An effective gas detection system protects on-site personnel from toxic and combustible gas releases. Gas detectors that are fit for purpose with the proper technology, measurement level, and location can provide an early detection and warning to personnel to evacuate or take appropriate action in the event of an accidental release.
Shortcomings of the Geographic Approach
The placement of gas detectors is never a straightforward analysis. A common rule of thumb for hydrocarbon releases (methane and propane) is the 5-meter cloud Geographic Approach; however, this approach is not valid for non-hydrocarbon releases and tends to over-specify the number of gas detectors required. A facility may require a gas detection system for varying chemicals with uncommon properties and parameters. The Geographic Approach does not consider building ventilation, geometry, or obstructions that can significantly impact the dispersion of gas plumes.
CFD Scenario Modeling
Computational Fluid Dynamics (CFD) scenario modeling is an alternative method to the Geographic Approach that delivers a more accurate number and placement of combustible and toxic gas detectors. Although there is a larger upfront investment, CFD modeling saves costs over the long term by reducing the number of gas detectors required and the expense of their calibration and maintenance.
CFD modeling also provides an auditable record of assumptions such as material properties, leak size, leak origin, wind speed, and building ventilation; this can be beneficial in the event of an actual leak incident since the original assumptions can be revisited and revised if the actual leak behavior deviates significantly from the modeled leak dispersion. The CFD modeling results may reveal unexpected behaviors from the vapors of concern compared to what was reasonably predicted. For example, a simple approach might place gas detectors near the roof for gases that are lighter than air. Yet, CFD modeling may demonstrate that ventilation effects pull the gas downward instead, requiring detectors to be placed at lower elevations. In similar fashion, it might be assumed that a remote warehouse building does not require a gas detection system if there are no chemicals of concern within the building; however, a gas leak in another area may disperse farther than expected, resulting in the gas cloud traveling to the unprotected warehouse building. In this case, gas detectors in the warehouse’s ventilation intakes might be advisable.
CFD scenario modeling is the best investment for large facilities with specialty gases to ensure gas detectors are appropriately located and perform optimally. It is the most effective tool to have high confidence in protecting personnel from toxic and combustible gas releases, while minimizing the long-term maintenance costs for the gas detection system. Dispersion modeling results may challenge the predictions of simpler, less quantitative methods, and documented modeling assumptions can be referred to and updated throughout the life of the facility.