PROCESS PLANT EQUIPMENT

PROCESS PLANT EQUIPMENTOperation, Control, and Reliability

Edited by

MICHAEL D. HOLLOWAYCHIKEZIE NWAOHAOLIVER A. ONYEWUENYI

A JOHN WILEY & SONS, INC., PUBLICATION

Cover photography: courtesy of Chikezie Nwaoha

Copyright 2012 by John Wiley & Sons. All rights reserved.

Published by John Wiley & Sons, Inc., Hoboken, New JerseyPublished simultaneously in Canada

No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical,photocopying, recording, scanning, or otherwise, except as permitted under Section 107 or 108 of the 1976 United States Copyright Act, without eitherthe prior written permission of the Publisher, or authorization through payment of the appropriate per-copy fee to the Copyright Clearance Center, Inc.,222 Rosewood Drive, Danvers, MA 01923, 978-750-8400, fax 978-750-4470, or on the web at www.copyright.com. Requests to the Publisher forpermission should be addressed to the Permissions Department, John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, 201-748-6011, fax201-748-6008, or online at http://www.wiley.com/go/permission.

Limit of Liability/Disclaimer of Warranty: While the publisher and author have used their best efforts in preparing this book, they make no representationsor warranties with respect to the accuracy or completeness of the contents of this book and specically disclaim any implied warranties of merchantabilityor tness for a particular purpose. No warranty may be created or extended by sales representatives or written sales materials. The advice and strategiescontained herein may not be suitable for your situation. You should consult with a professional where appropriate. Neither the publisher nor author shallbe liable for any loss of prot or any other commercial damages, including but not limited to special, incidental, consequential, or other damages.

For general information on our other products and services or for technical support, please contact our Customer Care Department within the UnitedStates at 877-762-2974, outside the United States at 317-572-3993 or fax 317- 5724002.

Wiley also publishes its books in a variety of electronic formats. Some content that appears in print may not be available in electronic formats. For moreinformation about Wiley products, visit our web site at www.wiley.com.

Library of Congress Cataloging-in-Publication Data:

Process plant equipment : operation, control, and reliability / edited byMichael D. Holloway, Chikezie Nwaoha, Oliver A. Onyewuenyi.

p. cm.Includes index.ISBN 978-1-118-02264-1 (cloth)

1. Pumping machineryMaintenance and repair. 2. PipelinesMaintenance and repair. 3. ValvesMaintenance and repair.4. CompressorsMaintenance and repair. 5. Storage tanksMaintenance and repair. 6. Mixing machineryMaintenance and repair.7. BoilersMaintenance and repair. 8. Filters and ltration. I. Holloway, Michael H., 1963 II. Nwaoha, Chikezie, 1984 III.Onyewuenyi, Oliver A., 1952

TJ900.P725 2012621.8dc23

2011028229

Printed in the United States of America

10 9 8 7 6 5 4 3 2 1

For the memory ofDenton Ward

student and friend

CONTENTS

Contributors xxiii

Preface xxiv

SECTION I PROCESS EQUIPMENT OPERATION

1 Introduction 3Michael D. Holloway

2 Valves 9Ali Ahammad Shoukat Choudhury, Chikezie Nwaoha, and Sharad Vishwasrao

2.1 Types of Control Valves, 102.1.1 Linear-Motion Control Valves, 102.1.2 Rotary-Motion Control Valves, 112.1.3 Nonreturn Valves, 122.1.4 Relief Valves, 12

2.2 Control Valve Actuators, 122.2.1 Pneumatic Valve Actuators, 122.2.2 Electric Valve Actuators, 132.2.3 Hydraulic Valve Actuators, 13

2.3 Control Valve Sizing and Selection, 132.3.1 Selecting a Valve Type, 142.3.2 Sizing and Selection: Letting the Computer Do It All, 15

2.4 Common Problems of Control Valves, 152.4.1 Control Valve Cavitation, 152.4.2 Control Valve Leakage, 162.4.3 Control Valve Nonlinearities, 17

2.5 Diagnosing Control Valve Problems, 192.6 Control Valve Reliability and Selection, 202.7 Control Valve Maintenance, 22

2.7.1 Detecting Control Valve Stiction, 23

vii

viii CONTENTS

2.8 Control Valve Troubleshooting, 24References, 24

3 Pumps 27Craig Redmond

3.1 Types of Pumps, 283.1.1 Positive-Displacement Pumps, 283.1.2 Dynamic Pumps, 30

3.2 Pump Applications, 343.2.1 Flooded Suction Applications, 343.2.2 Suction Lift Applications, 353.2.3 Staged Pumping, 353.2.4 Solids-Handling Applications, 36

3.3 Pump Sizing and Selection, 373.3.1 System Head Curve, 373.3.2 Pump Peformance Curves, 383.3.3 Actual Pump Sizing and Selection, 393.3.4 Net Positive Suction Head, 403.3.5 Net Positive Suction Head Available, 40

3.4 Pump Maintenance, 403.4.1 Bearing Lubrication, 413.4.2 Seal Maintenance, 413.4.3 Maintaining Performance, 433.4.4 Winterizing and Long-Term Storage, 433.4.5 Cold Temperature Installations, 43

3.5 Pump Troubleshooting, 44

4 Pipes 45Shaohui Jia

4.1 Types of Pipes, 454.1.1 Seamless Pipe, 454.1.2 Welded Pipe, 45

4.2 Pipe Selection, 464.2.1 Pipe Strength, 464.2.2 Pipe Toughness, 464.2.3 Pipe Weldability, 484.2.4 Piping Material, 48

4.3 Pipeline Network Design and Optimization, 484.4 Pipeline Failure, 50

4.4.1 Pipe External Corrosion, 514.4.2 Pipe Internal Corrosion, 524.4.3 Stress Corrosion Cracking, 53

4.5 Pipeline Inspection and Leak Detection, 544.5.1 Pipeline Inspection, 544.5.2 Pipeline Inspection Tools, 554.5.3 Pipeline Leak Detection, 56

4.6 Pipe Maintenance, 584.6.1 Pipeline Coatings, 584.6.2 Pipeline Repair, 59

4.7 Pipe Troubleshooting, 60References, 61

CONTENTS ix

5 Cooling Towers 63Zaki Yamani Zakaria and Chikezie Nwaoha

5.1 Cooling Tower Operation, 635.1.1 Cooling Tower Psychrometrics, 635.1.2 Principles of Cooling, 655.1.3 Heat Exchange, 675.1.4 Components of Cooling Towers, 67

5.2 Types of Cooling Towers, 695.2.1 Natural-Draft Cooling Towers, 695.2.2 Mechanical-Draft Cooling Towers, 72

5.3 Common Problems of Cooling Towers, 745.3.1 Scale Deposits, 745.3.2 Delignication of Wood, 745.3.3 Poor Pump Performance, 755.3.4 Poor Airow, 755.3.5 Makeup Water, 755.3.6 Clogging of Distribution Nozzles, 75

5.4 Measuring Cooling Tower Performance, 755.4.1 Performance Assessment, 76

5.5 Cooling Tower Maintenance, 77References, 79

6 Filters and Membranes 81Flora Tong and Chikezie Nwaoha

6.1 Types of Filters, 816.1.1 Gas Filters, 826.1.2 Liquid Filters, 83

6.2 Mechanisms of Filtration, 876.2.1 Depth Straining, 876.2.2 Surface Straining, 876.2.3 Depth Filtration, 876.2.4 Cake Filtration, 87

6.3 Filter Selection, 876.3.1 Chemical Compatibility, 876.3.2 Accurate Pore Size, 87

6.4 Particle-Size Measurement Techniques, 886.4.1 Image Analysis, 886.4.2 Direct Mechanical Measurement, 886.4.3 Ultrasonics, 886.4.4 Laser Scattering Technology, 88

6.5 Filter Location, 896.5.1 Pressure Line Filters, 896.5.2 Suction Line Filters, 896.5.3 Return Line Filters, 89

6.6 Membrane Filtration, 896.6.1 Ultraltration, 906.6.2 Microltration, 946.6.3 Reverse Osmosis, 956.6.4 Nanoltration, 102

6.7 Filter Maintenance, 1036.7.1 Preparation for Maintenance, 104References, 104

x CONTENTS

7 Sealing Devices 107Matt Tones and Jim Drago

7.1 Gaskets, 1077.1.1 Gasket Materials, 1077.1.2 Gasket and Seal Construction, 1137.1.3 Principles of Gasket Operation, 1197.1.4 Gasket and Metal Seal Applications, 1207.1.5 Gasket Selection, 123

7.2 Compression Packings, 1257.2.1 Packing Materials, 1267.2.2 Packing Construction, 1287.2.3 Packing Selection, 1297.2.4 Packing Installation, 130

7.3 Mechanical Seals, 1317.3.1 Considerations for Using Mechanical Seals, 1327.3.2 Types of Mechanical Seals, 1347.3.3 Mechanical Seal Applications, 1377.3.4 Environmental Controls, 1407.3.5 Failure Analysis, 1427.3.6 Troubleshooting Mechanical Seals, 144

7.4 Expansion Joints, 1477.4.1 Joint Construction, 1487.4.2 Principles of Joint Operation, 1497.4.3 Joint Selection, 1497.4.4 Industrial Use of Expansion Joints, 1527.4.5 Joint Installation, 1557.4.6 Joint Troubleshooting, 156

7.5 General Sealing Device Selection, 1577.5.1 Product Selection, 1577.5.2 Understanding the Forces, 1577.5.3 STAMPS Criteria, 158References, 161

8 Steam Traps 163Jacob E. Uche and Chikezie Nwaoha

8.1 Steam Trap Operation, 1638.2 Types of Steam Traps, 164

8.2.1 Thermodynamic Steam Traps, 1648.2.2 Mechanical Steam Traps, 1668.2.3 Thermostatic Steam Traps, 169

8.3 Steam Trap Installation, 1728.3.1 Outlets of Steam-Using Equipment, 1728.3.2 On Steam Lines, 172

8.4 Steam Trap Checking, 1728.4.1 Sight Method, 1738.4.2 Sound Method, 1748.4.3 Temperature Method, 1748.4.4 Fluid Conductivity Method, 174

8.5 Common Problems of Steam Traps, 1758.5.1 Air Binding, 1758.5.2 Dirt, 1758.5.3 Improper Sizing, 175

CONTENTS xi

8.5.4 Steam Trap Leakage, 1758.5.5 Steam Locking, 1768.5.6 Water Hammer, 1768.5.7 Erosion of Seat and Valve Sealing Faces, 1768.5.8 Life Expectancy, 176

