Lighting an oxy acetylene torch follows a specific sequence: purge both lines, open the acetylene valve first, ignite with a striker, then add oxygen and adjust to a neutral flame. Getting this order wrong risks backfires, flashbacks, or an unstable flame. Here’s the full process from setup to shutdown.
Safety Gear You Need First
Oxy acetylene torches produce intense heat and ultraviolet light, so you need proper eye and skin protection before you strike a flame. For gas welding, OSHA requires a minimum shade 4 lens for material under 1/8 inch thick, shade 5 for 1/8 to 1/2 inch, and shade 6 for anything thicker. If you’re cutting rather than welding, the shade requirements are slightly lower: shade 3 for material under 1 inch, shade 4 for 1 to 6 inches, and shade 5 for anything over 6 inches.
Wear leather gloves and a leather or flame-resistant apron. Keep a fire extinguisher within reach, clear the area of flammable materials, and make sure you have ventilation. Never use a disposable lighter to ignite the torch. Use a friction striker designed for the job.
Check for Leaks Before Anything Else
Before lighting up, test every connection for leaks. Apply a soapy water solution to hose fittings, regulator connections, and cylinder valve stems, then pressurize the system and watch for bubbles. One critical rule: never use a petroleum-based solution when checking oxygen fittings, because oil and grease can ignite violently in the presence of pure oxygen. If you find bubbles, tighten the fitting and retest. Don’t light the torch until every connection is clean.
Setting Your Regulator Pressures
With the torch valves closed, slowly open each cylinder valve. Open the acetylene cylinder valve no more than one full turn (so you can shut it fast in an emergency). Open the oxygen cylinder valve all the way.
Set your acetylene regulator to 3 to 5 PSI. This is not a suggestion you can push higher. OSHA prohibits using acetylene above 15 PSI because at higher pressures the gas becomes unstable and can spontaneously decompose and explode, even without a spark. In practice, most welding and cutting work keeps acetylene between 3 and 7 PSI. Set your oxygen regulator between 20 and 40 PSI, depending on your tip size and what you’re doing. For general cutting on 3/16 to 1/4 inch steel with a #0 tip, 30 PSI oxygen and 4 to 5 PSI acetylene is a reliable starting point.
One detail many beginners miss: set your final pressures with the torch valve open (and for cutting, with the oxygen lever depressed), because that reading reflects your actual working pressure under flow. The static reading on a closed system will be higher than what reaches the tip.
The Lighting Sequence Step by Step
This sequence applies to a standard equal-pressure (positive-pressure) torch, which is what most shops use.
- Purge both lines. Open the acetylene torch valve briefly to flush stale gas and air from the hose, then close it. Do the same with the oxygen valve. This prevents a mixed-gas pocket from sitting in the lines when you ignite.
- Open the acetylene torch valve about half a turn. Hold the striker at the tip and ignite the gas. You’ll get a yellow, smoky flame.
- Adjust the acetylene flow. Increase the acetylene until the flame jumps away from the tip, then back it off until the flame returns to the tip with no smoke. The flame should be orange-yellow and attached to the tip but not sputtering.
- Open the oxygen torch valve slowly. The flame will change from yellow to blue. Keep adding oxygen until you see a well-defined inner cone with no feathery edges. That’s your neutral flame.
If you’re using a cutting attachment, depress the oxygen cutting lever after you’ve set a neutral flame on the preheat flames. You may need to fine-tune the preheat oxygen valve so the flame stays neutral while the lever is down. The cutting oxygen stream should be visible and at least 4 inches long.
Reading the Three Flame Types
The flame tells you exactly what’s happening with your gas mixture, and using the wrong one will ruin your work or damage the metal.
A neutral flame is what you want for most steel welding and cutting. It has a bright bluish-white inner cone with a sharp, well-defined tip, surrounded by a lighter blue outer envelope. There’s no fuzziness or extra cones between the inner cone and the envelope. This flame adds no extra carbon or oxygen to the weld pool.
A carburizing flame (too much acetylene) has three visible zones instead of two. You’ll see the sharp inner cone, then a white feathery “intermediate” cone extending beyond it, and then the outer blue envelope. That feathery second cone is the giveaway. This flame pushes carbon into the metal, which makes steel brittle. You’d only want it for specific applications like hard-facing or brazing certain alloys.
An oxidizing flame (too much oxygen) has a smaller, pointed inner cone that looks slightly purple, and the flame hisses more aggressively. This flame burns hotter and introduces excess oxygen into the weld, causing porosity and oxidation in steel. It’s occasionally used on brass or bronze, but for steel work it’s harmful.
To move between these, you’re only adjusting the oxygen torch valve. Add oxygen to go from carburizing toward neutral. Add more and you cross into oxidizing. Back it off and the feather reappears. The sweet spot is right where that feathery edge disappears into the inner cone.
Backfires and Flashbacks
A backfire is a momentary flame pop. You’ll hear a sharp popping sound, and the flame may go out. This usually happens when the tip touches the work, gets clogged with spatter, or overheats. Relight the torch normally if it goes out. If backfires keep happening, check that your tip is clean, properly seated, and the right size for your pressures.
A flashback is far more dangerous. It’s an explosion that travels backward through the torch body, into the hoses, through the regulators, and potentially into the cylinders. The warning sign is a popping sound that transitions into a sustained high-pitched whistle or squeal. If you hear that whistle, shut off the torch oxygen valve immediately, then the acetylene valve, then close both cylinder valves. Do not try to keep working. Flashback arrestors installed between the hoses and regulators can stop this from reaching the cylinders, and they’re worth every dollar if your setup doesn’t already have them.
The Shutdown Sequence
Shutting down in the wrong order can cause a pop or a flashback, so follow this sequence consistently.
Close the oxygen valve on the torch handle first. The flame will go out or turn to a sooty acetylene flame. Then close the acetylene valve on the torch handle. Next, close both cylinder valves: acetylene first, then oxygen. Finally, open each torch valve one at a time to bleed the remaining pressure from the hoses and regulators. You’ll see the gauge needles drop to zero. Close the torch valves, then back out both regulator adjusting screws so there’s no spring tension on the diaphragms. This protects the regulators during storage and confirms the system is fully depressurized.