(102c) The Effect of Substrate on LNG Vapor Dispersion from An Impounded Area

In the 1980s, a set of LNG vapor dispersion tests (the ?Falcon series?) were performed, in which LNG was spilled into an area surrounded by vapor barriers [1]. The purpose of the Falcon tests was to evaluate the effect of the vapor barriers on the growth and dispersion of the LNG vapor cloud produced by the vaporization of the LNG spill. In these tests, an approximately constant vaporization flux was sought for the duration of the spill; for this reason, LNG was spilled onto a recirculating water pond so that the temperature of the substrate in contact with the LNG would remain approximately constant. In practical scenarios, however, LNG spills into impounded areas would occur onto land (e.g., concrete, compacted terrain, gravel) and not on water. Therefore, there is a critical difference between the Falcon tests and a hypothetical release at an onshore LNG terminal: when LNG is spilled onto land, the surface temperature in contact with the LNG pool decreases rapidly and, as a result, the LNG vaporization flux also decreases. Additionally, the turbulence introduced by the LNG boiling decreases with the vaporization rate, which has a significant impact on the growth of the vapor cloud inside the impoundment, as demonstrated in a previous study [2].

Recently, the results of the Falcon tests have been used by Havens [3] to argue against the vapor dispersion analysis methodology typically followed for onshore LNG terminals. The argument is based on evidence, from the Falcon tests, that the vapor cloud mixes with air inside the impoundment and grows to overflow the impoundment; the reason for the cloud mixing is attributed to wind-induced turbulence. While the general physics discussed by Havens are not incorrect, the practical implications of his argument, in terms of vapor dispersion distances, are greatly overstated by his failure to consider the effect of substrate (land vs. water) on the vapor cloud growth in a hypothetical release at an onshore LNG terminal.

This paper will demonstrate the effect of substrate on LNG vapor cloud dispersion for a spill on land inside an impoundment, by comparing the results previously obtained by the authors for the Falcon tests [4] with the results for an identical spill, into the same impoundment, but on a solid substrate.

References

[1] Brown, T.; Cederwall, R.; Chan, S.; Ermak, D.; Koopman, R.; Lamson, K.; McLure, J. & Morris, L., Falcon Series Test Data Report. 1987 LNG Vapor Barrier Verification Field Trials, Lawrence Livermore National Laboratory, 1990

[2] Gavelli, F.; Bullister, E. & Kytomaa, H., Application of CFD (Fluent) to LNG Spills into Geometrically Complex Environments, Journal of Hazardous Materials, 2008, 159, 158-168

[3] Jerry Havens, Tom Spicer, and Heather Walker, LNG Vapor Cloud Exclusion Zone Requirements Need Review, 2008 AIChE Spring National Meeting

[4] Gavelli, F.; Chernovsky, M.; Bullister, E. & Kytomaa, H., Validation of a CFD model for vapor dispersion from LNG spills into an impoundment, 2007 AIChE Spring National Meeting