by R. F. Wall Monsanto Chemical Co.
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W O R K B O O K
F E A T U R E
Reliability in Plant Process Control A carefully planned and executed program is essential
I N SOME advanced areas of control technology, as missile systems, control reliability has become possibly the most significant factor in determining the success or failure of a project. This has stimulated the development of a science of reliability estimation, measurement, and prediction that has been fundamental in the development of extremely complex systems that are at the same time reliable. As process control moves rapidly toward more intricate control loops, extensive use of process analyzers, data reduction, and computer control the selection and use of control equipment to obtain reliable performance becomes more difficult, and the quantitative estimation of the reliability of each part and of the assembly becomes important in eliminating the key points of unreliability and in obtaining a satisfactorily performing control system. Reliability calculation is a form of probability mathematics, and is simply a means of estimating the gamble that the new control system will perform adequately. The reliability of each individual system is determined by the combined effects of a number of contributing factors. One of the most important is the configuration of the component parts making the assembled system. Three types of configuration have been identified.
Configuration of Component Parts
The series or chain configuration is typified by the cascaded vacuum tube amplifier; the sequence of flapper, nozzle, linkage, and bellows in a pneumatic controller; or the sequence of sample system and
crease approximately 10 times in going from 1/t to full rating, but are appreciably lower than for tubes. The sensitivity and precision required for an instrument will considerably determine the effective reliability. A flow controller required to control flow to ± 1 %, or a process analyzer in the parts per million range at ± 1 % of scale required accuracy, will require more attention—i.e., be less reliable— than a ± 3% flow control or an analyzer in the per cent range with ± 3 % of scale required accuracy. Simplicity within the limits of conservative operation is conducive to reliability, but conservative operation is essential. A one-tube amplifier has half the number of parts but may be less reliable than a two-tube amplifier operated at lower rating. The details of assembly and installation of components and equipment may have a marked effect on reliability. Correct placement of parts and adequate cooling are very important for electronic assemblies; a capacitor placed next to a power resistor operated at rated dissipation under the chassis is not a candidate for longevity. The reliability of pneumatic equipment is completely dependent on a supply of clean, dry air, and an inadequate sample system will invalidate the accuracy and continuity of analysis of a process analyzer.
analyzer in a process analysis application. The reliability characteristics of this configuration are well illustrated by the series string of Christmas tree lights; one part fails and the system is down. The average on time between failures of the string is equal to the average life of a lamp divided by the number of lamps. Instruments are predominantly chain configuration, with the mean time between failures a similar function of the average life of the individual components. The redundant configuration has two or more units in parallel, each fully capable of preforming the required function. The use of a spare thermocouple or other spare instrumentation on a critical measurement is common practice. The distributed configuration has many units more or less in parallel. Any single unit can fail and operation will continue at a somewhat lower performance level. This type of configuration is probably most characteristic of process control systems. A process is seldom shut down if a limited number of thermocouples, flowmeters, etc., fail. There are usually enough other information points to permit operation to continue with less precision. Required Performance
Another factor determining reliability is the severity of the performance required of instruments and components. Harris and T a l l (/) list the failure rates of vacuum tubes as 9% per 5000 hours at 2 5 % of power dissipation and 50% rated voltage, and 2 6 % per 5000 hours at rated values. Failure rates for other common electronic components inI/EC
Maintenance
Reliability is a function of both frequency and duration of maintenance periods, and will be considerably determined by equipment design and construction. A n instrument can be very intricate and complex and still reasonably reliable,
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ORKBOOK FEATURES
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INSTRUMENTATION
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A Workbook
ANALYZER
Feature
Series of component parts of sample system and analyzer
RECORDER
TEMPERATURE POINT
as evidenced by the large computers. The extremely large number of components, particularly vacuum tubes, makes relatively frequent failures inevitable even with the best of components and design; however, self-checking and fault-finding features, plug-in assemblies, and/or easily changed components make repairs rapid and down time low. These maintenance features were essentially compelled by the impossibility of conventional trial-and-error trouble shooting on equipment of this complexity. Test circuits and trouble shooting aids can be built into process control equipment and should be used where their help will be worth the added cost. M u c h can be done i n design and construction to make maintenance easier with little i f any added cost, and easier maintenance equals better and faster maintenance. Accessibility for replacement or repair, logical component grouping, adjustments that are noncritical, and clear and complete maintenance manuals all tend to make maintenance easier and faster, and require less skill and training. I t is not necessary to miniaturization that size and accessibility should both be minimum. The organization and quality of maintenance provided can determine system reliability. The best maintenance can realize the potential maximum reliability of the control systems; poor maintenance can reduce it to zero. One of the most effective aids to reliability is a well organized program of preventive 74 A
maintenance. The scheduled replacement of vacuum tubes and/or other components subject to failure well within their expected normal life is an effective means of minimizing equipment failures. Maintenance technicians must be carefully selected for adequate basic skills, be thoroughly trained, and be a part of a well organized and smoothly functioning instrument maintenance department, if high quality maintenance is to be obtained. The design and construction of reliable control equipment present a difficult problem to the manufacturers. A l l of the above-mentioned factors plus economics must be considered. Simple equipment with few parts has fewer chances of failure, but those few components may be operated at high rating and the probability of failure may be high. Complex equipment is not necessarily unreliable, for with good design, high quality components conservatively rated, and circuits stabilized by high gain and inverse feedback, excellent reliability can be obtained at a cost and complexity that the user may object to or refuse to accept. The manufacturer has a difficult compromise to make among simplicity, complexity, and cost in obtaining the basic reliability of performance that is his responsibility. The user has a major responsibility in obtaining reliable process control through the proper application and maintenance of instruments. The instruments are the components used by the instrument engineer in developing a process control system.
the distributed configuration, arranging instrumentation so that if one instrument fails others can serve the function with less precision of control, wherever this is reasonably possible. Use redundancy where the consequences of failure justify the added expense and maintenance. Keep the planned instrumentation as simple and as conservative in performance requirements as is consistent with the control required. Don t ask for better control than you can ffectively use. Select instruments best meeting the needs of the job and within the capabilities of the maintenance staff. If it is possible to select instruments from previous experience, the selection can be made with considerable confidence. However, instrumentation is developing so rapidly that it is often necessary to select an instrument from new and unfamiliar equipment. A program of comparative evaluation is very helpful but has disadvantages of cost, is often prohibitively time-consuming, is dependent on the individual peculiarities of a single sample, and never seems to predict actual plant performance completely. An evaluation program should supplement, not replace, experienced judgment. Reliability can be accurately estimated by looking for overrated components, simplicity, convenience of maintenance, good maintenance manuals, etc., and this estimate alone is often adequate. Install instrumentation using the best practices reasonably possible. Particularly consider protection from elements and accessibility for maintenance. Provide well organized and adequate maintenance by skilled and trained craftsmen. Reliability of process control can almost always be obtained by a carefully planned and executed program of instrumentation. I t may not be cheaply attained, but is never as costly as unreliability. The returns are less down time, higher throughput, better quality, and better over-all efficiency from the process itself. Literature Cited
(1 ) Harris, V., Tall, M. M., "Progress Report on Reliability Measurement and Prediction," Proceedings of Second National Symposium on Quality Control and Reliability in Electronics, Inst. Radio Engrs., 1956.
Planning
Reliability begins w i t h the planning of an instrument installation or complete control system, and will be best when the process has been designed with control in mind. Plan to avoid the chain configuration, where the failure of one unit will throw a system out of operation. Emphasize
INDUSTRIAL AND ENGINEERING CHEMISTRY
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