Control Valve Selection and Sizing 6 - twanclik.free.frtwanclik.free.fr/electricity/IEPOPDF/1081ch6_1.pdf · Control Valve Sizing 1051 ... Determining the Valve Pressure Drop 1053

1045

Control Valve Selectionand Sizing

6

6.1APPLICATION AND SELECTIONOF CONTROL VALVES 1050

Introduction 1050Orientation Table 1051

Control Valve Trends 1051Globe vs. Rotary Valves 1051Valves vs. Other Final Control Elements 1051

Control Valve Sizing 1051Collecting the Process Data 1053Determining the Valve Pressure Drop 1053

Characteristics, Gain, and Rangeability 1054Characteristics and Gain 1054Valve Rangeability 1057

Actuator Selection 1058Piston Actuators 1058Actuator Speeds of Response 1059Actuator Power 1059Valve Failure Position 1059

Positioners 1060When to Use Positioners 1060When Not to Use Positioners 1060Positioners to Eliminate Dead Band 1061Split-Range Operation 1061Accessories 1061

Process Application Considerations 1061Pressure Considerations 1061High-Temperature Service 1063Low-Temperature Service 1066Cavitation and Erosion 1068Methods to Eliminate Cavitation 1068Control Valve Noise 1072

Flashing and Erosion 1073Corrosion 1075Viscous and Slurry Service 1075Valves That Can Be Sterilized 1076Valve Leakage 1076

Installation 1080Climate and Atmospheric Corrosion 1080

Control Valve Specification Form 1080References 1080Bibliography 1085

6.2ACCESSORIES AND POSITIONERS 1087

Introduction 1089Smart Valves 1089Positioners 1090

When to Use Positioners 1091When Not to Use a Positioner 1093Positioner Performance 1093Positioner Designs 1094Positioner Accessories 1095Position Indicators 1095

Transducers 1096I/P (Electropneumatic) Transducers 1096Digital Electropneumatic Transducers 1096

Relays 1096Booster Relays 1096Reversing and Other Relays 1097Quick-Exhaust Relays 1098Relays to Lock-up Valve Position 1099Failure Position Guaranteed by Stored Air 1100

© 2006 by Béla Lipták

1046 Control Valve Selection and Sizing

Energy Supplies 1100Air Sets 1100Hydraulic (High-Pressure) Operation 1101Hydraulic (Water) Operation 1101

Limit Switches 1101Solenoid Valves 1101

Three-Way Solenoids 1101Four-Way Solenoids 1102Solenoid Capacity 1102

Handwheels 1102Limit Stops 1103Bypass Valve 1103References 1104Bibliography 1104

6.3ACTUATORS: DIGITAL, ELECTRIC,HYDRAULIC, SOLENOID 1105

Introduction 1107Selection and Application 1107

Actuator Types 1107Actuator Features 1107

Digital Valve Actuators 1109Electromechanical Actuators 1110

Reversible Motor Gear Actuators 1111Rotary Output Actuators 1111Linear Output Actuators 1112

Electrohydraulic Actuators 1114External Hydraulic Source 1114Hermetically Sealed Power Pack 1114Motor and Pump Combinations 1115

Solenoid Valves 1118Modulating Solenoid Valves 1120

Smart Actuators 1121Applications 1121

References 1122Bibliography 1123

6.4ACTUATORS: PNEUMATIC 1124

Introduction 1126Definitions 1126Actuator Features and Selection 1126Spring/Diaphragm Actuators 1126

Steady-State Force Balance 1127Actuator Sizing Example 1128Actuator Nonlinearities 1129Dynamic Performance of Actuators 1129Safe Failure Position 1131Pneumatic Response Times 1132

Piston Actuators 1133High-Speed Actuators 1133Relative Merits of Diaphragm

and Piston Actuators 1135

Rotary Valve Actuators 1136Cylinder Type 1137Rotation by Spline or Helix 1138Vane Type 1138Rotary Pneumatic Actuators 1138

Other Pneumatic Actuators 1139Pneumohydraulic Actuators 1139Electropneumatic Actuators 1140

Reliability 1141Conclusions 1142References 1142Bibliography 1143

6.5ADVANCED STEM PACKING DESIGNS 1144

Introduction 1144History 1144

Bibliography 1149

6.6CAPACITY TESTING 1150

Introduction 11501. Scope 11502. Purpose 11503. Nomenclature 11504. Test System 1150

4.1 General Description 11504.2 Test Specimen 11504.3 Test Section 11524.4 Throttling Valves 11524.5 Flow Measurement 11524.6 Pressure Taps 11524.7 Pressure Measurement 11534.8 Temperature Measurement 11534.9 Installation of Test Specimen 11534.10 Accuracy of Test 1153

5. Test Fluids 11535.1 Incompressible Fluids 11535.2 Compressible Fluids 1153

Summary 1153

6.7CHARACTERISTICS AND RANGEABILITY 1154

Introduction 1154Valve Gain and Loop Gain 1154

Nonlinear Processes 1154Installed Valve Gain 1155

Theoretical Valve Characteristics 1155Valve Testing 1155Valve Characteristics 1155Valve and Process Characteristics 1155Selection Recommendations 1156Installation Causes Distortion 1157Distortion Coefficient 1157Correcting the Valve Characteristic 1158


Contents of Chapter 6 1047

Rangeability 1158Improved Definition of Rangeability 1159Why Traditional Rangeability Is Wrong 1159

Conclusions 1160Bibliography 1160

6.8DIAGNOSTICS AND PREDICTIVE VALVE MAINTENANCE 1161

Introduction 1161Diagnostics 1161

Instrumentation Used 1161Diagnostic Methods 1162Characteristics Tests 1162Valve Signatures 1162Analyzing Valve Signatures 1164


6.9DYNAMIC PERFORMANCEOF CONTROL VALVES 1165

Valve Response 1165Definitions 1165

Discussion 1166Valve System 1166Install Positioner 1167Increase Force 1168Reduce Friction 1168Defining Response 1168Measuring Response 1168Relationships 1169Determine Required Response

Specifications 1169Application Examples 1169Flow Control 1169Reactor Mixing 1170Neutralizing Waste Water 1170Antisurge Valve 1170Delay or Slowdown Valve Action 1170Safety Solenoid Valves 1170Troubleshoot Valve Response 1171

Bibliography 1171References 1171

6.10EMERGENCY PARTIAL-STROKE TESTINGOF BLOCK VALVES 1172

Introduction 1172The Partial-Stroke Test 1172

Mechanical Limiting 1173Position Control 1173Solenoid Valve 1173

Impact of PST on SIL 1173Block Valve Analysis 1175

Overall SIS Performance 1178Single Block Valve Case 1178Dual Block Valve Case 1179


6.11FIELDBUS AND SMART VALVES 1182

Introduction 1182Benefits and Savings 1183

Hart, Foundation Fieldbus,and Profibus-PA 1183

Valve Calibration and Configuration 1185Safety and Pollution 1188On-line Plant Asset Management 1189

Digital Valve Instrumentation 1189Second Generation 1189

Conclusions 1192References 1192

6.12INTELLIGENT VALVES, POSITIONERS, ACCESSORIES 1193

Introduction 1193Advantages of Intelligent Positioners 1193Typical Performance Specifications 1194Generating the Pneumatic Output 1194Valve Performance Monitoring 1195

Controlling the Process 1195Changing the Valve’s Characteristics 1195

Operation of Smart Positioners 1196Maintenance and Calibration 1197

Accessories 1197Flow Control by Smart Valve 1197

Limitations 1198References 1198Bibliography 1198

6.13MISCELLANEOUS VALVE AND TRIM DESIGNS 1199

Introduction 1200Miscellaneous Valve Designs 1200

Dynamically Balanced Plug Valves 1200Positioned Plug In-Line Valves 1201Expansible Valve Designs 1202Fluid Interaction Valves 1205

Special Valve Application 1206Cavitation and Flashing 1206Dirty Process Services 1208High Noise 1209High-Capacity Valves 1210



Cryogenic Valves 1210High-Temperature Valves 1210Steam Conditioning Valves 1211Tank-Mounted Valves 1211

Bibliography 1211

6.14VALVES: NOISE CALCULATION, PREDICTION,AND REDUCTION 1213

Introduction 1213Sound and Noise 1214

Speed of Sound 1214The Human Ear 1214

Loudness Perception 1215Limiting Valve Noise 1215

Valve Noise 1216Control Element Instability 1217Resonant Vibration 1217Hydrodynamic Noise 1218Aerodynamic Noise 1218

Controlling Noise 1218Path Treatment 1219Source Treatment 1220

Aerodynamic Noise Prediction 1223Standards 1224Calculations 1224Noise Calculation Example 1230Applying Distance Corrections 1232

Hydrodynamic Noise Prediction 1232Bibliography 1233

6.15SIZING 1234

Introduction 1234About This Section 1234Standards 1234

General Principles 1235The Flow Coefficient 1235

Liquid Sizing 1237Relative Valve Capacity Coefficient (Cd) 1237Factors FL, FF , FP , and FLP 1237

Example 1241Units Used in Valve Sizing 1241Sizing Example for Liquids 1243The Cavitation Phenomenon 1244Flashing 1248Laminar or Viscous Flow 1250

Gas and Vapor Sizing 1252Equations for Turbulent Flow 1252Constants for Engineering Units 1253Expansion Factor (Y ) 1253Choked Flow 1253Velocity of Compressible Fluids 1254Sizing for Compressible Fluids

(Example 12) 1255

Two-Phase Flow 1256Liquid-Gas Mixtures 1257Liquid-Vapor Mixtures 1258

Conclusions 1259Nomenclature 1259References 1260Bibliography 1261

6.16VALVE TYPES: BALL VALVES 1262

Introduction 1264Throttling Ball Valves 1264

Conventional Ball Valves 1265The Valve Trim 1266Flow Characteristics 1267

Characterized Ball Valves 1268Construction 1268Characteristics 1269

Ball and Cage Valves 1269Sizes and Other Features 1270Ball Unseated by Stem 1271Ball Gripped by Cage 1271

References 1271Bibliography 1271

6.17VALVE TYPES: BUTTERFLY VALVES 1273

Introduction 1274Conventional Butterfly Valves 1275

Operation 1276Construction 1276

High-Performance Butterfly Valves 1276Tight Shut-off Designs 1278Leakage Ratings 1279Fire-Safe Designs 1280

Torque Characteristics 1280Noise Suppression 1282Bibliography 1283

6.18VALVE TYPES: DIGITAL VALVES 1284

Introduction 1284History 1285

Balanced Piston Digital Control 1285Top-Entry Design 1285

Flow Metering 1288Gas Flow 1288Liquid Flow 1288

Conclusions 1288Reference 1289Bibliography 1289


Contents of Chapter 6 1049

6.19VALVE TYPES: GLOBE VALVES 1290

Valve Trends 1291Trim Designs 1292

Trim Flow Characteristics 1294Rangeability 1295Standard Trim Configurations 1296Special Trim Configurations 1296Trim Materials 1298Leakage 1298Plug Stems 1299

Bonnet Designs 1300Bolted Bonnets 1300Pressure Seal Bonnets 1301Bonnet Classification 1302Bonnet Packing 1303

Body Forms 1308Double-Ported Valves 1309Single-Seated Valves 1309Three-Way Valves 1315Lined and Thermoplastic Valves 1315

Valve Connections 1316Flanged Ends 1316Welded Ends 1317Threaded Ends 1317Special End Fittings 1318

Materials of Construction 1318Trademarks 1321Reference 1321Bibliography 1321

6.20VALVE TYPES: PINCH VALVES 1323

Introduction 1323The Sleeve 1326Pinch Valve Types 1328

Pressure Limitations 1328Shell and Tube Design 1328Throttling Characteristics 1334

Applications 1336Wastewater 1336Flue-Gas Desulfurization 1337Mine Slurries 1337Paper and Tile Manufacturing 1337Toxic Gas Applications 1337Pigments, Paint, and Ink 1337Glue 1337

Food 1337Powders and Grinding Compounds 1337Chemicals 1337

Cavitation 1337The Phenomenon 1338The Pinch Valves 1338Limiting or Eliminating the Damage 1338


6.21VALVE TYPES: PLUG VALVES 1341

General Characterisics 1342Plug Valve Features 1343Throttling and Actuator Considerations 1343

Design Variations 1343Characterized Plug Valves 1344V-Ported Design 1344Adjustable Cylinder Type 1345Semispherical Plugs for Tight Closure 1345Expanding Seat Plate Design 1345Retractable Seat Type 1346Overtravel Seating Design 1346Multiport Design 1347

Bibliography 1347

6.22VALVE TYPES: SAUNDERS DIAPHRAGM VALVES 1348

Introduction 1348Saunders Valve Construction 1348

Materials of Construction 1350Straight-Through Design 1351Full Bore Valve 1352Dual-Range Design 1352

Bibliography 1352

6.23VALVE TYPES: SLIDING GATE VALVES 1353

Introduction 1354Sliding Gate Valve Designs 1354

Knife Gate Valves 1354Positioned-Disc Valves 1355Plate and Disc Valves 1356

Bibliography 1357


1050

6.1 Application and Selection of Control Valves

B. G. LIPTÁK (1970, 1985, 1995, 2005) A. BÁLINT (2005)

Subjects Covered in this Section: Actuators; see also Sections 6.3 and 6.4Cavitation; see also Section 6.15Characteristics; see also Section 6.7Corrosion; see also Section 6.19Erosion; see also Section 6.19Fire safety; see also Sections 6.16 and 6.17Flashing and erosion; see also Section 6.15Gain; see also Section 6.7High pressure drop applications; see also Section 6.15High-pressure servicesHigh-temperature servicesInstallation considerationsIntelligent valve features; see also Section 6.12Jacketed valves Leakage; see also Sections 6.16–6.23Low-temperature services (cryogenics); see also Section 6.23Noise abatement; see also Section 6.14 and 6.17Packing designs; see also Section 6.19Positioners; see also Section 6.2Process dataRangeability; see also Section 6.7Selection chart for control valvesSequencing, split-rangingSizing; see also Section 6.15Small flow applications; see also Section 6.23Specification forms for control valvesToxic applications; see also Section 6.23Vacuum servicesViscous and slurry services

INTRODUCTION

In the field of control valve design, the most important devel-opments of the last decade occurred in the areas of electricand digital actuators (Section 6.3), in valve diagnostics (Sec-tion 6.8), dynamic performance evaluation (Section 6.9),safety shutdown systems (Section 6.10), fieldbus interaction(Section 6.11), intelligent positioners (Section 6.12), valvestatus detection and use for control (Section 6.13), and in theincreased availability of special valve designs (Section 6.12).Because each of these topics are covered in the noted separatesections, they are not treated in detail in this section.

While this section attempts to discuss all basic aspectsof control valve selection and application, in this area too,there exists some overlap with other sections. For example,as noted in the alphabetic listing above, the topics of valvecharacteristics and rangeability are discussed in more detail

in Section 6.7; noise and its reduction in Section 6.14; sizingin Section 6.15; valve actuators and accessories includingpositioners in Sections 6.2 and 6.4; and the features of theparticular valve designs in Sections 6.16 to 6.23. Therefore,the reader is advised to treat this section only as an overviewof the subject of control valve applications and refer to theindividual sections of this chapter for the detailed discussionof its many specific aspects.

It should also be noted that in most of the sections in thischapter English units are used with their SI equivalents givenin parenthesis. An exception is Section 6.14 on valve noisecalculation, which follows the general practice of acoustics anduses SI units. Appendices 1 and 2 (at the end of this handbook)give all the conversion factors that are required to go from oneto the other system of units.

So, for example, 1 lb/in.2 equals 6.89 kPa (kilo Pascals) or0.0689 bars. Hence, a 3–15 PSIG range is the approximate


6.1 Application and Selection of Control Valves 1051

equivalent of 20–100 kPa. The valve capacity coefficient usedin this chapter is the Cv, which is unity, if the valve passes 1.0gpm of cold water at a specific gravity of 1.0 at a pressure dropof 1.0 psid. The metric equivalent is the Kv, which correspondsto a valve passing 1 m3 of cold water per hour at a pressuredrop of 1 bar. Therefore, if the reader wishes to convert any Cvvalue into Kv, the multiplier is 1.17, and therefore Cv = 1.17 Kv.