8.6 Steam Trap Selection, 1768.7 Steam Trap Applications, 178

8.7.1 Protection Service, 1788.7.2 Process Service, 1788.7.3 Other Applications, 178

8.8 Steam Trap Sizing, 1788.8.1 Basic Considerations, 179

8.9 Steam Trap Maintenance, 181References, 181

9 Process Compressors 183N. Sitaram and Chikezie Nwaoha9.1 Types of Compressors, 1839.2 Continuous Compression Compressors, 184

9.2.1 Ejectors, 1849.2.2 Dynamic, Rotodynamic, or Turbocompressors, 185

9.3 Intermittent Compression Compressors, 1869.3.1 Positive-Displacement Compressors

(Intermittent Flow), 1869.3.2 Rotary Compressors (Continuous Flow), 187

9.4 Centrifugal Compressors, 1899.4.1 Major Components of Centrifugal Compressors, 1899.4.2 Thermodynamics of Centrifugal Compressors, 1959.4.3 Energy Transfer in Centrifugal Compressors, 1969.4.4 Slip in Centrifugal Impellers, 1979.4.5 Losses and Efciencies, 1989.4.6 Performance, Stall, and Surge, 1999.4.7 Drivers, 201

9.5 Reciprocating Piston Compressors, 2029.5.1 Compressor Operation, 2029.5.2 Thermodynamic Laws, 2039.5.3 Compression Cycles, 2039.5.4 Power Requirements, 2049.5.5 Multistage Compression, 2049.5.6 Cylinder Clearance and Volumetric Efciency, 2059.5.7 Valve Losses, 2069.5.8 Major Components of Reciprocating Piston

Compressors, 2069.5.9 Drivers, 207

9.6 Compressor Troubleshooting, 209References, 210Further Reading, 211

10 Conveyors 213Okenna Obi-Njoku10.1 Industrial Use of Conveyors, 21310.2 Types of Conveyors, 214

10.2.1 Belt Conveyors, 21410.2.2 Bucket Conveyors, 215

xii CONTENTS

10.2.3 Cart-on-Track Conveyors, 21510.2.4 Chute Conveyors, 21610.2.5 Gravity Wheel Conveyors, 21610.2.6 Monorails, 21610.2.7 Chain Conveyors, 21610.2.8 Pneumatic Conveyors, 21710.2.9 Roller Conveyors, 21710.2.10 Screw Conveyors, 21810.2.11 Slat Conveyors, 21810.2.12 Sortation Conveyors, 21910.2.13 Vane-Type Throwing Machines, 21910.2.14 Tow Conveyors, 21910.2.15 Trolley Conveyors, 21910.2.16 Vertical Conveyors, 22010.2.17 Vibrating Conveyors, 22010.2.18 Troughed Belt Conveyors, 22010.2.19 Magnetic Belt Conveyors, 22010.2.20 Power-and-Free Conveyors, 220

10.3 Conveyor Selection, 22110.4 Conveyor Safety, 22110.5 Conveyor Maintenance, 22210.6 Summary, 222

References, 223

11 Storage Tanks 225Marcello Ferrara and Chikezie Nwaoha

11.1 Types of Storage Tanks, 22511.1.1 Aboveground Tanks, 22511.1.2 Underground Tanks, 226

11.2 Storage Tank Classication, 22711.2.1 Aboveground Tanks, 22711.2.2 Underground Tanks, 227

11.3 Construction Materials, 22711.3.1 Tank Materials, 22711.3.2 Protective Coatings, 22811.3.3 Insulation, 229

11.4 Common Problems of Storage Tanks, 22911.4.1 Corrosion, 22911.4.2 Vapor Losses, 23011.4.3 Storage Tank Fires, 231

11.5 Storage Tank Maintenance, 23311.5.1 Tank Blanketing, 23311.5.2 Holiday Detection, 23311.5.3 Tank Cleaning, 234

11.6 Tank Appurtenances, 24111.6.1 Mixers, 24111.6.2 Dikes, 24111.6.3 Insulators, 24111.6.4 Platforms and Ladders, 24211.6.5 Gauging Devices, 24211.6.6 Valves, 24211.6.7 Manways, 24311.6.8 Diffusers, 243

CONTENTS xiii

11.6.9 Water Cannons, 24311.6.10 Vents, 24311.6.11 Grounding, 24311.6.12 Supporting Structures, 243

11.7 Storage Tank Maintenance, 243References, 244

12 Mixers 245Jayesh Ramesh Tekchandaney

12.1 Mixing Concepts: Theory and Practice, 24612.1.1 Batch and Continuous Mixing, 24612.1.2 Selection of Mixing Equipment, 24712.1.3 Design of Mixing Equipment, 24712.1.4 Scale-Up of Mixing Equipment, 247

12.2 Fluid Mixing, 24812.2.1 Fluid Mixing Applications, 24812.2.2 Mixing Regimes, 24912.2.3 Power Consumption in Agitated Vessels, 24912.2.4 Flow Characteristics, 25112.2.5 Liquid Agitation Equipment, 253

12.3 Solid Blending, 26412.3.1 Properties of Solids Affecting Blending, 26412.3.2 Types of Blend Structures, 26512.3.3 Mechanisms of Solid Blending, 26512.3.4 Segregation Mechanisms, 26512.3.5 Scale-Up of Solid Mixers, 26612.3.6 Solid Blending Equipment, 266

12.4 Mixing High-Viscosity Materials and Pastes, 27412.4.1 Dispersive, Distributive, and Convective Mixing, 27512.4.2 Power for Viscous Mixing, 27512.4.3 Scale-Up of High-Viscosity Mixers, 27512.4.4 Heat Transfer, 27512.4.5 Equipment for Mixing High-Viscosity Materials

and Pastes, 27512.5 Mechanical Components in Mixing Equipment, 284

12.5.1 Motors, 28412.5.2 Mixer Speed Reducers, 28512.5.3 Couplings, 28712.5.4 Bearings, 28812.5.5 Shaft Seals, 28912.5.6 Variable-Speed Operation Devices, 29112.5.7 Mixer Installation, Startup, and Maintenance, 29212.5.8 Mixer Specications, 295References, 295

13 Boilers 297Celestine C. G. Nwankwo

13.1 Types of Boilers, 29813.1.1 Water Tube Boilers, 29913.1.2 Fire Tube Boilers, 30013.1.3 Pot Boilers, 30013.1.4 Saddle Boilers, 30113.1.5 Packaged Boilers, 301

xiv CONTENTS

13.1.6 Fluidized-Bed Combustion Boilers, 30113.1.7 Stoker-Fired Boilers, 30213.1.8 Pulverized Fuel Boilers, 30213.1.9 Waste Heat Boilers, 30213.1.10 Thermic Fluid Heaters, 30213.1.11 Superheated Steam Boilers, 302

13.2 Boiler Accessories, 30313.2.1 Fittings and Accessories at the Boiler Unit, 30313.2.2 Steam Accessories, 30313.2.3 Combustion Accessories, 304

13.3 Boiler Selection, 30513.3.1 Costs, 30513.3.2 Boiler Sizing, 30513.3.3 Heating and Heating Fuels, 306

13.4 Common Problems of Boilers, 30613.4.1 Scaling, 30613.4.2 Corrosion, 30713.4.3 Boiler Water Carryover, 308

13.5 Boiler Failure Analysis and Welding Defects, 30813.5.1 Boiler Failure Analysis, 30813.5.2 Welding Defects, 309

13.6 Boiler Maintenance, 31313.6.1 Boiler Upgrading and Retrotting, 31513.6.2 Boiler Feed Water Treatment, 31613.6.3 Boiler Stack Economizer, 31713.6.4 Boiler Blowdown Control, 317

13.7 Boiler Troubleshooting, 31913.7.1 Combustion Problems, 31913.7.2 Draft Fan and Burner Problems, 32013.7.3 Fuel Pump and Fuel Pressure Problems, 320

13.8 Boiler Chemicals, 32113.8.1 Phosphates, 32213.8.2 Lime Softening and Sodium Trioxocarbonate, 32213.8.3 Chelates, 32213.8.4 Polymers, 32313.8.5 Oxygen Scavengers, 32413.8.6 Neutralizing Amines, 32513.8.7 Filming Amines, 325

13.9 Boiler Efciency and Combustion, 32513.9.1 Heat Losses, 32613.9.2 Types of Burners, 32613.9.3 Burner Control Systems, 327References, 327Further Reading, 328

SECTION II PROCESS PLANT RELIABILITY

14 Engineering Economics for Chemical Processes 331Alberto R. Betancourt-Torcat, L. A. Ricardez-Sandoval, and Ali Elkamel

14.1 Time Value of Money, 33114.2 Cash Flow Analysis, 333

14.2.1 Compound Interest Factors for Single Cash Flows, 333

CONTENTS xv

14.2.2 Compound Interest Factors for Annuities, 33414.2.3 Arithmetic and Geometric Gradient Series, 334

14.3 Protability Analysis, 33614.3.1 Payback Period, 33614.3.2 Minimum Acceptable Rate of Return, 33614.3.3 Present and Annual Worth Analysis, 33614.3.4 Internal Rate of Return, 337

14.4 Cost Estimation and Project Evaluation, 34014.4.1 Capital Investment, 34014.4.2 Cost Indexes, 34114.4.3 Capital Cost Estimates, 34214.4.4 Production Costs and Estimations, 34814.4.5 Estimation of Revenues and Cash Flow, 352References, 353

15 Process Component Function and Performance Criteria 355Robert Free

15.1 Material Classication, 35515.2 General Physical Quantities and Considerations, 356

15.2.1 Important Denitions, 35615.2.2 State Quantities, 35715.2.3 Phase, 35715.2.4 Isolation, 35715.2.5 Flammability, 35915.2.6 Viscosity, 35915.2.7 Volatility, 36115.2.8 Corrosive Substances, 36115.2.9 Conductivity, 36115.2.10 Composition, 36215.2.11 Morphology, 36315.2.12 Solid-Specic Considerations, 36315.2.13 Coefcient of Friction, 365

15.3 Material Transfer and Conveyance Equipment, 36615.4 Conveyors, 367

15.4.1 Conveyor Belts, 36715.4.2 Overhead Conveyors, 37715.4.3 Roller Conveyors, 38215.4.4 Chute Conveyors, 38815.4.5 Screw Conveyors, 38915.4.6 Other Conveyor Types, 392

15.5 Pumps, 39415.5.1 Head and Pressure: Fluid Flow Systems, 39415.5.2 Pump Construction and Operation, 39515.5.3 Selection by Application, 39815.5.4 Reliability, Maintenance, and Process Development, 39915.5.5 Additional Information, 400

