Process Control Basics - NET

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Process Control Basics

George Buckbee, PE

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NoticeThe information presented in this publication is for the general education of the reader. Because

neither the author nor the publisher has any control over the use of the information by the reader, both the author and the publisher disclaim any and all liability of any kind arising out of such use. The reader is expected to exercise sound professional judgment in using any of the information presented in a particular application.

Additionally, neither the author nor the publisher has investigated or considered the effect of any patents on the ability of the reader to use any of the information in a particular application. The reader is responsible for reviewing any possible patents that may affect any particular use of the information presented.

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Copyright © 2022 International Society of Automation (ISA)All rights reserved.

Printed in the United States of America.Version: 1.0

ISBN-13: 978-1-64331-130-2 (print)ISBN-13: 978-1-64331-131-9 (ePub)ISBN-13: 978-1-64331-132-6 (Kindle)

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Dedication

To my parents, George and Carol Buckbee

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vii

Contents

Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xiii

Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .xvii

About the Author . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xix

Chapter 1 What Is Process Control? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11.1 What Is a Process? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 What Is Control? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.3 Why Do We Need Process Control? . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.3.1 Process Safety and Reliability. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41.3.2 Process Quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51.3.3 Process Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61.3.4 Labor Costs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

1.4 How to Use This Book . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Chapter 2 Basic Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .92.1 General Principles of Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92.2 Pressure Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.2.1 What Is Pressure? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92.2.2 Units of Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112.2.3 Types of Pressure Sensors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

2.3 How Level Sensors Sense . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182.3.1 What Is Level? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182.3.2 Units of Level . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202.3.3 Types of Level Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

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2.4 How Flow Sensors Sense . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 242.4.1 What Is Flow? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 252.4.2 Units of Flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 252.4.3 Types of Flow Measurement. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

2.5 Temperature Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 312.5.1 What Is Temperature? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 312.5.2 Units of Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 312.5.3 Types of Temperature Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

2.6 Range, Span, and Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 362.6.1 Range and Span . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 362.6.2 Instrument Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 372.6.3 Truth in Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

Chapter 3 Control System Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 413.1 Closed Loop versus Open Loop Control . . . . . . . . . . . . . . . . . . . . . . . 41

3.1.1 Open Loop Control. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 423.1.2 Closed Loop Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

3.2 Single Loop Controller . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 433.3 Logic Control, Sequential and Batch Control . . . . . . . . . . . . . . . . . . . 44

3.3.1 Logic Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 443.3.2 Sequential Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 453.3.3 Batch Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

3.4 PLC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 463.4.1 Typical PLC Hardware . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 473.4.2 Typical PLC Software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

3.5 DCS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 533.5.1 Typical DCS Hardware . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 543.5.2 Typical DCS Software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

3.6 Hybrid Controllers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 563.7 Embedded Controllers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

Chapter 4 Communications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .594.1 Binary Communications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 594.2 Analog Communications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

4.2.1 Mechanical Communication . . . . . . . . . . . . . . . . . . . . . . . . . . . . 624.2.2 Communicating with Pressure: Pneumatics . . . . . . . . . . . . . . . 624.2.3 Communication with Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . 634.2.4 Communication with Current . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

4.3 Digital Communications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 644.3.1 Smart Device Communications . . . . . . . . . . . . . . . . . . . . . . . . . . 644.3.2 Wireless Communications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

4.4 Converting Between Signal Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . 664.4.1 I/O . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 674.4.2 Analog to Digital . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

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4.4.3 Digital to Analog . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 684.4.4 Transducers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

4.5 Displaying Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 684.5.1 Mechanical Displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 684.5.2 Electrical and Electronic Displays . . . . . . . . . . . . . . . . . . . . . . . . 694.5.3 Computer Displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

4.6 Retaining Data History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

Chapter 5 Final Control Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .735.1 Control Valves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

5.1.1 What Is a Control Valve? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 745.1.2 Types of Control Valves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 755.1.3 Control Valve Sizing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 785.1.4 Control Valve Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . 795.1.5 Performance Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 805.1.6 Control Valve Maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81

5.2 Dampers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 825.2.1 What Is a Damper? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 825.2.2 Damper Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 825.2.3 Damper Maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83

5.3 Actuators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 835.3.1 Type of Actuator Movement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 835.3.2 Actuator Control Action . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 845.3.3 Actuator Power Source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 845.3.4 Actuator Sizing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85

5.4 Switches and Positioners. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 855.4.1 Switches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 855.4.2 Positioners . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86

5.5 Variable Speed Drives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 865.5.1 What Is a Variable Speed Drive? . . . . . . . . . . . . . . . . . . . . . . . . . 865.5.2 Types of VSDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 865.5.3 Other Drives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 875.5.4 Drive Sizing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 875.5.5 Drive Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 885.5.6 Drive Maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88

5.6 Heaters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 885.6.1 Heater Sizing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 885.6.2 Heater Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 885.6.3 Heater Maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89

5.7 Other Final Control Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 895.8 Regulators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89

5.8.1 What Is a Pressure Regulator? . . . . . . . . . . . . . . . . . . . . . . . . . . . 895.8.2 Regulator Maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89

5.9 Limiting Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 895.9.1 What Is a Limiting Element? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89

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5.9.2 Types of Limiting Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 905.9.3 Maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90

Chapter 6 Continuous Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .936.1 Control Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 936.2 On-Off Control of Continuous Processes . . . . . . . . . . . . . . . . . . . . . . . 946.3 PID . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95

6.3.1 Proportional Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 966.3.2 Integral Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 986.3.3 Derivative Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1006.3.4 Controller Forms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1016.3.5 Controller Tuning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1026.3.6 Software Tools for Tuning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109

6.4 Selectors, Overrides, and Interlocks . . . . . . . . . . . . . . . . . . . . . . . . . . 1096.4.1 Selectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1096.4.2 Overrides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1106.4.3 Interlocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .111

6.5 Alarms, Messages, and Reports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1116.5.1 Alarms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1116.5.2 Messages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1126.5.3 Reports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112

Chapter 7 Logic Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1157.1 Binary Logic Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1167.2 How Binary Logic Control Works . . . . . . . . . . . . . . . . . . . . . . . . . . . . .116

7.2.1 Discrete Inputs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1167.2.2 Operations of Logic Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . .117

7.3 Binary Logic Diagramming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1217.3.1 Ladder Logic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1217.3.2 Boolean Logic Diagrams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1227.3.3 Sequential Function Chart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1227.3.4 Structured Text . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1237.3.5 Combining Programming Methods . . . . . . . . . . . . . . . . . . . . . 1237.3.6 PLC Programming Skills . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123

7.4 Implementation of Binary Logic Control . . . . . . . . . . . . . . . . . . . . . . 1247.4.1 Electromechanical Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1247.4.2 PLCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1267.4.3 Logic in DCSs and Hybrid Controllers . . . . . . . . . . . . . . . . . . . 127

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128

Chapter 8 Control System Documentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1298.1 Process Flow Diagrams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1298.2 P&ID Drawings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130

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xiContents

8.3 Instrument Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1328.4 Instrument Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1338.5 Piping Diagrams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1348.6 Control Narratives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1348.7 Loop Diagrams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1368.8 Logic Diagrams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1368.9 Control Loop Diagrams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1368.10 Network Diagrams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1378.11 Factory and Site Acceptance Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . 138References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139

Chapter 9 Practical Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1419.1 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .141

9.1.1 Wetted Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1419.1.2 Harsh Environments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142

9.2 Power Supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1429.2.1 Electrical Supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1429.2.2 Instrument Air . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143

9.3 Instrument Location . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1449.4 Accuracy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144

9.4.1 Electrical Signal Transmission . . . . . . . . . . . . . . . . . . . . . . . . . . 1459.4.2 Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1459.4.3 Other Settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146

9.5 Protecting the Instrument . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1469.5.1 Insulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1469.5.2 Enclosures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147

9.6 Maintenance and Reliability Considerations . . . . . . . . . . . . . . . . . . . 1479.6.1 Inspection and Monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1479.6.2 Automated Monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148

9.7 Management of Change . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1489.8 Cybersecurity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150

Chapter 10 Advanced Sensors and Analyzers . . . . . . . . . . . . . . . . . . . . . . . . . . 15110.1 Sensors for Process Fluids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151

10.1.1 pH and Oxidation-Reduction Potential . . . . . . . . . . . . . . . . . . 15210.1.2 Dissolved Oxygen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15310.1.3 Conductivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15310.1.4 Thermal Conductivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15310.1.5 Turbidity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15410.1.6 Infrared and Near-Infrared Spectroscopy . . . . . . . . . . . . . . . 15410.1.7 Consistency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15410.1.8 Humidity and Dew Point . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155

