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Would You Use a Safety PLC for Process Control?

By Invensys Operations Management |

With today’s state-of-the-art digital technology, the sky seems to be the limit for doing everything with one system platform. However, with the exception of a few very specific applications, using a safety PLC (programmable logic controller) for process control is not a good choice. Safety PLCs are designed and optimized to operate in a static environment, where action is only taken when a process variable approaches an unsafe region and cannot be controlled by a process control system, such as a distributed control system (DCS). Similarly, in the reverse situation, a DCS should not be used for ESD (emergency shutdown) safety applications. It is very important to understand that the design requirements for a safety PLC operating in a static, vigilant mode rely on a very high level of online auto-diagnostics while operating in a fail-safe manner. The internal auto-diagnostic routines are continually toggling the discrete values to assure that the devices will operate when a process demand requires action. Conversely, control systems are designed and optimized to operate in a dynamic environment, in which measurements and actions are continuous and operators and engineers have less rigorous constraints on access for tweaking…
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