# Compressed air dilemma

newbieny

## Main Question or Discussion Point

ok guys here is the scenario:
a moving car at an arbitrary speed carrying a cylinder of compressed air on the trunk. A car crashes into the rear of the car causing a fire mixed in with the leaking fuel from the gas tank. The tank ruptures on impact and releases the air..will this air make the explosion bigger and the fire worse?

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russ_watters
Mentor
Well, the tank can explode like any pressurized vessel, the gas forcing its way out and turning the tank into shrapnel.

But as far as the existing fire goes, it will have no positive effect and may even blow it out, the way you blow out a candle.

I can imagine a different scenario where the expanding air suddenly freed from the cylinder atomizes the gasoline from the car creating the conditions for a fuel-air explosive.

S_Happens
Gold Member
The scenario you give still has too many variables to give a simple and direct answer. Either situation could be plausible depending on how the vessel ruptures.

Although a gasoline fire would be much harder to blow out than a candle, it is probably the most likely to happen given how the vessel is ruptured from a collision instead of overpressurization of the vessel itself.

You would have to take into account the size of the fire when the vessel ruptures, how it ruptures (referring to how the escaping pressurized air is directed in relation to the fire and the size of the rupture vs pressure in the vessel), if the rupture causes shrapnel that can be affect the fire, etc. All this makes it, IMO, and poor problem to try and answer directly.

Try it. See what happens.

Averagesupernova
Gold Member
What type of compressed air tank rupture? Compressed air tanks won't necessarily release all they have at once. A rip in the tank, large crack, broken fitting, etc. could simply fan the fire which of course directs the heat as well as make it hotter by providing more oxygen.

What type of compressed air tank rupture? Compressed air tanks won't necessarily release all they have at once. A rip in the tank, large crack, broken fitting, etc. could simply fan the fire which of course directs the heat as well as make it hotter by providing more oxygen.
I think you're right. Mythbusters couldn't get one to explode by putting a bullet hole in it in their effort to recreate the finale of the movie Jaws.

IIRC they did get a fire extinguisher to explode by putting it in the middle of a bonfire. Maybe an air tank would explode under extreme heat. Or maybe the pipe fitting would blow off first, turning it into a rocket.

russ_watters
Mentor
I think you're right. Mythbusters couldn't get one to explode by putting a bullet hole in it in their effort to recreate the finale of the movie Jaws.

IIRC they did get a fire extinguisher to explode by putting it in the middle of a bonfire. Maybe an air tank would explode under extreme heat. Or maybe the pipe fitting would blow off first, turning it into a rocket.
Sounds like the safety factor is too high to allow it - that a regular air tank is able to withstand pressures MUCH higher than its operating pressure.

newbieny

what exactly is compressed air comprised of? and when ruptured what will it release ?

S_Happens
Gold Member

what exactly is compressed air comprised of? and when ruptured what will it release ?
If it's simply supplied by an air compressor then it is the same composition as the air you would be breathing, compressed to much higher density. Breathing air should be free of oil from the compressor, but if this is say an air suspension for a car, I have no idea of the quality of compressor used and whether or not there could be a substantial amount of oil in the tank. I'd ignore that for your little thought experiment.

What will be released is exactly what you have, air. The oxygen content is not higher, but because the pressure inside the tank is much higher, when it ruptures there is bunch of air (and therefore ~21% oxygen) that has to distribute itself. The only way you would cause a larger fire is if the "rupture" provided oxygen to the fire faster than it could naturally draw it in. This is the case that a few people are mentioning. There is also a delicate balance where the chemical reaction can be disrupted.

This is the reason that a simple specific answer can't be given. The tank would have to "rupture" in a way to provide more air to the fire without impeding the reaction to allow the fire to get bigger. That means you have a specific set of circumstances that must be met or at least pre-defined in your question to answer it with any definity. That pretty much kills your original general question.

IMO, using pressures from typical breathing air cylinders (~3000 psig) I would say that chances would be more in favor of snuffing out a direct flame or completely dispersing the flammabe material (gasoline), instead of making the fire larger.

f95toli
Gold Member
Sounds like the safety factor is too high to allow it - that a regular air tank is able to withstand pressures MUCH higher than its operating pressure.
It depends. A collegue of mine was involded in an accident a few years ago where he and another guy were using a helium cylinder (to transfer liquid helium). The other guy knocked the cylinder over and the regulator came off when it hit the floor; this turned the cylinder into a rocket (albeit not airborne) and it made a hole in one of the internal walls (luckily no one was hurt).
Cylinders can tolerate quite high pressures, but when they break bad things can happen and they can certainly explode if the ambient temperature gets high enough.

Wow, 11 speculation replies on a nonsense post. Science at its finest here.

Sorry to be rude, but this thread should just stop. We can all talk about the answers to this question to death, and no one is any closer to the truth than the next post.

Moral of the story: Do the experiment, find out for yourself.

Anything otherwise is speculation.

Last edited:
russ_watters
Mentor
Moral of the story: Do the experiment, find out for yourself.
Um, what exactly are you suggesting here...? :surprised

Don't worry, I told him to make sure he's in the trunk (Someone has to hold the tank).