Backfire

From GAWDAwiki

Backfire occurs when the flame burns back into the torch. It is characterized by a loud bang and is often a sign that there is a fault in the equipment or gas supply, e.g. incorrect pressure settings, gas cylinder running low, large pressure drops in the hoses, dirt or carbon blocking the nozzle, or the equipment becoming overheated. ^[1]

References

1. http://www.bocindustrial.co.uk/bocindustrial/technical/glossary/a.asp