Methodologies to determine fixed flammable gas detectors placement

Accidental loss of containment of flammable gases or vapors is one of the most feared events at industrial facilities. The ignition of a cloud of flammable gas can have serious consequences for the safety of people, installations and the environment. Fixed detection of flammable gas, by means of permanently installed detectors at specific points in the plant, is one of the barriers to preventing the risk of explosion.

Current standards and best practice in the installation of flammable gas detectors have helped to reduce the number of undetected flammable gas releases from industrial plants. Nevertheless, a significant number of accidental releases are not properly detected by the systems provided. An analysis by the Health and Safety Executive (HSE) in the UK, based on eight years of data (2001-2008) on releases of flammable products from offshore installations, reveals that only 60% of major releases (> 300 kg emitted into the atmosphere) were effectively detected.

This raises the question of the effectiveness of the detection system, and raises the question of the position and number of detectors. How many detectors do I need to install? Where should I position the detectors? These are certainly two of the most frequently asked questions, but also two of the most difficult to answer. To improve leak detection, one approach might be to increase the number of detectors. Such an approach, while capable of improving the level of detection if detectors are optimally placed, will never be able to detect all leak scenarios. On the other hand, increasing the number of detectors induces additional constraints that need to be carefully considered. In addition to the cost associated with the number of additional detectors and the design of the safety system, the addition of detectors entails constraints in terms of operation and maintenance. Moreover, increasing the number of detectors increases the risk of false detections, with unjustified alarms being triggered and, in some cases, inappropriate unit shutdowns. What's more, repeated false alarms can have serious consequences for operator safety. Operators may, for example, disable certain detectors in order to limit the number of unjustified alarms, or even, in the event of a real alarm, stop evacuating.

The aim of this article is to present the main methodologies for installing fixed flammable gas detectors. After a brief review of the issues involved in gas detection and the...