In my welding class we are learning safety precautions for Oxy- Acetylene processes. In this discussion we talked about how acetylene tanks are filled with a porous material and acetone to isolate the acetylene because it becomes unstable when stored together above
30psi. In this class we also discussed how we used splitters on the tanks to allow the use of two regulators on each tank. I noticed that the splitter, being located between the regulator and the tank contains raw acetylene at 400psi without anything to seperate the gas. How come the acetylene in the splitter does not become unstable, and what is the maximum space acetylene at 400 psi can occupy before it becomes unstable and may combust?
We debated this same issue in class and no one knew the answer.
It's not just the splitter, it's the entire top of the tank (above the acetone) and valve and the high pressure side of the regulator which is exposed to pure acetylene at pressure way over the 15 PSI safety limit in all tanks.
We need someone that understands the chemistry that's related to the 15 psi safety limit to explain it. None of the books I've seen explained it other than to say it _can_ become unstable and explode (or burn??) at the higher pressures. They way I've seen it worded left me thinking it was a rare thing to begin with and would only happen under exactly the right (wrong!) conditions.
The best I can come up with is the idea that for it to actually be a problem, there may need to be some O2 in the mix as might happen if air got mixed in it. In the hose, you always run the risk that it's filled with air when you first turn on the acetylene or might even have some pure O2 which leaked back from the torch if there were no check valves. Maybe, the real danger is in those situations when you add acetylene to the hose under pressures over 15 PSI and it's mixed with some O2. As long as there's not (or very little) O2, it's not a safety issue??
Then there's also the issue that you shouldn't use copper pipes in an acetylene manifold system because there's some sort of chemical reaction that takes place between copper and acetylene which can also create a safety issue. Maybe the 15 PSI limit is related to that type of problem as well. That is, maybe for there to be a problem, there needs to be some other type chemical in the mix other than just 02 which causes the acetylene to decompose so as long as the tanks and regulators are made of the correct materials, the higher pressure is not a problem????
It would be interesting to find the correct answer to what the danger is for higher pressure acetylene and why it's not a danger in the tank and on the high pressure side of the regulator.
"Ben" wrote: (clip) How come the acetylene in the splitter does not become unstable, and
^^^^^^^^^^^^^^^^^^ I think I know the answer, but it is based on my own understanding of chemistry--so it needs to be verified or refuted by someone who may know better.
There is a chemical reaction in acetylene itself which liberates heat. If the if the pressure is raised the reaction rate increases, because the molecules collide more frequently, Likewise, as the temperature goes up, the reaction rate goes up. The heat produced by this decomposition reaction tends to drive the temperature up, which, in turn, speeds the reaction. If the acetylene is contained in a small space, the heat is conducted away, and a stable, equilibrium temperature is reached. If the pressure and temperature go high enough, so that heat is generated faster than it can be dissipated, the reaction rate "runs away," resulting in rapid temperature rise leading to an explosion.
Presumably acetylene cylinders and regulators are designed so the heat dissipation in the spaces you are concerned about is sufficient to prevent a runaway reaction. IOW, small volumes with plenty of surface area.
First. let me explain the difference between a deflagration and a detonation.
A deflagration is when the reaction front moves slower [1] than the speed of sound in the material - if it moves above the speed of sound it's a detonation, that's the difference between a detonation and a deflagration. When a reaction front moves faster than the speed of sound some of the energy goes into making shock waves, which can be very damaging.
[1] for acetylene slower can be anything from very slow to very fast indeed! The effects of fast deflagrations can be hard to tell from detonations, both are what is normally called an explosion. For instance, a gunpowder pipe bomb doesn't detonate, if deflagrates - but it still goes bang and fires shrapnel about.
Next some acetylene chemistry. Acetylene can burn with air or oxygen, forming carbon dioxide and water:
C2H2 + 3O2 -> 2CO2 + H2O (+ heat)
The speed of the first reaction depends on the concentration of oxygen or air, as well as the temperature, pressure and volume (we'll get into the effects of volume later). The flammability limits for acetylene are very wide, and most concentrations will detonate if the volume is large enough.
