Comparison of Six Widely-Used Dense Gas Dispersion Models for Three Actual Railcar Accidents

Abstract

The simulations are compared of six widely-used dense gas dispersion models of downwind chlorine gas concentrations following three railcar accidents. The six models are ALOHA, HGSYSTEM, SLAB, SCIPUFF, PHAST and TRACE. The three railcar accidents, where as much as 60 t of chlorine were released, are Festus, MO (release from a ruptured 1 in. line while offloading), and Macdona, TX, and Graniteville, SC (release from a large hole due to an accident). Input data were obtained from public sources. Source emissions rates were refined based on source modeling with PHAST and TRACE and derivations using fundamental thermodynamic equations. When using the same source emissions rates, the models’ simulations of 10-minute averaged cloud centerline concentration, at downwind distances ranging from 0.1 to 10 km, agree with each other within plus and minus a factor of two most of the time. For a very large release (Graniteville), the 10-minute averaged 2000 ppm, 400 ppm, and 20 ppm contours are predicted to extend downwind about 1.3, 3.1, and 14 km, respectively, from the source. There is also agreement among the models simulations of the plume widths and heights to the 2000, 400, and 20 ppm contours. A major conclusion of the study is that estimation of the source or release term is important for reliable results.