December 1947
Instrumentation A s u m m a r y is given of t h e proceedings of t h e 1946 Texas A. & M. short course on i n s t r u m e n t a t i o n for t h e process i n d u s t r i e s .
by Ralph H. Munch
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NSTRUMENTATION can advance only if sufficient numbers of mathematicians, scientists, engineers, and technicians well trained in the field are available. Because it is a new field, there are no institutions with specialized courses leading to degrees in instrumentation. The experts in the field are those who started with good broad fundamental training in science and engineering and added considerable amounts of experience in the application of instruments in plants. As we have noted previously, educational institutions are giving a great deal of thought to the problem of how best to furnish men suitably trained to take advantage of the opportunities available in the field of instrumentation. Various possible means of solving the problem are being tried out. After these have been in operation long enough, they will probably supply the trained personnel needed. In addition to the need for men trained to enter the field of instrumentation, there is a very real need to give those already in the field an opportunity to better their knowledge of the subject. To fill this need, the School of Engineering of the Agricultural and Mechanical College of Texas has started a series of annual short courses. The first of these was held September 3 through 6, 1946. The proceedings of this symposium have recently been published as Bulletin 100 of the Agricultural and Mechanical College of Texas. This bulletin, entitled "Instrumentation for the Process Industries," can be obtained from the Texas Engineering Experiment Station, College Station, Tex., a t $2.00 per copy. To quote from the foreword of the bulletin, "In the 1946 course the object was to present a summary of the fundamentals of the subject and a survey of methods of application to actual measurement and control in plant practice. "The course is intended to be on the engineering level, with the major emphasis on the practical rather than the theoretical side. Since it is the theory, however, which largely provides the unifying thread, some theoretical aspects of instrumentation are covered. "The subject of instrumentation, if taken in its entirety, would be so broad that adequate coverage could not be given it. The field has been limited by placing primary emphasis on applications to continuous fluid flow processes. Measurement of gas flow is not done in great detail. Even with these limitations the field is so broad that it cannot be covered completely in any one year; hence it must be expected that thorough treatment of the entire subject will be obtained only in the course of several years' meetings." The symposium opened with a paper by C. S. Comstock of Monsanto Chemical Company entitled "Some Economic Considerations of Instrumentation." I t begins by citing the advantages of instrumentation—to make production possible, to improve product purity and uniformity, to reduce the dangers of operation, and to reduce costs of production. Next,
costs of instruments and labor are developed. I t is shown that the cost of owning an instrument—depreciation, return on investment, and cost of operating—amounts to $281 for the average control instrument, and t h a t the cost of a man per shift for a year is about $15,000. In spite of this, labor costs cannot be reduced beyond a certain point by substituting instruments for operators. The advisability of designing process equipment and instrumentation simultaneously to take full advantage of instrumentation is discussed. Finally the functions of an instrument department are described. "Measurement of Flow and Liquid Level" was the title of a paper by L. K. Spink of the Foxboro Company. I t is a, rather complete introduction to these two closely related subjects. The major types of flowmeters are described, but emphasis is put on variable head meters using the concentric flat-plate sharp-edged orifice. The piping hookup, arrangement of seal pots, and precautions to be observed to ensure accurate results are described in detail for many applications. Many piping diagrams are given as well as graphs showing Reynolds number corrections for various orifice-to-pipe diameter ratios and flows. A description of the commonly used types of liquid level instruments concludes the paper. V. L. Parsegian of the C. J. Tagliabue Division of Portable Products Corp. described the measurement of temperature and pressure. This paper gives an excellent résumé of the theory and practice of temperature measurement but does not cover pressure measurement thoroughly. The author makes a point in this paper which is all too often forgotten; he reminds his readers that the complexity and price of instruments go up very rapidly as the accuracy or speed requirements are made more rigid. For example, he states that, using the thermocouple method, one may measure the temperature of a furnace to within an accuracy of ="=3% at 1000° F . without spending more than a few dollars for a simple millivoltmeter. To increase the accuracy to within ± 0 . 5 % would multiply the cost of the meter by a factor of perhaps 10, while to increase it further to = t 0.2% would require the use of potentiometric type instruments and considerable care in the choice and use of the thermocouples, at a cost 20 to 30 times the original. Therefore one should choose equipment sufficiently good but not too good for any particular application. A survey of electrochemical measuring devices was presented by W. N . Greer of Leeds & Northrup Company, who gave an interesting survey of electrolytic conductivity methods, pH methods, and oxidation-reduction potential methods. He was careful to call attention not only to the potentialities of these methods but to their limitations. 85 A
Instrumentation The Taylor Instrument Companies' contribution to the program was a talk on some aspects of the theory of process control, by J. G. Ziegler. Ziegler points out that a good deal can be done to many processes to make them more easily con trollable, and t h a t an understanding of the effects of load changes in any process is necessary for the design of a good control system. He discusses methods for determining experi mentally the best settings for controllers using the response curve of the system. In an analysis of control of light ends fractionation E. D. Mattix of Cities Service Corporation discussed the control of fractionating equipment, known in petroleum refining circles as a debutanizer. He gives the liquid-vapor equilibrium dia gram for the n-butane-n-pentane system and also shows the liquid composition for each tray in the tower. The latter dia gram indicates that, to control this column, the temperature should be measured on the ninth tray rather than at the top or bottom. Mattix then goes on to describe two complete methods of controlling the operation of the debutanizer. J. B. McMahon and R. W. Landon of Republic Flow Meters Company spoke on the control of primary operating variables. They emphasized the fact that one must exercise considerable judgment in applying mathematical methods to instrumentation, since the neglect of what appear to be un important details may lead to completely erroneous results. They analyzed problems in flow, temperature, liquid level, and pressure control to demonstrate their point. Winfield B. Heinz of Penn Industrial Instrument Corpo ration is an exponent of pneumatic circuits for measurement and control. Few people are aware that pneumatic circuits are closely analogous to electronic circuits. Pneumatic relays can be used in suitable circuits in much the same way that vacuum tubes are used in electronic circuits. The author of this paper tabulates the analogous properties of pneumatic and electric circuits, and describes a group of useful pneu matic circuits. Much has been written on valve characteristics. At this symposium T. B. Burris of Fisher-Governor Company gave the subject a new twist in a paper entitled "The Relations of Valve Characteristics," in which he describes the many de tails of valve design which can completely destroy the desired equal percentage characteristic of a valve. Valve body de sign, stem and packing-gland design, and the design of the diaphragm motor all are important if the valve is to have an equal percentage characteristic. The final paper on the program was by D . M. Considine of the Brown Instrument Company on measurement as a factor in process instrumentation. I t is a survey of primary measur ing devices. Because most of those attending the meeting were familiar with the more conventional primary measuring devices, the author chose to emphasize some of the newer, less frequently used, primary measuring elements. Among these are the Pirani gage and the ionization gage for measuring low pressures, instruments for automatically recording dew point, the polarograph, infrared and ultraviolet spectrophotom eters, and the x-ray spectrometer. Those in charge of the Texas A. & M. short course on instrumentation for the process industries, and those who contributed papers for the program, are to be congratulated for the excellent job they have done. Almost everyone who is interested in instrumentation will find much stimulating new information in these proceedings.
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