Large Scale Testing Confirms Deflagration to Detonation Transition (DDT) Needs to be Considered in Facility Siting

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 and understand the potential for more devastating deflagration-to-detonation transitions (DDTs) on these facilities. While the likelihood of DDTs is lower than deflagrations, they have been identified in some of the most recent large-scale explosion incidents including: 2005 Buncefield explosion, 2009 San Juan explosion, and 2009 Jaipur event. The consequences of DDTs can be orders of magnitude larger than deflagration because they have the ability to self-propagate outside the region of high congestion/confinement. Hence, it is critical to understand how a facility’s geometry or equipment layout can affect explosion consequences and assist in their mitigation and/or prevention. However, most tools at present cannot accurately predict DDTs largely because they have been developed using experiments conducted at small scales.

Due to the inability to predict such devastating phenomena on the large scale, owners and designers cannot evaluate installations for risk of DDTs and provide “inherently safer” layout or mitigation measures to significantly reduce or eliminate such hazards. This paper will present the results of large scale testing that has been conducted in a newly developed test rig of over 50,000 ft³ (1,500 m³) gross volume to validate the necessary design tools used to predict the risk of DDT. The facility was designed in a modular-type fashion, whereby the congestion levels can be modified and adjusted to ensure detonations for fuels with different reactivities. These tests determined under what conditions two different reactivity fuels (propane and methane) transition from deflagrations to detonations at stoichiometric, lean and rich mixtures. The results indicate that DDTs are more likely to occur than originally anticipated at the large scale, and that current tools developed using smaller scale experiments require modifications. In addition, preliminary mitigation tests demonstrate that inhibitors are effective at slowing the propagation of a fast deflagration and help mitigate the transition to detonation.