Orientation Table

In order to provide some overall orientation about the relativemerits of the different valve designs, an orientation table,Table 6.1a, was prepared. In this table some of the more com-mon applications are described, together with some indicationsof the suitability of the various valve designs. At the end ofthis section, a standard control valve specification form, pre-pared by Instrumentation, Systems, and Automation Society,is provided together with an explanation of each of the entriesin that form. The discussion of the various topics related tovalve selection follows the approximate order of the entries inthat form. Therefore, after some introductory remarks, the dis-cussion begins with topics related to the service conditions(process data) and then continues with topics related to thefeatures and accessories of the valve and its installation.

CONTROL VALVE TRENDS

When this handbook was first published some 35 years ago,the overwhelming majority of throttling control valves werethe globe types, characterized by linear plug movements andactuated by spring-and-diaphragm operators. At that time, therotary valves were considered to be on/off shut-off devices.Today, globe valves are still widely used, but their dominanceis being challenged by the less expensive rotary (ball, butter-fly, plug) valves, which are usually actuated by cylinder oper-ators. This trend represents a mixed blessing and, therefore,is worth further discussion.

Globe vs. Rotary Valves

The main advantages of the traditional globe design includethe simplicity of the spring-and-diaphragm actuator; the avail-ability of a wide range of valve characteristics; the relativelylow likelihood of cavitation and noise; the availability of awide variety of specialized designs for corrosive, abrasive, andhigh/low temperature and pressure applications; the linear rela-tionship between control signal and valve stem movement; andthe relatively small amounts of dead band and hysteresis in itsoperation. These features make the globe valve usable withoutpositioners, which on fast processes is an advantage.

The main reason why rotary valves have been increasingtheir market share is their lower manufacturing cost and higherrelative flow capacity (Cd = 20–40 instead of Cd = 10–15, asfor globe). They also weigh less, can act as both control andshut-off valves, and are easier to seal at the stem to meetOSHA and EPA requirements.

The limitations of globe valves, in addition to their highercost per unit Cv (in Europe, the equivalent term Kv is used),include their relatively slow speed and low “stiffness” (plugposition is affected by dynamic forces in the process fluid),both of which can be improved by using hydraulic cylinderactuators operating at higher pressures.

Major disadvantages of rotary valves are their highertendency to cavitate and to produce excessive amounts ofnoise. They are also more likely, due to their smaller size perunit Cv (Kv), to have larger pipe reducers with the associatedwaste of pressure drop and distortion of characteristics. Theircontrol quality can suffer from the nonlinear relationshipbetween actuator linear movement and valve rotation, plusfrom the linkages, which can introduce substantial hysteresisand dead play. These characteristics result, in most cases, ina definite requirement for using a positioner, which on fastprocesses can cause the deterioration of control quality.

Valves vs. Other Final Control Elements

Before proceeding through the steps of selecting a controlvalve, one should evaluate if a control valve is truly neededin the first place, or if a simpler and more elegant systemwill result through some other means. For example, an over-flow weir can suffice to keep levels below maximum limits,and choke or restriction fittings can serve the function ofpressure letdown at constant loads. In other locations, it mightbe possible to reduce the investment by using regulatorsinstead of control valves.

The advantages of regulators include their high speed(high gain) and their self-contained nature, which eliminatesthe need for power supplies or utilities. If remote set pointadjustment is needed, regulators can be provided with air-loaded pilots to accommodate that requirement. While allregulators (being proportional-only controllers) will displaysome offset as the load changes, the amount of offset can beminimized by maximizing the regulator gain.

In still other applications, it is prudent to replace wholeflow control loops with positive-displacement meteringpumps or to replace the control valve with variable-speedcentrifugal pumps. The cost-effectiveness of the approach isusually found to be in lowered pumping costs, because thepumping energy that was “burned up” in the form of pressuredrop through the control valve is not being introduced, andtherefore it is saved.

CONTROL VALVE SIZING

Control valve sizing is discussed in depth in Section 6.15,and therefore only a few general recommendations are madehere.

One should first determine both the minimum and max-imum Cv (Kv in Europe) requirements for the valve, consid-ering not only normal but also start-up and emergency con-ditions. The selected valve should perform adequately over


1052C

ontrol Valve Selection and SizingTABLE 6.1aOrientation Table for Selecting the Right Control Valves for Various Applications

Control Valve Types

Features &Applications

Ball:Conven-

tional

Ball:Charac-terized

Butterfly:Conven-

tional

Butterfly:High-

performance Digital

Globe:Single-ported

Globe:Double-ported

Globe:Angle

Globe:Eccentric

disc Pinch

Plug:Conven-

tional

Plug: Charac-terized Saunders

Sliding gate:

V-Insert

Slidinggate:

Positioned disc

Special: Dynamically

balanced

Features:ANSI class pressure

rating (max.)2500 600 300 600 2500 2500 2500 2500 600 150 2500 300 150 150 2500 1500

Max. capacity (Cd) 45 25 40 25 14 12 15 12 13 60 35 25 20 30 10 30

Characteristics F G P F, G E E E E G P P F, G P, F F F F, G

Corrosive Service E E G G F, G G, E G, E G, E F, G G G, E G G F, G G G, E

Cost (relative to single-port globe)

0.7 0.9 0.6 0.9 3.0 1.0 1.2 1.1 1.0 0.5 0.7 0.9 0.6 1.0 2.0 1.5

Cryogenic service A S A A A A A A A NA A S NA A NA NA

High pressure drop (over 200 PSI)

A A NA A E G G E A NA A A NA NA E E

High temperature(over 500°F)

Y S E G Y Y Y Y Y NA S S NA NA S NA

Leakage (ANSI class)

V IV I IV V IV II IV IV IV IV IV V I IV II

Liquids:Abrasive service C C NA NA P G G E G G, E F, G F, G F, G NA E G

Cavitation resistance L L L L M H H H M NA L L NA L H M

Dirty service G G F G NA F, G F G F, G E G G G, E G F F

Flashing applications P P P F F G G E G F P P F P G P

Slurry includingfibrous service

G G F F NA F, G F, G G, E F, G E G G E G P F

Viscous service G G G G F G F, G G, E F, G G, E G G G, E F F F

Gas/Vapor:Abrasive, erosive C C F F P G G E F, G G, E F, G F, G G NA E E

Dirty G G G G NA G F, G G F, G G G G G G F G

Abbreviations:

A = AvailableC = All-ceramic design availableF = FairG = GoodE = ExcellentH = High

L = LowM = Medium

NA = Not availableP = PoorS = Special designs onlyY = Yes



a range of 0.8 Cvmin to 1.2 Cvmax. If this results in a rangeabilityrequirement that exceeds the capabilities on one valve, usetwo or more valves.

Control valves should not be operated outside their range-ability. Driskell (see Bibliography) properly points to the factthat all “fat” settles in the control valve. In constant speedpumping systems, each design engineer will add their ownsafety margin in calculating pressure drops through pipes andexchangers, and finally in selecting the pump.

Therefore, the control valve will end up with all thesesafety margins as added pressure drops, resulting in a much-oversized valve. A highly oversized valve will operate in anearly closed state, which is an unstable and undesirableoperating condition. In variable-speed pumping systems, thisproblem does not exist, because there the pump speed isadjusted to meet the load, and therefore the effect of accu-mulated safety margins is eliminated.

Collecting the Process Data

In order to select the right control valve, one must fully under-stand the process that the valve controls. Fully understandingthe process means not only understanding normal operatingconditions, but also the requirements that the valve must liveup to during start-up, shutdown, and emergency conditions.Therefore, all anticipated values of flow rates, pressures, vaporpressures, densities, temperatures, and viscosities must beidentified in the process of collecting the data for sizing.

In addition, it is desirable to identify the sources andnatures of potential disturbances and process upsets. Oneshould also determine the control quality requirements, so asto identify the tolerances that are acceptable in controlling theparticular variable. The process data should also state if thevalve needs to give tight shut-off, if the valve noise needs tobe limited, or any other factors that might not be known tothe instrument engineer. These can include subjective factors,such as user preferences, or objective ones, such as spare partsavailability, delivery, life expectancy, or maintenance history.

Lines 1–12 in the “Specification Form for ControlValves” (at the end of this section) describe the service con-ditions (process data) that must be provided for the controlvalve. It is important to carefully determine not only the“normal” values for this data but also the “minimum” and“maximum” values, because the valve must operate properlythroughout its range — not just under normal conditions.

Determining the Valve Pressure Drop

Assigning the sizing pressure drop for the valve is morecomplex than picking a number like 10% or 25% of the totalsystem drop or a number like 10 or 25 psi (0.69 or 1.72 bar).It requires an understanding of the interrelationships thatexist in pumping, fan, or compressor systems. If a systemconsists of nothing else but a pump, a control valve, piping,vessels, and an elevated destination, the energy profilethrough the system will be as illustrated in Figure 6.1b.

Note that the pressure drop available for the control valvedrops as the flow rate rises. This is because at higher flows,the pump discharge pressure will be lower, while the pressuredrop through the piping will rise. In other words, the controlvalve does not work with a fixed pressure drop, nor with afixed percentage of the total system drop, but it simply takeswhatever is left over from what is available and what isrequired by the rest of the system. Therefore, as shown inFigure 6.1c, the valve energy loss at any particular flow rate(load) is the difference between the corresponding points onthe pump and system curves.

As can be seen in Figure 6.1c, the available pressure dropincreases as the load (flow rate) drops, and it becomes the min-imum when the process flow rate (load) is the maximum. Thismeans that the actual control valve rangeability (in terms of Cv)must be much larger than the ratio of maximum and minimumflows. There is a similar effect on the valve characteristics, whichwill be discussed (together with rangeability) in both the para-graphs that follow and also in more detail in Section 6.7.

Because of the complexity of the problem, no simple ruleof thumb can be used in assigning the valve pressure drop;thus, it is important for the engineer to have a good under-standing of the pump and system curves together with therangeability and characteristics (gain) requirements of thevalve. In the process of deciding what pressure drop to assign,you must acknowledge the conflict between control qualityand energy conservation. The higher the pressure dropthrough the valve (relative to the rest of the system), the more

FIG. 6.1b Pressure profiles of a pumping system at high and low flow rates.

Statichead

Statichead

Pipepressure

drop

Control valvepressure drop

Pumppressure

rise

Control valvepressure drop

Pumppressure

rise

At low flow rates

At high flow rates



impact it will have on the process while, at the same time,the more pumping energy it will waste.

Some might argue that, in fact, no pressure drop needsto be assigned to the control valve during the design phasebecause as safety margins accumulate, the pump will beoversized anyway. There is some practical wisdom in thisattitude, because it is true that by the time the pump serviceconditions pass from the process engineer to the mechanicalengineer, then are sent out for bidding to the manufacturers,and finally a pump is selected, its flow and pressure capabil-ities will always much exceed the originally specified require-ments, and there will be plenty of pressure drop for the valve.While this argument sounds convincing and convenient, it iswrong. It is wrong because it results in unpredictable perfor-mance (possibly high noise) and usually also results in over-sized control valves, which tend not only to be unstable, butalso to have low rangeability.

Therefore, the proper approach to the selection of valvepressure drop is to first determine the total friction energy loss(excluding static energy) of the system at normal load (flow)and assign 50% of that to valve pressure energy drop. Basedon that assignment, one should next determine the resultingvalve drop at minimum and maximum loads (flows) and selecta valve that can handle the required Cv rangeability. As willbe discussed later, one should also select a valve characteristicthat, after being “distorted” by the change in valve drop asthe load varies, will give acceptable (stable) loop performance.

CHARACTERISTICS, GAIN, AND RANGEABILITY

Good control valve performance usually means that the valveis stable across its full operating range, it is not operating near

to one of its extreme positions, it is fast enough to correct forprocess upsets or disturbances, and it will not be necessary toretune the controller every time the process load changes. Inorder to meet the above goals, one must consider such factorsas valve characteristics, rangeability, installed gain, and actu-ator response. These topics will be separately addressed hereand in Section 6.7.

Characteristics and Gain

Characteristics and gain are discussed in more detail inSection 6.7, but they are also briefly covered here. The reasonprocess control engineers must be concerned with “selecting”the right valve characteristics (Figure 6.1d) is because the valveis part of the control loop, and the loop will be stable only ifthe products of all its gain components (the gains of the pro-cess, sensor, controller, and valve gains) is constant. Usually,the controller is tuned so that this gain product is 0.5, in orderto give quarter-amplitude damping. This was discussed in somedetail in connection with Figure 2.1x in Chapter 2.

If the gains of the loop components do not vary withload, but are constant, the desirable choice is to use a constantgain control valve. A constant gain valve is a linear valvewhose theoretical gain (change of flow per unit change oflift) is 1. If the gain of any of the loop components (such asthe process) decreases with load (flow through the valve), theproper choice of control valve gain is 1, which increases withload (equal percentage), because this combination will keepthe gain product of the loop relatively constant.

Installation Causes Distortion As was pointed out in con-nection with Figure 6.1c, in mostly friction systems, such aspumping through long pipes (where only a small portion of thetotal pump energy is used to overcome constant static pressure),

FIG. 6.1cThe difference between the pump discharge pressure curve and thesystem curve (which is the sum of the static head and the pipefriction loss) is the available valve differential.

100

100

80

80

60

60

40

40

20

200

0

Pump curve

System curve

Pipe frictionpressure loss

Sum, pipe andstatic pressure

Static head (pressure)

HeadFT

Flow GPMFmin Fmax

Fnorm

∆Pm

ax

∆P ∆Pm

in

FIG. 6.1d Inherent characteristics of control valves.

100

80

80 100

60

60

40

40

20

200

0% Lift or rotation

Equal percentage*

Linear(Gain = 1.0)

*Gain increases at a constant slope. �e rate of rise is a fixed % of the actual flow.

% C v

or fl

ow at

cons

tant

∆P



the pressure drop available for the control valve is dropping asthe load (flow rate) is increasing, and therefore more Cv isneeded for unit flow increases as the flow rises (Figure 6.1e).

This is a different condition from the condition at whichthe valve characteristics were established in the testing facil-ity of the manufacturer, where the flow rate through the valvewas measured under constant pressure drop conditions.Therefore, when such valves are installed, their gain (char-acteristics) shift, as will be discussed in Section 6.7. One wayto correct for such distortion is to obtain “near-linear”installed characteristics by installing a valve with “idealinherent” equal-percentage characteristics.

As will be discussed in connection with positioners(Section 6.2), it is also possible to use a cam in the posi-tioner to modify the installed characteristic of the valve,but this can dramatically both change the loop gain of thepositioner and limit its dynamic response. Therefore, it ispreferred to change the inherent characteristic of the valvetrim than to install cams in the valve positioner.

The installed characteristic of the valve can also be mod-ified by characterizing the control signal going to the posi-tioner. This characterization occurs outside the positionerfeedback loop, and therefore it has the advantage of notchanging the loop gain of the positioner. This method alsohas its dynamic limitations. For example, a 1% change in thecontroller output signal may be electronically narrowed to achange in the valve signal of only 0.1% (in the flat regionsof the valve characteristic), but such change is too small forsome valves to respond to at all.

Therefore, the best solution to obtaining a constant loopgain is to select the inherent characteristics of the valve trimto compensate for the nonlinearity of the process and, thereby,arrive at an installed flow characteristics, which is nearly linearover the operating range of the valve.

Selecting the Valve Characteristics Different engineershave approached the problems caused by process nonlinearity(drop in process gain) in different ways. One approach, thatof the old school, was to oversize the pump so that the ratiobetween the minimum and maximum energy loss in Figure6.1c will not be large, and therefore the gain of the processwill not change much with load. This approach works, but itwastes pumping energy.

Different engineers began to develop different rules ofthumb to be used in selecting valve characteristics for thevarious types of control loops. These recommendations varyin complexity. Shinskey, for example, recommends equal per-centage for temperature control and the use of linear valvesfor all flow, level, and pressure control applications (exceptvapor pressure, for which he recommends equal percentage).According to Driskell, one can avoid a detailed dynamic anal-ysis by just considering the ratio of the maximum and mini-mum valve pressure drops (∆pmax/∆pmin) and follow the rule ofthumbs listed for the most common applications in Table 6.1f.

