T-Shirt cannon pnuematic set ups

[Cody Carey]

Originally Posted by vivek16

Actually, 120 psi is a lot. There is a reason FIRST has us use only 60 psi. If you do make a pvc gun, please, please make sure that you don't use pvc for the air storage or charging.

Wrapping the tube (barrel) in duct tape a lot and putting a bigger tube around it makes it a little safer.

be safe, Vivek

When originally designing our T-shirt cannon, We looked at the pressure ratings listed in the Engineering toolbox.( http://www.engineeringtoolbox.com/pv...res-d_796.html )

It clearly lists that the maximum allowable operating pressure for 3" schedule 40 (which is what we used) is 158 Psi, and the minimum bursting pressure is 840 psi. Considering that that is 38 psi above what the exhaust valve on the pump we were using is set to, combined with the fact that we had the tank encased in a plexiglass shield and we wouldn't be operating it in temperatures above 140F (thank God)... I'd say we were perfectly within the confines of what would be considered "safe".

As for wrapping the barrel and/or tank in Duct tape, and placing it in a larger section of pipe, that would not only make it a little safer, it would virtually eliminate any safety hazard at all. No shrapnel means no major injuries... and the chances of the larger pipe bursting (if it is sealed) are minimal as well, because the largetr pipe would be exposed to a much smaller pressure PSI than the little one was.

As for the reasoning behind FIRST having the teams limited to 60 PSI, I doubt it has to do with the bursting pressures of PVC,but can more likely be attributed to them being overly stringent with their safety regulations... but as there have been no major pneumatic related injuries that I know of thus far in the program, I'd be an idiot to argue with their reasoning.

I'd say that as long as you have the tank well guarded from any shock, and you don't abuse the thing, you're safe to use PVC in a 120 PSI system.

[AndyB]

I know a team built a tshirt cannon for use at one of our competitions this year that used PVC for air storage. They shot shirts on Thursday and part of Friday I believe before they were asked to stop because of danger.

If you aren't concerned about your own safety, please be concerned for everyone else's.

[RyanN]

Originally Posted by Cody Carey

When originally designing our T-shirt cannon, We looked at the pressure ratings listed in the Engineering toolbox.( http://www.engineeringtoolbox.com/pv...res-d_796.html )

It clearly lists that the maximum allowable operating pressure for 3" schedule 40 (which is what we used) is 158 Psi, and the minimum bursting pressure is 840 psi. Considering that that is 38 psi above what the exhaust valve on the pump we were using is set to, combined with the fact that we had the tank encased in a plexiglass shield and we wouldn't be operating it in temperatures above 140F (thank God)... I'd say we were perfectly within the confines of what would be considered "safe".

As for wrapping the barrel and/or tank in Duct tape, and placing it in a larger section of pipe, that would not only make it a little safer, it would virtually eliminate any safety hazard at all. No shrapnel means no major injuries... and the chances of the larger pipe bursting (if it is sealed) are minimal as well, because the largetr pipe would be exposed to a much smaller pressure PSI than the little one was.

As for the reasoning behind FIRST having the teams limited to 60 PSI, I doubt it has to do with the bursting pressures of PVC,but can more likely be attributed to them being overly stringent with their safety regulations... but as there have been no major pneumatic related injuries that I know of thus far in the program, I'd be an idiot to argue with their reasoning.

I'd say that as long as you have the tank well guarded from any shock, and you don't abuse the thing, you're safe to use PVC in a 120 PSI system.

Even though these ratings are printed, for what type of application are they defining? Liquid or Gas?

Liquid does not compress very well, so the chance of PVC exploding with a liquid is very slim. However, gas does compress well, and therefore, the chance of the PVC exploding is much greater. You must also think about the size of the molecules and how they interact with the structural integrity of the PVC pipe. Liquid molecules in general are much larger than gas molecules, so liquid can be contained much easier.

Here is an example of what I'm getting to:
You have 2 balloons, fill one with water and one with gas. Wait a week and look at the size of both balloons. I bet the one with liquid is much larger than the one with gas. Also try popping both balloons. The one with air will be much more violent. This same concept applies to PVC pipe.

We've had our own accident with PVC exploding in a kids face, and he could have been killed very easily. He was well in the 120PSI zone (around 40PSI) when a coupling split in two pieces and made the whole tank shoot off like a rocket into the ceiling.

Also, PVC tanks do weaken over time from expanding and contracting. Water lines do the same, but most water lines remain compressed all the time, so the process is much slower. With an air cannon, it goes from 0PSI to xPSI every time it is shot.

