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Reliably Stopping a BG Backfire

Fire (combustion) requires three things to ‘burn’.
1. a fuel source (in this case, hydrogen)
2. oxygen
3. heat (including ignition energy)
In this case we are dealing with a gas (Brown’s Gas, HHO, BG, HydrOxy, etc.) that IS a perfect (stoichiometric) combustible fuel mixture (of hydrogen and oxygen) so fuel is no problem.
The BG can be ignited with static electrical spark or by compressing or by raising it’s temperature or with a shock wave or with something as simple as inserting a sharp point into the gas (or a combination of the above).  
Once ignited, the combustion provides it’s own heat to self propagate.
Note: When igniting a torch that is using BG as a fuel, the gas coming out of the torch orifice MUST exceed the open air burning velocity of the BG flame (about 22 fps) or the flame will burn back into the torch itself.  As long as the gas velocity is coming out of the torch faster than the BG can burn back, you are fine.
Once a BG flame is burning in an enclosed area it accelerates to detonation velocities (> 5000 fps) and the resulting ‘bang’ sounds like a rife shot.
NOTE: The BG really isn’t a ‘thermal’ flame as much as an ‘electrical’ flame.  Releasing it’s electrons into whatever it contacts… Which is why a tiny flame that measures less than 300°F with an infrared thermometer can melt tungsten at > 5000°F.  
The point is, if you have inadvertently allowed your torch flame to enter your torch, you are now trying stop the combustion of a high velocity (shock wave) high energy (electrons) gas.  
To stop the combustion of any flame you must remove one (or more) of it’s ignition requirements.
In our case, removing the fuel or oxygen is out of the question, because both are inextricably mixed together.
So our only option is to remove the heat and shockwave.
I’ve experimented (since 1986) on various ways to reliably stop this combustion.  
The traditional torch check valves are useless.  They are meant to stop combustible gasses from mixing.  BG is already mixed… And the BG is burning far too fast for a mechanical device to respond before the combustion is already past.
The traditional torch ‘backfire’ screen (designed to suck the ‘heat from the flame and thus ‘quench it’ is useless because the high energy combustion simply vaporizes the fine screen and the shock wave goes right through it in any case.
Even a tightly closed solenoid valve will not stop a BG backfire once the detonation reaches it’s full velocity (in about 2 feet of hose) because the shockwave goes right through the solid metal and ‘ignites’ the BG on the other side.  
Imagine my surprise when I learned that fact as I was designing systems for automatic shutoff of shapers (CNC controlled torch tables).  I did solve that issue BTW, using an ‘air purge’ apparatus.
Metal wool is a common solution to stopping BG backfires because it does absorb both heat and shockwave… But it is NOT reliable and is inconvenient.  You can increase reliability by making sure the wool is wet, so it absorbs more heat and shockwave.  But for it to work, it needs to be compressed tightly and very wet, so often restricts the flow of the BG.  Also, fine metal wool is FULL of sharp points and edges which can ignite the BG and finally, BG often has some electrolyte come out with it, which eventually plugs up metal wool filled backfire arresters.
And FYI, while the BG flame is MADE from water, it does not ‘burn under water’ any better than any other torch fuel gas… In fact, if you try it you will almost certainly cause your torch to backfire because the water mass and pressure will slow the gas velocity coming out of the torch orifice enough that the flame will ‘burn back’ into the torch.
So… The ONLY reliable and easily maintained backfire arrestor I’ve discovered (some of which are still operational since 1986) are liquid bath (usually water) bubblers that separate the gas into fine bubbles and bubble up through at least 4 inches of water.  Such bubblers reliably absorb both the heat and shockwave of the backfire.
NOTE: even a poorly designed bubbler will not work because the BG shockwave can ‘jump’ from bubble to bubble, thus traveling down through the liquid and past the bubbler.  This is why you must have fine bubbles, and enough water depth so that both the heat and shockwave are reliably dissipated.

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