Safety Instrumented System (SIS)
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How should this phrase be understood in the technical specifications for Safety Instrumented Systems (SIS)? All switch signals, including switches on the process system (such as pressure switches, level switches, etc.), manual switches, buttons, valve limit switches, relay contacts, or other control system output signals, shall use dry contacts (passive). Contacts located in hazardous areas must be completely sealed. If possible, process switches should be avoided. All interlock trip signals shall use transmitters. Other options shall be approved by the owner.
The reliability of switches is lower than that of transmitters, and transmitters have circuit diagnosis functions.
The SIS system focuses primarily on reliability; the reliability of switches is lower than that of transmitters, so process switches should be avoided if possible.
Are all chemical companies now required to have an SIS system?
To be precise, it’s not that the reliability of switches is inferior to that of transmitters. Instruments of the same SIL level are determined by hazardous failure and verification testing time. The drawback of switches is that it is inconvenient to conduct verification tests online, while offline verification tests introduce additional risks and labor costs.
No, first of all, SIS is not a panacea; safe production requires a comprehensive set of safety management measures, and in fact, human factors remain the most important element. Whether a SIS system is needed and which SIL level of safety instrumented system to select are usually determined by risk analysis. Risk is determined by the likelihood of a hazard occurring and the consequences it causes.
Starting from 01.01.2018, SIS is required for the two key areas and one major area.