2.4 Oxyfuel Cutting: Equipment, Safe Setup, and Cutting Practice
Key Takeaways
- OSHA 29 CFR 1910.253 prohibits opening an acetylene cylinder valve more than one and one-half turns, and preferably no more than three-fourths of a turn.
- Acetylene must never be used at a working pressure above 15 psig because it becomes chemically unstable.
- Stored oxygen cylinders must be separated from fuel gas cylinders by 20 feet or by a noncombustible barrier at least 5 feet high with a fire-resistance rating of at least one-half hour.
- A neutral flame has a clearly defined inner cone with no acetylene feather and is the flame used for most cutting and welding.
- Oxyfuel cutting works by preheating steel to its kindling temperature and then oxidizing it with a stream of pure oxygen, which is why it does not cut aluminum or stainless steel effectively.
How oxyfuel cutting actually works
Oxyfuel cutting is a chemical oxidation process, not a melting process. The preheat flames raise the steel to its kindling temperature — roughly 1,600°F, a bright cherry red — and then the cutting oxygen lever releases a high-purity oxygen jet that burns the iron into iron oxide. The oxygen stream blows the molten oxide out of the joint, forming the kerf.
That mechanism explains a fact the assessment likes to test: oxyfuel cutting works on carbon and low-alloy steels but performs poorly on aluminum, stainless steel, cast iron, and copper alloys, because their oxides melt at a higher temperature than the base metal and form a refractory skin that stops the reaction.
Cylinders, regulators, and hose
| Component | Key facts |
|---|---|
| Oxygen cylinder | Right-hand threads; green in common US practice; full cylinder about 2,200 psi at 70°F |
| Acetylene cylinder | Left-hand threads with a notched nut; packed with a porous filler saturated in acetone; must be stored and used valve end up |
| Regulators | Two-stage on the cylinder, showing cylinder pressure and working pressure; always back out the adjusting screw before opening the cylinder valve |
| Hose | Green for oxygen with right-hand fittings; red for fuel gas with left-hand notched fittings |
| Check valves and flashback arrestors | Reverse-flow check valves prevent gas mixing in the hoses; flashback arrestors stop a flame front from travelling back to the cylinder |
Handling and storage rules that appear verbatim on assessments:
- Transport cylinders with the valve protection cap in place, secured upright, using a cart or cradle. Never lift a cylinder by the cap and never use a magnet or a sling around the body.
- Never allow oil or grease to contact oxygen fittings. Oxygen under pressure ignites hydrocarbons spontaneously.
- Store oxygen cylinders at least 20 feet from fuel gas cylinders and combustible material, or separate them with a noncombustible barrier at least 5 feet high with a fire-resistance rating of at least one-half hour.
- If an acetylene cylinder is laid on its side, stand it upright for at least the time your employer's procedure requires — commonly a minimum of one hour and often longer — before use so the acetone settles.
- Never open an acetylene cylinder valve more than one and one-half turns, and preferably no more than three-fourths of a turn, so it can be closed quickly in an emergency. Leave the wrench on the valve stem if the cylinder uses one.
- Never exceed 15 psig working pressure with acetylene. Above that, acetylene can dissociate explosively even without air.
Lighting, flame adjustment, and shutdown
The lighting sequence is: open the fuel valve slightly, ignite with a friction striker (never a lighter or a match held in the hand), then add oxygen until the flame is correct. Shutdown reverses the emphasis: close the fuel valve first on the torch, then the oxygen, then close cylinder valves, bleed both lines, and back out the regulator adjusting screws.
Three flames, all testable:
| Flame | Appearance | Chemistry | Use |
|---|---|---|---|
| Carburizing (reducing) | Bright inner cone with a feathery acetylene plume around it | Excess fuel | Adds carbon to the puddle; used for hard-facing, not general cutting |
| Neutral | Sharply defined inner cone, feather just disappeared | Balanced 1:1 | Standard preheat for cutting and general welding |
| Oxidizing | Short, sharp, necked-in inner cone with a hissing sound | Excess oxygen | Burns carbon out; causes scale and porosity in steel |
Adjust by first setting a carburizing flame and then adding oxygen until the acetylene feather just disappears. That is neutral.
Cutting technique and quality indicators
Preheat the edge to a bright cherry red, then depress the cutting oxygen lever smoothly and begin travel. Quality is read off the cut face:
- Drag lines should be nearly vertical and evenly spaced. Curved, backward-trailing drag lines mean travel speed is too fast.
- Excessive slag adhering to the bottom edge usually indicates travel too slow, preheat too high, or a tip too large.
- A rounded, melted top edge indicates preheat too high or the torch held too close.
- Loss of cut partway through means travel too fast, oxygen pressure too low, or a clogged tip.
Tip size is selected from the manufacturer's chart based on material thickness, and each tip has published oxygen and fuel pressures. A tip cleaner is used to restore the orifice; a drill bit or welding rod enlarges and bells the orifice and ruins cut quality.
Backfire and flashback
- A backfire is a momentary loud pop at which the flame goes out or immediately reignites. Causes include touching the tip to the work, a tip that is overheated or dirty, or incorrect pressures.
- A flashback is a flame that travels back inside the torch, hose, or regulator, producing a shrill squeal or hiss. It is an emergency: close the torch oxygen valve first, then the fuel valve, then close both cylinder valves and take the equipment out of service until it is inspected.
Flashback arrestors and reverse-flow check valves exist precisely because the consequence of a flashback reaching an acetylene cylinder is catastrophic.
Hot work controls
Cutting inside a plant is hot work. A hot work permit, a fire watch maintained during the work and for at least 30 minutes afterward under common permit practice, removal or protection of combustibles within 35 feet, and closing of floor and wall openings are the controls a mechanic is expected to name.
How far may an acetylene cylinder valve be opened?
A mechanic adjusts the cutting torch by first setting a flame with a feathery plume around the inner cone, then slowly adding oxygen until that plume just disappears. What flame has been set?
An inspection of a finished cut shows drag lines curving sharply backward and the cut breaking out partway along the plate. What is the most likely cause?