(88a) Vapor Cloud Exclusion Zone Issues for Spills into Impoundments

Requirements of 49 CFR 193 for vapor cloud dispersion exclusion zones designed to protect the public from accidental releases of LNG from land-based storage facilities are outlined, and the mathematical modeling methods approved by the Department of Transportation for such determinations are reviewed. Use of the SOURCE5 model to determine LNG vapor overflow rates from spills into liquid impoundments, which assumes that no air mixes with the LNG vapor inside the impoundment, has been demonstrated by wind tunnel and field experiments to be in error. As a result, application of the DEGADIS model (as currently practiced) for determining exclusion zones by using SOURCE5 to determine the vapor flow rate input to DEGADIS is in error. As 49 CFR 193 does not allow the use of DEGADIS for determining the effects on dispersion of obstacles to the flow (such as dikes or impoundment walls), proposals have been filed to change NFPA 59A, which is incorporated in 49 CFR 193, to require the use of the FEM3A model if the applicant desires to account for the effects on dispersion of dikes and/or impoundments, other obstacles such as tanks, or terrain features. There are only two methods currently allowed by 49 CFR 193 for determining the effects on dispersion of vapor holdup in impoundments. Either the FEM3A model must be used to determine directly the downwind distance to the ½ LFL concentration, or the FEM3A model may be used to determine the rate and concentration of LNG vapor (in air) at which the impoundment overflows for input to DEGADIS to determine the downwind distance from the dike edge to the ½ LFL concentration. The authors recommend the latter method based on recent research suggesting that the FEM3A model is uniquely suited to determine the concentrations in the near field (such as at the downwind edge of the dike/impoundment), whereas the DEGADIS model is already accepted for determining the dispersion that would follow downwind of the dike/impoundment edge. The paper also describes other recent proposals to change NFPA 59A. It has been proposed that vapor cloud dispersion exclusion zones should be required to be determined at the wind speed at which the maximum distance would occur, just as is already done for thermal exclusion zones. Finally, inconsistencies in NFPA 59A, and with 49 CFR 193, regarding the specification of design spills for the requirements imposed for impoundment volume and the determination of vapor cloud dispersion exclusion zones are described, and proposals for change to eliminate those inconsistencies have been filed with NFPA. There will be important ramifications of such changes in the regulations, as the methods currently in practice can result in vapor cloud exclusion zones that are insufficient to protect the public, while increases in the required exclusion zones can be a determining factor in siting LNG facilities.