10.2 Physical and Mechanical Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . 15610.2.1 Photoelectric Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156

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xii Process Control Basics

10.2.2 Weighing Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15610.2.3 Vibration Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15710.2.4 Speed Detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157

10.3 Analyzers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15810.3.1 Sampling Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15810.3.2 Total Organic Carbon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15810.3.3 Mass Spectrometers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15910.3.4 Titration Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15910.3.5 Other Specialty Analyzers . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160

10.4 Combining Measurements: Virtual Sensors . . . . . . . . . . . . . . . . . . 160References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .161

Chapter 11 Beyond the Basics of Process Control . . . . . . . . . . . . . . . . . . . . . . 16311.1 Advanced Regulatory Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163

11.1.1 Cascade Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16311.1.2 Feedforward Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16611.1.3 Ratio Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16711.1.4 Signal Characterizers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16711.1.5 Gain Scheduling and Gap Controllers . . . . . . . . . . . . . . . . . . . 168

11.2 Model Predictive Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16911.3 Artificial Intelligence in Process Control . . . . . . . . . . . . . . . . . . . . . 171

11.3.1 Fuzzy Logic Controllers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17111.3.2 Neural Networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172

11.4 Safety Instrumented Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17311.5 Diagnostics and Analytics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173

11.5.1 Instrument and Valve Diagnostics . . . . . . . . . . . . . . . . . . . . . . 17311.5.2 Control Loop Performance Monitoring . . . . . . . . . . . . . . . . . . .174

11.6 Alarm Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .174References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175

Abbreviations/Acronyms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177

Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187

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xvii

Preface

Who Should Read This Book?If you work in or around process manufacturing, you certainly have had some expo-sure to process controls. Whether you are in operations, maintenance, sales, support, or management, this book will give you a basic understanding of process control. It will cover the terminology, principles, and applications of this field. If you are inter-ested in entering the career field of process control, this book will provide background on many aspects of the field, from design to operation and maintenance.

Process control is a critically important topic for modern manufacturing. Without modern control systems, it would be nearly impossible to efficiently produce commod-ities like pulp and paper, gasoline, plastic, and pharmaceuticals. The control system is like the eyes, ears, and nervous system of the plant. It senses, decides, and directs the activities of the pumps, valves, motors, and other equipment. The control system han-dles many routine tasks, freeing up the operator to oversee the operation and handle whatever new situations arise.

Yet most colleges, universities, and trade schools do not directly address the practical aspects of this field of study. University electrical and chemical engineer-ing programs focus on the theory and mathematics of controls but rarely discuss the practicalities of instrumentation and control system platforms. Most people learn pro-cess control through hands-on plant experience, accompanied by a healthy dose of self-study including reading books like this one, articles from trade magazines, and materials from industry organizations such as ISA.

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xviii Process Control Basics

This book covers the field of process instrumentation and provides many diagrams and practical examples to illustrate the application of those instruments. Reading this book will not make you an instrument engineer, but you will understand the basics and be able to ask the right questions, using the right lingo.

When it comes to control algorithms and equipment, you will learn the difference between discrete, continuous, and batch control. You will see that different control systems, programming languages, and documentation are needed for each. This book also addresses the management of control systems including discussions about main-tenance, change management, and documentation. Beyond the basics, one chapter introduces advanced control topics such as advanced regulatory control, multivariable control, and neural networks.

This book may be used in technical schools as an introduction to the basics of instrumentation, valves, and control. It may be useful to train salespeople, mainte-nance technicians, and operators. Or, it may help you cross the bridge into the exciting career field of process control.

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xix

About the Author

George Buckbee, PE, is a veteran of the process control industry. An experienced author and instructor, Buckbee has more than 30 years of practical experience improving pro-cess performance in a wide array of process industries including oil and gas, pulp and paper, pharmaceuticals, and consumer products. Buckbee holds a BS in chemical engineering from Washington University, and an MS in chemical engineering from the University of California, Santa Barbara.

Buckbee’s career includes direct industry experience at Procter & Gamble and Sanofi Pasteur, before entering the software development and control system con-sulting business at ExperTune. Currently head of performance solutions at Neles, he manages global digital solutions for valve monitoring, and the ExperTune family of products and services.

Buckbee was selected as an ISA Fellow in 2011, and he is a member of the ISA Publications Department. He is the coauthor (along with Joseph Alford) of Automation Applications in Bio-Pharmaceuticals, published by ISA. He wrote several chapters in The Instrument Engineer’s Handbook and authored Mastering Cascade Control and Mastering Split-Range Control. Buckbee has also written dozens of articles and white papers on control system performance.

Buckbee resides in Pennsylvania, where he and his wife Mary Ellen enjoy hiking, kayaking, golf, and traveling to visit their adult children.

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1

1What Is Process

Control?

1.1 What Is a Process?A process is any operation or sequence of operations involving a change in a substance being treated. Examples of processes include the following:

• A change of energy, such as from hot to cold

• A change of state, such as from liquid to gas

• A change of composition, as occurs in a chemical reaction or in mixing differentmaterials

• A change of dimension, as in grinding coal

This book considers processes in a broad sense. A process may be something as simple as pumping water from one place to another or as complex as deriving gasoline from the very complex mixture of chemicals in crude oil. Other industrial processes include pasteurizing milk, producing vaccines using bioreactors, crushing and grind-ing of mineral ores, generating steam to spin turbines and produce electricity, and turning trees into chips, then pulp, and finally into paper.

For all these processes, certain universal principles of measurement, control, and correction apply, although the hardware and techniques may differ greatly. The titles given to the people who supervise or direct the process may vary widely: plant opera-tor, anesthesiologist, airplane pilot, astronomer, radar technician, and many more.

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2 Process Control Basics

This book focuses on instrumentation and control as it is used in industry, par-ticularly the so-called process industries. These industries include oil refining, chemi-cal production, power generation, pulp and paper manufacturing, food and beverage processing, pharmaceutical production, mineral processing, and metal production. The hardware, software, and techniques mentioned in this book are typical for these industries.

Each process has several properties that may vary. Examples of properties are pres-sure, temperature, level, flow rate, acidity, color, quantity, viscosity, and many others. Each of these properties is known as a process variable. The changing values of the variables may be measured and sent to remote locations by means of signals. The measurements may be displayed, acted on for control, recorded or historized, trended, or analyzed.

1.2 What Is Control?A broad definition of control is to “determine the behavior or supervise the running of” some process. This definition applies directly to process control. As shown in Figure 1-1, there are three steps to all forms of control:

1. Measurement

2. Decision

3. Action

Measurement is accomplished with instruments and analyzers. The most common measurements in industrial plants are flow, level, pressure, and temperature. Other instruments measure acidity (pH), chemical composition, moisture content, color, and many other characteristics of the process.

Analyzers are typically more complex devices that may give a multivariable analy-sis of the process or of product quality. Chapter 2 covers basic instrumentation, and Chapter 10 covers more complex measurement devices, such as analyzers.

Figure 1-1. The three steps of control.

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9

2Basic

Measurements

2.1 General Principles of MeasurementYou cannot control what you do not measure. Measurement of the process is the basis for all forms of process control.

In this chapter, we look into the basics of process measurement. We will also intro-duce the four most common process measurements: flow, level, pressure, and tempera-ture. These measurements are found in all process industries.

2.2 Pressure Measurement2.2.1 What Is Pressure?Pressure is defined as the amount of force applied per unit of area.

If we place a 10-inch metal cube weighing 25 lb on a tabletop, the cube imposes a downward force of 25 lbf on the table. The contact area of the cube with the table is 10 in by 10 in, or 100 in2. The pressure, however, is the force divided by the contact area, 25 lbf divided by 100 in2, or 0.25 pounds per square inch (psi):

Pressure (P)Force (F)Area (A)

lbfin

= = = .25

1000 252 psi (2-1)

A technical note: 1 pound force, designated lbf, is defined as the force that a 1-pound mass exerts while resting at the surface of the earth, under standard gravita-tional acceleration of 32.1740 ft/s/s (foot per second per second). The mass of our metal

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cube does not change, but its weight (the measured force) could change from place to place because gravity varies over the surface of the earth. So, in some places, the cube could exert more or less than 1 lbf to the table.

If a person blows air through his or her lips, he or she creates a small, low-pressure wind with a force that can deflect a sheet of paper and not much more. An atmo-spheric wind with the same low pressure can move sailing ships because the pressure is exerted over the large area of the sail, thereby creating a very large force.

