The vapour of liquefied natural gas (LNG) is heavier than the ambient air at -160 deg C, but it becomes lighter than the ambient air at -60 deg C, which may be reached due to heat transfer from the air or from the ground. The lower flammable limit (LFL) concentration of methane is 5 %, if there is no fluctuation of concentration. But the US Federal Safety Standard of LNG facilities recommends 2.5 % as a critical value of LFL by assuming a factor of 2 for a concentration fluctuation in the atmosphere. The well-known conventional dispersion models DEGADIS and FEM3 cannot calculate concentration fluctuations. Therefore, concentration fluctuations were measured here for two cases with and without a building. The study considered many wind tunnel experiments using isothermal heavy gas and cryogenic gas of the same specific gravity as LNG, and compared the results of time-averaged concentration with those of field experiments, such as the Thorney Island Experiments (1987) and the China Lake Experiments (1987). The laboratory and field measurements agreed well, which suggests that a wind tunnel experiment can simulate important aspects of the diffusion of a cryogenic gas like LNG. These wind tunnel data were also compared with the models, DEGADIS and FEM3. We compared the standard deviation of concentration fluctuation with results calculated by our original numerical code of STD (standard deviation) model. The experimental and numerical results agreed well for unobstructed flows, but there were some discrepancies when a building was present. In addition to the comparison of the standard deviation of the concentration fluctuations, we compared the PDF (probability density function) produced by wind tunnel results, and found good agreement with that predicted by a DNS (direct numerical simulation). Atmospheric stability of stable condition can be simulated during dense gas modelling by satisfying the similarity rule of Richardson number. Cryogenic gas diffusion can be approximated near the source by using isothermal heavy gas with the same Froude number, while gas density changes cause overestimates far from the source.
Validation of heavy and light gas dispersion models for the safety analysis of LNG tank
Journal of Loss Prevention in the Process Industries ; 17 , 5 ; 325-337
2004
13 Seiten, 17 Bilder, 2 Tabellen, 12 Quellen
Aufsatz (Zeitschrift)
Englisch
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