valve positioners are of three types:

1. Digital Noncommunicating—A current signal (4–20 mA) is supplied to the positioner, whichboth powers the electronics and controls the output.

2. HART (highway addressable remote transducer) communications—This is the same as thedigital noncommunicating but is also capable of two-way digital communication over the same wiresused for the analog signal. (HART is a mark owned by HART Communications Foundation, Inc.)

3. Fieldbus—This type receives digital signals and positions the valve by using digital electroniccircuitry coupled to mechanical components. An all-digital control signal replaces the analog controlsignal. Additionally, two-way digital communication is possible over the same wires. The shift infield communications technology toward a fieldbus technology benefits the user by enabling improvedcontrol architecture, product capability, and reduced wiring.

There is a general trend toward greater use of digital valve controllers on control valves (Fig. 41).There are several reasons for this trend:

1. There is a reduced cost of loop commissioning, including installation and calibration.

2. Diagnostics are used to maintain loop performance levels.

3. There is an improved process control through reduced process variability.

4. Offset the decreasing mechanical skill base of instrument technicians.

Two aspects of digital valve controllers make them particularly attractive:

1. Automatic calibration and configuration: Considerable time savings are realized over traditionalzero and spanning.

2. Valve diagnostics: Through the DCS (distributed control system), PC software tools, or handheldcommunicators, users can diagnose the health of the valve while it is in the line (Fig. 42).

Electropneumatic Transducers

Electropneumatic transducers (Fig. 43) are devices that convert an electronic input into a pneu-matic output signal that is proportional to the input signal. Electropneumatic transducers are used in

FIGURE 41 Rotary control valve with digital valve controller (H. D. Baumann Inc.).

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FIGURE 42 Diagnostics programs can provide information to help plan predictive mainte-nance. (Fisher Controls International, Inc.)

electronic control loops to help operate pneumatic control valves. Most transducers convert a standard4- to 20-mA (milliampere) signal to a 3- to 15-psig (0.2–1.0 bar) pneumatic output. Devices also areavailable that can respond to digital signals and nonstandard analog inputs. The transducer functionis sometimes included with the valve positioner. If the transducer is included, the device is known asan electropneumatic positioner: the input is an electronic signal and the output is position.

Volume Booster. The volume booster is normally used in control-valve actuators to increase thestroking speed. These pneumatic devices have a separate supply pressure and deliver a higher-volumeoutput signal to move actuators rapidly to their desired positions. Special booster designs (Fig. 44) arealso available for use with positioners. These devices incorporate a dead-band feature to adjust theirresponse and eliminate instabilities. This booster, therefore, permits high actuator stroking speedswithout degrading the steady-state accuracy provided by positioners in the loop.

Trip Valves. Pressure-sensing trip valves are available for control applications where a specificactuator action is required when supply pressure fails or falls below a specific point. When supplypressure falls below the preadjusted trip point, the trip valve causes the actuator to fail up, lock inlast position, or fail down. When supply pressure rises above the trip point, the valve automaticallyresets, allowing the system to return to normal operation. Auxiliary power to provide for actuatoraction in case of trip is provided by pneumatic volume tanks. Fig. 45 shows a system installed on avalve actuator.

Limit Switches. Electrical position switches are often incorporated on control valves to provide theoperation of alarms, signal lights, relays, or solenoid valves when the control-valve position reachesa predetermined point. These switches can be either integrated, fully adjustable units with multipleswitches or stand-alone switches and trip equipment. Use special care in the selection of limit switchesfor harsh environments to assure functionality over time (Figs. 46 and 47).

Solenoid Valves. Small, solenoid-operated electric valves are often used in a variety of on-off orswitching applications with control valves. They provide equipment override, failure-mode interlock

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FIGURE 43 Electropneumatic transducers are com-mon accessories. They convert an analog electronic sig-nal to a pneumatic output signal. The best transducersare compact and accurate and consume little supply air.(Fisher Controls International, Inc.)

of two valves, or switching from one instrument lineto another. A typical application involves a normallyopen solenoid valve, which allows positioner out-put to pass directly to the actuator. On loss of elec-tric power, the solenoid valve will close the port tothe valve positioner and bleed pressure from the di-aphragm case to the control valve, allowing it toachieve its fail position.

Position Transmitters. Electronic position trans-mitters are available that send either analog or digi-tal electronic output signals to control-room devices.The instrument senses the position of the valve andprovides a discrete or proportional output signal.Electrical position switches are often included inthese transmitters (Fig. 48).

Manual Handwheels. A variety of actuator acces-sories are available which allow for manual overridein the event of signal failure or lack of signal previ-ous to start-up. Nearly all actuator styles have avail-able either gear-style or screw-style manual overridewheels. In many cases, in addition to providing over-ride capability, these handwheels can be used as ad-justable position or travel stops. Figure 49 shows theinstallation of a manual handwheel on a spring anddiaphragm actuator.

