Rainout assessment after a release of hazardous liquefied gas: Effect of physical and storage parameters

In process industry it is common practice to store and handle dangerous substances, which are gaseous at atmospheric temperature, as pressurized or refrigerated liquids. In case of accidental release, a fluid in these conditions gives rise to a two-phase discharge. In such conditions, the liquid breaks up in an aerosol cloud, whose behavior in atmosphere affects dispersion distances. The physical phenomena occurring after the release to the atmosphere are mainly three: expansion to ambient pressure, liquid atomization (break-up) and rainout. In particular, the rainout fraction is a crucial parameter in quantitative risk analysis and therefore models capable of estimating whether it occurs or not are of paramount importance. This study aims at emphasizing the influence of some physical parameters and storage related parameters on the rainout fraction, taking into account continuous and stationary releases of ammonia and LNG (Liquefied Natural Gas). These represent two different classes of substances: gas liquefied by pressure (superheated) and gas liquefied by refrigeration (subcooled), which undergo to different jet break-up mechanisms. The results obtained for these two compounds can then be extended to the whole range of pressurised liquids/liquefied gases. A simple model has been developed to evaluate rainout occurrence, the simple approach here proposed (which has been validated comparing its results to those of other simulation models as well as to some available experimental data) allows to investigate in a simple and effective way the importance of the different physical and storage parameters involved during rainout.