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What is a Flashback Arrestor and Why They Are Crucial in Welding

Date: 15-04-2024
Welding Safety

Alphaweld Insights - What is a Flashback Arrestor

It's no exaggeration to say that the development and use of flashback arrestors have prevented thousands of severe explosions with oxy-fuel welding and cutting. When the fire starts traveling back into the hose faster than the speed of sound, the operator can't react fast enough to cut off the gas. Without flashback arrestors, lethal explosions and burns can quickly occur.

What is Flashback and Backfire in Open-flame Welding?

When using any oxy-fuel setup for welding, brazing, cutting, preheating, or other flame treatments, the flame must be outside the torch's tip, never inside. However, the flame can sometimes regress inside the torch in one of two ways: backfire or flashback.

  • Backfire occurs when you get a momentary regression of flame into the torch tip, causing the flame to disappear and reappear at the torch tip. It's often followed by a popping or a banging sound.

  • Flashback is a much more serious issue in which the backfire flame doesn't stop at the torch tip and proceeds to burn inside the mixing chamber, hoses, and eventually reaches the gas cylinder, leading to the explosion. It's characterized by a high-pitched whistling sound. Flashbacks can also cause the hose to burst and flap violently in the air, with high-pressure burning fuel flying everywhere.

Flashbacks can cause severe operator injury and equipment damage. You can read the Government of Western Australia Safety Alert 02/2014 of how a plumber's truck with his gas cylinders exploded due to a flashback. Luckily, no one was injured. But, flashbacks shouldn't be underestimated. They are among the most serious safety hazards.

You should immediately stop working whenever you experience a backfire or a flashback with oxy-fuel. Follow the safety instructions of the blowtorch you are using on how to react in these situations. Different manufacturers recommend shutting the fuel or the oxygen valves first, so it's essential to know this before starting the job. Only continue working once the cause of the backfire/flashback is discovered, as they are usually equipment-related.

What Causes a Flashback?

There are hundreds of causes of flashbacks that can occur with your equipment during day-to-day welding and cutting operations. But, the bulk of the reasons come down to:

  • Damaged gas hoses from pedestrian and vehicular traffic.

  • Faults in gas hose fittings due to wear, dirt, carbonizing, and excessive heat.

  • Incorrect startup gas sequence.

  • Undersized or overheated blowpipe tip.

  • Blocked torch, blowpipe, and hoses.

  • Incorrect hose diameter.

  • Blocked tip with slag.

  • Holding the flame too close to the work surface or touching the work with the tip (this usually immediately causes a backfire, which can turn into a flashback).

  • Insufficient hose gas purging before ignition.

  • Inadequate gas pressure in the two supply hoses.

  • Gas tanks are almost empty and can't provide sufficient pressure to counteract the flame trying to burn back into the tip.

What is a Flashback Arrestor?

A flashback arrestor is a safety device designed to prevent the flow of flame back into the hoses, regulator, and gas bottle. Flashback arrestors quench the flame and close off the gas supply. Most arrestors include a non-return valve, flame arrestor, and thermal cut-off valve, providing multiple flame stopping methods. 

In Australia, it's mandatory to fit flashback arrestors to the regulator and torch on both fuel and oxygen lines. So, if a flashback were to start, you have two points of protection. The flame would travel from the tip into the torch blowpipe, and as it attempts to proceed into the gas hose, the flashback arrestor would stop it safely. But, if the first flashback arrestor were to fail, the flame would proceed into the hose. The next line of defence is the flashback arrestor at the end of the hose, where it's connected to the regulator. 

How Does a Flashback Arrestor Work?

Depending on the brand, flashback arrestors can have multiple lines of defence against the flame. 

  • Non-return valve (check-valve) cuts slow gas return and sudden gas return at high working pressure, which can help prevent the flashback in the first place. However, a check-valve alone can’t stop the flashback if it occurs.

  • Flame barrier cuts off the incoming flame and absorbs the pressure surge caused by the flashback.

  • Flame arrestor is the critical component made from specialized alloys that takes away the energy of the flame, rendering it unable to continue spreading the combustion.

  • Thermal cut-off valve is the last line of defence for situations where the entire component has heated excessively to the point where the flame arrestor can’t cool down the flame. In such a scenario, the thermal cut-off valve closes the gas flow as soon as the critical temperature is exceeded, making it almost impossible for the flashback to continue. 

What are the Best Flashback Arrestors?

It's critical to use high-quality flashback arrestors that conform to The Australian standard AS4603-1999 ("Flashback arresters — Safety devices for use with fuel gases and oxygen or compressed air"). 

Alphaweld provides the highest-quality flashback arrestors ranging from standard flow flashbacks to medium and high flow flashbacks. The specifics of your application determine the type of flashback you need. For example, the high and the medium flow flashback are used in applications where you need a higher gas flow rate than standard welding operations. Our Ibeda and Harris flashbacks offer exceptional protection against flame, giving you the protection you need for all open-flame operations.

High-quality flashbacks will protect you often without you even realizing it. A small pop you can notice infrequently would possibly turn into a full flashback without a flashback arrestor. However, flashback arrestors must be tested or replaced every 12 months to ensure their functionality. Repeated flashback arrestor activation makes them less effective in the future. 

Need Additional Help? Alphaweld is Here for You

If you are unsure about the required flashback size for your application, contact the experts at Alphaweld - call (08) 9456 8000 or send us an online enquiry and we'll be happy to help.