(93c) Defensible Rationale for Fire & Gas System Design

Jyoti Sharma, Kedar Kottawar, and Angela Summers,

SIS-TECH Solutions LP

12621 Featherwood Drive, Suite 120, Houston, TX 77034

jsharma@sis-tech.com, kkottawar@sis-tech.com

Abstract

Chemical releases or loss of containment may lead to a catastrophic event. Studies like PHA/LOPA are widely used to identify such release scenarios, estimate the severity, likelihood of an initiating event, and calculate the risk reduction factor. The severity level perceived for a consequence, most likely, could be based on prior site experience or could come from an experienced and loud personality in the team. As risk is dependent on severity and likelihood, an error in severity estimation could impact the risk reduction target, leading to inadequate mitigation strategies.

The risk reduction targets are used to design the protection layers; however, the effectiveness of a layer can be diminished if it is not implemented right. The FGS is used as a risk mitigation system for unexpected release events upon failure of preventive protection systems. To guide on implementing this layer, the industry mostly uses prescriptive standards like NFPA 72; however, there is no solid guideline on detector placements. Often, detector placement uses a philosophy from a similar existing facility or is based on design team’s experience. Detector placement needs more rigorous analysis than involving these heuristic approaches. The prescriptive approach towards the detector placement and qualitative approach towards release severity, questions the effectiveness of the FGS layer.

The effectiveness of this layer depends on detector placement for an early detection, its overall coverage, and architecture to eliminate spurious trips. This study provides a defensible rationale for the implementation of Fire and Gas Systems. It covers performance-based FGS design as discussed in ISA-TR84.00.07 with the support of engineering evaluation techniques such as dispersion modeling and 3D detector mapping addressing the following points:

• Bringing consistency in Risk Analysis to determine adequate IPL Risk Reduction Factor (RRF).
• Understanding the uncertainties with detector placement and meet the performance objectives.
• Determining the architecture to achieve the risk reduction target.

Keywords: PHA: Process Hazard Analysis, LOPA: Layer of Protection Analysis, FGS: Fire and Gas Detection, Consequence Analysis, Uncertainty