operating and safety instructions for a particular regulator and welding tip to define the pressure setting recommended. Never set pressures above 15 psi.
Torches
Most oxyacetylene welding systems are purchased as kits that include a torch handle, a cutting attachment, and several welding tips and cutting nozzles for various thicknesses of material. Some kits include a multi-flame heating head. This is similar to a large welding tip but instead of one hole with a single flame, it has multiple holes for multiple flames. This allows heating a wider area without a single hot flame melting the material.
Fig. 3.5. High-pressure gas cylinders, such as those used for oxygen, are made of high-strength steel and have a significant safety factor in their design. They are usually filled to about 2,500 psi, 170 times atmospheric pressure. Acetylene cylinders only contain 250 psi when full. They are filled with diatomaceous earth or a ceramic material and store the acetylene in acetone.
The torch handle has two valves used to adjust the flow rate of oxygen and acetylene. These require fine adjustment to obtain the proper flame properties. However, since the oxygen valve is also used for cutting where higher flow rates are required, it must also pass a high rate of gas flow when needed. The flow rates of gas may vary from 4 to 200 cfh (cubic feet per hour), a very wide range.
The welding tip often contains a mixer where the oxygen and acetylene are combined so they can burn. Two types of mixers are in use today. The first is called a medium-pressure type, where the gases are supplied at about equal pressures. The pressures, when using medium-pressure mixers, are typically similar for oxygen and acetylene, at about 2 to 7 psi depending on the tip size. The second is an injector type, where the oxygen is supplied at a high pressure (55 psi or higher) and the acetylene is supplied at a low 1 psi.
In the injector type, the oxygen passes through a very small orifice in the injector, and the expansion of the oxygen as it leaves the orifice pulls the acetylene into the mixing chamber. The mixer is located in the inlet of the welding tip because there is a relationship between the mixer and the tip size. A single mixer cannot satisfy all of the requirements.
Fig. 3.6. Most oxyacetylene welding systems are purchased as kits that include a torch handle, a cutting attachment, several welding tips, and cutting nozzles for various thicknesses of material. Some include a multi-flame heating head that allows heating a wider area with flames that do not melt the material.
Fig. 3.7. The torch handle has two valves that are used to adjust the flow rate of oxygen and acetylene. These require fine adjustment to obtain the proper flame properties and high-flow capacity for cutting. Flow rates of gas may vary from 4 to 200 cfh.
In addition, all the passages in the welding head must be designed so that if the flame is forced back into the head, it is extinguished without damage to the head or torch. This could occur by momentary contact of the torch tip against the work. This can lead to a flashback.
One advantage of an injector mixer design is it can use the last amount of acetylene left in the cylinder. One thing to watch for when the gas level is very low is it not only pulls out the acetylene gas but the acetone as well!
Cylinders
High-pressure gas cylinders, like those used for oxygen, are made of high-strength steel and have a significant safety factor in their design. They are usually filled to about 2,500 psi, which is 170 times atmospheric pressure. Gas cylinders are periodically pressure tested and examined for damage before they are allowed to be refilled.
A common, large oxygen cylinder is about 5½ feet high and 9 inches in diameter. When full it contains about 2,640 psi and 325 cubic feet of oxygen. The physical internal volume of the cylinder is about 1.8 cubic feet, yet it holds about 325 cubic feet of oxygen. That is the volume of the gas when it exits the cylinder and is at room temperature and atmospheric pressure of 14.7 psi, which is what you pay for.
That volume of gas is proportional to the absolute pressure, which is gauge pressure plus 14.7 psi. Therefore, the gas stored in a high-pressure oxygen cylinder (2,640 gauge reading + 14.7 psi ÷ 14.7 psi) is 180 times denser than if at atmospheric pressure. Then 1.8 cubic feet of physical cylinder volume times 180 higher density = 325 cubic feet of gas at what is called STP, standard temperature and pressure.
An empty cylinder weighs about 140 pounds. It is deep-drawn from a single piece of high-strength steel or forged from billet steel. The final shape is heat treated after forming. The cylinder is pressure tested at 3,360 psi before put in service. It must also be tested at least once every 10 years while it is in service. The U.S. Department of Transportation establishes the regulations that cover construction, testing, marking, filling, and maintenance issues.
On the top shoulder of the cylinder body is the date it was put in service and the dates when it was retested. The brass oxygen cylinder valve has a threaded outlet that is machined to Compressed Gas Association (CGA) standards, and the American National Standards Institute (ANSI) accepts these standards. All oxygen regulators sold in the United States and Canada have a mating outlet fitting that conforms to these standards. The connection is designated CGA 540 and is recognized for oxygen service only.
Never use an adapter to connect a regulator to any high-pressure cylinder. Oxygen regulators are specially made for that service. Every cylinder valve is also equipped with a bursting disk, which ruptures and allows the contents to vent in the event the cylinder reaches a pressure near the test pressure, as might occur in a fire.
Acetylene cylinders are constructed differently than oxygen cylinders. First, they are not under high pressure and only contain 250 psi when full. As mentioned, acetylene at any pressure above 15 psi is unstable and should never be used. In fact, acetylene at 29 psi becomes self-explosive, and it does not need oxygen for this explosion occur. It decomposes into carbon black and hydrogen.
How is it possible to have acetylene in a cylinder at 250 psi when the gas cannot be used above 15 psi? The gas in the cylinder is dissolved in acetone, and therefore, it does not exist as a gas within the cylinder.
The inside of the cylinder has a unique construction. It is completely filled with porous materials. Newer cylinders are filled with diatomaceous earth or a ceramic material. Older cylinders were filled with materials such as balsa wood, charcoal, and shredded asbestos. These fillers decrease the size of the open spaces in the cylinder.
Acetone, a colorless, flammable liquid, is added until about 40 percent of the porous material is filled. The filler acts as a large sponge to absorb the acetone, which absorbs the acetylene. In this process, the volume of the acetone increases as it absorbs the acetylene, while acetylene decreases in volume.
In the cylinder, there’s a safety plug with a small hole through the center that is filled with a metal alloy that melts at approximately 212 degrees F or releases the contents at 500 psi. When a cylinder is overheated, the plug melts and permits the acetylene to escape before a dangerous pressure can build up. The safety plug hole is too small to permit a flame to burn back into the cylinder if the escaping acetylene should become ignited.
Hose Types
Hose for oxyacetylene welding must be the correct type. The CGA defines these grades as follows:
• Grade R and Grade RM for acetylene use only
