SIL (Safety Integrity Level) SIL – Unit of Measurement for Risk Reduction

The question “What is hidden behind the term SIL?” has been the subject of many publications. At first glance it's easy to answer but the answer depends a lot on the point of view and focus.

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The question “What is SIL?” is very easy to answer: The Safety Integrity Level, or “SIL” for short, is a unit of measurement for quantifying risk reduction. Just as wind strength is quantified using units on the “Beaufort” scale, risk reduction can be quantified using the concept of SIL.

One example is the purchase of heating oil. First the desired amount is specified in the form of a number together with the unit “liter”. A dealer will then supply this heating oil using a suitable measuring method (usually a calibrated flow meter) to ensure that the quantity supplied corresponds to the quantity specified when the order was placed. Safety technology is similar, except that the need for safety is a top priority and satisfying the need for a material asset is not necessary.

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It is a well-known fact that technical equipment can pose a safety risk so dangerous that people should not be exposed to it. In such cases, the relevant risks must be reduced to meet the need for safe operation [Fig. 2]. It must be possible to quantify (and measure) risk reduction in order to satisfy this requirement. As already mentioned, this is achieved using the SIL “unit”, whereby only whole values are defined within a range from 1 to 4. When risk reduction is complete, it must be proven that the extent of the reduction corresponds to the minimum required parameters (i.e. the SIL requirement). This approach results in the following procedure:

• Establish the required risk reduction (specification of SIL requirement)

• Implement the risk reduction (install protective equipment)

• Confirm that the implemented risk reduction corresponds to the minimum required risk reduction (SIL evaluation of protective equipment).

Point 1 is commonly known as a risk analysis. Point 2 refers to the installation of protective equipment (also known as Z function). Point 3 is completed during the “assessment of functional safety”.

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