I've never seen a flash arrestor on a torch. the things on torches are
The function of a flash arrestor is to cover your arse after you've
screwed up. The screw up is to allow gas to mix in the hoses, which can
then ignite. The arse-cover is to prevent such a (scary) flashback
getting into the regulator or cylinder (fatal). There's little point in
putting a flash arrestor at the torch end, because there's no "flash" to
arrest - the torch has a thin wire mesh in there anyway and that's
enough to stop the minor pops from a light-back to a dirty mixer.
The non-return valve on a torch is to stop the mixed gas getting into
the hose in the first place. These have to be at the torch end, because
there's no point in putting them upstream of the hose - they'd still
allow gas mixing inside the hose. As they're intended for torch use,
they're only made in one direction too - put them at the regulator and
they'd simply be the wrong way round.
On Sat, 21 Jan 2006 21:09:00 GMT, firstname.lastname@example.org (Speechless) wrote:
Thanks. I'm in the UK - we rarely see Victor kit and I've not seen this
The idea of "built in flashback arrestors" strikes me as merely
pointless and just making the torch clumsy to handle. I don't like what
little Victor kit I've seen and (like much American gas welding kit) it
seems to me poorly designed and with bad ergonomics, compared to the UK
kit. It looks like 1930s-1950s kit, with similar balance.
As to the claim "No need for acessory flash arrestors", then I'd regard
that as dangerously misleading. The arrestors in the torch will do
_nothing_ to improve safety from a flashback in the hoses, and that's
the big accident you have to worry about.
If you are wise, you will have two sets of flash arrestors installed:
AT THE TORCH: This is to prevent the flame front from working its way
back into the hoses at the torch.
AT THE REGULATOR: This is to prevent the flame front from working its
way into the regulator via a SEVERED HOSE.
To "save money", people economize by installing only one set of flash
arrestors. If you have only one set, then the logical place to have
them is where the flame is: AT THE TORCH.
Most people operate their torches with the tanks placed at arms
length, which means that the hoses are not uncoiled and placed on the
floor where they might be severed by a falling, sparking chunk of
steel so, the compromise between cost and safety by not having flash
arrestors at the regulators is deemed to be acceptable for the average
I, personally, do not go near a torch unless it has flash arrestors
installed both at the torch and at the regulators -- my life is worth
more than the cost of an extra pair of flash arrestors. Just my
On Mon, 23 Jan 2006 20:17:15 GMT, email@example.com (Speechless) wrote:
Why would a flame from a burning acetylene hose propagate back through
the regulator ? Nor will a rubber hose burning in oxygen.
Acetylene burning in air is just not an energetic reaction, in
comparison to a rapid explosion of acetylene and oxygen mixture. Nor
will pure acetylene burn. If your acetylene is at a high enough pressure
to decompose on its own, then that's a different hazard.
No, this is a misunderstanding of what the hazard is.
The primary function of flash arrestors is to stop the _energetic_
explosion of a mixed gas explosion in a hose from propagating into the
regulator and cylinder. It's not about a flame from a correct gas mix
magically travelling back along an unmixed hose
Check valves at the regulators are pointless. They are unnecessary there
(why would they be?) and they cannot provide the first-line safety
precaution of avoiding gas mixing in the hoses.
Gas passages in a torch body are deliberately small, so as to limit the
total energy possible in a torch-body flashback. Obviously this is not
practical in long hoses -- one of our most important demands on safety
is to avoid mixed gases getting into a hose. Torch check valves are
important here, as is hose purging on startup.
Cats have nine lives, which is why they rarely post to Usenet.
You are correct in what you say for combustion.
"Flame front" is a term often used by those who study the processes
that take place at the fireball boundary in what a layman would call
an "explosion". If a mechanical shock to a gas in a confined space is
created, such as could happen if a sufficient amount of hose were
impacted with sufficient force, an instantaneous pressure wave within
the hose could be generated from the kinetic energy imparted. That
pressure wave could compress the gas, in this case acetylene, and
cause a localized spontaneous decomposition. The pressure wave,
initially generated by kenetic energy, is then sustained by chemical
energy derived from spontaneous localized decomposition as it
propagates down the hose. This is not spontaneous decomposition of
acetylene per se because the pressure wave acts on the gas
mechanically to initiate decompostion by compression as the process
proceeds down the hose.
