How can flashback be prevented?

Flashback is easily prevented by installation of flashback arrestors consisting of both a check valve and a flame arrestor. One flashback arrestor fits between each hose and the torch handle hose fitting. The check valves prevent the gas from one hose from entering the torch handle and then crossing to the other gas hose inside the back of the mixing chamber. Without the mixing of gases into an explosive mixture in the hoses, there can be no explosion in the hoses, regulator, or cylinder. The flame arrestor consists of a compressed stainless steel or sintered metal cylinder. The flame arrestor cylinder tends to stop fire from passing through it by both lowering the temperature of the flame front by absorbing its heat and by forcing the flame through small passages.

These devices are about the diameter of the gas hoses and about 1-3/4 inches long. Some newer torch designs incorporate check valves and flashback arrestors into the torch handle itself. Some arrestors fit between the regulator and the hose. See Figure 1–18. The best arrestors include a thermally-activated, spring-loaded shut-off valve which closes on sensing a fire.

Figure 1–18. Reverse-flow check valve flashback arrestor cross section.

What is backfire and what hazards does it present?

A backfire is a small explosion of the flame at the torch tip. The biggest hazard is that the detonation from the tip may blow molten weld metal five to ten feet from the weld and injure someone. Also, a series of repeated, sustained backfires, which can sound like a machine gun, may overheat the tip or torch, permanently damaging them.

How can backfire be prevented?

The most frequent cause of backfire is pre-ignition of the mixed acetylene and oxygen. Here are the most common causes of pre-ignition and their solutions:

            • The mixed welding gases are flowing out through the tip more slowly than the flame front burns and the flame front ignites the gas in the tip and/or mixing chamber causing a pop. Solution: Slightly increase both the oxygen and acetylene pressures and if this results in too large a flame for the job, reduce the torch tip size.

            • The tip may be overheated from being held too close to the weld or from working in a confined area like a corner. Solution: Let the tip cool off and try again holding the tip farther from the weld pool.

            • Carbon deposits or metal particles inside the tip act like spark plugs prematurely igniting the mixed gases. Solution: Let the tip cool, then clean it thoroughly with your tip cleaning kit.

You are using a rosebud tip that has a large flame for heating metal prior to welding, bending, or brazing. Soon after the torch is lit, it starts to pop (either once, or in a series of pops), or begins to squeal. What is the most likely problem and how is it best corrected?

This is flashback. Most likely low acetylene gas pressure is not pushing the oxygen/fuel mixture out of the tip faster than the flame can burn back on itself inside the tip. This allows the flame to burn inside the torch either in a single pop, a series of pops, or in a rapid series of pops that sounds like a squeal. Not only can one ruin a tip by allowing this to continue unchecked, but if the flame burning inside the tip reaches back into the hoses, these can explode and/or burn off and leave the weldor holding a burning rubber hose, a very serious condition. To avoid this hazard: Immediately turn off the torch, oxygen first, then the acetylene. Allow the torch to cool down for several minutes, then increase the acetylene regulator pressure setting to 15 psi (1 bar), reignite the torch and open the acetylene valve to obtain full flow, then adjust the oxygen.