In the process industries, we are mostly concerned about the pressure of liquids or gases in the process. For example, we may want to know about the pressure of steam being delivered to a heating system, or the pressure of water at the exit of a pump.

There is a difference between pressure and force. Figure 2-1 shows two vertical pipes on supports; one pipe has an inside diameter of 2.0 in, and the other has an inside diameter of 8.0 in. The bottom of each pipe is closed, and the tops are open. Both pipes are filled with cold water to a level of 27.68 in. We can assume that the water weighs 62.4 lbf per cubic foot. For each pipe, the downward force on its support is the same as its weight of water (ignoring the weight of the pipe). The force is 3.14 lbf for the 2-inch pipe and 50.24 lbf, 16 times as much, for the 8-inch pipe. However, the pressures at the bottom of both pipes are identical at 1 psi; that is because the pressure depends on the height of water, not the volume.

For the 2-inch pipe:

Area ( ) .A r= =π 2 23 14 1× in (2-2)

Figure 2-1. Pressure versus force.

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41

3Control System

Basics

Within the field of process control, there are many different types of control. For exam-ple, some processes operate continuously and must only maintain a constant tempera-ture. Other processes are more complex. A batch process requires a controller that can start, open, close, heat, cool, and stop the process in an organized, structured, and repeatable manner.

Likewise, several different types of specialized hardware and software are used to perform the various types of control. Programmable logic controllers (PLCs) and dis-tributed control systems (DCSs) are heavily used in the process industries. Although each type of control system has a specialty, their capabilities overlap. So, you may find it useful to understand these similarities and differences.

This chapter covers the basic control algorithms and the control systems that are used to implement them.

3.1 Closed Loop versus Open Loop ControlAs discussed in Chapter 1, there are three steps to control:

1. Measurement

2. Decision

3. Action

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Simply put, closed loop control means that all three steps are accomplished auto-matically. Open loop control implies that one or more of those steps is accomplished manually.

3.1.1 Open Loop ControlHolding the speed constant while driving a car is an example of an open loop process. Even though the car automatically measures the speed, it is up to the driver to decide and take action. The driver must decide if the car is too slow or too fast, and then adjust the pressure on the accelerator. The driver continues to check the car’s speed every so often and make adjustments during the ride.

Another example of open loop control is opening a window to change the temper-ature of a room. When you feel warm, you may open a window to let in a cool breeze. When you start to feel too cold, you close the window.

In process plants, open loop control may be used for many reasons, including:

• No electronic instrument is available.

• There is no automated valve or final control element.

• The instrument or valve is broken.

• Tight control of the process is not needed.

When control loops operate in an open loop, sometimes called manual mode, the operator must pay close attention to prevent the process from drifting out of control.

3.1.2 Closed Loop ControlSome cars are equipped with cruise control to automatically control the speed. When the cruise control is engaged, the car’s speed is in closed loop control. The desired speed is called the set point. The controller stays in a closed loop until the driver disen-gages it, by tapping the brake pedal or turning it off.

Process plants have flows of material, energy, or both. Each temperature, pressure, level, and flow might be controlled by a closed loop controller. The flow of material or energy is manipulated at the command of a controller to keep a process variable at a desired value; this value is termed the set point. For example, a pressure controller manipulates a flow of gas, a level controller manipulates a flow of water, and a tem-perature controller manipulates a flow of electrical energy.

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59

4Communications

Instruments, controls, and valves are generally separate devices or systems that must communicate with each other. Signals are used to transmit information between devices. There are three basic types of signals: binary, analog, and digital. Mechanical, pneumatic, electric, electronic, and wireless media can be used to send the signals. This chapter explains some of the many ways that communications take place.

Signals are often referred to as inputs and outputs. The output of one device becomes the input to another. Figure 4-1 shows how the information from a level measurement is communicated between various devices and systems.

4.1 Binary CommunicationsA binary signal is the simplest type of signal. It has two discrete values, such as On and Off, or True and False, One and Zero, or Yes and No. Binary instruments may output a signal to indicate a simple process condition, such as:

• A relay may indicate a pump is running (or not running).

• A level switch may indicate the tank level is high (or not high).

• A valve limit switch may confirm the valve is closed (or not closed).

Binary signals may be transmitted over mechanical, pneumatic, analog, or digital media.

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It is possible to perform process control, even with these simple binary communica-tions. The automatic home thermostat is an example of a device that uses a binary signal to control the temperature of a room. If the room temperature is set to 70°F but the actual temperature is 65°F, the thermostat sends an On signal to the heating system. This turns on the furnace, fan, or heater. The temperature in the room rises until the set point of 70°F is exceeded. Then, the thermostat signal turns off. As the room cools down, the cycle repeats. There is no finer adjustment to the control; the furnace is either on or off.

Binary signals are sometimes called discrete signals. Somewhat confusingly, they are also sometimes called digital signals, with their inputs and outputs referred to as digital ins and digital outs. This is not to be confused with true digital communications, whereby complex electronic signals are passed between devices, as discussed in Section 4.3.

Discrete signals in process control may be gathered, communicated, and processed at very high speeds. For example, electric eyes may be used on a bottling line to detect or even to count the bottles as they pass by.

Figure 4-1. The chain of information.

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73

5Final Control

Elements

Final control elements are the workhorses of control. Without them, control would simply be a bunch of signals bouncing around without any impact on the real world. When a control valve opens, fluid flow increases and the process is affected.

Final control elements are generally physical, mechanical devices. It is important to pay close attention to their sizing, materials, and capacity. Also note that as physical devices, they are subject to wear and tear. Mechanical maintenance is critical to main-taining the reliability and performance of these devices.

This chapter provides an introduction to the most common final control elements used in modern industrial process plants, including:

• Control valves

• Dampers

• Variable-speed drives

• Heaters

At the end of the chapter is a discussion of regulators and limiting elements. These are relatively independent physical devices that perform some direct control of the process.

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5.1 Control ValvesThe control valve is the workhorse of the process control business. The vast majority of process control happens, in the end, because a valve opens or closes. An on-off valve operates in binary fashion: it opens or closes fully when actuated. A modulating con-trol valve can be adjusted to any position, from 0% to 100% open, thereby affecting the flow through the valve.

Control valve selection and application can be surprisingly complex. Valves may cost a few dollars or hundreds of thousands of dollars, depending on their size, materi-als, quality, pressure, temperature, and performance specifications.

5.1.1 What Is a Control Valve?A common notion is that every valve may be used as a control valve. However, that is far from the truth. A control valve is a valve that can be manipulated to precisely regu-late the flow of process fluids. Control valves must meet a demanding set of require-ments to ensure accurate and precise control.

Manually operated valves are also used throughout plants. They can be opened or closed with a lever, a wheel, or chain. These valves are not considered control valves. However, it is possible for an attentive operator to achieve some control of the process using manually operated valves.

The control valve must be properly designed, sized for the normal range of flows and pressures, made of the correct materials, and perform properly across the full range of operation. Each of these aspects of control valves is examined in this section.

Control valves do not work by themselves. Typically, a complete valve assembly consists of a valve body, an actuator, and oftentimes a positioner, as shown in Figure 5-1.

The valve body is a pressure housing through which fluid passes. The body has connections for piping or tubing. A movable valve plug changes position to adjust the flow. The plug may be a disk, a ball, or some other specialized design. The valve stem is connected to the plug and provides a connection to the actuator.

The actuator provides the motive force to open or close the valve. There are sev-eral types of actuators, including spring-and-diaphragm and electric motor–operated actuators. Compressed air is often used as the motive force for valve actuators.

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93

6Continuous

Control

When people talk about process control, they usually mean continuous process control—that is, the measuring, monitoring, and control of hundreds of signals across a large industrial process plant.

This chapter examines the terminology and algorithms associated with continu-ous control of processes.

6.1 Control TerminologyLike many fields of specialty, process control has its own language. Control techni-cians and engineers might seem to be talking in some kind of code as they resolve issues. Some of the more common process control terms are addressed here.

Controlled variable – This is the process condition that you would like to control. Usually, the controlled variable is a temperature, pressure, level, or flow. The controlled variable may also be called the process variable, or PV.

Set point – The set point is the desired value for the controlled variable. For exam-ple, the room temperature may have a set point of 72°F. The cruise control on a car may be set to 100 km/h. The set point is often abbreviated as SP or sometimes SPT.