A typical example of this oxygen/acetylene detonation is a "pop" in the torch nozzle, but people also do stupid things like fill balloons with oxygen/acetylene mix - which can permanently damage your heating, or worse.
At room temperatures oxygen/acetylene mixes can explode (either detonate or deflagrate) at well below atmospheric pressure. At similar pressures acetylene/air mixes, and especially acetylene/oxygen mixes, are far more dangerous the acetylene alone.
Acetylene can also decompose exothermically (giving out a lot of heat) all by itself, with no oxygen present:
C2H2 -> 2C + H2 (+ heat)
(actually this reaction occurs in steps, and it can be reversible if it hasn't gone to completion - this is rare, but not unknown)
The speed of this depends on the temperature, pressure and volume. At room temperatures and pressures below 15 psig acetylene alone can't detonate, and deflagrations are fairly slow. However at pressures above
15 psig acetylene can detonate - this is one of the worst outcomes, so it's the one most talked about.
It's also the main reason why acetylene cylinders are filled with porous diatomaceous earth (in the UK, they use different materials elsewhere), and the acetylene is dissolved in acetone - the diatomaceous earth fills the cylinder completely, and the acetone comes about half way up. When the valve is opened the acetylene fizzes out of the acetone.
But that's not all - acetylene decompositions can also occur slowly. This is what causes burning in the torch body when the oxygen is turned off, melts the hose, and if it gets into the cylinder it starts rocking, as the deflagration continues inside the cylinder, which can explode - this usually takes some time, maybe half an hour or more.
Another example of a slow deflagration is in the regulator of an acetylene cylinder - this *always* happens to some extent when acetylene is used, caused by the expansion of the acetylene. Acetylene regulator passages are made small to limit the volume and thus lower the rate of reaction so that only a small proportion of the acetylene decomposes, and to ensure the deflagration does not become a detonation.
So don't use anything except a proper regulator designed for acetylene with acetylene!
Acetylene regulator manufacturers have another trick up their sleeves - remember I said the decomposition of acetylene can be reversible? The decomposition produces carbon in the form of soot, which is messy and can clog the hoses, flashback arrestors, etc, so they arrange things so that the partly-decomposed acetylene recombines. Clever chaps, those manufacturers!
Now on to volume.
Consider a sphere of reacting potentially-explosive material inside a volume of non-reacting potentially explosive material. The energy produced by the reacting material has to heat up let's say the next mm or so enough to make it react if the reaction is to continue. Now a sphere 1mm in diameter will have to heat up a sphere 3 mm in diameter, that's 9 times more than it's size - but a sphere 5 mm in diameter will have to heat up a sphere 7 mm in diameter, only 2.77 times as large.
The smallest radius at which a reacting sphere can heat up the next bit enough that the reaction continues is known as the minimum detonation radius (for a detonation). Below this radius the detonation can't propagate.
Now consider some explosive material in a tube, and detonate one end of it. The detonation wave will be slowed near the tube walls partly by cooling from the walls, and partly by other wall and edge effects which I won't go into. The effect of this is to curve the detonation wave so that it bulges out in the middle of the tube - and of the radius of that curve falls below the minimum detonation radius then the detonation can't propagate.
The situation is similar for different shapes than tubes, but the math is more complex!
Back to acetylene: detonations take a lot of energy being transferred to the unreacted material very quickly. Deflagrations on the other hand occur more slowly - this is why acetylene can decompose slowly at less than 15 psig eg in a hose or torch body, but can't detonate at 15 psig no matter how big the volume is.
Acetylene reacts with copper to form copper acetylide (and also to a lesser extent with silver, forming silver acetylide), which is a highly sensitive high explosive.
I forgot to mention minimum ignition energy: for acetylene/air and acetylene/oxygen mixes the minimum amount of energy required to set off a detonation is extremely small - the acetylene/oxygen reaction gives out a lot of heat, so the minimum radius is small (the ignition energy is approximately the energy required to heat up a sphere of the minimum radius).
