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GAS SUPPLIES AND GAS DISTRIBUTION SYSTEM

ACETYLENE

Acetylene is a colourless, flammable, and in the pure state odourless gas. It is can be manufactured by reacting Calcium Carbide with water in Acetylene generators. Ordinary commercial grades of Acetylene contain traces of impurities such as Phosphine, Arsine, Hydrogen Sulfide and Ammonia, and have a garlic-like odour. The gas is slightly lighter than air.

Acetylene alone burns in air with a very hot, bright and sooty flame. Mixed with air or Oxygen in the right proportion, Acetylene gives a concentrated, sootfree flame. When mixed with Oxygen the combustion is more intense than in air, and because of the Acetylene’s high carbon content (92.3% by weight) its maximum flame temperature is about 3100°C. Because of its high temperature, thermal value and speed of combustion (11.6 m/s), this is the most suitable gas flame both for welding and for cutting. Explosion and decomposition hazards Acetylene, is a highly flammable and explosive gas, and a mixture of Acetylene and air or Oxygen is explosive within wide limits. The explosive limits in air range from 2% to 82%, while if mixed with Oxygen the explosive range is from 2.5% to 93%. For this reason care must be taken to prevent an unnecessary or uncontrolled escape of Acetylene, and good ventilation must be assured in places where Acetylene is stored or used.

Should escaping Acetylene from an open top valve or from the regulator, catch fire, put out the flame by closing the top valve. For this eventuality a fire-resistant mitten should always be

kept handy. Should the flame make closing of the top valve difficult or impossible, it must first be put out by a Carbon Dioxide (CO2) or dry powder extinguisher.

Another property, which should be known and understood by personnel engaged in transporting or using Acetylene is that the gas can decompose into its constituents, Carbon and Hydrogen, if it is exposed to temperatures above 350°C (662°F) or if gaseous Acetylene at a pressure of more than 2 bar (200 kPa; 29 psig) is subjected to shock during storage or transportation. Decomposition proceeds rapidly and liberates a great deal of energy. Because of this property, Acetylene cannot be stored under pressure in the same way, for example, that Oxygen is stored. To avoid the decomposition characteristics of compressed, gaseous Acetylene the cylinders for storing the gas are filled with a porous mass, having minute cellular spaces. This eliminates the risk that any pocket of appreciable size remains, in which Acetylene in gaseous form may collect. The porous mass is saturated with Acetone, in which the Acetylene actually dissolves. The combination of these features - porous mass and solvent allows the Acetylene to be contained in cylinders at moderate pressure with greatly reduced danger of explosive decomposition occuring during normal handling and use. However, there still exists the chance that decomposition may be started by careless handling of the cylinder, such as hitting or dropping it, heating it, or by using incomplete or badly maintained welding equipment, thereby permitting a complete flash-

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