There have been many horror stories of PVC exploding and people saying not to do it:
http://www.osha.gov/dts/hib/hib_data/hib19880520.html
http://www.youtube.com/watch?v=AQo81nqx-W8
http://www.cnczone.com/forums/archiv...p/t-35978.html

[Bryan Herbst]

Okay, we finally got around to putting stuff together, and we are looking for tips on how to improve the cannon.

It actually has pretty decent range (shoots t-shirts about 30-40 feet), but it takes about a minute to fully pump up the tank. Now, this is pumping it to 80 PSI, but right now we are working at someone's house, not the school, so we should be able to lower the PSI and still be able to get shirts into the stands.

Currently, the barrel and tank are both made of 3 inch diameter PVC (please keep reading) and are both about 30 inches long. We are pumping the tank with two of the KOP air compressors. It takes about 1 minute and 40 seconds to pump the tank full with one compressor.

Now, as for the PVC tank- Yes, we are aware it is dangerous. However, we are limited on funds (currently have NO sponsors), and don't have many other options right now. We will be getting a different tank ASAP, but this is what we have right now. We have taken plenty of safety precautions (we stand around the corner from the tank, so if the tank explodes, it will explode into a wall and the person's lawn).


Now, if someone could suggest how to get a better pump, a better tank, or another way to do this more efficiently/effectively, we would be much obliged.

EDIT- Just remembered, and thought I would mention- We are currently using a 24v sprinkler valve hooked up to a standard FIRST control system (i.e. a 12v circuit). Would this make the solenoid open slower, and thus give us less bang per shot, or would it really not affect it at all?

[Andrew Schuetze]

take a look at our development of a similar system which has been documented here on C.D. The sprinkler valve you mention has also been discussed on C.D. and even a very nice animated graphic shows how it works. It is a relatively slow opening with a membrane that does flap during exhaust. Does your cannon honk when it shoots? Ours did.

We currently use 1 inch black pipe with npt thread for air storage as it is rated for pressurized air and is available at hardware stores. Stainless pipe is also available. One could even use a 5 gallon Craftsman air tank with only cost $20. On our system, the valve is immediately after the black pipe so that the pvc tubing and barrel do not maintain pressure except during the shot. We got better results when going to the manual ball valve but the best results are with the valve rated for this purpose. The local Parker dealer donated the valve to our team as it lists for $100.

Craftsman air tank


Ball Valve


Parker Valve

[R.C.]

We used ABS and could someone post material that is rated for a 1,000 psi. Ours works successfully, but we want to rebuild it. Also use a sprinkler valve, it works really well.

[RyanN]

We use 2 KOP compressors to charge our tank. It still takes a long time to charge from 0PSI to shooting pressure, but we can get 3 shots out of the tank before having to wait again.

We are using an old freon tank used to charge air conditioners that was empty. It's a bit large for our use, although I can't think of any other tanks that would work better.

For the sprinkler valve, we connected 3 9V batteries together in series to a relay (not a spike) and connected the relay to a spike. This gives the sprinkler valve 27V of power, which shouldn't hurt it at all (and hasn't hurt it over the past 4 years). We put a software limit on the amount of time the shooter can shoot which saves on air and charging time.

The example above me is pretty much the system we use.

[Mr. Freeman]

Originally Posted by RyanN

Liquid molecules in general are much larger than gas molecules

I don't think the size of molecules changes when changing states from liquid to gas.

[gorrilla]

pressure rated pvc pipe is perfectly fine to us as long as you dont exceed the rating

abs plastic is not pressure rated but when it fractures its like safety glass on a car YOU SHOULD NEVER USE ABS PIPE



steel pipe is the best but most people dont have lots of money for steel pipe



bottom line you can use pressure rated pipe as long as you are smart and dont exceed the rating

[RyanN]

Originally Posted by Mr. Freeman

I don't think the size of molecules changes when changing states from liquid to gas.

Is liquid not thicker than air? Try to move your arm through water, then do it through air. You will notice a big difference. Gas is much more free and energetic than liquid. That's my point. The actual size of the molecules don't change from changing states, but the different molecules in the gas form and in the liquid form are much different.

[NickE]

Originally Posted by RyanN

the different molecules in the gas form and in the liquid form are much different.

Actually, I believe that the molecules are the same. The difference is the spacing between the molecules.

