Oxy-fuel safety

What Causes a Flashback in Oxy-Fuel Welding & Cutting?

Safety Equipment·8 min read·Updated 24 Jul 2026

A flashback is the most dangerous fault in oxy-fuel work: the flame travels back through the torch into the hose — and can reach the cylinder. It is almost always caused by reverse gas flow meeting a fuel-and-oxygen mixture upstream. Here is exactly what causes it, and how to stop it.

The short answer

A flashback happens when the flame burns back past the torch mixing chamber and travels down the hose toward the regulator and cylinder. For that to happen, two things must line up: the flame front must be able to move upstream, and there must be a combustible fuel-and-oxygen mixture for it to follow — created when one gas flows back into the other’s line. The root causes are almost always incorrect pressures, a blocked or overheated tip, gas starvation, or a leak that lets the gases mix.

Backfire, sustained backfire and flashback — know the difference

These three faults escalate in severity. Telling them apart tells you how serious the situation is.

FaultWhat happensSeverity
BackfireThe flame pops out and either goes out or re-lights at the tip, usually with a loud snap.Warning sign
Sustained backfireThe flame disappears into the torch and keeps burning inside it with a hiss or whistle.Serious — shut off at once
FlashbackThe flame burns back through the torch into the hose, toward the regulator and cylinder.Dangerous — can rupture the hose or reach the cylinder

The real causes of a flashback

Most flashbacks trace to one of these — and nearly all are preventable.

CauseWhy it leads to flashbackPrevention
Incorrect (too low) gas pressureGas velocity drops below the flame speed, so the flame can travel back up the stream.Set the correct working pressures for the tip in use.
Blocked or overheated nozzleA blocked tip raises back-pressure and disturbs the flame; an overheated tip can ignite the mixture inside.Clean tips with proper reamers; don’t let the tip overheat or touch the work.
Worn or damaged tip / sealsLets gases mix or the flame seat move, destabilising the flame.Inspect and replace worn tips and O-rings.
Gas starvationA near-empty cylinder or drawing acetylene too fast starves the torch, dropping velocity.Never draw acetylene faster than 1/7 of cylinder contents per hour; change cylinders in time.
Reverse gas flowOxygen pushes back into the fuel hose (or vice-versa), creating an explosive mix in the hose.Fit non-return valves; set oxygen and fuel pressures correctly.
Wrong lighting / shutdown sequenceLeaves a fuel-rich or mixed condition the flame can chase back.Follow the correct light-up and shut-down order every time.
Damaged or leaking hosesCross-leaks let fuel and oxygen mix before the torch.Leak-test hoses; replace cracked or worn hose.

Why acetylene makes flashback especially dangerous

Acetylene has an extremely wide flammable range (about 2.5% to 100% in oxygen) and a high flame speed, so a flame finds it easy to travel back up an acetylene line. Acetylene can also decompose under pressure and heat even without oxygen. That is why an acetylene flashback that reaches the hose or cylinder is so serious — and why flashback arrestors and non-return valves are mandatory, not optional, on acetylene equipment.

What to do if a flashback happens

  1. 1
    Shut off oxygen first
    On most torches, closing the oxygen valve first starves the flame fastest. Then close the fuel-gas valve.
  2. 2
    Close both cylinder valves
    Shut the cylinders and let the equipment cool before touching the torch or hoses.
  3. 3
    Inspect before re-use
    Check the hoses, torch and any flashback arrestors. An arrestor that has taken a flashback, and any heat-damaged hose, must be replaced — not reused.
  4. 4
    Find the cause
    Do not relight until you know why it happened — wrong pressure, blocked tip, empty cylinder or a leak — and have fixed it.

How to prevent flashbacks

  • ✓Fit flashback arrestors and non-return valves at both the regulator and the torch, on both gases.
  • ✓Always set the correct working pressures for the tip; low pressure is a leading cause.
  • ✓Purge each hose separately before lighting up, and follow the correct light / shut-down sequence.
  • ✓Keep tips clean and cool; never let the tip touch the work or overheat.
  • ✓Respect the 1/7 acetylene withdrawal rule and change cylinders before they starve the torch.
  • ✓Leak-test hoses and connections regularly; replace anything worn.

Frequently asked questions

A flashback is caused by the flame travelling back through the torch into the hose. It needs two conditions: gas velocity dropping below flame speed (usually from low pressure or gas starvation), and a fuel-and-oxygen mixture upstream (from reverse flow or a leak). Blocked or overheated tips and the wrong lighting sequence are common triggers.
A backfire is a momentary pop where the flame goes out or re-lights at the tip. A sustained backfire keeps burning inside the torch with a hiss. A flashback is when the flame burns all the way back through the torch into the hose toward the cylinder — the most dangerous of the three.
Acetylene has a very wide flammable range and high flame speed, so flames travel back easily, and acetylene can decompose explosively under pressure and heat even without oxygen. A flashback that reaches an acetylene hose or cylinder can rupture it, which is why flashback arrestors and non-return valves are mandatory on acetylene equipment.
Shut off the oxygen valve first, then the fuel-gas valve, then close both cylinder valves and let the equipment cool. Inspect the hoses, torch and flashback arrestors before re-use, and find and fix the cause before relighting.
A flashback arrestor stops a flashback that has already started from travelling further up the line — it extinguishes the flame and, with a non-return valve, stops the reverse flow that feeds it. It is the last line of defence, but correct pressures, clean tips and good technique prevent the flashback in the first place.
Yes — low working pressure is one of the most common causes. If gas velocity at the tip falls below the flame’s burning speed, the flame can travel back up the stream. Always set the correct pressure for the tip size in use.

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