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Process Control
A Practical Approach
Process Control: A Practical Approach Myke King
© 2011 John Wiley & Sons Ltd. ISBN: 978-0-470-97587-9
Process Control
A Practical Approach
Myke King
Whitehouse Consulting, Isle of Wight, UK
Thiseditionfirstpublished2011
2011(cid:1)JohnWiley&SonsLtd
Registeredoffice
JohnWiley&SonsLtd,TheAtrium,SouthernGate,Chichester,WestSussex,PO198SQ,UnitedKingdom
Fordetailsofourglobaleditorialoffices,forcustomerservicesandforinformationabouthowtoapplyforpermissiontoreuse
thecopyrightmaterialinthisbookpleaseseeourwebsiteatwww.wiley.com.
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PatentsAct1988.
Allrightsreserved.Nopartofthispublicationmaybereproduced,storedinaretrievalsystem,ortransmitted,inanyformorby
anymeans,electronic,mechanical,photocopying,recordingorotherwise,exceptaspermittedbytheUKCopyright,Designs
andPatentsAct1988,withoutthepriorpermissionofthepublisher.
Wileyalsopublishesitsbooksinavarietyofelectronicformats.Somecontentthatappearsinprintmaynotbeavailablein
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Designationsusedbycompaniestodistinguishtheirproductsareoftenclaimedastrademarks.Allbrandnamesandproduct
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LibraryofCongressCataloging-in-PublicationData
King,Michael,1951-
Processcontrol:apracticalapproach/MichaelKing.
p.cm.
Includesbibliographicalreferencesandindex.
ISBN978-0-470-97587-9(cloth)
1.Chemicalprocesscontrol.I.Title.
TP155.75.K562011
660’.2815–dc22
2010034824
AcataloguerecordforthisbookisavailablefromtheBritishLibrary.
PrintISBN:9780470975879
e-PDFISBN:9780470976555
o-BookISBN:9780470976562
e-PubISBN:9780470976661
Setin10/12TimesRomanbyThomsonDigital,Noida,India
PrintedinSingaporebyFabulousPrintersPteLtd.
Contents
Preface ix
About the Author xv
1. Introduction 1
2. Process Dynamics 3
2.1 Definition 3
2.2 Cascade Control 9
2.3 Model Identification 11
2.4 Integrating Processes 20
2.5 Other Types of Process 22
2.6 Robustness 24
2.7 Laplace Transforms for Processes 27
References 28
3. PIDAlgorithm 29
3.1 Definitions 29
3.2 Proportional Action 30
3.3 Integral Action 33
3.4 Derivative Action 35
3.5 Versions of Control Algorithm 39
3.6 Interactive PID Controller 41
3.7 Proportional-on-PV Controller 43
3.8 Nonstandard Algorithms 50
3.9 Tuning 51
3.10 Ziegler-Nichols Tuning Method 52
3.11 Cohen-Coon Tuning Method 56
3.12 Tuning Based on Penalty Functions 57
3.13 Manipulated Variable Overshoot 60
3.14 Lambda Tuning Method 61
3.15 IMC Tuning Method 63
3.16 Choice of Tuning Method 65
3.17 Suggested Tuning Method for Self-Regulating Processes 66
3.18 Tuning for Load Changes 66
3.19 Tuning for Unconstrained MVOvershoot 71
3.20 PI Tuning Compared to PID Tuning 72
3.21 Tuning for Large Scan Interval 74
3.22 Suggested Tuning Method for Integrating Processes 76
vi Contents
3.23 Implementation of Tuning 78
3.24 Loop Gain 79
3.25 Adaptive Tuning 79
3.26 Initialisation 80
3.27 Anti-Reset Windup 81
3.28 On-Off Control 81
3.29 Laplace Transforms for Controllers 83
3.30 Direct Synthesis 85
References 88
4. Level Control 91
4.1 Use of Cascade Control 91
4.2 Parameters Required for Tuning Calculations 93
4.3 Tight Level Control 97
4.4 Averaging Level Control 100
4.5 Error-Squared Controller 105
4.6 Gap Controller 108
4.7 Impact of Noise on Averaging Control 111
4.8 General Approach to Tuning 113
4.9 Three-Element Level Control 114
5. Signal Conditioning 117
5.1 Instrument Linearisation 117
5.2 Process Linearisation 119
5.3 Constraint Conditioning 122
5.4 Pressure Compensation of Distillation Tray Temperature 124
5.5 Pressure Compensation of Gas Flow Measurement 125
5.6 Filtering 126
5.7 Exponential Filter 127
5.8 Higher Order Filters 129
5.9 Nonlinear Exponential Filter 130
5.10 Averaging Filter 131
5.11 Least Squares Filter 132
5.12 Control Valve Characterisation 136
5.13 Equal Percentage Valve 137
5.14 Split-Range Valves 140
6. Feedforward Control 147
6.1 Ratio Algorithm 147
6.2 Bias Algorithm 151
6.3 Deadtime and Lead-Lag Algorithms 152
6.4 Tuning 155
6.5 Laplace Derivation of Dynamic Compensation 161
7. Deadtime Compensation 163
7.1 Smith Predictor 163
Contents vii
7.2 Internal Model Control 166
7.3 Dahlin Algorithm 167
References 168
8. Multivariable Control 169
8.1 Constraint Control 169
8.2 SISO Constraint Control 170
8.3 Signal Selectors 171
8.4 Relative Gain Analysis 174
8.5 Steady State Decoupling 177
8.6 Dynamic Decoupling 180
8.7 MVC Principles 184
8.8 Parallel Coordinates 187
8.9 Enhanced Operator Displays 188
8.10 MVC Performance Monitoring 189
References 195