Manipulated variable – This is what you adjust to control the controlled variable. For example, you may open a steam valve to increase the product temperature. The product temperature measurement is the controlled variable. The steam flow is the

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manipulated variable: it is manipulated by the steam valve. The signal to the steam valve is called the controller output, or CO.

Control loop – The control loop contains all the physical devices and control func-tions needed for the automatic control of a variable. It includes the process sensor (measurement), controller function (decision), and final control element (action) func-tions that were shown in Figure 1-1.

Closed loop control – This is when a process measurement of the controlled variable is used to automatically determine how to adjust the manipulated variable. Operators may refer to the controller being in “auto” when it is in closed loop control. Most important process variables are designed to be in closed loop control, so the operator does not need to manage so many variables directly.

Open loop control – This is when the manipulated variable is adjusted manually to try to maintain the controlled variable. The operator decides how much movement is needed for the final control element and then adjusts it. These adjustments may be through direct operation of the valve handle or wheel, or they may happen by setting a desired valve opening in the distributed control system (DCS) or programmable logic controller (PLC). Operators may refer to the loop being in “manual” when it is in open loop control.

Controller mode – Controllers may have many different modes of operation. The most common are manual and auto modes. However, there are more complex modes such as cascade, ratio, or override. When the mode is manual, the loop is in open loop control. When in other modes, the loop is in closed loop control.

Control algorithm – The set of mathematical rules or programming that deter-mines how to adjust the manipulated variable based on the controlled variable’s move-ment. Proportional-integral-derivative (PID) is a common control algorithm that is discussed further in Section 6.3.

Controller tuning – Each control algorithm may be fine-tuned to get the best per-formance from the process. This usually involves setting several tuning constants for each control loop.

6.2 On-Off Control of Continuous ProcessesCan an on-off controller be used to control a continuous process? Absolutely, it is done all the time. In fact, room temperature is commonly controlled with an on-off control-ler known as a thermostat. The thermostat simply turns the heat on or off whenever

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115

7Logic Control

Despite being continuous processes, many parts of the plant operate in binary fashion. Motors can be on or off. Some valves are either fully open or fully closed. Even analog measurements can be “discretized,” or converted to binary form:

• Temperature can be high or not high

• Tanks can be full or not full

• Pressures can be within limits or over limits

Logic control uses these binary signals to make Yes/No, On-Off decisions and control actions, such as:

• Turning a motor on or off

• Opening or closing a valve

• Turning a heater on or off

Discrete, or logic, control has often been managed on a separate platform from contin-uous control. Traditionally, programmable logic controllers (PLCs) performed the logic control while distributed control systems (DCSs) performed the continuous control. However, modern DCSs and PLC systems have significant overlap in functionality, and it is now possible to accomplish both discrete and continuous control on the same platform.

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7.1 Binary Logic ControlAs discussed in Chapter 1, all controllers use the measure–decide–take action approach. Logic control uses this same approach, using discrete information to make yes/no decisions and take discrete action on the process.

Figure 7-1 illustrates the concept of logic control. Inputs to the controller may include operator actions, control system actions, and discretized process information. The control system makes discrete decisions by looking at the discrete inputs and then applying logical operators such as AND, OR, and NOT. This results in a set of discrete outputs. Logic operations follow a clear set of rules to determine the outcome. The mathematics were developed by George Boole in the nineteenth century, and are known as Boolean logic.

The resulting outputs, or control actions, include electric signals to activate motors, pumps, electric heaters, on-off valves, lights, and many other electrical, hydraulic, and pneumatic devices.

7.2 How Binary Logic Control Works7.2.1 Discrete InputsThe logic controller works only with discrete information, such as Yes/No, On-Off, or Zero/One. Some process information occurs this way naturally. A pump motor can be running or not running. A valve can be open or not open. A push button can be pressed or not pressed.

Figure 7-1. The concept of logic control.

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129

8Control System Documentation

Process operations are large and complex. Equipment, processes, and controls may be provided by many different vendors. It is likely that the logic control of a plant is spread out among many different systems and many types of devices: programmable logic controllers (PLCs), distributed controls systems (DCSs), safety PLCs, and net-works of relays.

Engineers and technicians must learn to work between many different systems to understand how the whole process operates. Documentation, whether in paper or electronic form, is critical to fast, safe, and efficient installation, troubleshooting, and maintenance.

Most of the documents mentioned in this chapter are developed and conveyed from the project team to the plant maintenance and operation teams. However, not all documents will continue to be maintained after the plant is started up. Chapter 9 discusses change management and document maintenance.

8.1 Process Flow DiagramsWhen a new plant is being designed, some of the first documents created are the pro-cess flow diagrams, or PFDs. PFDs show the material flows, energy flows, and separa-tions in the production process. Key design information, such as expected flow rates and energy consumption, are shown on PFDs. Both normal operations and abnormal operations information may be conveyed on a PFD; Figure 8-1 shows an example.

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These diagrams are useful for understanding the overall operation of the process. PFDs give context to the process and are often used for operations training. Minimum and maximum flow and operations data from PFDs become the basis for selection of instruments and valves.

Also, PFDs are often used as the basis for designing operator interface graphics. However, the PFD does not usually contain much information about the instrumenta-tion or valves themselves. For that, another layer of detail is required.

Plants with hazardous locations should also have a set of hazardous location draw-ings. These drawings are used to determine instrumentation and wiring requirements in each part of the plant.

8.2 P&ID DrawingsPiping and instrumentation diagrams, or P&IDs, are one of the most crucial sets of documents for instrumentation and control. The P&ID shows plant vessels, piping, valves, and instruments. It also shows the continuous control strategy for the plant. To allow for ample context, P&IDs are often drawn on large C-size drawing sheets, which are 18 by 24 inches, or on even larger D-size 24- by 36-inch sheets. Figure 8-2 shows an example section of a P&ID.

Note that P&IDs are not drawn to scale. In fact, instruments shown near each other on a P&ID may actually be quite far apart in the physical plant.

Figure 8-1. An example PFD.

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141

9Practical

Considerations

This chapter addresses practical matters for instrumentation, wiring, valves, and con-trols. Most process plants contain harsh environments. Equipment may be outdoors exposed to extremes of sunlight, wind, dust, heat, cold, wet or dry conditions, and cycles of freezing and thawing. The process itself may include extremes of tempera-ture, pressure, and vacuum. Process fluids may be corrosive, abrasive, sticky, thick, lumpy, or even explosive. This chapter discusses considerations for selecting, install-ing, and maintaining control system hardware, especially field devices such as instru-ments and final control elements.

9.1 MaterialsPiping, instruments, and valves come in contact with process fluids and vapors over a wide range of conditions. Therefore, these devices must be made of proper materials to ensure a long and reliable life, without contaminating the process, failing in place, or creating safety hazards.

9.1.1 Wetted MaterialsThe term wetted materials refers to the parts of equipment that come into contact with process fluids. There are many variations in materials available for contact with the process. No single material is the best solution for all applications. For example, stain-less steel is commonly used in some applications but will corrode when in contact with chlorine ions. Polymer materials, such as polytetrafluoroethylene (PTFE), are chemi-cally resistant in many applications but cannot withstand high temperatures.

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Design engineering firms and equipment vendors have extensive experience in matching materials to the needs of each process application. They can make recom-mendations of proper material selection and highlight issues that others have faced in similar applications.

9.1.2 Harsh EnvironmentsIt is not only the wetted parts that must be considered. Leaks and vapors from the pro-cess may escape into the ambient conditions at the plant. Piping often vibrates at high frequencies due to pumps, compressors, and other rotating equipment. Equipment located outdoors is subject to swings of temperature and humidity, sunlight, rain, hail, earthquakes, and whatever other meteorological phenomena occur in the plant location.

Again, instruments and valves must be properly designed and made of suitable materials to withstand these environments. Electronic components are more delicate and may need special housings or cabinets. Section 9.5 discusses common practices for protecting instruments.

9.2 Power SupplyInstruments and valves are generally powered either by electricity or by compressed air. This section addresses the importance of maintaining a reliable supply of power to all devices.

9.2.1 Electrical SupplyThe main supply of electricity to most process plants is medium-voltage alternating current (AC), ranging from 600 V to 69 kV. AC power is distributed through a vari-ety of switches, fuses, and transformers. Transformers reduce the incoming voltage to lower AC voltages for distribution throughout the plant. In this way, the entire plant electrical system is connected.

Control systems and programmable logic controllers (PLCs) generally convert low-voltage AC power to DC voltage for use within the control system. Some PLC and distributed control system (DCS) input/output (I/O) units act as the source of the 24 V power for instruments. Others may use a separate power supply. Many instruments are powered by their signal wiring and must be able to operate with a current as low as 4 mA at 24 VDC.