15.6 Valves, 40015.6.1 Valve Construction and Function, 40115.6.2 Selection Specication, 40415.6.3 Additional Information, 405

15.7 Pipes, 40515.7.1 Pipe Standards, 406

xvi CONTENTS

15.7.2 Pipe Classication, 40615.7.3 Common Pipe Materials, 40815.7.4 Pipe Sizing, 41015.7.5 Additional Information, 410

15.8 Conclusions, 410References, 410Corporation Web Sites, 411

16 Failure Analysis and Interpretation of Components 413Michael D. Holloway

16.1 Assessing the Situation, 41316.2 Failure Dened, 41416.3 Taking Advantage of Failure, 41516.4 Sources of Failure, 41616.5 Failure of Materials and of Machines, 41616.6 Types of Forces, 41716.7 Strength, 418

16.7.1 Stress and Strain, 41816.7.2 Yield, 419

16.8 Creep (Deformation), 41916.9 Fatigue (Material), 420

16.9.1 Rate of Fatigue, 42116.9.2 Fracture, 421

16.10 Wear, 42216.11 Property Changes, 42316.12 Temperature, 42316.13 Oxidation: Molecular Transitions and Chemical Inuences, 425

16.13.1 Corrosion, 42516.13.2 Corrosion in Nonmetals, 42516.13.3 Galvanic Corrosion, 426

16.14 Deposit Formation, 42616.15 Factors that Affect Deposit Formation, 428

16.15.1 Concentration and Pressure, 42816.15.2 Particle Size and Contaminant Type, 42816.15.3 Adsorption, 429

16.16 Documenting Failure, 42916.16.1 Root Cause Investigation, 43016.16.2 Failure Examination, 432References, 432

17 Mechanical Integrity of Process Vessels and Piping 435Oliver A. Onyewuenyi

17.1 Perspectives on Mechanical Integrity, Fitness for Service, andCondition Monitoring, 43717.1.1 Mechanical Integrity, 43717.1.2 Condition Monitoring, 43817.1.3 Fitness for Service, 438

17.2 Types of Flaws and Damage Mechanisms, 43917.2.1 Flaws or Discontinuities Versus Defects, 43917.2.2 Types of Flaws, 44017.2.3 Weld Flaws, 44017.2.4 In-Service Flaws and Environmentally Assisted Flaws, 440

CONTENTS xvii

17.2.5 In-Service Degradation and Susceptibility of VariousAlloys, 440

17.2.6 HAC and SCC Susceptibility of Various Alloy Systems, 44117.3 Inspection, Characterization, and Monitoring of Flaws, 442

17.3.1 General Metal Loss and Local Thinned Area Corrosion, 44217.3.2 Pitting and Crevice Corrosion, 44317.3.3 HIC, SOHIC, and Blister Damage, 44317.3.4 Cracklike and Sharp Flaws, 44317.3.5 Online Condition Monitoring of Damage, 443

17.4 Fracture Mechanics and Fitness-for-Service Assessment, 44317.4.1 Applicable Codes and Standards, 44417.4.2 When FFS is Needed, 44417.4.3 FFS Assessment Procedure, 446

17.5 Control and Prevention of Brittle Fracture, 45217.5.1 Denitions, 45217.5.2 Brittle Versus Ductile Fracture, 45217.5.3 Industry and Regulatory Codes and Standards for Brittle

Fracture Control, 45317.5.4 Determination of the Minimum Metal Temperature, 45317.5.5 Determination of the Lower Design Temperature, 45317.5.6 Toughness Requirements, 45517.5.7 Brittle Fracture Risk Assessment of Existing Systems, 45517.5.8 Assessment Approaches, 45617.5.9 LDT and Design CodeBased Assessments, 45617.5.10 FFS-Based Assessments, 45817.5.11 Assessment per API 579-1 and ASME FFS-1 Part 3, 45817.5.12 Full FFS Assessment, 458

17.6 Case Histories and Examples of FFS Applications to Cracks inProcess Plant Pressure Vessels, 459References, 464

18 Design of Pressure Vessels and Piping 467Maher Y. A. Younan

18.1 Modes of Failure, 46718.1.1 Failure Under Static Loading, 46718.1.2 Failure Under Dynamic Loading, 46818.1.3 Failure Under Other Types of Loading, 469

18.2 Basic Stress Analysis, 46918.2.1 Allowable Stresses, 470

18.3 Design of Pressure Vessels, 47018.3.1 Geometric Considerations, 47018.3.2 Design of Vessels Under Internal Pressure, 47118.3.3 Nozzles or Branch Connections, 47218.3.4 Design of Formed Heads, 47418.3.5 Vessels and Pipes Subjected to External Pressure, 47518.3.6 Design of Vessel Supports, 47818.3.7 Design by Rule Versus Design by Analysis, 479

18.4 Design of Piping Systems, 48118.4.1 Wall Thickness for Internal Pressure, 48118.4.2 Pipe Span Calculations, 48218.4.3 Pipe Supports, 48318.4.4 Expansion and Flexibility, 48318.4.5 Code Compliance, 485References, 486

xviii CONTENTS

19 Process Safety in Chemical Processes 489Jelenka Savkovic-Stevanovic

19.1 The Hazards, 49019.1.1 Special Hazards, 49019.1.2 Toxicology, 49019.1.3 Flammability, 49319.1.4 Explosions, 49419.1.5 Ignition, 49419.1.6 Ionizing Radiation, 49519.1.7 Pressure, 49519.1.8 Temperature Disturbance, 49619.1.9 Noise Disturbance, 49619.1.10 Fire and Explosion Index, 496

19.2 Hazard Analysis, 49919.2.1 Safety Checklists, 50019.2.2 Process Operation and Hazards, 501

19.3 Risk Analysis, 50319.3.1 Decision-Making System, 50319.3.2 Qualitative Risk Analysis, 50519.3.3 Qualitative Model Development, 505

19.4 Safety Ratings, 51119.4.1 Hazard Potential of a Volatile Substance, 51319.4.2 Hazard Potential from an Explosion, 51419.4.3 Evaluation of Hazardous Properties, 51519.4.4 Rating of Flammable and Explosive Substances, 516

19.5 Development and Design of a Safe Plant, 52419.5.1 Design and Construction Methods, 52619.5.2 Evaluation of Hazards by Probability of Occurrence, 53019.5.3 Reliability Analysis, 53419.5.4 Safety Based on Process Control, 53919.5.5 Damage-Minimizing Systems, 541

19.6 Safety Process Operation, 54319.6.1 Batch and Continuous Processes, 54419.6.2 The Human Aspect of Safety, 54519.6.3 Safety in Production Practice, 54619.6.4 Maintenance, 54819.6.5 Plant Safety Optimization, 55319.6.6 Plant and Process Modication, 55519.6.7 Hazard Impact Reduction, 556

19.7 Safety and Reliability Analysis, 55719.7.1 Process Safety Information, 55819.7.2 Project Safety Information, 55819.7.3 Design and Control Safety, 56319.7.4 Operating Procedures, 56319.7.5 Training, 56419.7.6 Process Hazard Analysis Revalidation, 56519.7.7 Emergency Flaring Systems, 57219.7.8 Computerized Hazard Identication, 57419.7.9 Risk Assessment, 578

19.8 Summary, 581References, 582

CONTENTS xix

SECTION III PROCESS MEASUREMENT, CONTROL,AND MODELING

20 Flowmeters and Measurement 587Gregory Livelli and Chikezie Nwaoha

20.1 Flow Measurement Techniques, 58720.1.1 Volumetric Totalizers, 58720.1.2 Turbine Flowmeters, 58820.1.3 Oval Gear Totalizers, 58920.1.4 Lobed Impeller Gas Meters, 58920.1.5 Vortex Flowmeters, 59020.1.6 Swirl Flowmeters, 591

20.2 Flow-Rate Meters, 59220.2.1 Differential Pressure Flowmeters, 59220.2.2 Variable-Area Flowmeters, 59320.2.3 Electromagnetic Flowmeters, 59420.2.4 Ultrasonic Flowmeters, 59520.2.5 Coriolis Mass Flowmeters, 59620.2.6 Thermal Mass Flowmeters, 598

20.3 Common Problems of Flowmeters, 59920.3.1 Liquid Carryover, 59920.3.2 Dirt, 59920.3.3 Viscosity Effects, 59920.3.4 Solids in a Fluid, 60020.3.5 Gas Content in a Liquid, 60020.3.6 Corrosion Risks with Aggressive Fluids, 60020.3.7 Vibration, 60020.3.8 Pulsation, 600

20.4 Flowmeter Installation and Maintenance, 60120.4.1 Flowmeter Installation, 60120.4.2 Flowmeter Maintenance and Operating Characteristics, 603

20.5 Calibration and Certication, 60620.5.1 Why Calibrate?, 60620.5.2 Flow-Rate Calibration Methods, 60620.5.3 Boundary Conditions and Measurement Fixtures, 607

20.6 LACT and Prover Descriptions, 60720.6.1 What Is a LACT Unit?, 60720.6.2 What Is a Meter Prover Used For?, 60820.6.3 Operation of a LACT Unit, 60820.6.4 LACT Unit Components, 60920.6.5 Liquid Displacement Provers, 613

20.7 Troubleshooting LACT and Prover Systems, 61420.8 Troubleshooting Flowmeters, 614

References, 617

21 Process Control 619John A. Shaw

21.1 Control System Components, 61921.2 Control System Requirements, 62021.3 Sensor Response, 620

21.3.1 Process Response, 620

xx CONTENTS

21.3.2 Controller/Actuator Response, 62421.4 Control Algorithms, 624

21.4.1 On/Off Switch, 62421.4.2 PID Algorithm, 62421.4.3 Control Modes, 625

21.5 Loop Tuning, 62521.5.1 Quarter-Wave Decay, 62521.5.2 ZieglerNichols Tuning Methods, 62621.5.3 Other Methods, 62821.5.4 Controllability of Processes, 629

21.6 Multiloop Control, 62921.6.1 Cascade Control, 62921.6.2 Ratio Control, 63221.6.3 Feedforward Control, 633

21.7 Final Control Elements, 63321.7.1 Time-Proportional Heating Elements and Solenoid

Valves, 63321.8 Process Controllers, 634

21.8.1 Distributed Control Systems, 63421.8.2 Programmable Logic Controllers, 634Reference, 634

22 Process Modeling and Simulation 635Mathew Chidiebere Aneke

22.1 Process Modeling, 63522.1.1 Steady State Versus Dynamic Models, 63622.1.2 Lump-Sum Versus Distributed Models, 63622.1.3 Shortcut Versus Rigorous Models, 636

