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A Real-Time Approachto Process Control

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A Real-Time Approach to Process Control, Second Edition W. Y. Svrcek, D. P. Mahoney and B. R. Young© 2006 John Wiley & Sons, Ltd. ISBN: 978-0-470-02533-8


A Real-Time Approachto Process ControlSecond Edition

William Y. SvrcekUniversity of CalgaryCalgary, Canada

Donald P. MahoneyBDMetrics Inc.Baltimore, USA

Brent R. YoungThe University of AucklandAuckland, New Zealand

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Copyright C© 2006 John Wiley & Sons Ltd, The Atrium, Southern Gate, Chichester,West Sussex PO19 8SQ, England

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Library of Congress Cataloging-in-Publication Data

Svrcek, William Y.A real time approach to process control / William Y. Svrcek. – 2nd ed.

p. cm.Includes bibliographical references and index.ISBN-13: 978-0-470-02533-8 (cloth)ISBN-10: 0-470-02533-6 (cloth)ISBN-13: 978-0-470-02534-5 (pbk. : alk. paper)ISBN-10: 0-470-02534-4 (pbk. : alk. paper)1. Process control–Data processing. 2. Real-time control. I. Title.

TS156.8.S86 2006670.42′75433–dc22 2006010919

British Library Cataloguing in Publication Data

A catalogue record for this book is available from the British Library

ISBN-13 978-0-470-02533-8 (HB) ISBN-13 978-0-470-02534-5 (PB)ISBN-10 0-470-02533-6 (HB) ISBN-10 0-470-02534-4 (PB)

Typeset in 10.5/12.5pt Times by TechBooks, New Delhi, IndiaPrinted and bound in Great Britain by Antony Rowe Ltd, Chippenham, WiltshireThis book is printed on acid-free paper responsibly manufactured from sustainable forestryin which at least two trees are planted for each one used for paper production.

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Tell me and I forget,

Show me and I may remember,

Involve me and I understand.

