I understand that oxygen speeds a fire.
So, lets say you have a garage and two cans of gas, one oxygen and one acetylene.
And one falls over and knocks its cap clean off.
And there’s a pilot light on the water heater.
Will the fire be as bad if it’s the oxygen tank that tips?
Oxygen doesn’t speed fire up, it is a necessary ingredient of fire. Fire requires an odxydizer (oxygen usually), fuel, and heat. In your example, the acetylene gas is the fuel. It will burn fine on its own because there is oxygen in the air. If you add a high concentration of more oxygen by releasing the oxygen tank, then the fire will burn much hotter and faster than without it. This will cause the fuel to be used up at a much faster rate, and the higher temperatures will cause the fire to spread faster, possibly even causing an explosion.
Hmm… probably depends on the construction of the water heater.
you ordinarily need three things for combustion or explosion… fuel, oxygen, and spark/heat.
the pilot light represents a spark. Oxygen is already available in the air, though the more concentrated variety in the can is more intensive than the 20% oxygen, mixed with nitrogen (which has some flame retardant effects IIRC) in the air.
So, of the can of acetylene (fuel) is knocked over, you’ve got a nice new quantity of fuel, spreading towards the spark, and oxygen in the air.
If the can of oxygen is knocked over, it might make the gas fuel in the water heater burn hotter, but the heater already has a regulator to supply only a little fuel at a time.
UNLESS… the flare-up from the pure oxygen is enough to damage the regulator so it starts spewing out natural gas fuel at high pressure, or some other fuel (like your basement floor) catches fire too. But in general I would say in this situation the acetylene is more dangerous.
I tend to disagree; certainly a major leak of acetylene is very dangerous, because it ignites and burns easily, but a major oxygen leak could easily be worse; lots of everyday things we normally think of as non-flammable suddenly become fuel sources (or explosives) in a high-oxygen atmosphere; the extra oxygen might even make it possible for the metal parts of the water heater to catch fire.
That pile of oily rags in the corner, that sack of damp charcoal briquets, oxygen can easily turn them into roaring blazes.
If the cap is knocked off the oxygen bottle, it will become a lethal missile, you really do not want to be in the vicinity, even a fairly small canister will propel itself at high velocity, as would the acetylene bottle.
Very likely a full bottle, such as used in Oxy-Acetylene pairs will rip straight through a plain brick wall, no problem whatsoever, if it does not hit anyone inside, then chances are it will when it ends up outside so the issue of confined gas will not arise, just a heavy gas propelled torpedo causing a serious public hazard.
Uncontrolled oxygen release in a closed environment could be extremely dangerous, greases, oils solvents and many other materials could spontaneously combust or, worse, explode.
An acetylene leak is not good, that’s why you would never store it in an unventilated space, or one that is lower than the surroundings, and particularly not anywhere near an unenclosed flame such as a pilot light, but the oxygen has a lethal capacity that is as great, or even greater still.
Actually, no.
You need an oxydizer, but it does not need to be oxygen. Fluoride and chloride (the elemental molecules, not the salts that contain the elements, your drinking water and your toothpaste are fine) are better oxydizers than oxygen… just less common ones.
Oxygen is a funny thing. Most molecules are “singlets”, meaning their most stable (and most common) version at room temperature and 1atm of pressure is the one where all their electrons go around in pairs. For oxygen, the common version is a triplet (two of the electrons are unpaired): this is the reason that 20% oxygen in regular air doesn’t just burn stuff up. If the common version of oxygen was the singlet, we wouldn’t exist. There is still some singlet oxygen at room temperature, but it’s very little. If there is a “high energy” point (spark, heat or extreme pressure), that’s a point where the concentration of singlet oxygen gets higher - so there is more risk of flame (explosive or not) than under regular conditions.
So, are the conditions you described dangerous? Yeah, you should never store “opposite pairs” together (acid/base, oxydizer/reducer, oxydizer/combustible) and you should never have an open flame in a place where you have all the combustible stuff you described.
But really, the oxygen from the bottle is not that much more dangerous than the one from the air… the pressure inside the bottle is not THAT high to make it go singlet.