CONTROL VALVE INSTALLATION [1]

Never install a valve where service conditions couldexceed those for which the valve was intended. Con-tact the manufacturer if you have questions concern-ing applicable service conditions.

Storage and Protection

Consider storage and protection early in the selection process, before the valve is shipped. Typically,manufacturers have packaging standards that are dependent upon the destination and intended lengthof storage before installation. Because most valves arrive on site some time before installation, manyproblems can be averted by making sure the details of the installation schedule are known and discussedwith the manufacturer at the time of valve selection. In addition, take special precautions when youreceive the valve at the final destination. For example, store the valve in a clean, dry place away fromany traffic or other activity that could damage the valve.

Installation Techniques

Always follow the control valve manufacturer’s installation instructions and cautions. Typical instruc-tions are summarized here.

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FIGURE 44 Volume booster that delivers added air volume for rapid actuator stroking. The booster shown isspecifically made for valve positioners. It uses a bypass to allow small output changes to pass through, but, whenthe signal changes exceed preset dead-band limits, it delivers high-volume output pressure for rapid stoking. (FisherControls International, Inc.)

Read the Instruction Manual. Before installing the valve, read the instruction manual. Instructionmanuals describe the product and review safety precautions to take before and during installation.Following the manual helps ensure an easy and successful installation.

Be Sure the Pipeline Is Clean. Foreign material in the pipeline could damage the seating surface ofthe valve or even obstruct the movement of the valve plug, ball, or disk so that the valve does not shutoff properly. To help reduce the possibility of a dangerous situation from occurring, clean all pipelinesbefore installing. Make sure pipe scale, metal chips, welding slag, and other foreign materials areremoved. In addition, inspect pipe flanges to ensure a smooth gasket surface. If the valve has screwedend connections, apply a good grade of pipe sealant compound to the male pipeline threads. Do notuse sealant on the female threads because excess compound on the female threads could be forcedinto the valve body. Excess compound could cause sticking in the valve plug or accumulation of dirt,which could prevent good valve shutoff.

Although valve manufacturers take steps to prevent shipment damage, such damage is possibleand should be discovered and reported before the valve is installed.

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FIGURE 45 Fail action for piston actuators can be accomplished byusing pneumatic trip systems. A switching valve transfers stored pressurefrom volume tanks to the piston to stroke the valve and maintain thepredetermined failure position. (Fisher Controls International, Inc.)

Do not install a control valve that has been damaged in any way.Before installing, check for and remove all shipping stops and protective plugs or gasket surface

covers. Check inside the valve body to make sure no foreign objects are present.

Use Good Piping Practices. Most control valves can be installed in any position. However, themost common method is with the actuator vertical and above the valve body. If horizontal actuatormounting is necessary, consider additional vertical support for the actuator. Be sure the body isinstalled so that fluid flow will be in the direction indicated by the flow arrow (Fig. 50) or instructionmanual.

Be sure to allow ample space above and below the valve to permit easy removal of the actuatoror valve plug for inspection and maintenance. Clearance distances are normally available from thevalve manufacturer as certified dimension drawings. For flanged valve bodies, be sure the flangesare properly aligned to provide uniform contact of the gasket surfaces. Gently tighten bolts afterestablishing proper flange alignment. Finish tightening them in a criss-cross pattern (Fig. 51). Propertightening will avoid uneven gasket loading and will help prevent leaks. It also will avoid the possibility

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FIGURE 46 Actuator, featuring externally adjustable travel stopsand integrally mounted cam-operated proximity limit switches. Alllinkages for switches and travel stops are fully enclosed. (Fisher Con-trols International, Inc.)

FIGURE 47 Limit switches are common actuator accessories. The unit shown hastwo limit switches, but similar designs can hold up to six switches and trip points canbe adjusted to any point of travel. (Fisher Controls International, Inc.)

of damaging, or even breaking, the flange. This precaution is particularly important when connectingto flanges that are not the same material as the valve flanges.

Pressure taps installed upstream and downstream of the control valve are useful for checking flowcapacity or pressure drop. Locate such taps in straight runs of pipe away from elbows, reducers, orexpanders. This location minimizes inaccuracies resulting from fluid turbulence.

Use 1/4- or 3/8-in. (6–10 mm) tubing or pipe from the pressure connection on the actuator tothe controller. Keep this distance relatively short and minimize the number of fittings and elbows toreduce system time lag. If the distance must be long, use a valve positioner or a booster with thecontrol valve.

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FIGURE 48 Stem position transmitters provide discrete or analog output of valve position for use bycontrol-room instrumentation. (Fisher Controls International, Inc.)