Benjamin FranklinScientist, Statesman

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Contents

Preface xi

Acknowledgements xiii

Endorsement xv

About the authors xvii

1 A brief history of control and simulation 11.1 Control 1

1.2 Simulation 3

1.3 References 10

2 Process control hardware fundamentals 132.1 Control system components 13

2.2 Primary elements 14

2.3 Final control elements 30

2.4 References 50

3 Fundamentals of single input−single output systems 513.1 Open-loop control 51

3.2 Disturbances 52

3.3 Feedback control overview 53

3.4 Feedback control: a closer look 56

3.5 Process attributes: capacitance and dead time 61

3.6 Process dynamic response 71

3.7 Process modelling and simulation 73

3.8 References 92

4 Basic control modes 934.1 On−off control 93

4.2 Proportional (P-only) control 95

4.3 Integral (I-only) control 101

4.4 Proportional plus integral (PI) control 104


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4.5 Derivative action 105

4.6 Proportional plus derivative (PD) controller 107

4.7 Proportional integral derivative (PID) control 110

4.8 Choosing the correct controller 111

4.9 Controller hardware 113

4.10 References 115

5 Tuning feedback controllers 1175.1 Quality of control and optimisation 117

5.2 Tuning methods 122

5.3 References 130

6 Advanced topics in classical automatic control 1316.1 Cascade control 131

6.2 Feedforward control 135

6.3 Ratio control 138

6.4 Override control (auto selectors) 140

6.5 References 146

7 Common control loops 1477.1 Flow loops 147

7.2 Liquid pressure loops 149

7.3 Liquid level control 151

7.4 Gas pressure loops 162

7.5 Temperature control loops 163

7.6 Pump control 170

7.7 Compressor control 170

7.8 Boiler control 177

7.9 References 180

8 Distillation column control 1838.1 Basic terms 183

8.2 Steady-state and dynamic degrees of freedom 184

8.3 Control system objectives and design considerations 186

8.4 Methodology for selection of a controller structure 188

8.5 Level, pressure, temperature and composition control 190

8.6 Optimizing control 198

8.7 Distillation control scheme design using steady-state models 202

8.8 Distillation control scheme design using dynamic models 213

8.9 References 214

9 Using steady-state methods in a multi-loop control scheme 2159.1 Variable pairing 215

9.2 The relative gain array 216

9.3 Niederlinski index 221

9.4 Decoupling control loops 221

9.5 Tuning the controllers for multi-loop systems 223


CONTENTS ix

9.6 Practical examples 223

9.7 Summary 234

9.8 References 234

10 Plant-wide control 23710.1 Short-term versus long-term control focus 237

10.2 Cascaded units 239

10.3 Recycle streams 241

10.4 General considerations for plant-wide control 246

10.5 References 247

Appendices

1 P&ID symbols 249

2 Glossary of terms 253A2.1 Reference 259

Workshops

1 Learning through doing 265

2 Feedback control loop concepts 269

3 Process capacity and dead time 275

4 Feedback control 283

5 Controller tuning for capacity and dead time processes 291

6 Topics in advanced control 297

7 Distillation control 307

8 Plant operability and controllability 315

Index 323


Preface

For decades, the subject of control theory has been taught using transfer functions,frequency-domain analysis, and Laplace transform mathematics. For linear systems(like those from the electromechanical areas from which these classical control tech-niques emerged) this approach is well suited. As an approach to the control of chemicalprocesses, which are often characterized by nonlinearity and large doses of dead time,classical control techniques have some limitations.

In today’s simulation-rich environment, the right combination of hardware and soft-ware is available to implement a ‘hands-on’ approach to process control system design.Engineers and students alike are now able to experiment on virtual plants that capturethe important non-idealities of the real world, and readily test even the most outlandishof control structures without resorting to non-intuitive mathematics or to placing realplants at risk.

Thus, the basis of this text is to provide a practical, hands-on introduction to thetopic of process control by using only time-based representations of the process andthe associated instrumentation and control. We believe this book is the first to treatthe topic without relying at all upon Laplace transforms and the classical, frequency-domain techniques. For those students wishing to advance their knowledge of processcontrol beyond this first, introductory exposure, we highly recommend understanding,even mastering, the classical techniques. However, as an introductory treatment of thetopic, and for those chemical engineers not wishing to specialize in process control,but rather to extract something practical and applicable, we believe our approach hitsthe mark.

This text is organized into a framework that provides relevant theory, along with aseries of hands-on workshops that employ computer simulations that test and allowfor exploration of the theory. Chapter 1 provides a historical overview of the field.Chapter 2 introduces the very important and often overlooked topic of instrumentation.In Chapter 3 we ground the reader in some of the basics of single input – single outputsystems. Feedback control, the elements of control loops, system dynamics includ-ing capacitance and dead time, and system modelling are introduced here. Chapter 4highlights the various PID control modes and provides a framework for understandingcontrol-loop design and tuning. Chapter 5 focuses specifically on tuning. Armed withan understanding of feedback control, control loop structures, and tuning, Chapter 6


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introduces some more advanced control configurations including feed-forward, cas-cade, and override control. Chapter 7 provides some practical rules of thumb for de-signing and tuning the more common control loops found in industry. In Chapter 8 wetackle a more complex control problem: the control of distillation columns. As withthe rest of this text, a combination of theory and applied methodology is used to pro-vide a practical treatment to this complex topic. Chapter 9 introduces the concept ofmultiple loop controllers. In Chapter 10 we take a look at some of the important issuesrelating to the plant-wide control problem. Finally, up-to-date information on computersimulation for the workshops can be found on the book website.

Although this text is designed as an introductory course on process control for senioruniversity students in the chemical engineering curriculum, we believe this text willserve as a valuable desk reference for practising chemical engineers and as a text fortechnical colleges.

We believe the era of real-time, simulation-based instruction of chemical processcontrol has arrived. We hope you’ll agree! We wish you every success as you beginto learn more about this exciting and ever changing field. Your comments on andsuggestions for improving this textbook are most welcome.

William Y. SvrcekDonald P. Mahoney

Brent R. Young


Acknowledgements

It would be impossible to mention all of the individuals who contributed to the ideasthat form the background of this text. Over the past 5 years, we have interacted withmany students, academics, and, perhaps most importantly, practitioners in the field ofprocess control. This, combined with the more than 50 years of cumulative experienceamong the authors, has led to what we believe is a uniquely practical first encounterwith the discipline of chemical process control.

