(153f) Effectiveness of Vapor Fences in Mitigating LNG Jetting and Flashing Releases

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. This decade also promises growth in LNG-fueled fleets of vehicles, railway transportation and marine vessels, as well as growth in other natural gas uses. The resulting expansion in the LNG distribution infrastructure will lead to an increased focus on managing the risks associated with releases of LNG.

At the same time as these developments are taking place, changes are also occurring in the interpretations of 49 CFR 193 regulations by the Federal Energy Regulatory Commission (FERC) and US Department of Transportation Pipeline and Hazardous Materials Safety Administration (PHMSA) on flammable cloud dispersion requirements.  These directly impact the design approach for new LNG terminals and expansion projects at LNG receiving terminals. Changes in the choice of leak scenarios that have to be considered have often resulted in larger and more severe worst-case LNG and refrigerant release scenarios.  The growth in the severity of the scenarios that need to be considered has necessitated the use of mitigation strategies, including vapor fences, to limit the extent of the siting exclusion zones.  As a result, modern LNG terminals are often designed with vapor fences that confine potential flammable gas releases within a well-defined dispersion exclusion zone.

Typically, vapor fences are placed along a large portion of the facility boundary, to contain the vapor cloud within to the facility. In order to be effective, these vapor fences need to be tall (up to 30 feet in some cases). Such long and tall fences are difficult to implement and can be subject to very high wind loads. An alternative geometry is investigated in this paper, using staggered fences to enhance mixing and dispersion of the cloud before it reaches the facility boundaries. These alternate vapor fences induce mixing of the vapor cloud instead of confining the vapor whithin the facility. This allows the use of shorter structures and reduces the concentration of flammable vapor inside the facility in a vapor release scenario.

A computational fluid dynamics (CFD) model of fence configurations was developed using the Star-CCM+ CFD software, to study the effect of various fence geometries to mitigate against high momentum jetting and flashing LNG releases. In this paper, the maximum extents of the ½-LFL methane vapor clouds are determined for releases involving no fence, continuous fence and staggered fence geometries. The results are used to quantify the effectiveness of each fence geometry to mitigate against the LNG releases.