Lytle’s recommendations are summarized in Table 6.1g;they are more involved, as they take more variables into account.

FIG. 6.1e In mostly friction systems, an increase in load (flow rate) results in a drop in the pressure drop, which is available for the control valve.Therefore, the same amount of increase in flow rate requires a larger increase in the valve capacity coefficient Cv (Kv). For such application,an equal-percentage valve is needed.

100

100

80

80

60

60

40

40

20

200

0Flow GPM

Availablevalve

differentialpressure

Avai

labl

e ∆P

100

100

80

80

60

60

40

40

20

200

0Flow GPM

Requ

ired

C v

TABLE 6.1f Valve Characteristics Selection Guide

ServiceValve (∆pmax /∆ pmin)

Under 2:1Valve (∆pmax /∆pmin)

Over 2:1 but Under 5:1

Orifice-type flow Quick-opening Linear

Flow Linear Equal %

Level Linear Equal %

Gas pressure Linear Equal %

Liquid pressure Equal % Equal %



Process Nonlinearity Yet another approach in overcomingthe process nonlinearity caused by the variation in valve pres-sure drop is to modify the controller output signal to eliminatethat nonlinearity (discussed in Section 6.7) or to replace thecontrol valve with a complete flow control loop (Figure 6.1h).By selecting a linear flow transmitter, the characteristics of theslave loop will also be linear, and therefore its gain will be 1.0.

This approach works fine, but it also increases the systemcost. In addition, while it eliminates a nonlinearity, it intro-duces a slave loop, which on fast processes can degrade thequality of control quality. By using intelligent control valves(Section 6.12), one can obtain flexibility in implementing oneor the other approach.

Process nonlinearity also exists for reasons other than vari-ations in the available valve pressure drop. The gain of heat-transfer processes, for example, always drops as the loadincreases, because the heat-transfer surface is constant, andtherefore the heat transfer is more efficient when small amounts

of heat need to be transferred. Consequently, in order to keepthe gain product of the control loop constant, it is necessary tocompensate the dropping process gain with an increasing valve

TABLE 6.1gRecommendations on Selecting Control Valve Characteristics for Flow, Level, and Pressure Control Loops‡

LIQUID LEVEL SYSTEMS

Control Valve Pressure Drop Best Inherent Characteristic

Constant ∆P Linear

Decreasing ∆P with increasing load, ∆P at maximum load > 20% of minimum load ∆P Linear†

Decreasing ∆P with increasing load, ∆P at maximum load < 20% of minimum load ∆P Equal-percentage

Increasing ∆P with increasing load, ∆P at maximum load < 200% of minimum load ∆P Linear

Increasing ∆P with increasing load, ∆P at maximum load > 200% of minimum load ∆P Quick-opening

PRESSURE CONTROL SYSTEMS

Application Best Inherent Characteristic

Liquid process Equal-percentage†

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

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

Linear†

Gas process, large volume, decreasing ∆P with increasing load ∆P at maximum load < 20% of minimum load ∆P

Equal-percentage

FLOW CONTROL PROCESSES

Best Inherent Characteristic

Flow Measurement Signal to Controller

Location of Control Valve Relation to Measuring Element Wide Range of Flow Setpoint

Small Range of Flow but Large ∆P Change at Valve with Increasing

Load

Proportional to Q In series Linear Equal-percentage†

Proportional to Q In bypass* Linear Equal-percentage

Proportional to Q2 (orifice) In series Linear† Equal-percentage

Proportional to Q2 (orifice) In bypass* Equal-percentage Equal-percentage

* When control valve closes, flow rate increases in measuring element.† Most common.‡ From Reference 1.

FIG. 6.1hValves can be linearized by replacing them with a complete slavecontrol loop.

FT

FC

FY

SP



gain. For this reason, all temperature control valves are alwaysequal percentage.

Composition processes can also be nonlinear, but theirnonlinearity is usually more complex, such as the titrationcurve of a pH control system. In these situations, the likelysolution is to use a linear valve (constant gain) and a nonlinearcontroller (Figure 2.19d), one whose gain varies as the mirrorimage of the process gain.

Valve Rangeability

Control valve rangeability is also discussed in more detail inSection 6.7. Here, it should suffice to state that the requiredrangeability should be calculated as the ratio of the Cv (Kv)required at maximum flow (and minimum pressure drop) andthe Cv (Kv) required at minimum flow (and maximum pressuredrop). The decision on whether a particular control valve iscapable of providing the required rangeability should be evalu-ated on the basis of a plot of valve gain vs. valve Cv. If the actualvalve gain is within 25% of the theoretical valve gain betweenthe minimum and maximum Cv, the rangeability is acceptable.As will be discussed in Section 6.7, the rangeability definitionsused by manufacturers are usually not based on valve gain.

One way to increase the rangeability is to have the control-ler operate more than one control valve. As will be discussedbelow, such multiple valves can be split-ranged, sequenced, oroperated in a floating mode.

Control Valve Sequencing When the rangeability require-ments of the process exceed the capabilities of a single valve,control valve sequencing loops must be designed that willkeep the loop gain constant while switching valves. Thisrequires careful thought.

Assuming that the task is to sequence two linear valves,with sizes of 1 and 3 in. (25 and 75 mm) having Cvs (Kvs)of 10 (8.62) and 100 (86.2), respectively, Shinskey’s recom-mendation is the following: If the large valve were to operatefrom 9 to 15 PSIG (0.6 to 1.0 bar) and the small one from3 to 9 PSIG (0.2 to 0.6 bar), the loop gain would change by10 when passing through 9 PSIG (0.6 bar).

The only way to keep the loop gain constant in thisexample would be to operate the small valve from 0 to 10%and the large valve from 10 to 100% of controller output.This would result in a 3–4.2 PSIG (0.2–0.28 bar) range forthe 1 in. (25 mm) and a 4.2–15 PSIG (0.28–1.0 bar) rangefor the 3 in. (75 mm) valve. Therefore, if more than one valveis required to increase rangeability, in most cases equal-percentage valve characteristics are needed.

Sequencing Equal-Percentage Valves The sequencing ofequal-percentage valves is done as follows: If the small valvehad a Cv of 10 and a rangeability of 50:1, its minimum Cv

would be 10/50 = 0.2. A line drawn on semilogarithmic coor-dinates connecting Cv (Kv) 100 (86.2) and 0.2 (0.172) appearsin Figure 6.1i. Observe that the Cv of 10 of the small valvefalls slightly above the mid-scale of the controller output (to

about 65%), providing a much more favorable span for thecalibration of the positioner.

In order to have the two valves act as one without dis-turbing the smooth equal-percentage characteristics at thepoints of switching, only one valve must be open at any onetime. Therefore, the large valve must be prevented from oper-ating at low flows, because in its nearly closed position itscharacteristics are not equal-percentage. For these reasons,only one valve must be open at any one time.

In the scheme shown in Figure 6.1i, the small valve alone ismanipulated until the controller output reaches the value cor-responding to its full opening. At this point the pressure switchenergizes both three-way solenoid valves, venting the smallvalve and opening the large to the same flow that the smallhad been delivering. Switching takes place in 1 sec or less,adequate for all but the fastest control loops.

When the controller output falls to the point of minimumflow from the larger valve (35%), the solenoids return to theiroriginal position. Thus, the switch has a differential gapadjusted to equal the overlap between valve positioners(30%). The range of the positioner for the large valve is foundby locating its minimum Cv in Figure 6.1i. A rangeability of50 would give a minimum Cv of 2.

FIG. 6.1i Sequencing two equal-percentage valves, while minimizing the upsetcaused by switching valves.

0.2

12

10

100

Cv

Large valve

Smallvalve

0 25 50 75 10035% 65%Controller output %

Controlleroutput

PS

Small Large

Set @ 65%Differential-30%



This same approach can be used to sequence three ormore valves. If linear characteristics are required, one shouldinsert a 10:1 multiplier relay in the controller signal to thesmall valve, so that a 0–10% controller output will result ina 0–100% signal to the small valve.

Split-Ranging or Floating Some process control engineersfeel that the switching scheme in Figure 6.1i is too abrupt orcould cause a maintenance problem. For these reasons, theyprefer the methods of valve sequencing illustrated in Figure 6.1j.

The split-ranging loop shown in the upper left of thefigure contains both gain and bias relays to provide the addedrangeability. Even better response can be provided by the useof a “floating” valve position controller, shown on the topright of Figure 6.1j. This controller slowly moves the largervalve so as to keep the smaller one near its 50% opening.This way, the small valve provides sensitivity and fastresponse to the loop within its capacity. The large valve is asort of an automatic bypass, which sets the capacity and hasa limited frequency response.

The “large/small valve selection” scheme shown at thebottom of Figure 6.1j differs from the one in Figure 6.1i only

in that it uses only one four-way solenoid pilot instead oftwo three-way units. The purpose of the 1:1 amplifier is toeliminate the bounce when the solenoid switches.

As the purpose of valve sequencing is to increase therangeability of the loop without upsetting its stability, theexistence of two or more valves should not be noticeable bythe controller. In other words, in a well-designed sequencingsystem, the controller would operate as if its final controlelement were a single valve, having the desired gain charac-teristics and a very wide rangeability.

In order to keep the gain characteristics of the valve paircorrect, there should be no “bumps” when the larger valve isopened. This requires that only one valve be throttled at a timeand the other be closed. From this perspective, the performanceof the small/large valve selection scheme in Figure 6.1j issuperior to that of the split-ranging or the floating methods.

ACTUATOR SELECTION

Sections 6.3 and 6.4 discuss the applications and relativeadvantages of the different pneumatic, electric, digital, andhydraulic actuator designs.

The popularity of the spring-and-diaphragm actuator isdue to its low cost, its relatively high thrust at low air supplypressure, and its availability with “fail-safe” springs. By trap-ping the pressure in the diaphragm case, it can also be lockedin its last position. It is available in various designs: spring-less, double diaphragm (for higher pressures), rolling dia-phragm (for longer strokes), and tandem, which providesmore thrust. One of the limitations of this design is the lackof actuator “stiffness” (resistance to rapidly varying hydraulicforces caused, for example, by flashing). For such applica-tions, hydraulic or electromechanical (motor gear) actuatorsare preferred, although a stiffer spring (6–30 PSIG, whichcorresponds to 0.41 to 2.59 bars) in a spring-and-diaphragmunit is sometimes sufficient to correct the problem.

Piston Actuators

Linear piston actuators provide longer strokes and can oper-ate at higher air pressures than the spring-and-diaphragmactuators. When used to operate rotary valves, the linearpiston or spring-and-diaphragm actuator does not provide aconstant ratio of rotation per unit change in air signal pres-sure. Therefore, the use of positioners is always advisable.

Rotary piston actuators operate at higher air pressuresand can provide higher torque, suitable for throttling largeball or butterfly valves. The double-acting version of thisactuator does not have a positive failure position, but such aposition can be added by extending the piston case and insert-ing a helical spring.

For higher torque (over 1000 ft lbf, which correspondsto 1356 Nm), heavy-duty transfer linkages are required(Scotch yoke or rack and pinion); such units cannot be easilydisassembled and maintained in the field. These actuators

FIG. 6.1j Alternate methods of obtaining high turndown through the use ofmultiple control valves.

FC

FC

3−15#

3−15#

(−3#)BIAS

(−15#)BIAS

3−9# 9−15#

AMP AMP

(2) 1:2 AMP+ BIAS

Small

Large

Set at 50%

“Floating” control oflarge valve

Split ranging(Gain plus BIAS)

1:2 1:2

± ±

P P

PP

Small

Large

P

P

1:1

S

AMP A

BVent

Set @ 65%Diff.: 30%PSH

Small/Large valve select



also require positioners, because the relationship between airsignal change and resulting rotation is not linear. The use ofpositioners on fast loops can deteriorate the loop’s perfor-mance, as it will need to be detuned.

Actuator Speeds of Response

In the family of pneumatic actuators, the spring-and-diaphragmactuators are the slowest (1–30 sec/stroke), spring-returned pis-tons with supply/exhaust at both ends of the piston are thefastest. Dual pistons can stroke valves in 0.5 sec.

In the family of electric actuators, the slowest are theelectromechanical motor-driven valves (5–300 sec/stroke);hydroelectric actuators can move at 0.25 in./sec (6 mm/sec)or faster if hydraulic accumulators are used. The fastest arethe small, on/off solenoids, which can close in 8–12 ms, whilethrottling solenoids require about 1 sec to stroke.

Valve actuators can be velocity limited because they can-not move faster than their maximum design speed. This istrue of both electric and pneumatic motors or actuators. Incase of the latter, maximum speed is set by the maximumrate at which air can be supplied or vented. If the full stroking(100%) of a valve takes 4 sec, then its velocity limit is 25%per second.

Valve signal changes usually occur in small steps, andtherefore the velocity limit does not represent a serious lim-itation because, for example, the time required to respond toa 5% change is only 0.2 sec. This is fast enough for mostloops. Figure 6.1k illustrates the response of velocity-limitedactuators to various types of control signals.

Actuator speeds can be increased by enlarging the airflow ports and by installing booster relays. On on/off valves,the addition of a quick-dump valve (Figure 6.2o) will dra-matically increase the venting rate. The dynamic performanceof the actuator can also be affected by modifying the tarevolume, pressure range, or dead band. In order to reduce thedead band, one usually needs to modify the piston seals,linkages, or rack and pinion connections.

Most valve actuators display some dead band or hyster-esis band due to packing friction (Figure 6.1l). This can causeinstability if the change in the control signal is small enoughto fall within the hysteresis band width.

Actuator Power

The actuator is sized on the basis of the power or thrust requiredto overcome the unbalanced forces in the valve body and theseating force, and on the basis of the “stiffness” necessary forstability. In pneumatic actuators, the thrust is a function ofpiston or diaphragm area times air pressure. While the controlsignal is usually 3–15 PSIG (0.2 to 1.0 bar), the actuatingpressure can be as high as the air supply pressure, if positionersor amplifier relays are installed. If their costs can be justified,electrohydraulic actuators will give the highest power and speedof response.

Valve Failure Position

In pneumatic actuators, the fail-safe action can be providedby a spring or from an air reservoir, but the latter representsadded expense, complexity, and space requirements.

Electric actuators are usually more expensive, except forsuch designs as the spring-loaded or modulating solenoidvalves discussed in Section 6.3. Electric actuators are there-fore most often used where air is not available, where thethrust required is less than 1000 lbf (4448 N), where it isacceptable to have the valve fail in its last position, and whereslow response is not a drawback.

Like most generalizations, however, these are not com-pletely true: Electromechanical motor and hydraulic actua-tors are available with high thrusts and can be provided withpositive failure. Hydraulic actuators are also available with

FIG. 6.1k Response of velocity-limited actuators to a step change (left) and to a high-amplitude sine wave (right) in the control signal, according toShinskey.

Time

Small-signal response

Large-signalresponse

Velocitylimit

Stem

pos

ition

Time

InputOutput

Am

plitu

de

FIG. 6.1lDead band in the valve (left) can result in limit cycling (right) whenthe loop is closed.

Stem

pos

ition

Motor pressure

Dead band

Flow

Time



fast response speeds. To gain an in-depth understanding ofthe capabilities of actuators, refer to Sections 6.3 and 6.4.

It is the responsibility of the process control engineer tospecify the valve failure position. It is the general practice tofail energy supply valves (steam, hot oil, and so on) closedand energy-removing valves (cold or chilled water) open. Theflow sheet abbreviations can be FC (fail closed), FI (failindetermined), FL (fail in last position), and FO (fail open).Spring-loaded actuators are the most convenient means ofproviding FC or FO action, while two-directional air or elec-tric motors will naturally tend to fail in their last positions.

In addition to anticipating the consequences of actuatorpower failure, one should also consider the results of othercomponent failures, such as the spring, diaphragm, piston, andso on. When such failures occur, the ultimate valve position willnot be a function of the actuator design, but of the process fluidforces acting upon the valve itself. The choices are FTO (flowto open), FTC (flow to close), or FB (friction bound—tends tostay in last position). FTO action is available with globe valves.FTC action can be obtained from butterfly, globe, and conven-tional ball valves. Rotary plug, floating ball, and segmented ballvalves tend to be friction bound, with the flow direction possiblyaffecting the torque required to open the valve.

