SIS Life Cycle – IEC 61511

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.

SIS Life Cycle

Analysis Phase

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.

Main steps:

  • Process Hazard Analysis.
  • Definition and Assignment of Protection Layers.
  • Determination of the SIL assigned to each Safety Instrumented Function (SIF).
  • Safety Requirements Specification (SRS).
  • Functional Safety Assessment (FSA-1).

Design & Implementation Phase

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).

Main steps:

  • Technology Selection.
  • Design of Safety Instrumented Functions.
  • Verification of compliance with the required SIL.
  • Review of the design of the SIFs that do not comply, and update of the SRS.
  • Procurement, Construction and Installation of products and equipment.
  • SIS Tests: FAT, SAT.
  • SIS Validation.
  • Functional Safety Assessment (FSA-2).

Operation & Maintenance Phase

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.

Main steps:

  • SIS Maintenance Plan.
  • Staff training.
  • Proof Testing and Inspections.
  • Management of bypasses.
  • Management of Repairing and Spare Parts.
  • Registration of failures.
  • Monitoring of SRS compliance.
  • Management of SIS modification.
  • Functional Safety Assessment (FSA-3).


Related links:

IEC 61511

Compliance with IEC 61511