SUMMARY CHECKLIST

The subject of control valves is complex and ever evolving. Valve styles are changing to meet changingprocess conditions and accessories and instrumentation continue to evolve to meet the requirements ofthe control systems. The key to the selection process is to understand both the needs of the process andthe needs of the controlling instrumentation. Tips for valve selection, sizing, and actuator selectionfollow.

1. Valve Body Selectiona. Sliding-stem valves provide the widest variety and best capability in the industry. Their perfor-

mance and versatility make them very popular. In large sizes they may be expensive, but forsizes NPS 3 or DN 80 and less they are a first choice.

b. Rotary-ball and eccentric-plug valves provide excellent control and are especially good valuesin sizes NPS 4 to 6 or DN 100 to DN 150. Erosion-resistant designs and trims are available toextend their life in many difficult applications.

c. Butterfly and high-performance butterfly valves are most popular and economical in sizes aboveNPS 6 or DN 150. In many large-size cases they are the only available choice.

d. Special requirements necessitate special valve solutions. Valve designs and special trims areavailable to handle high noise, cavitation, high pressure, high temperature, and combinationsof these.

2. Sizing of Valvea. The liquid sizing equation is simple to use and based on empirically determined sizing coeffi-

cients.b. A valve size should be selected which gives the required application Cv at 70–90% of travel.c. Sizing and trim selection are influenced by choked flow and the presence of cavitation. These

phenomena limit flow and may cause significant damage.

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FIGURE 49 The handwheel on this actuator can act as a travel stop or foremergency operation of the valve. The actuator is an air-to-open, spring-closeactuator. (Fisher Controls International, Inc.)

FIGURE 50 Be sure flow is in the same direction as the flow arrow onthe valve. (Fisher Controls International, Inc.)

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FIGURE 51 Tighten flange boltsor studs and nuts in a criss-crosspattern (Fisher Controls Interna-tional, Inc.).

d. Viscosity and piping corrections must be made in many sizing situations.Piping considerations are especially important when high-recovery valvesare specified.

e. Sizing valves for gas flow involves physical principles similar to liquid flow.However, effects of compressibility and critical flow factors must be consid-ered.

3. Actuator selectiona. Actuator selection must be based on a balance of process requirements, valve

requirements, and cost.b. Spring and diaphragm actuators are simpler, less expensive, and easier to

maintain. Consider them first in most situations.c. Piston actuators offer many of the advantages of pneumatic actuators, with

higher thrust capability than diaphragm styles. They are especially usefulwhere compactness is desired or long travel is required.

d. Electric and electrohydraulic actuators provide excellent performance. Theyare, however, much more complex and difficult to maintain.

e. Actuator sizing is not difficult, but the wide variety of actuators and valves makes this difficultto master. Vendor expertise is widely available.

REFERENCES

1. Control Valve Handbook, 3rd ed., Fisher Controls International, Inc., Marshalltown, Iowa, 1998.

2. ANSI B16.34-19, “Steel Valves,” American National Standards Institute, New York, 1988.

3. ANSI B16.1-19, “Cast Iron Pipe Flanges and Flanged Fittings,” American National Standards Institute, NewYork, 1989.

4. ANSI/FCI 70-2-1976 (R1982), “Quality Control Standard for Control Valve Seat Leakage,” Fluid ControlsInstitute, 1982.

5. ANSI/ISA S75.02-1988, “Control Valve Capacity Test Procedure,” Instrument Society of America, ResearchTriangle Park, North Carolina, 1988.

6. ANSI/ISA S75.01-1985, “Flow Equations for Sizing Control Valves,” Instrument Society of America,Research Triangle Park, North Carolina, 1985.

7. ANSI/ISA S75.05-1983, “Control Valve Terminology,” Instrument Society of America, Research TrianglePark, North Carolina, 1983.

8. ANSI/ISA S75.11-1985, “Inherent Flow Characteristic and Rangeability of Control Valves,”Instrument So-ciety of America, Research Triangle Park, North Carolina, 1985.

9. Control Valve Sourcebook—Power and Severe Service, Fisher Controls International, Inc., Marshalltown,Iowa, 1990.

10. Fitzgerald, W., Control Valves for the Chemical Process Industries, McGraw-Hill, New York, 1995.

11. Baumann, H. D., Control Valve Primer—A Users Guide, 3rd ed., Instrument Society of America, ResearchTriangle Park, North Carolina, 1998.

CONTROL VALVE TROUBLESHOOTING

Table 5 is a listing of a number of different valve problems or symptoms, with potential root causes,and recommended corrective action. It is presented in the form of a troubleshooting diagram thatreferences certain common procedures used in valve maintenance. These procedures are explained ingreater detail in the next section.

