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Posted By chrisroyal
If there is a backfire during oxyacetylene welding, the guidance note INDG297 specifically states that the oxygen valve on the blowpipe should be closed first, then the acetylene valve. However I've read other sources of information which state that the acetylene valve should be closed first.
Question: is there a definitive "best practice" for the sequence of valve closures, and could anybody explain the reasoning behind it? I'm just a simple fire-triangle bloke, me.
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Posted By Adrian Woon
Would it not be good practice to install flashback arrestors on both the oxygen and acetylene hoses to eliminate this potential hazard?
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Posted By SteveD-M
Chris
Firstly agree with the comment above..
Source first (oxygen bottle first then acetylene) this is in line with BCGA CP7.
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Posted By Crim
Prevention is better than cure - I would not use such equipment without flashback arrestors fitted.
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Posted By John Richards The procedure for turning the system off after a flashback depends upon whether the flashback extinguished the flame first, or not. It also changes depending upon whether the flame burned within the nozzle or hoses. See: Job knowledge for welders: http://www.twi.co.uk/content/jk27.htmlNote that the most common CAUSE of flashback is incorrect acetylene pressure at the welding torch, followed by incorrect operator procedure (pushing the flame onto the work). After a flashback occurs the operator should check that no heating of the cylinder occurs afterwards. Most users (welders/fabricators) check the cylinder for heating after use anyway, whether a flashback occurred or not.
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Posted By Anthony Edwards
If I remember correctly from when I used to teach welding (but it was over ten years ago). Flash back, and backfire or sustained back fire are different things. flashback occurring in the hoses or cylinder, which should be prevented by hose check valves and flash back arrestors.
Backfire or sustained backfire are gases igniting in the torch mixing chamber. I am sure we used to teach oxygen off acetylene off oxygen back on and immerse in bucket of water that was by welding bench.
regards
Tony
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Posted By Graham Bullough
The noise of a backfire is sudden and VERY LOUD. One occurred years ago when I was with HSE and inspecting a metal fabrication premises. Everybody in the vicinity, myself included, jumped out of their metaphorical skins. Cue prompt discussion about need for flashback arrestors and/or non-return valves, or perhaps replacing existing ones which were faulty.
I vaguely recall from a training course that one device was more reliable and effective than the other at preventing flashbacks and/or backfires, but needed to be located in the correct part of the system, probably at the torch/hose connections. However the training was almost 30 years ago, so my recollection is too vague to be reliable.
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Posted By John Richards
Flashbacks are extremely common. Especially when flame cutting. Flashback arresters will not stop a flashback, they are there to stop the flame proceeding up the hoses. They are commonly situated at the pressure regulator. Fitting them at the "torch" end of the hoses presents problem, namely that the da and oxygen hoses are adjacent to each other, leaving no room for anything else. They also are more likely to be damaged at the "torch" end of the hoses, as well as increasing the weight. In fact, some are so large that they can only be fitted at the regulator end of the hose/s.
Some are in-line, and require the hoses to be made-up so that the arresters can be inserted (yet more to go wrong or get broken).
The loud bang is also common in flame cutting, since when the flame is extinguished by accidentally "stubbing it out" on the metal then unburnt gas continues to flow, then it is ignited by the hot metal....with a loud bang....re-lighting the flame at the cutting nozzle.
It wouldn't surprise me if it was done on purpose anyway, it is SO easy to do !
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Posted By D. Hilton
Chris
Am only a simple tetrahedron of fire bloke myself but ............
A back fire is a scenario whereby a flame from the gas torch burns back into the torch and the flame can then extinguish or be reignited with no other consequence other than a pop or bang.
The scenario requiring the isolation of fuel or oxidant is a "Flash back".
I do not think that it matters which supply (oxygen or acetylene) is shut off first but best to shut off both.
For a flashback to occur, a reverse flow of either acetylene or oxygen must take place. This means that either the pressure of oxygen or the supply pressure of the fuel is greater than the other, thereby allowing one to reverse flow into the hose of the other.
This situation will result in the pre mixture of fuel and oxidant which can then be ignited by a "backfire" of flame into the torch body and or hose containing the premixed gases.
Regards
D
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Posted By John Richards
In any given situation the pressure at the hose of fuel gas versus oxygen will be widely different.
Typically, in oxygen/acetylene welding the oxygen pressure will be 10psi and the acetylene pressure 5 psi. Over 15 psi for acetylene is dangerous.
In flame cutting the pressures will be 35-50 oxygen and still only 5 for acetylene.
Flashbacks can also occur if the operator closes the valves on the cylinders at end-of-work and then opens the welding/cutting gun valves to purge the hoses of gas, a practice that happens at some workshops by ritual, and then fails to allow the hoses to be purged of AIR the next time the equipment is used....which leaves air/acetylene in the acetylene hose. An explosive combination.
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Posted By Andy Petrie
Don't assume that flashback arrestors will work, we had an incicent last year when on failed and caused a serious fire.
Even if you always have these things installed, you should still have a contingency plan in place should they fail.
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