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Air tank alternative: fiberglass-reinforced soda bottles

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Unread postAuthor: Gun Freak » Thu Jul 28, 2011 3:39 pm

You hit like a girl :flower:
Beat the crap out of it and see how much it can take! :violent1:
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Unread postAuthor: aEx155 » Thu Jul 28, 2011 4:15 pm

Gun Freak wrote:You hit like a girl :flower:
Beat the crap out of it and see how much it can take! :violent1:


I don't think my parents or my neighbors would like to hear anything blow up, but I'll go drop some bricks on it and see what happens.
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Unread postAuthor: Mr.Tallahassee » Thu Jul 28, 2011 5:18 pm

You could grease the bottle's top and put a section of pipe that fits around the outer lip under the lid and put another section of greased pipe into the bottle opening and pour in epoxy. It'll set to the threads and with the lubrication it should thread right off. The perfect fit adapter!
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Unread postAuthor: ramses » Thu Jul 28, 2011 7:28 pm

Mr.Tallahassee wrote:You could grease the bottle's top and put a section of pipe that fits around the outer lip under the lid and put another section of greased pipe into the bottle opening and pour in epoxy. It'll set to the threads and with the lubrication it should thread right off. The perfect fit adapter!


You'll end up with male threads that way. Pour the epoxy around the neck of a bottle.

Also, most caps have broken up threads. I actually successfully performed a "lost plastic" type casting in zinc of a bottle. (I burned the plastic out of the plaster with a torch first, though) The threads are pretty lousy, but I tapped the top for 1/4 NPT, which I connect to a sparkplug and ball valve. A 2l, 6x hybrid mix is quite loud...
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Unread postAuthor: aEx155 » Thu Jul 28, 2011 7:39 pm

Here is the most recent test video:

Image

This test was at 150 PSI. The brick fell 4 feet. I edited the video to remove the bad drops of the brick, and in the end I got tired of resetting the bottle so I just started whacking it with the level. I made so much noise my dad came outside.

After the test I inspected the bottle: there is a lot of scratches from when the brick slid along the side of the bottle, and there is one side that has a lot of scratches because it was touching the ground. The outer covering did exactly what it needed to and protected the bottle and the filament tape.

Hopefully this demonstrates how strong this kind of tank can be. It costs barely anything, it's very easy to get the materials, and it's a lot less dangerous if it fails. You also don't need any extra tools: if you don't have a heat gun, you can use a stove or barbecue to shrink the bottles.

When I get the chance I'll buy a 3/4" compression fitting and some epoxy so I can try two methods of making adapters. I'm also working on making a 1.5L tank which I hope will turn out better.

If I cast the bottle adapter inside a 1" female NPT fitting, the threads should help hold the epoxy in, right?
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Unread postAuthor: saefroch » Thu Jul 28, 2011 7:45 pm

Threads don't do much of anything compared to roughening up the surfaces. The tensile adhesive strength of most epoxies is usually 4,000+psi.

Just my opinion: I still don't feel comfortable with a thin-walled plastic pressure vessel. I'll stick to my steel and brass, that has a calculable failure point, and a generous safety factor.
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Unread postAuthor: aEx155 » Thu Jul 28, 2011 8:08 pm

saefroch wrote:Threads don't do much of anything compared to roughening up the surfaces. The tensile adhesive strength of most epoxies is usually 4,000+psi.

Just my opinion: I still don't feel comfortable with a thin-walled plastic pressure vessel. I'll stick to my steel and brass, that has a calculable failure point, and a generous safety factor.


Well, the area of the bottle is 0.95", so at 150PSI that's 143 pounds of force trying to remove the bottle. Would cast epoxy threads be able to hold that? Since I was just thinking, I put the bottle into a 1" fitting and put in epoxy around it. Should I use a slip fitting that is roughened up or a threaded fitting?

I was uncomfortable whacking a plastic bottle pressurized to 150PSI :?

