Beyond Surface Area and Packing Density: A Study of Traditional and Novel Congestion Parameters in Process Facilities

A large vapor cloud explosion (VCE) followed by a fire is one of the most dangerous and high-consequence events that can occur at petrochemical facilities. As the size and complexity of facilities increase, designs must consider the potential adverse effects associated with vapor cloud explosions in large congested areas. Designing topside structures to withstand a maximum credible event (MCE) by avoiding unacceptable escalation of events and damage to safety critical equipment is an essential part of the path toward safer designs. One main factor inhibiting inherently safer designs is the lack of detailed geometry information in the early design phase. When congestion due to small components is not integrated into the explosion pressure analysis model, the design blast loads will be severely underestimated. This paper will compare detailed CAD models obtained for both early design phase and “as-built” final detailed design, and demonstrate the gross under prediction of accidental explosion loads of the early phase models, as compared to as-built. In fact, the explosion overpressures can differ by an order of magnitude underlining the need for guidance on the necessary levels of congestion required to obtained reliable blast loads.

To this extent, Gexcon has evaluated traditional congestion parameters such as volume blockage ratio, surface area to volume ratio and packing density as indicators of predicted overpressure as they are commonly used with both detailed and simple tools for explosion siting studies. On their own, these parameters have not proven reliable indicators of predicted overpressure and makes assessing the amount of congestion that needs to be added to early design phase models challenging. This study will focus on evaluating alternative ways of quantifying congestion, such as special obstruction density gradients in order to evaluate combustion and eddy/turbulence decay downstream of obstructed areas. With these alternative parameters, we have further refined our anticipated congestion method (ACM) for petrochemical facilities for accurate predictions of explosion consequences with early design phase models (concept or FEED phase). This methodology will define what levels of anticipated congestion are necessary to supplement CAD models in early design phase to ensure accurate assessment of explosion hazards.

This paper will summarize the findings of our alternative congestion parameter analysis and include recommendations for required congestion level for supplementing early phase designs.