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Posted by Leo Lichtman on September 9, 2008, 4:06 pm
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"Ernie Leimkuhler" wrote: (clip) from your
> descriptions it sounds like a low velocity deflagration that never
> detonated. (clip)
^^^^^^^^^^^^^^^^^^^^
Quoted from Peter Farnbrother's earliest response: C2H2 -> 2C + H2 (+ heat)
From what I have gleaned from all the discussion, this reaction goes on very
slowly in acetylene all the time, but does not reach significant
proportions below 15 psi, nor in very small spaces.
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Posted by Peter Fairbrother on September 9, 2008, 6:34 pm
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Leo Lichtman wrote:
> "Ernie Leimkuhler" wrote: (clip) from your
>> descriptions it sounds like a low velocity deflagration that never
>> detonated. (clip)
> ^^^^^^^^^^^^^^^^^^^^
> Quoted from Peter Farnbrother's earliest response: C2H2 -> 2C + H2 (+ heat)
>
> From what I have gleaned from all the discussion, this reaction goes on very
> slowly in acetylene all the time, but does not reach significant
> proportions below 15 psi, nor in very small spaces.
>
>
Acetylene is generally regarded as being stable at normal temperature
and atmospheric pressure (or 15 psi), for all practical purposes, in the
absence of a source of ignition.
It isn't quite stable however, a slow partial decomposition does go on
all the time - but very slowly indeed [1]. It's hardly measurable at
atmospheric pressure.
It isn't significant even at bottle pressures unless there is a catalyst
present, or if the acetylene is flowing, when it could cause noticeable
sooting over a long period - turbulence (and sudden pressure drops) can
make it go faster. It's still very slow though.
Small spaces, eg a sintered arrestor, don't slow a slow decomposition
though, and would tend to increase the rate slightly - it happens more
at surfaces.
-- Peter
[1] I'm looking the rate and rate constants up. but it will take a while
to get an answer back from the library. However from memory and
theoretical considerations something in the parts per million per year
range seems about right. It's very slow.
The reaction doesn't go to completion either. The reaction takes place
in steps - the first, and rate-limiting, step is a fairly well
understood dimerisation, but after that it gets complicated.
At atmospheric pressure I *think* the major products are carbon and
butadiene, and very little hydrogen is produced - but I could very well
be wrong, and I'd have to look it up to be sure.
Plus the major reaction path will change in different conditions, and
there will be sub-paths going on too - ouch!, it gets very complicated
indeed, and there are at least billions of different possible end
products, some of which have never been characterised.
Nobody knows all the possible paths or products, and they probably never
will.
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Posted by Bob Engelhardt on September 10, 2008, 9:39 am
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Peter Fairbrother wrote:
[further elaboration on an already definitive explanation of acetylene
behavior]
This has got to be one of the all time greatest usenet threads!
Acetylene behavior is one of those perennially reoccurring subjects that
gets a lot of opinions expressed & then fades from currency until its
next re invocation. Thanks to Peter, the question can be considered
"settled"! Future queries on the subject need only be referred to this
thread.
Well done, Peter, & thank you.
Bob
--
Nota for President
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Posted by Peter Fairbrother on September 10, 2008, 10:36 pm
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Bob Engelhardt wrote:
> Peter Fairbrother wrote:
>
> [further elaboration on an already definitive explanation of acetylene
> behavior]
>
> This has got to be one of the all time greatest usenet threads!
> Acetylene behavior is one of those perennially reoccurring subjects that
> gets a lot of opinions expressed & then fades from currency until its
> next re invocation. Thanks to Peter, the question can be considered
> "settled"! Future queries on the subject need only be referred to this
> thread.
>
> Well done, Peter, & thank you.
>
> Bob
>
Thanks for that; but if only it *was* settled ..
In Japan for instance, they are introducing a 9 psi hose pressure limit,
not 15 psi. I suspect that may spread.
Detonations in oxygen-free acetylene have been recorded at less than
atmospheric pressure - though that's in tubes of 4" diameter, which is
unlikely to be of significance in modern acetylene equipment, and the
deflagration-to-detonation transition (and vice-versa) limits haven't
been properly characterised.
However as an external detonation, possibly of an acetylene/oxygen mix,
might impact acetylene in a hose or manifold, the DDT isn't that
important, and ..
The 15 psi limit may well go over time, probably to be replaced by an 8
or 9 psi limit. Still enough for most equal pressure torches though.
(I won't give any actual detonation limits, partly for safety reasons,
as they vary a lot as regards hose diameter, material, condition, etc. -
partly because they don't really apply to most acetylene equipment in
practical use - and partly because no-one *actually* knows them;
especially in relation to impure acetylene as supplied, with varying
impurities and impurity levels)
-- Peter Fairbrother
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Posted by Curt Welch on September 7, 2008, 9:16 pm
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> "Curt Welch" wrote: (clip) But the more complex question is that in the
> > tank valve and high pressure side of the regulator, there is higher
> > pressure acetylene. What keeps that safe? Is it the volume issue? Do
> > they have to take care to not use passage ways and pipes which are too
> > large to keep it safe? (clip)
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^
> Yes. Please go back and read my post, which discusses the heat trasfer
> issue.
Yeah, ok, I had read yours and saved it to read again and then read the
posts by Peter and basically forgot what you had written. What you write
sounds logical and I think it's consistent with what Peter was saying.
--
Curt Welch http://CurtWelch.Com/
curt@kcwc.com http://NewsReader.Com/
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> detonated. (clip)