A Study for the Prevention of Unconfined Vapor Cloud Explosions from Spilled LNG Confined in a Dike

摘要

Large-scale spills of LNG (Liquefied Natural Gas) can produce very cold vapor and the vapor will in general remain heavier than air until it absorbs heat from the surface of dike and surrounding air. If the cold vapor forms a fuel-air cloud heavier than air, then it may find an ignition source on the ground surface and cause an Unconfined Vapor Cloud Explosion (UVCE) during the process of dispersion. The object of this study is to investigate whether a flammable fuel-air cloud can be formed outside of the dike area in case of LNG spill made inside of the dike area. Evaporation and dispersion were modeled to predict the formation of a flammable vapor cloud. Some protection measures were also considered in the modeling to estimate the efficiency of those measures. Important input variables for dispersion modeling are size of LNG pool, wind direction and speed, insulation of dike floor, mitigation with high expansion foam, and vapor fence. The FLACS code, a computational fluid dynamics simulator, was used in dispersion modeling. The evaporation was modeled by analyzing heat transfer through conduction, convection and radiation. Results of the study show that a major leakage of LNG in the dike area could evaporate and flow over the dike wall and form a flammable cloud at the ground level outside the dike. This is different from what people have thought about natural gas. As NG is lighter than air at normal temperature, people have paid less attention to the UVCE risk of cold NG. Mitigation measures preventing from forming a large explosive cloud outside of the dike were also analyzed.