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Check Valve question

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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Check Valve question

Unread postAuthor: iturnrocks » Mon Jun 23, 2008 10:59 pm

Im in the process of building a gun which will have an air mattress fan and a check valve.

Since the flow direction of the check valve will be facing into the chamber, I imagine the force of the combustion will pull the valve open a little and pull in some air. I'm wondering what effect, if any, that will have on velocity of the spud leaving the barrel. I also wonder if by letting some air in, it would reduce the sound of the bang.

Has anyone noticed anything like this when using a check valve?
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Unread postAuthor: biggsauce » Mon Jun 23, 2008 11:11 pm

Im assuming you're using a pvc check valve? If you position the vavle so the "flap" hangs down, the vavle shouldn't have trouble sealing during combustion. If your fan is blowing into the chamber through the valve, the combustion shouldn't open the valve, it should seal it.

If air were allowed to leak from the chamber during combustion, you would loose some power behind your spud and I would imagine the boom may be less...

Does this answer your question?
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Unread postAuthor: hi » Tue Jun 24, 2008 12:04 am

i think it would actually force the valve to close even tighter rather than open it up. it might suck it open, but not till after the projectile is out the barrel, so it wont matter anyway.

if you are really worried about it just use a ball valve.
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Unread postAuthor: iturnrocks » Tue Jun 24, 2008 12:41 am

hi wrote: it might suck it open, but not till after the projectile is out the barrel, so it wont matter anyway.


Good point. I will try it and see what happens. Im sure I am overthinking this too much.
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Unread postAuthor: psycix » Tue Jun 24, 2008 10:17 am

Make sure the air mattress fan has enough pressure to blow through the checkvalve.
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Unread postAuthor: iturnrocks » Tue Jun 24, 2008 11:29 am

psycix wrote:Make sure the air mattress fan has enough pressure to blow through the checkvalve.


I did check that, and it does blow through, although there is quite a bit of resistance. I actually went up to a larger 2" check valve because the spring in the smaller one seemed stronger. This cannon is either going to look really cool, or really stupid. I just hope it all works good enough. I will post pics once I get it done, but its a slow process for me, so it might be a week or two.
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Unread postAuthor: iturnrocks » Wed Jun 25, 2008 7:50 pm

Ok, Ive got everything cut except the barrel. Here is my dry fit.

This will be my second gun. My first gun was a propane over/under 3" chamber- 1.5 inch barrel with BBQ igniter. I put it together pretty quick, and I didn't understand the whole chamber fan thing. I think not having that was my biggest problem. When it did fire, it was good, but most of the time it didn't. I was able to launch a few spuds, and I shot a slug through 1/2" plywood. with this one I can unscrew it in the middle for servicing if necessary.

Then I started planning to build a coaxial combustion. It has since evolved into this. The pass-thru barrel is just really long inside the chamber. I don't know if this counts as a coaxial or just an inline.

I may have gone overboard with the fittings, but i wanted to make it as thick as possible.

Image

1. 4" cleanout
2. 4" female
3. 4" x 2" wye
4. 2" 45 elbow
5. 4" street female
6. 2" male
7. 4" male
8. 4" x 2" cutout flush bushing
9. 4" coupling
10. 2" checkvalve
11. 2" male
12. 2" x 1.5" flush bushing
13. 4" coupling
14. 4" x 3" flush bushing
15. 3" street female
16. 3" male
17. 3" x 2" bored flush bushing

Here is my 4 x 2 cutout bushing. The 2" barrel seats here in the chamber. I over-dremeled a bit on one side.

Image

And here is my pass-thru barrel. 3x2 flush bushing bored out so the pipe would pass through. I'm going to fill the backside of this (inside the male fitting) with something. I just want to toughen it up since this is an ABS-PVC connection. Any suggestions? I'm thinking liquid nails or epoxy or something. This will be in contact with the combustion, so I need to make sure it can withstand that.

Image

Also will have Stun-gun ignition, spark strip, chamber fan, and direct propane injection. It has a rechargeable air bed fan on the checkvalve; I'm still working on how it will be connected.
I also got one of these one squeeze valves for the meter.

Image

Once I get everything glued up, I'll measure my chamber volume and decide how long my 2" barrel will be. I may also make a 3" barrel, since I do have 3" threads on the end.
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Unread postAuthor: psycix » Thu Jun 26, 2008 6:44 am

Wow, that are alot of fittings!
I like the complicated look.
What did all those fittings cost?

Some comments:
-Are you sure that that wye and 45 elbow (part 3 and 4) are pressure rated?
-The fuel will have to mix even into part 4, and also realize the flamefront also needs to go in there to make the combustion. This "semi"-dead space could drop off performance, make sure to place the fan and a spark gap near it.
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