Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Paul Lindfield on 06/02/2011 23:30:03
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Paul Lindfield asked the Naked Scientists:
I recently watched a TV programme where two people were locked in a bank vault with the water sprinklers on.
My question is, at what point would the water stop coming in because of the increase in air pressure?
What do you think?
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Don't know really, but at sea level, air is about 784 times less dense than water. Fresh water has a density of 1.0 while salt water has a density of 1.025 so depending on type of water? And the size of the room of course will matter too. The last question should be how much water can compress air before it dissolves into the water I guess?
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If you do it really fast I think the air should self-ignite too?
Now, that would be a sight :)
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When the air has been compressed enough that its pressure is equal to the static pressure of the water supply.
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Yeah, and I doubt the air could self ignite by water pressure, but it would be cool if it could :) Or hot? Assuming nobody is there that is. Otherwise it sounds a really nasty scenario.
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If the rate of change of pressure was great enough, it would get hot enough to ignite something, but there is nothing to ignite [:D]
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Oxygen can self ignite if the molecules starts moleculing really fast I think? The same phenomena if I remember right as in burning buildings where you can have a room bursting into fire because of the heat? Although I'm not sure I've seen it, but yeah. Australian wild fires jumping, but there you have the eucalyptus trees resin adding to oxygen, I think I read that somewhere?
I'm not sure of that one?
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Seems that there are two camps on that one. The one i liked (of course:)
"it does burn, quite dangerously so. If our air was not filled with nitrogen and various other gases, then the smallest spark would create an explosion. Ever heard of oxidization? That's where something reacts with oxygen to form an oxide. Under the right conditions (such as immersed in water or pure oxygen), you can readily accelerate this process. Burning is possibly the fastest method of oxidization."
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Naah I doubt it, it needs a catalyst to burn, I think? like something of matter as you said Geezer (not water:). But if anyone knows I'm interested :)
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You beat me to it.
That sounds like complete bollocks a tiny bit suspect. Oxygen is pretty good at oxidizing stuff, but oxygen is an element, so it can't break down into a fuel and an oxidizer because of an increase in pressure.
I suppose it can oxidize the nitrogen in the air though.
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Yeah, I think i read something about it. Nevermind, but it made me wondering. If you compress air really good it should become very hot at least, but I'm not sure what states it should wander through? A plasma? It should theoretically be able to end up in Black Hole I think, but before that?
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Maybe this was what you were thinking of?
http://en.wikipedia.org/wiki/Fire_piston
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Nice one :)
We had one discussing water under compression though.
http://www.thenakedscientists.com/forum/index.php?topic=7243.0
And check this one out Supercritical fluid. (http://en.wikipedia.org/wiki/Supercritical_fluid)
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At the high temperatures encountered in the cylinders of an IC engine (2000°C+) a certain degree of Nitrogen burning will occur.
Hence the much vaunted Hydrogen burning car is not pollution free.
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All bank vaults I have met do generally have a certain amount of ventilation, provided by a dedicated very heavily armoured grill or two, and in general the doors are not even dust tight, let alone watertight.
If the vault was truly airtight it would be impossible to open if closed on a day with a storm brewing ( low atmospheric pressure) and unlocked on a fine sunny day ( high pressure) as the pressure exerted on the door would be very high, certainly more than most people could overcome. The opposite would kill the unfortunate unlocker as the door springs open when the pressure inside is released.
Thus if the sprinklers activated the water would rise to a level where the incoming flow is balanced by the water leaking past the door and through the vents. Most vaults do not actually have a fire sprinkler, more a smoke detector, as there is no real way a fire can spread inside, due to a low air turnover inside, you are more likely to die from lack of oxygen inside the vault than from drowning. The fire extinguisher or hose reel would be outside, where they can be gotten ready to fight the fire when the door is opened, pointless to have them inside and unusable in a fire, as by definition you have stuff inside that is either very expensive or irreplaceable, and wetting it indiscriminately will not improve it.
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Great point Sean - air pressure would be immense. we very rarely encounter it in its full glory, but we would in this hypothetical situation.
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Great point Sean - air pressure would be immense. we very rarely encounter it in its full glory, but we would in this hypothetical situation.
Assuming the vault didn't actually "leak" (which, as Sean points out is pretty blinking unlikely) the air pressure would eventually reach the static pressure of the water supply to the sprinklers. That would probably only be something like 60 psi, or about four atmospheres.
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In this weird world where the vault was airtight and the water sprinkler worked properly until the air pressure reached 4 bar you would probably survive- the problems would be when someone opened the door.
Each litre of compressed air would store roughly 400 J of energy. A bank vault full would probably blow the door off when it was unlocked (assuming the locks didn't jam).
Also, the decompression would kill anyone in the vault
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This is a bit like the way you test a pressure vessel, like, for example, a boiler. You fill it full of water so that all the air is expelled. Then you seal it and continue to pump more water into it until you reach the test pressure. If the boiler happens to fail, you might get a lot of water pouring on to the floor.
It's not a good idea to substitute compressed air for the water.
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This is rather like the situation when submariners attempt to escape from their sunken vessel by entering an escape tube and allowing the water to enter until the pressure is sufficiently high to allow a hatch to be opened when they can float to the surface.
I understand they must make a big effort to expel the air from their lungs as they rise to the surface, it is a rather dicey business but has been known to work.
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Incidentally, with submariners if you're unlucky enough to be in a compartment that gets rapidly flooded (like in Kursk), you're in real trouble, even if you're in an airspace that remains after the pressures equalises- the air heats up as it is ("adiabatically") compressed and there's a real chance you'll be cooked.