Volume of flammable mixture resulting from the atmospheric dispersion of a leak or spill

Abstract

The investigators of an unconfined gas explosion typically derive some measure of the air blast, leading to the assignment of a “TNT equivalent.” This number is invariably small, ranging from 0 to 10% of the yield that one would have predicted from the heat of combustion of the fuel. The probable reason for this low value, as this paper seeks to show, is that only a small fraction of an atmospherically dispersed gas mixture can be within a flammable range of concentrations. This paper draws on measurements of the atmospheric dispersion of natural gas to test the applicability of the bivariant Gaussian distribution equation with standard deviations, u and z , derived from the air pollution literature. Three observations are discussed in relation to the dispersion of flammable gases: (1) The concentrations of interest (flammable limits) are much higher than most critical pollutant concentrations; (2) concentration peaks may well be an order-of-magnitude higher than time-averaged concentrations, which are derived from a statistical treatment; (3) most flammable vapors are heavier than air and form ground-hugging layers that extend the distances of ignition hazard. Calculations are presented of the volumes of vapor-air mixture within surfaces of equal con-centration. From these figures, it is evident that most of the flammable vapor is quickly dispersed to concentrations below the lower limit of flammability.