A test was conducted which demonstrates that a detonation wave, once formed due to a deflagration to detonation transition (DDT) within a congested region, will propagate as a detonation from the congested region into an uncongested region. This is the expected behavior based on the general behavior of detonation waves as well as other tests reported in literature. The impact of a detonation wave propagating beyond the congested volume in which it is initiated 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. The test rig was 16.5 m (54 ft) long with the first 9.1 m (30 ft) of the rig length comprised of a congested section 3.7 m (12 ft) in width and 1.8 m (6 ft) high. The congestion was made up of a regular array of vertical circular tubes [6 cm (2.375 in.) diameter, pitch-to-diameter ratio of 4.1, area and volume blockage ratios of 23% and 4.2%, respectively]. The last 7.3 m (24 ft) 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 a similar rig configuration had shown that this flammable mixture would undergo a DDT within the congested portion of the rig. (c) 2013 American Institute of Chemical Engineers Process Saf Prog, 2013