Overview

IEC 61508/ 61511 recommends a method to determine the required reliability of instrumented safety functions (SIFs) and ensures that SIFs are designed and operated correctly. A Safety Instrumented System (SIS) often includes ESD, PSD, HIPPS and F&G.

IRC have extensive experience in implementing IEC 61508/61511 guidelines in all phases for process industry projects and can offer assistance in:

· Classification;

· Verification; and

· Validation.


Classification

To determine the SIL for each SIF, a risk-based review of the protection systems using Layers of Protection (LOPA) method is commonly adopted.

The SIL is determined based on the semi-quantitative estimation of the frequency of the initiating event, and the development of the incident scenario, as well as the effective Independent Protection Layers (IPLs).


Verification

The verification of a SIF involves reviewingthe loop design and gathering the following information to calculate the probability of failure on demand (PFD) for the loop:

· Determininghardware failure rate;

· Commonmode failure percentage, β;

· Proof testing interval; and

· Proof test coverage etc.

Using PFD calculation software (aligned to IEC 61511), the loop PFD can be determined and compared to the SIL rating initially assigned to the SIF for verification.


Validation

The validation of a SIF is normallyperformed after the installation of the system.

The validation process may includethe following activities:

· Site acceptance test (SAT);

· Pre-startup safety review (PSSR); and

· Operational readiness review (ORR) etc.

Validation procedures should provide assurance that the Safety Instrumented System (SIS) safety requirements specification has been met and that any recommendations from earlierverification activities and reviews have been satisfactorily addressed. The extent and rigour of validation should be commensurate with the complexity of the SIS and the SIL rating involved.