A Unified Model for LNG Pool Spread and Vapor Dispersion: Is Wind Scooping Really a Factor?

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U.S. regulations for the siting of LNG facilities require LNG spills to be collected into impounded areas and the impoundments must be sited in such a way that LNG vapor clouds formed as a result of defined spills dissipate to below ½ LFL before reaching a property line that can be built upon [1]. Traditionally, the compliance of an impoundment with the flammable vapor cloud dispersion requirement was verified using the combination of the following models: SOURCE5 to calculate the LNG pool spread and vaporization rate within the impoundment; and DEGADIS to calculate the vapor cloud dispersion. In the last few years, however, several questions have been raised concerning this approach. First, some assumptions associated with LNG vapor holdup by the impoundment walls were disproved (the “wind scooping” argument) [2]. More recently, a study commissioned by the Fire Protection Research Foundation (FPRF) [3] identified several incorrect assumptions in the SOURCE5 model. As a result, LNG terminal applicants have been required to identify and utilize alternative methods for the calculation of the LNG pool spread and vapor source term for spills into impoundments.

GexCon's CFD model FLACS incorporates a fully two-dimensional shallow water-based model for the simulation of LNG pool spreading and vaporization, and a three-dimensional model for the simulation of LNG vapor cloud dispersion. As such, FLACS provides a unified environment in which the entire LNG spill and dispersion scenario can be simulated efficiently and accurately. In fact, FLACS was successfully validated against the entire Model Evaluation Protocol database for LNG vapor dispersion [4].

This paper will demonstrate the application of FLACS' unified pool spread and vapor dispersion model to LNG spills into impounded areas. The paper will examine the effect of impoundment substrate (i.e., concrete vs. water) on the rates of spreading and evaporation during a 10-minute spill. The paper will also evaluate the effect of wind-driven turbulence (the “wind scooping” effect) on impoundments of different sizes, to determine under which conditions higher wind speeds may lead to longer hazard distances, as sometimes argued.

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Filippo Gavelli
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