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High pressure hydrogen vessel

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High pressure hydrogen vessel

Unread postAuthor: DYI » Thu Dec 13, 2007 6:44 pm

I have a choice of either hydrogen or helium for the airsoft gun I'm making in the winter. Both would have an operating pressure of around 2kpsi. Obviously, if hydrogen was used, some safety precautions such as flushing the chamber with an inert gas prior to filling would be necessary. My question is, would the hydrogen embrittle the steel pressure vessel, assuming an exposure time of <30 seconds per shot? How long would it be before the steel pressure vessel became unsafe?

I likely will end up using helium, even though it limits muzzle velocity to ~3450 fps, but answers to the above questions could prove very useful for future reference.
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Unread postAuthor: BigGrib » Thu Dec 13, 2007 7:32 pm

The only problem with hydrogen is the extremely high flammability. other than that it is just a gas. Use helium. And a muzzle velocity of 3450 fps is more than enough.
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Unread postAuthor: DYI » Thu Dec 13, 2007 7:46 pm

The problem is that hydrogen can seep into pipe walls and react with carbon in the steel to form bubbles of methane (CH4). This weakens the pipe and makes it more brittle. Please don't post an answer without even reading the question, much less knowing the answer to it.

And you can never, ever have too much muzzle velocity.
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Unread postAuthor: bigbob12345 » Thu Dec 13, 2007 8:01 pm

Do you even now how fast 3450fps is cause you dont seem to be pleased with that number.Hydrogen would produce more but do you seriously need more 3450 is like 3 times as fast as a bullet.And helium is just safer because it is not extremely flammable. So use helium and try to get that 3450fps figure I doubt you will but it will be cool if you did.And I dont now the anser to that other question about the embrittlement.
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Unread postAuthor: Novacastrian » Thu Dec 13, 2007 8:23 pm

I would go with the Helium, purely because it cant blow you face apart!
I agree with your statement that you can never have too much velocity, even if 3450fps is about the speed at which grenades fragment.
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Unread postAuthor: DYI » Thu Dec 13, 2007 9:15 pm

Bigbob, the embrittlement wasn't "another question", it was the only question. I.e. singular. I never asked for your thoughts on whether I can achieve the speed of sound in helium, and you could have at least written your statement in coherent English.

Several rifle cartridges will exceed 3000 fps, and at least one regular production cartridge exceeds 4000. I know exactly how fast Mach 3 is, but the projectile will be rather light (about .2 - .5 grams), so even at that rather high velocity, its muzzle energy won't be very high (about 110 ft/lbs for the 0.5 gram at ~2600 fps, and about 90 ft/lbs for the .2 gram at >Mach 3). Please note that these are theoretical muzzle velocities. The actual velocities will be lower due to extreme air resistance in the barrel (which could be relieved by filling the barrel with helium). Due to the extra convenience and safety provided by helium, hydrogen would likely be a late upgrade, if anyone actually answers my question as to whether it is safe or not. Also, hydrogen would be a late addition because this will be handheld, and I want to avoid the aforementioned "blowing my face apart" thing.

And Nova, as far as I know, the initial velocities of the shrapnel from a detonating frag grenade are closer to 20 000 fps than 3 400.
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Unread postAuthor: bigbob12345 » Thu Dec 13, 2007 9:21 pm

Does it really matter you now what I mean
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Unread postAuthor: DYI » Thu Dec 13, 2007 9:37 pm

Punctuation is your friend. Without it and proper spelling, even the simplest statements become garbled. It only takes a few more seconds to reach for the comma or period, or use the spellcheck. That, and the fact that you never even attempted to answer my question.
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Unread postAuthor: bigbob12345 » Thu Dec 13, 2007 9:53 pm

That is because I do not now the answer to it.
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Unread postAuthor: BigJon » Thu Dec 13, 2007 10:01 pm

I think thirty seconds is enough time to embridle the steel, but could you put something on the inside of your chamber to keep the hydrogen from reacting with the carbon?

And what kind of velocities were you planning on reaching with the hydrogen anyway?
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Unread postAuthor: boilingleadbath » Thu Dec 13, 2007 10:01 pm

At 2000 psi, embrittlement might be an issue. I don't know much about the issue.

If it worries you, you might use an aluminum chamber... some of the aluminum alloys have tensile strengths comparable to pipe steels, arn't too expensive, and contain no iron to lead to embrittlement.

It'll be hard to find aluminum pipe in these alloys, so you may choose to get your chamber machined from a chunk of roundstock.
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Unread postAuthor: DYI » Thu Dec 13, 2007 10:05 pm

Thanks for some real answers. That's really what I had expected from the beginning, I just wanted to make sure. I may make a high pressure hydrogen gun at some point, but the extra work to avoid embrittlement makes it unlikely for now. I thought it was the carbon in steels that caused embrittlement?
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Unread postAuthor: Matheusilla » Fri Dec 14, 2007 11:54 am

Hydrogen embrittlement is more severe with pure hydrogen. One way that I have heard to decrease this is to have a mixed gas so that the hydrogen is bonded to something else. If you have access to hydrogen and helium, why not use something else like argon?
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Unread postAuthor: Novacastrian » Fri Dec 14, 2007 6:14 pm

And Nova, as far as I know, the initial velocities of the shrapnel from a detonating frag grenade are closer to 20 000 fps than 3 400.[/quote]

Rdx detonates at 8750fps, Tnt at 6900fps.
I was closer in my estimates than you :D
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Unread postAuthor: DYI » Fri Dec 14, 2007 6:42 pm

RDX does not detonate anywhere near 8750 fps. It detonates at 28 437 fps, which, likely not coincidentally, is 8750 m/s. Your source likely confused feet per second with meters per second. As further proof of this theory, TNT detonates at 22 425 fps, or 6900 m/s. Also, on a vaguely related note, according to my trusty table of explosive detonation velocities, Hexanitrohexaazaisowurtzitane (or CL-20), detonates at approximately 30 550 fps.

And to the person who suggested I use argon, it is far too dense, with a relatively low SOS, to be of any real use to me.
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