[TheForge] old propane tanks

[Bruce Freeman]

I have no first hand experience cutting into a propane tank, but the
idea of filling it with water first strikes me as good.  Water would
displace all the air and any free flammable gas, and, in addition,
provide a tremendous heat sink for any sparks.

There's a lot of rumors circulating about cutting into tanks.  I don't
know which to believe.  I suspect that most of the explosions reported
were due to ignorance of "flammability limits".

Take a gasoline can, for example.  Your typical gasoline can cannot be
emptied of every last drop of gasoline.  There's a reason for this.
As long as there's liquid gasoline present in the can, the vapor in
the can is too "rich" to burn.  Once that liquid all evaporates, the
vapor can become more "lean" and may enter the flammable range.

The flammable range is different for every different chemical.  For
acetylene and for hydrogen gas, the range is wide, making these gases
relatively dangerous.  For most hydrocarbons, the range is fairly low
-- which is why lawnmowers never want to start -- because they have
too much or too little gas to ignite in the cylinder.

There was a report recently on this forum of a soap barrel exploding
when cut (torched?).  I've been thinking about this and my only guess
is that the heat of cutting evaporated some soap (which is
combustible) and gave rise to a flammble concentration within the
barrel.

In principal, excluding air would entirely prevent such explosions.
This is not 100% reliable for two reasons -- the difficulty of
excluding all air, and the possibility of some other oxidizer being
present.  As I noted above, water not only excludes air, but damps any
possible ignition by carrying away the heat.

I'm not going to tell you what is or isn't safe when it comes to
cutting propane cylinders, but I hope this little dissertation might
help you see some of the possible dangers.

On Mon, Sep 26, 2011 at 12:26 PM, Ben Barrett <stircrazyben at gmail.com> wrote:
> I don't have that experience, in great part b/c of the danger;
> I've only ever heard of filling them with water prior to any cutting or
> dilling.
> Just in case there's any confusion.  Many of the DIY hang drum plans call
> for used propane tanks, IIRC, so you may find some more details among that
> info :)
>
> ~Ben
>
>
> On Mon, Sep 26, 2011 at 9:05 AM, Lucky7Steel <lucky7steel at gmail.com> wrote:
>
>> In an unrelated discussion about Propane ...
>>
>> Does anyone have experience welding or cutting old propane tanks? i have
>> been collecting a few empties but i'm a bit scared still to cut into them!
>> any tips?
>>
>> ~karen~
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-- 
Bruce
NJ