Contractors Unlimited

Safety Integrity Levels

The basis of a SIL review involves establishing the risk reduction required for each identifiable part, sometimes called sub-system, of a system e.g. loop. From this a safety system is selected with the required technical specification and architecture to satisfy the required reduction in risk for each of the subsystems.
The risk graph (sometimes called a SIL Tree) method is a common tool used to establish the SIL rating of a sub-system. A risk graph is shown below
SIL Tree
Definitions of terms used on the SIL Tree:


- No safety consequence. Exactly as it says.
- Slight Injury. Injuries not requiring immediate/emergency medical evacuation.
- Serious Injury or 1 death. All injuries requiring emergency medical evacuation and up to one fatality.
- Multiple deaths. All incidents resulting in more than one fatality and any number of injuries.
- Catastrophic. All incidents involving many fatalities.

Personnel Exposure

- Frequent. Personnel will be close enough to be affected and failure of the system precludes the possibility of mitigating action.
- Rare. Either personnel are not in the vicinity of the event, or if they are, fully independent means exist to initiate possible mitigation actions e.g. evacuation.

Alternatives to Avoid Danger

- Possible. The rate of escalation is such that personnel in the area will have time to get away from the immediate area or that there is sufficient warning from independent means to allow evacuation.
- Not Likely. All cases other than those explained previously in Possible.

Demand Rate

- Relatively High. Occurs between 1 and ten times a year.
- Low. Occurs between once per year and once in every 10 years.
- Very Low. Occurs less than once in every 10 years.
In general, the higher the specification of the safety system the higher will be its associated SIL, and the lower its probability of failure on demand, as described in the chart below:
 Probability of
Failure on
Demand (PFD)
in %
SIL 10.01 - 0.190 - 9910 - 100
SIL 20.01 - 0.00199 - 99.9100 - 1000
SIL 30.001 - 0.000199.9 - 99.991000 - 10000

Hazard Analysis Techniques for System Safety



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