For acetylene alone the minimum energy is quite a lot higher, as the sphere to be heated up is bigger, and the required temperature is higher
- which is why acetylene detonations are as rare as they are, even in higher pressures - but don't take the risk of relying on that!
Makers of acetylene compressors and the like do rely on the higher minimum energy, but they are experts and acetylene compressors are operated remotely, and even then they are in explosion-proof bunkers (or underwater).
Something else I might explain is why acetylene can deflagrate, but can't detonate, below 15 psig.
(the actual figure for pure acetylene is not 15 psi, but acetylene is seldom pure, and I'm not going to give any other figures for safety reasons - just keep it below 15 psi, or the rated pressure if it's from a carbide generator)
The answer lies in the mechanism whereby energy is transferred from reacting acetylene and unreacted acetylene. In a detonation energy is mostly transfered by shock compression, and to a lesser extent by thermal radiation. Shock compression isn't very efficient, and at low pressure it simply isn't up to the job.
(in shock compression a shock wave compresses the unreacted acetylene, thereby heating it up - the reaction then provides more energy to make more shock waves)
In a deflagration energy transfer occurs mostly by thermal conduction, and even mixing - these can't happen in a detonation, where the speed of the reaction front is greater than the speed of sound - and thermal radiation, but not shock compression (there are no shock waves).
Thermal conduction and mixing are more efficient, but slower than shock compression. Thus propagation by thermal conduction is possible at lower pressures than shock compression - but the result is a deflagration, not a detonation.
I'll shut up now, unless anyone has any questions.
This is all very good stuff and I feel I need to read it a few more times to really get it all. Thanks for posting it.
You talked about a lot of things that explains why we need to keep the pressure low to keep it safe. 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?
What about the guy that started this thread who is using a splitter on the high pressure side of the flow to allow one tank to feed two regulators? Is there danger in that if it's not designed correctly for that application?
Some wording I found in an MDS sheet for acetylene on the internet seemed to imply that flow rates were connected with the danger which implied to me part of the issue was controlling the amount of turbulence in the flow.
I need to re-read what you posted and think some more.
Obviously, the big point is that acetylene is just unstable stuff and wants to fall apart releasing lots energy in the process and the higher the pressure and the larger the volume, the more risk there is that it will do just that.
I've been trying to avoid answering these questions, because I don't the which answer is correct - but here goes.
Yes, the volume, or rather shape, and the materials the mainfold/bottle space is made from (bottle spaces are very small, the porous stuff almost completely fills the bottle) are important, and if small enough and the right shape, material etc this will prevent any detonation spreading. See my next email for more details.
The other possible answer is the minimum energy issue - it takes a minimum amount of energy to cause a detonation in acetylene, and it's unlikely that that amount of energy will ever be available in a manifold/bottle space.
The real answer is probably a mix of both, tending towards the first - the second is not reliable all by itself.
Two points about flow - the first point is the rate of flow from the bottle. The bottle is completely filled with porous stuff, with acetone coming about half-way up, in which the acetylene is dissolved. When the bottle is opened the acetylene fizzes out of the acetone, a bit like opening a coke bottle but more so.
The acetone is removed from the fizz by the extra porous stuff. However if the flow rate is high enough, more than 1/7 the capacity of the bottle per hour, acetone can be forced into the regulator, hoses etc, which has all sorts of nasty effects, some of which can be dangerous.
The second point about flow is flow in tubes. There are three main possible effects, the first is particulate impact, where a particle of dirt or dust gets accelerated by the flow then hits an obstacle - this can release a lot of energy locally, maybe enough to cause a deflagration or even a detonation, though the latter is unlikely.
The second effect is adiabatic heating, where some acetylene is at a lower pressure and then gets quickly raised to a higher pressure - this causes the gas to heat up. It's most common when opening valves, and is why you should open valves slowly.