POSITIONERS

The positioner is a high-gain plain proportional controllerthat measures the valve stem position (to within 0.1 mm),compares that measurement to its set point (the controlleroutput signal), and, if there is a difference, corrects the error.The open-loop gain of positioners ranges from 10 to 200(proportional band of 10–0.5%), and their periods of oscil-lation range between 0.3 and 10 sec (frequency response of3–0.1 Hz). In other words, the positioner is a very sensitivelytuned, proportional-only controller.

Positioners that are electronically and digitally con-trolled, or are intelligent and are capable of self-diagnostics,communication on fieldbuses, and other advanced features,are not discussed here, because they are described in detailin Section 6.12.

When to Use Positioners

The main purpose of having a positioner is to guarantee thatthe valve does, in fact, move to the position where the con-troller wants it to be. The addition of a positioner can correctfor many variations, including changes in packing frictiondue to dirt, corrosion, or lack of lubrication; variations in thedynamic forces of the process; sloppy linkages (dead band);or nonlinearities in the valve actuator. The dead band of avalve/actuator combination can be as much as 5%; when apositioner is added, it can be reduced to less than 0.5%. It isthe job of the positioner to protect the controlled variablefrom being upset by any of the above variations.

In addition, the positioner can also allow for split-rangingthe controller signal between more than one valve, can

increase the actuator speed or thrust by increasing the pressureor volume or the actuator air signal, and can modify the valvecharacteristics by cams or electronic function generators.

While the above positioner capabilities can be convenient,they can also be obtained without the use of positioners. Forexample, split-ranging can also be done by the use of split-ranged valve springs or by multiple/biasing relays in the airsignal line to the valve. Similarly, increasing the speed/thrustof the valve can be achieved by booster relays, and changesto the control valve characteristics can be obtained, not onlyby replacing the plug, but also by pneumatic or electroniccharacterizing of the controller signal. Therefore, these rea-sons do not necessitate the use of positioners.

Actuators without springs always require positioners. Whena valve is in remote manual (open loop) operation, it will alwaysbenefit from the addition of a positioner, because a positionerwill reduce the valve’s hysteresis and dead band while increas-ing its response. When the valve is under automatic (closedloop) control, the positioner will be helpful in most slow loops,which control analytical properties, temperature, liquid level,blending, slow flow, and large volume gas flow.

A controlled process can be considered “slow” if its periodof oscillation is three times the period at which the positionedvalve oscillates. In such installations as B in Figure 6.2b, theaddition of a positioner increases the open-loop gain and,therefore, the loop response. As a consequence, the tuning ofthe controller could also be improved by increasing the gain(making the proportional band narrower) and adding morerepeats per minute in the integral setting.

When Not to Use Positioners

In the case of fast loops, positioners are likely to degrade loopresponse, contribute to proportional offsets, and cause limitcycling (fast flow, liquid pressure, small volume gas pressure).

The positioner in effect is the cascade slave of the loopcontroller. In order for a cascade slave to be effective, it mustbe faster than the speed at which its set point, the masteroutput signal, can change. The rules of thumb used in thisrespect suggest that the time constant of the slave should beten times shorter (open-loop gain ten times higher) than thatof the master and the period of oscillation of the slave shouldbe three times shorter (frequency response three times higher)than that of the primary. The criteria for positioners need notbe this stringent, but still, it is recommended not to usepositioners if the positioned valve is slower than the processvariable it is assigned to control.

A controlled process can be considered “fast” if its periodof oscillation is less than three times that of the positionedvalve. In such situations, the positioned valve is one of theslowest components in the loop and, therefore, slows down theload (by limiting the open-loop gain of the loop and lengthen-ing the period of oscillation). Part A in Figure 6.2b illustratessuch a situation, where the loop can be tuned more tightly(higher gain, more repeats/minute) and, therefore, respondsbetter without a positioner. It might also be noted that after a



new steady state is reached, the positioned installation givesmore noisy control because of the hunting and limit cyclingof the positioner, which cannot keep up with the process.

Some will argue that all loops can be controlled using posi-tioned valves if they are sufficiently detuned. This is true, but“detuning” means that the controller is made less effective (theamount of proportional and integral correction is reduced),which is undesirable. A 0.2 gain (500% proportional) setting ona level controller means that the tank can be flooded or drainedbefore the controller fully strokes the valve. Also, there are caseswhere the gain must be so low (the proportional band so wide)that it is outside the available PB setting range of the controller.

Positioners to Eliminate Dead Band

All valves and dampers will display some dead band becauseof friction in their packing, unless positioners are used.Whenever the direction of the control signal is reversed, thestem remains in its last position until the dead band isexceeded, as shown in Figure 6.1l.

This figure on the right shows that if a sine wave controlsignal is driving the valve actuator (motor), it produces astem motion that is distorted and shifted in phase. This phaseshift, when combined with the integrating characteristic ofcertain processes and with the reset action of a controller,causes the development of a limit cycle. According to Shin-skey, widening the proportional band will not dampen theoscillation, but only make it slower.

The limit cycle will not appear if a proportional-only con-troller is used, and if the process has no integrating element.Processes that are prone to limit cycling in this way are liquidlevel, volume (as in digital blending), weight (not weight-rate),and gas pressure—all of which are related to the integrals offlow. Whenever one intends to control such a process with aproportional and integral (PI) controller, the use of positionersshould be considered. In case of level control, one can accom-plish the same goal by using a plain proportional controllerand a booster or amplifier instead of a positioner.

Positioners in general will eliminate the limit cycle byclosing a loop around the valve actuator. Positioners will alsoimprove the performance of valves on slow processes, suchas pH or temperature. On the other hand, dead band causedby stem friction should not be corrected by the use of posi-tioners on fast loops, such as flow or “fast” pressure.

The positioner’s function as a cascade slave, as wasexplained earlier, can cause oscillation and cycling on fast loopsif the controller cannot be sufficiently detuned (minimum gainis not low enough). Similarly, negative force reactions on theplug require an increase in actuator stiffness and not the additionof a positioner. Actuator stiffness can be improved by increasingthe operating air pressure or by using hydraulic actuators.

Split-Range Operation

The use of positioners for split-range applications is usuallyaccepted regardless of the speed of the process. This is not

entirely logical, because on fast loops the control perfor-mance can be degraded by the use of positioners. In suchcases, some instrument engineers do discourage the use ofpositioners to implement split-ranging. Instead, they recom-mend gain-plus-bias relays so that the positioner (the less-accurate device) will operate over its full range (Figure 6.1j).This also eliminates the need for a special calibration.

One can also consider accomplishing the split-range oper-ation through the use of different spring ranges in the valveactuators. In addition to the standard 3 to 15 PSIG (0.2 to 1 bar)range spring, valves can also be obtained with other springranges. These include 3–7 PSIG (0.2–0.5 bar), 4–8 PSIG (0.28–0.55 bar), 5–10 PSIG (0.34–0.68 bar), 7–11 PSIG(0.5–0.75 bar), 8–13 PSIG (0.55–0.9 bar), and 9–13 PSIG(0.62–0.9 bar).

Lastly, if split-range positioners are installed on fast pro-cesses, the resulting degradation of control quality can belimited by adding a restrictor or an inverse derivative relayin the control signal to it and, thereby, artificially making thecontroller appear to be slower than it really is. This techniqueis not highly recommended (except to reduce wear and tearon the valve in noisy loops), because restrictors are prone toplugging or maladjustment and because they both degradethe loop performance.

Accessories

If the need is to increase the speed or the thrust of the actuator,it is sufficient to install an air volume booster or a pressureamplifier relay, instead of using a positioner. Boosters willgive better performance than positioners on fast processes,such as flow, liquid pressure, or small-volume gas pressurecontrol, and they will not be detrimental (nor will offer advan-tages) if used on slow processes.

If the reason for adding a positioner is to alter or modifythe control valve characteristics, this is not a valid justificationon fast processes, because this aim can be satisfied by the useof dividing or multiplying relays in the controller output,which will not degrade the quality of control (see Section 6.7).

PROCESS APPLICATION CONSIDERATIONS

In selecting control valves, the properties of the process fluidmust be fully considered. The process data should be care-fully and accurately determined because even small variationsin temperature or pressure can cause flashing or cavitation.Considerations include such obvious variables as pressure,temperature, viscosity, slurry, or corrosive nature, or the lessobvious factors of flashing, cavitation, erosion, leakage, ster-ilization, and low flow rates. These are discussed in the para-graphs that follow below.

Pressure Considerations

The available design pressures for each valve type are listedin the feature summaries in the front of Sections 6.16–6.24.



In selecting the control valve for a particular application, oneshould pay particular attention to high pressure, high differ-ential pressure, and vacuum services. These will be discussedin the paragraphs below.

High-Pressure Services When designing valves for high-pressure services, the following features are of particularimportance:

1. Increased physical strength2. Selection of erosion-resistant material3. Use of special seals

Valve bodies can usually withstand higher pressures thancan the piping. Valve bodies for high-pressure services areusually forged to provide homogeneous materials free ofvoids and with good mechanical properties. The loads andstresses on the valve stem are also high. For this reason,higher strength materials are used with increased stem diam-eters. As shown in Figure 6.1m, the stems are usually keptshort and are well-guided.

High-pressure services will also increase the probabilityof noise, vibration, and cavitation, which will be discussedin later paragraphs.

High Differential Pressure With high flow rates and highpressure drops, a large amount of energy is dissipated inturbulence. A fraction of this energy is radiated as noise (seeSection 6.14). For most (but not all) gases and conditions,one result of the high pressure drop can be a very low outlettemperature. It is not always proper to use the gas laws topredict these temperatures, and instead, actual thermody-namic properties should be used. The very low temperaturewill cause some valve materials to become brittle, and carefulselection of alloys and other materials is always required.

Depending on the gas involved and other materials in theflowing fluid, hydrates or other solids may form in the valve.Liquid droplets may develop and cause erosion. It is neces-sary to investigate for any peculiarity of the flowing fluid. Ahigh-velocity jet leaving the valve can erode downstreampiping. Very high forces are developed on the valve body and

internal parts and can cause valve instability. With the changein magnitude and, often, direction of the fluid, substantialreaction forces are developed. Serious damage may result ifthe valve and piping are not properly restrained.

High operating pressure frequently involves high pres-sure drops. This usually means erosion, abrasion, or cavita-tion at the trim. These will be discussed in detail in thecoming paragraphs, but it should also be mentioned here thatcavitation and erosion resistance are usually not propertiesof the same metal. Materials resistant to erosion and abrasioninclude 440C stainless steel, flame-sprayed aluminum oxidecoatings (Al2O3), and tungsten carbide.

On the stem, where the unit pressure between it and thepacking is high, it is usually sufficient to chrome-plate the stemsurface to prevent galling. Special “self-energizing” seals areused with higher pressure valves (above 10,000 PSIG, or69 MPa, service) so that the seal becomes tighter as pressurerises. Popular body seal designs for such service include thedelta ring closure and the Bingham closure (Figure 6.1m).

As discussed in more detail in Section 6.19, the self-energizing seals are used in connecting the high-pressurevalves into the pipeline. These designs depend on the elasticor plastic deformation of the seal ring at high pressures forself-energization.

Special packing designs and materials are also requiredin high-pressure service, because conventional packing wouldbe extruded through the clearances. To prevent this, the clear-ance between stem and packing box bore is minimized, andextrusion-resistant material, such as glass-impregnated Teflon,is used for packing.

Some of the likely causes of valve failure in high pressuredrop services include:

1. Elastomer elements in Saunders or pinch valves (par-ticularly if they fail to open) can be ruptured.

2. The stem thrust can be excessive for globe valves. Ifglobe valves are flow to close, high ∆p can damage theseat or prevent the actuator from opening the valve. Ifthey are flow to open, they might open against theactuator.

FIG. 6.1mHigh-pressure valve designs.

Self-dragtrim



3. High ∆p can exceed the capabilities of plug or floatingball valves, and it can bend the shafts of butterflyvalves, damage the bearings of trunion-type ballvalves, or damage the seat of floating ball valves.

4. Pressure cycling generated by positive-displacementpumps can cause bolt fatigue if the number of cyclesis excessive.

Vacuum Service Low pressures can prevent some pres-sure-energized seals from properly operating, or they cancause leakage. In some processes, the in-leakage from theatmosphere results in overloading the vacuum source; in oth-ers, it represents a contamination that cannot be tolerated.Potential leakage sources include all gasketed areas and, toan even greater extent, the locations where packing boxes areused to isolate the process from the surroundings.

For vacuum service, valves that do not depend on stuffingboxes to seal the valve stem generally give superior perfor-mance. Such designs include Saunders valves and pinchvalves. These designs, unfortunately, are limited in theirapplication by their susceptibility to corrosion and their tem-perature and control characteristics. Their applicability tovacuum service is further limited by their design.

The jacketed pinch valve versions, for example, require avacuum source on the jacket side for proper operation, andthe mechanically operated pinch and Saunders designs arelimited in their capability to open the larger-size units againsthigh vacuum on the process side. The vacuum process tendsto keep the valve closed, and this can result in the diaphragm’sbreaking off the stem and rendering the valve inoperative.

For services requiring high temperatures and corrosion-resistant materials, in addition to good flow characteristicsand vacuum compatibility, conventional globe valves can beconsidered, with special attention given to the type of packingand seal used.

One approach to consider is the use of double packing,as shown in Figure 6.1n. The space between the two sets ofpacking is evacuated so that air leakage across the upper

packing is eliminated. The vacuum pressures on the two sidesof the lower packing are approximately equal, and thereforethere is no pressure differential to cause leakage across it.Usually, the space between the two packings is exposed to aslightly higher vacuum than the process, so that no in-leakageis possible.

Double packing provides reasonable protection againstin-leakage under vacuum, but it does not relieve the problemsassociated with corrosion and high temperatures. When allthree conditions exist (vacuum, corrosive flow, and high tem-perature), the use of bellows seals (discussed in Section 6.19)can be considered. The bellows are usually made of 316stainless steel and are tested by mass spectrometers for leak-age. They not only prevent air infiltration but also can protectsome parts of the bonnet and top-works from high tempera-ture and corrosion. Like all metallic bellows, these too havea finite life, and therefore it is recommended that a secondarystuffing box and a safety chamber be added after the bellowsseal. A pressure gauge or switch can be connected to thischamber between the bellows and the packing to indicate orwarn when the bellows seal begins to leak and replacementis necessary.

Considerations similar to those noted for high-vacuumservice would also apply when the process fluid is toxic,explosive, or flammable.

High-Temperature Service

The temperature limitations of each valve design are listedin their feature summaries in Sections 6.16–6.24.

All process conditions involving operating temperaturesin excess of 450°F (232°C) are considered high temperature.The maximum temperatures at which control valves havebeen successfully installed are up to 2500°F (1371°C).

High operating temperatures necessitate the review of atleast three aspects of valve design:

1. Temperature limitations of metallic parts2. Packing temperature limitations3. Use of jacketed valves

Metallic Parts High temperatures can cause galling, canaffect clearances, and can soften hardened trims. Temperaturecycling can cause thermal ratcheting and stress, resulting inbody or bolting rupture if the rate or frequency of temperaturecycles is high.

The high operating temperatures are considered in select-ing materials for both the valve body and trim. For the body,it is suggested that bronze and iron be limited to servicesunder 400°F (204°C), steel to operation below 850°F (454°C),and the various grades of stainless steel, Monel, nickel, orHastelloy alloys to temperatures up to 1200°F (649°C).

For the valve trim, 316 stainless steel is the most popularmaterial, and it can be used up to 750°F (399°C). For highertemperatures, the following trim materials can be considered:17–4 pH stainless steel (up to 900°F, or 482°C), tungsten

FIG. 6.1n Double packing is used to seal the valve stem if the process is undervacuum.

Connectionto vacuum

source



carbide (up to 1200°F, or 649°C), and Stellite or aluminumoxide (up to 1800°F, or 982°C).

At high temperatures, the guide bushings and guidepoststend to wear excessively, and this can be offset by the selectionof proper materials. Up to 600°F (316°C), 316 stainless steelguideposts in combination with 17–4 pH stainless steel guidebushings give acceptable performance. If the guideposts aresurfaced with Stellite, the above combination can be extendedup to 750°F (399°C) service. At operation over 750°F(399°C), both the posts and the bushings require Stellite.

