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
• Grade T for all fuel gases
Why are Grades R and RM for acetylene only? For many years, the predominant fuel gas in the industry was acetylene. Acetylene has little or no adverse effects on rubber and was only used at a maximum of 15-psi pressure. Therefore, no requirements for fuel gas compatibility were initially specified. Different types of fuel gases (propane, natural gas, methyl-acetylene-propadiene, propylene, hydrogen, etc.) became popular over time, particularly for cutting. Many of these fuel gases are detrimental to certain types of rubber. With the use of these fuel gases, combining the wrong hose and fuel could lead to premature hose failure.
Grades R, RM, and T are compatible with acetylene. If a fuel gas other than acetylene is used, Grade T hose must be used.
Fig. 3.8. Hoses for oxyacetylene welding must be of the correct type. The Compressed Gas Association defines three grades: Grade R, Grade RM, and Grade T. For fuel gases other than acetylene, only Grade T should be used.
Flame Types
In oxyacetylene welding, the torch tip never touches the material being welded; only the flame touches. The type of flame produced depends on the ratio of oxygen to acetylene.
A neutral flame is produced when there is a 1:1 ratio of oxygen to acetylene. This type of flame has no chemical effect on the weld metal so it does not oxidize the weld metal nor cause an increase in carbon. The excess acetylene flame is created when the proportion of acetylene in the mixture is higher than that required to produce the neutral flame. This is often called a carburizing flame.
An excess-acetylene flame causes an increase in the weld carbon content when welding steel.
An oxidizing flame is created when the proportion of acetylene in the mixture is lower than that required to produce a neutral flame. It ozonizes or “burns” some of the weld metal.
Chemistry of the Flame
When acetylene burns in the air, carbon dioxide and water vapor are the byproducts. It takes 2 cubic feet of acetylene and 5 cubic feet of oxygen or 2½ times as much oxygen as acetylene. Yet a neutral flame burns at 1:1 oxygen/acetylene ratio and a neutral flame does not have an excess of either gas. This is not a contradiction because the combustion process is more complex than simply the volume of oxygen and acetylene gas supplied to the torch. The actual combustion takes place in two stages. In the first stage, the mixture leaving the torch tip supplies the oxygen, and in the second stage, the air surrounding the flame supplies the oxygen.
In the first stage of combustion, the acetylene breaks down into carbon and hydrogen. The carbon reacts with the oxygen to form carbon monoxide, which requires one molecule of oxygen for each molecule of acetylene.
In the second stage of combustion, the carbon monoxide reacts with the oxygen from the air to form carbon dioxide. The hydrogen reacts with the oxygen from air to form water.
The two-stage combustion process produces the well-defined inner cone in an oxyacetylene flame. The first stage takes place at the boundary between the inner cone and the blue outer flame. The second stage takes place in the outer flame. If the proportion of acetylene supplied to the tip is increased, a white “feather” appears around the inner cone. This feather contains white-hot particles of carbon that cannot be oxidized to carbon monoxide in the inner cone boundary due to the lack of oxygen in the original mixture. On the other hand, if the proportion of oxygen fed to the tip is increased, the inner cone shortens noticeably and the noise of the flame increases.
Fig. 3.9. The neutral flame has a 1:1 ratio of oxygen to acetylene. Excess acetylene causes an increase in carbon content or carburizing when welding steel. An oxidizing flame is created when the proportion of acetylene in the mixture is lower than that required to produce a neutral flame, so it ozonizes or “burns” some weld metal.
Flame Adjustment
For most welding, a neutral flame is desired. Even a skilled oxyacetylene weldor has difficultly telling the difference between a true neutral flame and a slightly oxidizing flame. However, it is relatively easy to tell the difference between a neutral flame and a slight-excess acetylene flame. Therefore, it is always best to adjust the flame to neutral from a slight-excess acetylene flame.
Start with an excess-acetylene flame. Increase the flow of oxygen until the excess-acetylene feather is almost gone. This feather is visible in Figure 3.9, and it extends beyond the concentrated white flame cone at the torch tip.
Filler Rods
AWS classifies welding filler rods for oxyacetylene welding according to their chemical composition. AWS specification A5.2 designation of R45 is a common alloy. The rod contains low carbon and manganese alloy additions.