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 1–1 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.
Table 1–1 Combustion properties of fuel gases
Compressed Gas Cylinders
What is the difference between acetylene cylinder and oxygen cylinder construction?
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.
What materials other than acetylene are found inside acetylene cylinders?
Under certain conditions above 15 psi (1 bar), acetylene may spontaneously disassociate into its components of carbon and hydrogen. Acetylene cylinders are packed with an inert porous monolithic filler to prevent this dangerous disassociation. Acetylene cylinders are also contains acetone that can dissolve 25 times its own volume of acetylene per atmosphere of pressure. This greatly increases the cylinder’s acetylene capacity.
Figure 1–4Oxygen and acetylene cylinder cross sections
What is the purpose of safety valves and plugs in oxygen and acetylene cylinders?
Their purpose is to 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 3360psi (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 1–5Detail of pressure safety relief on oxygen valve
Why should the welder open the oxygen cylinder valve all the way, but open the acetylene cylinder valve just one turn?
Because the oxygen cylinder is filled to such a high pressure (2250 psi or 155 bar) to prevent leakage around the valve stem, oxygen and all other high-pressure cylinders have a second valve seat to make a solid seal around the valve stem when the valve is open. See Figure 1–6. Because the acetylene cylinder valve sees a relatively low pressure (225 psi or 15.5 bar), leakage around the valve stem in use is small and a single seat is used. Since the acetylene valve can deliver adequate volume with one turn open, opening the valve more just increases the closing time in an emergency. For similar reasons the welder must never remove the removable wrench from the valve of old-style acetylene cylinders while the cylinder is in use.
Figure 1–6Details a cross section of oxygen valve
Why should the welder position the cylinders between himself and the regulators when opening the cylinder valves?
If a regulator fails internally, releasing high-pressure gas from a cylinder into the regulator’s low-pressure side, the regulator housing and gauges may explode. Fatalities have resulted from such malfunctions.
If an acetylene cylinder has been incorrectly transported on its side, why should the welder avoid immediate use?
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.
What should the welder do knowing that a newly delivered acetylene cylinder has been incorrectly transported on its side?
Upright the cylinder 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.
How can one readily distinguish between the oxygen fitting swivel nut and one for acetylene?
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 1–7.
Figure 1–7Compares connector nuts used on oxygen and acetylene equipment
Are all oxygen cylinders painted green?
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.
What pressures should full oxygen cylinder and full acetylene cylinder gauges show at 70°F (21°C)?
The acetylene should show 225 psi (15.5 bar) and the oxygen 2250 psi (155 bar). Note that these pressures will fluctuate with ambient temperature.
What do the letters and numbers stamped on the neck of high-pressure cylinders indicate?
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