Packing Designs The ideal packing provides a tight sealwhile contributing little friction resistance to stem movement.With TFE packing, which is industry standard, the requiredstem finish is between 6 and 8 µ in. RMS (0.15 and 0.2 µm).

A common packing design might consist of Teflon V-rings,which are discussed in more detail in Section 6.19. Doublepacking with leak-off connection in between can be used ontoxic or vacuum service (Figure 6.1n). On toxic services, anadded seal can be provided by the injection of high-viscositysilicone or plastic packing, but this is rarely done.

Solid rings or ribbons of pure graphite (Graphoil), whilemore expensive than Teflon, are also popular because they aresuited for higher temperatures. On the other hand, they requiremore loading to energize the packing than does Teflon, andthe resulting friction can cause stem lockup. On less demand-ing services, O-rings are also used, but not frequently becauseunder pressure these elastomers will absorb gases, which candestroy the O-ring when depressurized rapidly.

Metallic bellows-type seals are seldom used because oftheir pressure limitations and unpredictable lives. On toxicservices, they should be provided with automatically moni-tored guard packing for security.

The bonnets are usually flanged and are extended on hotor cold services so as to bring the operating temperature of thepacking closer to the ambient. Screwed bonnets are not recom-mended for severe duty, and welded bonnets are not used at all,except as an extreme precaution on hazardous services. Thesliding stems can sometimes drag atmospheric contaminants orprocess materials into the packing, but this can be overcomeby close tolerance guide bushings or wiper rings. Packing con-tamination is less likely with rotary valves. In case of LPG, thepacking should be isolated from outboard roller bearings andthe intervening space vented to protect the lubricant.

Packing Limitations Packing and bonnet designs in gen-eral were discussed in the previous paragraph and will alsobe covered in Section 6.19. Here only their suitability forhigh-temperature service is reviewed.

The packing temperature limitation for most nonmetallicmaterials is in the range of 400–550°F (204–288°C), themaximum temperature for metallic packing is around 900°F(482°C), and Teflon should not be exposed to temperaturesabove 450°F (232°C). Pure graphite (Graphoil) can be usedfrom −400 to 750°F (−240 to 399°C) in oxidizing service

and up to 1200°F (649°C) in nonoxidizing service, with anultimate potential of 3000°F (1649°C).

Bonnets can be screwed, welded, or flanged. Screwedbonnets are not recommended for high-temperature service.Finned bonnet extensions were used in the past on high-temperature services, when packing material capabilitieswere more limited. These finned designs were not effective,and therefore with the introduction of Graphoil, their use waslargely discontinued on rotary valves. For sliding stem valves,Teflon V-rings within extension bonnets are frequentlyselected and used up to 850°F (454°C).

On high-temperature services, it can be effective tomount the bonnet below the valve. In liquid service, with thebonnet above the valve, the packing is exposed to the fullprocess temperature due to the natural convection of heat inthe bonnet cavity.

If the bonnet is mounted below the valve, no convectionoccurs, and the heat from the process fluid is transferred byconduction in the bonnet wall only. Therefore, by this methodof mounting, the allowable process temperature can be sub-stantially increased in some processes. This is not the casefor all applications, because some processes do not generateeffective condensate seals, and in other services the liquid-vapor interface line can cause metallurgy problems.

Figure 6.1o provides a method for determining packingtemperature, in gas or vapor service, with the bonnet abovethe valve for one particular valve design. With vapor service,it is likely that vapors will initially condense on the wall ofthe bonnet, lowering the temperature to the saturation tem-perature of the process fluid, but in some cases the heatconducted by the metallic bonnet wall will be sufficient toprevent this condensation from occurring.

In short, the packing temperature will be at or abovesaturation temperature (Ts) in vapor service, but if the bonnetis mounted below the valve, the packing temperature is sub-stantially reduced, due to the accumulated condensate. If thebonnet is below the valve, the relationship between processand packing temperature is not affected by the phase of theprocess fluid.

In case of ball or plug valves with double-sealing, it isimportant to vent the space between the seals to the line, sothat damage will not be caused by thermal expansion.

Jacketed Valves A number of control valve designs areavailable with heat-transfer jackets. Others can be traced orjacketed by the user. Jacketed valves can be installed foreither cooling or heating. When a cooling medium is circu-lated in the jackets, this is usually done to lower the operatingtemperature of the heat-sensitive working parts.

Such jacketing is particularly concentrated on the bonnet,so that the packing temperature is reduced relative to theprocess. For certain operations at very high temperatures,intermittent valve operation is recommended, such that whenthe valve is closed it is cooled by the jacket, and when it isopened, it is kept open only long enough to prevent temper-ature equalization between the valve and the process.



6000

40003000

20001500

1000700500

300

200150

1007050

30

20

10

Airsteam

CA

B

D

E

−.20

.4

.6

.81.0

.2

F

.4.3

.2.1

0

G

2600

2200

1800

2400

2000

1600

1300

1100

900

700

500

1400

1200

1000

800

600

450

H

Proc

ess fl

uid te

mperat

ure −

°F *

J

Tp m

ore t

han

450°

F(2

32°C

)(S

ee in

stru

ctio

n 3b

)

Tp -P

acki

ng te

mpe

ratu

re

Tp le

ss th

an 4

50°F

(232

°C)

(Use

teflo

n pa

ckin

g)

450°F(232°C)

Inle

t pre

ssur

e − p

sia*

*See Section A.1 for SI units

Instructions1. Determine constants from table at right corresponding to bonnet selection to locate points on scales “D” and “F.”2. Solve nomograph Key:

Line Up Straight Edge On Locate Intersection On

A TO B CC TO D EE TO F GG TO H J

3a. If Tp ≤ 450°F (232°C), use Teflon packing.3b. If Tp > 450°F (232°C), either use high-temperature packing or select another bonnet with smaller “F” value and recheck packing temperature.

Bonnet Characteristics

ValveSize

BonnetMaterial*

BonnetFactors

StandardBonnet

ExtensionBonnet

RadiationFin Bonnet

1" CS D 0.07 0.29F .83 .25

SS D .13 .33F .65 .15

11/2" CS D .07 .24 0.25F .83 .39 .35

SS D .12 .30 .31F .69 .22 .21

2" CS D .11 .29F .72 .25

SS D .17 .33F .55 .15

3" CS D .10 .24 .31F .74 .39 .20

SS D .17 .31 .34F .57 .21 .11

4" CS D .07 .25F .83 .36

SS D .14 .31F .65 .20

6" CS D .04 .23F .90 .40

SS D .08 .30F .80 .22

8" CS D .06 .23 .33F .86 .40 .14

SS D .11 .30 .35F .72 .22 .10

*CS: carbon steel, SS: stainless steel.

FIG. 6.1o Nomograph for packing temperature determination on hot gas or vapor services. (From “Determination of Proper Bonnet and Packing forHigh-Temperature Processes,” R.F. Lytle, Fisher Controls, Emerson Process Management.)



Heating jackets with steam or hot oil circulation are used toprevent the formation of cold spots in the more stagnant areasof the valve or where the process fluid otherwise would beexposed to relatively large masses of cold metal. Figure 6.1pshows one of these valves, designed to prevent localized freezingor decomposition of the process fluid due to cold spots.

Many standard globe pattern valves can be fitted with ajacket to allow heating or cooling as required (Figure 6.1q).Normally, these jackets are for services requiring steam.

Dowtherm or similar heating fluids prevent solidification orcrystallization of certain fluids. Often, the manufacturer canprovide these jackets, but where this is not available, thereare firms that specialize in designing and installing suchjackets on valves and other equipment.

These special jackets can be designed either to weld to thevalve as a permanent fixture (Figure 6.1r) or as separate devicesbolted or clamped to the valve body. In the latter case, it maybe necessary to use a heat-transfer paste between jacket andvalve body to give efficient transfer by eliminating the air gap.

Low-Temperature Service

Cryogenic service is usually defined as temperatures below−150°F (−101°C). Properties of some cryogenic fluids arelisted in Table 6.1s. Valve materials for operation at temper-atures down to −450°F (−268°C) include copper, brass,bronze, aluminum, 300 series stainless steel alloys, nickel,Monel, Durimet, and Hastelloy. The limitation on the varioussteels falls between 0 and −150°F (−17 and −101°C), withcast carbon steel representing 0°F (−17°C) and 31/2% nickelsteel being applicable to −150°F (−101°C). Iron should notbe used below 0°F (−17°C).

Conventional valve designs can be used for cryogenicservice with the proper selection of construction materialsand with an extension bonnet (as described in detail inSection 6.19) to protect the packing from becoming toocold. The extension bonnet is usually installed vertically sothat the boiled-off vapors are trapped in the upper part ofthe extension, which provides additional heat insulationbetween the process and the packing.

If the valve is installed in a horizontal plane, a seal mustbe provided to prevent the cryogenic liquid from entering theextension cavity. When the valve and associated piping areinstalled in a large box filled with insulation (“cold box”),this requires an unusually long extension in order to keep thepacking box in a warm area.

FIG. 6.1p Jacketed control valve for high-temperature service.

FIG. 6.1qSteam-jacketed valve. (Courtesy of Flowserve Corp.)

FIG. 6.1r Special steam jacket for retrofit installation on valve.

Heatingmedium

inlet

Drain



Cryogenic Valves A special design variation on the globevalve is the cryogenic valve. Section 6.19 shows a number ofcryogenic service designs, including the Y-valve design. Themost common design for this service is shown in Figure 6.1t.

This design is specific to cryogenic (down to −454°F, or−270°C) service and no other. Body configurations arestraight through, as shown, or angle body. Because of theneed for Charpy impact for the extremely cold service, thematerials are limited to bronze and austenitic stainless steelssuch as 304, 316, and 316L. Normally, the valves are weldedinto the piping or soldered in some cases with bronze. Smallvalves, 1 in. (25 mm) and 2 in. (50 mm), can be socket weldor butt weld, but butt weld in larger sizes up to the 10 in.(250 mm) maximum are available.

Body ratings through ANSI 600 and flange ends, eitherintegral or separable, are available depending upon manufac-turer. Seat rings may be integral hard-faced with Stellite,screwed-in metal, or soft seat for tight shut-off. The metal shut-offs will be ANSI Class III or IV, depending upon manufacturer,and the soft seat using Teflon or Kel-F will provide Class VI.

Cold Box Valves Cold box valves are designed to have alow body mass for fast cool-down and reduced heat transfer.The long extended bonnet is provided with a plug stem sealto minimize liquid “refluxing” into the bonnet and packingarea, thereby minimizing the heat loss due to conduction andconvection. Actually, the small amount of liquified gas pass-ing into the bonnet vaporizes and provides a vapor barrierbetween the liquified gas and the packing area. In addition,the pressure resulting from the vaporization of the liquidprevents additional liquid from passing into the bonnet area.Excess pressure vents back into the body. It is possible to fitthese valves with vacuum jackets where the applicationrequires this additional insulation.

TABLE 6.1s Properties of Cryogenic Fluids

Methane Oxygen Fluorine Nitrogen Hydrogen Helium

Boiling point (°K)(°C)

–259(–162)

–297(–183)

–307(–188)

–320(–196)

–423(–253)

–452(–269)

Critical temperature (°F)(°C)

–117(–83)

–181(–118)

–200(–129)

–233(–147)

–400(–240)

–450(–268)

Critical pressure (psia)[bar(A)]

67346.1

73750.5

80855.3

49233.7

18812.9

332.26

Heat of vaporization at boiling point (BTU/lbm)(J/kg)

219(5.09⋅105)

92(2.14⋅105)

74(1.72⋅105)

85(1.98⋅105)

193(4.49⋅105)

9(2.09⋅104)

Density (lbm/ft3) gas at ambient conditions (kg/m3)

0.042(0.673)

0.083(1.33)

0.098(1.57)

0.072(1.153)

0.005(0.080)

0.010(0.16)

Vapor density at boilingpoint

0.111(1.778)

0.296(4.74)

— 0.288(4.614)

0.084(1.346)

1.06(16.98)

Liquid density at boiling point

26.5(424.5)

71.3(1142)

94.2(1509)

50.4(807.4)

4.4(70.5)

7.8(125)

FIG. 6.1t Cold box valve with weld ends and welded bonnet. (Courtesy ofFlowserve Corp.)

Upper guide

Packing spacerBonnet

boltsBonnetflange

Plug

Body

Integralseat

Plug seal

Cold boxextension

Bonnetgasket

Upper packingBonnet



Features that are desirable for cryogenic valves includesmall body mass, which ensures a small heat capacity and,therefore, a short cool-down period. In addition, the innerparts of the valve should be removable without removing thebody from the pipeline, and if the valve is installed in a coldbox, no leakage can occur inside this box because there areno gasketed parts.

The most effective method of preventing heat transferfrom the environment into the process is by vacuum jacketingthe valve and piping (Figure 6.1u). The potential leakageproblems are eliminated by the fact that there are no gasketedareas inside the jacket.

For cryogenic services where tight shut-off is required,Kel-F has been found satisfactory as a soft seat materialbecause cold can cause many other elastomer materials toharden, set, or shrink. If used as seals, this can cause leaking.

Cavitation and Erosion

The cavitation and erosion phenomena in connection withglobe valves are discussed in detail in Section 6.19. Thechoking effect of cavitation and its influence on valve sizingis covered in Section 6.15. Sections 6.14 and 6.15 describehow the liquid pressure recovery factor (FL) is related to theratio between the valve pressure drop and the differencebetween the inlet and the vena contracta pressure.

As was shown in Section 6.15, the cavitation coefficientKc is the ratio between the valve pressure drop at whichcavitation starts and the difference between the inlet and thevapor pressure of the application. Section 6.15 also showshow the FLP factor can be calculated if the valve is placedwithin reducers, and it also shows the valve pressure differ-ential at which choking starts can be calculated.

The allowable maximum ∆p before cavitation begins is∆p = Kc (p1 – pv). As the FL and Kc values of the different valve

designs drop, the probability of cavitation increases. FL and Kc

values for fully open valves are also given in Section 6.15, andFL values for throttled valves are given in Figure 6.1v.

The high flow velocity at the vena contracta of the valveis reached by obtaining its energy to accelerate from the pressureenergy of the stream. This causes a localized pressure reductionthat, if it drops below the fluid’s vapor pressure, results in tem-porary vaporization. (Fluids form cavities when exposed to ten-sions equal to their vapor pressure.) Cavitation only occurs whenthe pressure in the vena contracta region drops below the vaporpressure of the flowing fluid. The vapor pressure is a functionof fluid temperature and chemical structure.

Cavitation damage always occurs downstream of the venacontracta when pressure recovery in the valve causes the tem-porary voids to collapse. Destruction is due to the implosionsthat generate the extremely high-pressure shock waves in thesubstantially noncompressible stream. When these wavesstrike the solid metal surface of the valve or downstreampiping, the damage gives a cinder-like appearance. Cavitationis usually coupled with vibration and a sound like rock frag-ments or gravel flowing through the valve.

Cavitation damage always occurs downstream of thevena contracta at the point where the temporarily formedvoids implode. In case of flow-to-open valves, the destructionis almost always to the plug and seldom to the seat.

Methods to Eliminate Cavitation

Because no known material can remain indefinitely undam-aged by severe cavitation, the only sure solution is to elimi-nate cavitation completely. Even mild cavitation over anextended time will attack the metal parts upon which thebubbles impinge. Hard materials survive longer, but they arenot an economical solution except for services with mildintermittent cavitation. Cavitation damage also varies greatlywith the type of liquid flowing.

FIG. 6.1u Vacuum jacketing of cryogenic valve.

