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PROCESS INSTRUMENTATION IN CHEMICAL ENGINEERING CURRICULUM 1. B. ARANT Stanolind Oil and Gas Company, Tulsa, Oklahoma
~NSTRUMENTSfor measurement and control of process variables have become of increasing importance in the design and maintenance of performance of modern chemical, petro-chemical, and petroleum refining plants. The idea of what can and cannot be done within practical limits of design factors is now closely tied in with the degree of automatic control that can be maintained on process variables. Unfortunately, many chemical engineers, and essentially all young chemical engineering graduates, have little or no conception of what constitutes the basic principles of process control and measurement, what can be done with instruments, and the relation with chemical engineering design. Because of this lack of knowledge of the basic requirements of design for adequate instrument installation, the economic and satisfactory operation of many plants is handicapped from the start. While the actual specification and design of process instrumentation is largely left in the hands of the instrument engineering specialist, he is often sorely tried to install a satisfactory system wheu little regard has been given to proper design and installation of equipment and lines for incorporating instruments and control equipment. First, let us briefly review the general background of the instrumentation movement. In the early part of the present century, chemical and design engineers had little concern for instruments, as only the most simple types were at their disposal. Pressure gages, thermometers, and manometers were almost the only available instrument equipment and plants were largely operated by cut and try manual valving of process fluids. Most processes were run batchwise, and instruments merely gave a crude indication of process variables a t critical points and served as a rough guide for hand valve settings. As the quality and variety of instruments progressed and control mechanisms were developed, progressive industries, such as the petroleum refineries, incorporated them more and more into their plants as an aid to greater efficiency in plant operation and increased quality of the product. Processing was changing from batchwise to continuous, when possible. As yet, instrumentation had no pronounced effect on process and plant designthe instruments were being added, where needed, in the field. Advancements in process design and instruments progressed hand in hand in this manner until a few years before the last World War. It was then that the increasing expansion of chemical process industries brought a more widespread acceptance of instruments as a major essential equipment item.
During the war, huge plants for the production of aviation gasoline, synthetics, essential chemicals, and the atom bomb were successful only because instrument control made complex processes simple to operate. Quality and quantity of productive output were entirely dependent on automatic control and measurement. Today, every major process industry finds that it must rely heavily on instruments to meet competition in quality of product and economy of operation. Instruments are becoming more and more an essential factor in plant design, for many recent developments in chemical and refining processes, such as catalytic cracking, would be extremely difficult, if not impossible, if critical process variables were not closely controlled. This will be equally true in the future wheu modern electronic instruments and advancements in application of control and measurement will make possible even greater refinements in chemical design and operation. Where, then, is the place of the chemical engineer in this new field of specialized engineering? The general relations that have evolved between chemical design and development and process instrumentation have been brought out above. The purpose of this article is to show some of the basic principles of process instrumentation in relation to chemical engineering and to point out various factors that definitely show a great need for a process instrumentation course in the basic chemical engineering curriculum. The basis of most process instrumentation lies in a thorough understanding of unit operations and their applications in chemical processes. The ability to study and analyze a process for significant control variables and to understand process peculiarities will enable an instrument engineer to do a more satisfactory job in specifying a control system and specific control points. In like manner, the understanding of where, how, and when control equipment can be used is of great value to the chemical engineer in designing his equipment and planning hi process flow sheet and plant layout. The relationships and problems involved cannot be stated as simply as this, however. They are many and varied. Some of the more essential points are listed as follows: I . What Are the Process Requirements? The essential problems here have to do with what the process is to accomplish, what equipment is required, what is the sequence of operations, how many units will be needed, and how can savings in operation and increased efficiency be obtained. Whiie most of these problems primarily concern the process engineer, the decisions
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made will affect and be affected by the requirements for control as set forth by the instrument engineer. Often, two or more schemes of processing are devised, and mutual study by both engineers will bring to light factors that make one of the methods more economical than the others by better control and increased efficiency. Many savings in equipment, cost of processing, etc., become more readily apparent when viewed in this manner. 9. What Special Problems Are to Be Solved? Extremes in temperature and pressure as well as corrosion problems are often encountered in any process design. These problems, in many cases, directly affectpurity of product and ability to control critical and essential process variables. This will call for knowledge of ranges of various pressure and temperature devices and corrosion resistant materials. Ways and means of special handling of such problems often concern both process and instrument engineers. Climatic conditions usually have to be considered as a special problem. The effect of heat and cold on the process often dictates the use of lagging or insulation and control equipment will have to be modified accordingly. Likewise, protection against freezing must he given the instruments. 3. What Are the A c m r a y Requirements for Control? T i e and capacity lags as well as measurement lags in the process are often a limiting factor on the degree of control that can be maintained on a process. While instruments incorporating floating action and rate action are available to help minimize these factors, the effects usually cannot be entirely eliminated. A process demandmg a temperature variation of only a Z°F. span most certainly will not produce the desired results if the control installation can do no better than a 4°F. or 6'F. span. Should the process require extremely close control such as above, to produce the desired product, then close specification of the control installation or some modification of equipment or arrangement may have to be resorted to for adequate control. Close study by both process and instrument engineers is necessary to specifically plaoe the point of control measurement. 4. What Space I s AuailableI The amount of space available will directly affectthe arrangement of process buildings and equipment together with the location of control stations. As pointed out in Item 3, this arrangement will sometimes have a hearing on the control equipment and its ability to function properly. Location of control stations adjacent to sources of vibration such as compressors, reciprocating pumps, large centrifugal pumps or other machinery is poor practice. This does not necessarily imply that the control requirements are the limiting factor on arrangement, for compromises are usually made in such situations. 6. What Are the Operating Variables? Operating variables such as feed and reflux rates, pressures, temperatures, etc., will usually fixthe type and quality of instruments and controls in each operation. These data together with specific gravity, composition of line fluids, maximum and minimum flows and other data
