Guide
Oxy-acetylene welding and cutting the equipment, the three flames and the settings
Oxy-acetylene burns at about 3,100 °C, the hottest common flame, which is why it still welds, cuts, brazes and heats where other gases cannot. The flame is set by the oxygen-to-acetylene ratio: neutral for most work, carburising for hardfacing, oxidising rarely. Line pressure never exceeds 0.62 bar g.
- Oxy-acetylene flame temperature
- ≈3
- Maximum acetylene line pressure
- 0.62 bar g normal; 1.03 bar g only with CCoE clearance
- Neutral flame ratio
- Approximately 1:1 oxygen to acetylene
- Maximum withdrawal rate
- One seventh of cylinder contents per hour
- Flash back arrestors required
- Both lines
Why oxy-acetylene is still used
Electric processes took most of the welding that oxy-acetylene used to do. It has held on anyway, for three reasons that have not changed.
The flame is the hottest available
About 3,100 °C with oxygen, against roughly 2,800 °C for oxy-propane. Acetylene also needs less oxygen per unit of gas than propane, so less of the flame's heat is spent warming surplus combustion products, and the hot zone sits in a tight inner cone a few millimetres from the tip, which is where the work is.
It needs no electricity
A trolley with two cylinders works on a construction site, a ship's hull, a pipeline trench and in a power cut.
It pierces
Starting a cut in the middle of a plate rather than from an edge is where acetylene beats every alternative, and it is why fabrication shops that cut mostly with LPG still keep acetylene for piercing.
The outfit, item by item
| Item | Detail that matters |
|---|---|
| Acetylene cylinder | Maroon under IS 4379, dissolved in acetone, upright always |
| Oxygen cylinder | Black under IS 4379, to IS 309 |
| Regulators | Two-stage on each gas. The acetylene regulator has a left-hand thread, which is the convention for fuel gases |
| Hose | Colour-coded, correct bore for the nozzle, no oil or grease anywhere near either line |
| Flash back arrestors | At the regulator and at the torch, on both lines. See flash back arrestors |
| Torch | Welding handle plus a cutting attachment, with nozzles sized to the work |
The arrestors are the item most often absent, and the one that decides whether an incident is a bang at the torch or a ruptured cylinder.
The three flames, and what each is for
The flame is set by the ratio of oxygen to acetylene, and the three settings are not interchangeable.
Neutral — roughly 1:1
A sharply defined inner cone and no feather. It neither adds carbon to the weld nor burns it out, so it is the flame for most welding and for all cutting preheat. If you are unsure, this is the one.
Carburising — excess acetylene
A feathery secondary cone around the inner cone. The excess carbon it introduces is wanted for hardfacing and for welding high-carbon steels. On mild steel it makes the weld brittle.
Oxidising — excess oxygen
Shorter, sharper and noisier, with a smaller inner cone. Correct for brazing some brasses and bronzes, where a slight oxide film suppresses zinc fuming, and wrong for almost everything else. On steel it burns out carbon and leaves a porous weld.
Setting the flame is done by eye against the cone, not by the gauges. The gauges get you into range; the cone tells you where you are.
Cutting is a different process from welding
This is the point most often misunderstood. In welding, the flame melts the parent metal and fuses it, with or without filler.
In cutting, the flame only preheats. Once the steel is at its ignition temperature, the cutting lever releases a jet of pure oxygen, the steel burns in that jet, and the stream blows the molten oxide out of the kerf. The cut is an oxidation reaction, not melting.
Two consequences follow. It works on carbon steel and poorly or not at all on stainless, aluminium and cast iron, whose oxides melt higher than the metal or form a protective skin. And the quality of the cut depends on the oxygen stream, which is why a worn or spattered cutting nozzle ruins an edge no amount of preheat will fix.
The oxy-acetylene setup, as a diagram
Drawn in the order the gas travels. Both lines are identical in principle; what differs is the thread, the colour and the fact that nothing on the oxygen side may carry oil or grease.
Rules that matter
What it is good at
Advantages and disadvantages of oxy-acetylene welding
- Portability. No electricity, no generator, no cable run. A trolley goes anywhere.
- The hottest common flame, so it welds, brazes, hardfaces, heats and cuts with one set of equipment.
- Piercing. Starting a cut away from a plate edge, where no alternative fuel gas keeps up.