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TABLE 5 Troubleshooting Diagram

Problems andSymptoms Causes Solutions

1. Body erosion. 1a. Velocity. 1a. Increase valve trim size to slow fluid.1b. Particulates in flowstream. 1b. Switch to streamlined design to reduce fluid1c. Cavitation and flashing. impingement.

1c. Switch to C5 body material.1d. Switch to torturous-path trim to slow fluid.1e. Switch to low-recovery valve and trim to conrol

cavitation.1f. Repair by welding up with stainless material.

2. Trim erosion. 2a. Velocity. 2a. Increase valve and trim size to slow fluid.2b. Particulates in flowstream. 2b. Switch to hardened trim.2c. Cavitation and flashing. 2c. Switch to torturous-path trim to slow fluid.

2d. Switch to low-recovery valve and trim tocontrol cavitation.

2e. Switch to streamlined design to reduceimpingement.

3. Seat ring-to-plug 3a. Low load (benchset, calibration, 3a. Use proper surface preparation (lapping).leakage. friction, etc.). 3b. Correct actuator and valve setup

3b. Poor surface condition (lapping, (benchset, calibration, friction, etc.).materials).

4. Seat ring-to-body 4a. Low load (inadequate torque, parts 4a. Correct bolt load, parts stack-up, gasketing.leakage. stack-up, improper gasketing). 4b. Recut, clean up gasket face.

4b. Surface condition (cleanlinness, 4c. Porosity in casting can sometimes result infinish). leakage around gaskets. Check for porosity.

4c. Porosity in body. Grind out and weld up.

5. Packing leakage. 5a. Stem finish/cleanliness. 5a. Clean up and polish stem to 4 rms finish.5b. Bent stem. 5b. Straighten stem to within 0.002 in. over5c. Low packing load. stroking length.5d. Wrong packing type or configuration. 5c. Retorque bolting or use live-loading.5e. Excessive packing stack height 5d. Check packing type and configuration

(graphite.) against application. Repack as necessary.5f. Corrosion and pitting (graphite). 5e. Install spacers to minimize packing height.5g. Seized or cocked packing follower. Repack valve.

5f. Use sacrificial washers. Remove graphitepacking if valve is to be inactive for morethan 2 to 3 weeks.

5g. Inspect and replace any damaged parts suchas flanges, nuts, and followers.

5h. Switch to high-performance packing system.

6. Sliding wear. 6a. High cycling (unstable loop?). 6a. Tune loop; reduce friction to reduce6b. Excessive contact stress. instability.6c. Misalignment. 6b. Increase bearing size.6d. Surface finish not to specification. 6c. Remachine parts to correct alignment.6e. Incorrect materials choice 6d. Polish surfaces.

6e. Review materials choice in light of application.6f. Switch to sliding-stem globe-style

valve because of better guiding.

(Continues)

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7. Bonnet-to-body leakage 7a. Low load from bonnet bolting 7a. Retorque bolting. Check parts stack-up(torque, internal parts, stack-up, against drawings.spring rate in gasket). 7b. Retouch, clean up gastket faces.

7b. Surface finish. 7c Sometimes casting porosity can let process7c. Stud leaks. fluid seep into bottom of stud holes.

Leakage around studs looks like leakagepast bonnet gasket (see Fig. 52).Grind out and weld up porosity.

8. Loose stem connection. 8a. Improper torque or pinning. 8a. Purchase the stem and plug as an assembly.or broken stem. 8b. Vibration or instability. 8b. Review trim-style application.

8c. Reduce clearances between cage and plug.8d. Switch to a welded plug or stem connection.

9. Excessive leakage 9a. Cage finish too rough. Cage 9a. Polish cage bore, check I.D. against drawings.past piston seal. I.D. too large. 9b. Replace seal, follow installation instructions.

9b. Improper installation: 9c. For some seals such as graphite piston rings,graphite rings, omniseal. high leakage is normal.

9d. Is leakage normal for the 9d. Change to high temperature design.type of seal? 9e. Replace seal. Address loop stability if cycling

9d. Exceeding temperature limitations is caused by this.for seal.

9e. Seal simply worn out due to cycling.

(Continues)

FIGURE 52 Porosity in body masquerading as bonnet jointleakage.

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10. Valve will not respond 10a. No air supply or low air 10a.Check the system in accordance with the P&IDs.to signal. supply. Verify that all air supply valves are open.

