Process Control Systems.pdf

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Application

F. G. SHINSKEY

Systems Design Engineer, The Foxboro Company

“\

M C GRAW-HILL BOOK COMPANY a

New York

Toronto

London

Sydney

San Francisco

. . .

viii

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 I ix

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

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