JOURNAL OF CHEMICAL EDUCATION
as required are pertinent information of interest to both the process and instrument engineers. Whether a gas is wet or dry will dictate placement of orifice plates, for instance. Sediment in the line fluid, flashing and hydrate forming characteristics and viscosity are other examples of operating conditions that might need to be considered. 6. What Will Happen If S'peeifGed Operating Conditions Are Not Maintained? If the operating conditions necessary to produce a product of desired purity are not maintained, then not only will the product suffer, but possible damage to equipment and upsetting of . the process will occur. High temperatures may burn out furnace tubes and linings and high pressures may rupture equipment and lines, and cause a shutdown for major repairs. An upset in the system may ruin an entire run of material unless noted in time for corrections to be made. A central control system for a continuous process is not only good practice but often eliminates many operating headaches. A11 critical operating measurements and controls grouped together in one centralized location is the best possible insurance for 6fficient and economic operation. Time and money lost in rerunning materials and lining out the process are reduced to a minimum. 7.. What Are the Safety Requirements? High temperatures and pressure must be controlled as well as possible so as to protect equipment and personnel. In the event of air failure, control valves must be specified to open or shut, as the case may be for maximum protection. Emergency shutdowns for fire or other causes will necessitate venting of flammable gases and vapor to a distant flare, and relief valves should be sized to protect vessels from rupture. It is a good idea for the process and instrument engineer to discuss the possibilities and safety requirements so that emergency switches, alarms, valves, etc., can be located to the best advantage. 8. What Changes Can Be Made to Increase Controllability? Often, changes in sequence or type of equipment will offer increased possibilities for control of a process. The point of measurement might be shifted further up or down the line for more efficient control. The possibilities are many and only mutual study by both process engineer and instrument engineer will bring items such as these to light. Many savings can be made in this way and efficiency is usually greatly enhanced. 9. What About Instrument Installation? The proper installation of instruments and controls requires a great deal of coordination between the process and design section and the instrument section. It is often the case that the effectiveness of a control system is greatly reduced by improper xttention paid to the requirements of installation. Are transmitters and controllers placed adjacent to control valves so that manual by-passing is easily accomplished? Are pipe runs laid out in a manner conducive to correct installation of orifices? Can temperature bulbs and thermocouples he placed so as to indicate correctly? Are clearances adequate
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for servicing control equipment? Are gage glasses placed where they can be easily seen and read? These are only a few of the questions and problems that conceln correct instrument installation. It is to the process engineer's advantage to consider such questions equally important as other design factors. This is an indication of the problems and relationships that exist between chemical engineering and process instrumentation, and the present intimate relationship becomes more readily apparent. It is because of this relationship that it has now become necessary to incorporate basic instruction in process instrumentation to the student working on a chemical engineering degree. Such instruction running concurrently with courses in chemical engineering unit operations 'nd plant design would be of tremendous value in understanding operation of equipment and processes from the standpoint of how and where they are controlled t o produce the desired results. Actual practice in the use of the more frequently encountered types of instruments, such as flow, temperature, and pressure recorders and controllers, could be .obtained in the unit operations laboratory. Many young chemical engineers can recall from their undergraduate days that they often wondered how one would know how much steam was going t o a fractionator reboiler, how the overhead product was split into the desired reflux and net overhead streams, and what means were used to hold the tower pressure constant. Little mention, if any, was ever made of instrument control and equipment, and it was usually sometime after entering the field before an understanding was obtained on how a process was operated by instrument control., Such a comparatively simple item as a pneumatic control valve was a source of fascination, and the operation of a controller was indeed the greatest of all mysteries. The author must confess that he was under the impression that a plant was operated almost wholly by manual valve setting and, from personal observation, has found that the same idea is in the mind of practically
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every young chemical engineer just entering the field. The extent of training on process instrumentation in an engineering school is of necessity limited in scope for several reasons. First, of course, is the limited amount of equipmeat that can be incorporated in the unit operations laboratory. Second is the inability to adequately duplicate actual control systems as found in the field. Third is the lack of time that can be devoted t o a more thorough study of this subject. The mere study of a basic course in process instrnmentation does not necessarily imply that it will fit the young engineer to desigq or install control systems any more than he would be able to design a fractionator or lay out a process. Such things come only with thorough field training tempered by experience. The course would fulfill two purposes, however. First, it will help do away with the basic distrust that all engineers have with operating equipment not familiar to them; and it will enable the engineer t o have a better understanding of plant operation under instrument control, as well as understand the problems involved. Secondly, it will give those chemical engineers who desire to enter the instrument engineering field the background necessary to absorb the training and detailed instruction necessary to qualify them for work in this specialized field. The diversified training given the chemical engineer qualifies him to understand what takes place, and why, when certain process variables are controlled, and so he is by far the most logical one to take a prominent part. It is becoming obvious that the increasingly important role that instruments must and will play necessitates the inclusion of a t least one semester of work on this subject in the curriculum of every progressive chemical engineering school. The demands of our high-geared modern technology require that young engineers be given a t least fundamental training on each subject necessary to provide the tools for understanding the requirements of modern chemical processes. The value in the field to industry is a proved fact.
