Automated Continuous Process Control

Preface xi

1 Introduction 1

1-1 Process Control System 1

1-2 Important Terms and Objective of Automatic Process Control 3

1-3 Regulatory and Servo Control 4

1-4 Transmission Signals, Control Systems, and Other Terms 5

1-5 Control Strategies 6

1-5.1 Feedback Control 6

1-5.2 Feedforward Control 8

1-6 Summary 9

2 Process Characteristics 11

2-1 Process and Importance of Process Characteristics 11

2-2 Types of Processes 13

2-3 Self-Regulating Processes 14

2-3.1 Single-Capacitance Processes 14

2-3.2 Multicapacitance Processes 24

2-4 Transmitters and Other Accessories 28

2-5 Obtaining Process Characteristics from Process Data 29

2-6 Questions When Performing Process Testing 32

2-7 Summary 33

Reference 33

Problems 34

3 Feedback Controllers 38

3-1 Action of Controllers 38

3-2 Types of Feedback Controllers 40

3-2.1 Proportional Controller 40

3-2.2 Proportional–Integral Controller 44

3-2.3 Proportional–Integral–Derivative Controller 48

3-2.4 Proportional–Derivative Controller 50

3-3 Reset Windup 50

3-4 Tuning Feedback Controllers 53

3-4.1 Online Tuning: Ziegler–Nichols Technique 53

3-4.2 Offline Tuning 54

3-5 Summary 60

References 60

Problems 60

4 Cascade Control 61

4-1 Process Example 61

4-2 Implementation and Tuning of Controllers 65

4-2.1 Two-Level Cascade Systems 65

4-2.2 Three-Level Cascade Systems 68

4-3 Other Process Examples 69

4-4 Closing Comments 72

4-5 Summary 73

References 73

5 Ratio, Override, and Selective Control 74

5-1 Signals and Computing Algorithms 74

5-1.1 Signals 74

5-1.2 Programming 75

5-1.3 Scaling Computing Algorithms 76

5-1.4 Significance of Signals 79

5-2 Ratio Control 80

5-3 Override, or Constraint, Control 88

5-4 Selective Control 92

5-5 Designing Control Systems 95

5-6 Summary 110

References 111

Problems 112

6 Block Diagrams and Stability 127

6-1 Block Diagrams 127

6-2 Control Loop Stability 132

6-2.1 Effect of Gains 137

6-2.2 Effect of Time Constants 138

6-2.3 Effect of Dead Time 138

6-2.4 Effect of Integral Action in the Controller 139

6-2.5 Effect of Derivative Action in the Controller 140

6-3 Summary 141

Reference 141

7 Feedforward Control 142

7-1 Feedforward Concept 142

7-2 Block Diagram Design of Linear Feedforward Controllers 145

7-3 Lead/Lag Term 155

7-4 Extension of Linear Feedforward Controller Design 156

7-5 Design of Nonlinear Feedforward Controllers from Basic Process Principles 161

7-6 Closing Comments on Feedforward Controller Design 165

7-7 Additional Design Examples 167

7-8 Summary 172

References 173

Problem 173

8 Dead-time Compensation 174

8-1 Smith Predictor Dead-Time Compensation Technique 174

8-2 Dahlin Controller 176

8-3 Summary 179

References 179

9 Multivariable Process Control 180

9-1 Pairing Controlled and Manipulated Variables 181

9-1.1 Obtaining Process Gains and Relative Gains 186

9-1.2 Positive and Negative Interactions 189

9-2 Interaction and Stability 191

9-3 Tuning Feedback Controllers for Interacting Systems 192

9-4 Decoupling 194

9-4.1 Decoupler Design from Block Diagrams 194

9-4.2 Decoupler Design from Basic Principles 196

9-5 Summary 197

References 197

Problem 198

Appendix A Case Studies 199

Case 1: Ammonium Nitrate Prilling Plant Control System 199

Case 2: Natural Gas Dehydration Control System 200

Case 3: Sodium Hypochlorite Bleach Preparation Control System 201

Case 4: Control System in the Sugar Refining Process 202

Case 5: Sulfuric Acid Process 204

Case 6: Fatty Acid Process 205

Reference 207

Appendix B Processes For Design Practice 208

Installing the Programs 208

Process 1: NH 3 Scrubber 208

Process 2: Catalyst Regenerator 211

Process 3: Mixing Process 213

Index 215