What distinguishes a safety instrumented system (SIS) from a basic process control system (BPCS)?

A safety instrumented system (SIS) is distinguished from a basic process control system (BPCS) by its primary purpose, integrity requirements, and architecture. A BPCS is designed to maintain process variables (e.g., temperature, pressure, flow) within a desired operating range to optimize production and efficiency. A SIS, on the other hand, is specifically designed to prevent or mitigate hazardous events, protecting personnel, equipment, and the environment. While a BPCS aims to control the process under normal conditions, a SIS acts only when the BPCS fails or when abnormal conditions arise that could lead to a hazard. The integrity requirements for a SIS are much higher than those for a BPCS. A SIS must be designed to achieve a specific safety integrity level (SIL), which is a measure of its reliability and performance in preventing or mitigating hazardous events. This requires the use of highly reliable components, redundant systems, and rigorous testing and maintenance procedures. A BPCS typically does not have specific SIL requirements. The architecture of a SIS is also different from that of a BPCS. A SIS is typically designed with a separate and independent architecture from the BPCS to avoid common-cause failures. This means that the SIS has its own sensors, logic solver, and final elements that are independent of the BPCS. This independence ensures that the SIS can function even if the BPCS fails. A BPCS, conversely, often shares sensors and final elements, prioritizing cost and efficiency over complete independence. Furthermore, SIS logic solvers are often designed with higher diagnostic coverage to detect internal failures, ensuring the system's readiness.