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3 The triple bond which makes the oxy-acetylene flame the hottest of all gas flames is also responsible for two rather exceptional properties of acetylene gas which you should always remember. The first is this: that free gaseous acetylene, depending on confinement conditions, is potentially unstable at pressures above 15 psig (103kPa). If subject to severe shock, or a source of ignition, some of the triple bonds may break, releasing enough energy to cause all the other molecules in the enclosed volume to decompose into carbon and hydrogen with explosive force. The force of such an explosion is not so great as that released by the explosion of most mixtures of acetylene and oxygen, or acetylene and air, but it is substantial, and can be withstood only by extra-heavy-wall steel tubing. The maximum free acetylene pressure permitted by safety codes is 15 psig. All oxy-acetylene equipment is designed and manufactured to permit the use of acetylene at less than 15 psig. How it is possible to ship acetylene in cylinders at a pressure of 250 psi (1725 kPa) or more is something we’ll get to a bit later in this chapter. The other property of acetylene which you must remember is this: that the flammability range of mixtures of air and acetylene is broader than that of any other fuel gas/air mixture. Let’s explain that more fully: Acetylene/air mixtures can be ignited when they contain anywhere from 2.5 percent acetylene to 80 percent acetylene. Mixtures of methane (the principal component of natural gas) and air are flammable when they contain as little as 5 percent methane and not more than 15 percent methane. The hazards resulting from acetylene leaks are therefore somewhat greater than the hazards involved in leaks of other fuel gases. Any leak in a fuel gas system is a hazard; acetylene is noticeably more hazardous than other gases only at the upper end of the flammability range. Except in an acetylene generator, the chances of creating a mixture which contains more acetylene than air are relatively small. Treat ALL fuel gases with respect and you’ll have no trouble. (In passing, we should note that flammability range is sometimes called explosive range. There’s really no difference.)