12.2 OFC Gasses and Cylinders
Karl Fulton
Cylinders and General Cylinder Safety
Your OFC setup may be stationary or portable. You will most likely be utilizing a high-pressure oxygen bottle and a fuel bottle. For stationary setups in shops, there will likely be cryogenic oxygen tanks.
Acetylene Cylinders
The acetylene cylinder is probably the most unique fuel bottle that you will come across and the one you will want to be the most cautious with. This is because acetylene becomes explosive and extremely shock-sensitive if stored in its free state at 29.4 pounds per square inch (psi).
Warning
Never use acetylene at a pressure above 15 psi. Acetylene becomes explosive and shock-sensitive as pressure increases. Abide by all warnings on acetylene regulators.
Because of these hazards, the acetylene fuel tank is specially designed and can safely compress up to 275 psi in a porous material like fire brick, balsa wood, charcoal, and other materials. In the past, finely shredded asbestos was used. The purpose of the porous material is to prevent the formation of any high-pressure pockets in the tank. The tank is then filled with acetone, which the acetylene is then dissolved into. Acetone is able to dissolve a large amount of acetylene without changing the chemical composition of the gas. Once filled with liquid acetone, it is imperative that the cylinder be kept upright; if not, the acetone can be drawn into the hose and torch. This will contaminate the regulator and hose and affect the flame. If you notice that a cylinder has been on its side, make sure to put it in the upright position and let it settle for at least two hours. A disadvantage of acetylene being dissolved into acetone is you will not be able to tell how much acetylene is left by the pressure gauge. The bottle pressure will remain fairly consistent until the majority of the gas is used.
As an additional safety feature, the cylinder is equipped with fusible plugs designed to relieve pressure if the cylinder is exposed to excessive heat.
Oxygen Cylinders
Pure oxygen is a noncombustible gas that is slightly heavier than air and is tasteless, colorless, and odorless. It does support the combustion of other elements, which is what makes it useful in welding and cutting. Our atmosphere consists of about 21% oxygen and 78% nitrogen. The white, flaky corrosion on aluminum, rusting of ferrous metals, and discoloration of copper are all examples of oxidation—the chemical reaction that happens when oxygen from the atmosphere comes into contact with these metals.
There are two ways that oxygen is collected for commercial use: the liquid air process and the electrolytic process. The liquid air process starts with collecting and compressing air from the atmosphere and cooling it to a temperature of -375 degrees Fahrenheit, at which point it becomes liquid. Then the temperature of the liquid is elevated to approximately -320 degrees Fahrenheit, which allows the nitrogen to off-gas while the oxygen stays in its liquid form. The liquid oxygen is again elevated in temperature, this time to -297 degrees Fahrenheit, where it becomes gaseous again. This is oxygen is compressed into bottles and further purified.
The electrolytic oxygen collection process involves running an electrical current through water with an alkali acid added. This separates the water’s hydrogen and oxygen composition. Oxygen collects at the positive terminal and is collected and bottled from there.
Most often, oxygen for welding purposes is supplied in seamless steel cylinders. The cylinder walls are over 0.25-inches thick, and the bottle will hold over 2,000 psi at 70 degrees Fahrenheit. The 244 cubic feet (ft3) bottle is the most common size, although there are larger and smaller bottles available.
The cylinder will have a removable metal cap to protect the valve head when it is not in use and during transportation. This valve head is made of bronze and is the most vulnerable point on the bottle. When in operation, the valve must be opened all the way, not partially opened; it is a double-stage valve, so it will be sealed when all the way open but will leak if only partly opened. The valve is equipped with a safety fuse plug and disk to rupture and release excessive pressure before the bottle itself fails.
Safety When Working With Oxygen Bottles and Cylinders
It is important to practice safety around oxygen bottles and cylinders to protect yourself and the people working around you from their inherent hazards, especially combustion and explosion. Here are some safety practices to employ when working with oxygen cylinders:
- Never weld on any oxygen cylinders.
- Oil and grease should never come in contact with an oxygen cylinder, as it can lead to spontaneous combustion and explosion.
- Oxygen is not air and should never be referred to as such. Therefore, oxygen should never be used as a replacement for compressed air. Never use oxygen to clean work areas or dust yourself off. This is extremely dangerous.
Attributions
- Figure 12.3: 2023 Taunton track renewals (02) cutting the old rail by Geof Sheppard is released under CC BY-SA 4.0
- Figure 12.4: Travaux d’excavation dans le section Louis-Hubert/Morgines au Petit-Lancy. Découpage de poutrelle métallique au chalumeau. by MHM55 is released under CC BY-SA 4.0