I don't know if that clarifies it or not; however, I think we both
agree that a flash arrestor should be installed at the regulator.
British practice is to install check valves at the torch and one set
of flash arrestors at the regulators. The argument for having flash
arrestors at the regulators is to prevent flashback in the hoses (no
matter what caused it to be in the hoses) from propagating into the
regulators and into the tank. This does not address safety issues
arrising from leaving hoses unprotected from torch body flashback.
North American practice is to install one set of flash arrestors at
the torch and check valves at the regulators. (The opposite of
British practice.) The industry argument (not mine) for having flash
arrestors at the torch is to prevent torch body flashback from
propagating into the hoses, with the reasoning being that if the hoses
are protected from torch body flashback, the absence of flashback in
the hoses will also protect the regulators. Some consider this
reasoning to be fallacious because the problem of flashback in the
hoses being initiated by sources/conditions not attribuatable to torch
body flashback is not addressed.
In short, neither the British practice nor the North American practice
is perfect so, the evolving consensus on both sides of the pond is to
recommend that flash arrestors be installed both at the torch and at
the regulators, with the British appearing to be more vociferous about
it as evidenced by the fact that this recommendation is beginning to
appear in government sponsored safety publications in the UK.
Any torch I own has a set of flash arrestors installed both at the
torch and at the regulators.
The industry argument (not mine) in North America is that check valves
block the reverse flow of gas, so therefore, it doesn't matter which
end of the hose they are on.
My personal opinion is that British practice is more sane and that
check valves should be as close as possible to the location where gas
mixing can be initiated, which means at the torch.
Any torch I own has check valves installed at the torch.
Cats have nine lives because they always purge their hoses...not in my
Is there any drawback, apart from extra cost and extra physical space,
to having combination flashback arrestors and check valves at both ends
of the hose? For instance, would it introduce enough extra constriction
in the hoses so that the gas pressure drops too much between the
regulator and torch?
Yes, provided you also have a set of flash arrestors installed at the
torch. (See my other message in this thread.)
Please understand that we are talking about flash arrestors here,
which should not be confused with check valves. You also need a set
of check valves installed. The ideal location for check valves is at
the torch but, in the interests of making the torch less cumbersome,
check valves are commonly installed at the regulators.
a) Try to find a torch you like that has both the flash arrestors and
check valves built in. These tend to be more compact but, not
necessarily lighter in weight.
b) Try a torch design that might be more ergonomically suitable for
what you are doing. I've heard a lot of favorable comments about the
Dillon/Henrob/Cobra Torch from long term users but, haven't used it
myself. People who spend a lot of hours per day with a torch seem to
like it. Have a look at:
Flash arrestors are designed to permit the flow of gas in one
direction only and to block the flame front from travelling in the
The only difference is whether there is a male fitting or a female
fitting on the input side or output side of the flash arrestor. You
CAN NOT remove a set of flash arrestors attached to the torch and
mount them on the regulators (or vice versa) because this would orient
them the wrong way and leave you with NO PROTECTION whatsoever.
If you are not familiar with torch plumbing, it would be wise for you
to take it in to a factory authorized dealer or repair center licensed
to modify gas torch apparatus to ensure that whatever it is you want
done, is done properly. Unlike with bathroom plumbing, there is a lot
more to know about torch plumbing than simply how to operate a
wrench/spanner on some pipe fittings. You do not get a second chance
to fix torch plumbing errors.
Official British version of how to work safely with acetylene:
The article contains some interesting photos of what happens when an
acetylene tank blows up.
Note that, unlike in North America, they recommend check valves be
placed at the torch, and when one set of flash arrestors is in use, to
place the flash arrestors at the regulators. In the event of a flash
back, this protects the regulators and the tanks but, leaves the hoses
unprotected. In the event of a flash back, this setup will prevent a
lot of property damage, but does nothing to protect the torch operator
from exploding hoses that can permanently maim, in the end, perhaps
making the operator wish that the tank did explode and take him out
To their credit, they too, advise using flash arrestors at both the
torch and at the regulators.
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