By Andrew Sloley, Chemical Engineer, Process Consulting Services Inc.
Control and instrumentation have benefited greatly from the electronic technology that lets us build very complex devices and systems. Measurement devices and instrumentation improve constantly, and sophisticated equipment from OEMs gives advanced control and optimization options as never before to the plant floor. The reliability of elements within control systems has improved. These are the benefits of complex systems.
But complex systems can cause problems as well. Just because we can do something does not mean we should. Complex systems require a higher level of conceptual understanding to apply them correctly. Complex systems also require more skilled maintenance for both the hardware and especially the software.
Overall fitness of entire control systems has only modestly improved. As advances improve the effectiveness of individual components, system complexity has risen and introduced new problems that threaten to take back much of the reliability advances. System complexity continues to advance faster than the users capability to always understand.
Systems cannot be considered separately from their elements. From my perspective, I know that process performance and equipment performance are intimately linked. An OEM cannot make sensible equipment design choices without knowing the processing objective. The same is true for control systems; you cannot analyze system performance without knowing how the sensors, indicators, actuators, and other components work; you cannot sensibly select components without knowing the system objective.
Few people understand how all the individual items tie together to determine control system performance. Even fewer can take the next step and tie it to the process being controlled. As an engineer on the receiving end of control integration, I must make a plea to keep control systems and elements simple. Do what is necessary, but dont add extra wrinkles just because you can. Both machine builder and customer can mutually benefit from this design philosophy.
As a practical guideline, the control system builder might ask some questions. The right answers would make systems work better, and make life easier and more profitable for many. Have you designed for poor maintenance? At the moment, the economy is relatively good. Maintenance departments have sufficient budgets to keep everything working. When money becomes tighter, maintenance suffers and systems stop working.
Have you designed for reliable operation when your customers are working with only a quarter of their normal maintenance budget? Have you designed for low performers? Sooner or later, someone who is not the best engineer or operator in the world will run your system. Even after it becomes obvious that someone does not have what it takes for a job, it may be a while before they are replaced.
Have you planned for a system that can be understood and maintained by someone with below average performance capabilities?
Do you have too many system messages? The advanced capability of modern equipment creates manpower savings opportunities for OEM customers, perhaps meaning fewer operators and engineers. Newer instrumentation also makes more information available. More information to fewer people is the result, and too much information, given too often, can overload operators. Important alarms and warnings become lost in the forest of detail. The net result is that people tend to ignore most of the instrument warnings. Make an effort to categorize information from the control instruments. Keep diagnostic messages separate from warning messages and troubleshooting information. Let critical items speak for themselves by their rarity.
Can the system be easily run on manual? Systems that run well on manual are usually systems that have a good underlying linkage between the independent (control knobs) and dependent (control targets) variables. If the system cannot be easily run on manual, check the design again. Something is wrong.
Control components and systems are critical parts of operating plant machinery. While modern technology has improved them, needless complexity has robbed us of the profits of much of the available improvement. Keeping simplicity in mind makes systems more reliable. Reliability equals profits just as much as new capabilities.
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