22.2 Process Simulation, 63622.3 Process Optimization, 63622.4 Commercial Tools for Process Modeling, Simulation, and

Optimization, 63722.4.1 Modular Mode Process Simulators, 63722.4.2 Equation-Oriented Process Simulators, 637

22.5 Process Modeling Case Studies, 63822.6 Concluding Remarks, 650

References, 650

Appendix I Methods for Measuring Process Temperature 653Chikezie Nwaoha

Appendix II Airow Troubleshooting 659Chikezie Nwaoha

Appendix III MIG Shielding Gas Control and Optimization 663Jerry Uttrachi

Appendix IV Rupture Disk Selection 665Chikezie Nwaoha

Appendix V Pressure Gauge Selection 669Chikezie Nwaoha

CONTENTS xxi

Appendix VI Corrosion and Its Mitigation in the Oiland Gas Industries 673Krupavaram Nalli

Appendix VII Mixers 681Jayesh Ramesh Tekchandaney

Glossary of Processing Terms 685Garlock Sealing Technologies

Index 693

CONTRIBUTORS

Mathew Chidiebere Aneke, Department of Built Environ-ment, Northumbria University, Newcastle upon Tyne,England

Alberto R. Betancourt-Torcat, Department of ChemicalEngineering, University of Waterloo, Waterloo, Ontario,Canada

Ali Ahammad Shoukat Choudhury, Department ofChemical Engineering, Bangladesh University of Engi-neering and Technology, Dhaka, Bangladesh

Jim Drago, P.E., Sr. Manager Marketing, Intelligence,GPT, Palmyra, New York

Ali Elkamel, Department of Chemical Engineering, Uni-versity of Waterloo, Waterloo, Ontario, Canada

Marcelo Ferrara, ITW S.r.l., Innovative TechnologiesWorldwide, Augusta, Italy

Robert Free, Department of Engineering Physics, Univer-sity of Oklahoma, Norman, Oklahoma

Michael D. Holloway, Certied Laboratories, NCH Cor-poration, Irving, Texas

Shaohui Jia, PetroChina Pipeline R&D Center, Langfang,Hebei, China

Gregory Livelli, ABB Measurement Products, Warminster,Pennsylvania

Krupavaram Nalli, Tebodin & Co., Al-Athaibab, Muscat,Oman

Celestine C. G. Nwankwo, Federal University of Technol-ogy, Owerri, Nigeria

Chikezie Nwaoha, Control Engineering Asia, Ten AlpsCommunications Asia, Aladinma, Owerri, Imo State,Nigeria

Okenna Obi-Njoku, Owerri, NigeriaOliver A. Onyewuenyi, MISOL Technology Solutions,

Katy, Texas

Craig Redmond, The Gorman-Rupp Company, Manseld,Ohio

L. A. Ricardez-Sandoval, Department of Chemical Engi-neering, University of Waterloo, Waterloo, Ontario,Canada

Jelenka Savkovic-Stevanovic, Department of ChemicalEngineering, Faculty of Technology and Metallurgy,Belgrade University, Belgrade, Serbia

John A. Shaw, Process Control Solutions, Cary, NorthCarolina

N. Sitaram, Thermal Turbomachines Laboratory, Depart-ment of Mechanical Engineering, IIT Madras, Chennai,India

Jayesh Ramesh Tekchandaney, Unique Mixers and Fur-naces Pvt. Ltd., Thane, Maharashtra, India

Matt Tones, Director, Marketing Intelligence, GarlockSealing Technologies, Palmyra, New York

Flora Tong, Dow Chemical (China), Shanghai, ChinaJacob E. Uche, Port Harcourt Rening Company, Eleme,

Nigeria

xxiii

xxiv CONTRIBUTORS

Jerry Uttrachi, WA Technology, Florence, South CarolinaSharad Vishwasrao, Vigilant Plant Services, Yokogawa

Engineering, Asia Pte. Ltd., Singapore

Maher Y. A. Younan, Department of Mechanical Engi-neering, School of Sciences and Engineering, AmericanUniversity in Cairo, Cairo, Egypt

Zaki Yamani Zakaria, Department of Chemical Engi-neering, Faculty of Chemical Engineering, UniversitiTeknologi Malaysia, Skudai, Johor, Malaysia

PREFACE

The nature of human beings has been to control theimmediate environment for safety and comfort. Buildingshelter, nding food and water, and staying warm anddry are the reasons for our success. None of this wouldor could be possible if we did not develop a means tocommunicate and retain information. Consider the fact thathumans do not possess great physical strength or agilitycompared to other animals or the ability to withstandharsh environments without the use of an extension of ourbodies (clothing and shelter). We truly rely on each othersexperiences to help us accommodate to what the worldthrows at us. This manuscript is an extension of just thatameans by which humans can share ideas and experiences inorder to live our lives more comfortably. From petroleum,pharmaceuticals, and various chemicals and food products,to energy and power production, processing plants produceproducts essential to our survival. Without these plants wewould not have the ability to get much older than ourprehistoric ancestors. Developing the competence to runthese plants efciently, reliably, safely, and protably istherefore a prime human objective.

The effort that proceeds over the next several hundredpages is nothing short of a miracle! Rarely can you get oneor two people to commit to such a monumental project. Thisparticular undertaking has herded over thirty (yes, thirty!)of the worlds top academic and engineers to embark on aproject that is encyclopedic in nature. Talented folks fromAsia, Africa, Europe, the Middle East, South America, andNorth America have each put forth the effort to take on atopical chapter in order to build this work, all intended toprovide readers with the information that will lead them tounderstand and implement best practices in process plantequipment operations, reliability, and control.

The initial idea for the book sprang from the musings ofa very talented and promising young engineer, ChikezieNwaoha. His vision to provide a comprehensive text

that would enable readers to have access not only tothe fundamental information concerning process plantequipment but also to practical ideas, best practices, andexperiences of highly successful engineers from aroundthe world. Nwaoha, being a smart man, decided todelegate some of the work. He broke the book intothree parts. He edited the rst section, Process EquipmentOperations, Michael D. Holloway edited the second section,Process Plant Reliability, and the third section, ProcessMeasurement, Control, and Modeling, was edited byOliver A. Onyewuenyi, a world-renowned engineer.

The work incorporates the latest information, bestpractices, and trends. The sound foundations of engineeringprinciples for a process facility provided in this book havesolid roots from which the tree of productivity is constantlybearing fruit. If the reader chooses to put any of theseideas into practice, improvement in equipment operation,reliability, and control should be witnessed and enhancedsafety, protability, and performance will ensue. Only goodthings can happen.

Like any experienced bushman on the savanna knows,you can only eat an elephant one bite at a time. It issuggested that you take your time and read and digest eachchapter carefully. Feel free to write in the margins, highlightpassages, and quote as you see t (but please use soundjudgment concerning copyright laws!). Most important,use this work as a tool. Employed with care, informationcan develop into knowledge. With proper application andsound judgment, wisdom can spring forth. This work is thebeginning of a very wise approach.

Chikezie NwaohaMichael D. HollowayOliver A. Onyewuenyi

xxv

SECTION I

PROCESS EQUIPMENT OPERATION

1INTRODUCTION

Michael D. HollowayNCH Corporation, Irving, Texas

A process is an amalgamation of machines, methods,materials, and people working in concert to produce some-thing. Generally, the end product is something tangible:fuel, food, textiles, building materialsthe list is exhaus-tive. The end product from a process can also be intangible:a bond, software, laws. It is difcult to say where a personbegins and a process ends. Human beings are dependent onprocesses to live, as we are dependent on water to live. Therst known process was probably irrigating elds to growcrops. Many argue that this process began over 20,000 yearsago, others that it was closer to 50,000 years ago. Everyfew years a discovery is made that puts the date back evenfurther as well as the place of origin: Africa, Asia, the Mid-dle East? Needless to say, humans have been trying for avery long time to reduce labor and add comfort through thesystematic use of materials and machines to implement aprocess to achieve a desired goal. Consider the followingincomplete list of materials and machines. All required aprocess.

Machines Primary machines: simple machines that rely on

their own structure to complete work: lever, pulley,inclined plane, hammer

Secondary machines: simple machines that rely onan accompanying machine: screw, wheel, axle, saw

Tertiary machines: complex machines that require acontribution from a compliant machine: gear, valve,pump, furnace, bearing, engines, boiler

Process Plant Equipment: Operation, Control, and Reliability, First Edition. Edited by Michael D. Holloway, Chikezie Nwaoha, and Oliver A. Onyewuenyi. 2012 John Wiley & Sons, Inc. Published 2012 by John Wiley & Sons, Inc.

Materials Primary materials: material used in the unpro-

cessed state: water, wood, pitch, clay, stone, sand,wax, bone, ber

Secondary materials: material developed from acombination or treatment of primary materials:leather, cement, paint, pigments, cloth, metal, glass

Tertiary materials: materials made from chemicalmanipulation: alloys, polymers, semiconductors,composites

A process does not become successful without observa-tion and communication. One of the most important devicesdeveloped for a process was the pump. The rst pistonpump was invented by Ctesibius of Alexandria, a Greekphysicist and inventor born around 300 b.c. One of hisbetter known engineering efforts was improvement of thewater clock. A water clock keeps time by means of drippingwater maintained at a constant rate. His ideas of rene-ment of the water clock allowed for accurate timekeeping.The accuracy of his water clock was not improved uponfor 1500 years. The second invention he is noted for isthe water organ, the precursor of the hydraulic pump. Thiswas a mechanized device in which air was forced by waterthrough organ pipes to produce sounds. At rst glance onewould be in error not to think of the vast number of appli-cations such a device could have. There are hundreds ofdifferent pumps in any given process plant. The conceptof conveying gas or liquids without a pump is unheard of

3

4 INTRODUCTION

today. This invention resulted from observation of one ofhis rst inventionsa counterweighted mirror.

Ctesibius was born the son of a barber, and like manygood sons he tried to follow in his fathers footsteps.Perhaps it was a good thing that he spent more time thinkingabout how to improve his fathers trade than in clippingbangs. He invented a device: a mirror placed at the end ofa tubular pole, with a lead counterweight of the exact sameweight placed at the other end that allowed the mirror tobe adjusted for each customer. He noticed that when hemoved the mirror, the weight bounced up and down whilemaking a strange whistling noise. He theorized that thisnoise was air escaping from the tube. He tinkered withvarious dimensions and escape holes, which led to otherobservations and inventions using the power of pressure,gases, and liquids to achieve certain results. Without thesemusings the piston pump might never have came into being.

