Analysis of accidents involving hazardous materials
Considerable quantities of toxic industrial chemicals (TICs) are produced and stored in the chemical and process industries or transported by railway, road or sea. These can be stored either in liquid form under pressure at near ambient temperatures, or liquefied by cooling the chemical to or below its boiling point.
Accidental releases of TICs may cause significant hazard to both human health and environment far away from release point. Reliable methods are therefore needed for estimating the release rates and the atmospheric dispersion of TICs following, for example, a process failure, accident or terrorist attack. The methods can be used in, for example, the emergency contingency planning and in the safety analyses of industrial installations.
The ESCAPE model
Consequences of chemical accidents in air quality can be estimated by dispersion models developed in Finnish Meteorological Institute. The dispersion model ESCAPE (Expert System for Consequence Analysis and Preparing for Emergencies) is a validated assessment tool for consequence analysis of accidents involving hazardous materials. The model can be used for evaluating releases, the formation of source terms, atmospheric dispersion and consequences of hazardous materials. The model is applicable to evaluate both continuous and instantaneous ground-level releases of toxic and flammable gases into the atmosphere.
The model can be utilized in release cases, which are a consequence of a sudden rupture (catastrophic failure) of a container or the rupture of a pipe or container wall, either from the liquefied or gaseous state of a refrigerated or pressurized container. An aerosol or gaseous plume or puff will be formed, and in certain conditions, if the release scenario results in the formation of a liquid pool, its evaporation may produce either a gas puff or plume. Predicted concentration distribution near ground level caused by an assumed leak from a railway chlorine container is illustrated in the example figure.
Web-browser based user interface
A graphical user interface for the ESCAPE model based on web-browser technology has been developed for the needs of the Finnish emergency authorities to be readily applicable for rescue service in operational cases. Its primary purpose is to provide estimates of the spatial extent of some common hazards associated with chemical spills for emergency response personnel.
The software has been designed to utilize directly and in real-time the meteorological data produced by numerical weather prediction (NWP) models. Direct utilization of e.g. NWP-model products simplifies the data a user has to provide while providing more reliable and accurate predictions. The user only needs to specify input data regarding the characteristics of the release and the environment (the type of release and its associated details, the location and time of the accident, and the contaminant).
Limitations of the model
The model is not designed for evaluating the dispersion of buoyant plumes originating from, for example fires. A separate model (BUOYANT) can be used to estimate the dispersion of pollutants emitted from warehouse and wind-land fires. The ESCAPE model is limited in its ability to account for the detailed effects associated with complex terrain, buildings and other obstacles. Further, time-varying releases are outside of the scope of the model.
Model evaluation and validation
The ESCAPE model has been scientifically evaluated, for example, within the EU-project SMEDIS (Scientific Model Evaluation of Dense Gas Dispersion Models). Comparison of model predictions against measured experimental data from six field campaigns indicates a fairly good agreement between predictions and measurements.
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