Practical modelling of gas dispersion in low wind speed conditions, for application in risk assessment

Abstract

Risk assessments for hazardous installations, in which loss of inventory could result in offsite risk, will generally require the modelling of the dispersion of toxic or flammable gases for several realistic scenarios in a range of representative wind conditions. Hazard ranges — the distances affected by hazardous gas clouds — are usually greater for low wind speeds, but these are frequently omitted from the assessment. It is therefore useful to be able to model such conditions in an appropriate manner, and also to understand the nature and frequency of these low wind speed conditions. This paper presents the results of part of a study which considered the whole problem of using low wind speed conditions in risk assessments. A brief review is given of the current status of the modelling of gas dispersion in low wind speeds, with particular reference to releases of dense gas from low level, and the problems associated with using dispersion models beyond their range of validation or stated application are discussed. Different types of models are reviewed, and the potential for using models specifically developed for low wind speed conditions is assessed. A brief discussion is then given of the likely effects of using such models on the results of risk assessments and safety cases.