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Anonymous
Process control used to be performed in pneumatic, analog, single loop controllers. Safety functions were performed in different hardware, typically hardwired relay systems. Electronic distributed control systems (DCSs) started to replace single loop controllers in the 1970s. Programmable logic controllers (PLCs) were developed to replace relays in the late 1960s. Since both systems are software programmable, some people naturally concluded that there would be benefits in performing both control and safety functions within the same system, usually the DCS. The typical benefits touted included single source of supply, integrated communications, reduced training and spares, simpler maintenance, and potentially lower overall costs. Some believe that the reliability, as well as the redundancy, of modern DCSs are "good enough" to allow such combined operation. However, all domestic and international standards, guidelines, and recommended practices clearly recommend separation of the two systems. The authors agree with this recommendation and wish to stress that the reliability of the DCS is not the issue.
Critical systems require testing and thorough documentation. It's debatable whether normal process control systems require the same rigor of testing and documentation. When the US government came out with their process safety management (PSM) regulation (29 CFR 1910.119) in 1992, many questioned whether the mandated requirements for documentation and testing applied to both the control systems as well as the safety systems. For example, most organizations have documented testing procedures...
