Abstract
Chemical and petrochemical facilities will have substances that may need to be released to atmosphere, either all the time or on special occasions like system maintenance situations or as a part of safety procedures during unwanted process safety incidents. As the substances are often toxic and denser than air, the typical solution has been to vent the substances from stacks at a significant elevation. While this solution may work well when there is significant wind, the gas plume may fall to the ground during low wind scenarios (< 1-2 m/s) exposing workers or neighbours of the facility to problematic gas concentrations. To ensure safe design of stack venting systems it is common to use integral models for dispersion predicting downwind distribution of gas. While standard integral consequence models may predict gas concentrations for wind dominated scenarios well, generally they cannot predict plume behaviour and hazards for the more critical low wind scenarios. For these scenarios gas plume density, slumping to the ground, effects of buildings, process plant geometry and topology are important. In this work CFD modelling has been utilized to better understand stack dispersion during low winds to be able to predict the hazards and identify the safe operational window. Among several interesting findings of the study was that worst-case concentrations at ground level may significantly increase with reduced stack flow rate, as the vertical upwards velocity and mixing with air are reduced. Several mitigation possibilities to reduce ground level gas concentrations during low wind scenarios exist. This study concluded that a frequently used approach of extending the stack had a very limited beneficial effect, while strong vertical fans next to the stack during low wind conditions could have a very beneficial effect reducing ground level concentrations.