Pumping water for consumption, irrigation, and washingchanged human society. If a stable water source was found,the water could be transported with minimal laborallthat was needed was a pump. People no longer had tomove repeatedly to new areas to nd food and water. Theycould stay put, farm, and live. In doing so, cities wereestablished. With a high concentration of people, the oddsof more improved processes increased exponentially. Withthe increased demand for improved comfort and greatercommercial prots came a higher concentration of thinkers.Some people despise the modern city, but it must beadmitted that cities are responsible for generating many ofthe ideas that make the rest of society ourish.

Mechanical means to move gases and uids are essentialin any process plant, but so is chemical manipulation.Perhaps the rst known form of manipulating somethingchemically would be the cooking of food. With cooking,meats, grains, and vegetables become easier to digest andtransport, and spoilage is reduced. Adding heat requiresa fuel source and a means to control the thermal output.Being able to heat a substance in a controlled fashion ona larger scale introduced materials such as alloys, glass,and a whole host of chemicals. This process requiredfurnaces and valves, among other devices. The second greatfeat of chemical manipulation is fermentation followedby distillation. Fermention of grains and berries has beencarried out for tens of thousands of years. Humans arenot the only creatures to enjoy a good buzz. Manyanimals will have a party ingesting fermented berries andfruit. The ethanol produced provides a feeling of euphoria.One cannot blame any creature for wanting to feel better,but hopefully, it doesnt get in the way of the successof a species. To be able to separate alcohol from waterrequires observing condensation, fashioning a controllableheat source, and qualitative analysis. Alcohol is not justfor drinking; it is actually a very valuable solvent, and theprinciples needed to understand how to make and distill

alcohol are the very reasons that humans have becomeso successful. Without knowledge of the principles offermentation and distillation, our heat, shelter, clothing,transportation, medicines, food, and materials would notexist as we know them.

The most inuential industry to date is petroleumrening. Distillation is the main process in petroleumrening. Pharmaceuticals, building materials, solvents,plastics, and various fuels are all a result of the controlleddistillation of crude oil. All this came about from therenement of fermented grain. In fact, it is fair to say thatwithout fermentation, we would not have progressed muchfurther than the Cro-Magnons. Think about that the nexttime you sip a beer or enjoy a glass of wine or Scotch.

The effort that unfolds over the next several hundredpages is an undertaking that convinced over thirty ofthe worlds top academic and engineers to embark ona project that is encyclopedic in nature. Talented andpracticing experts in process plant engineering from Asia,Africa, Europe, the Middle East, and North America havecontributed chapters to this book: all intended to helpthe reader to understand and implement best practices inprocess plant equipment operations, reliability, and control.The book is a comprehensive text that will provide thereader with access not only to fundamental informationconcerning process plant equipment but also with accessto practical ideas, best practices, and experiences ofhighly successful engineers from around the world. Thebook is divided into three sections: Section I, ProcessPlant Equipment Operations; Section II, Process PlantReliability; and Section III, Process Measurement, Control,and Modeling. An overview of the main highlights of thevarious chapters follows.

Section I: Process Equipment Operation

Chapter 2: Valves This chapter provides an introduc-tory description of control valves, their types, and selec-tion criteria, sizing procedures, operating principles, andmaintenance and troubleshooting methods. It also describescommon problems suffered by control valves and theirremedies. Procedures for preventive and predictive main-tenance of control valves and nonintrusive methods fordetection of valve stiction are also discussed briey.

Chapter 3: Pumps Water and other liquids are thelifeblood of many industrial processes. If those uids are theblood, the plumbing system makes up the veins and arteries,and the pump is the heart. This chapter touches briey onseveral types of industrial pumps, but deals primarily withthe most common type, the centrifugal pump. Most of theprinciples apply to other types of pumps, but regardlessof the type of pump in use, the pump manufacturersmanual and recommendations should always be followed.

INTRODUCTION 5

The chapter also provides the following: general termscommonly used in the pump industry; brief informationon several different types of pumps that may allow a userto identify what type of pump is either in use or neededfor a particular application; basic component descriptionscommon to centrifugal pumps; instructions on how to reada typical pump performance curve; categories of differenttypes of pump applications; how to size and select a pumpproperly, including net positive suction head calculationsand considerations; proper pump maintenance; and basicpump troubleshooting guidelines.

Chapter 4: Pipes Pipelines are one of the main meth-ods of transporting oil and gas worldwide. Historically,pipelines have been the safest means of transporting nat-ural gas and hazardous liquids. The integrity, safety, andefciency of a pipeline system is important and key tooperators. Based on these considerations, this chapter cov-ers mainly pipe types and pipe selection strategy, includingpipe strength, toughness, weldability, and material; pipelinenetwork design; pipe problems; pipeline inspection; andpipe maintenance.

Chapte 5: Cooling Towers Cooling towers are the mostbasic type of evaporative cooling equipment used primarilyfor process water cooling purposes in many chemical plants.Their principal task is to reject heat to the atmosphereand they are deemed a relatively inexpensive and reliablemeans of removing heat from water. Basically, hot waterfrom heat exchangers or other units will be sent to acooling tower and the water exiting the tower (which iscooler) will be sent back to the heat exchanger for coolingpurposes.

Chapter 6: Filters and Membranes Filters and mem-branes are used in vast industrial processes for the separa-tion of mixtures, whether of raw process media materials,reactants, intermediates, or productscomprising gases,liquids, or solutions. This chapter identies gas and liquidltration covering solidliquid separations, solidgas sep-arations, solidsolid separations, liquidliquid separations,and liquidgas separations. It includes membrane technol-ogy such as microltration, reverse osmosis, ultraltration,and nanoltration. It is a complete reference tool for allinvolved in ltration as well as for process personnel whosejob function is ltration.

Chapter 7: Sealing Devices This chapter covers a varietyof gasket types, compression packing, mechanical seals, andexpansion joints. Discussed are materials of construction,principles of operation, and applications of sealing products.Wherever there are pumps, valves, pipes, and processequipment, there are sealing devices. Although relativelylow in cost, sealing devices can have huge consequences if

they dont work as needed or if they fail. All these devicesare used in process industries and are critical to plant safetyand productivity.

Chapter 8: Steam Traps A steam trap is a deviceattached to the lower portion of a steam-lled line or vesselwhich passes condensate but does not allow the escape ofsteam. It is also a piece of equipment that automaticallycontrols condensate, air, and carbon dioxide removal froma piping system with minimal steam loss. Hot condensateremoval is necessary to prevent water hammer, which iscapable of damaging or misaligning piping instruments. Airin the steam system must be avoided, as any volume ofair consumes part of the volume that the system wouldotherwise occupy. Apart from that, the temperature ofthe airsteam mixture normally falls below that of puresteam. It has been proven that air is an insulator andclings to the pipe and equipment surfaces, resulting in slowand uneven heat transfer. This chapter covers the varioustypes and classication of steam traps and their installation,common problems, sizing, selection strategies, application,and maintenance.

Chapter 9: Process Compressors This chapter dealswith compressors used in the process industry. Basic theorywith practical aspects is provided in sufcient detail for theuse of process industry personnel.

Chapter 10: Conveyors This chapter takes into accountthe types of conveyors been manufactured by modernindustries to meet the current challenges encountered inconveying operations. It enumerates their usefulness, whatconveyors are, industries that use them, conveyor selectionand types, and safety and maintenance.

Chapter 11: Storage Tanks Storage tanks pose a com-plex management problem for designers and users. Becauseof the wide variety of liquids that must be stored, some ofwhich are ammable, corrosive, or toxic, material selectionfor tanks is a critical decision. This chapter provides gen-eral guidelines that will aid in the selection of the propertype of storage to be used in a particular application. Var-ious codes, standards, and recommended practices shouldbe used to supplement the material provided. Manufacturersshould be consulted for specic design information pertain-ing to a particular type of storage.

Chapter 12: Mixers Effective mixing of solids, liquids,and gases is critical in determining the quality of food, phar-maceuticals, chemicals, and related products. It is thereforeessential that research and development scientists, pro-cess and project engineers, and plant operational personnelunderstand the mixing processes and equipment. Mixingprocesses may be batch or continuous and may involve

6 INTRODUCTION

materials in combination of phases such as liquidliquid,liquidsolid, liquidsolidgas, liquidgas, and solidsolid(free-owing powders and viscous pastes). An understand-ing of mixing mechanisms, power requirements, equipmentdesign, operation and scale-up, and maintenance will leadto maximizing the mixing performance and enhancing busi-ness protability.

Chapter 13: Boilers A boiler is process equipmentcomprising a combustion unit and boiler unit, which canconvert water to steam for use in various applications.Boilers are of different types and generally work withvarious ttings, retrots, and accessories. Boiler efciencyis achieved by skillful maintenance practices, includingpreventive and repair maintenance, in addition to use ofonly suitably conditioned water as feed water.

Section II: Process Plant Reliability

Chapter 14: Engineering Economics for Chemical Pro-cesses This chapter presents basic tools and methodsused traditionally in engineering to assess the viabilityand feasibility of a project. Presented rst are the toolsavailable to represent money on a time basis. Next, themathematical relationships frequently used to model dis-crete cash ow patterns are presented. The equivalencebetween the different discrete models is included on thissection. The various indexes available to select the mostprotable project between a set of alternatives are then pre-sented. In this section, the payback period, the minimumacceptable rate of return, and the internal rate of return areintroduced. An illustrative case study showing the appli-cation of these concepts is presented at the end of thissection. The methods available to perform cost estimationand project evaluation are presented next, including severalexamples to show the application of cost estimation tech-niques. Companies execute engineering projects based onthe revenues expected. Accordingly, they invest time andmoney in the process of selecting the project that wouldreturn the maximum revenues and satisfy such projectconstraints as environmental and government regulations.Therefore, the tools, techniques, and methods presented inthis chapter would be used by engineers to assist themin the selection of the most suitable engineering projectand to accurately estimate the costs associated with theproject.

Chapter 15: Process Component Function and Per-formance Criteria This chapter explores the basic andadvanced concepts of material transfer and conveyanceequipment for air, steam, gases, liquids, solids, and pow-ders. Also included are the engineering considerations forthe component construction for material transfer. Eachcomponent section consists of a portion dedicated to

selection specications, reliability and cost savings, vari-ous maintenance approaches, and process development andimprovement of transfer systems.

