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