I won't be able to find the actual burst pressure, though my conservative guess is 200 PSI. But now that I know that it can take that kind of beating, I feel safer knowing that I can stick with 80-100 PSI (max of my bike pump)
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Unread postAuthor: warhead052 » Thu Jul 28, 2011 8:28 pm

Might I suggest hydro testing it? Just fill it with water and fill with air until it bursts. When it fails it should just split in half rather then spitting plastic at you.
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Unread postAuthor: aEx155 » Thu Jul 28, 2011 9:08 pm

warhead052 wrote:Might I suggest hydro testing it? Just fill it with water and fill with air until it bursts. When it fails it should just split in half rather then spitting plastic at you.



All of the tests I've performed are with the bottle filled with water, since it's in-compressible and doesn't store energy like air. (which is why PVC is recommended for water use but not air) The only thing that has air in it is the supply hose, which I know is rated to 300 PSI.

I know I don't have enough pressure to find the bursting point, although if I happen to come across some dry ice and a 500PSI gauge, I'll figure something out and tell you.
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Unread postAuthor: Mr.Tallahassee » Thu Jul 28, 2011 10:06 pm

How would that make male threads :?: :?: :?: I meant pouring epoxy around the greased bottle threads. That would make female threads.
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Unread postAuthor: aEx155 » Thu Jul 28, 2011 11:04 pm

Mr.Tallahassee wrote:How would that make male threads :?: :?: :?: I meant pouring epoxy around the greased bottle threads. That would make female threads.


The greased bottle would be inside the fitting, and I would pour epoxy between the bottle and the fitting. That way I would have a bottle-PVC adapter instantly.

I don't know what would hold better though: a sanded slip-fitting, or a threaded fitting.

I will probably be using Loctite brand 5min epoxy, although if there is something better I can pick up at Home Depot I'll try that instead.
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Unread postAuthor: Technician1002 » Thu Jul 28, 2011 11:05 pm

You are using a measuring device as a hammer? :shock: Don't be surprised if it is no longer accurate when you are done. Oh, and never ask to borrow any of my tools. End of rant for proper tool for the right job.

The bottle with no modification will often handle 135 PSI with no modification.

Dry ice in water can often produce pressure over 500 PSI depending on the temperature of the water and the amount of dry ice used. Some of the gas will dissolve into the water to make fizzy water. Lots of dry ice with no water works slower, but can reach higher pressures.

You do want to follow the safe handling of the stuff. It can be dangerous.
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Unread postAuthor: Gun Freak » Thu Jul 28, 2011 11:07 pm

I still dont get why you wanna use a plastic bottle... PVC is way easier and strong enough.
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Unread postAuthor: Gippeto » Thu Jul 28, 2011 11:14 pm

[quote="aEx155"]
I was uncomfortable whacking a plastic bottle pressurized to 150PSI :?
quote]

You should be. If I caught someone "whacking" anything with my level I'd be kicking their ass. A good level costs $$$ ... don't abuse it.

Would suggest you stop using your dads level as a "beatin tool" before you destroy it....if it's not already too late.

If you want to do some hydro testing...buy a cheap grease gun...and eliminate the air from the system. Use the pressure to cause the failure...you don't need to be beating on it as that proves nothing.

My .02 (as the owner of a quality level :wink: )
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Unread postAuthor: aEx155 » Thu Jul 28, 2011 11:38 pm

EDIT: Okay, okay, I admit it was a really, really bad choice to use a level. Sorry if I angered anyone. If it helps at all, the level shows no signs of damage and I promise not to do it again. :oops: (I would like to point out that my dad was present while I was doing this, so interpret that how you like)

I did want to try abusing it in some way, though, to see if I could get it to fail, but it didn't work. Next time I'll use something more appropriate.

-----------

Technician1002,
I know that some bottles can handle 135 PSI, but the question is: how many times will it handle 135 PSI?

Gun Freak,
I was trying to find a compromise between the price and availability of PVC and the strength of a metal tank.

Personally I just want to stay away from PVC because of its failure mode. Even if you take care of it to the highest extent you have no guarantee that it won't fail. (that can be said for anything used in an unintended way). By going with a bottle I know it'll fail in a way that's less dangerous than PVC.

Gippeto,
If I had access to a grease gun I would try that, but for now I'll stick with lower pressures.
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