What is a Flashback Arrestor and What Does it Do?

Safe handling and use of Oxy- Fuel gas equipment is vital - danger is always just one incident away.

Flashback happens when the flame moves back from the welding nozzle into the torch, and potentially into the gas supply hoses. It is generally caused by one of the following:

• Lack of system purging before lighting up the torch - 90% of flashbacks occur due to this reason.
• Incorrect pressures - if the pressure is too low or the flow rate exceeds the nozzle capacity, the gas at the higher pressure then flows into the lower-pressure gas line. This will occur if incorrect pressures are used or if nozzles, cutting attachments and welding torches are incompatible, damaged or blocked.
• Lighting up incorrectly with both torch control valves open. One of the main reasons for this is short-cutting safety procedures to save time.
• A drop in pressure of either the Fuel gas or Oxygen due to kinks in the supply hose, leaks in the regulator, hoses or connections.
• The reverse flow of gases during temporary storage or shutdown, incorrect shutdown / closedown procedures or malfunctioning valves or regulators.

When a flashback occurs, the flame rapidly travels up the gas supply hose followed by a pressure wave to the regulator and possibly into the gas cylinders, causing damage and even an explosion.

It’s no exaggeration to say the consequence of this can be terrible or even fatal injuries, as well as causing serious damage to the welding environment.

Therefore, it is essential to have flashback arrestors on both gas hoses in all Oxy-Fuel welding, cutting or heating operations.

There are three types/categories of flashback arrestor:

• Resettable - Automatically shuts off the gas supply following flashback, but can be reset. It features a pressure-sensitive cut-off device and visible indicator lever. This product is ideal if you’re welding at some distance from the cylinders, and also in difficult industrial environments.
• Standard in-line - A simple and compact unit that includes a non-return valve; flame-arresting sintered element; inlet filter; and pressure-sensitive thermal cut-off device. The arrestor also includes a drop-down shroud that indicates the cut-off component has been activated.
• Premier in-line – Resettable. Designed in sleek chrome and brass, featuring a drop-down shroud that lets the welder know when the pressure-sensitive cut-off device has been activated.

Flashback arrestors contain several vital elements, depending on the type of unit you choose. All must conform to EN730-1/ISO5175-1.

The global standard DIN EN ISO 5175-1 describes the necessary safety elements:

• a gas non-return or check valve
• a flame arrestor
• a temperature-sensitive cut-off valve
• pressure-sensitive cut-off device that isolates gas supply in the case of a flashback

It is important not to confuse an arrestor with a non-return valve, which doesn’t possess flame-stopping properties.

Quality is the key. A faulty flashback arrestor is life-threatening. The valve must meet the requirements of DIN EN ISO 5175-1. The highest level of safety is certified by the German BAM (Federal Institute for Materials Research and Testing). BOC flashback arrestors have this BAM certification for your maximum safety, and you’ll find our full range online.

If unwanted gases flow through the pipeline in a hazardous event, the spring-loaded non-return valve is activated. This prevents the return flow in an instant, so an explosive gas mixture cannot occur.

But if the gases do mix, the unit will still deal with the flashback, as the flame arrestor kicks in. This porous cylinder prevents the flame from entering the regulator and a dangerous situation from occurring, with incoming flames broken up, cooled and extinguished in fractions of a second.

Finally, depending on the type of flashback arrestor, the plastic component in the temperature-triggered cut-off valve will melt meaning the accompanying spring is deployed, automatically interrupting the gas supply.

Each type of flashback arrestor is a durable and robust unit made to internationally recognized standards which can be easily installed whatever the required orientation. Typically, you’ll want to install it at the connection point between the pressure regulator and gas hose.

It’s essential to ensure that a flashback arrestor is fitted to all fuel gas supply lines, and to oxygen supply lines when they are used in conjunction with fuel gases.

There are several important steps to follow:

1. Check the unit is correct for the gas type you’re using.
2. Wipe the flashback arrestor clean to remove foreign bodies.
3. Check that the fittings, and that threads and gas seating faces, aren’t damaged.
4. Tighten fittings with the right spanner; do not force or over-tighten the nuts.
5. Point the flashback arrestor away from the cylinder and leave room around it to connect the supply hose easily.
6. When using a resettable type of flashback arrestor, check that the reset button or lever is in the closed position.
7. When the system is completely assembled, set the pressure on the regulator and leak-check all fittings with a suitable approved leak-detection fluid.

As they perform such a vital role in protecting the welder, you’ll want to ensure any flashback arrestors you install remain fully operational, and therefore safe, at all times. To that end it’s crucial that you invest in an annual CP7 safety inspection. The process is cost-effective and can mitigate against potential problems, building safety and efficiency into your welding activities in one go. Discover more about BOC’s Gas Equipment Safety Inspections service.

If this flashback arrestor article has stopped you in your tracks and it’s time for you to install or change yours, call in to your local Gas & Gear store where you can discuss the best option for you or find our full range of Flashback Arrestors which are available to buy online when you visit BOConline.co.uk/shop.