FIG. 6.1v In this figure, the pressure recovery factor (FL) of different valvedesigns is shown as a function of their discharge coefficients (Cd

values, which in the metric system are defined as ε = Kv / DN2) asthese valves are throttled from their full open positions.2

Seatupstream

Seatdownstream

Valve sizes 2"−36"

Ball

Gate valve

Globe valve

BFV

SYM disc0.9

0.4

0.5

0.6

0.7

0.8

1 2 4 6 8 10 20 40 60 80

Cd = Cv/d2

FL

Butterflyvalve



The greatest damage is caused by a dense pure liquidwith high surface tension (e.g., water or mercury). Densitygoverns the mass of the microjet stream, illustrated inFigure 6.1w, and surface tension governs the more importantjet velocity. Mixtures are least damaging, because the bubblecannot collapse as suddenly. As the pressure increases, par-tial condensation in the bubble changes the vapor composi-tion, leaving some vapor to slow the collapse. Some appli-cations of cavitating mixed hydrocarbons show nomechanical damage or high noise level. Cavitation can bereduced or eliminated by several methods, listed in the fol-lowing paragraphs.

Revising the Process Conditions A reduction of operatingtemperature can lower the vapor pressure sufficiently to elim-inate cavitation. Similarly, increased upstream and down-stream pressures, with ∆p unaffected, or a reduction in the∆p can both relieve cavitation. Therefore, control valves thatare likely to cavitate should be installed at the lowest possibleelevation in the piping system and operated at minimum ∆p.Moving the valve closer to the pump will also serve to elevateboth the up- and downstream pressures.

If cavitating conditions are unavoidable, then it is pre-ferred to have not only cavitation but also some permanentvaporization (flashing) through the valve. This can usuallybe accomplished by a slight increase in operating temperature

or by decreasing the outlet pressure. Flashing eliminates cav-itation by converting the incompressible liquid into a com-pressible mixture.

Revising the Valve Design Where the operating conditionscannot be changed, it is logical to review the type of the valvein terms of its pressure recovery characteristics. The moretreacherous the flow path through a particular valve, the lesslikelihood exists for cavitation. Inversely, the valves mostlikely to cavitate are the high recovery valves (ball, butterfly,gate) having low FL and Fc coefficients (Section 6.15).Figure 6.1x illustrates some of the ways available to eliminatecavitation.

Figure 6.1y shows a number of anticavitation valve designsthat combine multiple-port and multiple-flow-path features.

If cavitation is anticipated, the engineer should selectvalves with low recovery and, therefore, high Fc and FL coef-ficients. Different valve designs react differently to the effectsof cavitation, depending upon where the bubbles collapse. Ifthe focus is in midstream, materials may be unaffected. Forexample, in the “Swiss cheese”-type design, small holes in theskirt or cage are arranged in pairs on opposite sides of thecenterline of the valve. Streams from opposing holes impingeon each other, causing the cavities to collapse in the liquidpool (theoretically). This method, illustrated in Figure 6.1z hasbeen used successfully for mild cavitation.

FIG. 6.1w Cavitation occurs when downstream of the vena contracta the pressure rises. When it reaches the vapor pressure of the process fluid, thevapor bubbles implode and release powerful microjets that will damage any metallic surface in the area.

Vaporizationstarts as

pressure dropsbelow the vaporpressure of theflowing fluid

Cavitation starts asbubbles collapse

when pressure risesabove Pv

P1 = InletpressureP2 = Outlet

pressurePv = Vapor

pressurePvc = Vena-contractapressure

Distance

Microjet

Pressure

P1 P2 Flow



Labyrinth-type valves avoid cavitation by a very largeseries of right-angle turns with negligible pressure recoveryat each turn, but the narrow channels are subject to pluggingif particulate matter is in the stream (Figure 6.1aa).

The multistep valves at the bottom of Figure 6.1aa canavoid cavitation by replacing a single and deep vena con-tracta, as would occur in a single-port valve, with severalsmall vena contracta points as the pressure drop is distributedbetween several ports working in series. If the vapor pressureof the process fluid is below the outlet pressure of the valve(Condition A), this valve is likely to work.

On the other hand, if Pv is greater than P2 (Condition B),this valve is likely to cavitate in its noted intermediate port.One might note that if Condition B occurred in the conven-tional valve (noted by the dotted line), no cavitation wouldoccur, because some of the vapor formed at the vena contractawould never recondense but would stay in the vapor state(flashing). Therefore, these multistep valves are not recom-mended for flashing applications.

Gas Injection Another valve design variation that can alle-viate cavitation is based on the introduction of nonconden-sible gases or air into the region where cavitation is antici-pated. The presence of this noncompressible gas prevents thesudden collapse of the vapor bubbles as the pressure recoversto values exceeding the vapor pressure, and instead of implo-sions, a more gradual condensation process occurs. As shownin Figure 6.1bb, the gas may be admitted through the valveshaft or through downstream taps on either side of the pipe,in line with the shaft and as close to the valve as possible.Because the fluid vapor pressure is usually less than atmo-spheric, the air or gas need not be under pressure.

Revising the Installation In order to eliminate cavitation, itis possible to install two or more control valves in series.Cavitation problems can also be alleviated by absorbing someof the pressure drop in restriction orifices, chokes, or in par-tially open block valves upstream or downstream to the valve.The amount of cavitation damage is related to the sixth powerof flow velocity or to the third power of pressure drop. This isthe reason why reducing ∆p by a factor of two, for example,will result in an eightfold reduction in cavitation destruction.

In some high-pressure let-down stations, it might not bepossible to completely eliminate cavitation accompanied byerosion or corrosion. In such installations, one might considerthe use of inexpensive choke fittings (shown in Figure 6.1cc)instead of (or downstream to) control valves.

A single, fixed-opening choke fitting is applicable onlywhen the process flow rate is relatively constant. For variable-flow applications, one can provide several choke fittings ofdifferent capacities isolated by several full bore on/off valves,providing a means of matching the process flow with theopening of the required number of chokes. If the chokesdischarge into the vapor space of a tank, this will minimizecavitation damage because the bubbles will not be collapsingnear to any metallic surfaces.

Material Selection for Cavitation While no material knowntoday will stand up to cavitation, some will last longer thanothers. Table 6.1dd shows that the best overall selection forcavitation resistance is Stellite 6B (28% chromium, 4% tung-sten, 1% carbon, 67% cobalt). This is a wrought material andcan be welded to form valve trims in sizes up to 3 in. (75 mm).Stellite 6 is used for hard-facing of trims and has the same

FIG. 6.1x The pressure profiles shown in dotted lines illustrate some of the options available to the process control engineer to eliminate cavitation.

∆P

P2

P1

P1

PV

PVC

Valve withless recovery

higher FL & KC

∆P

∆P′

P2

P1

PV

PVC

P2

P1

PV

PVC

Two valvesin series

∆P

Reducevalve ∆P

∆P

P2

PV

PVC

Move valve closerto pump or to lower

elevation

P1

PVC

∆PP2

PV

PV′PVC

Lower thetemperature



FIG. 6.1y Control valve designs that are less likely to cavitate due to their multipath and multiturn flow paths.

Cageretainer

Cageretainer

Plug Cavitrolcage

Balance holesin plug

Seat ring

Valve plug

Valve stem

Seal ring

Cage

Seat ring

Bonnet

Lower stem guideBonnet flange

BoltingPlugseals

Cartridge

Seatring

Body

Seatgasket

Bonnetgasket

Plug

Sleeve

Sleeve gasketPackingPacking spacer

Frictional losses

Sudden expansion,turbulent mixing,

mutual impingement

Outer stageMiddle stage

Inner stage

Channel stream trim detail and valve installation.(Courtesy of Flowserve Corp.)

P2 = 800 psi(5.5 Mpa)

P1 = 5000 psi(34.5 Mpa)

800 (5.5 Mpa)1500 (10.4 Mpa)2200 (15.2 Mpa)2900 (20.0 Mpa)3600 (24.8 Mpa)4300 (29.7 Mpa)5000 (34.5 Mpa)

Flow

Typical pressure drop chartCascade-Trim step type plug for high pressure breakdown.

(Courtesy of Copes Vulcan, Inc., SPS Process Equipment.)

Step plug and orifice trim for liquid service.(Courtesy of Masoneilan, Division

of Dresser Flow Control.)Flash-Flow trim element and valve installation. (Courtesy of Hammel Dahl ConoFlow.)

VRT (Variable Resistance Trim) element, plates, and assembly in a valve. (Courtesy ofMasoneilan, Division of Dresser Flow Control.)

Cavitrol trim element and valve installation. (Courtesy of Fisher Controls, Emerson Process Management.)



chemical composition but less impact resistance. Correspond-ingly, its cost is lower.

In summary, the applications engineer should first reviewthe potential methods of eliminating cavitation. These wouldinclude adjustment of process conditions, revision of valvetype, or change of installation layout. If none of these tech-niques can guarantee the complete elimination of cavitatingconditions, the design engineer should install chokes or spe-

cial anticavitation valves that can last for some reasonableperiod, even if some cavitation is occurring.

Control Valve Noise

The calculation of noise levels generated by control valvesand the methods of lowering these noise levels are bothcovered in Section 6.14 and, therefore, will not be repeated

FIG. 6.1y (Continued).

Glandflange

Packingfollower

Stempacking

Bonnetflange

Plugseal

Plug

Stem

Disk stackassembly

Flexgasket

Seat ring

Body

Bonnetseal

Bonnet

Yoke clamp

Hush trim element and plug flow is into plug bore andout. (Courtesy of Copes Vulcan, Inc., SPC Process Equipment.)

Turbo-cascade trim element and valve installation.(Courtesy of Yarway , Tyco Valves & Controls.)

Self-drag valve with example of disk element. (Courtesy of CCI-Control Components, Inc.)



here. As can be seen in that section, many of the features oflow-noise control valves are similar to the features of theanticavitation valves shown in Figure 6.1y.

Flashing and Erosion

Cavitation occurs when (in Figure 6.1w) p2 > pv, while flashingtakes place when p2 < pv. When a liquid flashes into vapor,there is a large increase in volume. In this circumstance, thepiping downstream of a valve needs to be much larger thanthe inlet piping in order to keep the velocity of the two-phasestream low enough to prevent erosion. The ideal valve to usefor such applications is an angle valve with an oversized outletconnection. In Section 6.15, the method for calculating the exitvelocity in such two-phase flashing applications is illustrated

FIG. 6.1z The “Swiss cheese” design can withstand mild cavitation.

FIG. 6.1aa The labyrinth (top) and multistep (bottom) valve designs help to reduce the probability of cavitation.

Lift

A

CCI-Courtesy Control Components, Inc.

Courtesy Masoneilan, Division of Dresser Flow Control.

Section A-A

Labyrinth turn

Multi-step valve

Multi-step valve

Conventionalvalve

Conventionalvalve

Condition “A”

Condition “B”

P1

P1

Pv

Pv

P2

P2

A



by an example. In addition, the piping must be designed sothat it is not damaged by slug flow.

The impingement of liquid droplets can be erosive if thevelocity is great enough (> 200 ft/s, or 60 m/s, across theorifice), such as in applications involving high-pressure let-

down of gas or vapor with suspended droplets. On high-pressure let-down applications, the ideal valve to use is thedynamically balanced plug valve provided with a hard-facedplug.

Section 6.15 gives an example on how the exit velocityfrom a steam let-down valve can be calculated. Erosion causedby high exit velocities can also cause corrosion problems.

FIG. 6.1bb Cavitation can also be alleviated by the admission of air into theflowing stream.

Special ball valve

Gas inspiration

Air

Air

Air

Standard ball modified

FIG. 6.1cc The probability of cavitation can also be reduced by installing achoke fitting downstream of the valve.

TABLE 6.1ddRelative Resistance of Various Materials to Cavitation

Trim or Valve BodyMaterial

Relative CavitationResistance Index

Approximate RockwellC Hardness Values

CorrosionResistance Cost

Aluminum 1 0 Fair Low

Synthetic sapphire 5 Very high Excellent High

Brass 12 2 Poor Low

Carbon steel, AISI C1213 28 30 Fair Low

Carbon steel, WCB 60 40 Fair Low

Nodular iron 70 3 Fair Low

Cast iron 120 25 Poor Low

Tungsten carbide 140 72 Good High

Stellite #1 150 54 Good Medium

Stainless steel, type 316 160 35 Excellent Medium

Stainless steel, type 410 200 40 Good Medium

Aluminum oxide 200 72 Fair High

K-Monel 300 32 Excellent High

Stainless steel, type 17-4 pH 340 44 Excellent Medium

Stellite #12 350 47 Excellent Medium

Stainless steel, type 440C 400 55 Fair High

Stainless steel, type 329, annealed 1000 45 Excellent Medium

Stellite #6 3500 44 Excellent Medium

Stellite #6B 3500 44 Excellent High



Some metals do not corrode due to a self-regenerating pro-tective surface film; however, if this film is removed by ero-sion faster than it is formed, the metal corrodes rapidly. Forlining, either nobler metals or ceramics should be consideredin such situations. The preferred arrangement for flashingservice is to use a reduced port angle valve dischargingdirectly into a vessel or flash tank.

Corrosion

Some information data on corrosion is contained in Table 6.1dd.A detailed tabulation of the chemical resistance of materials isgiven in Appendix A.3 of this volume. In evaluating a particularapplication, one should also consider the facts that most processfluids are not pure and that the corrosion rate is much influencedby flow velocity and the presence of dissolved oxygen.

On corrosive services, one can also consider the use oflined valves (tantalum, glass, plastics, and elastomers), butone should consider the consequences of lining failure. Dam-age can be caused by accidentally exposing the lining to highconcentrations of inhibitors or line cleaning fluids. Gasabsorption in elastomer linings can also cause blistering.

Viscous and Slurry Service

When the process stream is highly viscous or when it containssolids in suspension, the control valve is selected to providean unobstructed streamline flow path.

The chief difficulty encountered with heavy slurrystreams is plugging. Conditions that can contribute to thisinclude a difficult flow path through the valve, shoulders,pockets, or dead-ended cavities in contact with the processstream. Valves with these characteristics must be avoidedbecause they represent potential areas in which the slurry canaccumulate, settle out, and gel, freeze, solidify, decompose,or as most frequently occurs, plug the valve completely.

The ideal slurry valve is one that

1. Provides full pipeline opening in its open position2. Provides for unobstructed and streamlined flow in its

throttling position3. Has high pressure and temperature ratings4. Is available in corrosion-resistant materials5. Is self-draining and has a smooth contoured flow path6. Will fail safe7. Has acceptable characteristics and rangeability8. Has top works that are positively sealed from the process

Unfortunately, no one valve meets all of these require-ments, and the instrument engineer has to judge which fea-tures are essential and which can be compromised.

If, for example, it is essential to provide a full pipeopening when the valve is open, there are several valves thatcan satisfy this requirement. They include the various pinchvalves (A in Figure 6.1ee), the full opening angle valves (B

FIG. 6.1ee Valves for viscous and slurry services.

controlsignal

Metallic jacketA Jacketed pinch valve

Connectionfor

flushing

B Full opening angle valve C Saunders valve

D Characterized ball valve E Self-draining valve F Eccentric rotating plug valve G Sweep angle valve

Flexiblesleeve

Inlet



in Figure 6.1ee), some of the Saunders valve designs (C inFigure 6.1ee), and the full-ported ball valves (Section 6.16).While these units all satisfy the requirement for a fully openpipeline when open, they differ in their limitations.

The pinch valves, for example, are limited in their mate-rials of construction, pressure, temperature ratings, flowcharacteristic, speed of response, and rangeability, but theydo provide self-cleaning streamlined flow, which in somedesigns resembles the characteristics of variable venturi.

The Saunders valves and pinch valves have similar features,including the important consideration that the sealing of theprocess fluid does not depend on stuffing boxes. They are supe-rior in the availability of corrosion-resistant materials, but theyare inferior if completely unobstructed streamline flow isdesired. Pinch valves are suitable for very low pressure dropservices only, while Saunders and wedge plug valves can operateat slightly higher pressures. Lined butterfly valves are a goodchoice if the process pressure is high while the valve drop is low.

The angle valve with a scooped-out plug satisfies mostrequirements except that its flow characteristics are not thebest, and it is necessary to purge it above the plug in orderto prevent solids from migrating into that area.

Full-ported ball valves in their open position are as goodas an open pipe section, but in their throttling positions boththeir flow paths and their pressure recovery characteristic areless desirable.

Valves that do not open to the full pipe diameter but stillmerit consideration in slurry service include the followingdesigns: characterized ball valves (D in Figure 6.1ee), variousself-draining valve types (E in Figure 6.1ee), the eccentricdisc rotating globe designs (F in Figure 6.1ee), and the sweepangle valves (G in Figure 6.1ee).

