Practical Process Control for Engineers and Technicians PDF

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Practical Process Control for Engineers and Technicians Dedication This book is dedicated to Wolfgang who fought a courageous battle against motor neurone disease and continued teaching until the very end. Although he received his training in Europe, he ended up being one of Australia’s most outsta...

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Practical Process Control for Engineers and Technicians

Dedication

This book is dedicated to Wolfgang who fought a courageous battle against motor neurone disease and continued teaching until the very end. Although he received his training in Europe, he ended up being one of Australia’s most outstanding instructors in industrial process control and inspired IDC Technologies into running his course throughout the world. His delight in taking the most complex control system problems and reducing them to simple practical solutions made him a sought after instructor in the process control field and an outstanding mentor to the IDC Technologies engineers teaching the topic. Hambani Kahle (Zulu Farewell) (Sources: Canciones de Nuestra Cabana (1980), Tent and Trail Songs (American Camping Association), Songs to Sing & Sing Again by Shelley Gorden) Go well and safely. Go well and safely. Go well and safely. The Lord be ever with you. Stay well and safely. Stay well and safely. Stay well and safely. The Lord be ever with you. Hambani Kahle. Hambani Kahle. Hambani Kahle. The Lord be ever with you.

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Contents

Practical Process Control for Engineers and Technicians Wolfgang Altmann Dipl.Ing Contributing author: David Macdonald BSc (Hons) Inst. Eng, Senior Engineer, IDC Technologies, Cape Town, South Africa

Series editor: Steve Mackay FIE (Aust), CPEng, BSc (ElecEng), BSc (Hons), MBA, Gov.Cert.Comp., Technical Director – IDC Technologies Pty Ltd

AMSTERDAM • BOSTON • HEIDELBERG • LONDON NEW YORK • OXFORD • PARIS • SAN DIEGO SAN FRANCISCO • SINGAPORE • SYDNEY • TOKYO Newnes is an imprint of Elsevier

Newnes An imprint of Elsevier Linacre House, Jordan Hill, Oxford OX2 8DP 30 Corporate Drive, Burlington, MA 01803 First published 2005 Copyright © 2005, IDC Technologies. All rights reserved No part of this publication may be reproduced in any material form (including photocopying or storing in any medium by electronic means and whether or not transiently or incidentally to some other use of this publication) without the written permission of the copyright holder except in accordance with the provisions of the Copyright, Designs and Patents Act 1988 or under the terms of a licence issued by the Copyright Licensing Agency Ltd, 90 Tottenham Court Road, London, England W1T 4LP. Applications for the copyright holder’s written permission to reproduce any part of this publication should be addressed to the publishers British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library Library of Congress Cataloguing in Publication Data A catalogue record for this book is available from the Library of Congress

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Contents

Preface..................................................................................................................................xi 1

Introduction................................................................................................................. 1 1.1 Objectives ................................................................................................... 1 1.2 Introduction .................................................................................................. 1 1.3 Basic definitions and terms used in process control ..................................... 2 1.4 Process modeling......................................................................................... 2 1.5 Process dynamics and time constants.......................................................... 5 1.6 Types or modes of operation of process control systems ........................... 13 1.7 Closed loop controller and process gain calculations ................................. 15 1.8 Proportional, integral and derivative control modes .................................... 16 1.9 An introduction to cascade control.............................................................. 16

2

Process measurement and transducers .................................................................. 18 2.1 Objectives .................................................................................................. 18 2.2 The definition of transducers and sensors .................................................. 18 2.3 Listing of common measured variables ...................................................... 18 2.4 The common characteristics of transducers ............................................... 19 2.5 Sensor dynamics........................................................................................ 21 2.6 Selection of sensing devices ...................................................................... 21 2.7 Temperature sensors ................................................................................. 22 2.8 Pressure transmitters ................................................................................. 28 2.9 Flow meters................................................................................................ 35 2.10 Level transmitters ....................................................................................... 42 2.11 The spectrum of user models in measuring transducers............................. 44 2.12 Instrumentation and transducer considerations .......................................... 45 2.13 Selection criteria and considerations .......................................................... 48 2.14 Introduction to the smart transmitter ........................................................... 50

