Loop Checking

About the Authors xiii

Preface xv

Acknowledgments xvii

1 Introduction to Loop Checking 1

1.1 The Opportunity 1

1.2 Loop Checking: Introduction 3

1.2.1 Defining the Loop 3

1.2.2 Elements of the Loop 4

1.2.3 Sensor/Transmitter 4

1.2.4 Controller 5

1.2.5 Final Control Element 5

1.2.6 Other Loop Types 6

1.2.7 Loop Checking 7

1.2.7.1 Process Overview 7

1.2.7.2 Technology Improvements 8

1.3 Process Control Example 8

1.3.1 Measurement 8

1.3.2 Control System 8

1.3.3 Final Control Element 9

1.4 Other Loop Checking Considerations 10

1.4.1 Keeping Score 10

1.4.1.1 Loop Variability 10

1.4.1.2 Standard Deviation 10

1.4.1.3 Heading Variability 12

1.4.2 Loop Nonlinearities and Dead Time: The Bad Guys 13

1.5 Control Loop Design Guidelines 17

References 17

Review Questions 18

2 The Factory Acceptance Test 19

2.1 Documentation 19

2.2 Test Planning 20

2.3 Performing the FAT 20

2.3.1 Hardware and I/O Check 20

2.3.1.1 Section 1: Introduction 21

2.3.1.2 Section 2: Test Method Instructions 21

2.3.1.3 Section 3: Bill of Materials (BOM) 21

2.3.1.4 Section 4: System Components 21

2.3.1.5 Section 5: Grounding and Wire Shielding 22

2.3.1.6 Section 6: Alternating and Direct Current (AC/DC) Power 22

2.3.1.7 Section 7: I/O Check (Optional) 22

2.3.2 Loop Checking 22

2.3.2.1 Section 1: Introduction 24

2.3.2.2 Section 2: Test Method Instructions 24

2.3.2.3 Section 3: Base-Level Analog and Discrete Elements 24

2.3.2.4 Section 4: Basic Regulatory Control Loop Elements 25

2.3.2.5 Section 5: Discrete Element Control (Motor Starts and Stops, On-Off/Open-Close

Devices) 26

2.3.2.6 Section 6: Display Testing 27

2.3.2.7 Section 7: Test Results 28

2.4 Process Simulation 28

2.5 Process Control Example 30

2.5.1 Documentation 30

2.5.2 Test Plan 30

2.5.3 Test Equipment 30

2.5.4 Test Methods 30

2.5.5 Checklists 31

References 36

Review Questions 37

3 Start-Up 39

3.1 Documentation 39

3.2 Loop Check Test Plan 40

3.3 Checking the Loop 40

3.3.1 Traditional Instruments (Non-Smart) 40

3.3.2 Smart Instruments 42

3.3.3 Smart Commissioning 42

3.4 Process Control Example 45

3.4.1 Documentation 45

3.4.2 Test Plan 45

3.4.2.1 Traditional Instruments (Non-Smart) 45

3.4.2.2 Smart Instruments 47

3.5 Example Forms 47

References 50

Review Questions 50

4 Performance Benchmarking 51

4.1 Designing the Test 51

4.1.1 Documentation 51

4.1.2 Interview Key Personnel 52

4.1.3 Loop Check Planning 52

4.2 Performing the Test 53

4.2.1 TheWalk-Through 53

4.2.2 Setting Up for the Loop Check 55

4.2.3 Connecting to the Process Signals 56

4.2.4 Running the As-Found Loop Time-Series Test 58

4.2.5 Running the Open-Loop Check or Bump Test 60

4.3 Analyzing and Reporting the Test Results 61

4.3.1 As-Found Data Collection Analysis 61

4.3.2 Bump Test Analysis 62

4.3.3 Loop Tuning 64

4.3.4 Reporting the Test Results 66

4.3.5 Fix, Tune, and Re-Run the Loop Check 67

4.3.6 Next Step 67

4.4 Process Control Example 67

4.4.1 Designing the Test 67

4.4.1.1 Documentation 67

4.4.1.2 Interviews 68

4.4.1.3 Monitoring Variables 69

4.4.2 Performing the Test 69

4.4.2.1 TheWalk-Through 69

4.4.2.2 Setting Up for the Test 70

4.4.2.3 Running the As-Found Time-Series Test 70

4.4.2.4 Running the As-Found Bump Test 70

4.4.3 Analyzing and Reporting the Test Results 70

4.4.3.1 As-Found Data Collection Analysis 70

4.4.3.2 Bump Test Analysis 70

4.4.3.3 Reporting the Test Results 74

References 74

Review Questions 75

5 Sustaining Performance 77

5.1 Maintenance Strategies 77

5.2 Operator Efficiency 78

5.3 Where to Start? 78

5.4 Selecting the Loop Performance Program Scope 78

5.5 Loop Performance Monitoring and Analyzing 79

5.5.1 Transmitters 79

5.5.1.1 Traditional Transmitters 79

5.5.1.2 Smart Transmitters 80

5.5.2 Controller PID Algorithm 82

5.5.2.1 Incorrect Mode 83

5.5.2.2 Limited Control 83

5.5.2.3 Uncertain Input 83

5.5.2.4 Large Variability 84

5.5.2.5 Economic Assessment 84

5.5.3 Final Control Device 84

5.5.4 Traditional Valves 85

5.5.5 Smart Valves 86

5.6 Performance Reporting 87

5.7 Loop Performance Program Architecture 88

5.7.1 Traditional Control System 89

5.7.2 Smart Field Devices and Loop Performance Software 89

5.7.3 Online Device Monitoring 90

5.7.4 Integrated Performance and Control System 90

5.8 Loop Performance Program Summary 92

References 92

Further Information 92

Review Questions 93

6 Loop Checking Existing Installations 95

6.1 Why Loop Checks Are Essential for Calibrations 95

6.1.1 Example 96

6.2 Simulating Signals and Processes 98

6.3 Common Problems with Existing Installations 99

References 99

Appendix A Tuning 101

A.1 Author's Note 101

A.2 Loop Classification by Control Function 101

A.2.1 Controlled Variable 101

A.2.2 Manipulated Variable 102

A.2.3 Indicated Variable 102

A.2.4 Set Point 104

A.3 Control Algorithms 104

A.3.1 Basic Algorithms 104

A.3.2 The PID Algorithm 104

A.3.2.1 Proportional 105

A.3.2.2 Integral 106

A.3.2.3 Derivative 109

A.3.2.4 Complete PID Response 110

A.4 Loop Tuning 111

A.4.1 Tuning Criteria or "How DoWe Know When It's Tuned?" 111

A.4.1.1 Informal Methods 111

A.4.1.2 Mathematical Criteria: Minimization of Index 112

A.4.1.3 What's Really Important 112

A.5 Tuning Methods 112

A.5.1 Experience-Based Tuning 113

A.5.2 The Ziegler-Nichols Tuning Method: Open-Loop (Reaction Rate) 113

A.5.3 The Ziegler-Nichols Tuning Method: Closed-Loop (Ultimate Period) 114

A.5.4 Controllability of Processes 115

A.5.5 Flow Loops 115

Reference 115

Appendix B Answers to Review Questions 117

Chapter 1 117

Chapter 2 118

Chapter 3 119

Chapter 4 119

Chapter 5 120

Abbreviations 121

Bibliography 123

Index 125