SIL allocation

In the clause 9 of the IEC 61511 standard lifecycle, it is fundamental importance to recognize the levels of protection already existing and how much risk reduction they can provide.

If a SIS (Safety Instrumented System) protection layer is required, a determination of the SIL level of each SIF assigned to the SIS will be required.

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The SIS is, therefore, the container of all “safety loops”, from the sensor to final element, which takes the name of Safety Instrumented Function (SIF) and where the SIS is used to implement one or more SIFs.

The peculiarity of the SIF is that it refers to a single function to be controlled, such as a high or low pressure trip (dangerous situations), avoiding dangerous events and driving the process towards a state of safety.

The SIL level corresponds to the reduction of the required risk, which is determined by comparing the process risk without the SIS with the tolerable risk (also known as “SIL allocation, SIL study, SIL analysis” etc).

Responsibility: a team of specialist (HSE, Process, Instruments, Mechanical etc)

For each client/project a dedicate procedure for the classification will be implemented (normally provided by HSE)

Reference standard IEC61511 part 3 (Guidance for the determination of the required safety integrity levels)

Different methods can be used to determine  SIL level, anyway the most used are:

  • Qualitative method: Risk Graph (Annex E)
  • Semiquantitative method: Layer of Protection Analysis (LOPA) (Annex F)

For more information and Request of Proposal of SIL allocation, contact MYND staff over