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AC/OX CUTTING/WELDING/BRAZING

Flashback

Flashback is a phenomenon that may occur when, for some reason or other, the speed of combustion at one or more places in the flame hole becomes greater than the speed of gas flow, allowing the flame to burn back into the blowpipe. The degree of safety against flashback in a blowpipe depends on its design.

Various types of flashback: 1) A Backfire implies that the flame burns back into the torch with a sharp bang. Either the flame is extinguished, or it is reignited at the nozzle opening. A backfire is fairly harmless in itself, but it can be a sign of some fault in the equipment or gas supply. 2) In a Sustained Backfire the flame bums back into the torch with continued burning in the mixer, often at the mixing point itself. A sustained backfire is characterised by an initial bang (backfire) followed by a whistling or screeching sound from the continued combustion. If the sustained backfire is not quickly interrupted, melting will occur in the torch, and escaping combustion products can cause injuries.

3) Flashback implies that the flame burns back through the torch and into the gas supply system, i.e. the hoses and in the worst cases even the regulators. If a flashback reaches an Acetylene cylinder, which lacks the necessary safety equipment a serious accident can occur. Flashback is mostly caused by reverse flow, e.g. flow of oxygen into the Acetylene hose, so that an explosive mixture is present in the hose. This mixture can then be ignited by a backfire, which occurs when the torch is lit. The hose will then explode.

Handling of blowpipes in the event of flashback: In the event of sustained backfire the oxygen valve of the blowpipe must be shut first, then the Acetylene valve. Shut them as quickly as possible. Sustained backfire is the result of incorrect handling and/or poor maintenance of the blow pipe. Before re-lighting after a sustained backfire the blowpipe must be cooled. Under no circumstances should welding or cutting be continued before both the equipment and the handling routines have been checked thoroughly. A full fashback occurs only from very faulty handling, bad maintenance of the blowpipe or by wrongly set working pressure.

A backfire (popping) can easily occur if the flamehole is dirty or damaged – see separate chapter on maintenance of blowpipes.

Flashback can result in the hose exploding, and/or catching fire. In extreme cases, the regulator may catch fire.

To protect against a full flashback and the dangers this entails, the welding equipment should be fitted with flashback arrestors and non-return valves.

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