10b. Leaks in actuator. 10b.Measure and verify sufficient air supply pressure.10c. Solenoid closed on inlet lines. 10c. Listen for blowby at the seals or diaphragm.10d. No controller input signal. Repair or replace defective parts.10e. Crimped, broken air lines. 10d. Actuate solenoid valve. Replace if defective.10f. Leaking air fitting. 10e.No controller input may indicate a fuse10g. Incorrect flow direction has blown. Replace.

causing excessive loads on plug. 10f. Check all air lines to see they are not10h. Incorrect air line connections. crimped or broken. Repair or replace.10i. Packing parts binding on stem 10g. Check fittings for leaks. Tighten or repalce

or shaft. 10h. If the valve was just installed, check the flow10j. Defective positioner or I/P. arrow to ensure the process is flowing in the proper10k. Packing overtightened. direction. Flow above the seat can add pressures the10l. Trim is seized. actuator may not be able to overcome. Reverse10m. Plug stuck in seat. flowing direction, if appropriate.

10i. Check the air to and from a piston actuator toensure the supply is not connected to the exhaustand vice versa. Check all connections.

10j. Check the packing gland. Improper glandconfiguration is a primary cause of rod binding.Replace parts and polish trim.

10k. Check the positioner and/or the I/P to see ifthe output can be changed manually. If not, it isdefective. Repair or replace.

10l. Overtightened packing or binding in guidescan cause excessive friction that blocks valve.Loosen, lubricate, cycle, and retorque.

10m. Replace or repair seized trim. Damage may bepolished out.

10n. Pull or machine plug out of seat. Repair orreplace affected parts.

11. Valve will not open to 11a. Insufficient supply pressure. 11a. Verify adequate supply pressure.rated travel. 11b. Leaks in the actuator or 11b. Stop all leaks in actuator, air lines fittings,

accessories. and accessories.11c. Incorrect positioner or I/P 11c. Correct positioner and/or I/P calibration.

calibration. 11d. Readjust valve travel.11d. Incorrect travel adjustment. 11e. Change actuator spring.11e. Incorrect actuator spring rate. 11f. Adjust benchset.11f. Incorrect benchset. 11g. Replace bent stem or shaft.11g. Bent stem or shaft. 11h. Replace damage trim.11h. Damaged valve trim. 11i. Clean out valve trim.11i. Debris in trim. 11j. Reverse flowing direction.11j. Incorrect flow direction. 11k. Replace actuator.11k. Actuator is too small. 11l. Loosen packing, cycle, lubricate and retorque.11l. Excessive packing friction. 11m. Readjust manual operator or travel stop.11m. Incorrect position of manual

operator on travel stop.

(Continues)

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TABLE 5 Troubleshooting Diagram (Continued)


12. Valve travel sluggish 12a. Excesive packing friction. 12a. Readjust or replace packing.or slow. 12b. Stem or shaft bent. 12b. Replace bent shaft or stem.

12c. Inadequate supply pressure. 12c. Increase supply pressure.12d. Inadequate supply volume. 12d. Go to bigger supply line or add12e. Undersized accessories. capacity at valve.12f. Excessive friction in piston-type 12e. Increase flow capacity of

actuator. accessories.12g. Bearing friction. 12f. Clean out, polish cylinder I.D.,12h. Poor positioner response. remove excess lubricant.

12g. Repair or replace defective bearings.12h. Repair or replace positioner.

13. Valve travel is jumpy. 13a. Stick-slip action in packing seals 13a. Loosen, lubricate packing. Replaceor bearings. or repair seals and bearings.

13b. Volume booster bypass may need to 13b. Adjust booster bypass.be adjusted. 13c. Repair or repalce positioners.

13c. Positioner may be defective. 13d. Adjust positioner gain. Replace13d. Positioner gain may be too high. with lower gain model.

14. Rotary valve will not Rotary valves have some unique 14a. Readjust actuator stops.rotate problems. In addition to those 14b. Replace shaft.

items already covered in items 10 14c. Replace damaged parts, readjustand 11: travel.

14a. Actuator stops set wrong, 14d. Replace or clean parts.stopping the valve mechanically 14e. Recheck actuator sizing and valvebefore it fully rotates. service limits. Change valve and/or

14b. Broken shaft. actuator, as appropriate.14c. Overtravel can cause severe 14f. Loosen line bolting.

damage to eccentric valves;valves can jam.

14d. Dirt or corroded valve seatscan cause broken stems or valvecan jam.

14e. Changing service conditions,higher pressures and greaterpressure drops may stop thevalve from rotating due to insuffi-cient torque, high bearing loads.

14f. Overtightened line boltingcan increase friction between theball and seal.

15. Poor flow control See items 12 and 13 relating to sluggish 15a. Replace cage.and (rotary and sliding response “jumpy” travel. Other causes 15b. Replace piston rings.stem). include: 15c. Resolve sources of damage. Replace

15a. Deformed cage. parts.15b. Damaged piston rings. 15d. Replace Shaft.15c. Erosion, corrosion, and cavitation can alter 15e. Reverse valve in line.

trim profile. 15f. Correct flow characteristic.15d. A twisted shaft will indicate a position that is 15g. Select valve assembly

untrue in regard to disk and seat. The valve with conrol requirementsmay indicate full open or full closed taken into account.and may really be mid-range.

