A backfire is the loud popping noise that occurs when a welding or cutting torch flame burns back into the torch tip and is extinguished. Backfires are caused by improper gas pressures; the torch tip touching the work to molten metal; an incorrect, loose, overheated tip; or dirt on a connection or one of the seats. Although they are more of any annoyance than anything else, backfires can be eliminated by following proper procedures.

Flashbacks are more serious. Instead of going out, the flame burns back into the torch beyond the mixing chamber, possibly even into the hoses, regulator, and cylinder. The result can be a disastrous torch or hose explosion. More serious flashbacks may cause the regulator, or even the cylinder, to explode.

A flashback requires immediate action. It is often accompanied by a shrill, squealing sound. If a flashback occurs, shut off the torch fuel-gas valve immediately; then shut off the torch oxygen valve. Quick action is needed because prolonged exposure to a flashback may cause the torch to burn. Allow a hot torch to cool before relighting. Purge all hose lines before relighting the torch.

The use of flashback arrestors will provide additional protection and help prevent flashbacks and backfires. Flashback arrestors can be used in both cylinder and pipeline applications.

It should be noted that no device can ever replace the necessity for safe operating practices and properly maintained equipment.

The same conditions that cause backfires may also cause flashbacks. Correct, well-maintained equipment and proper procedures should be used. Flashbacks are most frequently caused by the failure to purge the hose lines before lighting the torch.

One way to prevent the propagation of a flashback into the equipment supplying the torches is to install check valves between the torch gas inlets and the incoming oxygen and fuel gas hoses to keep unwanted gases from entering the hoses. For additional protection, check valves are often placed at the regulator hose connections as well. Check valves do not eliminate the need to purge hoses individually and to follow the proper setup and working procedures. These safety devices will prevent backflow into the lines only if they are maintained in serviceable condition.