1. Home
  2. ⚡ Welding Forums
  3. Splitter on Acetylene tank
  4. Page 2

Splitter on Acetylene tank

A flashback arrestor works at hose pressure, not bottle pressure, and it is mainly designed to prevent non-detonating flashbacks spreading. You shouldn't get detonating flashbacks in hoses unless there is an acetylene/oxygen mix in them.

It works by sucking the heat out of a deflagration, causing it to stop propagating. The fine holes are there to give a large surface area, causing the heat to be sucked out quickly.

The holes will also be too small for a detonation to propagate in, so they also prevent detonations spreading. The gauzes on a Davy's lamp work in a similar way.

One problem with flashback arrestors, especially old ones, is that a detonation, or to a lesser extent the pressure wave from a deflagration, can cause the sintered metal bit to get either pushed out of position, or crack. The very rapid and uneven heating also can cause them to crumble.

I think (but I'm *not* sure) that modern arrestors are made to be capable of taking an acetylene/oxygen detonation or a pressure wave (and the rapid heat pulse) from a non-detonating flashback. You should at least check them after one of these events, if not replace them as standard practice - but as I said, I'm not sure about this. I'm a bit out-of-date here. Ernie?

The porous stuff in a bottle works slightly differently. There is simply too much acetylene in a cylinder (higher pressure, plus the dissolved acetylene) for the porous stuff to absorb all the heat it can produce - a flame arrestor only has to cope with a little acetylene at hose pressure.

Apart from it's role in controlling the acetone, the porous stuff first breaks the space up into little chunks to prevent detonations. Second, it slows deflagrations (but doesn't always stop them) because it physically gets in the way of the flame front, and slows heat transfer.

It has another effect in that the reacted acetylene, now hydrogen gas and soot, is kept next to the now-hot porous stuff, and more acetylene is slow to diffuse to the hot bits; which further slows, but usually doesn't stop, a slow deflagration in a cylinder.

That's not practical.

I'll have a look, always interested in old chemistry.

-- Peter Fairbrother

Reply to
Peter Fairbrother
Loading thread data ...

Ernie, you know so much about welding, yet can take in new stuff. That's admirable.

Steve

Reply to
SteveB

"SteveB" wrote: Ernie, you know so much about welding, yet can take in new stuff. That's

^^^^^^^^^^^^^^^^^^^ Agreed. Steve, how do you think he got to know so much? At the other extreme, you have people that you want to ask, "How can you know so much, when no one can tell you anything?"

Reply to
Leo Lichtman

"Ernie Leimkuhler" wrote: (clip) from your

^^^^^^^^^^^^^^^^^^^^ 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.

Reply to
Leo Lichtman

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.

Reply to
Peter Fairbrother

Duh !!!

2 C2H2 + 5 O2 -> 4 CO2 + 2 H2O (+ heat)

My apologies,

-- Peter Fairbrother

Reply to
Peter Fairbrother
4C, 4H, 10O -> 4C, 8O, 4H, 2O. Matches. No fair using quarks and imaginary particles. :-)

Martin

Mart> Peter Fairbrother wrote:

----== Posted via Pronews.Com - Unlimited-Unrestricted-Secure Usenet News==----

formatting link
The #1 Newsgroup Service in the World! >100,000 Newsgroups

---= - Total Privacy via Encryption =---

Reply to
Martin H. Eastburn

But if we are using quarks and imaginary particles, and presumably quantum foam as well, then (very) small microdetonations are going on all the time! Everywhere!

:-)

- Peter F

Reply to
Peter Fairbrother

and how could it be unfair - it's how the universe behaves, and fairness doesn't come into it.

(this has nothing whatsoever to do with acetylene safety, or welding - maybe)

-- Peter not-fair-brother

Reply to
Peter Fairbrother

:-) That is why we use brass connections - the lead shields us !

If you see blocks of 'wax' - in a wall - watch out - that is a neutron shield. Welding in some locations can be different.

Martin

Mart> Mart>> 4C, 4H, 10O -> 4C, 8O, 4H, 2O. Matches.

----== Posted via Pronews.Com - Unlimited-Unrestricted-Secure Usenet News==----

formatting link
The #1 Newsgroup Service in the World! >100,000 Newsgroups

---= - Total Privacy via Encryption =---

Reply to
Martin H. Eastburn

Aaaaurugghh! I thought it was bees!

Reply to
Peter Fairbrother
[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

Reply to
Bob Engelhardt

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

Reply to
Peter Fairbrother

PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.