The gas detector layout is designed by the technical safety discipline using experience and following standard guidelines, which for the most cases will result in an adequate layout. Gas dispersion analysis may be used for verification purposes of a gas detector layout. The verification activity may be performed during engineering/construction phase (in order to avoid deficiencies emerging during operation) or in some cases, as a result of inadequacies suspected/proven during operation.
The verification activity may uncover parts of areas with inadequate gas detector coverage, and different detection limits (high / low alarm or action, as % of LEL) and different choices of voting may be evaluated.
The CFD tool FLACS is used in these analyses, where a 3D model of the installation is made, including both point and line gas detector in their exact positions. Once modelled, the gas detectors may be turned on or off, and detector limits or voting may be altered in order to evaluate the effect of these detectors or parameters. A representative choice of time-dependent gas dispersion simulations is run, and features like time to detection and probability of detection is found for each simulation. The percentage of gas clouds detected before they become dangerous (e.g. giving explosion pressures above design loads) is evaluated.
The method may be applied to different type of gas detector, e.g. hydrocarbon gas, H2S or other toxic gases.
Example of 3D plot showing the extent of a toxic gas dispersed in a hazardous area, for determining location of gas detector around the equipment: