T-shirt Launcher

[chapman1]

I share everyone's rightful concern for safety, and I certainly lack credentials on PVC and air pressure vs. water pressure. At the same time, I have to question whether PVC knows what's inside it (water or air). Pressure, it seems, is pressure.

The documented examples I have seen of PVC failure have occurred at low temperatures, where I will agree, PVC gets brittle and dangerous. If inside a warm building at 1/2 rated pressure, I would tend toward not worrying excessively.

[GeeTwo]

Originally Posted by chapman1

The documented examples I have seen of PVC failure have occurred at low temperatures, where I will agree, PVC gets brittle and dangerous. If inside a warm building at 1/2 rated pressure, I would tend toward not worrying excessively.

We'd never get permission to put our tires on the gym floor. The main place we use our air cannon is outdoors during football season. Even in the deep south, I'm getting a bit queasy.

[Daniel_LaFleur]

Originally Posted by chapman1

I share everyone's rightful concern for safety, and I certainly lack credentials on PVC and air pressure vs. water pressure. At the same time, I have to question whether PVC knows what's inside it (water or air). Pressure, it seems, is pressure.

The documented examples I have seen of PVC failure have occurred at low temperatures, where I will agree, PVC gets brittle and dangerous. If inside a warm building at 1/2 rated pressure, I would tend toward not worrying excessively.

You are incorrect that pressure is pressure. Liquid (which PVC is rated for) is incompressible where gasses (That PVC is not rated for) are compressible.

The issue with PVC is the failure mode. When PVC fails with liquid in it the internal pressure relieved immediately (due to liquid being incompressible) thus there is little time to accelerate the shards that are generated during a failure. Gasses, though, relieve pressure only as fast as the gas can escape, thus giving the force plenty of time to accelerate the PVC shards.

Hope this helps to explain the difference.

[Chris_Ely]

Originally Posted by chapman1

I share everyone's rightful concern for safety, and I certainly lack credentials on PVC and air pressure vs. water pressure. At the same time, I have to question whether PVC knows what's inside it (water or air). Pressure, it seems, is pressure.

The documented examples I have seen of PVC failure have occurred at low temperatures, where I will agree, PVC gets brittle and dangerous. If inside a warm building at 1/2 rated pressure, I would tend toward not worrying excessively.

PVC also deteriorates with exposure to UV radiation. This may not be a problem immediately, but years down the road this will become a concern.
The cement used to bond PVC is also not rated for air pressure.

[hrench]

We've built a t-shirt cannon, and I'm going to be really unpopular here and mention something that the non-PVC'ers haven't said.

PVC is rated for high enough pressure--it just isn't rated for air pressure. The reason is because of water-hammer--go figure--that it's fine to use PVC with water and not with air.

"Water hammer" if you didn't know, is a pressure spike that occurs when you close a valve quickly on moving flow, which can cause instantaneous spikes of pressure as high as 400psi or more. Water hammer--with air--is what explodes PVC pipes. I used to design railroad pneumatics, so yes, I've exploded PVC pipes before. It's loud and dangerous and I don't like loud and dangerous. Please forgive me, we've most all done stupid stuff before.

But think about this...If I use a sprinkler valve that's plumbed with steel NPT fittings--I have all metal in the 'firing' section of the system.

I only need to use PVC for the barrels. The barrels CAN NEVER see a spike caused by water hammer, because you cannot shut off the air you've put in them. If the barrels were to fail--even catastrophically, the worst thing that could happen is that it shoots a t-shirt. One end of the barrel is open to the world. Even if they developed a huge crack in them, the air would still exit the open end with the t-shirt.

People have been making potato guns and t-shirt cannons with PVC barrels for many years. Do you know how much a three-inch ID piece of steel or copper costs? Or how much it weighs? Just to be safe, we never run more than 60 psi, but our 'gun' is four years old and still not broken. Also, the design of a sprinkler valve--which is piloted--prevents it from being closed quickly. Another built-in protection from spikes.

We built a working 2-PVC-barrel t-shirt cannon and the shooting parts of it cost less than $50.

The air-bubble, the Thomas compressor and the kit-frame all came from 'inventory' in the shop.

I think all of the no-PVC'ers here are going to prevent many teams from building t-shirt guns.

[Chris_Ely]

Originally Posted by hrench

snip

I agree with you that using PVC for the barrel is fine because the barrel is not storing pressure. I am more concerned about people using PVC as the pressure storage tank.

[GeeTwo]

Originally Posted by hrench

"Water hammer" if you didn't know, is a pressure spike that occurs when you close a valve quickly on moving flow, which can cause instantaneous spikes of pressure as high as 400psi or more. Water hammer--with air--is what explodes PVC pipes.

Water hammer produces such high pressures because the water is incompressible and massive, so stopping it suddenly requires a large, quickly applied force - hence a hammer. Air is light and compressible. In fact, small vials of air are used as water hammer suppressors. Bottom line is, that I don't think that the problem with air in PVC is really water hammer.

I'm still planning to replace those PVC barrels.

[Daniel_LaFleur]

Originally Posted by hrench

We've built a t-shirt cannon, and I'm going to be really unpopular here and mention something that the non-PVC'ers haven't said.

PVC is rated for high enough pressure--it just isn't rated for air pressure. The reason is because of water-hammer--go figure--that it's fine to use PVC with water and not with air.

"Water hammer" if you didn't know, is a pressure spike that occurs when you close a valve quickly on moving flow, which can cause instantaneous spikes of pressure as high as 400psi or more. Water hammer--with air--is what explodes PVC pipes. I used to design railroad pneumatics, so yes, I've exploded PVC pipes before. It's loud and dangerous and I don't like loud and dangerous. Please forgive me, we've most all done stupid stuff before.

But think about this...If I use a sprinkler valve that's plumbed with steel NPT fittings--I have all metal in the 'firing' section of the system.

I only need to use PVC for the barrels. The barrels CAN NEVER see a spike caused by water hammer, because you cannot shut off the air you've put in them. If the barrels were to fail--even catastrophically, the worst thing that could happen is that it shoots a t-shirt. One end of the barrel is open to the world. Even if they developed a huge crack in them, the air would still exit the open end with the t-shirt.

People have been making potato guns and t-shirt cannons with PVC barrels for many years. Do you know how much a three-inch ID piece of steel or copper costs? Or how much it weighs? Just to be safe, we never run more than 60 psi, but our 'gun' is four years old and still not broken. Also, the design of a sprinkler valve--which is piloted--prevents it from being closed quickly. Another built-in protection from spikes.

We built a working 2-PVC-barrel t-shirt cannon and the shooting parts of it cost less than $50.

The air-bubble, the Thomas compressor and the kit-frame all came from 'inventory' in the shop.

I think all of the no-PVC'ers here are going to prevent many teams from building t-shirt guns.

This is not correct.

Water hammer spikes pressure as high as it does because it is not compressible. And while Air hammer will occur, the pressure spike is much smaller.

The issue, again, with PVC and gasses is the FAILURE MODE. A barrel will see at least enough pressure to move the ammunition, if not significantly more pressure. Should the barrel fail, it will fail catastrophically and explosively. Please, DO NOT USE PVC for ANY gas pressure.

and just because some people have made potato guns out of PVC and pressurized air, does not make it right or safe. Remember, these t-shirt cannons are, by their very nature, designed to be in front of the public where a catastrophic failure could cause injury to people near by. Think of this before you design your cannon.

[hrench]

Originally Posted by Daniel_LaFleur

This is not correct.

Water hammer spikes pressure as high as it does because it is not compressible. And while Air hammer will occur, the pressure spike is much smaller.

The issue, again, with PVC and gasses is the FAILURE MODE. A barrel will see at least enough pressure to move the ammunition, if not significantly more pressure. Should the barrel fail, it will fail catastrophically and explosively..

n.

Okay, I have quite a few people disagreeing with me saying that the failure mode for PVC isn't water hammer, but no one presenting evidence. If you simply read the wikipedia entry on water hammer, you'll see that is it common with air too. With water it isn't explosive as it is with air because water isn't compressible. Even the engineers studying this airplane crash attributed it to water hammer.

And it is true that in a water-system you can introduce compressible trapped-air to prevent water hammer because you've introduced a 'spring' and suddenly you have a second-order differential equation-- that system won't be able to find 'reflection' to 'hammer' the end into a pressure spike.

