Guide written by:
Michael, Professional and passionate welder, Sheffield
Whether it's brazing, welding or autogenous welding you're up to, gas is an essential ingredient for all these processes. Welding gas can come in a drum or a pressurized bottle, and can be as diverse as acetylene, butane, propane or propylene. Enough gassing, over to our expert...
- Oxidizing gas
- Combustible gas
Combustible and oxidizing gases, which is which?
To make sure you buy the right product, you need to determine your needs in terms of the metals you're working with, including the filler metal.Gases can be classified according to theircombustion power, and in case the fuel (or combustible) gas isn't capable of reaching the required temperature alone, you can add a second, oxidizing gas to boost the combustion power.
Flame welding, as the name suggests, requires a flame. In order to produce a flame, you need: activation energy (to light the flame), fuel (which is consumed, e.g. welding gas), an oxidizer (a substance that allows combustion, e.g. oxygen). These three elements make up the Fire Triangle. As firefighters are quick to remind you, remove one of these three elements and there is no fire.
What are the combustible gases?
So, combustible gases generate a flame in the presence of an oxidizer, thanks to an initial burst of activation energy. There are several types:
- Butane is a liquefied petroleum gas (LPG). It is stored in the liquid state under pressure (maximum 2 bars) and then used in the relaxed gas phase at 0.28 millibars. Its evaporation temperature is around 5°C, making it perfect for use indoors but not ideal outside. It has a characteristic odour that makes it easy to detect;
- Propane is also a liquefied petroleum gas (LPG). It vaporizes at negative temperatures (as low as -44°C), making it an ideal fuel for outdoor work even in winter. Its internal pressure is around 7 bars;
- Propylene (C3
H6) is a synthetic gas which vaporizes at -47.6°C. Often combined with butane and propane in disposable drums to improve combustion power;
- Acetylene (C2
H2) is a synthetic gas obtained from the reaction of calcium carbide with water and is stored in pressurized bottles (at 15 bars) after being dissolved in acetone. In use, these bottles must not be laid down so as not to burn the acetone which remains at the bottom of the bottle. Acetylene is a highly flammable gas with exceptional calorific value, making it an all-time favourite with flame welders.
What are the oxidizing gases?
While no combustion at all can occur without an oxidizer (i.e. a fire in a closed jar will die), adding an additional oxidizer improves combustion and thus increases the temperature of the flame.
Oxygen is the most commonly used oxidizer. It is used in bi-gas kitsalong with butane, propane or acetylene because it allows the temperature of the flame to be raised to high temperatures for brazing and welding.
- Oxygen (O2) in a bottle is the same as what you breathe every day. It is obtained in pure form by liquefying air and distilling it to separate out the nitrogen, argon, neon etc. It is packaged in disposable drums, small recyclable bottles or steel cylinders of 2, 7 or 10m3 under a pressure of 150 bars. Bottles of smaller volumes (about 1m3) can be purchased without any kind of restricton, but in larger volumes a contract may be required between user and the distributor.
Important: never lubricate pressure gauges or oxygen supply valves, or you'll get a nasty shock: they catch fire without even being exposed to a flame!
How to distinguish the different types of gas bottle?
The warhead (not the nuclear kind) of a gas bottle is the top part that connects up to supply your equipment. These are colour-coded for ease of recognition - white for oxygen and brown for acetylene.
Store the bottles securely against a wall or even better, in a dedicated trolley. When connecting the pressure regulators, you can't go wrong since oxygen and acetylene bottles have oppositely pitched screw threads, same as the torch fittings.
Fire-resistantbackstops are designed with a failsafe system to prevent accidents. Sometimes bottles come with a quick coupling system - very practical if you want to change torch without closing the bottles.
Any safety tips?
First off, an age-old piece of advice: never open gas bottles all the way! ¼ turn is enough to release the pressure, and in case of cockups they can quickly be closed.
For oxyacetylene welding, place fireproof backstops just at the mouth of the torch to prevent flame recoil.
Never assemble your hoses with adhesive: if one length caught fire, the next would ignite immediately and could quickly spread out of control.
Pay attention to hose colours - blue for oxygen, red for acetylene.
Stay within recommended operating pressures (1–1.2 bar for O
and 0.2–0.5 for C
2). Excessive oxygen pressure makes the metal melting process unstable, while excessive acetylene pressure gives a porous, brittle carbide solder.
When your work is done, close the valves on the bottles and let the pressure gauges down to 0 bar. This is professional best practice!
Help! My acetylene bottle has caught fire!... Keep calm and throw a wet rag onto the warhead to avoid getting burned while you close the bottle. Better than jumping up and down screaming while your workshop goes up in flames.
Don't hesitate to consult gas Safety Data Sheets (MSDS). These are available for each type of gas and will inform you of the risks and the precautions to take when using them.
A final tip to finish?
As you might have heard from an annoying schoolteacher or two, "keep it simple stupid!"
An "oxygen-acetylene" assembly is sufficient for all types of welding and brazing since it has the greatest combustion power.
Otherwise, depending on your budget and your plans, a single-gas torch or bi-gas kit may be just as useful. Happy welding!
Learn more about metalworking equipment...
To find out more about welding equipment, follow our editors' advice and check out their other guides:
- How to choose your protective eyewear?
- How to choose your protective gloves?
- How to choose your safety shoes?
Guide written by:
Michael, Professional and passionate welder, Sheffield, 74 guides
I was trained as a pipe worker and a pipe-welder and after having traveled for 35 years working around the UK, I became the head a metal shop and then a designer and in the end the head engineer. I have designed and built a workshop where I make metal sculptures: I managed to find a piece of paradise where I can to let my imagination run wild. Auctions and garage sales are no secret to me. I find unusual objects and old tools there that I collect or transform into works of art. I also like decoration, painting on canvas, and gardening. I am developing new technologies concerning tools. To share my passion and humbly advise you in your choice of materials is a real pleasure.