In this article, we will discuss general features and operating principles of flame cutting metal of the two most popular torch fuel types: acetylene and propane.
OXYGEN/ACETYLENE TORCH
This is a handheld gas torch that burns a mixture of acetylene and oxygen to produce a high-temperature flame (5,800°F). It is one of the most popular tools for cutting ferrous metals, not stainless steel, bronze, or copper. It does not do well with heavily rusted metal. Have an angle grinder as part of your torch kit to remove rust.
One tank is filled with acetylene while the other is filled with oxygen. The two gases are then combined and passed through the torch tip to produce an extremely hot, narrow stream of fire that can weld or cut. Never allow the torch tip to touch the work piece during this process. Typical flame coupling distance is about inch, depending on the tip size, ideally just close enough so the tips of the preheat flames contact the work piece.
Acetylene is extremely unstable. The gas is shipped in special cylinders designed to keep it stable. The cylinders are packed with porous materials such as diatomaceous earth, then filled to around 50% capacity with acetone. The cylinder is then topped off with acetylene, which is absorbed into the acetone, stabilizing it and making it safer for handling. This method is necessary because above 15 pounds per square inch (psi), acetylene is unstable and may explode. Only open the acetylene valve a third to a half open so you can close it quickly in an emergency.
Always keep the acetylene bottle upright, especially during cutting operations. The torch is made to burn gas, not liquid. If laid on its side, the liquid acetylene/acetone will be drawn in the regulator and forced down the hose to the tip system (which burns the gas) and may cause an explosion. If the bottle is delivered (unknown amount of time) on its side, it must be turned upright for at least 12 hours before use, to allow the liquid to drain back to the bottom of the tank. If you lay it down, note the time, and before you begin torch operations, have the bottle upright for the same amount of time. Do not withdraw more than 1/10 of the bottle’s volume in less than an hour. You can risk drawing liquid acetylene into the torch system. The tip chart can help in determining rate of consumption vs. bottle volume.
Never use oil/lubricants on any torch threads/connections. Be careful when handling the tank. Even denting it can cause an explosion.
There are many different styles of torch/bottle threads, so keep some adaptors with your torch kit (photo 1). Many acetylene bottles will have left-handed thread and male or female valves (photo 2). Left hand or reverse thread is noted by the notches on the corners of the nut. Note the double male adapter for the propane tank to regulator.
A full tank has about 250 psi. Acetylene, when combined with oxygen, burns at 5800°F to 6,300°F. The primary disadvantages of acetylene, in comparison to other fuels, are high cost, chemical instability, and hazardous transport issues.
Acetylene is lighter than air, which means it can rise into roof spaces and empty areas. Detecting acetylene gas is easy because it has a distinctive garlic-like smell, even in concentrations below 2%.
1 A brass thread adapter allows propane threads to fit a torch/fuel regulator. (Photos by author.)
2 A female-to-female thread adapter. Note the notches, indicating a left-hand thread.