Process Control Systems: Application, Design, and Adjustment

376 Pages·1988·4.26 MB·English

by  F. Greg Shinskey

Application
F. G. SHINSKEY
Systems Design Engineer, The Foxboro Company
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vi.i .i . I Preface
are not communicated to the people who must apply them. Control
problems arise in the plant and must be solved in the plant. Until plant
engineers and control designers are able to communicate with each
other, their mutual problems await solution. I do not mean to imply
that abstract mathematics is not capable of solving control problems, but
it is striking how often the same solution can be reached by using good
common sense. High-order equations and high-speed computers can
be manipulated to the point where common sense is dulled.
Some months ago I was asked to give a course on process control to
a large group of engineers from various departments of The Foxboro
Company. Sales, Product Design, Research, Quality Control, and
Project Engineering were all to be represented. If the subject were
presented through the traditional medium of operational calculus, the
effort would be wasted, because too few of the students would have this
prerequisite. Rather than attempt to teach operational calculus, I
chose to do without it altogether. It then became necessary to approach
control problems solely in the time domain. Once the transition was
begun, I was surprised at the fresh point of view which evolved. Some
situations which were clouded when expressed in frequency or in complex
numbers were now easily resolved. Dead time, fundamental to any
transport process, is naturally treated in the time domain.
The value of this new approach was evident at once. In the very
first session the student was able to understand why a control loop behaves
the way it does: why it oscillates at a particular period, and what deter-
mines its damping. The subject was tangible and alive to many students
for the first time. Interest ran high, and the course was an immediate
success. The great demand for notes prompted the undertaking of
this book.
Through the years, I have observed many phenomena about control
loops which have never been explained to my satisfaction. Why does
a flow controller need such a wide proportional band, whereas a pressure
controller does not? Why is derivative less effective in a loop contain-
ing dead time than in a multicapacity loop? Why are some chemical
reactors impossible to control? What makes composition control
SO
difficult? Why cannot some oscillations be damped? These and many
other observations are explained in this book and perhaps nowhere else.
It is always very satisfying to learn the reasons behind the behavior
of things which are familar, or to see accepted principles proven in a new
and different way. Therefore i expect that those who are accustomed
to the more conventional approaches to control system design will find
this treatment as interesting as those who are not familiar with any.
In spite of the simplicity of this presentation, we are not kept from
Preface ix
I
applying the most advanced concepts of automatic control. Feedfor-
ward control has proven itself capable of a hundredfold improvement
over what conventional methods of regulation can deliver. Recent
developments in nonlinear control systems have pushed beyond tradi-
tional barriers-achieving truly optimum performance. These advances
are not just speculation-they are paying out in increased throughput
and recovered product. Although their impact on the process industries
is as yet scarcely felt, the revolution is inevitable. The need for economy
will make it so.
But the most brilliantly conceived control strategy, by itself, is noth-
ing. By the same token, the most definitive mathematical representa-
tion of the process, alone, is worthless. The control system must be
the embodiment of the process characteristics if it is to perform as
intended. Without a process, there can be no control system. Anyone
who designs controls without knowing what is to be controlled is fooling
himself. A pressure regulator cannot be used to control composition.
Neither can a temperature controller on a fractionator perform the same
function as one on a heater. For these reasons this entire text is written
from the viewpoint of the needs of the process. Each type of physical-
chemical operation which has a history of misbehavior is treated in-
dividually. Not every situation can be covered, because plants and
specifications differ, and so do people. If for no other reason, this book
will never be complete. But enough attention is given to basic prin-
ciples and typical applications to permit extension to a broad area of
problems. The plant engineer can take it from there.
In appreciation for their assistance in this endeavor, I wish to express
my gratitude to Bill Vannah for providing the initiative, to Molly
Dickinson, who did all the typing, and to John Louis for his thoughtful
criticism.
Greg Shinskey
Preface vii
PART UNDERSTANDING FEEDBACK CONTROL
1. Dynamic Elements in the Control Loop 3
Negative Feedback 4
The Difficult Element-Dead Time 6
The Easy Element-Capacity 18
Combinations of Dead Time and Capacity 31
Summary 35
Problems 35
2. Characteristics of Real Processes 37
Multicapacity Processes 38
Gain and Its Dependence 44
Testing the Plant 55
xi
xii I Contents
References 59
Problems 59
3. Analysis of Some Common LOOPS 61
Flow Control 62
Pressure Regulation 67
Liquid Level and Hydraulic Resonance 71
Temperature Control 74
Control of Composition 80
Conclusions 86
References 87
Problems 87
PART SELECTING THE FEEDBACK CONTROLLER
4. Linear Controllers 91
Performance Criteria 92
Two- and Three-mode Controllers 95
Complementary Feedback 103
Interrupting the Control Loop 110
Direct Digital Control 118
References 122
Problems 123
5. Nonlinear Control Elements 124
Nonlinear Elements in the. Closed Loop 125
Nonlinear Dynamic Elements 128
Variations of the On-off Controller 131
The Dual-mode Concept 136
Nonlinear Two-mode Controllers 144
Problems 149
PART MULTIPLE-LOOP SYSTEMS
6. Improved Control through Multiple Loops 153
Cascade Control 154
Ratio Control Systems 160
Selective Control Loops 167
Adaptive Control Systems 170
Summary 179
References 180
Problems 180
Contents I x-iii
7. Multivariable Process Control 181
Choosing Controlled Variables 182
Pairing Controlled and Manipulated Variables 188
Decoupling Control Systems 198
Summary 202
References 202
Problems 203
8. Feedforward Control 204
The Control System as a Model of the Process 206’
Applying Dynamic Compensation 211
Adding Feedback 219
Economic Considerations 224
Summary 227
References 228
Problems 228
APPLICATIONS
9. Control of Energy Transfer 233
Heat Transfer 23.4
Combustion Control 241
Steam-plant Control Systems 243
Pumps and Compressors 250
References 256
Problems 256
10. Controlling Chemical Reactions 257
Principles Governing the Conduct of Reactions 268
Continuous Reactors 269
pH Control 275
Batch Reactors 282
References 286
Problems 286
Il. Distillation 288
Factors Affecting Product Quality 289
Arranging the Control Loops 295
Applying Feedforward Control 307
Batch Distillation 319
Summary 323
References 323
Problems 324
xiv I Contents
12. Other Mass TransferOperations 325
Absorption and Humidification 326
Evaporation and Crystallization 332
Extraction and Extractive Distillation 338
Drying Operations 343
Summary 346
References 347
Problems 347
Appendix: Answers to Problems 349
Index 355
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