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Old 06-28-2009, 04:25 AM #1
gunangel
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Sorry guys quick LAW questions

Alright there's a lot here and I appreciate anyone who is taking the time to read this. Kind of in a hurry since we are experimenting with it tomorrow

Short Story:
We built a launcher, all fittings are brass, what other safety precautions should be taken?
What are people using as their delivery system?
What size and material expansion chamber are people using?
What are things to look out for?

Long Story:
So my buddy came to me with a request for a confetti cannon, and I basically built him a launcher . After building the launcher I realized I had some concerns for the safety of the design and function. As of now it is safely stowed with no air source so no problems there .

Basically I used one port on an on/off asa,
used a 1/8th to 3/8th adapter,
attached that to a 3/8 brass 1/4 turn valve,
which goes back to a 1/8th braided stainless steel hose,
which feeds into an expansion chamber,
which has another 1/8th to 3/8th adapter,
which feeds into another 3/8th brass 1/4 turn valve,
which finally feeds into the base of the launcher (3 inch pvc plug).

All this is powered by a 20 oz co2 tank.

We tested it and it works fairly well. The gas is pressurized from the on/off then cut off with the 1/4 turn valve so the gas has the ss hose and expansion chamber (AGD bike grip) to expand and is finally released with the second quarter turn valve. I figure this should keep the operating pressure of the cannon fairly low. If he needs more oomph he could always siphon it.

I've looked up the specs for brass pieces and it looks like they are suppose to with hold 1200 psi, and the ss hose itself can handle 3000psi so I'm not really worried there (should i be?).

My questions are what other safety things should I suggest? I would like to make this idiot proof.
I suggested drilling a small hole in the pvc launcher to have a hole to bleed air in case the item to be launched gets stuck in the tubing (we are using dixie cups right now, what are other people using?).
Would a burst disk be overkill?

And for later down the road after the confetti cannon has seen its use, I've read the larger the expansion chamber the bigger the shot (for some nerf rocket fun ), I don't really see an issue with using a 3/8 inch pipe to connect the two valves making a bigger expansion chamber, if the fittings can handle the 800-1000 psi of co2 (this is in a fairly controlled environment) a brass pipe should as well. Would I be grossly mistaken?

And if someone would check my calculations.
I am using
P = pressure in atmospheres, where 1 atm = 14.7 PSI at sea level
V = volume of gas in liters, where 1L = 61.02 in3
m = mass in grams of CO2
MW = molecular weight of CO2; MW = 12+32=44
R = 0.082 liter*atm/K
T = temperature in kelvins [which is 298 K at 25 C or 77 F]
24grams = 1 oz
16 oz = 1 pound

(pi*R^2)*H (R=radius, H=height)
so as it sits now (3.14*1.5in^2)*24in = the launcher has 169.56 cubic inches of volume or 2.77 liters

so to bring it to an operating pressure of roughly 120 psi or 8.163 atm
PV=nRT PV/RT=n
(8.163 atm)(2.77 liters) / (0.082liter*atm/K)(298K) = I would have to use 0.925 moles of co2 or 40 grams or 1.7 oz of co2

co2 exists at roughly 900psi or 14.7 atm at room temp so the volume i should be using V=nRT/P
(0.925 moles)(0.082liter*atm/K)(298K) / (14.7 atm) = 1.53 liters = which means i need an expansion chamber of roughly 94 cubic inches?

If we cut the launcher down to 12 inches that should be 47 cubic inches need to be filled with co2 to 900 psi which would mean i would need a 15 inch length of inch length of 2 inch piping. O_o that seems like a ginormous expansion chamber, what are people using? I know PVC is super unsafe, and I'm not sure how to connect a 3/8th valve to this expansion chamber.
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Last edited by gunangel : 06-29-2009 at 01:20 PM.
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Old 06-28-2009, 08:07 AM #2
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A photo of the Launcher would help with ideas. I believe the setup you current have may work for launching confetti fine once you convert it to a Nerf launcher you'll want to convert it to use a larger air chamber. My understanding is you are using two ball valves. One to air up and the 2nd one to trigger the launcher. If you are keeping this same setup for your Nerf launcher you'll want to use steel pipe (cheapest) for the Air Chamber. This will operator like a 12gram launcher.

