A fuel gas combined with pure oxygen will produce a flame with an extremely high temperature and an enormous quantity of heat.
On farms and ranches across Canada, compressed gas cylinders supply fuel gases and oxygen to specially designed torches that are used for heating and cutting, but also welding, brazing, or soldering.
There are some basic safety precautions that everyone using these gases should know.
The most common fuel gases in use are acetylene and propane. For some applications, the fuel gas alone is combusted with air using a specially designed venturi torch that siphons in extra air and mixes the gases in a swirl chamber. These torches are suitable for heating, brazing, or soldering applications with the major advantage of not needing a second cylinder of oxygen.
These applications do not need the highest flame temperatures with a precise concentration of heat. Some large versions of these torches are designed for heavy-duty heating applications, typically burning propane, and produce vast quantities of heat but at a relatively low flame temperature.
For applications requiring a high temperature concentrated flame, such as cutting or welding steel, torches designed to mix acetylene and pure oxygen dominate the industry. For this reason and due to the versatility of the set-up, most operations will have an oxy-acetylene torch available for equipment repair or new steel fabrication work.
Although the fuel gas acetylene had been known since 1836, it wasn’t until the accidental discovery of an inexpensive way to produce calcium carbide by the Canadian inventor Thomas “Carbide” Willson in 1892 that it came to common use. When calcium carbide is reacted with water, acetylene gas is produced, and this became the operating principle for early lamps in mines, automobiles, and rural homes. Experimentation in this period found that this gas produced a very high temperature concentrated flame when mixed with oxygen, suitable for fusion welding and flame cutting of steel.
There was just one problem: Acetylene is unstable and explodes when highly pressurized.
Only after the invention of a special steel cylinder, which stores the acetylene as a dissolved gas in liquid acetone soaked into a porous mass, did it become a portable tool. An early application of these cylinders was to mount them on vehicles to fuel the headlamps. Even today, the smallest cylinders of acetylene are designated “MC” for motorcar and “B” for buses.
This method of dissolving the acetylene in the liquid acetone presents a couple of safety challenges for users. If you imagine a carbonated beverage and the way that the carbon dioxide bubbles out slowly from the liquid, this is the same principle by which acetylene is released from the cylinder for use. However, if you release the cylinder contents too quickly, then both the gas and the liquid are released. In an acetylene torch, this means withdrawing the acetone liquid and the acetylene gas and this can damage the regulator and create a hazardous condition.
This problem can be avoided by controlling the rate at which the acetylene is consumed to no more than 1/10 of the cylinder capacity per hour. A large multi-flame (rosebud) acetylene heating tip can easily outstrip this rate of supply from a single cylinder, so without grabbing a calculator, it would be best to select an alternative fuel system, such as propane or propylene for large heating jobs. (These alternative fuel cylinders contain liquid, and have withdrawal rate limits, however they are less sensitive than acetylene.)
Another issue with acetylene cylinders and the liquid contents, is that they should be stored, transported, and used in the upright position. If an acetylene cylinder is left on its side, it should be stood upright for at least an hour before use.
Due to the explosive nature of acetylene if over-pressurized, when connecting these cylinders, a special pressure regulator must be used with a 15-psig (103 kPa) red-line gauge. Regulators for use with other fuel gases using the same inlet connections but that don’t have the 15-psig red-line gauge are marked “not for use with acetylene”.
The presence of commercial acetylene, which is lighter than air, can be detected by a distinct garlic-like smell.
Although not a fuel gas and naturally present in the air, oxygen poses a fire risk by rapidly accelerating fires. Any small fire can quickly get out of control when the atmosphere is enriched with additional oxygen. For this reason, never use oxygen as a substitute for compressed air, to dust-off clothing, or to help to start an engine.
Even handling oxygen regulators with oily hands or gloves can lead to trouble. Oil or grease in the presence of pressurized oxygen can ignite easily and burn vigorously. All components in an oxygen system, from the cylinder to the torch tip must be kept free of oil, grease, and other flammable substances. Equipment used for oxygen service such as regulators and hoses must not be used for any other gas.
