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Scope

1.1 ISA-TR84.00.02-2002 – Part 3 is intended to be used only after achieving a thorough understandingof ISA-TR84.00.02-2002 ? Part 1, which defines the overall scope. This technical report addresses:

a) technical guidance in Safety Integrity Level (SIL) Analysis;
b) ways to implement Safety Instrumented Functions (SIF) to achieve a specified SIL;
c) failure rates and failure modes of SIF components;
d) diagnostics, diagnostic coverage, covert faults, test intervals, redundancy of SIF components; ande) tool(s) for SIL verification of SIF.

1.2 ISA-TR84.00.02-2002 – Part 3 is considered informative and does not contain any mandatoryrequirements. The User should refer to ISA-TR84.00.02-2002 ? Part 1, which defines the generalrequirements for the verification of SIL for SIF.

1.3 ISA-TR84.00.02-2002 – Part 3 is intended to provide guidance on the application of Fault TreeAnalysis (FTA) to SIF. FTA is one possible technique for calculating SIL for a SIF installed per ANSI/ISA-84.01-1996(1).

1.4 ISA-TR84.00.02-2002 – Part 3 covers the analysis of a SIF application from the field sensorsthrough the logic solver to the final elements.

1.5 Common cause failure and systematic failure are an example of important factors readily modeledin FTA.

1.6 Part 3 assumes that the complex analysis of the failure rate for a programmable logic solver is doneby another method (see Part 5) or is provided by a vendor as an input PFDL or MTTFspurious into thisanalysis (per Clause 7.3.2 of ANSI/ISA-84.01-1996, the failure rate of the logic solver should be suppliedby the logic solver vendor). Calculation of the PFDavg and MTTFspurious of electrical/electronic/programmable electronic systems can be performed using FTA by applying the techniques presented inthis part.

1.7 This part does not cover modeling of external communications or operator interfaces. The SIL analysis includes the SIF envelope as defined by ANSI/ISA-84.01-1996 (see Figure I.2).

1.8 The ultimate goal for the FTA is to determine the following:

– The PFDavg, Safety Integrity Level (SIL), and
– The MTTFspurious of the SIF

This analysis aids in the design of an effective SIF by allowing the User to determine where weaknessesexist within the SIF. This technique is applicable when the failure of the SIF can be caused by more thanone pathway, when strong interactions exist between multiple SIF, or when several support systems(instrument air, cooling water, power, etc.) are involved.

Product Details

Published:

06/17/2002

Number of Pages:

72

Part of:

ISA TR84.00.02-2002 – SET