I/EC A
INSTRUMENTATION W O R K B O O K
F E A T U R E
by Robert Wall, Plastics Division Monsanto Chemical Co.
Modern Process Instrumentation Maintenance Requirements For best results, maintenance should be done by skilled mechanics assisted by engineers who know the process and instrumentation
THE ADVENT of modern
process
instrumentation has been accompanied by an increase in problems of instrument maintenance, requiring changes in maintenance procedures and in the organization of instrument groups. The use of process instrumentation has increased considerably, from roughly 5% of plant cost in 1945 to about 15% at the present time. Instrument maintenance problems have increased out of proportion to the increase in the budget, and have not been satisfactorily solved by an equivalent increase in maintenance manpower. Process instrumentation has become far more complex, both in the individual control units and in applications. Maintenance Now Requires Skilled Instrumentation Engineers
Pressure and temperature measurements, etc., are supplemented or replaced by more selective and more sensitive sensing elements—such as process analyzers, including the infrared, gas chromatograph, and mass spectrometer. The conventional controls have been developed to a high degree of speed and precision, and a diversified selection, including the modern electronic instruments, is available. These are used in more complex control systems and in interconnected loops, including some recent examples of limited
computer control. This marked increase in the complexity of control equipment appears to be continuing, and this alone presents problems in obtaining or training skilled maintenance craftsmen. A more subtle and considerably more significant change is that process instrumentation is far along on a transition from a skilled craft to a technical profession. A particularly important part of this change is that the process has become a significant factor in control and in instrument maintenance. Effective instrumentation and instrument maintenance of modern complex plants requires a technical understanding of the process as well as instrumentation, as problems arise that cannot be adequately solved by skilled application of standard test procedures to the instrumentation. The maintenance of modern instrumentation is usually provided by some combination of maintenance, engineering, and research departments; the form of organization differs for each company. Where it is most successful, the maintenance has been provided by skilled craftsmen aided by engineers having an understanding of both instrumentation and the process. The need for skilled instrumentation engineers parallels the rate of use of new control techniques and the newness of the control methods
and equipment used. Instrument maintenance by skilled craftsmen is almost always very satisfactory for proved processes that are operating routinely, even though the instrumentation is complex. Many problems of instrument maintenance may be avoided by using only those instruments that have been well proved by experience, but at the price of lagging several years behind in the application of new control techniques. As progress in instrumentation is directed at increasing the efficiency of plant operation, this is equivalent to accepting a lower level of plant efficiency. The cost of this is usually prohibitive. Special Maintenance
The organization of instrument maintenance departments varies widely among companies, but is commonly based on skilled craftsmen assisted and guided by engineers having process know-how. Instrumentation development has proceeded much more rapidly than the reorganization into modernized instrument departments. A rather loose integration of instrument maintenance with the engineering and research functions, which have been operating in the planning and development of advanced instrumentation, is common. Some general procedures have proved best for the maintenance VOL. 49, NO. 9
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SEPTEMBER 1957
67 A
I/EC
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of modern instrumentation and for rapidly becoming competent in new developments as they appear. T h e use of selected skilled craftsmen in specialized groups has proved most effective for the maintenance of the more complex instruments— the process analyzers, data reduction equipment, etc. T h e use of these specialized groups is successful in facilitating the competent use of newly developed equipment as soon as possible. Although nonspecialization has many advantages as regards simplicity of organization, it is impractical to train all men in the intricacies of such devices as the mass spectrometer or infrared analyzer, and it is much faster to train a few men for the new developments, as, for example, electronic controls. As plants mature, additional men can be trained and at the same time simpler maintenance procedures will be evolved, with some decrease in specialization following.
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Effective trouble shooting and maintenance procedures must be developed within each group working on an area of advanced instrumentation. These procedures are based on logical evaluation of test information and tend to isolate the difficulty to limited areas, followed by either replacement or more intensive trouble shooting. Replacement alone is generally unsatisfactory as advanced instrumentation is far too complex; there are too many units to replace, and often the replacement unit requires careful adjustment to be tuned to the system. As an example from our own experience, the trouble shooting of infrared analyzers by the trial and error replacement of component parts has more often confused than solved the problem. T h e most effective method is a logical analysis of the symptoms and carefully thought out systematic tests directed at isolating the difficulty. This demands highly competent, well trained, and experienced personnel. T h e use of a carefully retained reference sample fh checking analytical instruments has proved very valuable. If the instrument re-
Circle No. 68 A on Readers' Service Card, page 119 A 68 A
INDUSTRIAL A N D ENGINEERING CHEMISTRY
A Workbook
Feature
sponse to this test sample remains consistent, the analyzer itself is probably satisfactory. If the analysis still appears to be in error, the trouble may lie in the process. T h e stream composition may have changed so that unsuspected interfering materials are present in the sample. Also, there may have been an error in sampling or in the laboratory check analysis. It is best not to draw definite conclusions or to take corrective action until definite confirmation of the trouble is obtained. There is a tendency to short-cut systematic test procedures under the urgency of returning needed equipment to service, and to attempt to j u m p to a solution of the difficulty, but, in the most cases, the result is simply a delay in the use of systematic procedures and the solution of the trouble.
S u p p l y of Expert Craftsmen Is Basic P r o b l e m
T h e problem remains of where to obtain the skilled and experienced men needed to staff adequately for the rapid developments. T h e supply of skilled personnel is short arid only occasionally can a good m a n be hired. Therefore, the instrument department must develop and train its own expert craftsmen, and most instrument departments have well organized and systematic training programs. A careful selection of the best men for training is particularly important. Instrument engineers must also be trained, for as yet academic curricula have paid little attention to process control. This discussion has emphasized advanced and developmental instrumentation, as this has been responsible for most of the problems and changes in maintenance procedures. In meeting these problems, instrumentation has become noted for marked progress and development.
Our authors like to hear from readers. If you have questions or comments, or both, send them via The Editor, l/EC, 1155 16th Street N.W., Washington 6, D. C. Letters will be forwarded and answered promptly.
