THE AUTOMATIC CONTROL PROBLEM F. J. VAN ANTWERPEN 60 East 42nd Street, New York, N. Y.
UTOMATIC control of industrial processes, both mechanical and chemical, is slowlyapproaching the status of a science and is growing out of the stage in which applications to and analysis of problems were more an art than a science. In the past the words “automatic control” were sheer magic, and blurring the true picture of what could be done were the visions of larger profits if processes were made automatic. This created a sudden stampede to put a control on every type of process, machine, and pipe line. Analysis of control factors was a relatively unknown science, and the best way to solve a control problem was a
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Vol. 34, No. 4
Micromax Conductivity Recording Controllers o n Carbonising Baths in a Worsted Mill Are S h o w n o n t h e Panel Below.
T h e Micromax Recorder in t h e Circle Indicates and Records t h e Temperature of a Resin Kettle, and a Thermocouple Is Shown in t h e Above Picture Installed on Top of t h e Kettle. A t t h e Left Are Conductivity Cells in t h e Worsted Mill Carbonizing Bath. Courtesy, Lee& & Northrup Company
April, 1942
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comdete mess of misatmlied instrumentation, triesand processes were successfulin adapting controliers, and in many instances gratifying dividends were realized. In many other instances, however, there was complete failure to change procedures to automatic operation. It is now believed by the best minds engaged in adapting instrumentation to chemical processes that the scientific principles of control will some day show just why there were failures and that these principles will indicate solutions of the difficulties. The most important advance registered by control device manufacturers, since the inception of the idea of control, has taken place in the past few years; that advance is a study of the basic principles of control apparatus and, more important, of the basic principles of the process the apparatus is expected to control. Already there have been substantial benefits from the application of these scientific principles, and there will be more benefits as the snowball of accumulated knowledge grows. However, the development of every new scienceand this is a new science-has always been hindered by efforts to conjugate theorems and to materialize initial information into something immediately usable; caution must be exercised in translating new facts into commercial realities. It is safe t o say that the rapid present-day progress in industrial processing and the attempts to add continuous automatic operation to these processes has introduced complications much faster than commercial control developments can solve them. The few individuals and committees attempting to find some workable solution in analytically weighing all the factors entering the vast problem of instrument control are in danger of being overwhelmed and discredited before there is a fair chance to apply their studies. Once again chemical industry must expend patient money to ensure the growth of an infant science, and this article hopes to point a way t o do this. One of the major precepts in the vast field which is now opening to students of control devices is the knowledge that correct primary design of processes and equipment is a necessary adjunct to the correct application of the control instrument. Primary design is the field of the chemical engineer, especially design of processes. One of the major considerations of chemical engineering design is that operations, or processes, retain a state of static balance, and enormous amounts of detailed and accurate engineering are done to assure this condition. The success of many processes depends upon the maintenance of steady-state conditions ; preliminary calculations are based upon this and there is seldom a thought as to what the dynamic characteristics or changing conditions are. Provisions for change are seldom made and the same concept, that of considering automatic control as static, has led to many misapplied and unfortunate applications. This may seem strange in view of the fact that the alleged purpose of a control installation is to hold constant a certain process variable. This is true in so far as it is accomplished only by a series of recoveries from threatening unbalances, such as variations of flow, pressure, and liquid level. The continuity of these unbalances is usually gradual; process variables are eliminated. To date, chemical engineering has designed on a static basis and is a t cross purposes with the control engineer who is always concerned with the dynamic characteristics of the process and continually re-establishing the desired state of static balance. Control engineering is based upon transient characteristics, and let it be noted that extreme changes in variable can seldom be satisfactorily corrected in a complex process by control apparatus alone; there must also be a basic design which helps the control to maintain its method of continual adjustment, or averaging, of upsets. Static engineering design enables only the determination of the range of variables and makes no provisions for minimizing their
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mining the time cvcles and duration of the maximum and minimu;. It mig& also be stated that there is little realization, outside of the small circle of control engineers, of the extreme changes that take place in most conventional chemical equipment and the effects which these changes cause. Automatic control manufacturers have few opportunities to carry out analyses of such changes in chemical processing; yet to a great extent they are the only industry that has tried to do so. Chemical industry has much to gain in experiments of this sort and only a few tons of raw material to lose. Pilot plant technique does not seem to be adaptable to these problems, as has been discovered by the companies who have conducted studies in their laboratories. A further difficulty is found in the realization that practical experience, based on technique other than that expended in control work, is of little value because most unsolved control problems concern reactions and reasoning solved only by second-order mathematics. All the problems have forced the applications of controls to be made in an experimental way and have depended to a great extent upon experience gained in similar installations in which the problems have been solved. It has also meant that instruments and control devices have always been added to equipment after specifications as t o processes, arrangement, size, and capacity have been written. Distinct advantages are to be gained by a preliminary consideration of control factors and the subsequent adjustment of specifications to meet the demands which such considerations require. Parallel with the recent knowledge that control may be facilitated by design are the beginnings of several studies on the basic principles of automatic control which may, in time, enable control problems to be solved on paper instead of by actual installations. There must be a radical change in the attitude of chemical industry as to the type of men it entrusts with control engineering, however, before this is brought about. If industry is to benefit by the investment of patient money in control work, it must begin to appoint trained, imaginative engineers who can and will coordinate research and plant practice. Further than this, present trends seem to indicate that economic studies between most efficient engineering practice and most efficient control practice are necessary, for very often the two are a t odds, The law of diminishing returns applies in this field just as in any other.
