How Does Concrete Affect Evaporation of Cryogenic Liquids: Evaluating LNG Plant Safety

Abstract

With the impending natural gas boom in the U.S., many companies are pursuing DOE approval for exporting liquefied natural gas (LNG), which is a cryogenic liquid. The next decade also promises to demonstrate growth in LNG-fueled fleets of vehicles and marine vessels, as well as growth in other natural gas uses. The future expansion in the LNG infrastructure will lead to an increased focus on managing the risks associated with spills of LNG. Risk analysis involving LNG spill scenarios and their consequences requires the determination of the size of resulting ignitable flammable vapor clouds. This in turn depends strongly on the rate of evaporation of the spilled LNG. The evaporation of a cryogenic LNG spill (and thus the flammable vapor cloud hazard) can be quite a complex process, and it is primarily controlled by the rate of spreading of the pool and by the transient conductive heat transfer from the ground to the spilled liquid. Radiative and convective heat transfer are also present, but the conductive heat transfer rate dominates in the evaporation of a cryogenic liquid spilled into a trench or sump initially at ambient temperature. The time dependent evaporation rate can be calculated using a variety of models, such as the built-in model in PHAST (DNV) or other proprietary models that account for pool spreading, heat conduction within the substrate, and phase change. Trenches and sumps used to contain LNG spills are normally lined with various types of concrete, including insulated or aerated concrete. We have found that for a cryogenic liquid, the choice of thermal properties for concrete can greatly affect the source term. In this work, we perform a sensitivity study of the effects of substrate properties on the evaporation rate of LNG. The study will look at the dependence for a range of sump diameters. The PHAST model results will be compared to results obtained using an in-house Shallow Water Equation (SWE) liquid propagation and heat transfer model. The results of this work will provide guidance for the selection of substrate properties during modeling; as well as a comparison of the relative evaporation rates expected for different surfaces, such as regular concrete and insulated concrete.