The potential for a deflagration to detonation transition (DDT) to occur in an unconfined vapor cloud explosion (VCE) with high reactivity flammable gases (e.g., ethylene, hydrogen, etc.) under conditions relevant to chemical processing and petroleum refining plants has been demonstrated in multiple test programs. For comparatively small flammable gas clouds which are restricted to a congested volume (e.g., a process unit), the VCE blast loads away from the cloud are similar whether the VCE occurs as a high speed deflagration or a detonation, although the loads in and near the cloud are much higher with a detonation. However, the blast loading from these two types of VCEs can be very different in the case of a large gas cloud extending well outside the congested volume. The deflagration flame front velocity will decrease rapidly outside the congested volume such that the portion of the flammable gas cloud outside the congested volume contributes little to the resulting blast load. In contrast, a detonation wave will propagate through the portion of the flammable gas cloud outside the congested volume, increasing the explosion energy and decreasing the standoff to targets away from the congested volume.
This paper discusses a test which demonstrates that a detonation wave, once formed due to a DDT within a congested volume, will propagate from a congested region into an uncongested region. The test rig was 54 feet long and 6 feet high, with the first 30 feet of the rig length comprised of a congested section 12 feet in width. The congestion was made up of a regular array of vertical circular tubes (pitch-to-diameter ratio of 4.1, area and volume blockage ratios of 23% and 4.2%, respectively). The last 24 feet of the test rig length was completely uncongested. The test rig was configured without any confinement (i.e., no wall or roof sections). A near-stoichiometric ethylene-air mixture completely filled both the congested and uncongested portions of the test rig. Prior testing with this rig configuration had shown that this flammable mixture would undergo a DDT within the congested portion of the rig. It was shown that, as expected based on other tests reported in literature as well as the general behavior of detonation waves, a detonation wave will propagate through a flammable cloud which extends into an uncongested volume.
The impact of a detonation wave propagating beyond the congested volume in which it is triggered on the resulting blast load was evaluated parametrically. As would be expected, the impact on the blast load is large for flammable clouds which extend well beyond the congested volume.
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