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The use or flame arresters to prevent the emission of propane-air explosions from cubical vessels has been further investigated. External protection of the arresters with bursting diaphragms or magnetic panels has been shown to be practicable, and the effect of the covers on the explosion pressures was measured. The presence of two-dimensional contents within the vessels, such as a shelf or partition, increased the pressure particularly when more than 25 per cent of the cross-section of the vessel was blocked. Three-dimensional obstacles, such as bulky components, increased the pressure to a lesser extent. Means of minimising the increases were described. Flame arresters made from thermally resistant metals showed marked improvement over those reported previously, and permitted reduction in the required vent area; the area would then usually be governed by the maximum pressure permissible rather than by the need to prevent thermal damage to the arresters. Increased explosion pressures were also caused by multiple ignition sources, but the increases were relatively small in the presence or obstacles.