Answers to common questions about running a modular torch on propane, propylene or natural gas instead of acetylene.
Which fuel gas should I choose besides acetylene?
The common alternatives to acetylene are propane, propylene and natural gas. Acetylene has the hottest flame and pierces fastest, followed by propylene, then propane, with natural gas having the lowest flame temperature and the slowest piercing. Propane is widely used for preheating and heating work, and propylene is widely used for cutting.
The best choice depends on the job, the plate thickness and the torch. Whichever fuel you pick, use mixers, heads and nozzles that are made for that fuel, and follow the maker's instructions for gas pressures.
Can I use my acetylene nozzles and mixers with propane?
Not unless the maker says so. Nozzles and mixers are designed around one fuel gas, because the fuels need very different amounts of oxygen and burn differently. TWI notes that nozzle choice depends on the fuel gas, and propane needs roughly three to four times more oxygen per volume of fuel than acetylene.
An acetylene part on propane typically gives a poor or unstable flame. Look for the fuel marked on the nozzle, mixer or head, and check the maker's literature. If in doubt, our team can help you identify the correct part.
How does cutting and preheating with propane differ from acetylene?
Propane puts much less of its heat in the inner cone and more in the outer flame, so the usual acetylene technique of holding the inner cone just above the plate is slow. Hold the torch a little higher so the outer flame does the preheating.
Propane takes noticeably longer to start a cut or pierce than acetylene, but TWI reports that cutting speed is about the same once the cut is running. Propane also tolerates a greater nozzle-to-work distance. Follow the nozzle maker's flame setting and height advice for the size you are using.
Why does propane use so much more oxygen?
Each volume of propane needs far more oxygen to burn than acetylene does. TWI gives approximate oxygen-to-fuel ratios of about 4.3 to 1 for propane, 3.7 to 1 for propylene, 1.8 to 1 for natural gas and 1.2 to 1 for acetylene, and these are maximum-flame ratios that vary between sources.
This means oxygen cylinders empty faster and you must have enough oxygen supply and a regulator able to supply it. The recommended oxygen and fuel pressures for each nozzle are in the maker's chart, so use those rather than reusing acetylene settings.
What is the difference between an injector torch and an equal-pressure torch when using alternative fuels?
Both torch types mix oxygen and fuel gas, but they do it differently. In an injector torch, a fast jet of oxygen passing through the mixer draws the fuel gas in, so the fuel gas can be used at a low pressure. In an equal-pressure torch, both gases reach the mixer at similar pressures.
Because the mixing method differs, mixers and heads are made for one type of torch and for particular fuel gases, and they are not interchangeable between types. Check the markings on your torch, mixer and head, and use the maker's manual to find which fuels and pressures are approved. Do not run a fuel gas through a torch unless the maker states it is suitable.
What are the main safety differences between propane and acetylene?
Propane is heavier than air, so a leak sinks and collects at floor level or in low areas, whereas acetylene is lighter than air and rises. Propane and propylene are liquefied petroleum gases and do not have the decomposition risk that acetylene has, but all fuel gases are highly flammable and form explosive mixtures with air and oxygen. Incomplete combustion produces carbon monoxide, so ventilate well, especially in enclosed spaces.
That does not mean you can relax. Check for leaks with a leak-detection solution made for oxy-fuel use, keep cylinders upright and secured, and use hoses, regulators and fittings made for the fuel gas. Follow the manufacturer's instructions and your risk assessment.