Safety Integrity Level (SIL): is a measure of the safety system performance, related to the Probability of Failure on Demand (PFD). Four numbered levels has been defined from 1 to 4, the higher the SIL number, the higher the cost and complexity of the safety system, and the better the performance is expected. Each SIF is assigned a SIL, As per IEC 61511, each SIF shall have an associated SIL.
| SIL | PDF avg | Availability % | Consequences |
|---|---|---|---|
| 1 | 0.01 – 0.01 | 90 - <99 | minor injury |
| 2 | 0.001 – 0.01 | 99 – <99.9 | major injury |
| 3 | 0.0001 – 0.001 | 99.9 | multiple fatality |
| 4 | 0.00001 – 0.0001 | >99.99 | community fatality |
It has been observed that 95% of implemented SIF have SIL 1, while 5% of implemented SIF have SIL 2, and only near 1% of SIF have SIL 3. SIL 4 is for nuclear plant SIF, and SIL 3 for off-shore SIF applications.
Probability of Failure on Demand (PFD avg): is the probability of a functional unit failing to respond to a demand for action upon hazardous condition. It is measured in failures per hour.
Availability: probability that a functional unit will perform its task.
Hardware Failure Tolerance (HFT): the ability of a hardware to continue to perform a safety function in presence of failures or errors. An HTF of N, means that N + 1 faults will cause loss of safety function.
Safe Failure Fraction (SFF): ratio of average failure rates
To what extent can a device or process be expected to perform safely, and, in the event of a failure, to what extent can the process be expected to go to the safe state? These issues are addressed by the term safety integrity level (SIL). Therefore SIL gives a measure of safety risk or risk reduction to a tolerable limit for a given process.
See previous article Differences between SIF and SIS
References
IEC 61508/IEC 61511 Safety Integrity Level, Functional safety in the process
industryerisk reduction with safety instrumented systems, Endress þ Hauser catalog