In this final instalment of the Shop Class Series, we’re drawing on technical expertise from a variety of sources. They include Cal Shaw, a welding instructor who teaches at SIAST’s Palliser Campus, published text books, and technical training material as guides.
This time we’ll take a look at two problems — backfires and flashbacks — both of which can occur when using an oxyacetylene system for cutting. And we’ll finish off with a look at how to shut the system down when you’re done.
While cutting, you may notice the torch making a popping sound. This is known as a backfire. It happens when the flame momentarily burns inside the tip and goes out with a popping sound. There are also other times when the flame may go out and reignite with a popping sound.
Backfires occur for a few reasons, like the cutting tip accidentally touching the work, or because a small piece of metal gets lodged in the tip orifice or the tip gets too hot through misuse. Also, using the torch in a confined space may cause a backfire as the limited surrounding oxygen is used up causing the flame to pop out momentarily.
Backfires typically don’t cause damage to the torch, but if they become repetitive, it’s time to check the settings, clean the tip or reassess your technique.
A flashback is much more dangerous. A flashback occurs when the torch flame recedes back to where the gasses mix. After a flashback occurs, the tip and torch become very hot. Black smoke, red sparks and a screeching sound will come out of the torch tip. The operator must shut the oxygen torch valve off right away, followed by the acetylene torch valve. The whole system must be fully shut down immediately, bled off and the torch should be disassembled and inspected for damage.
Flashbacks can occur for a variety of reasons, which include not matching the gas flow to the tip size, allowing the tip to get too hot or touching it to the workpiece.
WRAPPING IT UP
Once all the cutting is completed, oxyacetylene systems must be shut down properly. To extinguish the flame, turn off the acetylene torch valve first. Then turn off the oxygen torch valve. But don’t put the torch away just yet; first you need to purge the hoses.
Start that process by closing the acetylene cylinder valve. Next, open the acetylene valve on the torch and watch as the regulator gauge pointers drop to zero on both the line and tank pressure sides. Then close the torch valve and back out the acetylene pressure adjusting screw on the regulator.
Now, a similar process is required for the oxygen. Close the cylinder valve and depress the torch lever while you watch both gauges drop to zero. Then open the torch body valve to ensure the line pressure is reduced to zero. Let go of the lever and close the torch valve. Finally, back out the regulator adjusting screw and wrap the hoses around the handle of the cylinder cart.
If you’ll be using the system again before too long, you’re now finished with it. But if it will be a day or two before you need to cut again, you should remove the regulators and install the protective caps over the cylinder valves. That adds another level of protection against accidental damage. If something falls against the cylinder valves, it could cause a small leak you may not notice. That could allow the shop to slowly accumulate gas in the atmosphere. Remember, acetylene is flammable in concentrations between two and 80 percent, making it the most explosive substance you may ever handle.
This is the last instalment in our oxyacetylene series. We’ve covered a lot, but clearly we couldn’t explain everything you need to know in detail. If you use an oxyacetylene system but have never taken any formal training, you should consider doing so. Aside from reducing your risk of injury, learning good technique from a qualified and experienced instructor is priceless.
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