3

Basic principles of control valves and actuators ....................................................... 52 3.1 Objectives .................................................................................................. 52 3.2 An overview of eight of the most basic types of control valves.................... 52 3.3 Control valve gain, characteristics, distortion and rangeability .................... 67 3.4 Control valve actuators............................................................................... 71 3.5 Control valve positioners ............................................................................ 76 3.6 Valve sizing ................................................................................................ 76

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Contents

4

Fundamentals of control systems ............................................................................ 78 4.1 Objectives................................................................................................... 78 4.2 On–off control ............................................................................................. 78 4.3 Modulating control ...................................................................................... 79 4.4 Open loop control ....................................................................................... 79 4.5 Closed loop control ..................................................................................... 81 4.6 Deadtime processes ................................................................................... 84 4.7 Process responses ..................................................................................... 85 4.8 Dead zone .................................................................................................. 86

5

Stability and control modes of closed loops ............................................................. 87 5.1 Objectives................................................................................................... 87 5.2 The industrial process in practice................................................................ 87 5.3 Dynamic behavior of the feed heater .......................................................... 88 5.4 Major disturbances of the feed heater......................................................... 88 5.5 Stability....................................................................................................... 89 5.6 Proportional control..................................................................................... 90 5.7 Integral control............................................................................................ 93 5.8 Derivative control........................................................................................ 95 5.9 Proportional, integral and derivative modes ................................................ 98 5.10 I.S.A vs ‘Allen Bradley’................................................................................ 98 5.11 P, I and D relationships and related interactions ......................................... 98 5.12 Applications of process control modes........................................................ 99 5.13 Typical PID controller outputs ..................................................................... 99

6

Digital control principles ......................................................................................... 100 6.1 Objectives................................................................................................. 100 6.2 Digital vs analog: a revision of their definitions.......................................... 100 6.3 Action in digital control loops .................................................................... 100 6.4 Identifying functions in the frequency domain ........................................... 101 6.5 The need for digital control ....................................................................... 103 6.6 Scanned calculations................................................................................ 105 6.7 Proportional control................................................................................... 105 6.8 Integral control.......................................................................................... 105 6.9 Derivative control...................................................................................... 106 6.10 Lead function as derivative control............................................................ 106 6.11 Example of incremental form (Siemens S5-100 V) ................................... 107

7

Real and ideal PID controllers ................................................................................ 108 7.1 Objectives................................................................................................. 108 7.2 Comparative descriptions of real and ideal controllers .............................. 108 7.3 Description of the ideal or the non-interactive PID controller..................... 108 7.4 Description of the real (Interactive) PID controller..................................... 109 7.5 Lead function – derivative control with filter .............................................. 110 7.6 Derivative action and effects of noise ....................................................... 110 7.7 Example of the KENT K90 controllers PID algorithms............................... 111

Contents

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8

Tuning of PID controllers in both open and closed loop control systems ................ 112 8.1 Objectives ................................................................................................ 112 8.2 Objectives of tuning.................................................................................. 112 8.3 Reaction curve method (Ziegler–Nichols) ................................................ 114 8.4 Ziegler–Nichols open loop tuning method (1) .......................................... 116 8.5 Ziegler–Nichols open loop method (2) using POI...................................... 117 8.6 Loop time constant (LTC) method ............................................................ 119 8.7 Hysteresis problems that may be encountered in open loop tuning .......... 120 8.8 Continuous cycling method (Ziegler–Nichols) .......................................... 120 8.9 Damped cycling tuning method ................................................................ 123 8.10 Tuning for no overshoot on start-up (Pessen) .......................................... 126 8.11 Tuning for some overshoot on start-up (Pessen) ..................................... 127 8.12 Summary of important closed loop tuning algorithms................................ 127 8.13 PID equations: dependent and independent gains ................................... 127

