Flammable gas concentrations in ex-tank volumes

A simple two-volume model was defined and used for calculating flammable gas concentrations within headspace volumes of single-shell tanks, and within smaller ex-tank volumes connected to the headspace. Assumptions and parameters used to characterize the headspace portion of the model were taken from the GRE Analysis Tool (AT) for simulating gas release events. Additional assumptions used to construct the ex-tank portion of the two-volume model were conservative extensions of those made within the AT, and chosen to simulate headspace to ex-tank gas-flow conditions that would maximize ex-tank concentrations. Numerical evaluations of the two-volume model were performed over a range of headspace GRE conditions and representative ex-tank parameters. To assure consistency with the AT, the range of headspace parameters was taken from 1000 simulated GREs generated by the AT computer code RESOLVE. Based upon waste level fill factors, three tanks (TX-102, SX-103, and TX 112) were chosen to represent typical large, medium, and small headspace volumes available in actual SSTs. Engineering drawings of these tanks were used to determine values of their ex-tank parameters (V2`s and estimates for the gas-flow fraction ``a`` into the specific V2). The results of these evaluations were used to compare time periods for which flammable gas concentrations in the tank headspace and the ex-tank volumes exceeded the lower flammability limit for upward flame propagation. These results indicate that even for relatively small flow fractions, headspace concentrations that exceed the LFI, can cause delayed ex-tank concentrations to also exceed the LFLU. The extent to which this occurs is determined mostly by the geometrical aspects of the model, as expressed in the effective volume fraction parameter