Chapter 16: Failure Analysis and Interpretation ofComponents This chapter highlights the fact that under-standing how a component or device fails is essential indeveloping a scheme as to how to increase reliability andsystem robustness and ultimately reduce operational costs.There are essentially only four reasons for failure: the mate-rial, the methods, the machine, or the man. To identify thesource of failure requires an understanding of the signs ofthe various sources. This chapter provides a fundamentalexplanation of failure by helping organize information tomake the failure assessment a logical process.

Chapter 17: Mechanical Integrity of Process Vesselsand Piping This chapter builds a focused and practicalcoverage of engineering aspects of mechanical integrityas it relates to failure prevention of pressure boundarycomponents in process plants. Principal emphasis is placedon the primary means of achieving plant integrity, whichis the prevention of structural failures and failure ofpressure vessels and piping, particularly any that could havesignicant consequences. It provides practical concepts andapplicable calculation methodologies for the tness-for-service assessment and condition monitoring of processpiping systems and pressure vessels.

Chapter 18: Design of Pressure Vessels and PipingThis chapter covers the basic principles behind the designequations used in pressure vessels, and piping design codes.The design procedures for vessels and pipes are outlined.Numerical examples have been used to demonstrate someof the design procedures. This chapter is not intended toreplace design codes but rather to provide an understandingof the concepts behind the codes.

Chapter 19: Process Safety in Chemical Processes Inthis chapter risk analysis and equipment failure are pro-vided; process hazard analysis and safety rating are studied;safe process design, operation, and control are highlighted;and risk assessment and reliability analysis of a processplant are examined.

Section III: Process Measurement, Control,and Modeling

Chapter 20: Flowmeters and Measurement There aremany different methods of measuring uid ow, which areuseful but can be very confusing. The objective of thischapter is to unravel some of the mysteries of ow technol-ogy selection and teach how different owmeters work andwhen and when not to use them. This chapter covers the

INTRODUCTION 7

basics, including terminology, installation practices, owproles, ow disturbances, vercation techniques, owme-ter selection, and troubleshooting.

Chapter 21: Process Control Process control is used tomaintain a variable in a process plant at a set point or causeit to respond to a set point change. The most commonmethod used in process control is the PID (proportional,integral, and derivative) control algorithm. This algorithmand how it is used are discussed in this chapter.

Chapter 22: Process Modeling and Simulation Thiswork serves as a guide and deals with the basic require-ments for developing a model of a process. It covers thebasic steps necessary for developing either a dynamic orsteady-state model of a process. The case studies provided

are made as simple as possible and make it possible forstudents and nonexperts to develop a simple model ofa process that will help them investigate the behaviorof either the entire process plant or a unit operation ofinterest.

As any experienced bushman on the Savannah knows,you can only eat an elephant one bite at a time. It issuggested that you take your time and read and digest eachchapter carefully. Feel free to write in the margins, highlightpassages, and quote as you see t (but please use soundjudgment concerning copyright laws!). Most important,use this work as a tool. Information can develop intoknowledge with proper application. With proper applicationand sound judgment, wisdom can come forth. This work isthe beginning of a very wise approach.

2VALVES

Ali Ahammad Shoukat ChoudhuryBangladesh University of Engineering and Technology, Dhaka, BangladeshChikezie NwaohaControl Engineering Asia, Ten Alps Communications Asia, Aladinma, Nigeria

Sharad VishwasraoVigilantPlant Services, Yokogawa Engineering, Asia, Singapore

Control valves are the most commonly used actuatorsor nal control elements in process industries. Theymanipulate the owing uids to keep the variables beingcontrolled in the desired positions. A control valve is knownas the nal control element because it is the elementthat ultimately manipulates the value of the variable inthe control process. It is dened as a mechanism that altersthe value of the variable being manipulated in responseto the output signal from a controller, whether automatic,manual, or by direct human action. It is the element thatimplements the decision of the controllers. Controllers canbe set in either automatic or manual mode control. A cross-sectional diagram of a typical pneumatic control valve isshown in Fig. 2-1. The purpose of the valve is to restrict theow of process uid through the pipe that can be seen at thevery bottom of the gure. The valve plug is attached rigidlyto a stem that is attached to a diaphragm in an air pressurechamber in the actuator section at the top of the valve.When compressed air is applied, the diaphragm moves upand the valve opens. The spring is compressed at the sametime. The valve illustrated in Fig. 2-1 is a fail-closed type ofvalve because when the air pressure is reduced, the springforces the valve to close.

A control valve has three basic components:

1. Actuator . Most actuators are pneumatic. Usually, anactuator works with the help of a diaphragm and instrument

Process Plant Equipment: Operation, Control, and Reliability, First Edition. Edited by Michael D. Holloway, Chikezie Nwaoha, and Oliver A. Onyewuenyi. 2012 John Wiley & Sons, Inc. Published 2012 by John Wiley & Sons, Inc.

air. This is the device that positions the throttling element(i.e., the valve plug inside the valve body).

2. Valve body subassembly . This is the part where thevalve plug, valve seats, and valve casing are located.The valve body and the valve plug differ in geometryand material construction. The combined body and pluggeometry determines the ow properties of the valve.There are through-ow, blending, and stream-splitting typesof congurations. Similarly, valve seats also differ inconstruction. There are conventional and contoured valveseat types with parabolic and quick-opening plugs whoseinternals can be inspected only during servicing.

3. Accessories . These include positioners, I/P (current-to-pressure) transducers, and position sensors.

In the process industries, hundreds or even thousands ofcontrol loops are in use to produce marketable end products.Many of these valves are housed in an attractive fashion,as shown in Fig. 2-2.

Typically, a control loop consists of three major ele-ments: a sensor and transmitter, a controller, and a controlvalve. A feedback control loop is shown in Fig. 2-3. Thecontrol loop is a closed system consisting of selectedinstruments that work together as a unit with the singleobjective of controlling an identied variable. A loopconsists of a sensor that can be an orice, a thermocouple,

9

10 VALVES

Diaphragm

Stiction

Spring

Instrument air

PlugValve seat

Fluid outFluid in

P

Packing

Vent cap

Figure 2-1 Cross-sectional diagram of a pneumatic control valve.

Figure 2-2 Assembly of control valves.

or a venture meter; a transmitter, which can be either adifferential pressure electropneumatic or pneumatic trans-mitter; an indicator, which can be a pressure gauge, a levelgauge, or a temperature gauge; and a transducer, whichconverts the signal reported from the form manipulatedto a form understandable to the controller. The controllermakes the decision and sends it to an I/P converter that

converts the electric signal to a pneumatic signal and sendsit to the nal control instrument, or a positioner that givesproportional positional action to the valve stem so as toposition the plug correctly in the valve body and, nally,regulates the ow (Fig 2-3).

2.1 TYPES OF CONTROL VALVES

A variety of types of control valves are used in all sectorsof the process industries, depending on the suitability of avalve for a process. Two general types of control valvesare based on their motion: linear-motion valves and rotary-motion valves.

2.1.1 Linear-Motion Control Valves

Linear-motion valves have a tortuous ow and lowrecovery. They can be offered in a variety of special trimdesigns and can throttle small ow rates. Most linear-motion valves are suitable for high-pressure applications.They are usually anged or threaded and have separablebonnets. Examples of linear-motion valves are gate valves,diaphragm valves and globe valves.

2.1.1.1 Gate Valves Gate valves are generally usedwhen a straight-line ow of uid and minimum restriction

TYPES OF CONTROL VALVES 11

Pipeline

Control valveSensor

FC

Figure 2-3 Components of a typical control loop arranged in a feedback conguration.

are desired. They are so named because the part that eitherstops or allows ow through the valve acts somewhat likethe opening and closing of a gate. When the valve iswide open, it is fully drawn up into the valve, leaving anopening for ow through the valve of the same size asthe pipe in which the valve is installed. Therefore, thereis little pressure drop or ow restriction through the valve.Gate valves are not usually suitable for throttling purposesbecause ow control would be difcult, due to the valvedesign, and the ow of uid slapping against a partiallyopen gate can cause serious damage to the valve. Gatevalves used in steam systems always have exible gates[26]. The reason is to prevent binding of the gate within thevalve when the valve is in the closed position. When steamlines are heated, they will expand, causing some distortionof valve bodies. If a solid gate ts snugly between the seatof the valve in a cold steam system, when the system isheated and pipes elongate, the seats will compress againstthe gate, wedging the gate between them and clampingthe valve shut. This problem is overcome by the use ofa exible gate. This allows the gate to ex as the valveseat compresses it, thus preventing clamping [27].

2.1.1.2 Diaphragm Valves In a diaphragm controlvalve, operating air from the pilot acts on the valvediaphragm. The substructure that contains the diaphragmis direct acting in some valves and reverse acting inothers. If the substructure is direct acting, the operatingair pressure from the control pilot is applied to the top ofthe valve diaphragm. If the substructure is reverse acting,the operating air pressure from the pilot is applied to theunderside of the valve diaphragm [26]. Diaphragm valvesare lined to pressures of approximately 50 psi. They areused for uids containing suspended solids and can beinstalled in any position. In this valve, the pressure dropis reduced to a negligible quantity. The only maintenancerequired in this valve is the replacement of the diaphragm,which can be done without removing the valve fromthe line.

2.1.1.3 Globe Valves These are probably the mostcommon valves in existence. The globe valve derives its

name from the globular shape of the valve body. However,positive identication of a globe valve must be madeinternally because other valve types may also have globularbodies [26]. Globe valve inlet and outlet openings are usedextensively throughout the engineering plant and other partsof the ship in a variety of systems. In this type of valve, uidpasses through a restricted opening and changes directionseveral times. It is used extensively for the regulationof ow.

2.1.2 Rotary-Motion Control Valves

Rotary-motion control valves have a streamlined ow pathand high recovery in nature. They have more capacity thanthat of linear-motion valves. This type of valve has anadvantage in handling slurries and abrasives. They are easyto handle because they are angeless and have an integralbonnet. Rotary-motion valves are designed to have highrangeability. Examples of this type of valve are butteryvalves, ball valves, and plug valves.

2.1.2.1 Buttery Valves The buttery valve is used in avariety of systems aboard vessels. These valves can be usedeffectively in saltwater, lube oil, and freshwater systems[25]. Buttery valves are light in weight, relatively small,quick acting, provide positive shutoff, and can be used inthrottling. This valve has a body, a resilient seat, a butterydisk, a stem, packing, a notched positioning plate, and ahandle. The resilient seat is under compression when it ismounted in the valve body, thus making a seal around theperiphery of the disk and both upper and lower points wherethe stem passes through the seat. Packing is provided toform a positive seal around the stem for added protectionin case the seal formed by the seat should become damaged.Buttery valves are easy to maintain [26]. The resilient seatis held in place by mechanical means, and neither bondingnor cementing is necessary. Because the seat is replaceable,the valve seat does not require lapping, grinding, ormachine work.