And I've seen some other people saying that the barrel will "fail catastrophically" but it's not clear to me how even that can result in shrapnel from the back end of the PVC when the barrel-end of the system is OPEN. And again, I point out, I've done this and I've seen PVC fail using it for air pressure on a test system for train brakes. I didn't shrapnel, it just blew at a joint. I still maintain that the barrel won't introduce shrapnel, even in a catastrophic failure. It will blow a joint or split.

Also, when I blew up that accumulator tank that I was using, it was while quickly opening then CLOSING 1-inch dia piloted valves. I maintain you have to close a valve to get water hammer and with these systems, you never close a valve under high flow.

If I'm wrong...and I sometimes am...provide me with a logical explanation of why and how PVC--rated to usually 100+psi for water--fails with air.

[hrench]

Also, if you don't want liability, don't build a t-shirt launcher. The purpose of this machine is to aim projectiles at the crowd.

Projectiles are dangerous and litigious.

[twetherbee]

The pyrotechnics industry also highly discourages using PVC pipe. When PVC fails, it shatters and creates splintered shrapnel which can seriously injur or kill someone. They use EPDM pipe or fiberglass for mortar tubes from 1.75" up to 12" diameter (for the big 4th of July shows). EPDM pipe is more expensive and a little harder to find that PVC pipe, but fails in a more ductile (vs. brittle) manner.

http://www.pyrouniverse.com/FAQ.htm#9 (this has some good examples of failed PVC pipe)

This is for information only. I don't encourage anyone to build a t-shirt canon with any plastic.

Originally Posted by hrench

Also, if you don't want liability, don't build a t-shirt launcher. The purpose of this machine is to aim projectiles at the crowd.

Projectiles are dangerous and litigious.

+1

[Landonh12]

This is a little off to the side of the main conversation, but here is the T-shirt launcher we made last summer. It uses a balloon tank with a sprinkler valve and 2 Viair compressors. It has a water separator to keep moisture out of the tank (our last one rusted through the bottom). It also has a analog pressure sensor so I can set the pressure shutoff psi.

It runs off a cRIO-II. We will be putting addressable LEDs on soon.

http://youtu.be/H_IjhJFgQIY

[FrankJ]

Originally Posted by hrench

Okay, I have quite a few people disagreeing with me saying that the failure mode for PVC isn't water hammer, but no one presenting evidence. If you simply read the wikipedia entry on water hammer, you'll see that is it common with air too. With water it isn't explosive as it is with air because water isn't compressible. Even the engineers studying this airplane crash attributed it to water hammer.

...

First of all any rigorous discussion on fluid dynamics that mentions water hammer and air in the same sentence gets points off. But this is CD and not rigorous. :0
Taking the second reference first. The "water hammer" reference was discounted by the NSTB. The accepted conclusion was aluminum has a fatigue life & the plane exceeded it by the high number of take off & landings relative to the relatively short flight miles of the air frame. See the Dehavilland Comet failures for another classic example of the fatigue of aluminum and stress risers which BTW effectively handed the commercial airplane market to the United States.

First example second. You should really read the article. Water hammer requires a quick closing valve on the exit of the fluid column. The pressure spike comes from the rapid deceleration of the fluid column. The spike is increased by the weight (directly related to the density) & the velocity of the column. It is decreased by the compressibility of the elements involved. So when you shoot a tee shirt, you might see actually see a vacuum in the barrel when the tee shirt exits the tube. Hard to predict what happens upstream of the valve without the specifics. I expect we see a vacuum spike in the accumulator of our cannon. it is a relatively small volume & we precharge it to 20-120 psi & fully discharge it on each shot. The cannon will constantly shoot a 70 yard field goal.

The reason why you get a large number of "PVC" works for me is that the pressure involved is well within the working pressure of PVC pipe. Failure is not a regular occurrence. The issue comes from the severity of the failure mode & the documentation from both the pipe manufacture & OSHA saying don't do it.

[hrench]

Originally Posted by Chris_Ely

PVC also deteriorates with exposure to UV radiation.

The 'gray' PVC is 'UV stabilized', but don't forget that simply painting PVC protects it from most UV.