Flashback is the dangerous phenomenon in which the oxygen / acetylene / air mixture burns back into the hose and cylinder that supplied the oxygen or acetylene. It produces a hissing or squealing sound as well as flame disappearance. If allowed to continue, a flashback can cause an explosion of the cylinder or other damage of equipment.
Many of the causes of flashback overlap with the causes of backfire, and include restrictions in gas lines, extremely long small-diameter hoses, placing the torch tip directly on metal, improper gas pressures for the tip being used, damaged or over-tightened needle valves, and improper seating of the acetylene-cylinder valve. All of these conditions cause torch starvation, which results in overheating and flashback.
Flashback can be prevented by using properly functioning check valves and flashback arrestors.
