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Tank Materials

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
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Tank Materials

Unread postAuthor: fogus » Tue Oct 28, 2008 11:44 pm

Hello All,

I have been doing a lot of thinking towards my next cannon, which I plan to be an over/under (i.e., non-inline), 2 inch diameter barrel and 2 inch diameter combustion tank, both of equal length (approx. 36”). The weapon would be held and fired like a SMAW. Recently, I have been wrestling with what to build it out of. Below is a discussion of what I have considered. I invite anyone to post their comments, suggestions, or responses to some of the questions that will arise in the discussion.

PVC
Tried and true, I could go this route. My main concern is that I will not be able to complete my design with this material. What is the most reliable way to join two parallel lengths of PVC together? Would you recommend normal PVC glue and some kind of constructed separator (such as is used on extended magazines on pump/semi-auto shotguns)? This question applies to the other materials as well, although welding is an option on the metals. My secondary concerns are strength in wear and tear and the overall mediocrity of a PVC spud gun: everyone has made one.

Aluminum
With the connections I have to make, I would either have to weld it or get fittings. Does anyone know where to get 2” aluminum pipe fittings? What wall thicknesses should I use for a normal combustion gun (I am assuming I would be using 6061)? The downsides to this material are that I would have to get some welding equipment. I have an argon/CO2 tank and wire for steel welding and a MIG welder. I could go out and get a pure argon tank and some aluminum wire, but would it be worth it?

Plain steel (not normal black, no seams)
I could weld this stuff, but how thick (and therefore heavy) would plain (i.e., mild, non seamed) steel pipe have to be to hold the kinds of pressures (I would be using propane as a fuel, I assume there is some maximum pressure for this type of combustion. Was it 150 psi?). I want to avoid an overly heavy gun, so building in schedule 40 steel would be out of the question. What could I safely get away with?

Also, what would I ask for? I am not very familiar with the terminology used in steel piping for this kind of stuff. I would be planning on coating the whole thing in black hockey tape to give it a good look and feel and to ward off corrosion. If I wanted better corrosion resistance, what type of paint should I paint it first with (I assume it would be something other than a generic oil based paint)?

Stainless steel
Not sure what it takes to weld this stuff, but it is expensive. It would cost $200+ to build one cannon out of this, not taking into account anything but the piping itself. Would I be using 304 or a different alloy?

One final note on this gun: I was hoping to build single shot canisters filled a high pressure mix of propane/air. I am not exactly sure how to calculate the right mix (I did read http://www.spudfiles.com/forums/hybrid- ... 13602.html and I did watch http://youtube.com/watch?v=SelKyVBaVkg , but I have been unable to combine these into a useful way of deciding where the needles on the pressure gauges should be after the propane injection and then of the HPA injection). Perhaps someone could write HYBRID FUELING 201: Applications :) . These canisters would contain enough fuel to fill my chamber and then some (they would inject via a male/female Schrader valve coupling on the canister/combustion tank, respectively). I was thinking somewhere between a 1.5x and a 2x chamber volume equivalent shoved into a small canister. My question is this: if I were to use black steel canisters, how big (and therefore what true mix (10x, 20x…?) they would contain) should I make them to avoid a rupture if one of them accidentally ignited during fueling?

The fueling process would be something like this: attach the propane tank to the fuel meter (described in “how to build a propane meter” video), inject fuel until the needle gets to the right point. Inject air from an HPA tank until the needle gets to the right point (again, I don’t know what these points are, or how to find them). Then open the injection valve and fill my canisters. Repeat this process for all canisters. Press a given canister into the front of the combustion chamber (where the female Schrader valve is) and allow it to fill the chamber. Fire away. Has anyone else done this?

After I work out the final design I will post it (as it does not depend heavily on what material I will be making it out of).

Cheers
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Unread postAuthor: frankrede » Wed Oct 29, 2008 12:29 am

To weld aluminum you need to use a spool gun if you want to wirefeed it.
Or you would have to TIG weld it.
I would go with Steel, normal sch-40 is more than enough to hold hundreds of psi without worry.
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Unread postAuthor: mega_swordman » Wed Oct 29, 2008 12:59 am

If I am reading this correctly, you are building a hybrid cartrige system. If this is the case, I would go with Frankrede's suggestion of using Steel.

However, if this is simply for a combustion cartridge purpose, then I would suggest building the cartridges out of PVC. This is due to the fact that PVC is strong enough to resist the force produced by the combustion and cheap enough that making several cartridges would not be a problem.
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Unread postAuthor: Lentamentalisk » Wed Oct 29, 2008 1:00 am

If you are just trying to make your standard combustion cannon, then go for PVC. It will hold just fine. To connect two parallel pieces of pipe, just use a coupler.
Metal is only needed if you are planning high X mixes with a hybrid.

