The image shows the main phases of the SIS Life Cycle (Life Cycle of Safety Instrumented System), as well as the technical disciplines that lead each part.
In this phase, a rigorous analysis of the hazards of the process is carried out, comparing the probability that a risk scenario will occur with its consequences. The end user must define the maximum tolerable risk in the Plant. Each risk scenario must be analyzed in detail, and assigned the necessary protection layers (relief valves, control system, etc.). When the “non-SIS” protection layers are not sufficient, then an SIS protection layer, called “Safety Instrumented Function”, will be assigned, with the SIS level required in each case.
The SRS is the most important document of this phase of SIS Life Cycle.
In this phase we start from the Safety Functions (SIF) defined in the previous phase of SIS Life Cycle. We must carry out the design so that the SIL level, required in the SRS, is met. The most important part is the Validation of the SIS (usually coincides with the SAT – Site Acceptance Tests).
It is the longest phase of the SIS Life Cycle. Preparing a good SIS Maintenance Plan is one of the main keys, as well as its correct execution and that there is a good safety culture in the Plant, starting with the Management.
