Vapor Dispersion: How Safe Is Your Safety Factor?

Risk analysis, consequence analysis, building siting, demonstrating compliance with regulatory permitting criteria – these activities all rely heavily upon mathematical modeling of physical phenomena and flammable vapor dispersion. The common safety factor or rule of thumb, especially as applied in permitting of LNG facilities through the Pipelines and Hazardous Materials Safety Administration (PHMSA), applies a value of 2. The hazard threshold is often modeled as ½ of the lower flammable limit (LFL). But who came up with the safety factor and how useful is this factor of 2? In the case of the PHMSA Model Evaluation Protocol, this was loosely based upon standard models’ concurrence with a set of large scale field test results. In this manuscript, we will briefly introduce those results but use a different and new data set to evaluate the significance of this safety factor selection for flammable gas dispersion. Downwind concentration time histories for multiple propane releases in a controlled wind tunnel environment will be analyzed both individually and as an ensemble average resulting in an experimentally significant safety factor for comparison to the existing guidance.