Welding gases may come in small nonrefillable cylinders that are meant to be emptied once and discarded. Refillable cylinders are larger and generally have steel or aluminum walls ¼-to-½-inch thick, with a reinforced neck and bottom. There are three general types of refillable cylinders. The standard cylinder with an open interior is used with oxygen and a wide variety of other compressed gases. Cylinders with an internal siphon tube are for gases such as carbon dioxide that are used both in liquid and vapor form.
Welding gas cylinders for acetylene fuel gas have a unique interior design. Acetylene gas is unstable and prone to spontaneous explosive combustion unless stored under high pressure in specially designed cylinders. Acetylene cylinders are filled with a porous foam-like material to prevent formation of unstable acetylene pockets. Acetylene is dissolved in acetone to help keep the gas stable. Acetylene cylinders must be stored upright or corrosive acetone may get into the valve or welding rig.
The most common cylinder used for acetylene fuel gas stands 40 inches tall, is 12 inches in diameter and holds 225 cubic feet of compressed acetylene at 250 pounds of pressure. This same size cylinder, without the internal foam, is also used for MAPP gas (methylacetylene propadiene), which offers almost as much heat as pure acetylene and lasts more than twice as long. MAPP gas also is stable to handle. Acetylene and MAPP gas cylinders typically are yellow. The other component of gas welding is oxygen. The most common welding-rig cylinder for holding oxygen stands 51 inches tall, is 9 inches in diameter and holds 200 cubic feet of oxygen at 2,000 pounds pressure. Oxygen cylinders typically are green.
All welding gas cylinders must bear certain identification markings stamped into the metal. They include a Department of Transportation type number that designates whether a cylinder is steel or aluminum and the maximum service pressure in pounds per square inch. This is followed by a serial number identifying the specific cylinder. Other markings include the name of the original owner, date of manufacture, manufacturer’s inspection approval stamp, date of last pressure safety test and the cylinder’s empty weight. Cylinders also must bear a stick-on label identifying the contents and may have a bar code sticker identifying the specific cylinder for tracking through the refill process.