Safety Instrumented Systems / Layer of Protection Analysis

Risk concepts are at the heart of the ANSI/ISA-S84.01 standard Application of Safety Instrumented Systems for the Process Industries and the related international standard IEC 61508. A critical aspect of conformance with the standard is the establishment of Safety Integrity Level (SIL) performance requirements for individual safety instrumented systems (SISs). An SIL (level 1, 2, or 3) is a metric of the reliability of an SIS, reflecting the probability that it will meet its design intent on demand.

The appropriate SIL for an SIS is dependent on

the likelihood of a process event that would call for actuation of the SIS,

the reliability of any non-SIS safeguards that would prevent or mitigate that process event, and

a pre-established risk tolerance criterion that each process event scenario needs to achieve.

The tools typically used to support SIL determination (identified, for example, in the technical report ISA-TR84.02) are classical reliability/risk tools, such as fault tree analysis, reliability block diagram analysis, Markov analysis, and simplified, related methods such as Layer of Protection Analysis (LOPA).

We support clients in assessing SIL selection through

systematic construction of hypothetical loss scenarios and identification of prospective SIS and non-SIS safeguards

use of our ePHA tool to provide an analysis environment well-suited to scenario identification, Layer of Protection Analysis, and preliminary SIS and non-SIS reliability quantification

use of our eFTA fault tree tool for detailed modeling of more complex SIS logic and SIL categorization

application of numerous generic component reliability databases we have accumulated and refined, used to supplement any plant-specific failure experience when estimating SILs.

We also support SIL assurance through:

confirmatory reliability analyses to guide conceptual design, detailed design, and installation, and

establishment of test and maintenance activities and frequencies to meet SIL criteria.