2.1.2.2 Ball Valves These are stop valves that use a ballto stop or start the ow of uid [25]. When the valve

12 VALVES

handle is operated to open the valve, the ball rotates toa point where the hole through the ball is in line withthe valve body inlet and outlet. When the valve is shut,which requires only a 90 rotation of the hand wheelfor most valves, the ball is rotated so that the hole isperpendicular to the ow openings of the valve body, andow is stopped. Most ball valves are of the quick-actingtype, but many are planetary gear operated [26]. This typeof gearing allows the use of a relatively small hand wheeland operating force to operate a fairly large valve butincreases the valve operating time. Ball valves are normallyfound in the following systems: desalination, trim and drain,air, hydraulic, and oil transfer. They are used for generalservice, high-temperature conditions, and slurries.

2.1.2.3 Plug Valves These are quarter-turn valves thatcontrols ow by means of a cylindrical or tapered plugwith a hole through the center which can be positionedfrom open to close by a 90 turn. They are used for generalservices slurries, liquids, vapors, gases, and corrosives [26].

Other types of control valves are used either to controlthe ow of uids or to control the pressure of uids:nonreturn valves and relief valves.

2.1.3 Nonreturn Valves

Also known as reux valves or check valves , these valvespossess automatic devices that allow water to ow in onedirection only (Fig. 2-4). They are made of brass or gunmetal. Usually, a valve is pivoted at one end and can reston a projection on the other end. This valve is provided inthe pipeline that draws uid from the pump [27]. Whenthe pump is operated, the valve is open and the uidows through the pipe. But when the pump is suddenlystopped or fails due to a power failure, the valve is closedautomatically and the uid is prevented from returning tothe pump [28].

2.1.4 Relief Valves

Relief valves are also known as pressure relief valves ,cutoff valves , or safety valves [25]. These are automatic

Projection

Pivot

Fluid inFluid out

Figure 2-4 Nonreturn valve.

valves used on system lines and equipment to preventoverpressurization. Relief valves normally have a spring,and the power of the spring is adjusted such that avalve always remains in the closed position up to somepermissible uid pressure in the pipeline. When the pressureof the uid suddenly exceeds the permissible pressure, thevalve opens (lifts) automatically and the excess pressureis released instantaneously and then resets (shuts). Thus,the pipeline is protected from bursting. These valves areprovided along the pipeline at points where the pressureis likely to increase. Other types of relief valves are high-pressure air safety relief valves (PRVs) and bleed air surgerelief valves. Both are designed to open completely at aspecied lift pressure and to remain open until a specicreset pressure is reached, at which time they shut [25].However, the PRV is also the one piece of equipment thatwe hope never needs to operate. Because the PRV is thelast line of defense against the catastrophic failure of apressurized system, it must be maintained in like newcondition if it is to provide the condence necessary tooperate a pressurized system.

2.2 CONTROL VALVE ACTUATORS

A control valve, typically outtted with an actuator,provides the nal control element in many process systems.The actuator accepts a signal from an external sourceand, in response, positions (opens or closes) the valve tothe position required or designed. Valve actuators enableremote operation of control valves, which is essential forworker safety in many application environments. Actuatorscan be moved into position by either hydraulic, air/gas, orelectric signals. Typical control valve position commandsinclude more closed, more open, fully closed, and fullyopen. There are different types of control valve actuators,and they are classied according to the power supplyrequired for activation. Types of valve actuators includepneumatic valve actuators, electric valve actuators, andhydraulic valve actuators.

2.2.1 Pneumatic Valve Actuators

A pneumatic valve actuator is a control valve actuatorthat can adjust the position of the valve by convertingair pressure into rotary or linear motion. Rotary motionactuators are used on buttery valves, plug valves, andball valves, and they position from open to closed bya 90 turn [30]. Meanwhile, linear motion actuators areused on globe valves, diaphragm valves, pinch valves,angle valves, and gate valves, and they employ a slidingstem that controls the position of the element (closure).Pneumatic valve actuators can be single- acting, in thatair actuates the valve in one direction and a compressed

CONTROL VALVE SIZING AND SELECTION 13

spring actuates the valve in the other direction. Single-acting devices can be either reverse-acting (spring-to-extend) or direct-acting (spring-to-react). The operatingforce is generated from the pressure of the compressedair. Choosing between reverse-acting and direct-acting isdependent on the safety requirements (in the event of acompressed supply air failure), response/activation time, airsupply pressure, and so on. For example, for safety reasonssteam valves must close upon failure of the air supply.Pneumatic valve actuators have the advantage of simpleconstruction, requiring little maintenance, and a quick valveresponse time to changes in the control signal.

2.2.2 Electric Valve Actuators

An electric valve actuaor is a compact valve actuator witha large stem thrust. Electric valve actuators are typicallyemployed in systems where a pneumatic supply is notneeded or available. An electric valve actuator is morecomplex than a pneumatically operating valve actuator.When control valves are spread out over large distances, asis often the case in pipeline applications, an electric valveactuator should be chosen for purely economic reasons (i.e.,because electrical energy is cheaper and easier to transportthan instrument air and/or hydraulic uid). Electric valveactuators rely on an electrical power source for theirposition signal [31]. They employ single- or three-phaseac/dc motors to move a combination of gears to produce thedesired level of torque. Subsequently, the rotational motionis converted into a linear motion of the valve stem via a gearwheel and a worm transmission. Electric valve actuators areused primarily on linear motion valves, globe valves, andgate valves. They are also used on quarter-turn valves suchas buttery valves and ball valves. Linear electric valveactuators are installed in systems where tight tolerancesare required, whereas rotary electric valve actuators aresuitable for use in packaging and electric power. Electricvalve actuators have the disadvantage of valve response,which can be as low as 5 s/min.

2.2.3 Hydraulic Valve Actuators

Hydraulic valve actuators usually employ a simple designwith a minimum of mechanical parts. Hydraulic valveactuators convert uid pressure into linear motion, rotarymotion, or both. Like electric actuators, they are alsoused on both quarter-turn and linear valves. In quarter-turn valves, the hydraulic uid provides the thrust, whichis converted mechanically to rotary motion to adjust thevalve. For linear valves, the pressure of the hydraulicuid acts on the piston to provide the thrust in a linearmotion, which is a good t for gate or globe valves[31]. Hydraulic valve actuators are used particularly insituations where a large stem thrust is required, such as

the steam supply in turbines or the movement of largevalves in chimney ues. In a situation where very largevalves are to be actuated, it is often advisable to installthe actuators on mechanical gearboxes to provide increasedoutput (torque). There are different types of hydraulic valveactuators that convert linear motion to rotary motion. Forexample, whereas diaphragm actuators are generally usedwith linear motion valves, they can also be used for rotarymotion valves if they are outtted with linear-to-rotarymotion linkage. Similarly, lever and link actuators transferthe linear motion of a piston. Rack-and-pinion actuatorstransfer the linear motion of a piston cylinder to rotarymotion, and scotch yoke actuators convert linear motion torotary motion as well. For safety reasons, most hydraulicactuators are provided with fail-safe features: either failopen, fail close, or fail stay put.

For a control system to be effective, the control valvemust adjust to its desired position as quickly and efcientlyas possible. To achieve this, the right valve actuator must beselected for the application. Therefore, it could be said thatthe valve actuator specication process is more importantthan the selection of the control valve itself. To ensurethat the right valve actuator is chosen for a given process,critical site information, such as the availability of powersupply, hydraulic uid pressure, and air pressure, must beconsidered. In addition, the stroke time of the valve, fail-safe position, control signal input, and safety factors mustbe given due consideration.

2.3 CONTROL VALVE SIZING AND SELECTION

Control valve sizing is a procedure by which the dynamicsof a process system are matched to the performancecharacteristics of a valve. This is to provide a control valveof appropriate size and type that will best meet the needsof managing ow within that process system.

The task of specifying and selecting the appropriatecontrol valve for any given application requires an under-standing of the following principles [23]:

How uid ow and pressure conditions determinewhat happens inside a control valve

How control valves act to modify pressure and owconditions in a process

What types of valves are commonly available How to determine the size and capacity requirements

of a control valve for any given application How actuators and positioners drive the control valve How the type of valve inuences the costs

Selecting the right valve for the job requires that theengineer is able to:

14 VALVES

Ensure that the process requirements are denedproperly

Calculate the ow capacity required over the operatingrange

Determine any limiting or adverse conditions, such ascavitation and noise, and know how to deal with them

Know how to select the valve that will satisfythe constraints of price and maintainability whileproviding good performance during process control.

Valve sizing involves several steps. They can bedescribed briey as follows [34]:

1. Dene the system . To begin, the system should bedened properly, based on information regarding the uidand its density, the temperature, the pressures, the designow rate, the minimum ow rate, the operating ow rate,and the pipe diameter.

2. Dene the maximum allowable pressure drop. Themaximum allowable pressure drop across the valve shouldbe determined from the difference between the net positivesuction head available and the net positive suction headrequired. Its important to remember the trade-off: Largerpressure drops increase the pumping cost (operating), andsmaller pressure drops increase the valve cost because alarger valve is required (capital cost). The usual rule ofthumb is that a valve should be designed to use 10 to 15%of the total pressure drop, or 10 psi, whichever is greater.

3. Calculate the valve characteristic.

Cv = Q

G

P

where Q = is the design ow rate (gpm), G = the specicgravity relative to water, and P = the allowable pressuredrop across a wide-open valve.

4. Make the preliminary valve selection . The Cv valueshould be used as a guide in the valve selection along withthe following considerations:

a. Never use a valve that is less than half the pipe size.b. Avoid using the lower 10% and upper 20% of the

valve stroke. The valve is much easier to control inthe stroke range 10 to 80%.

Before a valve can be selected, one needs to decidewhat type of valve will be used. Is it a globe valve ora buttery valve? Equal percentage or quick opening?Depending on the valve type, the appropriate valvechart supplied by the manufacturer should be used toget the valve size or the diameter.

