Comparison of EPA Guidelines Tables With a Commercial Model

The USEPA has provided a set of guidelines to aid compliance with Risk Management Planning (RMP) regulations, one specific for ammonia releases and a generic guideline for all other materials. These guidelines were developed using specific models, the SACRUNCH and SLAB model respectively. This paper compares predictions made by a proprietary model, PHAST/SAFETI Version 5.2 with both set of guidelines. Evaporation rate predictions are shown to be highly time-dependent. Differences with the EPA guidelines occur primarily from the process of averaging or making discrete approximations to the time-varying curve. Our predictions are less sensitive to storage temperature than EPA guidelines because in our model the cooling effect of an evaporating pool rapidly washes out the initial pool temperature effects. PHAST/SAFETI hazard zone predictions are found to differ from the guidelines primarily by way of the averaging time adjustment applied. Hazard distances are found to depend importantly on variables not considered in the guidelines: the release direction and the actual phase of a release, whether from the liquid or vapor space. However, release height, also not considered in the guidelines, was found to be less important, at least for volatile toxic materials such as chlorine.