Your Oxygen tank is very dangerous. You could get a flash fire
with a leak. If your work area is on fire, the fire department
fears your O2 tank more than the acetylene.
A leaky oxygen tank won’t START a fire. it only will make an
existing one burn hotter and faster. You can leak oxygen all you
like, even with an ignition source like a pilot light or spark, and
unless there is some fuel already there to catch fire from that
ignition source, the oxygen won’t have much effect, other than
perhaps to make a pilot light burn hotter too. The big danger from
an oxygen tank, as opposed to an acetylene tank is the pressure.
Oxygen tanks, when newly filled, are sitting there with over 2000
pounds per square inch pressure. If you catch one in a fire that
might cause it to burst from yet increased pressure from the heat,
then the result is not a ruptured tank, but the release of all that
pressure. Might as well have a bomb go off. The tank explodes. This
would be true even if the tank were inert gas. Now, if the cause of
an oxygen tank exploding is an existing fire, then not only do you
have that explosive force being release, but it will then suddenly
and dramatically increase the ferocity of the existing fire. The fire
department thus has good reason to be concerned with an oxygen tank
that’s in the neighborhood of a fire. But this is very different from
any risk posed by a leaky oxygen tank. A small leak in an oxygen tank
is not likely to do very much unless there is an existing fire
already. By itself, it doesn’t start fires. The atmosphere does not
normally contain anything that will burn. So a pilot light, for
example, might burn quite bright and hotter than normal, but with no
harm done, since there may not be anything flammable in the vicinty
for that now hotter pilot light to ignite. With higher concentrations
of oxygen in a room, such as you might get with a really rapid leak,
then things that are flammable will be easier to ignite, but they
don’t automatically (with a few exceptions) ignite on their own.
Now in the case of a fuel gas like acetylene, the situation differs.
If an acetylene tank leaks, and lacking ventillation is able to build
up a concentration of gas in a room, then by combining with the
oxygen already in the atmosphere, it can be explosive. Any ignition
source, such as the small spark when an electrical switch is thrown
or a motor runs, not to mention actual flame sources like pilot
lights, could then ignite an explosion. But even this is not always
easy to arrange, since acetylene dissipates quite rapidaly, and if a
room has any sort of decent ventilation, then it will be difficult to
get a sufficient concentration of acetylene in the air to exceed the
minimum amounts needed for ignition. And while logic might suggest
that an acetylene tank would be very dangerous in a fire, again the
oxygen tank in that situation might exceed the risks of acetylene,
for two main reasons. One is that the degree of combustion or
explosive force that can occur is limited not just by the amount of
fuel, but also by the amount of available oxygen. If you’ve got a
nice hot building fire going, enough to rupture an acetylene tank,
then the existing fire may already be consuming much or most of the
available oxygen. Adding more fuel, even in the form of an explosive
fuel gas like acetylene, might not actually accelerate the fire as
much as you’d think, since it can only burn at a rate determined by
the amount of oxygen it can combine with, and any oxygen it burns
with is then taken away from the oxygen available to take part in the
original fire. Certainly, it would not be a good thing, but it might
in situations of a close environment like an indoor fire, be somewhat
self limiting. Recall that oil well fires are often put out by use of
explosives, that when detonated use up all the available oxygen at
the well fire, thus depriving the oil of the oxygen it needs to
burn, actually putting the fire out with another fire (the
explosion).
The other thing to keep in mind about acetylene tanks is that,
unlike oxygen tanks, they are not under high pressure. You might
rupture them in a fire, releasing all that fuel gas, but they’re not
going to quite so violently explode from the pressure in the tank
because the acetylene is stored in the tank by being in solution in
liquid acetone, rather than being compressed to high pressures. This
also limits, somewhat, the rate at which the acetylene would be
released in the event of a rupture, since it has to come out of
solution. That’s still pretty fast, but not the equal of the explosive
release of an oxygen tank bursting.
While any fuel gas container has it’s safety risks, all told,
acetylene is not that bad, at least not as compared to one specific
danger with propane, another popular bottled fuel. Propane is also
stored as a low pressure liquid, so again, the high pressure of the
tank is not a risk. But the sheer volume of propane that fits in a
tank is much more than an acetylene tank can hold. Plus, and
importantly, propane not only dissipates in air at a rather slow
rate, but it is also heavier than air, tending to sink to the floor
and “pool”. That means that a leaky propane tank might build up an
explosive level of gas at floor level, with only modest amounts of
gas at “nose” level, and since things like furnace and water heater
pilot lights tend to also be at floor level, basement workshops run
the risk of a small propane leak being able to build up an explosive
mix at floor level without being noticed in time, and the result can
literally “raise the roof” and redecorate your yard with the remnants
of your house… Add to this potential that many propane tanks are
fairly cheaply made affairs, easily refilled at lots of filling
stations where the tank may or may not be checked for condition (You
think the kid at U-Haul refilling your propane tank knows what to
look for?), a tank can sit around a workshop sometimes for years
between fillings, getting rusty, maybe developing a leak…
For my money, I’d rate the 20 pound propane bottles as the higher
danger between acetylene, oxygen, or the propane tanks. Just my
unscientific opinion.
Now, that said, for the record I’ll also mention that my own
workshop torches are fueled by a propane tank, exactly as warned
against here. I’m very careful to be sure the tank, when refilled
about every year or so, is new or in new condition. I have both the
tank shut off valve and secondary shutoff valves between the tank and
the torches, reducing the risk of a leak. And I installed a propane
detector next to where the tank is stored, hooked up to my alarm
system, so if the detector sees a leak, I get a warning long before
it could reach an explosive level. Is this totally safe? No. But I
like it better than running the propane line in from an outdoor
location, given the wet and rusting nature of Seattle’s climate in
winter. I might be better using bottled natural gas instead of
propane, since natural gas is not heavier than air, and dissipates
faster. The main thing, for safety, is that I’m very aware of the
dangers of this system, and take great pains to be sure it’s working
properly, shut off completely when not in use, and all the rest. I’d
be happer if the house had a natural gas line I could use, but it
doesn’t. Ah well. Nothing in life is totally without risk, but I feel
I’ve minimized that risk in my own case. Still, I’d hesitate to
recommend this setup for anyone else, since I’ve no idea whether they
would also be as paranoid and careful with the system as I am.
Oh, and as for my oxygen tank, it frankly doesn’t worry me too much.
Not like the Propane does. It’s securely chained in position, so it
cannot fall over (a common cause of accidents with oxygen tanks,
including a cause in those building fires when whatever supports the
tank burns and fails, allowing the tank to tumble and crack open the
neck…)
cheers
Peter Rowe
here’s two more