Safety Instrumented System Design: Lessons Learned

Engineering is a bold discipline. Engineers are constantly reaching for new heights, searching for new materials and greater efficiency. Unfortunately, part of that process means we occasionally exceed known boundaries. It is regrettable, but it would appear that human nature requires that we learn the hard way. While this is an obviously painful process, we can learn more from our few mistakes than from our many successes. Our many successes may contain flaws that are never revealed under normal conditions, and we may go on repeating them over and over. It is only when expected conditions are exceeded) and failure is the result, that we learn where we went wrong [12]. Valuable lessons can be learned from failures, and there are plenty of examples from industry in general as well as specific details regarding failures of safety control systems. For example, the UK HSE (Health and Safety Executive) issued a publication in 1995 [2] that reviewed 34 accidents that were directly caused by control and safety system failures. The HSE published the reviews so that engineers could learn from and hope fully not repeat the mistakes discussed in the book. The IEC and ISA standards on this subject, as well as the CCPS Guidelines, are based upon a "Safety Life Cycle" which is a set of steps one should go through in the overall design process in an effort to ensure that nothing falls through the cracks. As responsible engineers, we should not have to learn the hard way. Plants have become too large and the risks have become too great for us to learn by mere trial and error. Because we cannot do a recall on all refineries we need to get things right the first time. We can, however, learn from the mistakes of others without re-inventing the wheel or operating in isolation.