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SIL

The concept of safety integrity levels (SILs) is a measure of the quality or dependability of a system which has a safety function – a measure of the confidence with which the system can be expected to perform that function.

The standards recognize that safety functions can be required to operate in quite different ways. In particular they recognize that many such functions are only called upon at a low frequency / have a low demand rate. On the other hand there are functions which are in frequent or continuous use.

  • For functions with a low demand rate: 
                The accident rate is a combination of 2 parameters:
    • The frequency of demands
    • The probability the function fails on demand (PFD).
  • For functions which have a high demand rate or operate continuously:
                The accident rate is the failure rate, λ, which is the appropriate measure of performance


Our SIL application can calculate SIL based on a number of methods:

  • Qualitative Methods

    • Modified HAZOP

    • Consequence only
    • Risk Matrix

  • Semi-Quantitative Methods

    • Risk Graph
    • Layer of Protection Analysis (LOPA)

  • Quantitative Methods
    • Detailed Calculated SIL method (IEC 61508 AND IEC 61511 standards)

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