From: "Barry L. Ornitz" <ornitzz@dpnet.net>
Newsgroups: rec.crafts.metalworking,sci.engr.joining.welding
Subject: Re: Need to extend my Acetylene spud
Date: Tue, 12 Oct 1999 15:59:20 -0400

mulligan@advinc.com wrote in message <7tvr08+ACQ-2s2+ACQ-1@nnrp1.deja.com>...
>In article <FJGrE4.29n.0.omega-3@interaccess.com>,
>  rnichols@interaccess.com (Robert Nichols) wrote:
>
>> Maybe I'm misinterpreting what you are saying, but wouldn't the free
>> acetylene in the fitting at high pressure be in danger of detonating?
>
>Arrgh. Not this again.  OK, I'll bite.  There's lots more knowledgeable
>folks here on this topic (check back with dejanews) but the jist
>of it is that a) the gas in the neck of the tank and inside the
>fitting is saturated with acetone vapor.  Which inhibits the
>decomposition. And b) there is a mean free path length issue - ie
>if the gas is in a smaller diameter tube, there is less chance for
>detonation.


The Germans developed much of the acetylene synthesis technology during
W.W.II.  This was because of the lack of petroleum feedstocks  and the
availability of coal.  They pioneered the use of small diameter tubing, or
large tubes filled with small tubes to safely transport acetylene at high
pressures.  This is not a mean free path issue but rather a heat transfer
one.  If decomposition is initiated, heat transfer to the wall of the
tubing will be so fast that the decomposition cannot propagate down the
tube.  Today sintered metal is used to provide flashback protection in much
the same way.  I hope all the welders in the group have such flashback
protectors installed in their systems.  They cost about $25 each but a
cheap insurance in the long run.

Mean free paths approaching even the smallest capillary tubing dimensions
are only obtained at pressures well below atmospheric.

        Barry L. Ornitz     ornitzNOSPAM@NOSPAMdpnet.net

From: Gary Coffman <ke4zv@bellsouth.net>
Newsgroups: rec.crafts.metalworking
Subject: Re: This is driving me crazy
Date: Mon, 22 Nov 1999 13:12:46 -0500

On Mon, 22 Nov 1999 12:22:22 GMT, Hoyt McKagen <batwings@i-plus.net> wrote:
>I'd like to hear from someone who was cheerfully torching away and had a
>spontaneous hose exploion. My money is on it being impossible; it would
>have to have happent at fire-up. Seriously, if this happent to you, tell
>me, but don't tell me about it happening to a friend.

You already have heard it. I described the incident. The torch had been purged,
lit, and in use for 20 minutes. The mixture formed in the hose when a bit of slag
popped back and plugged the tip, the flame went out, and oxygen backfilled the
acetylene hose. It then exploded, whether spontaneously from the high pressure
of the O/A mixture in the hose, or from the hot tip, I don't know, but explode it did
about 2 seconds after the flame was snuffed (I was just standing there wondering
"What the hell?" and holding the cutting lever down).

It took out a bit over 40 feet of the 50 foot hose, shredding it. The remaining 10 feet
(paired with 10 feet of burnt in two oxygen hose) pirouetted around the tanks like a
rocket, spewing flame, until I crawled over and shut off the tank valves. At that point,
the tanks were hot to the touch. It was very close to being a real disaster. As it was,
it was just scary as hell.

My 30 year old torch didn't have check valves. It does now. If I hadn't been
dumbfounded and continued to hold down the cutting lever after the flame went
out, this probably wouldn't have happened. But it can't happen again now that
I have the check valves in place to prevent it.

Normally, the high pressure oxygen in a cutting torch is fed separate from the
heating mixture in the blowpipe. I theorize that the bit of slag capped the tip in
such a way that the oxygen could backflow into the mixing chamber, and on into
the acetylene hose. I can't prove this because the tip was blown clean in the
explosion, but it seems the most likely explanation.

This event wasn't at all like the backfire you can get when lighting an improperly
purged torch. That gives a "pop" sometimes followed by a rattling buzzing noise
as the flame burns in the hose. This was a very strong "BOOM", followed by a
roar as the hose ends played rocket around the tanks.

I don't know how long I stood there gaping, but it couldn't have been more than a
few seconds before I went for the tank shutoff valves. I've been pretty close to the
edge several times in my life, slipping on an icy tower, shaking hands with Reddy
Kilovolt in a transmitter, but when I felt how hot those tanks had gotten in those
few seconds, I realized this was much too close.

Gary
Gary Coffman KE4ZV  | You make it  |mail to ke4zv@bellsouth.net
534 Shannon Way     | We break it  |
Lawrenceville, GA   | Guaranteed   |

From: Gary Coffman <ke4zv@bellsouth.net>
Newsgroups: rec.crafts.metalworking
Subject: Re: This is driving me crazy
Date: Tue, 23 Nov 1999 10:25:28 -0500

On Wed, 24 Nov 1999 00:36:08 -0500, "kenneth knaell" <knaell@erols.com> wrote:
>Gary,
>I am surprised that the tanks got hot.  What do you suppose made them heat
>up.  Did they both get hot?
>Actually I would expect them to get cooler with a rapid discharge of gas.
>ken knaell

They got hot because that 20 foot long tongue of flame from the rocket-like
whipping of the remaining hose ends (held together at the outer end by one
of the siamese hose bands) played across them. The snakes were whipping
every which way, but they bathed the tanks in flame often enough to get them
uncomfortably hot in a hurry.

(I'm not sure the tongue of flame was actually 20 feet long. I didn't stop to
measure it. But it was long, loud, and hot. My welding jacket and gloves
got badly scorched fending off the snakes while I shut off the tank valves.)

Gary
Gary Coffman KE4ZV  | You make it  |mail to ke4zv@bellsouth.net
534 Shannon Way     | We break it  |
Lawrenceville, GA   | Guaranteed   |