The SIL value is a measure of the reliability and availability of a safety system. It is the measurement of performance of a safety system under all the stated conditions within a stated period of time.
LOPA is a quantitative method for determining the SIL level for the safety system. The LOPA process uses information such as initiating event frequencies, the probabilities of failures of all safeguards, and the tolerable frequency of risk mitigation to determine the required probability of failure of the safety system. The required probability of failure is a number representing the probability that a safety system will fail in a dangerous scenario.
The SIL value for the safety system is determined by comparing the required probability of failure to the international standards of functional safety, defined in IEC 61508 and IEC 61511.
Based on the demand rate of the safety system, the SIL standards are classified into the following two types:
As per the industry standards, the required probability of failure are related to one of the four safety integrity levels contained in the following table:
| Required Probability of Failure | ||
|---|---|---|
| Safety Integrity Level (SIL) |
Low Demand Mode Probability of Failure on Demand (PFD Avg) |
High Demand or Continuous Mode Probability of Failure per Hour (PFH) |
| 4 | ≥ 10-5 to < 10-4 | ≥ 10-9 to < 10-8 |
| 3 | ≥ 10-4 to < 10-3 | ≥ 10-8 to < 10-7 |
| 2 | ≥ 10-3 to < 10-2 | ≥ 10-7 to < 10-6 |
| 1 | ≥ 10-2 to < 10-1 | ≥ 10-6 to < 10-5 |
For example, if the required probability of failure for a safety system is 0.02, which is a value between 0.01 and 0.1, the Safety Integrity Level for the safety system is 1.
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