Flashback arrestor

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File:Flashback arresters.png
Nineteenth century examples of dry and wet types of flashback arrestors, for use with oxy-hydrogen.
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A flashback arrestor or flame arrestor is a device most commonly used in oxy-fuel welding and cutting to stop the flame from burning back up into the equipment and causing damage or explosions. The two main types are dry and wet. Each has its own advantages and disadvantages. Most oxy-fuel flashback arrestors are the dry type.

Dry type

Dry flashback arrestors typically use a combination of methods to stop flashback. This is the type that is typically found in most workshops, homes and portable oxy-fuel kits as they work just as effectively with any orientation, need very little maintenance, and are often small and light enough to be installed between the torch and hoses. They include:

  • Flame trap to cool the flame front. They are designed to allow free flow of gas through them but to take the heat out of the flame front to get it below the ignition temperature of the burning gas mixture. The most common types are:
  • Temperature-triggered valves to stop the gas flow completely. Because it relies on extracting heat from the flashback to stop it continuing, most arrestors have a temperature-controlled valve that will cut off the gas flow when the unit reaches around 90°C, until either the unit cools (if the unit is automatic) or the reset button is pressed (if the unit is manual).
  • A check valve that closes due to the back pressure.

Wet type

Liquid seal flame arrestors are liquid barriers following the principle of a siphon where the liquid stops the entering deflagration and/or detonation and extinguishes the flame, they work by bubbling the gas through a non-flammable and ideally non-gas-absorbing liquid, which is typically water. They stop the flame by preventing it from reaching the submerged intake.

These devices are normally very effective at stopping flashbacks from reaching the protected side of the system. They have the disadvantages of only working in one orientation and tend to be much larger than dry type arrestors. This makes them mainly only suitable for large or fixed installations and the liquid level needs to be constantly checked.

On smaller units having a pressure release valve to prevent the unit from bursting under a severe flashback, the fluid level should be monitored to keep it always above the intake and not so high that the liquid could splash or overflow into the outlet.

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