The advantages claimed for the blau-gas process are safeness, cheapness, compressibility of the gas to a liquid, high B. T. U. (1800 per cubic feet), and convenience. On the other hand, the oxy-blau gas flame is not so hot as the oxy-acetylene and must be larger to do the same work. This is sometimes a disadvantage. This system is well established in Europe but has been adopted in this country only a few years and has not proved a very serious competitor of the other systems. Blau gas is distilled from fuel oil at only 600 degrees centigrade; so the hydrocarbon gases are not broken up and very little tar or methane is formed. During liquefaction for the market, the distillate is first subjected to two compressions which liquefy the low pressure gases and these are drawn off with the cooling water. The heavier gases are then compressed in two more stages and absorb most of the permanent gases when they liquefy.
The apparatus used for blau-gas welding consists of the regulation gas cylinders containing blau gas at 100 atmospheres pressure, cylinders with compressed oxygen, a gas expansion cylinder, pressure indicating and reducing gages, tubing, and high pressure torches. Owing to the blau gas being composed of gases of different critical pressures it must not be drawn from the top of the cylinders direct; so a tube extends through the liquid gas to the bottom of the cylinders and the heavier liquid is drawn off first. This expands in the expansion cylinder and blowpipe before entering the flame, and the lighter gases are drawn off last. The customary glasses should be provided for the workmen's eyes.
The welding torch has several sizes of tips and the cutting torch is so arranged that there is a preheating flame around the oxygen inlet. Liquid blau gas is first let into the expansion chamber at about 50 pounds pressure and then into the torch at from 10 to 20 pounds, depending upon the work to be done. The oxygen is led directly to the torch at pressures of 15 to 30 pounds for the various operations. Cutting requires higher pressures than welding.
The process of welding with the oxy-blau gas apparatus is similar to the other hot-flame systems excepting that its heating value is less and a larger spread of flame is required to give the required amount of heat units and temperature for the work. The explosive range of blau gas is 4 per cent and ranges from 4 parts gas to 96 parts air up to 8 parts gas and 92 parts air. Up to the present time its principal application has been to cutting, but it has been used for welding practically everything and a large amount of steel piping and tank work has been done with it. It is comparatively safe to use because it is chemically inert, non-poisonous, and leaves no deposit in the pipes.
Water-gas welding is done by mixing oxygen with the gas and using it in a manner similar to the other gases. The gas is drawn from the mains and carried to the torch and there mixed with the oxygen for use. Its application is not very general because of (its low heating value, but it is cheap when it can be obtained at all. It can also be obtained compressed into cylinders like the other gases but there is very little to recommend it for general use in view of the gradual reductions in cost of acetylene.
Fig. 121. Davis-Bouraonvilla Cutting Blowpipe.
Fig. 123. Crosshead Slots Cut Out of Drop Forging.
Courtesy of Oxweld Acetylene Company.