COBRA - A HEAVY GAS-LIQUID SPILL AND DISPERSION MODELING SYSTEM

A heavy gas/liquid spill and dispersion modelling system has been developed for estimating the hazard zones due to accidental spills of flammable or toxic chemicals such as propane, butane, chlorine and ammonia. It consists of three basic modules: the spill rate module for estimating the direct source term for an accidental release from refrigerated or pressurized storage; the pool spread and gas generation module for estimating the rate of spread of any liquid spill on a surface and the rate of gas generation (indirect source term) as a result of evaporation or boil-off from the pool; and the heavy gas spread and dispersion module for estimating time dependent downwind concentration distributions and hazard zones. The third module is based on advanced similarity modelling concepts. It incorporates the effects of gravity spread, atmospheric dispersion modified by the presence of the heavy gas cloud, heat transfer from the substrate, diffusion in the x-direction, and cooling and heating of the cloud by physical reaction and phase change of humidity in the air and heavy gas-liquid droplets. The latter effect is particularly important for releases of liquefied gases from pressurized storage where a large part of the spilled material ends up in aerosol form suspended in the gas cloud. The objective of this paper is to describe the basic equations used in the dispersion module in relation to the physical phenomena important in the behaviour of heavy gas clouds released from pressurized liquefied storage. Model comparisons with available field data from Maplin sands, Thorney Island, China Lake and Frenchman Flats are presented. Results of comparative sensitivity runs are given for various scenarios such as refrigerated versus pressurized storage, diked versus unconfined spills, etc., demonstrating the capabilities of the modelling system and its usefulness in emergency planning and also as a safety design tool.