Each of these has some features that represent an improve-ment over some other design. The characterized ball valve,for example, exhibits an improved flow characteristic in com-parison with the full-ported ball type. It is well-suited forprocess fluids containing fibers or larger particles. The self-draining valve allows slurries to be flushed out of the systemperiodically. Complete drainage is guaranteed by the fact thatall surfaces are sloping downstream.

The sweep angle valve, with its wide-radius inlet bendand its venturi outlet, is in many ways like the angle slurryvalve. Its streamlined nonclogging inner contour minimizeserosion and reduces turbulence. In order to prevent the processfluid from entering the stuffing box, a scraper can be fur-nished, which if necessary can also be flushed with somepurge fluid. The orifice located at the very outlet is built likea choke fitting. Both orifice and plug may be made of abra-sion-resistant ceramic or hard metals.

For slurries with large solid particles, the ideal orificeshape is a circle, such as that of an iris valve or a jacketedpinch valve (A in Figure 6.1ee).

Orifice size, particle size, and rangeability are interre-lated. For any particle size and orifice shape, there is a min-

imum opening below which plugging can be expected. Toget good rangeability (control at low flow rates) the valve ∆pshould be made small. One way to accomplish this is by useof a “head box” (Figure 6.1ff). The selection of the valvestyle and the piping configuration around the valve inlet mustbe guided by the intractability of the particular slurry.

Valves That Can Be Sterilized

In food processing applications, in addition to the above, valvesmust not contain pockets where process material can be retained,and they should be constructed so that they can be easily steril-ized and disassembled for cleaning. Materials of constructionshould not contain compounds that are prohibited by the FDA,such as some of the elastomer compounding materials.

Valve Leakage

Any flow through a fully closed control valve when exposedto the operating pressure differentials and temperatures isreferred to as leakage. It is expressed as a cumulative quantityover a specified time period for tight shut-off designs and asa percentage of full capacity for conventional control valves.

According to ANSI B16.104, valves are categorizedaccording to their allowable leakage into six classes. Theseleakage limits are applicable to unused valves only:

Class I valves are neither tested nor guaranteed forleakage.

Class II valves are rated to have less than 0.5% leakage.Class III valves are allowed up to 0.1% leakage.Class IV valves must not leak more than 0.01% of their

capacity.

FIG. 6.1ff In slurry service, the use of a head box can provide a small andconstant pressure drop across the control valve.

Head box



Class V valves are specified to have a leakage of 5 ×10−4 ml/min water flow per inch (25.4 mm) of seatdiameter, per 1 psi (0,0685 bar) differential pressure.

Class VI is for soft-seated valves, and leakage isexpressed as volumetric air flow at rated ∆p up to50 psi (3.45 bar).

Generally the functions of tight shut-off and those ofcontrol should not be assigned to the same valve. The bestshut-off valves are rotary on/off valves, which are not nec-essarily the best choices for control.

Table 6.1gg gives a summary of the ANSI leakageclasses, and Table 6.1a lists the leakage capabilities of thevarious control valve designs. For added details on soft-seated designs and other special designs, refer to the featuresummaries at the front of each section.

Some valve manufacturers list in their catalogs the valvecoefficients applicable to the fully closed valve. For example,a butterfly valve supplier might list a Cv of 13.2 (Kv = 11.4)for a fully closed, metal-to-metal seated 24 in. (600 mm) valve.It should be realized that such figures apply only to new, cleanvalves operating at ambient conditions. After a few years ofservice, valve leakage can vary drastically from installation toinstallation as affected by some of the factors to be discussed.

It should also be noted that some fluids are more difficultto hold than others. Low-viscosity fluids such as Dowtherm,refrigerants, or hydrogen are examples of such fluids.

Soft Seats One of the most widely applied techniques forproviding tight shut-off over reasonable periods of time isthe use of soft seats. Standard materials used for such servicesinclude Teflon and Buna-N. Teflon is superior in its corrosionresistance and in its compatibility to high-temperature ser-vices up to 450°F (232°C). Buna-N is softer than Teflon butis limited to services at 200°F (93°C) or below.

Neither should be considered for operating conditionssuch as static pressures of 500 PSIG (34.5 bar) or greater,for use with fluid containing abrasive particles, or if criticalflow is expected at the valve seat.

The leakage of double-ported valves is much greater thanthat of single-ported ones, and it can be as high as 2–3% offull capacity in metal-to-metal seated designs.

Temperature and Pipe Strain It is frequently the case thateither the valve body is at a different temperature than thetrim or the thermal expansion factor for the valve plug isdifferent from the coefficient for the body material. It is usualpractice in some valve designs (such as the butterfly) toprovide additional clearance to accommodate the expansionof the trim when designing for hot fluid service. The leakagewill, therefore, be substantially greater if such a valve is usedat temperatures below those for which it was designed.

Temperature gradients across the valve can also generatestrains that promote leakage. Such gradients are particularlylikely to exist in three-way valves when they are in combiningservice and when the two fluids involved are at differenttemperatures. This is not to imply that three-way valves areinferior from a leakage point of view. Actually, their shut-offtightness is comparable to that of single-seated globe valves.

Pipe strains on a control valve will also promote leakage.For this reason, it is important not to expose the valve toexcessive bolting strains when placing it in the pipeline andto isolate it from external pipe forces by providing sufficientsupports for the piping.

Seating Forces and Materials The higher the seating forcein a globe valve, the less leakage is likely to occur. An averagevalve has a seating force of 50 lbf per linear inch (8750 N/m)of seat circumference. Where necessary, a much increased seat-ing force will create better surface contact by actually yieldingthe seat material. Seating forces of this magnitude (about tentimes the normal) are practical only when the port is small.

Seating materials are selected for compatibility with serviceconditions, and Stellite or hardened stainless steel is an appro-priate choice for nonlubricating, abrasive, high-temperature, andhigh pressure drop services. These hard surface materials alsoreduce the probability of nicks or cuts occurring in the seatingsurface, which might necessitate maintenance or replacement.

Small-Flow Valves Valves with small flow rates are foundin laboratory and pilot plant applications. Even in industrial

TABLE 6.1gg Valve Seat Leakage Classifications per ANSI B16.104-1976 (FCI 70–2)

Class Maximum Leakage

I No test required

II 0.5% of rated valve capacity

III 0.1% of rated valve capacity

IV 0.01% of rated valve capacity

V 5 × 10–4 ml/minute of water per inch of orifice diameter per psi differential

VI ml per minute of air or nitrogen vs. port diameter per the following tabulation

Nominal Port DiameterMaximum Seat

Leakage, ml/minutein. mm

1 25 0.15

11/2 38 0.30

2 50 0.45

3 75 0.90

4 100 1.70

6 150 4.00

8 200 6.75



installations, the injection of small quantities of neutralizers,catalysts, inhibitors, or coloring agents can involve flows inthe range of cubic centimeter per minute.

Valves are usually considered miniature if their Cv is lessthan 1 (Kv = 0.862). This generally means a 1/4 in. (6.25 mm)body connection and a 1/4 in. (6.25 mm) or smaller trim. Thetop works are selected to protect against oversizing, whichcould damage the precise plug.

The field of small-flow control valves is a highly special-ized area, unlike any other application. The mechanicaldesign and fluid flow constraints encountered essentiallymake these valves custom applied for the service. Small-flowvalves are used in laboratories, process pilot plants, and someareas of full-scale plants.

The design and building of these valves test the ingenuityof any manufacturer. There are several design types available,and great care must be taken to match the application withthe valve design. While the manufacturers publish Cv (Kv)ratings for their valves and trims, these should be treated withextreme caution and used for reference purposes only (below0.01 Cv [0.00862 Kv]).

In many cases, the flow may shift from laminar to turbulentwith flowing conditions and valve stroke changes. It is quitecommon for a laminar flow pattern to predominate, particularlywith viscous fluids or in low-pressure applications. Laminarflow means the flowing quantity will vary directly with pres-sure drop instead of with the square root of pressure drop.

It is wise to devise a test procedure simulating the serviceapplication to evaluate performance of a specific valve beforeusing it in actual service. It is not uncommon for two “iden-tical” small valves to exhibit somewhat different Cv(Kv)capacities and flow curves under the same test conditions.

Needle Valves There are at least three approaches to thedesign of miniature control valves: (1) the use of smooth-surfaced needle plugs, (2) the use of cylindrical plugs witha flute or flutes milled on it, and (3) positioning the plug byrotating the stem.

One of the most common designs looks like a miniatureversion of the standard globe control valve (Figure 6.1hh).The trim consists of a precision honed and close-fitting plugfitted into an orifice made of a hard alloy. The control areaconsists of a fine taper slot milled into the outer surface ofthe piston-shaped plug or a long shallow taper plug. In smallerCv (Kv) trims, this slot may be a calibrated scratch in the surface.

It is not uncommon to find up to 30 trims available in agiven body to cover the C range of 0.000002 to 0.1 (K from0.00000172 to 0.0862 Kv). The rangeability, i.e., the ratiobetween maximum and minimum controllable flow, can belimited for this type of valve due to the inherent leakage flowbetween piston and orifice. The smaller the trim size, thelower the rangeability.

Needle plugs give more dependable results than the oneswith grooves, scratches, or notches because the flow is dis-tributed around the entire periphery of the profile. This results

in even wear of the seating surfaces and eliminates sidethrusts against the seat. The trim is machined for very smallclearances, and hard materials or facings are recommendedto minimize wear and erosion.

Needle plugs are available with equal-percentage (downto Cv = 0.05 [Kv = 0.043]), linear, and quick-opening char-acteristics (Figure 6.1hh). Some manufacturers claim theavailability of valves with coefficients of Cv = 0.0001 (Kv =0.000086) or less. At these extremely small sizes it is verydifficult to characterize the plugs (equal-percentage is notavailable), and the valve rangeability also suffers.

It is easier to manufacture the smaller cylindrical plugswith one or more grooves (Figure 6.1hh) and obtain thedesired flow characteristic by varying the milling depth. Boththe needle and the flute plugs are economical, but it is difficultto reproduce their characteristics and capacity accurately.

Ball Valves In contrast to the relatively long stroke pistonand orifice valve discussed, there is another valve design forsmall-flow applications that has a very short and variableadjusted stroke (Figure 6.1ii). Here, a synthetic sapphire ballis allowed to lift off a metal orifice and throttle the flow. Theparticular advantage of this valve is that the diaphragm strokecan be adjusted to produce various stem lifts with a standard3–15 PSIG (0.2–1.0 bar) signal.

Two versions are available. One can be adjusted to cover0.07 to 0.00007 Cv (0.06 to 0.00006 Kv) and the other can beadjusted from 1 to 0.001 Cv (0.862 to 0.000862 Kv). Thesecan be used for high-pressure, high-drop applications rangingfrom 3000 to 30,000 PSIG (207 to 2068 bar), depending uponthe model.

FIG. 6.1hh Needle-type small-flow valve and plugs with flutes milled on cylin-drical surface.



Stem Rotation Type Another short-stroke valve especiallysuitable for high-pressure service (up to 50,000 PSIG, or3447 bar) is shown in Figure 6.1jj. Variation in Cv (Kv) ratingof identical plugs and seats is achieved by mechanical adjust-ment of a toggle arrangement. The toggle can change thevalve stroke between 0.010 and 0.150 in. (0.25 to 3.75 mm).With different trim inserts, a Cv range of 1 to 0.000001 (Kv

range of 0.862 to 0.00000086) can be covered.In this design, the lateral motion of the plug is achieved

by rotating the stem through a lead screw. The linear diaphragmmotion is transferred into rotation by the use of a slip ball joint.Valve capacity is a function of orifice diameter (down to0.02 in., or 0.5 mm), number of threads per inch in the leadscrew (from 11 to 32), amount of stem rotation (from 15 to60°), and the resulting total lift, which generally varies from0.005 to 0.02 in. (0.125 to 0.5 mm).

The extremely short distance of valve travel makes accu-rate positioning of the plug essential, and this necessitates apositioner. The combination of a long stem and short plugtravel makes this valve sensitive to stem load and temperatureeffects. Because this differential thermal expansion can causesubstantial errors in plug position, this valve is limited tooperating temperatures below 300°F (149°C).

Laminar Valve Finally, the latest addition to the low-flowvalve family is shown in Figure 6.1kk. This valve is designed

FIG. 6.1ii Low-flow ball valve with adjustable short-stroke actuator. (Courtesyof A.W. Cash Co.)

FIG. 6.1kk Low-flow valve using laminar flow element. (Courtesy of EmersonProcess Management.)

H

L h

FIG. 6.1jj Small-flow plug positioned by stem rotation.

Slip ball joint



to operate on the laminar flow principle. The flow is con-trolled by forcing the fluid through a long and narrow path,formed between two parallel surfaces. The actuator varies thelaminar gap through a diaphragm and O-ring-sealed hydrau-lic ram. Because, in the laminar regime, flow varies linearlywith pressure drop across the valve and with the third powerof the gap width between the two surfaces (valve travel), thisdesign has extremely high control rangeability along with awide Cv range of 0.02 to 0.000001 (Kv from 0.017 to0.00000086). The laminar principle also eliminates cavitationeffects with liquids and sonic choking velocities with gases.This design can be used with inlet pressures and pressuredrops up to 2675 PSIG (185 bar).

While it is not commonly thought of for low-flow valveapplication, the labyrinth disc valve design (see Figure 6.1aa)can be manufactured for this purpose. Its most commonapplication is for reducing high-pressure fluid samples foranalyzers, but obviously it can be used in other services. Onecommon factor for this design, as well as for all of the othersdiscussed, is the need for the fluid to be extremely clean.These valves are not tolerant of dirt or sediment due to thesmall passage and close clearances. Unless the fluid is knownto be clean, it is necessary to provide for a high level offiltration upstream.

INSTALLATION

When a valve is larger than 4 in. (100 mm) or sometimeswhen it is more than one size smaller than the pipe, it isadvisable to use pipe anchors to minimize force concentra-tions at the reducers and more frequently to relieve flangestress loading due to valve weight. The end connections onthe valve should match the pipe specifications. If weldedvalves are specified, the nipples should be factory-weldedand the welds should be stress-relieved. If lined valves arespecified, their inside diameters should match that of the pipeto avoid extrusion. On flangeless valves, the bolting and thetightness of the gaskets can be a problem if the valve bodyis long.

If valves are fast closing (or fail) in long liquid lines,water hammer can result in the upstream pipe or vacuum candevelop in the downstream line. Fast-opening steam valvescan thermally shock the downstream piping. Steam trapsshould be provided at all low points in a steam piping net-work. Anchors should be provided in all locations wheresudden valve repositioning can cause reaction forces todevelop.

Flow-to-close single-seated valves should not be usedbecause if operated close to the seat, hydraulic hammer canoccur. If the damping effect of the actuator alone will notovercome the vertical plug oscillation, then either the actuatorshould be made “stiffer” (higher air pressure operation) orhydraulic snubbers should be installed between the yoke andthe diaphragm casing.

Climate and Atmospheric Corrosion

In humid environments such as the tropics, moisture willcollect in all enclosures, and therefore drains should be pro-vided. Electrical parts should all be encapsulated where pos-sible or be provided with suitable moistureproof coating.

Vent openings should be provided with storage plugs andinsect screens. Even with such precautions, the vents andseals will require preventive maintenance and antifungustreatment in some extreme cases.

Cold climates can produce high breakaway torque ofelastomers, and in general, metals and plastics will becomemore brittle. Electrohydraulic actuators will require heatingbecause oils and greases can become very viscous.

In high-temperature environments, the weak link is usuallythe actuator, but liners, plastic parts, and electric componentsare also vulnerable. The damage is not only a function of thetemperatures but also of the lengths of time periods of exposure.Diaphragm temperature limits are a function of their materials:Neoprene — 200°F (93.3°C), Nordel — 300°F (148.7°C),Viton — 450°F (232.2°C), silicone glass—500°F (260°C).For higher temperatures, one can replace the diaphragms withpistons (or with metallic bellows in some extreme cases) oradd heat shields.

In power plant applications, valves frequently must bedesigned to withstand anticipated seismic forces. If theatmosphere contains corrosive gases or dusts, it is desir-able to enclose, purge, or otherwise protect the more sen-sitive parts. The stem, for example, can be protected by aboot.

