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SIL Determination Methods

Definition - What does SIL Determination Methods mean?

Safety Integrity Level (SIL) Determination methods are analytical methodologies. They are used to assess the SIL level at which an industrial application or process must be rated so that the safety hazards created by the application or process can be reduced to an acceptable or desired level.

SIL levels are a measurement of the level of risk reduction that is provided by a safety instrumented system (SIS)active safety functions that prevent industrial processes from violating safe operating conditions.

Safeopedia explains SIL Determination Methods

The level of risk reduction associated with each SIL is defined by the general International Electrotechnical Commission standard IEC 61508. Additional standards are available to describe the application of SIL-rated SISs to specific industries. For instance, IEC 61511 describes how SISs are used in the process industry.

SILs are only relevant to systems that require an SIS in order to achieve a necessary level of risk reduction. As such, the first element of an SIL Determination method is to determine if an SIL-rated function is necessary. If an initial analysis shows that non-SIS hazard-reduction tools are able to reduce the hazard frequency of an industrial system to an acceptable level, the SIL determination would conclude that SIL-rated functions are unnecessary.

However, if the analysis showed that the risk of a hazardous event was unacceptably high, an SIS system would need to be applied. For example, if the acceptable level of risk for a given industrial process is one hazardous event per 10 years—and after applying non-SIS safety methods, the process is found to have a risk of 30 hazardous events per single year—then an SIS that reduces risk by at least a factor of 300 must be applied to the process.

The SIL rating that is required by an SIS system is determined by the level of hazard reduction required by the system. SIS systems with an SIL 1 rating reduce risk by a factor of 10 to 100, SIL 2 by a factor of 100 to 1,000, SIL 3 by 1,000 to 10,000, and SIL 4 rated systems must reduce risk by a factor of 10,000 to 100,000. In the above example, an SIS with a risk-reduction factor of 300 is necessary. Therefore, the system would be determined to require an SIL 2 rating, and the hazard associated with the industrial process would be referred to as an SIL 2 hazard.

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