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Chapter 7 SILect – SIL Selection
The use of the exSILentia SILect (SIL Selection) phase will be described in this chapter. This
chapter will provide an overview of the SILect tasks and options. It will explain how you can select
between three different SIL selection techniques, i.e. Risk Graph, Hazard Matrix, and Frequency
Based Targets. Based on the SIL selection technique applied, this chapter will explain how you can
perform Safety Integrity Level selections for Safety Instrumented Functions. The first part of the
selection process is to calibrate the tolerable risk to be considered during the SIL selection that fits
your plant / company. The second part of the selection process is to specify the severity and
likelihood of the hazard that the Safety Instrumented Function is protecting against. The tolerable
risk specification and severity and likelihood selections will be described per SIL selection technique.
7.1 Tolerable Risk
In Safety Integrity Level selection there are two key aspects, i.e. inherent risk of the process versus
the tolerable risk:
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The Process inherent risk or unmitigated risk is determined by the Severity
(Consequence) and Frequency (Likelihood) of the Hazard that the Safety Instrumented
Function will be protecting against.
The safety integrity that the SIF should provide is determined by dividing the unmitigated risk
by the tolerable risk which yields the required risk reduction. The required risk reduction
directly relates to a PFDavg value which in its turn relates to a required or target SIL level for
the Safety Instrumented Function.
You will only be able to determine the required risk reduction given a certain level of process risk
after you have specified the tolerable level of risk. If you try to calculate a Target SIL level before you
have specified the tolerable risk, exSILentia will give you a warning that no tolerable risk calibrations
have yet been specified.
For each of the SIL selection methods in exSILentia the first step will be define the tolerable risk
criteria. Once a tolerable risk calibration is defined it can be saved in a separate etr (exSILentia
Tolerable Risk) file through the SILect - Save Tolerable Risk menu. An existing etr file can be used to
load the tolerable risk criteria in an exSILentia project.
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exSILentia® User Guide
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