The third effect is about turbulence, as you guessed. This can cause local areas to get hot, or higher pressure, or both, when opposing subflows meet. It's complicated, as it involves mathematically unpredictable ("chaotic") phenomena, but basically keep flows slow. This in contradiction to the need to keep passageways small, but the manufacturers have worked it out, usually by hard experience.
Indeed.
Though it can also release energy slowly, which is more likely to kill or injure than a fast detonation nowadays, what with all the safety stuff the acetylene equipment manufacturers do to prevent detonations.
But then you usually have time to do something about it (turn off the valve if you know it's safe to do so, eg in a hose blowback, it usually is, then get out and call the fire brigade: however if the bottle is rocking and getting hot, just run!), unlike a detonation :)
Not trying to be contradictory, but I'm going to have to be :( - it's not really heat transfer, there are other edge effects which for a detonation are usually more important.
In a detonation heat transfer doesn't happen much, the detonation is too quick - and it's detonations which are the worst result, though overheating cylinders failing is a more common cause of injury or death than detonations nowadays, due to the safety precautions built into acetylene equipment.
I'll describe one edge effect, one of the more important ones, but there are more; for instance what happens to the shockwave at the edge can be very important.
Consider a speck of explosive just in front of a flat shock wave. It gets energy from the wave from all directions - mostly from directly in front, where the wave is closest, but from the sides as well.
If the speck is at an edge of the explosive then the total energy it gets is less, as there is no energy from the direction outside the explosive, and the rate of transfer is slower - this will cause the detonation to slow, and may even be enough to cause the detonation to stop.
In a cylindrical shape like the inside of a tube this also causes the reaction front to bulge forward in the middle, and if the radius of detonation becomes too small the detonation dies out.
So it's not just the volume, it's the shape - plus the material the manifold is made of and how thick it is (this affects the way shockwaves bounce off it), and more - so don't make your own manifolds, buy proper ones.
I'm sorry if this is a lot to take in, but there is a lot that can happen!
"Peter Fairbrother" Not trying to be contradictory, but I'm going to have to be :( - it's
^^^^^^^^^^^^^^^^^^^^^^^^^ Shouldn't we be talking about the processes that occur BEFORE detonation conditions are reached?
In a cylinder sitting quietly in my shop, there is no detonation. After I open the valve, there is acetylene at a pressure well above 15 PSI in the valve, the high side of the regulator and inside the regulator up to the diaphragm-activated valve. No detonation is going on, so I don't see why you refer to the speed of the detonation wave being so high that heat transfer hasn't time to occur.
The model I am using is the classic heat balance that establishes ignition temperature. There is a family of curves in which the temperature is low, so the reaction rate is low, so the heat generation rate is low, and the gas reaches an equilibrium temperature, slightly above ambient, but it does not ignite. If the temperature is raised, the reaction rate is raised, so more heat is generated, and a new, higher equilibrium temperature is reached. At some temperature, the heat generation goes high enough to cause the temperature and heat generation to rise more than the heat dissipation can handle, so the reaction "runs away." Ignition. The family of curves branches--the low ones stay low, and the ones that attain the ignition temperature go nearly vertical, as combustion takes place.
Even with all of the effects you cite inside a space filled with acetylene, is the one I am describing not applicable at all?
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.
The part I'm having trouble following you on is that you are talking about what happens after a detonation gets started. You talk about how things have to be just right to keep them going, with the idea that if you design the system correctly, detonations won't keep going. And that's all fine and good. Better to have a small pop than an explosion that kills everyone and burns down the shop. But the real question here is what causes it to start in the first place?
Are you saying that it's normal for micro-sized detonations to be starting all the time, but they instantly die out or something like that?
Leo's answer makes sense to me because he was addressing how we keep the parameters from reaching the point where the detonation would start in the first place which seems to me are the most important aspect here.
