Benefits of CFD for Onshore Facility Explosion Studies

Significant releases (>50 kg/s) of hydrocarbons, whether as flashing liquid or dense gas, combined with moderate winds can in less than one minute generate very large flammable vapor clouds in an onshore facility. This potential has been previously seen in accidents like Flixborough (1974), but the potential to obtain very large vapor clouds have also been confirmed in a number of recent explosion accidents (e.g. Jaipur, San Juan). In onshore siting studies for facilities, whether driven by the API RP-752 or Seveso-II directive, the typical approach will be to use blast curve methods like the Multi-Energy method (MEM), the Congestion Assessment Method (CAM2) or the Baker-Strehlow-Tang (BST). When applying such blast curves, the typical approach will be to only consider blast energy for the flammable vapor located inside one unit of the plant at a time (one congested area). This approach may be acceptable if the spacing between units is sufficient, and there is no risk for possible escalation in a neighboring unit and for deflagration-to-detonation-transition (DDT). Accidents like the Buncefield explosion tell us that DDTs cannot be ruled out, and as such, the typical blast-curve approach will be very far from conservative. Another major weakness with the blast-curve approach is that the toolbox to assess the effects of mitigation is nonexistent or quite limited. In this paper a CFD-based approach is presented which evaluates i) the potential to generate large gas cloud sizes through a dispersion study, ii) the conditions when neighboring units cannot be evaluated as independent single units, iii) critical gas cloud sizes necessary to achieve DDT for different areas of an onshore facility, and iv) various mitigation possibilities (e.g., soft barriers, confinement, flange guards, deluge) to limit the potential of DDTs, if required.