Gas Explosions in Long Closed Vessels

Abstract

The majority of experimental data on which current vent design practice is based, has been obtained in compact vessels of length lo diameter ratio (L/D) less than 3. This leads to great uncertainty when designing vents for larger L/D vessels. The need for more information on explosions in long enclosures has been noted in recent critical reviews and official guides. In this work methane/air explosions were investigated in closed vessels of large L/D with the aim of providing experimental data relevant to the explosion protection of such vessels. A 76mm diameter tube of an L/D of 21.6 and a 162 mm diameter tube of variable L/D (6.2-18.4) were employed. High rates of pressure rise, associated with fast flame speeds and significant overpressures, characterized the very early stages of these explosions, indicating the need for fast and effective pressure relief within the initial 10% of the total explosion time, Lower flame speeds and lower rates of pressure rise succeeded the fast initial phase. Small changes in the spark position were found to have a significant effect on the initial flame speeds and rates of pressure rise. The rates of pressure rise measured in this work did not scale in accordance with the ‘cube root’ law and therefore, the Ka concept cannot be used for large L/D vessels. Different scaling trends were determined on a tentative basis, and the need for more data was pointed out. Recent literature on this type of explosion has concentrated on phenomena occurring in the later stages, such as the tulip flames, pressure wave effects, acoustic instabilities and transition to turbulence. These later stages or the explosion arc irrelevant, from a safety point of view, unless the vessel can withstand overpressures of 1.5 to 2 times the initial pressure, generated in the earlier stages. Thus, the present work has shown that the very early phases are the most important, and should dominate vent design considerations because of the high rates of pressure rise and high overpressures measured in this period.