Vapor Cloud Explosions at Nil Wind

Current regulations for siting of LNG facilities specify the range of wind speeds to be considered as 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 these requirements, the bounding cases in most LNG facility siting studies are based on wind speeds of 1 or 2 m/s, depending on the wind data for each site.

Since the consequences of vapor dispersion scenarios, including the size of flammable vapor clouds that may result in explosions, are generally assumed to become worse as the wind speed decreases, the argument has been raised that a worst-case study should consider wind speeds down to zero (i.e., ‘nil-wind’). However, before regulations are modified to include additional burdensome requirements, consideration should be given to the frequency of occurrence of nil-wind conditions, as well as the likely change in hazard consequences as the wind speed decreases.

Blue Engineering and Consulting recently completed a PHMSA-sponsored research project to determine whether the change in flammable dispersion and overpressure hazards for credible accidental scenarios at LNG facilities, as the wind speed decreases to zero, can be sufficiently severe to warrant their inclusion in LNG facility siting studies. The evaluation was performed both under the current prescriptive regulations as well as the risk-based siting methodology outlined in NFPA 59A-2019.

This paper will discuss the assumptions made during the study and present the results and the recommendations made to PHMSA.