Process plants contain large numbers of electric motors. Starting large motors can temporarily reduce the line voltage and create some electromagnetic interference for nearby equipment. This may result in noise or interference with low-voltage signals.

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151

10Advanced Sensors

and Analyzers

The basic sensors covered in Chapter 2, “Basic Measurements,” typically make up more than 80% of the process measurements for process control. The remaining 20% consists of a wide variety of specialized instruments and analyzers. There are analyz-ers for chemical composition, moisture content, color, particle size, conductivity, and many other characteristics.

Each industry has its own special needs. Refining and chemicals industries, for example, must deliver products of a specific chemical composition. Special analyzers are used throughout the plant to ensure that each part of the process is delivering the right composition to the next downstream process.

This chapter introduces some of the more common sensors and analyzers used in the process industries.

10.1 Sensors for Process FluidsMany properties of process fluids are useful to improve process control. These proper-ties can be measured with specialized sensors, which are generally installed in-line, in direct contact with process fluids. When direct in-line measurement might damage the instrument, a sample line, or sampling system, can be used to condition the fluid before measurement.

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10.1.1 pH and Oxidation-Reduction PotentialA pH probe measures the hydrogen ion concentration or activity. A pH value below 7 indicates an acidic solution; pH above 7 indicates a basic solution. Good pH control is important for biological processes and for certain chemical reactions. Furthermore, plant effluents are often regulated for environmental protection reasons, making pH control and reporting a critical part of that protection.

The oxidation-reduction potential, or ORP, is another measure of chemical properties. It is important for wastewater treatment and many metal-processing industries, and in certain types of bleaching.

pH and ORP sensors use electrodes, as shown in Figure 10-1. These electrodes are often made of glass and require special handling and protection.

The pH measurement and its effect on the process are notoriously nonlinear, as illustrated in Figure 10-2. Most processes are operated near the inflection point of the pH curve. Tight control in such nonlinear conditions is difficult. A characterizer, as discussed in Chapter 11, may be used to linearize the process in the view of the con-troller, making it easier for a standard proportional-integral-derivative (PID) controller to obtain tight control.

Figure 10-1. pH electrode.

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163

11Beyond the Basics of Process Control

In this chapter, you will learn about more advanced tools and techniques that can help you extract even more profit and performance from control systems. These advanced process control (APC) methods are heavily employed in some industries and seldom employed in others.

11.1 Advanced Regulatory ControlAdvanced regulatory control (ARC) refers to the use of control strategies that employ basic controls in some combination to provide a better overall solution for the process operation. For example, in these strategies you will often see some combination of multiple proportional-integral-derivative (PID) controllers.

These techniques can be readily performed in most distributed control systems (DCSs). In some programmable logic controller (PLC) systems (depending on the man-ufacturer) ARC can still be employed, although it may take extra engineering effort to handle the finer points, such as transition between controller modes.

ARC also requires special attention to commissioning, operator training, tuning, and troubleshooting.

11.1.1 Cascade ControlWith cascade control, two controllers work together to stabilize the process, as shown in Figure 11-1. The two loops are called the outer loop (sometimes primary loop) and the inner loop (sometimes secondary loop).

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Cascade control is most often used when the outer control loop, if present alone, responds too slowly to disturbances. With cascade control, the inner loop quickly detects and corrects for those disturbances before they can have a large impact on the outer loop.

A common example of cascade control is a heat exchanger, shown in Figures 11-2 and 11-3. The steam header supplies steam to many different processes in the plant.

Figure 11-1. Cascade control of tank level.

Figure 11-2. Heat exchanger control without cascade.

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177

Abbreviations/Acronyms

Abbreviation/Acronym Definition

A/D analog-to-digital

AC alternating current

ACFM actual cubic feet per minute

AI analog input

AI artificial intelligence

ANSI American National Standards Institute

AO analog output

APC advanced process control

ARC advanced regulatory control

CFM cubic feet per minute

CO controller output

COTS commercial off-the-shelf

CPU central processing unit

CV controlled variable

D/A digital-to-analog

DC direct current

DCS distributed control system

DI discrete input

DO discrete output

DO dissolved oxygen

DP differential pressure

DV disturbance variable

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181

Bibliography

ANSI/ISA-12.04.04-2012. Pressurized Enclosures. Research Triangle Park, NC: ISA (International Society of Automation).

ANSI/ISA-18.2-2016. Management of Alarm Systems for the Process Industries. Research Triangle, NC: ISA (International Society of Automation).

ANSI/ISA-5.1-2009. Instrumentation Symbols and Identification. Research Triangle Park, NC: ISA (International Society of Automation).

ANSI/ISA-50.1-1975 (R2012). Compatibility of Analog Signals for Electronic Industrial Process Instruments. Research Triangle Park, NC: ISA (International Society of Automation).

ANSI/ISA-61511-2-2018 / IEC 61511-2:2016. Functional Safety – Safety Instrumented Systems for the Process Industry Sector – Part 2: Guidelines for the Application of IEC 61511- 1:2016 (IEC 61511-2:2016, IDT). Research Triangle Park, NC: ISA (International Society of Automation).

ANSI/ISA-62381-2011 (IEC 62381 Modified). Automation Systems in the Process Industry – Factory Acceptance Test (FAT), Site Acceptance Test (SAT), and Site Integration Test(SIT). Research Triangle Park, NC: ISA (International Society of Automation).

ANSI/ISA-75.01.01-2012 (IEC 60534-2-1 MOD). Industrial-Process Control Valves – Part 2-1: Flow Capacity – Sizing Equations for Fluid Flow Under Installed Conditions. Research Triangle Park, NC: ISA (International Society of Automation).

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ANSI/ISA-88.01-2010. Batch Control – Part 1: Models and Terminology. Research Triangle Park, NC: ISA (International Society of Automation).

Baumann, Hans D. Control Valve Primer, A User’s Guide. Research Triangle Park, NC: ISA (International Society of Automation), 2009.

Boyes, Walt, ed. Instrumentation Reference Book. 4th ed. Oxford, UK: Butterworth- Heinemann, 2010.

Buckbee, George. Mastering Cascade Control. Clarks Summit, PA: PID Tutor, 2010.

Buckbee, George. Mastering Split-Range Control. Clarks Summit, PA: PID Tutor, 2010.

Caro, Dick. Wireless Networks for Industrial Automation. 4th ed. Research Triangle Park, NC: ISA (International Society of Automation), 2014.

Center for Chemical Process Safety. Guidelines for the Management of Change for Process Safety. American Institute of Chemical Engineers (AIChE). Hoboken, NJ: John Wiley & Sons, 2008.

Del Buono, Amanda. “Top 10 Worst Cyberattacks of the Decade.” ControlGlobal.com Off-site Insights (blog). December 30, 2019. https://www.controlglobal.com/blogs/off-site-insi ghts/ top-10-worst-cyber-attacks-of-the-decade/.

Dunn, Alison. “The Father of Invention: Dick Morley Looks Back on the 40th Anniversary of the PLC.” Manufacturing Automation. June 12, 2009. Accessed June 6, 2020. https://www.automationmag.com/855-the-father-of-invention-dick-mor-ley-looks-back-on-the-40th-anniversary-of-the-plc/.

Fisher Controls Company. Control Valve Handbook. 2nd ed. Marshalltown, IA: Fisher Controls Company, 1977.

Fortuna, Luigi, Salvatore Graziani, Alessandro Rizzo, and Maria G. Xibilia. Soft Sensors for Monitoring and Control of Industrial Processes. Advances in Industrial Control series. Springer-Verlag London Limited, 2007.

Goble, William. “Learn to Trust Safety PLCs.” Control Design. May 30, 2003. Accessed December 6, 2020. https://www.controldesign.com/articles/2003/249/.