Some who deserve special mention for their influence include Bjorn Tyreus and EdLongwell from DuPont, and Paul Fruehauf from Applied Control Engineering. Thesegentlemen share a passion for the field and a commitment to the practical approach toboth teaching and practising process control.

As with any text, many more names were involved in its creation than the threeprinted on the cover. To those who put in such generous effort to help make this text areality, we express our sincerest of thanks.

To Dr Barry Cott, Global R&D Leader, Process Control and Optimization, ShellGlobal Solutions for contributing the section on ‘Screening control strategies via steady-state simulation’ in Chapter 8.

To Shannon Peddlesden, consulting engineer, for her capable assistance in editingand revisions to the second edition.

To Joanna Williams, consulting engineer, we would express our gratitude for hermany helpful suggestions. In particular, her careful editing of the original text andenhancements to the workshops is most appreciated.

To Dr Wayne Monnery, consulting engineer, for preparing the section on controlvalve sizing. We thank him for this excellent expose.

To Dr Martin Sneesby, consulting engineer, for the excellent effort in reviewing,testing, and suggested changes to the original group of workshops.

To Ken Trumble and Darrin Kuchle of Spartan Controls for facilitating the provisionof the detailed hardware schematics and photographs shown in the book. In particular,Ken’s many helpful comments on the text are much appreciated.

To the 1997, 1998, and 1999 fourth-year chemical engineering students at the Uni-versity of Calgary for their constructive comments on the book and, in particular, theworkshops.


Endorsements forthe first edition

‘As plants are pushed beyond nameplate, it is increasingly obvious that the importanceof process control has grown to the point where it is the single biggest leverage pointfor increasing manufacturing capacity and efficiency. The process engineer, who is bestposed to use his process knowledge for getting the most from better control, typicallyhas had just a single course in control. Furthermore, the approach was based on theoryrather than on practice, and was immersed in the frequency domain. Real processes arediverse and complex and the view into their behavior is by means of real time trendrecordings. This book provides a building block real time approach to understanding andimproving process control systems. Practical examples and workshops using modelsdrive home the points and make the principles much more accessible and applicable.’

Gregory K. McMillan, Senior Fellow, Solutia Inc.

‘At the undergraduate chemical engineering level, the traditional, highly mathematicalapproach misses the point of what knowledge of control and dynamics the practicingprocess engineer requires. If BS graduates in chemical engineering simply understoodthe basics of time based process dynamics and control (capacitance, dead time, PIDcontrol action and controller tuning, inventory, throughput, and distillation control), theimpact on process design and plant operations throughout the CPI would be immense.Today, these skills are among the least developed in BS chemical engineering gradu-ates, despite having taken the requisite traditional process control course. This text isparticularly suitable for any college, university, or technical training program seeking toprovide its graduates with a truly practical and applied background in process dynamicsand control. With today’s widespread commercial availability of high fidelity processsimulation software, the understanding gained from this text can be immediately anddirectly applied.’

Thomas C. Hanson, Senior Engineering Associate, Praxair, Inc.

‘Several years ago, a recruiter from a major chemical company told me that his com-pany was hesitant to interview students that indicated a first preference in the area of


xvi ENDORSEMENTS FOR THE FIRST EDITION

process control because his company “did not have any jobs that made use of Laplacetransforms and frequency domain skills”. This was an excellent example of the mis-match between what is frequently taught in universities, and what often gets applied inindustry. After teaching chemical process control for over 30 years, I feel strongly thatgood process control is synonymous with good chemical engineering. Industry wouldbe well served if all chemical engineering graduates, regardless of career paths, had abetter, more practical working knowledge of process dynamics and control. I think theapproach taken in this text is right on target, and is consistent with how we teach at theUniversity of Tennessee. It provides a good hands-on feel for process dynamics andprocess control, but more importantly, it presents these concepts as fundamentals ofchemical engineering. For undergraduate programs looking to transition away from thetraditional mathematical-based approach to a more applied, hands-on approach, thistext will be an invaluable aid.’

