A situation in oxyfuel gas welding when oxygen or fuel gas reverse their flow and travel back up the hoses or other equipment. Several conditions can cause this reverse flow of gases, including a clogged torch tip, a malfunctioning check valve, improper shutdown and/or startup procedures, allowing cylinder pressures to become too low, when one cylinder goes empty during operation. Backflow is one of the most common causes of flashbacks. See also Oxyfuel Welding Torch, Check Valveand Flashback Arrestor.

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