9. Inferentials and Analysers 197
9.1 Inferential Properties 197
9.2 Assessing Accuracy 203
9.3 Laboratory Update of Inferential 208
9.4 Analyser Update of Inferential 210
9.5 Monitoring On-stream Analysers 212
Reference 214
10. Combustion Control 215
10.1 Fuel Gas Flow Correction 215
10.2 Measuring NHV 220
10.3 Dual Firing 222
10.4 Inlet Temperature Feedforward 223
10.5 Fuel Pressure Control 225
10.6 Combustion Air Control 227
10.7 Boiler Control 236
10.8 Fired Heater Pass Balancing 237
11. Compressor Control 243
11.1 Polytropic Head 243
11.2 Flow Control (Turbo-Machines) 246
11.3 Flow Control (Reciprocating Machines) 251
11.4 Anti-Surge Control 252
12. Distillation Control 259
12.1 Key Components 262
12.2 Relative Volatility 263
12.3 McCabe-Thiele Diagram 266
12.4 Cut and Separation 271
viii Contents
12.5 Effect of Process Design 281
12.6 Basic Controls 285
12.7 Pressure Control 285
12.8 Level Control 299
12.9 Tray Temperature Control 315
12.10 Pressure Compensated Temperature 325
12.11 Inferentials 335
12.12 First-Principle Inferentials 342
12.13 Feedforward on Feed Rate 344
12.14 Feed Composition Feedforward 348
12.15 Feed Enthalpy Feedforward 349
12.16 Decoupling 350
12.17 Multivariable Control 352
12.18 On-stream Analysers 360
12.19 Towers with Sidestreams 361
12.20 Column Optimisation 364
12.21 Optimisation of Column Pressure 366
12.22 Energy/Yield Optimisation 368
References 370
13. APC Project Execution 371
13.1 Benefits Study 371
13.2 Benefit Estimation for Improved Regulatory Control 373
13.3 Benefits of Closed-Loop Real-Time Optimisation 380
13.4 Basic Controls 382
13.5 Inferentials 384
13.6 Organisation 385
13.7 Vendor Selection 389
13.8 Safety in APC Design 391
13.9 Alarms 392
References 393
Index 395
Preface
Sowhywriteyetanotherbookonprocesscontrol?Therearealreadymanypublished,but
they are largely written by academics and intended mainly to support courses taught at
universities.Excellentassomeofthesebooksareinmeetingthataim,thecontentofmany
academic courses has only limited relevance to control design in the process industry.
Thereareafewbooksthattakeamorepracticalapproachbuttheseusuallyprovideonlyan
introduction to the technologies. They contain enough detail if used as part of a wider
engineeringcoursebutnotenoughforthepractitioner.Thisbookaimsmoretomeetthe
needs of industry.
Mostengineersresponsibleforthedesignandmaintenanceofcontrolapplicationsfind
dauntingmuchofthetheoreticalmathematicsthatiscommonintheacademicworld.In
this book we have aimed to keep the mathematics to a minimum. For example, Laplace
transformsareonlyincludedsothatthereadermayrelatewhatisinthisbooktowhatwill
be found in most theoretical texts and in the documentation provided by many DCS
(distributed control system) vendors. They are not used in any of the control design
techniques.Andwhilewepresentthemathematicalderivationofthesetechniques,toshow
that they have a sound engineering basis, the reader can skip these if too daunting and
simply apply the end result.
The book aims to present techniques that have an immediate practical application. In
additiontothedesignmethodsitdescribesanyshortcutsthatcanbetakenandhowtoavoid
commonpitfalls.Themethodshavebeenappliedonmanyprocessesonawiderangeof
controllers. They should work!
Inadditiontoprovidingeffectivedesignmethods,thisbookshouldimprovetheworking
practices of many control engineers. For example, the majority still prefer to tune PID
(proportional, integral, derivative) controllers by trial and error. This is time-consuming
andrarelyleadstocontrollersperformingaswellastheyshould.Thismightbebecauseofa
justifiedmistrustofpublishedtuningmethods.Mostdohaveseriouslimitations.Thisbook
addresses this and offers a method proven to be effective in terms of both controller
performance and engineering effort.
DCSincludeawidearrayofcontrolalgorithmswithmanyadditionalengineer-definable
parameters.TheDCSvendorsarepooratexplainingthepurposeofthesealgorithmswith
the result that the industry is rife with misinterpretation of their advantages and
disadvantages. These algorithms were included in the original system specification by
engineers who knew their value, but this knowledge has not passed to the industry. The
resultisthattherearesubstantialimprovementsthatcanbemadeonalmosteveryprocess
unit, surpassing what the control engineer is even aware of – let alone knows how to
implement. This book addresses all the common enhancements.
Thisbooktakesaback-to-basicsapproach.TheuseofMVC(multivariablecontrollers)
is widespread in industry. Control engineering staff and their contractors have invested
