The effects of release of hazardous material will originate from the release point, and will diminish with distance from the site. The greatest potential for significant off-site effects exists when a gas or vapour is released which may be either toxic or flammable. This means that an important part of any consequence analysis study must be the calculation of gas dispersion. In many cases, the action of the atmosphere in dispersing a gas will serve to reduce the hazard to nearby personnel. This particularly true for passive or positively buoyant releases from elevated sources and is used to good effect in the design of stacks for the discharge of normal emissions. However, for releases at ground level, or where the gas is denser than air, dispersion is much slower and hazard ranges could be significant.
This paper briefly reviews the nature of atmospheric turbulence and the way in which this affects dispersion. Particular features of dense gas dispersion are discussed and compared with the dispersion of neutrally buoyant gases. Methods for incorporation of topographical and obstruction effects are also discussed, together with a range of mathematical models available for the prediction of dense gas dispersion. Case studies from a range of industries are presented in order to demonstrate the use of the methods discussed.