In hazardous areas, all electrical devices should either bereplaced by pneumatic ones or be made intrinsically safe orexplosionproof.

CONTROL VALVE SPECIFICATION FORM

Compiling the information necessary to specify a control valveis best done with the aid of a tabulation sheet. Many largecompanies have their own customized forms. Figure 6.1ll showsa general-purpose form (ISA Form S20.50 Rev. 1) standard-ized by the Instrumentation, Systems, and Automation Society.After the form, general instructions are provided to assist incompleting the form.

A similar data sheet standard has been published by IECas publication IEC 60534-2-3, “Industrial Process ControlValves, Part 7: Control Valve Data Sheet.”

References

1. Lytle, R. F., “Equipment Selection for Control System Performance,” paperpresented at PUPID-SECON Conference in Birmingham, U.K., 1974.

2. Rahmeyer, W., “The Critical Flow Limit and Pressure Recovery Factorfor Flow Control,” InTech, November 1986.



FIG. 6.1llISA S20 Specification Form For Control Valve

* Copyright ISA © 1981, reprinted with permission of the Instrumentation, Systems, and Automation Society (ISA).









Bibliography

Adams, M., “Control Valve Dynamics,” InTech, July 1977.ANSI/ISA-TR75.02-2000, “Control Valve Response Measurement from

Step Inputs,” Research Triangle Park, NC: Instrumentation, Systems,and Automation Society, 2000.

Aubrun, C., Robert, M., and Cecchin, T., “Fault Detection in a ControlLoop,” Control Engineering Practice, Vol. 3, No. 10, pp. 1441–1446,October 1995.

Arant, J. B., “Fire-Safe Valves,” InTech, December 1981.Arant, J. B., “Positioner Use Is Myth-Directed,” InTech, November 1992.Ball, K. E., “Final Elements: Final Frontier,” InTech, November 1986.Baumann, H. D., “A Case for Butterfly Valves in Throttling Applications,”

Instruments and Control Systems, May 1979.Baumann, H. D., “How to Assign Pressure Drop across Control Valves for

Liquid Pumping Services,” Proceedings of the 29th Symposium on Instru-mentation for the Process Industries, Texas A&M University, 1974.

Baumann, H. D., “Effect of Pipe Reducers on Valve Capacity,” Instrumentsand Control Systems, December 1968.

Baumann, H. D., “The Introduction of a Critical Flow Factor for ValveSizing,” ISA Transactions, Vol. 2, No. 2, April 1963.

Baumann, H. D., “Control-Valve Sizing Improved,” InTech, 46(6):pp. 54–57, June 1999.

Beard, C. S., Final Control Elements, Philadelphia, PA: Chilton, 1969.Biegacki, S. and VanGompel, D., “The Application of DeviceNet in Process

Control,” ISA Transactions, Vol. 35, No. 2, pp. 169–176, 1996.Bishop, T., Chapeaux, M., Jaffer, L., et al., “Ease Control Valve Selection,”

Chem. Eng. Prog. 98, (11): 52–56 November 2002.Boger, H. W., “Recent Trends in Sizing Control Valves,” 23rd Annual Sym-

posium on Instrumentation, Texas A&M University, 1968.Boger, H. W., “Flow Characteristics for Control Valve Installations,” ISA

Journal, November 1966.Bolek, W., Sasiadek, J., and Wisniewski, T., “Two-Valve Control of a Large

Steam Turbine,” Control Engineering Practice, Vol. 10, No. 4,pp. 365–377, April 2002.

Borden, G. and Zinck, L., “Control Valve Seat Leakage,” InTech, November1986.

Brunone, B. and Morelli, L., “Automatic Control Valve-Induced Transientsin Operative Pipe System,” Journal of Hydraulic Engineering,Vol. 125, No. 5, p. 534, May 1999.

Buckley, P. S., “Design of Pneumatic Flow Controls,” Proceedings of the31st Annual Symposium on Instrumentation for the Process Industries,Texas A&M University, 1976.

Buckley, P. S., “A Control Engineer Looks at Control Valves,” Proceedingsof 1st ISA Final Control Elements Symposium, Wilmington, DE, 1970.

Buresh, J. F. and Schuder, C. B., “The Development of a Universal GasSizing Equation for Control Valves,” ISA Transactions, Vol. 3, No. 4,October 1964.

Cain, F. M., “Solving the Problem of Cavitation in Control Valves,” Advancesin Instrumentation and Control—Proceedings of the ISA/91 Interna-tional Conference, Vol. 46, Part 2, pp. 1133–49, Paper #91-0462, 1991.

Cain, F. M. and Barnes, R. W., “Testing for Cavitation in Low Pressure Recov-ery Control Valves,” ISA Transactions, Vol. 25, No. 2, pp. 61–67, 1986.

“Control Valves — Globe, Plug, Pinch, Needle, Gate,” Measurements andControl, February 1993.

“Control Valves, Regulators,” Measurements and Control, June 1992.“Control Valve Market to Reach nearly $3.5 Billion by 2008,” Control Eng.,

April 2003.Coughlin, J. L., “Control Valves and Pumps: Partners in Control,” Instru-

ments and Control Systems, January 1983.“Control Valves Deserve Your Respect,” Control Engineering, Vol. 49, No. 4,

p. 64, April 2002.Cunningham, E. R., “Solutions to Valve Operating Problems,” Plant Engi-

neering, September 4, 1980.“Digital Hydraulic Valves Gaining Momentum,” Design News, Vol. 58,

March 3, 2003.

Dobrowolski, M., “Guide to Selecting Rotary Control Valves,” InTech,December 1981.

Driskell, L. R., “Sizing Control Valves,” ISA Handbook of Control Valves,ISA, 1976.

Fagerlund, A. C., “Recommended Maximum Valve Noise Levels,” InTech,November 1986.

Gassmann, G. W., “When to Use a Control Valve Positioner,” Control,September 1989.

George, J. A., “Sizing and Selection of Low Flow Control Valves,” InTech,November 1989.

Gibson, H. I., “Variable-Speed Drives as Flow Control Elements,” ISA Trans-actions, Vol. 33, No. 2, pp. 165–169, July 1994.

“Growth Forecasted for the Control Valve Market,” Control Solutions74 (12):8, December 2001.

Hammitt, D., “Key Points about Rotary Valves for Throttling Control,”Instruments and Control System, July 1977.

Hammitt, D., “How to Select a Valve Actuator,” Instruments and ControlSystems, February 1977.

Hanssen, A. J., “Accurate Valve Sizing for Flashing Liquids,” Control Engi-neering, February 1961.

Hanson, C. L. and Clark, J. C., “Fast-Closing Vacuum Valve for High-CurrentParticle Accelerators,” Review of Scientific Instruments, January 1981.

Hegberg, M. C., “Control Valve Selection for Hydronic Systems,” ASHRAEJ., 42 (11): 33, November 2000.

Hegberg, M. C., “Control Valve Selection Response,” ASHRAE J., 43(3):24, March 2001.

Hill, A. G. and Lau, K. H., “Artificial Intelligence in Control Valve Selec-tion,” ISA Transactions, Vol. 28, No. 1, pp. 37–44, 1989.

Horch, A., “A Simple Method for Detection of Stiction in Control Valves,”Control Engineering Practice, October 1999, pp. 1221–1231.

Hutchison, J. W. (ed.), ISA Handbook of Control Valves, 2nd ed., ResearchTriangle Park, NC: Instrumentation, Systems, and Automation Society,1976.

“Installing Smart Positioners–A Wise Move,” Chemical Engineering Mag-azine, December 1, 2000.

“Intelligent Actuator Allows Programmable Valve Control,” Water World,Vol. 15, No. 5, p. 101, May 1999.

Jämsä-Jounela, S.-L., Dietrich, M., Halmevaare, K., and Tiili, O., “Controlof Pulp Levels in Flotation Cells,” Control Engineering Practice,Vol. ll, No. 1, pp. 73–81, January 2003.

Jury, F. D., “Positioners and Boosters,” Instruments and Control Systems,October 1977.

Kam, W. Ng., “Control Valve Noise,” ISA Transactions, Volume 33, No. 3,pp. 275–286, September 1994.

Karpenko, M., Sepehri, N., and Scuse, D., “Diagnosis of Process ValveActuator Faults Using a Multilayer Neural Network,” Control Engi-neering Practice, Vol. 11, No. 11, pp. 1289–1299, November 2003.

Kayihan, A. and Doyle, F. J., III, “Friction Compensation for a ProcessControl Valve,” Control Engineering Practice, Vol. 8, No. 7,pp. 799–812, July 2000.

Keagle, J., “It’s 18 mA, Do You Know Where Your Positioner Is?” InTech,May 1992.

Keles, O. and Ercan, Y., “Theoretical and Experimental Investigation of aPulse-Width Modulated Digital Hydraulic Position Control System,”Control Engineering Practice, Vol. 10, No. 6, pp. 645–654, June 2002.

Keskar, P. Y., “Analysis of Lightning-Related Damages to Instrumentationand Control System for Water and Wastewater Plants,” ISA Transac-tions, Vol. 35, No. 1, pp. 9–15, May 1996.

Kimura, T., Hara, S., Fujita, T., and Kagawa, T., “Feedback Linearizationfor Pneumatic Actuator Systems with Static Friction,” Control Engi-neering Practice, Vol. 5, No. 10, pp. 1385–1394, October 1997.

Kirsner, W., “Control Valve Selection,” ASHRAE J., 43(3): 24, March 2001.Lee, J. G. and Kim, O. H., “Development of a New Hydraulic Servo Cylinder

with Mechanical Feedback,” Control Engineering Practice, Vol. 7,No. 3, pp. 327–334, March 1999.

Lipták, B. G., “Control Valves in Optimized Systems,” Chemical Engineer-ing, September 5, 1983.



Lipták, B. G., “Control Valves for Slurry and Viscous Services,” ChemicalEngineering, April 13, 1964.

Lipták, B. G., “How to Size Control Valves for High Viscosities,” ChemicalEngineering, December 24, 1962.

Lipták, B. G., “Valve Sizing for Flashing Liquids,” ISA Journal, January1963.

Louleh, Z., Cabassud, M., and Le Lann, M.-V., “A New Strategy for Tem-perature Control of Batch Reactors: Experimental Application,”Chemical Engineering Journal, Vol. 75, No. 1, pp. 11–20, August1999.

Monsen, J. F., “Spreadsheet Sizes Control Valves for Liquids/Gas Mixtures,”InTech, December 1990.

Moore, R. L., “Flow Characteristics of Valves,” in ISA Handbook of ControlValves, 2nd ed., Pittsburgh, PA: Instrumentation, Systems, and Auto-mation Society, 1976.

Morgenroth, J., “Quarter-Turn Plug, Ball, and Butterfly Valves,” Plant Engi-neering, July 24, 1980.

“1992–94 Catalog of U.S. & Canadian Valves & Actuators,” Value Manu-facturers Association of America.

O’Keefe, W., “Learn Fluid-Handling Lessons from Nuclear Isolation Valvesand Actuator Systems,” Power, January 1981.

Page, G. W., “Predict Control Valve Noise,” Chem. Eng., New York, 107 (9):23–26, August 2000.

Pham, D. T., Jennings, N. R., and Ross, I., “Intelligent Visual Inspection ofValve-Stem Seals,” Control Engineering Practice, Vol. 3, No. 9,pp. 1237–1245, September 1995.

Price, V. E., “Smart Valve Intelligence Takes Many Forms,” InTech, August 1992.Pyotsia, J., “A Mathematical Model of a Control Valve,” 1992 ISA Confer-

ence, Houston, TX, October 1992.Rahmeyer, W., “Cavitation Testing of Control Valves,” Instrument Society

of America, paper no. C.I. 83-R931, presented at the InternationalConference in Houston, TX, October 1983.

Rahmeyer, W., “The Critical Flow Limit and Pressure Recovery Factor forFlow Control,” InTech, November 1986.

“Recommended Voluntary Standard Formulas for Sizing Control Valves,”Fluid Controls Institute, Inc., FCI 62-1, May 1962.

“Revised Control Valve Standard,” Hydrocarb. Process., 82(9): 114, Sep-tember 2003.

Restrepo, A., González, A., and Orduz, S., “Cost-Effective Control Strategyfor Small Applications and Pilot Plants: On/Off valves with Tempo-rized PID Controller,” Chemical Engineering Journal, Vol. 89,No. 1–3, pp. 101–107, October 2002.

Riveland, M. L., “The Industrial Detection and Evaluation of Control ValveCavitation,” Instrument Society of America, paper no. C.I. 82-909,presented at International Conference in Philadelphia, PA, October1982.

Roth, K. W. and Stares, J. A., “Avoid Control Valve Application Problemswith Physics-Based Models: Kinetic Energy Criteria have Many Lim-itations,” Hydrocarb. Process., 80(8):37, August 2001.

Ruel, M., “Control Valve Health Certificate,” Chem. Eng., New York,108(12):62–65, November 2001.

Sanderson, R. C., “Elastomer Coatings: Hope for Cavitation Resistance,”In Tech, April 1983.

Scott, A.B., “Control Valve Actuators: Types and Application,” InTech,January 1988.

Sharif, M.A. and Grosvenor, R.I., “The Development of Novel Control ValveDiagnostic Software Based on the Visual Basic Programming Lan-guage,” P. I. Mech. Eng. I.-J. Sys., 214(12):99–127, 2000.

Shinskey, F. G., “Control Valves and Motors,” Foxboro Publication No. 413–8.Singleton, E. W., “Control Valve Sizing for Liquid Viscous Flow,” Engineer-

ing Report No 7, Introl Limited.Spreter, R.,, “Valve Application Benefit from Technology Upgrades,” Power

Engineering, Vol. 103, No. 9, p. 46, September 1999.“Standard Control Valve Sizing Equations,” ANSI/ISA-S75.01-1977.Thornhill, N. F., Cox, J. W., and Paulonis, M. A., “Diagnosis of Plantwide Oscil-

lation through Data-Driven Analysis and Process Understanding,” ControlEngineering Practice, Vol. 11, No. 12, pp. 1481–1490, December 2003.

Tullis, J. P., Hydraulics of Pipelines: Pumps, Valves, Cavitation, Transients,New York: John Wiley and Sons, 1989.

“Valve Actuator Roundup,” InTech, January 1988.Wendy, E. J. and Stanton, C. C., “Industry Corner: A World View of Indus-

trial Valves,” Business Economics, Vol. 32, No. 2, p. 56, April 1997.Weir, W., “Control Valve Market Analyzed in New Study,” Hydrocarbon

Processing, Vol. 75, No. 9, p. 27, September 1996.Weirauch, W., “Electric Actuators ‘Leading the Way’ in Device Network

Use,” Hydrocarbon Processing, Vol. 75, No. 10, p. 31, October 1996.Weirauch, W., “Valve Automatization Gains Renewed Interest,” Hydrocar-

bon Processing, Vol. 80, No. 7, p. 25, July 2001.“What’s New in Valves and Valve Operators,” Pipe Line & Gas Journal,

Vol. 227, No. 11, p. 43, November 2000.“What’s New in Valves and Valve Operators,” Pipelines & Gas Journal,

Vol. 229, No. 11, p. 61, November 2002.Wilton, S. R., “Control Valves and Process Variability,” ISA Transactions,

Vol. 39, No. 2, pp. 265–271, April 2000.Wolter, D. G., “Control Valve Selection,” InTech, October 1977.Yang, J. C. and Clarke, D. W., “The Self-Validating Actuator,” Control

Engineering Practice, Vol. 7, No. 2, pp. 249–260, February 1999.Yu, W. X., Lin, J., Wang, C.W., et al., “A Thermo-Control Valve Fabricated

from Shape-Memory Alloy for Use in the Oil Field,” Rare Metal Mat.Eng., 31(5): 393–396, October 2002.

Yu, F. C., “Easy way to estimate realistic control valve pressure drops,”Hydrocarb. Process., 79(8): 45–48, August 2000.


Documents

Control Valve Selection and Sizing 6 - twanclik.free.frtwanclik.free.fr/electricity/IEPOPDF/1081ch6_1.pdf · Control Valve Sizing 1051 ... Determining the Valve Pressure Drop 1053