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https://www.controlglobal.com/blogs/off-site-insights/top-10-worst-cyber-attacks-of-the-decade/

https://www.automationmag.com/855-the-father-of-invention-dick-mor�ley-looks-back-on-the-40th-anniversary-of-the-plc/

https://www.automationmag.com/855-the-father-of-invention-dick-mor�ley-looks-back-on-the-40th-anniversary-of-the-plc/

https://www.controldesign.com/articles/2003/249/

187

Index

Aabbreviations, 177–179absolute pressure, 13absolute zero, 31accuracy, 38

calibration, 145–146dual in-line package (DIP)

switches, 146electrical signal transmission, 145precision and, 39–40process signals, 144–146statements of instrument, 39

acid-base titration, 159acronyms, 177–179action, control step, 3actuator, 74

control action, 84control valve, 74, 75double-acting piston, 83final control element, 83hydraulic, 125–126linear, 83piston, 83pneumatic, 125–126power source, 84–85rotary, 83sizing, 85solenoid valves, 83, 125–126spring-and-diaphragm, 83, 84type of movement, 83

A/D (analog-to-digital), converter, 68address, 53

AGA. See American Gas Association (AGA)AI. See artificial intelligence (AI)air, instrument, 143–144alarm

management, 174–175process control, 111–112

Alarm Identification and Rationalization (ISA-TR18.2.2-2016), 175

Alarm Philosophy (ISA-TR18.2.1-2018), 175alternating current (AC)

drive controller, 86electrical supply, 142–143power supply, 47signal transmission, 145variable frequency drive (VFD), 87

American Gas Association (AGA), 28American Society of Mechanical Engineers

(ASME), 28ampere, 33Ampere, Andre M., 33analog communications, 61–64. See also

communicationscurrent, 63–64mechanical, 62pneumatics, 62–63voltage, 63

analog input (AI), 48analog output (AO), 48analog-to-digital (A/D), converter, 68analytics

big data, 148, 174diagnostics and, 173–174

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analyzers. See also sensorsconductivity, 153mass spectrometers, 159measurement, 2sample conditioning, 159sampling systems, 158specialty, 160titration systems, 159total organic carbon (TOC), 158–159

AND gatelogic control, 117–119truth table, 118

ANSI/ISA-88, 46anticipatory control, 100APC (advanced process control), 163ARC (advanced regulatory control), 163–169

cascade control, 163–165feedforward control, 166gain scheduling and gap controllers, 168–169ratio control, 167signal characterizers, 167–168

Archimedes, 22artificial intelligence (AI)

fuzzy logic controllers, 171–172neural networks, 172–173process control, 171–173

ASME. See American Society of Mechanical Engineers (ASME)

atmospheric pressure, 14atmospheric tanks, measuring level, 24automated monitoring, 148automated titrators, 159Automation Systems in the Process Industry (ANSI/

ISA-62381-2011), 139auto-tuning, 109

Bbackplane, 47ball valve, 77–78batch control, 46Bedford Associates, 46bellows, pressure sensor, 16, 17bias, 96big data analytics, 148, 174bimetallic temperature sensor, 32–33binary communications, 59–61. See also

communicationsadvantages and disadvantages of, 61on-off valve, 61

binary logic control, 116AND gate, 117–119DCSs, 127discrete inputs, 116–117

electromechanical devices, 124hybrid controllers, 127implementation of, 124–127logical NOT, 119logic diagram, 120–121maintenance, 126operations of logic control, 117operators, 120OR gate, 119, 120PLCs, 126, 127pneumatic and hydraulic actuation, 125–126relays, 124–125

binary logic diagrammingBoolean logic diagrams, 122combining programming methods, 123Ladder Logic, 121, 122PLC programming skills, 123–124Sequential Function Charts (SFCs), 122–123Structured Text (ST), 123

Binary Logic Diagrams for Process Operations (ISA-5.2-1976), 136

Bluetooth connection, 146Boole, George, 116Boolean control, 44Boolean logic, 116Boolean logic diagrams, 122Bourdon, Eugene, 16Bourdon tube

filled thermal system, 35pressure sensor, 15–16

butterfly valve, 75, 76

Ccalibrate, 37calibration, 37

accuracy and, 145–146capacitance probe, level sensor, 23cards, 47cascade control, 163–165

heat exchanger, 164, 165cavitation

butterfly valve, 75control valve, 81

Celsius, Anders, 32characteristic, 79

drive, 88heater, 88

characterizerspH, 168proportional-integral-derivative (PID)

controller, 152signal, 167–168as X-Y lookup table, 169

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189Index

chemical titration, 159closed loop control, 42–43cold junctions, 34commercial off-the-shelf (COTS), 54communication cards, PLCs, 49communications

analog, 61–64binary, 59–61chain of information, 60converting between signal types, 66–68digital, 64–66displaying information, 68–70pneumatic, 62, 63retaining data history, 71wireless, 66

compressed air, 144computer display information, 70conductance probe, level sensor, 23conductivity, sensor, 153cone and belt system, variable speed drives, 86cone and pulley system, variable speed drives, 86consistency, sensors, 154–155constantan, 34continuous process control, 93

on-off control of, 94–95control, 2–3

definition of, 2steps of, 2, 41–42terminology, 93–94

control action, actuator, 84control algorithm, 94controlled variable, 93controller gain (Kc), 96–98, 104, 106, 168–169controller mode, 94controller output (CO), 94, 96controller tuning, 94

criteria, 103–104lambda tuning method, 107–108PID, 102–108software tools for, 109techniques, 106–108units, 104–105Ziegler-Nichols reaction curve method,

106–107Ziegler-Nichols ultimate gain method, 106

control loop, 3, 43, 44, 94cascade control of, 165diagram, 3performance monitoring, 174

control loop diagrams, 136–137control loop performance, 174control narratives, 134, 136control system(s). See also documentation

alarms, 111–112

batch control, 46closed loop versus open loop control, 41–43cybersecurity, 149distributed control systems (DCSs), 41,

53–56embedded controllers, 56hybrid controllers, 56logic control, 44–45management of change, 148–149messages, 112programmable logic controllers (PLCs), 41,

46–53reports, 112sequential control, 45–46single loop controller, 43–44

control valve(s), 74–81actuator, 74, 75assembly, 75ball valve, 77–78butterfly valve, 75, 76characteristics of, 79–80eccentric disc valve, 78globe valve, 75–76, 77maintenance, 81performance factors, 80–81positioner, 75sizing, 78–79types of, 75–78valve body, 74

conversions, pressure, 12Coriolis flowmeters, 30corner taps, 27costs, automation, 7counter, logic element, 120CPU (central processing unit), 49cruise control for car, control loop, 3current communication, 63–64current loop tuning, drive, 88current-to-pressure (IP) transducer, 68cybersecurity, 149

DD/A (digital-to-analog), converter, 68daisy-chain topologies, 137dampers, 82–83

characteristics, 82maintenance, 83

data compression, 71data historian, 71DCS. See distributed control systems (DCSs)deadband, 174dead time (Td), 105, 107decision, control step, 3

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degrees Celsius, 32degrees Rankine, 32delta P (DP), differential pressure, 13derivative control, 100–101derivative filter, 100derivative kick, 100–101dew point, 156

sensor, 155–156diagnostics

analytics and, 173–174instrument and valve, 173–174

diaphragm, pressure sensor, 17differential gap, 95differential pressure, 13, 14

flow measurement, 26–28digital, binary signal, 60digital communications, 64–66. See also

communicationsadvantages and disadvantages of wireless, 66smart device, 64–65wireless, 66

digital input, 48digital ins, 60digital output, 48digital outs, 60digital positioners, 86digital-to-analog (D/A), converter, 68dioxin, analyzer, 160direct-acting controller, 97–98direct current (DC)

drive controller, 86power supply, 47signal transmission, 145

discrete, binary signal, 60discrete control, 115discrete input (DI), 48

logic control, 116–117discrete output (DO), 48displacement sensors, level, 22displacer, level, 22displaying information

computer, 70electrical and electronic, 69, 70mechanical, 68–69

dissolved oxygen (DO), sensor, 153distributed control systems (DCSs), 3, 41, 49,

53–56, 65advanced regulatory control (ARC), 163computer displays, 70custom programming, 56DCS configuration, 55–56electrical supply, 142engineering stations, 55input/output (I/O), 54

logic in, 127network configuration, 54open loop control, 94operator stations, 54processors, 54racks, 54shared resources, 55typical hardware, 54–55typical software, 55–56

disturbance variable (DV), 166documentation

control loop diagrams, 136–137control narratives, 134, 136factory acceptance test (FAT), 138–139instrument index, 132–133instrument specifications, 133–134, 135logic diagrams, 136loop diagrams, 136network diagrams, 137–138piping and instrumentation diagrams

(P&IDs), 130, 131, 132piping diagrams, 134process flow diagrams, 129–130site acceptance test (SAT), 138–139

double-acting piston, actuator, 83DP (delta P), differential pressure, 13drive, 86dry bulb temperature, humidity, 155–156dual in-line package (DIP) switches, 146