15e. Valve may be installed backward.15f. Incorrect selection of flow characteristic.15g. Low performance valve package.

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Common Valve Maintenance Procedures

Packing Maintenance. Valve packing is one of the more troublesome elements of control valveoperation. As a result, the end user is often faced with the prospect of pulling it out and installing anew set. The best way to do this is to take the bonnet off of the valve and then push the old packingout from the bottom, using the following procedure. Note that this procedure covers a sliding-stemglobe-style valve, and, as such, it can be done in line. For rotary valves, the procedure differs inthat there is no bonnet, so the valve has to be taken from the line to extract the packing as indicatedbelow:

1. Apply enough air pressure to the actuator to put the valve in an intermediate position so that thereis no residual stem load. Disconnect the actuator and valve stems. Relieve the air pressure, anddisconenct the actuator supply and any leakoff piping.

2. Remove the yoke coupling, yoke locknut, or the yoke bolting, and remove the actuator from thebonnet.

3. Loosen the packing flange nuts so that the packing is not tight on the valve plug stem. Remove anytravel indicator disk and stem locknuts from the valve plug stem threads. Safety note: When liftingthe bonnet, be sure that the valve plug and stem assembly remains on the seat ring. This avoidsdamage to the seating surfaces as a result of the assembly dropping from the bonnet after beinglifted part way out. The parts are also easier to handle separately. Use care to avoid damaginggasket sealing surfaces. If the cage cannot be held in the body due to gasket adhesion, control itso that it will not cause equipment damage or personal injury should it fall unexpectedly.

4. Unscrew the bonnet bolting and carefully lift the bonnet off the valve stem. If the valve plug andstem assembly start to lift with the bonnet, use a brass or lead hammer on the end of the stem andtap them back down. Set the bonnet on a cardboard or wooden surface to prevent damage to thebonnet gasket surface.

5. Remove the valve plug, the seat ring, and the cage. Note: All residual gasket material must beremoved from the cage gasket surfaces. If the gasket surfaces are scored or damaged during thisprocess, smooth and polish them by hand, sanding with 360-grit paper and using long, sweepingstrokes. Failure to remove all residual gasket material and/or burrs from the gasket surfaces willresult in leakage.

6. Clean all gasket surfaces with a good-quality degreaser. Remove and residual tin or silver fromall gasket surfaces.

7. Cover the opening in the valve body to protect the gasket surface and to prevent foreign materialfrom getting into the body cavity.

8. Remove the packing flange nuts, packing flange, upper wiper, and packing follower. Carefullypush out all the remaining packing parts from the body side of the bonnet using a rounded rod orother tool that will not scratch the packing box wall.

9. Clean the packing box and the related metal packing parts: packing follower, packing box ring,spring or lantern ring, special washers, etc.

10. Inspect the valve-stem threads for any sharp edges that might cut the packing. A whetstone oremery cloth may be used to smooth the threads if necessary. They can also be chased with a die.

11. Remove the protective covering from the body cavity, and install the cage using new top gaskets.Install the plug and then slide the bonnet over the stem and onto the studs. Lubricate the studthreads and the faces of the hex nuts. Replace hex nuts and torque the nuts in a crisscross pattern tono more than one-quarter of the nominal torque value specified. When all the nuts are tightened tothat torque value, increase the torque by one-quarter of the specified nominal torque and repeat thecrisscross pattern. Repeat this procedure until all the nuts are tightened to the specified nominalvalue. Apply the final torque value again and, if any nut still turns, tighten every nut again.

12. Install new packing and the metal packing box parts according to the appropriate arrangementin the instruction manual. If desired, packing parts may be prelubricated for easier installation.

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Slip a smooth-edged pipe over the valve stem, and gently tamp each soft packing part into thepacking box.

13. Slide the packing follower, wiper, and packing flange into position. Lubricate the packing flangestuds and other related parts and the faces of the packing flange nuts. Replace the packing flangenuts. For spring-loaded TFE V-ring packing, tighten the packing flange nuts until the shoulder onthe packing follower contacts the bonnet. For other standard packing types, tighten the packingflange nuts to the recommended torque. For high-performance packing sets, adjust the live-loadingsprings as indicated in the instruction manual.

14. Mount the actuator on the valve body assembly, and reconnect the actuator and valve stemsaccoding to the procedures in the appropriate instruction manual.

