Acetylene is a colourless, flammable gas with a garlic-like odour. Under compressed conditions, it is highly explosive; however, it can be safely compressed and stored in high-pressure cylinders if the cylinders are lined with absorbent material soaked with acetone. Users are cautioned not to discharge acetylene at pressures exceeding 15 psig (103 kpa), as noted by the red line on acetylene pressure gauges.
CHEMICAL CHARACTERISTICS
Acetylene is a hydrocarbon with a chemical structure expressed in the formula c²h², which indicates that two atoms of carbon are are combined with two atoms of hydrogen. By atomic weight, it is 92.3% carbon and 7.7% 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, which means it absorbs heat during its production and liberates heat when it is decomposed.
Acetylene is combined with oxygen to intensify the heat of the flame. An oxyacetylene flame creates intense heat. The theoretical maximum for this flame is 7878ºf (4359ºc), although the working temperature is approx. 6000ºf (3316ºc). The temperature of the oxyacetylene flame cannot be approached by any other gas, and is only exceeded by the heat produced in arc welding or with the laser and electron beam process.
METALWORKING WITH ACETYLENE
Oxygen and acetylene, mixed in equal amounts and burned at the tip of a welding torch, create a so-called neutral flame (fig.4). This flame is identified by the luminous, well-defined white cone at the torch tip and by a fairly long almost colourless outer envelope that is blue or orange at its leading edge. The neutral flame is the correct flame to use for welding many metals.
If excess oxygen is fed into the torch, an oxidizing flame results. This flame is characterised by a short inner cone and a short outer envelope (fig.5). The flame is hotter than a neutral flame.
When this situation is reversed and an excess of acetylene is used, the resulting flame is called carburizing. This flame appears as a greenish feather-shaped form between the inner cone and the outer envelope (fig.6). There are carbon particles in this feather, which are dissolved to some extent in molten metal during welding.
With its intense heat and controllability, the oxyacetylene flame can be used for many different welding and cutting operations including hardfacing, brazing, beveling, gouging and scarfing. The heating capability of acetylene also can be utilised in the bending, straightening, forming, hardening, softening and strengthening of metals.
Cylinder stabilisers
In 1897, the value of acetone was demonstrated. This colourless, flammable liquid, when added to the porous material in the cylinder, is capable of absorbing 25 times its own volume of acetylene for each atmosphere of pressure applied. For example, at a full-cylinder pressure of 250lb/in.² at 70ºf, it can absorb more than 400 times its own volume of acetylene.
In 1958, a lightweight calcium-silicate filler was introduced with 92% porosity. Composed of sand, lime and asbestos, the new filler decreased cylinder weight by 30% while increasing cylinder capacity.
When it became known that asbestos fibers small enough to enter the lungs are carcinogenic, research was conducted to find an asbestos-free filler. The result of that research was a non-asbestos, alkaline-resistant glass fiber filler developed in 1982.
Safe handling
Increased pressure and decreased temperature can liquefy acetylene, and at extremely low temperatures, acetylene can solidify. The danger at the point of liquefaction or solidification is that high pressure creates a very unstable product. At the slightest provocation, compressed acetylene will dissociate into its chemical components, carbon and hydrogen, and the resultant drastic increase in temperature and pressure can cause an explosion. Acetylene should not be discharged at pressures exceeding 15 psig.
The acetylene distributor and the user of acetylene gas must observe the following precautions: