ISA-TR84.00.02:2015

ISA-TR84.00.02-2015 is informative and does not contain any mandatory clauses. ISA-TR84.00.02 is intended for use by those with a thorough understanding of ISA-84.00.01-2004 Part 1. This document assumes that a SIS is required. It does not provide guidance on the hazard and risk assessment used to identify the need for a SIS. The user is referred to ISA-84.00.01-2004 Part 3, and CCPS’s Hazard Evaluation Procedures and Layers of Protection Analysis: Simplified Risk Assessment for guidance on assigning the SIL.
Prior to proceeding with use of ISA-TR84.00.02-2015, the hazards and risk assessment and the allocation of safety functions to protection layers should be completed and the following information provided:
At least one SIF is required
The functional requirements of the SIF
The integrity requirements of the SIF
ISA-TR84.00.02-2015 provides guidance on different issues that impact SIL verification:
Assessing random and systematic failures, classifying failure modes, and estimating the failure rates for individual devices of an SIF;
Assessing the impact of diagnostic and mechanical integrity choices on the performance of the SIF and its devices;
Assessing and estimating the potential for common cause and common mode failures; and
Verifying that the SIF achieves a specified SIL and spurious trip rate.
ISA-TR84.00.02 provides guidance on techniques for evaluating the following:
Average probability of failure on demand for low demand mode
Spurious trip rate
There are four topics that are being held until the next revision of ISA-TR84.00.02. Until then, the reader should refer to ISO 12489 for appropriate methodologies.
Modeling of continuous and high demand mode systems
Understanding proof test effectiveness and how this is addressed by design and mechanical integrity practices
Understanding how to model common cause and systematic contribution to the failure of subsystems and systems and to the occurrence of the hazardous event
Expanding the discussion of common cause and systematic error to address these issues across the entire lifecycle