15. Cycle the valve 20 to 30 times and recheck packing load.

Packing can be replaced with the valve in the line, but it is not recommended due to the increasedrisk of stem or packing box damage. If it must be attempted, follow the above procedure with thechanges noted below:

1. Remove the packing loading parts so that the top of the packing rings can be seen.

2. Very carefully insert a corkscrew packing extraction tool into the packing box and twist it into thetop of the packing until it can be used to pull the top packing ring out.

3. Repeat this procedure until all the upper packing has been removed. If there is a spacer or bushingbelow the packing or between the upper and lower packing sets on a double arrangement, it usuallyhas some type of slot or extraction hole. If it does not, it will have to be left in place. Assumingthat it can be extracted, pull it out, and continue the above process with any packing left below thespacer.

4. Once all the packing and internal parts have been removed, do your best to clean the box out andinspect for any signs of damage. This cleaning and inspection will be very difficult to accomplishwith the bonnet in place.

5. Normally you should remove the stem connector and the actuator so the rings can be slid downover the stem. If this is not possible, split rings can be used, and they can be forced onto thestem by twisting them until the opening is large enough to slide over the stem. Split rings are notrecommended due to their propensity to leak. If they are used, make sure to stagger the splits toreduce the potential for leakage.

6. If any damage is found, the valve should be disassembled and the situation corrected at the firstopportunity. Effective corrective action cannot be taken with the bonnet on the valve, and repackingwith the bonnet on the valve will improve packing performance for a limited time, at best, if thestem or box is damaged in any way.

7. Repack and reassemble as noted above, using split rings if the stem connector was not removed.

Lapping the Seats

Lapping is procedure used to provide a better fit and surface finish between the valve plug and themating seat. It applies only to metal-to-metal seating and is normally used for Class IV or V shutoffon control valves. Classes I, II, and III don’t require it and Class VI nearly always requires softseats. The plug and seat in their as-machined state do not always fit together perfectly around theircircumference. Imperfections in fit result in excess leakage, so lapping is required to eliminate theseimperfections and to make sure that the two parts fit together as closely as possible. Lapping shouldbe carried out as follows:

1. Lapping should be done with the standard guiding in place to make sure that the parts are lappedin the positions that they will be in once the valve is fully assembled. For this reason, it is normallydone with the bonnet in place.

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FIGURE 53 Lapping tool used with spring.

2. With the seat ring in place in the body, apply a light coating of corase grinding compound (600grit) to both the seat ring and the plug. If the seating surfaces are made of stainless steel, use somewhite lead in the grinding compound to keep it from tearing or galling. Insert the plug and steminto the body and assemble the bonnet onto the body opening. The bonnet does not have to thebolted into place as long as the guiding simulates actual service.

3. Lapping requires that a very light load be applied to keep from tearing the metal, so if the plug isheavy, a spring should be used to support some of the load. The spring can be inserted over thestem and then a piece of strap iron can be locked into place on the stem and used as a grindinghandle (Fig. 53).

4. Gently rotate the plug and stem four or five times, over about a 45◦ arc. Pick it up and move to anew position and repeat. Continue this procedure, lapping over the entire circumference at leastonce. Pull the assembly apart, clean he surfaces, and look for a fine continuous lap line on both theplug and the seat. Using a mirror will make the line easier to see on the seat ring inside the valvebody.

5. If the lap lines look good, reassemble and repeat the procedure with a fine grit compound. If the laplines are not continuous, repeat with the course compound. If they are still not continuous, repeatwith the course compound. If they are still not continuous, try coining the surfaces by hitting thetop of the stem two or three times with a heavy, but soft hammer, and lap again. If this still doesn’tprovide the desired results, the plug and seat should be remachined to provide a better initial fitand the process restarted.

6. When the fine grinding is done, thoroughly clean the surfaces and reassemble, torquing the bonnetin place. If possible, a seat-leak test should then be carried out to ensure tight shutoff.

7. High-temperature valves should be heated, if possible, before beginning this process to betterduplicate actual guiding and fit in service.

8. Double-ported bodies will never seal as well as a single-port design, but they can still be lapped toimprove shutoff. Special considerations for these valves include:

The top seat grinds faster than the bottom. Use a coarser grit on the bottom ring to help correct forthis.

Never leave one seat dry while grinding the other one. This will tear the metal and hurt the shutoff.

Heavy grinding on one seat may be required to get the two seats to contact at the same time.

9. Note that despite claims to the contrary, blue-lining to check for seat contact will not providethe same tight shutoff seen with lapping. Tests have shown that there can still be relatively

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large imperfections present even though the blue-line shows continuous contact between the twosurfaces.

