If you shut off the fuel first, there's a good chance the torch will backfire and the flame can travel backwards towards the regulator. The backflow preventer is supposed to ... prevent this.
The same thing can happen if you turn up the oxygen too high. Not allowed in my shop.
Quote:
How to Properly Light, Adjust and Shut Down an Oxy–Acetylene Torch
Lighting, adjusting and shutting down an oxygen-acetylene torch is easy – especially when following the proper procedures. In addition to these easy to follow instructions, you must always be sure to follow the torch manufacturer's operating procedures.
First - before you attempt to light the torch follow these checks:
Make sure regulator pressure adjustment screws are backed out!
Make sure torch valves are closed!
Stand away from front of regulator
Separately and slowly open the oxygen and acetylene cylinder valves
Adjust regulator p/a screws to tip pressure settings
Open/close torch valves separately and fine tune pressure settings on regulators
Depress cutting lever and adjust pressure if necessary
Lighting and adjusting the torch (with a positive/equal pressure mixer):
Separately purge both oxygen and fuel gas lines
Open fuel gas valve 1/2 turn
Ignite flame with striker
Increase fuel gas flow until flame leaves end of tip and no smoke is present
Decrease until flame goes back to tip
Open oxygen valve and adjust to neutral flame
Depress oxygen lever and make necessary adjustments
Shutting down the torch (with a positive/equal pressure mixer):
Close oxygen torch valve
Close fuel gas torch valve
If the torch/regulators and gases are done being used for a while, follow these procedures:
Close oxygen and fuel gas cylinder valves
Separately purge oxygen and fuel gas lines
Make sure all regulator gauges read 0
Back out regulator pressure adjustment screws!
If you are using in a commercial environment, report any damage, etc. to your supervisor
The procedures for using a Harris torch with an injector mixer for alternate fuels vary slightly from the information listed above. These procedures can be found in our alternate fuel section. There you can also find informative procedural and application videos.
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How to Properly Light, Adjust and Shut Down an Oxy–Acetylene Torch | The Harris Products Group
Also, no lubrication allowed on the fittings or o-rings that could come in contact with the O2, and never use acetylene unless the tank is vertical and has been that way for at least a half hour.
Otherwise the acetone can hit the torch which on a good/bad scale is way down at the bottom, near the bad mark.
And never over 25 psi on the reg. I usually don't go over 15 psi.
