Mitigating Hazardous Scenarios

The main purpose of a Fire and Gas (FGS) mapping study is to identify and assess the placement and performance of gas flammable, toxic, and fire detectors. The proposed approach links the robust risk-based quantitative assessment results with zone definition and categorization. It provides the exact location of FGS for ensuring detection coverage (i.e., detector coverage verification) and addresses which is the required reliability for ensuring the pursued risk reduction (i.e., functional safety availability verification). Even though the present manuscript focuses on detection coverage verification, both mapping and functional safety concepts are addressed. Functional safety requirements are fully addressed in a dedicated related reference covering Safety Instrumented Systems (SIS) [5].

Introduction

Reference [1] provides an overview of layers of protection suitable to reduce the risk level of a process facility; i.e., measures intended to prevent and/or mitigate the identified hazardous scenarios. The cited paper [1] illustrates that based on the development and results of a risk-based quantitative assessment, the regions, zones or even more detailed locations (e.g., occupied buildings, critical process equipment) with an intolerable risk level can be identified and the associated hazardous scenarios that most contribute to the risk of these zones. It is confirmed that a sensitivity analysis and cost-benefit analysis can be performed with the aim to find which safeguards achieve the reduction of risk to an acceptable level at the most reasonable cost.

From all layers of protection considered in reference [1], the Safety Instrumented Systems (SIS) and performance-based Fire and Gas Detectors Systems (FGS) were found to be the safeguards that should comply with very specific requirements stated in related standards; i.e., IEC 61508 [2], IEC 61511 [3] and ISA 84.00.TR.07 [4], respectively. While SIS selection and verification guidance can be found in reference [5], the main purpose of this manuscript is to address performance-based FGS intended to comply with standard [4] by introducing a risk-based Fire and Gas Mapping approach.

The proposed Fire and Gas Mapping Study is based on detailed results obtained during the development of a risk-based quantitative assessment. All introductory criteria for ensuring a complete and robust risk-based quantitative assessment are illustrated in references [1], [6], [7], [8], [9], [10], [11] and [12]. Formally, the FGS mapping study is justified after comparing the actual risk levels in the process facility with worldwide recognized tolerability criteria and evidencing that risk reduction is required. Note that the magnitude of the risk reduction required (i.e., Risk Reduction Factor, RRF) is directly correlated to the Safety Integrity Level (SIL) and associated Probability of Failure of Demand (PFD) of the Safety Instrumented Function (SIF). While all these concepts are necessary to be introduced in the present manuscript, it is important to mention that “Functional Safety” requirements are fully develop in reference [5], which allows the present paper to focus on how to link risk-based quantitative assessment results with zone definition, categorization and detector coverage verification.

Figure 01: Example of Individual Risk Contours for a Given Facility