9

Controller output modes, operating equations and cascade control ....................... 131 9.1 Objectives ................................................................................................ 131 9.2 Controller output....................................................................................... 131 9.3 Multiple controller outputs......................................................................... 132 9.4 Saturation and non-saturation of output limits........................................... 133 9.5 Cascade control ....................................................................................... 134 9.6 Initialization of a cascade system ............................................................. 136 9.7 Equations relating to controller configurations .......................................... 136 9.8 Application notes on the use of equation types......................................... 139 9.9 Tuning of a cascade control loop.............................................................. 140 9.10 Cascade control with multiple secondaries ............................................... 141

10

Concepts and applications of feedforward control .................................................. 142 10.1 Objectives ................................................................................................ 142 10.2 Application and definition of feedforward control....................................... 142 10.3 Manual feedforward control ...................................................................... 143 10.4 Automatic feedforward control .................................................................. 143 10.5 Examples of feedforward controllers......................................................... 144 10.6 Time matching as feedforward control...................................................... 144

11

Combined feedback and feedforward control ......................................................... 147 11.1 Objectives ................................................................................................ 147 11.2 The feedforward concept.......................................................................... 147 11.3 The feedback concept .............................................................................. 147 11.4 Combining feedback and feedforward control........................................... 148 11.5 Feedback–feedforward summer ............................................................... 148 11.6 Initialization of a combined feedback and feedforward control system...... 149 11.7 Tuning aspects......................................................................................... 149

12

Long process deadtime in closed loop control and the Smith Predictor .................. 150 12.1 Objectives ................................................................................................ 150 12.2 Process deadtime..................................................................................... 150

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Contents

12.3 12.4 12.5 12.6 12.7

An example of process deadtime.............................................................. 151 The Smith Predictor model ....................................................................... 152 The Smith Predictor in theoretical use ...................................................... 153 The Smith Predictor in reality.................................................................... 153 An exercise in deadtime compensation..................................................... 154

13

Basic principles of fuzzy logic and neural networks ................................................ 155 13.1 Objectives................................................................................................. 155 13.2 Introduction to fuzzy logic ......................................................................... 155 13.3 What is fuzzy logic? ................................................................................. 156 13.4 What does fuzzy logic do? ....................................................................... 156 13.5 The rules of fuzzy logic ............................................................................. 156 13.6 Fuzzy logic example using five rules and patches .................................... 158 13.7 The Achilles heel of fuzzy logic................................................................. 159 13.8 Neural networks........................................................................................ 159 13.9 Neural back propagation networking......................................................... 161 13.10 Training a neuron network ........................................................................ 162 13.11 Conclusions and then the next step .......................................................... 163

14

Self-tuning intelligent control and statistical process control ................................... 165 14.1 Objectives................................................................................................. 165 14.2 Self-tuning controllers ............................................................................... 165 14.3 Gain scheduling controller ........................................................................ 166 14.4 Implementation requirements for self-tuning controllers............................ 167 14.5 Statistical process control (SPC) .............................................................. 167 14.6 Two ways to improve a production process .............................................. 168 14.7 Obtaining the information required for SPC .............................................. 169 14.8 Calculating control limits ........................................................................... 173 14.9 The logic behind control charts ................................................................. 175

Appendix A: Some Laplace transform pairs ....................................................................... 176 Appendix B: Block diagram transformation theorems......................................................... 179 Appendix C: Detail display ................................................................................................. 181 Appendix D: Auxiliary display............................................................................................. 185 Appendix E: Configuring a tuning exercise in a controller .................................................. 188 Appendix F: Installation of simulation software .................................................................. 190 Appendix G: Operation of simulation software .................................................