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Unread postAuthor: ramses » Wed Oct 29, 2008 3:02 pm

a coupler is used if the chamber and barrel were inline. they will not be. I would suggest elbows. steel would be fine for a small hybrid.
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Re: Tank Materials

Unread postAuthor: mega_swordman » Wed Oct 29, 2008 5:41 pm

fogus wrote:What is the most reliable way to join two parallel lengths of PVC together?


One technique I used was to take two T-joints, sand the insides so the joints slips over the pipe, and connect the two tops to each other with a small piece of pipe. This will connect two parallel pipes with the fittings together looking like a large "H".
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Unread postAuthor: rp181 » Wed Oct 29, 2008 5:54 pm

ramses: when he said coupler, i think he meant have it vertical, supporting the barrel, not actually used as a coupler.
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Unread postAuthor: fogus » Wed Oct 29, 2008 10:17 pm

To clarify, the gun will fire without the canister attached (the canister is used to put fuel into the combustion chamber). This is not a hybrid gun, but the fuel canisters will hold a hybrid mix at high pressures.

frankrede wrote:To weld aluminum you need to use a spool gun if you want to wirefeed it.
Or you would have to TIG weld it.
I would go with Steel, normal sch-40 is more than enough to hold hundreds of psi without worry.


Yes, I have a spool gun (MIG), but not the right wire or gases currently.

Schedule 40??!
fogus wrote:I want to avoid an overly heavy gun, so building in schedule 40 steel would be out of the question. What could I safely get away with?

No, please no. It's 3.65lbs/foot. I will have 8 feet. That's almost 30lbs. What is the maximum pressure I should get my pipe rated for? This is purely a combustion cannon. Could I use schedule 10 (half the wall thickness and half of the weight, or 1/10th of an inch instead of 1/5)?

I could lug 15lbs around, not 30.

mega_swordman wrote:
fogus wrote:What is the most reliable way to join two parallel lengths of PVC together?


One technique I used was to take two T-joints, sand the insides so the joints slips over the pipe, and connect the two tops to each other with a small piece of pipe. This will connect two parallel pipes with the fittings together looking like a large "H".


I think I will build this out of PVC first (low on cash in school). This seems like as good idea as any to join two pipes together. I was hoping for a connection without this much space between the pipes. I would like the top of the top pipe to be not higher than 6.5 inches from the bottom of the bottom pipe.

rp181 wrote:ramses: when he said coupler, i think he meant have it vertical, supporting the barrel, not actually used as a coupler.


Yes, as a support for the barrel, not a path for expanding gasses to take.

BTW, I thought this FF addon was worth the try: http://mac.softpedia.com/progDownload/S ... 40132.html
Seems pretty neat, give it a try.

I will make a drawing soon.

Thanks all!
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Unread postAuthor: Gippeto » Thu Oct 30, 2008 11:43 am

Biged uses exhaust pipe in his pneumatics. He has pressure tested the one anyway to 400psi. (2.75" exhaust pipe)

Edit: Goofed up. :oops: The barrel is exhaust pipe, not the chamber, and it was 300psi.

Did the calculation for 2" pipe with .062 wall, using a yield strength of 36 000psi, burst pressure comes to 2400psi.

Use gaseq to determine the pressure you will be exposing your parts to, determine your comfort level, and go from there.

Get gaseq here;

http://www.arcl02.dsl.pipex.com/
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Unread postAuthor: fogus » Sat Nov 01, 2008 1:37 am

Does anyone know what the pressure rating on schrader valves is?

Trust me, I googled it, but there were conflicting answers. I'm looking for the kind that fits into the 1/4" NPT. Does anyone have first hand experience with one blowing out at a certain PSI?

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Unread postAuthor: MrCrowley » Sat Nov 01, 2008 1:55 am

They've been used well past 700PSI i'm sure. You'll be fine with anything under that.
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Unread postAuthor: Demon » Sat Nov 01, 2008 7:06 am

I'm looking for the kind that fits into the 1/4" NPT


You'l better use a replacement schraeder valve, more there is pressure, more it seals.
No mess with threads and epoxy.
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Unread postAuthor: Gippeto » Sat Nov 01, 2008 11:58 am

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Unread postAuthor: fogus » Sat Nov 01, 2008 6:07 pm

Demon wrote:
I'm looking for the kind that fits into the 1/4" NPT


You'l better use a replacement schraeder valve, more there is pressure, more it seals.
No mess with threads and epoxy.


What do you mean by a replacement schraeder? What is it replacing?
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