With the pressure-adjusting screw fully open, very slowly open the cylinder contents valve. One suggestion is to not look at the contents gauges while opening the valve. In fact, take an extra precaution and stand to the side of the regulator. In lab tests, when an explosion occurs, most (but not all) of the flame comes out of the front and sometimes the rear of the regulator. One reason for opening slowly is to avoid a shock to the pressure gauge, but another reason is that a regulator explosion might be caused by high pressure rapidly entering the small gauge tube that bends when pressure is applied.
If even a small amount of hydrocarbon contaminant enters the tube, and the pressure suddenly increases from 14.7 to 2,600 psi of pure oxygen, spontaneous combustion can occur. It is like a Diesel engine combustion chamber. No spark is needed, just fuel and high pressure.
Fig. 3.4. Acetylene regulators only allow pressures up to 15 psi because, beyond that, pressure acetylene gas is unstable. At 29 psi the gas can spontaneously explode. When acetylene is contained in a cylinder, the gas is dissolved in acetone.
A Diesel engine with a high compression ratio of 25:1 has a maximum cylinder pressure of 368 psi of air (25 × 14.7). That is sufficient to ignite Diesel fuel in air. With 2,600 psi of oxygen, it takes little fuel to cause ignition! Once the fire starts, all materials burn in an oxygen environment—the brass regulator body and even a stainless steel diaphragm.
An oxygen regulator burnout is not an everyday occurrence. There are hundreds of thousands of oxygen regulators on cylinders in the United States and just a few burns occur each year. Evidence collected over the years shows that when one does occur the inlet filter was often missing. While filters are necessary for all uses and in all environments, clean filters are especially important in auto body shops, garages, or any place where hydrocarbon products are on benches or floors.
When a full cylinder is being exchanged for an empty cylinder, the oxygen regulator might be placed on a dirty bench or floor. The inlet nipple could pick up some grease or oil and enter the opening. Burnouts have occurred in coal mines where safety precautions are always emphasized and taught to all workers. Could it be the regulators were exposed to coal dust when swapping cylinders?
Most burnouts occur when a new full cylinder is being installed. If the inlet filter is always checked before installing an oxygen regulator, the pressure adjusting screw is backed all the way out, and the cylinder valve is opened very slowly, an oxygen regulator burnout or explosion should never be experienced.
There are also oxygen regulators designed to contain an explosion should one occur. Ask your welding supplier about them.
Acetylene Regulator
Acetylene is potentially unstable at pressures over 15 psi. To increase a cylinder’s capacity, while providing a safe environment, the acetylene is dissolved in acetone and held in a porous media contained in the cylinder. Cylinder pressures of 250 psi can then be used. The acetylene regulator lowers the pressure to the 5 to 8 psi required for welding. Consult the manufacturer’s 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