Designers of fire and gas systems have long struggled with the question of how many detectors are required and where those detectors should be located.  Historically, decisions about the number and location of detectors were made based on heuristics, expert judgement, or rule-of-thumb.   The first step forward in gas detector placement came in 1992 with the release of a report from the UK Health & Safety Executive (HSE) titled OTO 93 002 – Offshore Gas Detector Siting Criterion Investigation of Detector Spacing.  A major finding of this report is that a 6 meter gas cloud could result in flame speeds greater than 100 m/sec, which can result in damaging blast effects on unprotected equipment/buildings.  As a result of these findings, HSE recommended that fixed gas detection systems be installed in offshore oil & gas facilities on a grid with 5 meter spacing.  This was adopted by industry and has been used as a basis for design on many combustible gas detection systems on offshore installations throughout the world.  Since 1992, additional research has been conducted (MERGE, etc.) that provide additional insight into the size of gas accumulation (volumetric accumulation) that can result in a hazardous blast; however, the 5 meter spacing recommendation remains constant.  Applying this criterion often results in a very large number of detectors and a costly design.

In 2010, the ISA 84 committee made a great step forward in answering these questions with the release of the ISA-TR84.00.07 – Guidance on the Evaluation of Fire, Combustible Gas and Toxic Gas System Effectiveness

[2].  The aim of this technical report was to provide a framework for evaluating the performance of fire and gas detection systems in terms of detector coverage.  Two methods were proposed for evaluating coverage; geographic or scenario-based.  The geographic coverage method for gas detection still requires a specification of the size of a gas accumulation of concern, e.g., 5 meter spherical gas cloud, etc.  Kenexis has been working on improving on this by studying gas scenarios using Computational Fluid Dynamics (CFD) software to evaluate the near-field effects of gas dispersion in a complex process environment.  Historically, CFD has been difficult to apply to gas dispersion other than special applications.  With advances in technology, we can now apply CFD more broadly.  In addition, Kenexis is working on coverage methods consistent with TR84.00.07 that identify the most beneficial locations to place detectors using the results of CFD.  Soon we will be able to prioritize each proposed location for either point or open path gas detectors.  This exciting new work has been well received by the technical community.

I am excited to publish this work in a technical paper entitled, Performance-Based Gas Detection System Design Using Computational Fluid Dynamics (CFD) Modeling Of Gas Dispersion.   This is being presented at the 2015 ISA Process Control & Safety Symposium. The conference is being held in Houston TX at the Marriott Westchase from 9 – 12 November, 2015.  I encourage you to attend this event.  For more information, please contact Kenexis.