Because of this fire-accelerating property, combining pure oxygen with a fuel gas such as acetylene in a controlled manner creates a powerful tool, especially for cutting steel.
An oxy-fuel cutting torch uses a ring of small flames to preheat the steel to a kindling temperature of 870 C and then directing a stream of pure oxygen, or a cutting jet, at the spot. The resulting rapid oxidation of the metal releases a tremendous amount of heat.
Alternative fuels (such as propane and propylene) when combined with oxygen can also produce a suitable cutting flame, however these gases do consume more oxygen. Some changes in equipment (such as the cutting torch tips) and techniques will be required if one of these fuel gases is used. When combined with oxygen, the only function that these alternative fuels are not suitable for is fusion welding. For many end users, this may simplify their operations and there may be an overall cost savings if the quality of the cuts performed is not critical.
Equipment setup and use
When hooking up an oxy-fuel torch, there are many other safety precautions to follow:
- Traditionally, the term “cracking a cylinder” refers to the process of quickly opening and closing the cylinder valve to clear dust and debris before installing the regulator. This action should not be performed on any fuel gas cylinder due to the risk of ignition. If required, cylinder valves can be wiped clean with a clean cloth free of lint and oil.
- Oxygen cylinder valves are double-seated and must be opened fully when in use. Most acetylene valves should only be opened about 1½ turns so that they can be closed quickly (unless the cylinder is labelled otherwise). Some acetylene cylinders have a t-handle key (wrench) that must be kept on-hand.
- Select regulators, torches, and components (such as cutting tips) that are designed to work with the gas that you intend to use and your applications. Check the manual for your equipment for the proper flow rates and pressure settings.
- Check your gas hoses. The current standard for oxy-fuel gas lines is the oil and flame-resistant T-grade hose. Older hose should be replaced, and any damaged hoses should be cut and properly spliced or replaced (and never taped). Oxygen hoses should be coloured green and fuel gas hoses should be red. When in-use, always keep the hoses away from the heat and cutting dross.
- Reverse flow check valve and with a flame arresting element must be installed on both the oxygen and fuel gas lines for oxy-fuel systems. These safety devices stop the gases from flowing in the wrong direction in the system and resist flames burning in the lines and backward toward the regulators and the cylinders. The manufacturer’s recommendations for these safety devices must be followed to ensure that they are properly sized and located. Some models of torches have these devices already built-in and installing additional accessory devices can cause problems with gas flow restrictions. These devices are not required for fuel gas line for single-line fuel-air only systems.
- Check the system for leaks after assembly. Commercial leak detection fluids are available, but if you decide to mix your own, do not use a soap solution that is based upon oils or fats. Never use a flame to test for leaks.
- Never light a torch with a match or a cigarette lighter. Use a proper friction lighter (flint striker) to prevent burns to hands and fingers.
- Be cautious with what you are heating and where you are heating it. Too many structure fires have been caused by welding or cutting “hot work”. Even cutting directly on a concrete floor can cause a serious steam explosion. Welding or cutting on any sort of drum or container can lead to injury or death unless proper precautions are taken.
- Keep your equipment in good repair. A basic routine maintenance function is to keep the torch tips clean with a proper tip cleaner.
The innocent looking oxy-fuel torch rig in the corner of the workshed is a fantastic and indispensable tool. It can also too easily be a deadly and destructive force if not properly set-up, maintained, and used.
It is recommended that everyone using this equipment understands it and gives it the respect it deserves. Following the manufacturer’s specific instructions on the set-up, operating pressures and tip selection, torch use, and safe shut-down sequence is critical.
Jim Galloway teaches welding technology at Conestoga College in Cambridge, Ont. He also volunteers on several CSA technical committees, including CSA W117.2 — Welding Safety