T-Shirt cannon pnuematic set ups

[Akash Rastogi]

Originally Posted by artdutra04

PLEASE DO NOT USE PVC FOR AIR CANNONS. PERIOD.

PVC is NOT AT ALL safe for pressurized use, as it is a very brittle material. If there ever is a leak in the material, the result will be a catastrophic explosion of PVC shrapnel. Just because it may have been used without problems does not mean that it is safe.

Please, for the sake of innocent bystanders who may be near the robot if the PVC explodes, don't do it. Spending a little more money up front on better materials is well worth it if someone is severely injured from PVC shrapnel and decides to sue.

And please make sure its UL certified no matter what you use.

(Underwriters Laboratory)

[SSMike]

Originally Posted by artdutra04

PLEASE DO NOT USE PVC FOR AIR CANNONS. PERIOD.

PVC is NOT AT ALL safe for pressurized use, as it is a very brittle material. If there ever is a leak in the material, the result will be a catastrophic explosion of PVC shrapnel. Just because it may have been used without problems does not mean that it is safe.

Please, for the sake of innocent bystanders who may be near the robot if the PVC explodes, don't do it. Spending a little more money up front on better materials is well worth it if someone is severely injured from PVC shrapnel and decides to sue.

I agree. As I read my previous post, I realized that it could be seen that I was advocating a PVC air cannon. In no way am I doing so. I was simply stating how I built an air cannon. (Just a little disclaimer)

PVC is dangerous folks, and shouldn't be used for super-atmospheric pressures

[vivek16]

Originally Posted by pyro20911d

CRT 306 has had a safely functioning t-shirt cannon for some time now..using non-pressure rated pvc is perfectly fine for the minimal pressures that you're going to be working with..120 psi=not much..hit up any of our team members for more info..aim/ pyro20911..my cell # 814-462-4904

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

[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

[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

[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.

[Spartan151]

Okay. I've been following this for a couple days. I figured that a t-shirt cannon would be a nice idea for our team, since at least half of us are graduating this year. This leads me to ask some questions before attempting a build.

Question 1: In regards to the air tanks, what kind of material could I use for pressurized air storage?

Question 2: Is it possible to use a high-pressure rated tank or such for a "rifle-style" t-shirt cannon without the need of having to carry around a storage tank?

Question 3: On the barrel part, since I DON't plan to use PVC for storage, can I use a strong duct tape-wrapped PVC pipe for just a place to load the t-shirts?

Question 4: What is the best way to design one that is safe? (We don't have a huge budget, so buying one is a bit of a problem.)

Question 5: What kind of sprinkler valve/solenoid should I use and where can I get one from?

[Mr. Freeman]

I don't know what thread it was in, but someone on this forum posted awhile back about doing some tests and finding that duct tape doesn't prevent shrapnel.

[Daniel_LaFleur]

Originally Posted by Spartan151

Okay. I've been following this for a couple days. I figured that a t-shirt cannon would be a nice idea for our team, since at least half of us are graduating this year. This leads me to ask some questions before attempting a build.

Question 1: In regards to the air tanks, what kind of material could I use for pressurized air storage?

Question 2: Is it possible to use a high-pressure rated tank or such for a "rifle-style" t-shirt cannon without the need of having to carry around a storage tank?

Question 3: On the barrel part, since I DON't plan to use PVC for storage, can I use a strong duct tape-wrapped PVC pipe for just a place to load the t-shirts?

Question 4: What is the best way to design one that is safe? (We don't have a huge budget, so buying one is a bit of a problem.)

Question 5: What kind of sprinkler valve/solenoid should I use and where can I get one from?

1> Use steel or aluminum tanks rated for 3x higher than your working pressure. Other materials may be used but stay away from PVC as it fails explosively.

2> You can use a smaller High pressure tank. The compressed air will weigh the same and the tank will weigh less. Again I suggest a 3x safety margin from storage pressure. Also, you will need to regulate down from this high pressure and that may cause restriction in the air flow.

3> Duct tape does virtually nothing if the PVC fails. DO NOT USE PVC. I cannot stress more that PVC does not like the shock loads that an air cannon provides and when it fails (not if) it will fail explosively.

4> Understand the forces used and the gas laws that apply (especially that gas exerts the same force in all directions ... not just out the barrel). Design with safety in mind and always err on the side of caution.

5> You need a valve with a large orifice that unseats quickly. Many have chosen sprinkler valves for this type of use ... but I'd look closer at vane valves myself.

Good luck and play safe.

[Tom Line]

Originally Posted by 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

It terrifies me when people make posts like this without understanding they may very well be placing somebody in harms way.

I wonder what type of professional engineering or material engineering degree this person has. I wonder what license and what expertise are this person bases his statement on.

OSHA and the manufacturers say NO.

http://www.osha.gov/dts/hib/hib_data/hib19880520.html