Separation the norm in process industries
"One of the most controversial topics in the industry is whether control and safety systems should be combined within the same system. In other words, should all safety functions be combined in the process control system? Proponents would argue that nowadays both systems are programmable, and process control systems are reliable and can be redundant, so why not? The answer is rather simple and does not hinge on 'reliability.' As we shall see, all of the standards, recommended practices, and guidelines in the industry recommend that separate systems be provided for process and safety control.
"Trevor Kletz, one of the industry leaders in terms of process safety, has stated: 'Safety systems such as emergency trips should be completely independent of the control system and, when practicable, hard wired. If they are based on a computer, it should be independent of the control computer.' "
The preceding quote is from Safety Shutdown Systems: Design, Analysis and Justification by Paul Gruhn, P.E., and Harry L. Cheddie, P.E., published by ISA. Continuing, the authors note that England's Health and Safety Executive, the American Institute of Chemical Engineers, and the International Electrotechnical Commission all favor or require complete separation of safety and process controls.