Case study: Investigating the effect of the A2 Pasquill atmospheric condition on the dispersion modeling of heavy gases

Abstract

This study provides a defensible argument for the inclusion of the A2 Pasquill weather parameter in dispersion modeling, in addition to the typical F2 and D5. The A2 parameter conservatively models gas releases that concentrate at ground level, thus allowing for a more robust design of vent heights, surrounding structures/equipment and safety measures. For this investigation, 12 case studies of common hydrocarbon and aromatic gas mixtures were developed in PHAST (Process Hazard Analysis Software Tool) version 8.6. Each case varied in temperature, release pressure (velocity), and molecular weight (MW) to simulate dense gases likely to tend toward ground level. Subsequently, each case study was modeled with the Pasquill Atmospheric Stability Classes, A2, D5, and F2, to visualize the dispersion of dense gases under different weather conditions and evaluate which weather parameter would be most inclusive of high‐severity scenarios. Results demonstrate that dense (colder than dew point, heavy, pressurized) gases yield highest ground‐level concentrations using the A2 atmospheric condition, and the further the release temperature falls below the mixture's dew point, the greater the mixture concentration at ground level. Consequence modeling recommendations are discussed, and specific gas properties are addressed that necessitate using a model that is conservative in its estimation of ground‐level concentrations.