While the main focus of the SIL number is the determination of process safety, an important byproduct of the statistics used in calculating SIL ratings is the statement of a product’s reliability.
In order to determine if a product can be used in a given SIF, the product must be shown to “BE AVAILABLE” to perform its designated task. In other words, how likely is it that the device in question will be up and functioning when needed to perform its assigned task?
Considerations taken into account when determining “AVAILABILITY” include: Mean Time Between Failures (MTBF), Mean Time To Repair (MTTR), and Probability to Fail on Demand (PFD). These considerations, along with variations based upon system architecture (i.e. 2oo2 versus 2oo3, or TMR installation), determine the reliability of the product.
Subsequently, this reliability data, combined with statistical measurements of the likelihood of the product to fail in a safe manner, known as Safe Failure Fraction (SFF), determine the maximum SIL environment in which the device(s) can be used.
SIL ratings can be equated to the Probability to Fail on Demand (PFD) of the device in question. The reciprocal of the PFD is known as the Risk Reduction Factor (RRF).