5. Check the Cv and stroke percentage at the minimumow . If the stroke percentage falls below 10% at theminimum ow rate, a smaller valve may have to be usedin some cases. Judgment plays a role in many cases. For

example, is the system more likely to operate closer tothe maximum ow rates more often than close to theminimum ow rates? Or is it more likely to operate nearthe minimum ow rate for extended periods of time? Itsdifcult to nd the perfect valve, but you should nd onethat operates well most of the time. At the minimum owrate, the Cv value should be recalculated. Then from thevalve chart, the stroke percentage should be determined.If the valve stroke is within 10 to 90%, the valve isacceptable. Note that the maximum pressure drop is tobe used in the calculation. Although the pressure dropacross the valve will be lower at smaller ow rates, usingthe maximum value gives the worst-case scenario and theconservative estimate. Essentially, at lower pressure drops,Cv would only increase, which would be advantageous inthis case.

6. Check the gain across the applicable ow rates . Gainis dened as

gain = ow stroke or travel

Valve gains should be calculated for the minimum ow rate,the operating ow rate, and design ow rate conditions. Forthese three ow conditions, two gains can be calculated,taking the operating ow as basis. The difference of thesetwo gains should not be more than 50% of the higher value.Also, the gain value should never be less than 0.50.

2.3.1 Selecting a Valve Type

When speaking of valves, it is easy to get lost inthe terminology. Valve types are used to describe themechanical characteristics and geometry (gate, ball, globevalves). From the ow characteristics, there are threeprimary types of control valves:

1. Equal percentage. Equal increments of valve travelproduce an equal percentage in ow change.

2. Linear . Valve travel is directly proportional to thevalve stoke.

3. Quick opening . In this type, a large increase in owis coupled with a small change in valve stroke.

So how do you decide which control valve to use? Hereare some rules of thumb for each:

1. Equal percentage (the most commonly used valvecontrol)a. Used in processes where large changes in pressure

drop are expectedb. Used in processes where a small percentage of the

total pressure drop is permitted by the valvec. Used in temperature and pressure control loops

COMMON PROBLEMS OF CONTROL VALVES 15

2. Lineara. Used in liquid level or ow loopsb. Used in systems where the pressure drop across

the valve is expected to remain fairly constant (i.e.,steady-state systems)

3. Quick openinga. Used for frequent onoff serviceb. Used for processes where an instantly large ow

is needed (i.e., safety systems or cooling watersystems)

2.3.2 Sizing and Selection: Letting the Computer DoIt All [2]There are two types of valve sizing software. The rst letsyou pick the valve type and then gives you the specics,such as rated Cv , FL, and Fd values, allowing you tocarry out the correct sizing calculations. The second typelets you enter only the ow conditions, and the computercalculates the Cv and selects the right valve, usually thebest economical choice. This software is vendor-specicand usually valid for valves from the same manufacturer.When selecting a program, the following things are to bekept in mind:

1. Valve sizing should accord with current ISA standardS75.01 or the corresponding IEC standard.

2. The noise equation should follow ISA standardS75.17 or IEC 534.8.3.

3. The required maximum Cv should not be more than85% of the rated Cv of the valve selected.

4. The minimum Cv should be greater than the Cv ofthe valve selected at 5% of valve travel.

5. Pay attention to the cavitation or ashing warnings.They may indicate trouble.

Following is a partial list of vendors offering computerprograms for control valve sizing and selection.

H. D. Baumann Inc.35 Mironal RoadPortsmouth, NH 03801

Eckardt AGPostfach 50 03 47D-7000 Stuttgart 50Germany

Engineered Software, Inc.P.O. Box 2514Olympia, WA 98507

Fisher Controls International Inc.295 South Center StreetMarshalltown, IA 50158

Gulf Publishing Co.Houston, TX 77252

Instrumentation Software, Inc.P.O. Box 776Waretown, NJ 08758

ISAP.O. Box 12277Research Triangle Park, NC 27709

Masoneilan DresserDresser Valve and Controls Division275 Turnpike StreetCanton, MA 02021

Neles-Jamesbury, Inc.P.O. Box 15004Worcester, MA 01615

Valtek, Inc.P.O. Box 2200Springville, UT 84663

2.4 COMMON PROBLEMS OF CONTROLVALVES

Control valves suffer major problems when in use. Themost common problems are described below.

2.4.1 Control Valve Cavitation

Cavitation is a two-stage transformation of the initialformation of vapor bubbles by the owing uid and thenthe reverse: bubbles back to the liquid at high downstreampressure. For the bubbles to form the static pressure of theowing liquid falls below the uid vapor pressure. Thebubbles eventually collapse at the downstream pressure,which is higher than the vapor pressure of the liquid.

In a pure liquid distribution system involving a highpressure differential and high ow rates, automatic controlvalves tend to vibrate and make excessive noise. Thenoise and vibration problems are safety hazards. In valves,cavitation is therefore caused by a sudden and severe fallin pressure below the vapor pressure level and consequentvapor bubble formation as a result of excessive uidvelocity at the seating area. The energy released by theeventual collapse of vapor bubbles eats away the surfaces

16 VALVES

of the valve plug and seat. This causes loss of owcapacity and erosion damage. The collapse of vapor bubblescan cause local pressure waves of up to 1,000,000 psi. Fluidmicrojets are also formed, due to the asymetrical bubblecollapse. The high-intensity pressure waves combine withthe microjet impingement on the valve surface to causesevere damage to the valve.

2.4.1.1 Types of Cavitation It is usually not difcult todetermine whether a valve is cavitating. One has merelyto listen. However, to determine if the cavitation intensityis high enough to cause damage requires quantifyingthe intensity and comparing it with available experimen-tal cavitation reference data for the valve of interest.Cavitation intensity can be quantied relative to fourlevels.

1. Incipient cavitation refers to the onset of audible,intermittent cavitation. At this lower limit, cavitationintensity is slight. The operating conditions that fosterincipient cavitation are conservative and are seldom usedfor design purposes.

2. Critical cavitation , the next stage, describes thecondition when the cavitation noise becomes continuous.The noise intensity is often difcult to detect abovethe background ow noise. Critical cavitation causes noadverse effects and commonly denes the no cavitationcondition. This level is referred to as critical becausecavitation intensity increases rapidly with any furtherreduction in .

3. Incipient damage refers to the conditions underwhich cavitation begins to destroy the valve. It is usuallyaccompanied by loud noise and heavy vibration. Thepotential for material loss increases exponentially as drops below the value that initiates incipient damage.Consequently, this is the upper limit for safe operationwith most valves. Unfortunately, its the limit that is mostdifcult to determine, and experimental data are availablefor only a few valves.

4. Choking cavitation is a ow condition in whichthe mean pressure immediately downstream from thevalve is the uids vapor pressure. This represents themaximum ow condition through a valve for a givenupstream pressure and valve opening. It is a conditionthat damages both valve and piping. Choking cavitationis an interesting and complex operating condition. Eventhough the valve outlet is at vapor pressure, the downstreamsystem pressure remains greater. Reducing the downstreampressure increases the length of the vapor cavity but doesntincrease the ow rate. The noise, vibration, and damageoccur primarily at the location where cavity collapseoccurs.

2.4.1.2 Cavitation Prevention Strategies1. Applying two valves in series . In the case of extreme

high-pressure differentials, two valves installed in serieseffectively mitigate the incidence of cavitation. The secondvalve acts as a backup when the rst valve fails and alsoensures pressure reduction to some level [10]. The problemsassociated with this method are lack of enough space toinstall two valves and the cost of the second valve.

2. Applying orice plates . Devices that produce back-pressure, such as orice plates, can be used downstreamof a valve to prevent cavitation. The orice plate has theadvantages of low ongoing and installation costs. It hasthe disadvantage of being only effective within a narrowow range and can cause reduction of ow capacity withinthe system. This can possibly cause cavitation, thereby cre-ating a potential for damage to downstream ttings, soabsolute care must be taken to follow the manufacturersspecications [10].

3. Applying anticavitation valves and trim . The mosteffective approach to controlling valve cavitation is toinstall an anticavitation valve. Where an anticavitationvalve is in existence, it should be retrotted with ananticavitation trim. Equipping valves with anticavitationtrim is considered most often in systems where extremepressure differentials and high-velocity orates are present.The cavitation solution is self-contained either for anexisting valve equipped with anticavitation components orfor a new valve with trim [10]. This provides a widerrange of ow rates and smooth operation with low levelsof vibration and noise.

4. Designing a cavitation-free system . The best methodfor preventing cavitation is the inclusion of cavitationprevention measures as an integral part of the design ofthe distribution system. This involves a complete cavitationstudy before selection, purchase, and installation of valvesin a pipeline. Consulting valve manufacturers is necessaryto specify the proper size of a valve equipped withanticavitation trim or another option [10]. This methodoffers the advantages of lower maintenance cost, fewerequipment failures, less downtime, and optimum efciencyof systems.

2.4.2 Control Valve Leakage

There are two types of valve leakage:

1. Stem leakage. A loose or worn stem packing causesexternal leakage of the process uids, which may violateV.S. Environmental Protection Agency regulations. On theother hand, tight packing may cause excessive friction,which can make the loop performance unsatisfactory [6].

2. Valve seat leakage. Control valves are not shutoffvalves. Often, there may be uid leakage through the valve

COMMON PROBLEMS OF CONTROL VALVES 17

seat. Depending on the quantity of uid that passes throughthe leakage, valve seat leakages are classied into one ofsix categories.

Control valves are designed to throttle but they are notshutoff valves, so will not necessarily close 100%. A controlvalves ability to shutoff has to do with many factors, suchas the type of valve. A double-seated control valve has verypoor shutoff capability. The guiding, seat material, actuatorthrust, pressure drop, and type of uid can all play a partin how well a particular control valve shuts off. There areactually six different seat leakage classications, as denedby ANSI/FCI-70-2-1976 (rev. 1982). An overview of theseclassications is provided in Table 2-1.

2.4.3 Control Valve Nonlinearities

There are two types of nonlinearities that may be relatedto control valves. The rst type arises from the nonlinearcharacteristics of valves, such as their equal percentage,quick opening, and square-root characteristics. Usually, theeffect of these types of nonlinearities is minimized duringthe installation of valves, so that their characteristics arelinear. The second type of nonlinearity may appear due tomanufacturing limitations or gradual development of faults,Among these faults, deadband, hysteresis, backlash, andstiction are problems commonly found in control valvesand other instruments.

Control valves frequently suffer from such problems asstiction, leaks, tight packing, and hysteresis. Bialkowski [1]reported that about 30% of the loops are oscillatory, due

to control valve problems. In recent work, Desborough etal. [11,12] reported that control valve problems accountfor about one-third of the 32% of controllers classiedas poor or fair in an industrial survey [1]. If the controlvalve contains nonlinearities (e.g., stiction, backlash, anddeadband), the valve output may be oscillatory, which inturn can cause oscillations in the process outp