Addressing Hazardous Vapor Cloud Dispersions

A detailed risk-based approach is proposed for addressing flammable and toxic dispersions impacting occupied buildings. The approach is based on the results from a complete quantitative risk-based assessment (QRA), which provides the following information per each outcome impacting the target location under analysis: (1) individual frequency of occurrence; (2) outdoor concentration; (3) exposure time; and (4) indoor concentrations by considering building air infiltration. Exceedance curves per hazard type (flammability or toxicity) and per chemical can be generated, which allow the user to decide whether a target building should be introduced in the mitigation plan as per API Recommended Practice 752 and API Recommended Practice 753, or if the target location can be within the acceptable risk region. Additionally, the proposed approach allows the construction of dedicated FN curves per location being impacted by toxic dispersions. The results from the proposed approach allow decision-makers to decide if there is the need to install the most cost-effective risk reduction measures based on the identified target locations to be in an unacceptable risk region.

Introduction

This manuscript focuses on the impact of Loss of Containment scenarios (LOCs) of hazardous materials that could lead to flammable and toxic dispersions to portable and fixed buildings. The outcomes are based on the source term models which consider released material properties and behavior, conditions of the release and various phenomena that accompany the release of hazardous materials under such conditions (e.g., expansion, choked flow, two-phase flow, aerosolization, rainout, etc.) [1]. These models are important because they provide input data to the dispersion models and the accuracy from these models is dependent upon the accuracy in the source term computation. Detailed information on the mathematical source term and dispersion models can be found in reference [2].

During the development of the quantitative risk-based assessment, it is critical to properly locate all structures/buildings present in a process facility [3]. Accordingly, all identified LOCs [4] are analyzed and modeled following criteria established in references [5], [6], [7], [8] and [9]. Note that these cited references provide the knowledge for quantitative risk-based assessment development, which is the basis of the proposed approach [9]. In this manuscript, a risk-based approach is focused on identifying which occupied buildings in a process facility could be impacted by hazardous dispersions due to toxic and flammable releases is defined and characterized.