Acetylene, a hydrocarbon (C2H2), is a colorless, flammable gas shipped dissolved in a solvent. It has a garlic-like odor. Users are cautioned not to discharge acetylene at pressures exceeding 103 kPa (15 psig), as noted by the red line on acetylene pressure gauges. Other specifications of acetylene are:
Molecular weight: 26.038
Specific Gravity (Air = 1): 0.91 at 0°C (32°F)
Specific Volume: 0.09 m3 kg at 156°C (14.5 ft3/lb at
Critical Temperature: 35.2″C (95.3″F)
Critical Pressure: 6139.3 kPa (890.4 psia)
Acetylene is said to have an endothermic quality because it absorbs heat in formation and liberates it during combustion. In this respect, acetylene differs from most hydrocarbons: they are exothermic and give off heat during formation. As a fuel gas, acetylene generates 1433 Btu per cuft; 277 are derived from hydrogen combustion, 928 Btu result from the combustion of carbon into carbon dioxide, and 228 Btu result from its endothermic quality.
Chemical Characteristics
The chemical structure of acetylene is given in the formula C2H2, showing that two atoms of carbon (atomic weight 12) are combined with two atoms of hydrogen (atomic weight 1.008), which can be expressed as 92.3% carbon and 7.7% hydrogen. The nearest gaseous hydrocarbon is ethylene (C2H4), which consists of 85% carbon and 15% hydrogen.
Acetylene contains the highest percentage of carbon of all the gaseous hydrocarbons and is the only one of the unsaturated hydrocarbons with endothermic properties (absorbs heat during its production, and liberates heat when it is decomposed). Because of these characteristics, the oxyacetylene flame creates intense heat. The theoretical maximum for the oxyacetylene flame is 4359°C (7878″F), although the working temperature is about 3316°C (6000°F). The temperature of the oxyacetylene flame cannot be approached by any other gas, and is only exceeded by the heat produced in the electric arc or electron beam and laser processes.
Metalworking with Acetylene
Acetylene is usually combined with oxygen to intensify the heat of the acetylene flame for welding. It can also be combined with air, but with a much lower flame temperature. The principal application for the air-acetylene mixture is in soldering operations.
Mixed in equal amounts and burned at the tip of a welding torch, oxygen and acetylene create the so called neutral flame. This flame can be identified by the luminous, well-defined white cone at the torch tip, and by a fairly long, almost colorless outer envelope that is blue or orange at its leading edge. See Figure A-1. The neutral flame is the correct flame with which to weld many metals. See OXYACETYLENE .
If excess oxygen is fed into the torch, an oxidizing flame results. This flame is characterized by a short inner cone and a short outer envelope. The flame is hotter than a neutral flame, burning acetylene at the same rate. When this situation is reversed and an excess of acetylene is used, the resulting flame is termed carburizing. This flame appears as a greenish feather-shaped form between the inner cone and outer envelope. There are white-hot carbon particles in this feather which are dissolved to some extent in molten metal during welding.
Applications
Because of its intense heat, and because it can be accurately controlled, the oxyacetylene flame can be applied to literally hundreds of welding and cutting operations, including hardfacing, brazing, beveling, gouging, and scarfing. The heating capability of acetylene is utilized extensively in bending, straightening, forming, hardening, softening, and strengthening many types of metals.
Historical Background
Acetylene gas was discovered by Edmund Davy in 1836, but it was not until 1862 when Woehler’s discovery that acetylene gas could be produced from calcium carbide that the gas became well known. These developments were of little consequence, however, until 1892, when Thomas L. Wilson, of Spray, N. C., invented a process for producing calcium carbide and established facilities to produce it. He and James Morehead devised an economical commercial production method, and by 1895 acetylene gas was becoming recognized as a valuable gas for lighting.
However, acetylene producers’ hopes for widespread use of acetylene for illumination of streets and buildings were dashed by the growing use of incandescent lamps. Acetylene’s potential in metalworking became apparent in World War I, when welding was adopted as the most effective and expedient method of constructing and repairing war ships and merchant vessels.
(A) Carburizing
(B) Neutral
(C) Oxidizing
Figure A-1- Types of Oxacetylene Flames
Producing Acetylene
Acetylene is produced either in generators, by the reaction of calcium carbide and water, or by the cracking of hydrocarbons in a chemical plant.
In the generator method, water is allowed to react with calcium carbide (CaC,), a chemical compound produced by fusing lime and coke in an electric furnace. The reaction between water and carbide is instantaneous, and as a result, the carbon in the carbide combines with the hydrogen in the water, forming slaked lime, or calcium hydrate.
There are two methods of generating acetylene: (1) carbide-to-water, and (2) water-to-carbide. The carbide-to-water method is generally used in the United States, while the water-to-carbide method is favored to a large extent in Europe. A carbide-to-water generator operates on a “batch” basis, with a ratio of one gallon (8.3 lb) of water to one pound of carbide. This mixture is designed, in some generator models, to produce one cubic foot of acetylene per hour per pound of carbide hopper capacity. Some stationary generators are “double-rates’ for capacities of 2 ft3/hr per pound of carbide hopper capacity.
There are two further classifications for acetylene generators: low pressure and medium pressure. The low-pressure generator carries out the calcium carbide-to-water reaction process at pressures below 7 kPa (1 psi). A medium-pressure generator produces acetylene at between 7 and 103 kPa (1 and 15 psi).
Calcium carbide used for acetylene generation in the U. S. normally produces gas containing less than 0.4% impurities other than water vapor. Because of this favorable factor, there is no need for further purification of acetylene used for welding and cutting.
The welding supply distributor receives and resells acetylene in its most common form: dissolved in acetone and compressed in cylinders. These rugged acetylene cylinders have nominal capacities of 0.28, 1.1, 2.8, 6.4, or 8.5 m3 (10, 40, 100, 225 or 300 cu ft) and hold the gas at a pressure of 1724 kPa (250 psi).