Abstract
In 2005, the Buncefield accident showed us that a tank overfilling scenario cannot be treated as a simple liquid spill. After extensive research conducted by HSE, it was established that liquid cascades tended to fragment into droplets and a proportion of the release would form a vapour that could subsequently disperse and if confined, explode. HSE published a Technical Note in 2013 that included a simple mathematical approach to simulate the vapour cloud generation due to evaporation from a cascading liquid. Unfortunately, this empirical approach was limited in its application to a few types of fuels, while the associated explosion prediction also restricted its usage to zero wind conditions and specific congestion situations.
Because the tank-overfilling risks can be relevant to other materials and (weather) conditions as well, Gexcon has expanded the HSE model to calculate the vapour cloud generation due to the overfilling of a tank containing a volatile liquid. The model has been extended to account for the evaporation rate from the generated liquid pool at the bottom, and now incorporates a pure thermodynamic method to calculate the vapour cloud generation of any overflowing flammable liquid.
This way, it has become a pure “source term prediction” method, and now allows the result to be used as input for a subsequent dense gas dispersion phase (which may be subject to certain wind conditions), to evaluate potential explosion risk in any meteorological and confinement situation. Furthermore, the “tank overfilling” model predicts both cloud and pool contents, providing the possibility to calculate the subsequent pool fire event due to direct ignition of the remaining liquid spill.
Typical results of the extended “tank overfilling” method, such as vapour concentrations at the bottom of the liquid cascade, and “cascade” vaporisation rate have been compared with the experimental data and showed very good agreement. The extended “tank overfilling model for cascading liquids” is now available in the Gexcon EFFECTS consequence modelling software. Because the method now also includes the pool evaporation source rate and uses a thermodynamic “droplet evaporation” method, its applicability has been expanded to “non-zero” wind situations, while suitable for any chemical or Hydrocarbon mixture.
