Acetylene will not explode under low pressure at normal temperatures. However, it becomes unstable and spontaneously combustible when compressed to a pressure over 15 psi. Beyond 29.4 psi, it becomes self explosive, and a slight shock may cause it to explode even in the absence of air or oxygen. Since acetylene is shock sensitive and explodes above 30 psi, cylinders of acetylene contain acetylene dissolved in acetone, which lowers the danger of storing acetylene under high pressure. Acetylene cylinders are not hollow. They are packed with porous rock or crushed firebrick that is saturated with acetone. Acetylene happens to dissolve readily into acetone, and the dissolved gas is no longer in contact with gaseous O2, which does not tend to dissolve in acetone, and therefore is not as prone to decomposition by O2. The porous rock helps by minimizing the free volume of the cylinder, cooling and controlling any thermal decomposition before it gets out of control. Each decomposition of acetylene gives off heat. Acetone is capable of absorbing acetylene to approximately 25 times its own volume. Incidently, placing acetylene cylinders on their sides can cause the acetone and binders to dislodge. The result may be the formation of an acetylene pocket that is subject to polymerization and the possibility that liquid acetone will be released into the regulator. If storing upright is not possible, it is recommended that the cylinder be placed upright and left to stand for one half hour before using, although I read somewhere else that it should be standing upright for a period of eight hours. Restoring to the upright position allows the acetone to settle to back to the bottom of the cylinder, which in turn would prevent liquid acetone from running through the regulator. Anyway, as far as your pressures and limits on tip sizes 3 and above, the Victor chart I have shows that for welding and multi flame heating nozzles using acetylene, the acetylene pressure range for a #3 and above tip is 3 psi minimum, and 6 psi maximum. The acetylene ranges on the chart go from 3 psi minimum to 15 psi maximum, for material thicknesses of up to 1/32" to 4". Incidently, the "cutting with acetylene" chart shows ranges of 3 psi to 15 psi.