Factors in Processing Design There would seem to be a place in this article for a description of the control apparatus now available, their functions, and workings; but nothing would be gained, for it is of no importance, The most important problem facing this science is the determination of process characteristics, the effect the change of one variable has upon another ahd upon the time, and the changes this effect has upon the system as a whole. The mathematical equations and characteristics of most control apparatus are known, but this knowledge is of little avail if the corresponding equation of the system upon which it works is unknown. The two are dependent upon each other. Added to this is the unpleasant fact that process analysis has advanced to such a small degree that one large control manufacturer has only one stated rule and that is “when in doubt use a flow controller”. Such a small amount is known about the effects of change upon systems that they must first seize upon the easiest thing to stabilize, flow, and work on from there. Here we see in operation how important it is to fix the causes of upset, and usually the flow controller is placed upon the primary ingredient such as flow of charge to a fractionating tower or steam to a dryer. Only recently has there been an indication of the most important factors to be considered in preliminary design for
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Temperature, Pressure, Flow, and Quantity Are Variables Which Must Be Controlled i n Reaction Kettles ( l e f t ) . T e m perature Control Is Important in Drying Textiles; the Illustration Below Shows a Typical Installation. Recording and Control Instruments Are Often Placed Apart f r o m the Controlled Equipment Shown i n the Photograph a t the Bottom. Courtesy, The Foxboro Company
automatic control and these factors, which follow, are probably the only certain rules that this new science has yet been able to formulate. The first consideration is whether the variable is susceptible to measurement; is it possible to measure accurately the change taking place? An example might be found in the drying of amorphous pigments, where it is desired to measure the moisture content of the material and to control from that. There is no industrial means of measuring the moisture content of materials as such. Humidity may be used as the controlling measurement if there is a relation between the moisture content of the material and the humidity of the surrounding atmosphere. If no such relation exists, then judgment must be used and this sagacious factor cannot be built into a control apparatus. Bound up in the problem of the susceptibility of measurement is the significance of the measurement once it is made and the necessity of the measurement remaining significant over the full working range. This is easily shown by the problem of controlling the heat input to a tank of water a t the boiling point. Up to the boiling point, temperature is an accurate measurement of heat input. At the boiling point, however, latent heat effects prevent temperature from being utilized as an indication of heat input. Here we see that the measurement used in the early stages of heating has failed to meet
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the requirement of being consistently significant. At such a critical point it becomes necessary to use a measurement which is significant, the significance depending upon the process. The third important consideration, if the system meets the foregoing requirements, is whether the response to control action is consistent. As an example, it is desired to control the flow of a liquid or gas pumped by a reciprocating pump. Flow can be measured easily, and it may seem possible t o control i t by using flow change to regulate a valve in the steam line to the pump and thus vary pump speed in direct response to flow change. The mechanics of this problem are easy as far as control is involved. However, the amount of valve movement, on the steam supply side of the pump, between complete shutdown and full speed may be very small and ever changing. Control by this method would entail difficulty, especiallywhere pumps are misapplied or taken from a service for which they were designed and used in another installation. I n another case it may be desired to put the controlled valve in a line which is by-passed back to the suction of a centrifugal blower or pump. This is a common installation for controlling the flow through a discharge line. Success in such a case will depend upon the characteristic curve of the blower or pump; for flow changes in response t o pressure drop
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and a flat characteristic curve will vary discharge pressure only slightly in response to valve opening. If the curve has a definite slope, changing the valve opening will change the discharge line inlet pressure and control can be secured. One other factor of great importance is the relation of pressure drop to controllability. Pressure drop through valves installed directly in a flow line must bear a sufficiently large ratio to the variable friction drop through the line so that the response of flow to valve port changes will be consistent over the required range of flows. If this friction drop through a system is 200 pounds per square inch, and the drop through a valve under normal operation is 10 pounds (a widely used number for pressure drop), the valve can increase the flow only in the ratio of the square roots of the system pressure plus valve pressure to that of the system (dm/d%).
Acknowledgment Many persons have aided the author in the difficult task of describing this new scientific thought, so new i t was feared that an early expression, necessarily devoid of immediate commercial value, would do more to hinder than to help. To them he extends thanks and hopes that the reaction is one of greater help from the vast chemical industry.
Photograph by Robert Yarnall Richie
Control Panel in the Power Plant of Texas Gulf Sulphur Company, Newgulf, Texas
