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Click Below for Additonal SIL Information
Topics
 Articles: SIL
101 | SIL
102 | SIL
103
Calculating
SIL Suitability Levels
Functional
Safety Standards
LOPA:
Layer of Protection Analysis
Choosing an SIL System
The identification of risk tolerance is subjective and site-specific.
The owner / operator must determine the acceptable level of risk to
personnel and capital assets based on company philosophy, insurance
requirements, budgets, and a variety of other factors. A risk level
that one owner determines is tolerable may be unacceptable to another
owner.
When determining whether a SIL 1, SIL 2, or SIL 3 system is needed,
the first step is to conduct a Process Hazard Analysis to determine
the functional safety need and identify the tolerable risk level.
After all of the risk reduction and mitigation impacts from the Basic
Process Control System (BPCS) and other layers of protection are taken
into account, a user must compare the residual risk against their
risk tolerance. If there is still an unacceptably high level of risk,
a risk reduction factor (RRF) is determined and a SIS / SIL requirement
is calculated. The RRF is the inverse of the Probability of Failure
on Demand for the SIF / SIS (see table below).
Selecting the appropriate SIL level must be done carefully. Costs
increase considerably to achieve higher SIS / SIL levels. Typically
in the process industry, companies accept SIS designs up to SIL 2.
If a Process Hazard Analysis indicates a requirement for a SIL 3 SIS,
owners will usually require the engineering company to re-design the
process to lower the intrinsic process risk.
| Safety Integrity Level |
Risk Reduction Factor |
| SIL 4 |
100,000 to 10,000 |
| SIL 3 |
10,000 to 1,000 |
| SIL 2 |
1,000 to 100 |
| SIL 1 |
100 to 10 |
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