QRA Study of an Activated Carbon Filter Safeguard System

The Mary Kay O'Connor Process Safety Center (MKOPSC) at Texas A&M University conducted a Quantitative Risk Analysis (QRA) for subsystems of the VX neutralization process. The process is conducted in a negative pressure containment system. The very large air handling system also acts to direct any fugitive emissions or small leaks to the carbon adsorption filter systems, the final safeguard in preventing highly toxic chemicals from escaping to the atmosphere. The activated carbon filter system concentrates the low level fugitive emissions, thereby creating a potential for a more significant catastrophic release if the carbon filter system fails. The risk tradeoff between capturing low-level fugitive emissions and the potential for a large-scale toxic chemical release must be compared. The fault trees for the air handling system and the carbon filter systems are highlighted. In particular, the uncertainty in the reliability data and their respective impact on the overall failure rate and system availability are emphasized. The importance of this study was to point out a specific failure mode that was not adequately addressed with safeguards in the original process design, thereby creating an unacceptable risk and requiring additional safeguards. The study also verified that the safeguards in the original design for all other identified failure modes reduced the risks to generally acceptable levels.