Method Process Safety

SIL Determination

In the Functional Safety Lifecycle, the next step after hazard and risk assessment is “allocation of safety functions to protection layers”. This is a process whereby suitable control measures are specified for each of the potential hazardous events found during the H&RA Hazard & Risk Assessment (usually a HazOp) . This includes mechanical safeguards such as pressure relief valves, and also Safety Instrumented Systems (SIS).

The process of “allocation” is deciding which protective measures will be put in place, and how reliable they need to be. If a safety instrumented function (SIF) is to be used, the reliability required from it will be expressed as a safety integrity level, or SIL. There are four SILs – 1, 2, 3 and 4, which give increasing levels of reliability, as follows:

SIL PFDavg Risk Reduction Factor (RRF)
From From To To
1 0.1 0.01 10 100
2 0.01 0.001 100 1,000
3 0.001 0.0001 1,000 10,000
4 0.0001 0.000001 10,000 100,000

A key task during “allocation” is therefore to carry out “SIL determination”; in other words to decide what SIL is required from each SIF.

The most common technique used for SIL determination is layer of protection analysis (LOPA).  See “what is a LOPA?” for more information.

What =Method can do for you

  • Provide experienced LOPA study facilitators
  • Train and give mentoring support to aspiring LOPA study leaders

Process Safety Lifecycle

SIL Determination

Most recent projects for SIL Determination

Update to earlier LOPA following review by HSE.

Industry: Mining / Metal / Cement
Topic: Layer of Protection Analysis (LOPA)

77 LOPA scenarios were reviewed as part of an ongoing functional safety project.

Industry: Agrochemical
Topic: Layer of Protection Analysis (LOPA)

A LOPA “Layer of Protection Analysis” study for three events relating to a hydro-power system: flood gates opened not in accordance with procedures, flood gates not opened when required and insufficient flow. The purpose of the LOPA study was to establish the effectiveness of existing and intended protection layers and safety functions. The LOPA study considered four scenarios derived from the three events and identified a set of actions where there were uncertainties in quantifying the initiating causes and frequency modifiers.

Industry: Power
Topic: Layer of Protection Analysis (LOPA)

A Layer of Protection Analysis of a drinks industry demonstration area and laboratory, following an earlier HazOp (P-6164).

Industry: Brewing and Distilling
Topic: Layer of Protection Analysis (LOPA)

LOPA of oil storage site (based on typicals approach)

Industry: Water + Waste
Topic: Layer of Protection Analysis (LOPA)

View all of our recent projects here

Initial Conceptual Design & Feasability Study

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