- Control. The welder sees the pool and the flame directly and can back off heat instantly.
- Low capital cost against any electric process, and it repairs in the field with hand tools.
- Thin sheet and dissimilar work, where arc processes blow through or need more setup than the job is worth.
What it is bad at
- Slow on thick section. Heat input is low against MIG or submerged arc, so production welding moved away from it decades ago.
- A wide heat-affected zone, which means more distortion than a concentrated arc produces.
- It will not cut stainless, aluminium or cast iron. Oxy-fuel cutting is an oxidation reaction, and those metals' oxides do not oblige.
- Operator skill. Flame setting, pool control and filler feeding are all manual and all take time to learn.
- Two cylinders of hazard on the job. Acetylene has the widest flammable range of any common gas, 2.5 % to 82 %, and oxygen enriches everything around a leak.
- Running cost per cubic metre is higher than LPG, which matters on long straight cuts and not much else.
The honest summary: for fabrication volume, an electric process wins. For maintenance, site work, repair, piercing and anywhere without power, oxy-acetylene is still the right tool, which is why it has outlived every prediction of its disappearance.
Pressures, and the limit you cannot exceed
Set working pressures from the torch manufacturer's chart for the nozzle in use. Nothing else is a valid source, because the right pressure is a property of the nozzle orifice and not of the job.
There is one number that overrides any chart. Acetylene must never exceed 0.62 bar g at the point of use, under IS 14814 cl 7.2.1. A maximum of 1.03 bar g, 15 psig, applies only where special safety measures are in place and the Chief Controller of Explosives has cleared it. Above that, acetylene can decompose explosively with no oxygen present at all, and a regulator wound up past the limit is a hazard regardless of what the torch chart says.
Two more rules sit alongside it. Do not draw more than one seventh of the cylinder's contents per hour in continuous use; beyond that the acetone comes out with the gas. And where a high continuous rate is needed, manifold several cylinders into a cascade rather than over-drawing one.
The pressures that matter, in one table
Semrush and Search Console both show people looking for an acetylene pressure chart. There is no single chart, because working pressure belongs to the nozzle. These are the figures that do not move.
| Pressure | Figure | Where it comes from |
|---|---|---|
| Acetylene at the point of use | 0.62 bar g maximum | IS 14814 cl 7.2.1 — the hard ceiling |
| Acetylene at the point of use, special case | 1.03 bar g (15 psig) | Only with safety measures and CCoE clearance |
| Acetylene gas cylinder, settled maximum | 15.495 atm at 15 °C with acetone (≈14.7 bar g) | IS 14814 cl 7.2.2 |
| Acetylene storage | No pressure applies — cylinders are stored as filled, upright, ventilated at high level | IS 14814 |
| Oxygen cylinder, filled | Typically 150 bar g at 15 °C | Cylinder specification |
| Working pressure at the torch | From the nozzle chart, nowhere else | Torch manufacturer |
| Maximum continuous draw | One seventh of cylinder contents per hour | KIGL product specification |
Three things follow that catch people out.
Do not quote 20 bar for an acetylene cylinder. It sits above the 15.495 atm the standard permits for a cylinder settled at 15 °C, and it is in wide circulation because it is a filling-line figure that gets copied across as though it were the pressure in the cylinder. Ours is one of them: KIGL fills at 20 bar g at the filling line. That is a number for the compressor and the manifold, not for the cylinder in your shop, and the two should never be used interchangeably. Quote the standard.
The acetylene cylinder gauge is not a contents gauge. It reads the vapour pressure of the acetone, which moves with temperature. A cylinder that reads lower on a cold morning has not lost gas. Weigh it against the tare stamped on the shoulder.
Storage has no pressure setting. The question "what is the safe storage pressure of acetylene" has no answer in bar: a cylinder is stored at whatever it was filled to. What makes storage safe is upright, ventilated at high level, away from oxygen and oxidisers, away from heat, and secured.
The 0.62 bar g ceiling is not a recommendation. Above roughly 1.5 bar g free acetylene can decompose with no oxygen present, and the Indian code sets the working limit well below that with margin. A regulator wound past it is a hazard whatever the torch chart says.
Lighting and shutting down, in order
- Check both regulators are wound fully out before opening the cylinder valves.
- Open the acetylene cylinder valve slowly, no more than about one and a half turns, and leave the key on the valve.
