Flammable Vapor-Gas Dispersion Protection [49 CFR 193.2059]

(A)A. Each LNG container and LNG transfer system must have a dispersion exclusion zone in accordance with sections 2.2.3.3 and 2.2.3.4 of NFPA-59A-2001 (incorporated by reference, see §6713) with the following exceptions.#

(1)1. Flammable vapor-gas dispersion distances must be determined in accordance with the model described in the GTI-04/0049, "LNG Vapor Dispersion Prediction with the DEGADIS 2.1 Dense Gas Dispersion Model" (incorporated by reference, see §6713). Alternatively, in order to account for additional cloud dilution which may be caused by the complex flow patterns induced by tank and dike structure, dispersion distances may be calculated in accordance with the model described in the Gas Research Institute report GRI-96/0396.5 (incorporated by reference, see §6713), "Evaluation of Mitigation Methods for Accidental LNG Releases. Volume 5: Using FEM3A for LNG Accident Consequence Analyses". The use of alternate models which take into account the same physical factors and have been validated by experimental test data shall be permitted, subject to the Commissioner's approval. [49 CFR 193.2059(a)]#

(2)2. The following dispersion parameters must be used in computing dispersion distances.#

(a)a. Average gas concentration in air = 2.5 percent. [49 CFR 193.2059(b)(1)]#

(b)b. Dispersion conditions are a combination of those which result in longer predicted downwind dispersion distances than other weather conditions at the site at least 90 percent of the time, based on figures maintained by National Weather Service of the U.S. Department of Commerce, or as an alternative where the model used gives longer distances at lower wind speeds, Atmospheric Stability (Pasquill Class) F, wind speed = 4.5 miles per hour (2.01 meters/sec) at reference height of 10 meters, relative humidity = 50.0 percent, and atmospheric temperature = average in the region. [49 CFR 193.2059(b)(2)]#

(c)c. The elevation for contour (receptor) output H = 0.5 meters. [49 CFR 193.2059(b)(3)]#

(d)d. A surface roughness factor of 0.03 meters shall be used. Higher values for the roughness factor may be used if it can be shown that the terrain both upwind and downwind of the vapor cloud has dense vegetation and that the vapor cloud height is more than ten times the height of the obstacles encountered by the vapor cloud. [49 CFR 193.2059(b)(4)]#

(3)3. The design spill shall be determined in accordance with section 2.2.3.5 of NFPA-59A-2001 (incorporated by reference, see §6713). [49 CFR 193.2059(c)]#

AUTHORITY NOTE: Promulgated in accordance with R.S. 30:551.C.#

HISTORICAL NOTE: Promulgated by the Department of Natural Resources, Office of Conservation, LR 44:1050 (June 2018).#