The most obvious example of an existing detonation coming into contact with bottle pressure acetylene is when there is detonating acetylene/oxygen mix in the tube and the arrestor (and then often the regulator) fails or is absent. Acetylene/oxygen mixes detonate very easily even at atmospheric pressure. Note that there is not likely to be an acetylene/oxygen mix in the bottle-pressure parts.
The *overriding* safety requirement for manifolds and bottle spaces is to prevent detonations in bottle pressure acetylene spreading - detonations reach much higher pressures, and are much more destructive by shock effect, than even very fast deflagrations.
Stopping fast deflagrations (which thermal transfer to the walls can sometimes do) would be nice, but even in professional equipment it doesn't always happen - this is usually how bottles start a slow deflagration inside, which is the most common cause of damage, serious injury and death from acetylene equipment nowadays - but only because detonations are prevented from spreading.
On reflection, I'm not so sure it's the most common cause of death - it is the situation which is most worrying to the fire brigades, and I maybe wrongly extrapolated that to it being the most common cause. I don't know the actual statistics.
The fire brigades won't be there when a detonation occurs due to bad equipment - it's almost instantaneous, and you're probably dead long before they arrive. Whereas a slow deflagration inside a bottle can take hours before it causes cylinder failure.
Is the real reason for keeping the pressure below 15 psi is that prevents the possibility of an external ignition source causing it to detonate? That is, it will only burn at that point and likely do less damage to the people around it? That is, as long as it's pure acetylene because once you mix enough air or o2 with it, it can detonate even at the low pressure?
So at higher pressures, there's no real risk of it exploding on it's own, but the real danger is that the gas is in the range where it could detonate instead of just burn if something were to trigger it (like a flash back). So, when you have high pressure acetylene such as the valve and regulator, the major design concern (for safety) is to keep the volume small enough so that a detonation can't spread turning it into a fast deflagration at worse?
And the porous material in the tank doesn't just keep the acetone from bubbling out, but it also reduces the volume and density of the acetylene to the point that a detonation won't spread through the high pressure tank?
So even though there might normally be some reaction tacking place in the pure acetylene (as Leo was talking about), that alone won't produce enough heat to start a fire of any type? Or at least, once you design the system to limit a detonation, you also end up with a system that can't possibly produce the run-away heat effect Leo talked about?
Yes. also deflagrations are not self-sustaining in acetylene at 15 psi without a flame holder or hot spot.
Yes. That is, as long as it's pure acetylene because once you
Yes - but a low pressure detonation is far less dangerous than a high pressure detonation, as there is more gas in a high pressure detonation.
Not entirely correct - there is a risk of it detonating without a detonation causing it - and it's a big risk, almost certain, if the shape is big.
Yes.
Sort-of. The porous material prevents detonations, It also prevents fast deflagrations - only slow deflagrations are possible inside tanks which are properly filled with porous stuff, this *is* due to thermal transfer effects.
But the individual pores are very small, much smaller than typical passages in acetylene equipment.
I haven't heard of this happening to any significant extent without a catalyst present. Then it can form carbon nanotubes, polyacetylene (which is a polymer which conducts electricity, but unfortunately is unstable in air), soot and hydrogen, or various other things, depending on catalyst and conditions.
Or at least, once you design the system
A detonation-proof system can still have deflagrations.
I don't know about the runaway effect, it's not something I have heard of happening in relation to acetylene without a catalyst present, though it does happen in other things.
Yes it will happen to some extent in acetylene, but I don't think it's very significant as a source of ignition for today's acetylene equipment. Plus I'm not entirely sure what he meant.
I intend to reply to reply to Leo's post in detail, but it will take some time (which I haven't got a lot of right now) for research.
ps if you want a reply, please send to me as well, as I don't normally follow this list
I was wondering if the sintered metal in the flashback arrestors worked similarly to the porous filler in the acetylene bottle -- working by disipating the energy from the flash in a torch head preventing it from propagating down the hose. Same with the metal gauze on the designs for carbide-acetylene coal miner's lamps. (Of course there's no O2 in the bottle (hopefully), but the porous filler would still disipate energy.)