How is the 3" PVC Plug connect. Are you using the one with tread?
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Old 06-28-2009, 08:23 AM #3
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it seems that you have quite a few resources at your disposal to make this. I would swap out PVC for aluminum. CO2 and PVC do not belong together. If the aluminum were to fail from over pressurization, it doesn't fail in the form of thousands of sharp shards of shrapnel like PVC does. There are pressure relief valves available commercially that will work better than a burst disk.
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Old 06-28-2009, 03:03 PM #4
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pics should be up later today.

we drilled a hole and used a 3/8th adapter and put it in from one side and threaded it into the ball valve. for extra rigidity we used some jb weld to keep it in place.

I do have a bit of resources at my disposal (a local material shop to be exact) what would be the diameter I should be looking for for my launcher? how did people connect the piping (expansion chamber) to the valve? i'd be very very wary of using jb weld or some other type of unthreaded cap, and since i do not have access to a lathe cutting threads into large diameter pipes would be quite difficult.

Thank you very much for the help!
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Old 06-28-2009, 05:08 PM #5
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I just realized with my setup I'm pressurizing the system to 900 psi. Since I'm using a co2 tank I should probably be running a regulator before installing an expansion chamber. Or are people finding other ways to overcome this?
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Old 06-28-2009, 10:56 PM #6
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A regulator is the way to go.

You will also want to install a safety relief valve on your expansion chamber. something set slightly above your operating pressure.


Link to Grainger
http://www.grainger.com/Grainger/wwg...l?CatPage=3240

Link to McMaster Carr (more sizes and pressure ratings) (search Pop-safety valves)
http://www.mcmaster.com/#
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Old 06-29-2009, 08:57 AM #7
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If you are keeping the ball valves you can use thread steel pipe. Don't have to be very large. It'll be heavy.

If you want one that has range and accuracy get rid of the ball valves and use a pneumatic valve (Sprinkler Valve). To do that you'll need to bring down the pressure to a safe pressure around 70-110psi with a regulator. You'll also want to install a Pop-safety valves like the one "Q" mentioned. You'll also want to go with HPA instead of CO2. Better on the valve.
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Old 06-29-2009, 11:17 AM #8
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Ah my apologies, I left out context. This is going to be mainly used for a church so co2 is much easier for them to purchase and find. It is going to be mounted yo a stand so weight should not be a significant issue. Most likely this won't be used for tank kills, but more of launching nerf balls down a field to play flier's up or something. And to my understanding ball valves can handle higher pressure? I really want to make this pretty idiot proof so in case it is in the hands of an unknowing individual it can handle being misused.

Once again thanks guys you guys have been a great help.
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Old 06-30-2009, 12:36 AM #9
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There's the pic. I figure the current expansion chamber/foregrip should be replaced with a regulator, a expansion chamber added before the release valve and a relief valve drilled and tapped into the expansion chamber.
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Old 06-30-2009, 09:58 PM #10
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PRESSURE GAUGE! Add one of these because the more info you have about what the air is doing, the safer you are! A gauge is especially crucial on a hot day when pressure can spike rapidly.
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Old 07-02-2009, 02:51 AM #11
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thats definitely one of the things we are adding

the remake should be up when i get the pictures sent to me. we have a roughly 10-13 inch by 1 inch diameter expansion chamber with a regulator regulating the psi down to about 300 or so.

I plan on putting in a 4 point fitting 1/8th fitting using 1 for the air in (after the regulator), 1 for the air out (to expansion chamber), one for a gauge, and one for a pressure relief valve.

The problem I have is finding a pressure relief valve that goes up to 600+psi that is not a custom order that doesn't set me back an arm and a leg.
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