•Unscrew the regulator pressure adjustment screws on both cylinders in preparation for the next use of the equipment. The regulator screws should be loose but not about to fall from their threads.
Gases
What is acetylene gas?
It is a clear gas having a specific gravity slightly lighter than air at 0.906 (air = 1.000). Acetylene’s chemical formula, C2H2, indicates that each molecule of this hydrocarbon compound contains two carbon atoms and two hydrogen atoms.
What is the odor of acetylene gas?
It has a distinctive garlic odor. Because the liquid acetone in the acetylene cylinder also has an odor, this acetone odor is frequently mistaken for that of acetylene, when in fact it is the odor of the mixture of both acetylene and acetone.
How is acetylene made?
Acetylene results from dissolving calcium carbide in water and capturing the resulting gas. One pound of calcium carbide generates about 10 cubic feet of acetylene (1 kg calcium carbide generates about 618 liters of acetylene).
Where does calcium carbide come from?
Calcium carbide results from an industrial process where lime and coke are smelted in an electric furnace. A gray, hard solid, it is supplied in a variety of forms: bricks, powders, pellets, or granules.
What is the chemical equation of combustion of acetylene in a neutral flame?
C2H2 + 2.5O2 → 2CO2 + H2O
What does this equation tell us?
One part acetylene and two and a half parts oxygen combine to produce a neutral flame (a neutral flame has just the right amounts of fuel and oxygen so there is neither an excess of oxygen or fuel after combustion). What the equation does not tell us is that equal volumes of acetylene and oxygen from the compressed gas cylinders combine with another one and a half parts of oxygen from the atmosphere to make the flame.
How is oxygen made for welding?
Atmospheric air is repeatedly cooled and compressed until it becomes a very cold liquid. This liquid is gradually warmed, and as each component gas of the liquid air reaches its vaporization temperature, it comes out of the liquid air, and separates itself. This is the fractional distillation of liquid air. Other gases important in welding—nitrogen, carbon dioxide, and argon—are also made using this process. Oxygen can also be made by electrolysis of water, but this is not a cost-effective process to make industrial quantities.
What are the two main ways of supplying welding shops with oxygen?
Compressed gas cylinders are used in smaller shops; liquid oxygen cylinders in larger shops. The liquid oxygen flows from its cylinder into a radiator that warms the liquid oxygen, and converts it into gaseous oxygen.
How are large welding shops supplied with acetylene?
Multiple cylinders are manifolded together and their output piped around the plant to each welding or cutting station.
What is peculiar about the filling and draining of acetylene cylinders?
Because the acetylene is dissolved in acetone, not just pumped into the pressure vessel, the filling process takes seven hours as the absorption process occurs. Similarly, an acetylene cylinder can only deliver one-seventh of its capacity per hour as the acetylene will not come out of solution in the acetone faster. More acetylene capacity will require cylinders manifolded together. This can become an important issue when using large multi-flame heating tips (in the industry called a rosebud tip) which consume many times more gas than a welding tip.
Why is acetylene potentially so dangerous?
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.
Can other fuel gases be used in place of acetylene?
Certainly, 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 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