Eeccentric disc valve, 78efficiency, process control, 6–7electrical display, information, 69, 70electrical relay coil, 125electrical signal transmission, accuracy, 145electrical supply, power, 142–143electric motor, logic control, 120–121electric room heater, control loop, 3electric solenoids, 85electromagnetic flowmeters, 30electromechanical devices, binary logic

control, 124electromechanical relays, binary logic control,

124–125electronic display, information, 69, 70elevated-zero range, 37embedded controllers, 56enclosures, 147encoder, 48engineering stations, 55erosion, control valve, 81error, 43, 96

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191Index

Ffactory acceptance test (FAT), 138–139, 146Fahrenheit, Gabriel D., 32false, 117FAT. See factory acceptance test (FAT)feedback control, 166feedforward control, 166final control elements, 73

actuators, 83–85control valves, 74–81dampers, 82–83heaters, 88–89limiting elements, 89–90positioners, 86regulators, 89switches, 85variable speed drives, 86–88

flame detectors, 160flashing, control valve, 81float

level, 21, 22rotameter, 29

flow, common units for, 25flow controllers (FC), 108flow measurement

Coriolis flowmeters, 30differential pressure, 26–28magnetic flowmeters, 30Pitot tube, 28, 29positive displacement, 30rotameter, 29sensors, 30–31thermal, 30turbine, 30ultrasonic, 31vortex, 30weight, 31weir and flume, 31

flowmeters, instrument location, 144flow rate, 25flow sensors, units of flow, 25–26force, pressure versus, 10Foundation Fieldbus, 64full scale, 38fuzzification, 171fuzzy logic, 171fuzzy logic controllers, 171–172

Ggain scheduling, 168–169gap controllers, 168–169gauge glass, 23gauge pressure, 13, 14

glass, level sensor, 23globe valve, 75–76, 77graphical user interface (GUI), 49

Hhardware

for DCS, 54–55for PLCs, 47–50

harsh environments, 141materials for, 142

HART communications, 64–65head, pressure, 11head sensors, level, 22–23heater

characteristics, 88maintenance, 89sizing, 88

heat exchanger, cascade control, 164, 165high, 117historian, 60, 71HMI. See human-machine interface (HMI)hot junctions, 34human-machine interface (HMI), 49

computer displays, 70inspection and monitoring, 147PLC, 49–50

humidity, sensor, 155–156hybrid controllers, 56, 127hydraulic actuation, 125–126hydrogen sulfide, analyzer, 160

Iideal form, PID algorithm, 101–102information. See displaying informationinformation technology (IT) security, 149infrared (IR) spectroscopy, sensors, 154inherent valve characteristics, 80in-line blending application, 108inner loop, cascade control, 163, 164input/output (I/O)

cards, 67–68DCS, 54flexible, 48PLC, 48, 126, 127signals, 67–68smart, 48

inputs, 59inspection, monitoring and, 147–148installed flow characteristic, 80instrument air, 134

power supply, 143–144Instrumentation Symbols and Identification (ANSI/

ISA-5.1-2009), 132

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instrument calibration, 37instrument diagnostics, 173–174instrument index, documentation, 132–133instrument location, 144Instrument Loop Diagrams (ISA-5.4-1991), 136instrument protection, 146–147

enclosures, 147insulation, 146

instrument specificationsdocumentation, 133–134sample, 135

insulation, instrument protection, 146integral constant (Ti), 105integral control, 98–100integral gain (Ki), 98integral time, 98interlocks, process control, 111International Society of Automation (ISA), 5,

46–47International System of Units (SI), 20I/O cards, 67–68. See also input/output (I/O)IP (current-to-pressure) transducer, 68iron-constantan (ISA Type J), thermocouple, 34ISA. See International Society of Automation (ISA)

KKc (controller gain), 96–98, 104, 106, 168–169kelvin, 32Kelvin, William T., 32

Llabor costs, process control, 7Ladder Logic, 47

AND, 51hybrid controllers, 56motor start-stop, 121, 122OR, 51for PLCs, 50–51, 126

lambda tuning method, 107–108large valve gain, 78level gauge, 23level sensors

capacitance probe, 23displacement sensors, 22float sensors, 21glass, 23head sensors, 22–23measurement, 18–24optical, 23radiation, 23rotating paddle, 23thermal, 23types of, 21–24

ultrasonic sensor, 23units of level, 20–21upper limits for level, 19vessel, 18weight, 23zero references for level, 19

light-emitting diode (LED), 69limiting elements, 89–90

maintenance, 90rupture disks, 90safety values, 90types of, 90vacuum relief valves, 90

linear actuators, 83, 85linear relationship, 28linear thrust, 85liquid crystal displays (LCDs), 69liquid crystals, temperature sensors, 35live zero, 62logic control, 44–45, 115

binary, 116. See also binary logic controlconcept of, 116electric motor, 120–121

logic diagram, 120–121documentation, 136electric motor, 45

loop, control, 3, 44loop checks, 132loop diagrams, documentation, 136louvers, 82low, 117lower range limit, 36lower range value (LRV), 37

Mmachine learning, 172magnetic flowmeters, 30maintenance

automated monitoring, 148control valve, 81drive, 88heater, 89inspection and monitoring, 147–148limiting elements, 90regulator, 89relays, 126reliability considerations, 147–148

Management of Alarm Systems for the Process Industries (ANSI/ISA-18.2-2016), 111, 175

Management of Change (MOC), 148–149manipulated variable, 93–94manometer, 13, 15manual mode, open loop control, 42

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193Index

mass spectrometers, 159materials

harsh environments, 142wetted, 141–142

measurementaccuracy, 38, 39calibration, 37control step, 2flow sensors, 24–31general principles of, 9level sensors, 18–24precision, 39–40pressure, 9–18range and span, 36–37temperature sensors, 31–36truth in, 37–40

measuring junction, 33mechanical displays, information, 68–69mercury, analyzer, 160mesh networks, 137messages, process control, 112middle-of-three selector, 109–110millivolts (mV), 33model predictive control (MPC), 169–171

multivariable controller, 170objective function, 170

monitoringautomated, 148control loop performance, 174inspection and, 147–148

mood rings, 35Morley, Dick, 46, 47MPC. See model predictive control (MPC)multidrop, 137multivariable control, 169–170

NNAND (NOT and AND), logic element, 120National Bureau of Standards, 37National Electrical Code (NEC), 147National Electrical Manufacturers Association

(NEMA), 147National Institute of Standards and Technology

(NIST), 37near-infrared (NIR) spectroscopy, sensors, 154network diagrams, 137–138neural networks, 172–173Nichols, Nathaniel, 106NIR (near-infrared) spectroscopy, sensors, 154nitrogen oxide, analyzer, 160noise, 76, 81, 147, 148

electrical, 100, 142electromagnetic, 63, 64, 145

high-frequency, 100instrument, 71, 100, 173

normally closed, 125normally open, 125NOT, logic control, 119not high, 117

Oobjective function, 170off, 117of range, term, 38of reading, term, 38of span, term, 38ohm, 33Ohm, Georg Simon, 33on, 117on-off control

continuous processes, 94–95room temperature, 95

on-off valve, binary signals, 61open loop control, 42, 94operations, logic control, 117–121operator stations, 54optical, level sensor, 23OR gate

logic control, 119truth table, 120

orifice platedifferential pressure, 26flow measurement, 27

ORP. See oxidation-reduction potential (ORP)outer loop, cascade control, 163, 164outputs, 59overrides, 111

process control, 110–111overshoot, tuning, 103, 104oxidation-reduction potential (ORP)

sensor, 152titration, 159

ozone, analyzer, 160

PP&ID. See piping and instrumentation diagram

(P&ID)parallel form, PID algorithm, 101–102partial stroke testing, 90pause, 120performance factors, control valve, 80–81pH

nonlinear measurement, 168sensor, 152, 153

photoelectric sensors, 156photo eyes, 156

4977_Book.indb 193 29-6-21 5:51:04 PM




physical model, 46PID. See proportional-integral-derivative (PID)

controllerpipe taps, 27piping and instrumentation diagram (P&ID)

documentation, 130, 132example section of, 131instrument location, 144

piping diagrams, documentation, 134piston, actuator, 83Pitot, Henri, 28Pitot tube, flow measurement, 28, 29plant air, 143PLCs. See programmable logic controllers (PLCs)pluggage, control valve, 81pneumatic actuation, 125–126pneumatic communications, 62–63. See also

communicationsadvantages and disadvantages of, 63

positionercontrol system, 86control valve, 75

positive displacement, flow sensor, 30pounds of force, 21power source, actuator, 84–85power supply, 47–48, 142–144