[RyanN]

Originally Posted by NickE

Actually, I believe that the molecules are the same. The difference is the spacing between the molecules.

I guess that's what I'm trying to explain. Gas likes to escape faster than liquid. Liquid does not compress well while gas does.

Basically the point is that PVC should never be used to store air unless you are expecting an explosion at any time. I would never recommend using PVC. I saw what it can do, and don't want it to explode in my face. If it's a very far from anyone and inside a steel container I would then feel safe using it. Otherwise I would never use it. The kid had a failure at 40PSI while he was still charging it up to 60PSI. It was not thin pipe at all. It was very thick. It failed at a joint. It cracked the whole piece, then released the top section like a canon straight up into the ceiling. This happened at 40PSI. I know the water pressure in my home is right at 60PSI. PVC is not designed to hold air. It's not safe. It fails unexpectedly unless you drop it, and if you do that, you better run like a mad man, because it's very likely to go off. A PVC pipe with 60PSI of air, or 100PSI, or whatever is a bomb. Note that I did not use like a bomb. It is a bomb. A grenade. Unsure of when it will go off.


Hopefully I'm getting my point across. Don't use PVC for the tank. The way they're describing they're using it sounds pretty safe, but make sure no one, and I mean no one, is around it at all. There shouldn't be anyone within 100 yards of your tank when it's pressurized. Be behind the wall and don't go out unless it's not pressurized anymore.

[artdutra04]

Originally Posted by gorrilla

pressure rated pvc pipe is perfectly fine to us as long as you dont exceed the rating

"Pressure Rated PVC" is only rated for liquids under pressure. That means it is NOT rated for gases under pressure.

When liquids are under pressure, and if a crack in the PVC appears, the worst that may happen is a crack appears that releases the water safely and slowly. When gas is under pressure, and is a crack in PVC appears, the result is catastrophic and spontaneous explosion of shrapnel in all directions.

If you want a demonstration of this, take a balloon and fill it with water. Then take another balloon and inflate it with air. Take a knife or a thumb tack and pop the water balloon. Then pop the air-filled balloon.

The water balloon will most likely just kind of split open and spill water everywhere. The air balloon will violently pop and pieces of the latex balloon will fly everywhere. The same thing happens with PVC.

Just because it may say "Rated to 150 PSI" or something on PVC does not mean it works for everything under pressure.

[gorrilla]

take a soda bottle down about 100 feet under water and fill it with air and let it float the the surface



by the by water is just as dangerous as air under pressure and water under pressure explodes things too


they test scuba tanks by filling them with water to 3000psi

[artdutra04]

Originally Posted by gorrilla

take a soda bottle down about 100 feet under water and fill it with air and let it float the the surface

This hypothesis does not make PVC safe to use with gases under pressure.

If you do that, you will have to fill the water bottle to a high enough pressure to match (or exceed) the water around it, otherwise it will be crushed by the water pressure. If you make the air pressure in the bottle match the ambient water pressure around it, then at 100 feet down everything will be in equilibrium and nothing spectacular will happen.

Despite being at the same pressure, the mass of air inside the bottle is less than equivalent mass of water at the same volume, so the bottle will float upward. As it hits the surface, the relative pressure difference between the bottle and the water around it will begin to skyrocket. At it hits the surface, the pressure of the water bottle will be very high compared to the water/air around it, and then has a high chance of violently exploding. The resulting possible explosion would be very much like that if you filled a water bottle 1/3 with water, put in some dry ice, screw on the lid, then run like there's no tomorrow - e.g. a very violent and VERY LOUD explosive decompression.

The same thing can happen to the dissolved gases in the blood stream of divers who surface too fast, resulting in the bends.

Originally Posted by gorrilla

by the by water is just as dangerous as air under pressure and water under pressure explodes things too


they test scuba tanks by filling them with water to 3000psi

I never said water [or more generally, liquids] under pressure are not dangerous, as in fact it can cut through objects six or more inches thick in the form of a waterjet.

How materials react to these differences between pressurized liquids and gas is what I am concerned about. PVC reacts much more violently during a fracture with pressurized gases rather than pressurized liquids, which is why PVC is banned by OSHA for any pressurized gas applications.

Avoiding PVC for pressurized gases is like never pointing a gun towards anyone at any time, or always using your seatbelt in a car. Sure, you can always claim a few times where one has ignored these concerns and nothing bad happened. But accidents are just that - accidents - and by not following safety guidelines you seriously increase the risk of serious bodily harm to both yourself and anyone around you.