Chemical Engineering September 2018 - 50

Safety Integrity Level (SIL) - Risk Parameters
Consequence
Minor injury
Possible under certain conditions
Serious, permanent injury to one or
more persons; death of a person
Rare to more often exposure
Frequent to permanent exposure
Rare to more often exposure
Death of several persons
Frequent to permanent exposure
Rare to more often exposure
Very many deaths
Frequent to permanent exposure
Possible under certain conditions
Almost impossible
Possible under certain conditions
Almost impossible
Almost impossible
Possible under certain conditions
Almost impossible
SIL
1
SIL
2
SIL
3
SIL
4
A single
E/E/PES is
insufficient
Frequency and
Exposure Time
Possibility of Failing to
Avoid Hazard
RELATIVELY HIGH
No special
safety
requirement
Probability of Occurrence
SLIGHT
No safety
requirement
No special
safety
requirement
SIL
1
SIL
2
SIL
3
SIL
4
VERY SLIGHT
No safety
requirement
No safety
requirement
No special
safety
requirement
SIL
1
SIL
2
SIL
3
FIGURE 3. Safety integrity level (SIL) is defined as a relative level of risk-reduction provided by a safety function; a SIL (1, 2, 3 or 4) can be specified as a facility's
target level of risk reduction. SIL is a measurement of performance required for a safety instrumented function (SIF), as defined in IEC 61508
straints of a product development
process - including appropriate
quality control, process management
and verification and validation
methodologies, as well as failure
modes, effects and diagnostic analysis
- so that one can reasonably
justify that the final system attains
the required SIL.
Further guidance related to SIL is
provided by IEC 61511, Functional
safety - Safety instrumented systems
for the process industry sector.
According to this standard, facilities
must follow these guidelines:
1. Conduct a hazardous operation
analysis and risk assessment to
identify and evaluate problems
that may represent risks to personnel
or equipment. This evaluation
will help in determining the
facility's target SIL.
2. Develop auditing, verification and
validation activities to improve
the integrity of the safety-related
functions.
3. Develop post-incident and postaccident
activities for root-cause
analysis and corrective actions.
In the case of SIL product testing,
third-party organizations document
the design of the process and test
both hardware and software in order
to provide a more complete evaluation
of product operation. Compared
to their non-SIL counterparts, products
that have achieved third-party
SIL certification generally offer improved
diagnostics (by providing information
on failure modes) and are
likely to cut maintenance costs by
reducing the frequency of calibration
and testing.
Categories of risk
Given that the purpose of a hazardous-area
gas-safety system is risk
mitigation, it is appropriate to review
the risks associated with combusReliability
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Chemical Engineering September 2018

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