electrical, 142–143instrument air, 143–144

pre-act control, 100precision, accuracy and, 39–40pressure, 9–11

absolute, 13atmospheric, 14communicating with, 62–63conversions, 12definition, 9differential, 14force versus, 10gauge, 13, 14measurement, 9–18pneumatic communication, 62–63types of sensors, 14–18units of, 22–30

pressure differential, 14pressure drop, control valve, 80pressure loss, 27pressure regulator, 89pressure sensors

bellows, 16, 17Bourdon tube, 15–16diaphragm, 17manometer, 15strain gauge, 17–18types of, 14–18

pressurized tanks, measuring level, 24primary loop, 163procedural model, 46process, 1–2process control, 2–3, 4–7, 93

alarm management, 174–175alarms, 111–112artificial intelligence in, 171–173control loop performance monitoring, 174diagnostics and analytics, 173–174efficiency, 6–7interlocks, 111labor costs, 7messages, 112model predictive control (MPC), 169–171overrides, 110–111quality, 5–6reliability, 4reports, 112safety, 5safety instrumented systems (SIS), 173selectors, 109–110

process flow diagrams (PFDs), 129–130process fluids

conductivity, 153dissolved oxygen, 153pH and oxidation-reduction potential, 152sensors for, 151–156thermal conductivity, 153

process gain (Kp), 107process history, 71process industries, 2process model, 46processor, CPU (central processing unit), 49process variable, 2PROFIBUS, 64programmable logic controllers (PLCs), 3, 44, 45,

65combining programming methods, 52–53,

123communications cards, 49computer displays, 70control system, 46–53electrical supply, 142graphical user interface (GUI), 49human-machine interface (HMI),

49–50, 126input/output (I/O) cards, 48, 126, 127Ladder Logic, 50–51, 121, 122, 126open loop control, 94power supply, 47–48processor, 49programming skills, 123–124Sequential Function Charts (SFCs), 52

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195Index

Structured Text (ST), 52, 123tags and addresses, 53typical hardware, 47–50typical software, 50–53

proportional control, 96–98proportional-integral-derivative (PID) controller,

43, 94advanced regulatory control (ARC), 163algorithm forms, 101–102characterizer, 152control, 43controller forms, 101–102controller tuning techniques, 106–108controller units, 104–105derivative control, 100–101integral control, 98–100proportional control, 96–98software tools for tuning, 109tuning, 102–108tuning criteria, 103–104

proportional-only (P-only) control, 97–98pyrometers, temperature sensors, 35

Qquality, process control, 5–6Quality Standard for Instrument Air (ISA–7.0.01–

1996), 143quarter-amplitude decay ratio, tuning, 104, 105

RRadiation, level sensor, 23radius taps, 27ramp rate, drive, 88range, measurement, 36–37rangeability, 78Rankine, William J. M., 32rate control, 100ratio control, 167reference junction, 33regen, 87regeneration, 87regulator

maintenance, 89pressure, 89

relative humidity, 155relay coil (electromechanical), 124–125relays

binary logic control, 124–125maintenance, 126

reliability, process control, 4–5repeatability, 40reports, process control, 112reset, 120

reset action, 98resistance temperature detector (RTD), 34, 36resistance temperature sensor, 34–35resistance thermometers, 34restart, 120reverse-acting controller, 97–98ring topologies, 137rise time, tuning, 104rotameter, flow measurement, 29rotary actuators, 83, 85rotary valve, advantages of globe valves over, 76rotating paddle, level sensor, 23rotor, rotameter, 29RTD. See resistance temperature detector (RTD)rupture disks, limiting elements, 90

SS88, 46safety, process control, 5safety instrumented functions (SIFs), 173safety instrumented systems (SISs), 5, 111, 173safety valves, limiting elements, 90sample conditioning, 159sampling systems, 158SAT. See site acceptance test (SAT)SCADA (supervisory control and data

acquisition) system, 70secondary loop, 163security, cybersecurity, 149Seebeck, Thomas J., 34Seebeck effect, 34selectors, process control, 109–110self-tuning, 109semiconductor sensors, temperature, 35sensors. See also analyzers

conductivity, 153consistency, 154–155dew point, 155–156dissolved oxygen, 153humidity, 155–156infrared and near–infrared spectroscopy,

154oxidation–reduction potential, 152pH, 152, 153photoelectric, 156physical and mechanical, 156–158for process fluids, 151–156speed detectors, 157–158thermal conductivity, 153turbidity, 154vibration, 157virtual, 160–161weighing systems, 156–157

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sequential control, 45–46Sequential Function Chart (SFC), 52

programming PLCs, 122–123series form, PID algorithm, 101–102set point, 42, 93shielded twisted pair cable, 145Siemens, Sir William, 34SIFs. See safety instrumented functions (SIFs)signal characterizers, 167–168signal types

analog to digital, 68chain of communication, 67converting between, 66–68digital to analog, 68input/output (I/O), 67–68transducers, 68

single loop controller, 43–44SISs. See safety instrumented systems (SISs)site acceptance test (SAT), 138–139, 146sizing

actuator, 85drive, 87heater, 88

slow sampling, 71smart device, communications, 64–65smart input/output (I/O), 65smart positioners, 90smart sensors, 31soft sensors, 160software

for DCS, 55–56for PLCs, 50–53tools for tuning, 109

solenoids, 85solenoid valves, 83, 125–126span, measurement, 37specification, instrument

documentation, 133–134sample, 135

specific gravity, 12spectrometer, 159speed detectors, 157–158speed loop, drive, 88spring-and-diaphragm actuator, 83, 84stability, tuning, 103stack analyzers, 160star topology, 137steam header override control, 110stiction, 174strain gauge, pressure sensor, 17–18Structured Text (ST), 52, 123Stuxnet attack, 149suppressed-zero range, 37switches, control system, 85

Ttag, 53tag names, 53, 55, 132tank level, cascade control, 163, 164tank level low, 117taps, 27Taylor Instrument Companies, 106temperature, 31

absolute zero, 31common measurements, 32fuzzification of, 171units of, 31–32

temperature sensors, 31–36bimetallic, 32–33filled thermal system, 35liquid crystals, 35pyrometers, 35resistance, 34–35semiconductor sensors, 35thermocouple, 33–34thermowells, 36types, 32–36ultrasonic sensors, 35

thermalflow sensor, 30level sensor, 23

thermal conductivity, sensor, 153thermistors, thermal plus resistor, 34thermocouple, 33–34thermostat, 94thermowells, 363D CAD (three-dimensional computer-aided

design) program, 134Timer, logic element, 120titration systems, 159total carbon, 158total organic carbon (TOC), analyzer, 158–159toxic gases, analyzer, 160transducers, signal type, 68true, 117truth table, 118

AND gate, 118OR gate, 120

tuning, 94, 102–108. See also controller tuningturbidity, sensors, 154turbine, flow sensor, 30

Uultimate gain (Ku), 106ultimate period of the oscillation (Pu), 106ultrasonic

flow sensor, 31level sensor, 23

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197Index

temperature sensors, 35uniform, relationship, 28units, pressure, 11–14universal I/O cards, 68upper range limit, 36upper range value (URV), 37

Vvacuum relief valves, limiting elements, 90valve

ball, 77–78butterfly, 75, 76diagnostics, 173–174eccentric disc, 78equal percentage, 79globe, 75–76, 77linear, 79quick opening, 79sizing, 174

valve body, 74valve characteristics, 79–80valve damage, 78valve sizing, 78–79variable-area meter, 29variable frequency drive (VFD), alternating

current (AC), 87variable speed drives (VSDs), 86–88

cones and belts, 86drive characteristics, 88drive maintenance, 88drive sizing, 87

types of, 86–87vena contracta taps, 27Venturi, G. B., 27Venturi tube, 27vibration sensors, 157virtual sensors, 160–161volt (V), 33Volta, Alessandro, 33voltage communication, 33, 63volume, 25vortex, flow sensor, 30

Wwater gauge, 14weigh belt, 157weighing systems, 156–157weight

flow sensor, 31level sensor, 23

weir and flume, flow sensor, 31wet bulb temperature, humidity, 155–156wetted materials, 141–142wireless communications

advantages and disadvantages of, 66digital, 66

Zzero an instrument, 37Ziegler, John, 106Ziegler-Nichols reaction curve method, 106–107Ziegler-Nichols ultimate gain method, 106

4977_Book.indb 197 29-6-21 5:51:04 PM



Process Control Basics - NET

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