Back fire in a gas torch

this is to request you to let me know that what are the reasons for a back fire in a gas torch and how i can rectify it

regards
manoj patil product engineer (india) manik

A BACKFIRE is a small explosion of the flame at the torch tip which may be the result of several avoidable conditions. The most frequent cause is preignition of the gases. Some causes of backfiring are:

1. The gas is flowing out too slowly and the pressures are too low for the size tip (orifice) used. The gases are therefore burning faster (flame propagation) than they can flow out of the tip. This trouble may be corrected by adjusting to a slightly higher pressure for both the oxygen and the acetylene.

2. The tip may become overheated from overuse, from operating in a hot corner, or from being too close to the weld. Cool the tip.

3. The inside of the tip may have carbon deposits or a hot metal particle may be lodged inside the orifice. These particles become overheated and act as ignitors. Correct this by cleaning the tip. Backfiring happens rarely but could occur when the inner cone of the flame is submerged in the puddle.

This comes from one of the college textbooks "Modern Welding" by Althouse, Turnquist, Bowditch, and Bowditch.

If you are depositing filler metal first, and then applying enough heat to "wet it out", that will also make the torch 'pop'.

A key step in brazing is to bring the base metal up to a tinning temperature without melting the base metal. If the base metal is allowed to melt, the filler will mix with it and cause a weak spot. When using brass filler, a flare up can also occur which is the zinc burning out of the alloy. You will notice a white powdery substance when this occurs. It's the same thing you will see when welding galvanized metal.

You also want to be sure that enough flux is used. Flux should flow slightly ahead of you as you progress along the joint.

Try this procedure Mr. Patil. Clean the metal surface by grinding. Heat the metal to a dull red. If you are using a fluxed rod, melt a little flux off of it and in the spot you wish to begin brazing. If you are using a powder flux, heat the end of the rod a bit and dip it in the flux. Then melt some of that flux off as previously mentioned.As soon as the flux flows, melt a small amount of the rod and see if it flows. If it does, that means the proper temperature and level of cleanliness has been achieved. Steve

In regard to your safety apparatus: If you have a later model Victor torch, it already has reverse flow check valves built-in. The new Vangaurd series has flash arrestors and check valves built in. If you don't have one of these torches, you should install check valves at the torch handle and flash arrestors at the regulators. Some people use flash arrestors at both ends of the hose.
Steve

The other, more (read VERY dangerous) dangerous condition is flashback. Flashback occurs when the flame goes out, relights itself and is pushed back into the torch. Black smoke and a squealing sound comes from the tip. The results are a ruptured hose, or a regulator or cylinder explosion. Anti-flashback valves minimize the danger.
reasons:
1. tip is clogged
2. tip cone submerged in weld puddle
3. failure to purge hoses before lighting

Hope this helps, and I'm sure Steve and Fred will add anything I've missed.