\"Code for Design of Safety Instrumented Systems in Petrochemical Industries\"

If the process is safely evaluated as SIL 2, then the instrument points and control systems involved in the safety interlocks must also have a SIL 2 certification. In other words, it is necessary to use the SIS system, which comes with its own redundant configuration as per SIL 2 certification. Moreover, conventional instruments must have SIL 2 certification.

The process requires a safety rating of SIL2 – is this a requirement for the system? Are it still circuit requirements? It is necessary to conduct calculations to determine whether a specific instrument meets the SIL2 standard; not all instruments require SIL2 certification. The SIL level can be increased through redundancy, and uncertified instruments can meet the requirements by calculating their PFD value and applying it to achieve the SIL standard

If a safety loop is rated as SIL2, then all components in that loop must also be SIL2 (including measuring instruments, safety instrumented systems, and actuating elements such as valves). The instruments on the SIL circuit must meet SIL2 requirements; this can be achieved with a single SIL2 instrument, or through redundancy using SIL1 instruments. For SIL1 instruments, it is important that the SC indicated on their certificates be SIL2 – otherwise, even with a redundant configuration, the requirements will not be met.

The contents of this standard should be easy to understand. In a typical SIS loop, field instruments, logic controllers, actuators, etc., are required to have SIL2 certification. However, according to the IEC60511 standard, it is possible to rely on experience before actual use; that is, if a field instrument does not have SIL2 certification but its reliability has been proven through practical use, such an instrument can still be used. It’s not necessary to obtain a SIL2 certification; of course, having such a certification is definitely better.

Safety instrument functions can generally also be implemented by control systems. However, in the past decade or so, safety instrumented systems have become increasingly popular. Especially in recent years, amid the strict demands and urgency surrounding safe production, these systems have been adopted without hesitation in accordance with various regulations, directives, and management rules, and evaluations of them are also required. The safe operation that enables the automation of tank areas has been forced to rely on safety instrumented systems, which in turn brings into play relevant standards and practices. If one falls into the \"traps\" of safety instrumented systems, it is necessary to comply with standards such as GB50770, GB21109/IEC61511, and GB20438/IEC61508. Based on these criteria, when implementing a safety instrumented system, a HAZOP assessment must be conducted first to determine the SIL level for the process. After this is done, the safety instrumented system itself needs to be evaluated to verify whether it meets the required SIL level. Based on the above criteria, when conducting a Hazop analysis, adjustments can be made in accordance with the process and disaster prevention and mitigation strategies. Methods such as processes, workflows, instrumentation, and equipment can be employed to reduce or increase the SIL level required for the process, with reduction being the common approach. This is an important application of IEC standards in disaster prevention and mitigation, aimed at reducing risk levels and cutting costs associated with safety instrument systems. I encountered this situation during the tank farm design for a foreign project; I hired an assessment and analysis company to carry out a Hazop study, aiming for the lowest possible SIL level. The result was SIL0. The DCS was used to handle process control, while also fulfilling the functions of the safety interlock system. Unfortunately, our understanding of IEC61508 and IEC61511 comes from foreign suppliers of safety instrumented systems; in recent years, certification companies have also avoided these applications, which prevents us from fully understanding and applying these standards and thus creates some confusion. -Transferred from Mr. Ye Xiangdong’s views in the petrochemical industry.

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SC is the abbreviation for System Capability, that is, system capacity. If the system’s capability does not reach SIL2, multiple redundancy designs are of no use.