(7b) Case Study: Investigating the Effect of the A2 Pasquill Atmospheric Condition on the Dispersion Modelling of Heavy Gases

Understanding the potential consequence of loss of primary containment is the key to a safe and robust design. By understanding the worst credible scenario from a hydrocarbon or chemical releases, adequate safeguards and mitigations can be put in place for a safer, more robust design. In the case of dispersion modelling, typically, the D5 and F2 Pasquill weather parameters are used to represent the entire range of weather conditions in hazardous effects modelling. D5 is used because it is typically the most common atmospheric stability case during the day, and F2 typically represents the worst-case atmospheric condition that would result in the highest pollutant concentration furthest downwind of the release. However, these conditions do not usually account for cases of high pollutant concentrations at ground level where most people and environments are at risk, especially for releases at height

The A2 Pasquill case describes unstable weather conditions with the steepest vertical temperature gradients due to which dense gases are pushed downward and cause pollutants to concentrate at ground level. Although the A2 case is low in likelihood, due to the vertical temperature gradient it has the greatest potential to cause a vapor plume to disperse downward to the grade level. Thus, adding the A2 Pasquill case as another standard weather parameter will generate models that predict the worst-case scenarios for ground-level concentrations, especially for scenarios where the release is at height, e.g. atmospheric vents

The goal of this study is to demonstrate the advantage of using the A2 parameter to encompass the most severe dense gas release events. For this investigation, 21 case studies of common hydrocarbon and aromatic mixtures were developed in PHAST (Process Hazard Analysis Software Tool). Each case varied in temperature, release pressure (velocity), and molecular weight (MW) to simulate dense gases that are likely to tend towards ground level. Subsequently, each case study was modelled with the A2, D5, and F2 conditions to visualize the dispersion of dense gases under the different weather conditions and to 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 concentration using A2 atmospheric condition. Moreover, the further the release temperature falls below the dew point of the mixture, the greater the liquid content of the release, and the greater the mixture concentration at ground level, therefore making it even more critical to use a model that is conservative in its estimation of ground-level concentrations.

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. It conservatively models gas releases that concentrate at ground level, thus allowing for a more robust design of vent heights and surrounding structures/equipment, as well as the administration of more robust safety measures.