Flashback Arrestors

15. (1994)

Flashbacks are unintentional and uncontrolled burning of gas back into an oxygen-fuel gas system, resulting in possible damage to the equipment. This can range from deposit of carbon within the torch tip, valves, and hoses, which effects their operation, to substantial and expensive damage to the regulator and possibly the cylinder. A flashback may result in serious personal injury.

A Flashback Arrestor is a device designed to quench a flashback. Some models also incorporate an automatic shut off value, to stop the gas flow, and some can be reset by the user. The main factor in choosing the model of flashback arrestor is the required gas flow rate, for example, when the welding equipment is used for cutting.

Flashbacks can and do occur due to -

• Excessive or incorrect pressures. The gas at the higher pressure flows into the lower pressure line. This can occur if incorrect pressures are used or if incompatible items of equipment are connected together.
• A leak from a regulator, hose or connection which results in a drop in pressure, and gas from the higher pressure line back-feeds into the other line.
• Leaking values allowing gas to mix when the equipment is not in use.
• Lighting up with both torch control values open, but one cylinder closed.
• When an oxygen cylinder becomes empty, the fuel gas may back-feed into the oxygen line, regulator, and cylinder. If the regulator is then placed on a new cylinder, and the cylinder value is opened too rapidly, the pressure can increase the temperature of the mixed gas enough to ignite it.
• Nozzle blockage or faulty equipment.

Note: Should a flashback occur, first turn off the blowpipe oxygen and then the acetylene. Inspect the hoses and replace if necessary. Purge the hose before lighting up, but never in a confined space. The range of flammability of acetylene is 2.5 to 85% in air, while for LPG it is 1.8 to 9.5%.

Flashback arrestors are required to be fitted to fixed or manifold systems operating on oxygen-fuel mixtures. For example, on the regulators supplying Atomic Absorption Spectrophotometers. It is also strongly recommended that portable oxygen-acetylene welding systems be fitted with flashback arrestors. Flashback arrestors should be fitted to the regulator of both the fuel and oxygen cylinders. Additional flashback arrestors fitted to the blowpipe of welding equipment is also recommended.

Additional information is available from the OHS Unit.

References:

Welding Safety, Department of Labour, New Zealand, Wellington, 1984.

Safe Handling of Compressed Gases, The Commonwealth Industrial Gases Limited, 1993.

Gas Welding and Cutting Equipment Guide, CIGWELD, Comweld Group Pty Ltd, 1993.

Dangerous Goods Inspectorate, ACT, conversation, 3/94.

Centre for Terotechology of Australia, Eutectic Castolin, Sydney, Facsimile and discussion, 3/94

For further information contact:
OHS Officer, e-mail: OHS.Officer@anu.edu.au