Guidelines for Jetting and Flashing LNG Vapor Exclusion Zone Analysis

AIChE Spring Meeting and Global Congress on Process Safety
April 4, 2012 - 12:00am

Liquefied Natural Gas (LNG) has been a growing energy resource in the U.S., for which significant import and storage infrastructure was developed in the past decade. The infrastructure for liquefaction and export has also been developing as a result of recent shale gas finds in North America. In the U.S., the construction and operation of LNG facilities are regulated by the Pipeline and Hazardous Materials Safety Administration (PHMSA) of the U.S. Department of Transportation (DOT), and the regulations are administered by the Federal Energy Regulatory Commission (FERC). As part of the regulatory process (Title 49, Code of Federal Regulations, Part 193.2059), regasification and liquefaction facility owners are required to perform prescriptive calculations to estimate the potential consequences of major releases of LNG prior to permitting. One of the criteria is that the ½-LEL boundary of resulting flammable vapor clouds cannot cross the property boundaries (exclusion zone). The consequence analysis requirements have recently been broadened to include new spill scenarios in addition to the previously required LNG spills into impoundment or “sumps” and to allow the use of alternative computational methods to calculate the ½-LEL exclusion zones for these scenarios. Specifically, in 2010 PHMSA and DOT issued a written interpretation regarding the flammable vapor exclusion zone requirements requiring analysis of “jetting and flashing” phenomena accompanying a pressurized release of cryogenic LNG, as well as spills into containment trenches and troughs. This paper focuses on the new requirement for the analysis of jetting and flashing events.

Jetting and flashing events can be modeled with relative ease by using integral models; however, the selection of appropriate scenarios can add complexity to the analysis. This paper presents typical scenarios that are required for this analysis, including high pressure flange and valve packing failure scenarios. This paper then parametrically presents the consequences studies of a broad range of LNG jetting and flashing release scenarios to provide facility owners with a basis for selecting appropriate scenarios and estimates of the consequences of such releases. This paper finally reviews the benefits and limitations of the simplifying assumptions associated with integral models. The data presented in the paper can be used by facility owners as a preliminary aid in the decision making process for facility upgrades and modifications.

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