Replacing the Actuator Diaphragm. After isolating the valve assembly from all pneumatic and/orfluid pressures, relieve spring compression in the spring, if possible. (On some spring and diaphragmactuators for use on rotary-shaft valve bodies, spring compression is not externally adjustable. Initialspring compression is set at the factory and does not need to be relieved in order to change thediaphragm.) Remove the upper diaphragm case. On direct-acting actuators, the diaphragm can belifted out and replaced with a new one. On reverse-acting actuators, the diaphragm head assemblymust be dismantled to change the diaphragm.

Most pneumatic spring and diaphragm actuators utilize a molded diaphragm for control valveservice. The molded diaphragm facilitates installation, provides a relatively uniform effective areathroughout the valve’s range, and permits greater travel than could be possible if a flat-sheet diaphragmwere used. If a flat-sheet diaphragm is used in an emergency repair situation, it should be replacedwith a molded diaphragm as soon as possible.

When reassembling the diaphragm case, tighten the cap screws around the perimeter of the casefirmly and evenly to prevent leakage. Be careful not to tear the diaphragm in the area of the bolt holesduring reassembly. Avoid reusing a diaphragm since they are prone to leak if reused.

Replacing threaded-in seat rings. Threaded-in seat rings are no longer the preferred design forcontrol valves in the chemical process industry. Nevertheless, this design is encountered fairly oftendue to its popularity in the past. The main reason this design has fallen from favor is that the seat ringscan be very diffcult to get out. Adhering to the following recommended practice should help extractthe seat ring with a minimum of effort and risk of personnel:

1. Before trying to remove the seat ring(s), check to see if it has been tack-welded into the body. Ifit has, grind out the weld.

2. To make disassembly easier, soak the ring and threads with penetrating oil and allow them to sitfor some time so that the oil can do its job in loosening up the threads.

3. Insert a seat ring puller like that shown in Fig. 54 against the lugs or in the slots of the ring. Becareful to hold the puller down against the ring while applying torque, and any rounded edges onthe lugs or slots should be corrected to keep the puller from slipping past the lugs or slots.

FIGURE 54 Seat ring puller. (Fisher Controls Interna-tional, Inc.)

1.8P1: xxx



4. The torque can be applied manually or with the aid of a hydraulic torque wrench. If the powerwrench is used, be extra careful to avoid slippage due to the high torques and the safety risk topersonnel if something slips or breaks. If the valve has been pulled from the line, a lathe or boringmill may be the easiest way to apply the torque to back the ring out.

5. The bonnet bolting can be used as a reaction point for the torque and to hold the puller down intothe body.

6. On particularly stubborn rings, using an impact wrench can help to break them loose.

7. As the ring starts to come out, the bolts holding the puller in the body must also be loosened topermit the ring to move up.

8. Once the ring is out, thoroughly clean and chase all threads.

9. Apply a heavy coat of lubricant or pipe compound to all threads and reinstall and torque to specifiedlevels. The ring may be tackwelded in place, as necessary.

10. On double-ported valves, the port the farthest distance from the actuator is the smallest and needsto be installed first.

REFERENCES

1. Preckwinkle, S. E., Maintenance Guide for Air Operated Valves, Pneumatic Actuators & Accessories, ElectricPower Research Institute, Palo Alto, Calif., 1991.

2. Ozol, J., “Experiences with Control Valve Cavitation Problems and Their Solutions,” Proceedings of EPRIPower Plant Valves Symposium EPRI, Palo Alto, Calif., 1987.

3. McElroy, J. W., Light Water Reactor Valve Performance Surveys Utilizing Acoustic Techniques, PhiladelphiaElectric Co., Philadelphia, Pa. 1987.

4. Fitzgerald, W. V., “Automated Control Valve Troubleshooting: The Key to Optimum Valve Performance,” ISA,Proceedings, ISA, Research Triangle Park, N.C., 1991.

5. Ferguson, Brian, “Air-Operated Valve—Preventive Maintenance Program,” Proceedings of the 2d NRC/ASMESymposium on Pump & Valve Testing, Washington, D.C., 1992.

6. Hutchison, J. W., ISA Handbook of Control Valves, ISA, Research Triangle Park, N.C., 1971.

7. Control Valve Handbook, 1st ed., Fisher Controls, Marshalltown, Iowa, 1977.

8. Instruction, Manual, EHD, EHS, & EHT, Form 5163, Fisher Controls, Marshalltown, Iowa, 1985.

CONTROL VALVE CAVITATION:AN OVERVIEW

by Marc L. Riveland∗

INTRODUCTION

Cavitation is of significant concern to the process control industry. It can be the source of unac-ceptable noise, vibration, material damage, and a decrease in the efficiency of hydraulic devices. Left

* Sr. Engineering Specialist, Applied Research, Fisher Controls International, Inc., Marshalltown, Iowa 50158.

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