PS: MS in Chem Eng (specialty Organic), MS in Quantum Chem, MS in Industrial Safety. So either I know what I’m talking about or my brain got blown to pieces ages ago. Probably by a triplet molecule, too…
You realize that’s not because it’s Oxygen, but merely because it’s gas at high pressure, right? You’re talking about shaking a champagne bottle, then taking off the cork and grabbing the cork while the bottle speeds away (probably to smash itself upon your foot, poor foot).
We all realize that. The point was, that if you have a pressurized bottle of anything tip over and break open, you’ve got bigger problems than any fire.
Which is why we can be thankful that cylinders are extremely sturdy items. Indeed, the valve is the most fragile part of the assembly. I know of an accidentally run-over 4.5Kpsi SCBA cylinder which passed a hydrostatic test.
Just for clarification, although it supports burning, oxygen does not burn.
No one has said that it does burn but I just thought it was worth mentioning this subtle distinction.
(Then again, since this is the SDMB, everyone probably knew this already.)
Hehe, you’re right; you’re also right that most people here probably appreciate that. That oxygen facilitates burning is exactly the reason I feel a serious oxygen leak has more hazard potential than a leak of some flammable gas or other; the oxygen makes all sorts of things burn nicely - even things like reels of wire; boxes of nails (the actual nails, not just the boxes), etc.
Not *exactly the same thing, but I remember reading an article about a guy dumping a load of liquid oxygen on a lit charcoal grill; the whole thing - metal grill, coal pan, legs and all, burned (violently) away to ash.
You certainly have a problem, but I question the assumption that it’s worse than any fire. I’ve seen pictures of the aftermath of the rupture of a fair-sized nitrogen cylinder: it caused considerable damage, but the shop in which it happened survived more or less intact. There was much less total damage than would have resulted from even a moderate fire.
The LOX application to a charcoal grille was originally hosted on the Purdue University website, now you may find it here.
For an explicit example of combustion in air vs. pure O[sub]2[/sub]:
As part of a chemistry experiment in high school, we had to burn a sample of phosphorus in a bottle (non-pressurized) of pure oxygen.
We started first by filling bottles with pure O[sub]2[/sub] that was a product of some chemical reaction, then capping the bottles with a wet glass slide.
We took a little ladleful of phosphorus and held a flame to it until it started to burn. This didn’t last long; it simply went to smoldering after the flame was removed. Then we slid the wet slide on a bottle of O[sub]2[/sub] aside, stuck the smoldering phosphorus in, and slid the slide closed. Almost instantly I had what seemed to be a 200 Watt lightbulb beneath my hands.
Er… Are we sure that an acetylene bottle would do that, and decompress as strongly and rapidly as an oxygen bottle? I thought that oxygen bottles held compressed oxygen, but that acetylene bottles held acetone that had acetylene dissolved in it?
You’re mostly correct - acetylene must be stored at a lower pressure. At high pressure it can explode spontaneously, without oxygen or other oxidiser, or a spark or flame. The energy comes from a decomposition reaction - acetylene has a highly stressed triple bond that releases a fair bit of energy when it breaks down.
Acetylene tanks contain an inert absorbent material such as mineral wool, soaked in acetone. The acetylene is dissolved in the acetone at about 125 psi. I’ve read that the refilling process takes several days, with the tanks being slowly rotated on rollers.
The pressure in an acetylene tank is therefore rather lower than in a simple pressurised gas cylinder, but it is still non-trivial.
I’m not sure we can properly rank the dangers of a ruptured high pressure gas cylinder on the loose, a typical garage flooded with oxygen, and a typical garage flooded with acetylene in proximity to a pilot light. In all cases, I think the rule is “be somewhere else”!
Something similar has been the subject of an IgNobel Prize award, in this case dumping LOX on an unlit barbecue and then igniting it (with a taper on a very long pole). It was a project to speed barbecue lighting, and it can indeed get a charcoal barbie ablaze in about three seconds; the barbecue itself tends not to be fit for re-use (really sturdy examples can take this brutal treatment maybe three times before being good only for scrap).