- Open the oxygen cylinder valve fully.
- Set the working pressures to the nozzle chart.
- Purge each line separately.
- Open the torch acetylene valve and light it. Increase until the smoke clears.
- Open the torch oxygen valve and bring the flame to neutral.
Shut down in reverse: acetylene off first at the torch, then oxygen. Closing oxygen first leaves a sooty acetylene flame burning at the tip. At the end of the shift close both cylinder valves, bleed both lines and wind the regulators out.
What goes wrong, and what it means
Popping or the flame going out
A blocked or damaged tip, pressures off the chart, or over-drawing the cylinder. Treat persistent popping as a warning rather than a nuisance: it is the condition that precedes a flashback.
A squealing or shrieking flame
A sustained backfire burning inside the torch. Shut the torch off immediately, acetylene first.
Flashback
The flame travelling back into the hose or beyond. The arrestor is what stops it reaching the cylinder, and an arrestor that has acted once is spent and must be replaced. What causes a flashback covers the conditions.
Soot on the work
Usually a carburising flame where a neutral one was wanted.
What to buy, and what it costs
The gas side is dissolved acetylene to IS 308 and industrial oxygen to IS 309, both filled at KIGL's Guwahati works, with flash back arrestors built to IS 11006. Indicative gas prices and the six things that move a quotation are on the industrial gas prices guide.
For the choice between acetylene and LPG on a cutting job, which turns on whether you are piercing or running long straight cuts, see acetylene or LPG for cutting.
Oxy-acetylene welding — frequently asked questions
What is oxy-acetylene welding?
A process that melts the parent metal using a flame produced by burning acetylene in pure oxygen, with or without a filler rod. It is the oldest gas welding process still in general use, and it survives because the flame is the hottest of any common fuel gas at about 3,100 °C and because the equipment is portable and needs no electricity.
What is the difference between oxy-acetylene welding and cutting?
Welding melts and fuses the parent metal. Cutting does something chemically different: the flame preheats steel to its ignition temperature, then a separate stream of pure oxygen is released that burns the steel and blows the oxide out of the kerf. That is why a cutting torch has an extra lever and why cutting works on carbon steel but not on stainless or aluminium, whose oxides do not behave that way.
What are the three oxy-acetylene flames?
Neutral, with roughly equal oxygen and acetylene, which is what most welding and all cutting preheat uses; it has a sharply defined inner cone and does not alter the chemistry of the weld. Carburising, with excess acetylene, recognised by a feathery secondary cone, used for hardfacing and for welding high-carbon steel. Oxidising, with excess oxygen, a shorter and noisier flame used for brazing some brasses and bronzes and wrong for almost everything else.
What pressure should I set for oxy-acetylene welding?
Set it to the torch manufacturer's chart for the nozzle you are using, and never above 0.62 bar g on the acetylene line, which is the limit IS 14814 sets for free acetylene at the point of use. Above that, acetylene can decompose with no oxygen present. Anybody quoting a universal pressure without asking which nozzle you have is guessing.
In what order do I light and shut down the torch?
Light on acetylene first with the oxygen closed, then add oxygen until the flame is set. Shut down in reverse: close acetylene first, then oxygen. Closing oxygen first on a lit torch leaves a sooty acetylene flame burning; closing acetylene first puts it out cleanly. Both cylinder valves close at the end of the shift, and the lines are bled.
What equipment does an oxy-acetylene set need?
Two cylinders, acetylene and oxygen; a two-stage regulator on each, acetylene with a left-hand connection; hose in the correct colours; flash back arrestors at both the regulator and the torch on both lines; and the torch with its welding or cutting attachment and nozzles. Nothing on that list is optional, and the arrestors are the item most often missing.
Can I use acetylene for cutting thick plate?
Yes, and it pierces faster than any alternative, which is what makes it the usual choice for starting a cut away from an edge. On long straight cuts in thick plate the picture changes: LPG costs less per cubic metre and keeps pace once the cut is running. The comparison is set out in acetylene or LPG for cutting.
Why does the flame keep popping or going out?
Usually the nozzle, the pressure or the draw rate. A blocked or damaged tip, pressures set outside the nozzle's chart, or withdrawing more than the cylinder can safely give — which pulls acetone out with the gas and destabilises the flame. Persistent popping is a warning, not a nuisance: it is the condition that precedes a flashback.
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