What types of equipment are needed?
Compressed Gas Cylinders
Both oxygen and acetylene are shipped and stored in special cylinders. Oxygen cylinders are seamless vessels of special high-strength alloy steel. They are made from a single billet by a draw-forming process and they contain no welds. Acetylene cylinders are fabricated and contain welds.
Frequently, oxygen cylinders are painted green or have a green band, but the only sure way to determine the contents of a compressed gas cylinder is to read the adhesive label on it. This label is required by law and should not be removed. Do not go by its color as there is no color code. Unlike civilian industry, the US armed forces do color code their cylinders.
Figure 3-6 Here is a basic OAW setup. It includes cylinders of oxygen and acetylene, regulators for the tanks, hoses, igniter, and welding torch with interchangeable tips.
Photo courtesy of Hobart Welders.
Acetylene will form explosive mixtures with air at all concentrations between 2.5 and 80%. This is the widest range of any common gas and almost insures an explosion if leaking gas is ignited. At 70°F (21°C), the acetylene should show 225 psi (15.5 bar) and the oxygen 2,250 psi (155 bar). Note that these pressures will fluctuate with ambient temperature.
Acetylene, like most other fuel gas handling equipment, has a notch or groove cut in the middle of the edges of the hexagonal faces of the swivel nut. This is a flag for a left-handed thread. See Figure 3-7.
Figure 3-7 This illustration compares connector nuts for acetylene and oxygen equipment.
Tank Cross Sections
Safety valves and plugs prevent the cylinder bursting from overpressure when it is heated. Oxygen cylinders have a small metal diaphragm in a section of the valve which ruptures, releasing cylinder pressure to the atmosphere and preventing a cylinder burst. Disk rupture occurs above 3,360 psi (232 bar), the cylinder test pressure.
Acetylene cylinders contain one to four fusible safety plugs depending on their capacity. These fusible plugs, made of a special metal alloy, melt at 212°F (100°C).
They also release the cylinder contents to atmosphere to prevent rupturing (and then exploding) when the cylinder is exposed to excessive temperatures, usually from a fire. Acetylene cylinders may have the plugs on the top, or top and bottom.
Figure 3-8 Detail of pressure safety relief on oxygen valve
Figure 3-9 Cross sections of an oxygen and acetylene cylinders
Transporting Cylinders. If an acetylene cylinder has been incorrectly transported on its side, the welder avoid using it immediately. The acetylene gas and the acetone in which it is dissolved may become mixed in the area just below the valve, resulting in both gaseous acetylene and liquid acetone at the top of the cylinder. This is where acetylene exits the cylinder and goes through the valve to enter the regulator. Both acetylene gas and liquid acetone will be drawn into the regulator possibly ruining the rubber components of the regulator and torch and creating a safety hazard. The weld metallurgy may also be contaminated.
To use the cylinder, stand it upright and wait at least one-half hour before connecting and using the cylinder to allow the liquid phase of the acetone to separate from the acetylene gas in the upper portion of the cylinder. That way no acetone will be drawn into the regulator possibly damaging its seals. Also, acetone in the weld flame will contaminate the weld pool and spoil the weld.
Reading High-Pressure Cylinders
The stampings indicate which US Department of Transportation specifications the cylinder meets, what type steel was used, who fabricated it, and when.
•Steel stamp markings such as “DOT-3A-2400” indicate the cylinder was made to US Government Department of Transportation (DOT) specifications, the “3A” denotes chrome manganese steel (or “AA” for molybdenum steel), and the “2400” the maximum filling pressure in psi.
The oldest date indicates the month and year of manufacture. Subsequent dates, usually at five year intervals, indicate when mandatory hydrostatic pressure testing was performed and by whom. See Figures 3-10 and 3-11.
Figure 3-10 High-pressure cylinder markings
Figure 3-11 Acetylene cylinder markings
Common Cylinder Sizes
A 55 ft3 (1557 liter) oxygen cylinder would last under two hours cutting 1/8 inch (3 mm) steel plate. For the larger cylinders, their size and weight can be major drawbacks. In general the mid-sized cylinders offer the best compromise of economy and convenience.
Figure 3-12 Oxygen cylinder sizes
Figure 3-13 Acetylene cylinder sizes
Other Fuel Gases can be used in place of acetylene, but their maximum heat potential is below that required for welding steel. Acetylene is the best gas for welding because it:
•Has the highest temperature of all fuel gases.
•Acetylene delivers a higher concentration of heat than other fuel gases.
•Has the lowest chemical interaction with the weld pool’s molten metal than all other gases.
However, other gases such as natural gas, methylacetylene-propradene stabilized (also called MPS or MAPP® gas), propane, hydrogen, and proprietary gases based on mixtures of these are frequently used for other non-welding processes for cost reasons. They work well for soldering, brazing, preheating, and oxygen cutting, and are seldom used for welding. Small changes, like different torch tips, may be necessary to accommodate alternate fuel gases. Table 3-1 shows the maximum temperature achievable with different fuel gases. Where even lower temperatures are needed (sweating copper tubing and many small soldering tasks) a single cylinder of fuel gas using only atmospheric oxygen is effective and economical.
Figure 3-14 Two-stage regulator
Regulators
Regulators reduce the pressures of welding gases from the very high cylinder pressures to the low pressures needed by the torch to function properly. Also, as the cylinder pressure falls with gas consumption,