Maybe you have to appeal to the Ideal Gas law PV=3DnRT and limiting any of the variables in the equation is a way of limiting the energy that could set off an explosion --- controlling the thermodynamics.
Links I've found on the web for this topic are pretty old (turn of the century), but I'll bet the chemistry that they explain is still sound.
1911 Encyclopedia Britannica entry for =93acetylene=94:
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Longer text from 1909: Acetylene, The Principles Of Its Generation And Use
All this talk about deflagrations makes me wonder about something I found a few years back. While teaching at South Seattle we got all new regulators for our Oxy/Acetylene welding manifold. This manifold was inside the classroom and ran 12 gas torches. Outside it was fed by a manifold of large acetylene and oxygen cylinders.
When I pulled all the old acetylene regs off the pipe I was horrified to find a mass of carbon inside the pipe. I thought this meant they must have had a flashback, but from your descriptions it sounds like a low velocity deflagration that never detonated.
Here are two other possible causes of sooting in a manifold.
First, very slow decomposition, perhaps catalysed by the wall or the soot - what Leo talked about. This does happen, but it's not a deflagration, there is no flame front, and apart from making soot it isn't particularly dangerous - at bottle/manifold pressure, unless the catalytic action is extreme, it's most unlikely to cause ignition.
If acetylene was stored at high pressure, say 3,000 psi like oxygen, then this slow decomposition would be a very significant concern as a cause of ignition or runaway reaction (as it is in eg ethene cylinders)
- but at the lower bottle pressures used by dissolving it in acetone it is not likely to run away or cause ignition.
This is one reason why acetylene is stored in solution to give lower bottle pressures - the main one however being that, once ignited by whatever means, high pressure acetylene makes a very effective explosive, and preventing the transition from deflagration to detonation is very difficult indeed.
Because there is many times more acetylene in a space at 3,000 psi than at 250 psi, the amount of energy in such a space is many times higher. I looked for a figure for "many", but there is little data available for acetylene at 3,000 psi - I wonder why :) Say 12 times as a first guess, I can't be bothered to do the exact calculation.
Even if detonation was somehow prevented, the pressure caused by a fast deflagration in 3,000 psi acetylene would be at least 10,000 psi
(by extrapolation, a detonation in 3,000 psi acetylene could create shock transient pressures in excess of 1 million psi - however as far as I know no-one has actually measured it).
At bottle pressure however, a fast deflagration (eg a non-detonating flashback) won't reach more than 1,100 psi, and in practice it would be less, and not last long - a manifold ought to withstand that. The real danger of a non-detonating flashback is if it gets into the bottles and turns into a slow deflagration.
A bottle pressure detonation on the other hand would have significant amounts of energy at a pressure of around 3,000 psi, with possible shock transient pressures of 50,000 psi or more. These tend to break things.
(the total energy release is the same for a deflagration and a detonation, but a detonation is much faster - in a deflagration the pressure rises as a slowish pulse, in a detonation it rises very rapidly and to higher, sometimes very much higher, pressures)
If both fast deflagrations (flashbacks) and detonations sound a bit like explosions, it's because they are; ?An explosion may be defined as a loud noise accompanied by the sudden going away of things from the places where they were before.?
I digress.
A second possible cause is that acetylene tends to partly decompose when the pressure is changed quickly, as in regulators, perhaps when a new cylinder is connected to a manifold, or if the cylinder valve is only a little open and the acetylene is flowing fast.
Generally regulators are arranged so that the acetylene recombines (mostly to avoid soot production clogging them), but this may well not be true (or even possible) for a manifold.
Again it's not particularly dangerous, apart from clogging issues - there is no flame front unless the pressure transition is very abrupt indeed.
The usual advice is to open cylinder valves slowly, one-and-a half turns, no more, no less. If opened fast it can cause adiabatic heating: if opened less than one-and-a-half turns the pressure change can cause sooting; if opened more, the opening is more likely to let a deflagration get into the bottle.
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