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