Fire Safety Science Digital Archive

Measurements of the rates of pressure rise and a photographic study of the flames emerging from the maximum experimental safe gap apparatus have shown that although relief venting may reduce the maximum explosion pressure it may increase the rate of pressure rise within the time interval when the reaction zone and following hot combustion products are flowing through the flange gap. In some explosions the presence of the relief caused acoustic standing waves, and the resultant movement of gas interfered with the combustion wave. Schlieren and direct photographs of the transmitter explosions have shown that, in vented explosions, transmission through a flange gap need not necessarily occur nearest to the igniting source, and in some cases ignition took place in two locations simultaneously. With ethylene/air and propane/air mixtures the transmissions occurred during the initial stages of the explosion. With hydrogen/ air mixtures the transmission of the explosion occurred when the explosion pressure reached 25 per cent of the peak value. As the location of the explosion transmission through a flange gap is largely unpredictable, the timing of the newly formed flame front outside the vessel with ionisation detectors opposite the igniting source may be inaccurate.