Charles F. Moore, Professor of Chemical Engineering, University of Tennessee

‘What BS degree chemical engineers need is a base level understanding of differentialequations, process dynamics, dynamic modeling of the basic unit operations (in thetime domain), basic control algorithms (such as PID), cascade structures and feedforward structures. With these basic tools and an understanding of how to apply them,they can solve most of their control problems themselves. What they do not need isthe theory and mathematics that usually surround the teaching of process control suchas frequency domain analysis. Graduate education in process control is the place tointroduce these concepts.’

James J. Downs, Senior Engineering Associate, Eastman Chemical Company


About the authors

William Svrcek is a Professor of Chemical and Petroleum Engineering at the Univer-sity of Calgary, Alberta, Canada. He received his BSc (1962) and PhD (1967) degreesin Chemical Engineering from the University of Alberta, Edmonton. Prior to joiningthe University of Calgary he worked for Monsanto Company as a senior systems en-gineer and as an Associate Professor (1970–1975) in the Department of Biochemicaland Chemical Engineering at the University of Western Ontario, London, Ontario.Dr Svrcek’s teaching and research interests centre on process simulation control anddesign. He has authored or co-authored over 150 technical articles/reports and has su-pervised over 30 graduate students. He has been involved for many years in teachingthe continuing education course titled ‘Computer Aided Process Design – Oil and GasProcessing’ that has been presented world-wide. Most recently this course has beenmodified to include not only steady-state simulation, but also dynamic simulation andcontrol strategy development and verification. Dr Svrcek was also a senior partner inHyprotech, now part of Aspen Technology, from its incorporation in 1976. As a Princi-pal, Director, and President (1981–1993) he was instrumental in establishing Hyprotechas a leading international process simulation software company. He is currently pro-viding leadership and vision in process simulation software as the President of VirtualMaterials Group Inc. He is a registered Professional Engineer, in both Alberta andOntario, and a member of professional societies that include The Canadian Societyfor Chemical Engineering, American Institute for Chemical Engineers, Canadian GasProcessors Association and the Instrument Society of America.

Donald Mahoney is co-founder and Chief Operating Officer with BDMetrics, Inc.,a company that develops and markets web-based analytics software. Mr Mahoneyearned a Bachelor’s Degree in Mechanical Engineering from Penn State, a Master’sDegree in Control Theory from Purdue University, and an MBA from the Universityof Delaware. Mr Mahoney has held research and teaching positions at the US Navy’sApplied Research Lab and Purdue University, where he was awarded the staff’s ‘Out-standing Teaching Award’. He has also lectured extensively on process simulation andcontrol topics, and has published a number of journal articles in the field. Prior to join-ing BDMetrics, Mr Mahoney was Vice President with AEA Technology EngineeringSoftware/Hyprotech where he led the introduction and launch of more than a half dozen


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design, modelling and optimization software products. He has held industrial positionsat General Motors and DuPont as a control systems engineer and process modelling andcontrol consultant. While at DuPont, Mr Mahoney was involved in the development andsupport of the chemical industry’s first object-oriented dynamic simulation package,TMODSTM.

Brent Young is Senior Lecturer of Chemical and Materials Engineering at the Univer-sity of Auckland, New Zealand. He received his BE (1986) and PhD (1993) degrees inChemical and Process Engineering from the University of Canterbury, New Zealand.Prior to his graduate studies, he worked as a Chemical Engineer for Ravensdown Fertil-izer Coop’s Super Phosphate Plant in Christchurch and developed a process model forthe simulation of a rock phosphate grinding circuit. In 1991, he joined the Universityof Technology in Sydney, Australia, as a lecturer, received tenure in 1994 and was pro-moted to Senior Lecturer in 1996, continuing his research in the areas of modelling andcontrol of processes, particularly industrial processes. He was an Associate Professor ofChemical and Petroleum Engineering at the University of Calgary from late 1998 to theend of 2005. He joined the University of Auckland in January 2006. He is a registeredProfessional Engineer and a member of a number of professional societies. His researchis centred on the two major areas of process simulation and control, and process designand development – particularly the processing of carbonaceous substances.

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