Air Cannon

[FourPenguins]

Our school hasn't forced us to do a safety run. We just talked to the athletic director and he thought it was cool. Before the first game, he may require some demos though.
As for proving it's safe, I'd say do some test fires, maybe have people catching them up in the bleachers, prove they aren't getting hurt.

[artdutra04]

Originally Posted by FourPenguins

Our school hasn't forced us to do a safety run. We just talked to the athletic director and he thought it was cool. Before the first game, he may require some demos though.
As for proving it's safe, I'd say do some test fires, maybe have people catching them up in the bleachers, prove they aren't getting hurt.

Is any part of it made out of PVC? If so, you may want to look into shielding it (in case of rupture) or using higher-PSI rated fittings. (PVC, though often labeled on the pipe itself as able to hold enough PSI to be considered "safe", is never supposed to be used for any sort of pressure application.)

Replacing the PVC with safer alternatives, such as air hoses and metal fittings rated for high pressure gas use with sufficient PSI rating (including a safety factor), would be a smart idea.

[FourPenguins]

No PVC is used for stored pressure (that's in a craftsman pressure tank).
The barrel and a couple of fittings are PVC, but these will never be under high pressure as the barrel is open.
(Any further questions, comments criticisms, PM Mike Nawrot. It's his baby.)

[Otaku]

I strongly suggest that you remake the barrel out of ABS plastic. Unless you have a burgoiuse amount, it should be pretty cheap (I got 5 feet of 2 inch dia ABS plastic for about 6$).

Even though the PVC will not be subjected to pressures over 60 (probably) for more than one second at most, if PVC fails it will shatter, whereas ABS will split or crack.

So, in terms of safety, Cracking > Shrapnel.

[Gabe]

Believe it or not, simply painting the cannon very nicely can be enough to convince people of its safety, if you are having trouble with that. People are easily swayed by appearance.

[Pat Arnold]

MARS Team 1523 created a t-shirt launcher over the summer too and has also run into school approval issues. Initially it was approved for use as long as it was mounted to a robot. Now the high school will not allow the "launcher's" use without team liability insurance. (We've found that calling it a "launcher" rather than "shooter" or "air canon" garners greater support. The earlier post about painting it is also true)

As far as safety, we discussed the design with an ME, EE, & PhD Chemist. (MARS used heavy wall schedule 40 PVC (whole thing) and fill the launcher to 80 psi.) Here were their thoughts : Regarding PVC bursting, it is dependent on the pressure and size being used. Typically what will happen is a crack will occur and slowly propagate at lower pressures. At the pressures being used in our launcher, crack propagation causing more than a leak would probably only occur if a weakness was present (such as the PVC was poorly made - which could be said of any material being used) or if mistreated (pouring a solvent on it, using a sledge on it, etc.). Where care must occur is in the glue joints, which if not done properly could leak or, if very poorly done, come off rapidly.

With others in the CD community suggesting PVC explosion, shrapnel, etc, I'm curious if they still feel this is a concern with launchers built using designs similar to MARS (heavy wall schedule 40, 80 psi fill).

[Otaku]

I'd still see it as an issue. Since the chance for failure is always present (and mostly the damage is microscopic but at those pressures you will magnify the problems) and PVC does tend to react in the way it does, it would really be your best bet to switch to ABS plastic.

Search on YouTube for Potato Cannon explosions. You'll notice all of those ones are PVC. That's why I built mine out of ABS.

[Pat Arnold]

Originally Posted by Otaku

I'd still see it as an issue. Since the chance for failure is always present (and mostly the damage is microscopic but at those pressures you will magnify the problems) and PVC does tend to react in the way it does, it would really be your best bet to switch to ABS plastic.

Search on YouTube for Potato Cannon explosions. You'll notice all of those ones are PVC. That's why I built mine out of ABS.

I don't think a total rejection of heavy wall PVC is warrented. Schedule 40 is rated for 200 psi and that's a conservative number (probable factor safety of 2). Household water pressures can run 70 psi (think of all the pvc used without a problem, i.e. water exploding from pipes) and I know of businesses using PVC tubing for compressed air running 120 psi.

[Otaku]

You'll also notice that all proper usages for PVC are underground, and that's for a reason...

[lukevanoort]

Originally Posted by Otaku

You'll also notice that all proper usages for PVC are underground, and that's for a reason...

Probably because it weakens with long term exposure to UV light. A quick coat of paint should fix that right up. (assuming it is the correct type of paint, one that doesn't weaken the PVC)

[artdutra04]

Originally Posted by Pat A.

I don't think a total rejection of heavy wall PVC is warrented. Schedule 40 is rated for 200 psi and that's a conservative number (probable factor safety of 2). Household water pressures can run 70 psi (think of all the pvc used without a problem, i.e. water exploding from pipes) and I know of businesses using PVC tubing for compressed air running 120 psi.

It's actually against OSHA guidelines to use PVC for any kind of (non-buried) compressed gas use.

And easier way to think of the PVC vs. non PVC debate for air cannons is to compare acrylic to Lexan. We all know that acrylic may work for many uses on our robots, but we one needs to baby it in order to prevent it from cracking and shattering. If they used Lexan on the other hand, they don't have to worry as much about catastrophic shattering.

[Schnabel]

Originally Posted by GeeForce

Psch, your guys' cannon is dangerous. The high school from IRI has a few new holes in its ceiling

[FourPenguins]

I've decried the use of PVC tanks before, and I stand by that. I would never use PVC for storage of any more than atmospheric pressure. I've seen what it can do, and i know someone with the scar to prove it.
However, the odds of damage to the barrel of the cannon are fairly low as the air is bound to follow the path of least resistance. In a sealed tank, this would be any crack or weakness in the tank. In an open barrel or the pipe to an open barrel, this would be the opening in the barrel, with only the resistance of a light shirt.
Is there some risk of a leak, a crack, or a blowout? Yes. Would that risk exist with any material? Also yes, although perhaps it would be a smaller risk with heavier/stronger materials. Like everything, it's a case of acceptable risk.

[Rob2713g]

The air cannon we built is pretty simple. We already had a sprinkler valve (which limits flow to 110PSI) and used PVC rated at 280PSI. I'll bring up changing to ABS. I uploaded a picture here. Thanks for the advice!

[Jim E]

Liability Insurance.
Thats a biggie. Is it worth $1500/yr or more for $1M policy to shoot a T-shirt into the stands? I think not!

I've seen industrial accidents happen with stored pressure. They ARE NOT PRETTY. Fortunately, nobody was seriously injured, but the insurance is there for a reason.

For tank safety, there is an aluminum-welded tank on ebay for about $16.00. This should solve that issue. Contact me if interested,for the link.

Of Course the kids-will-be-kids factor is always in play. At my high school, they fired an air cannon after touchdowns. It was located on the flat roof of the announcer's booth just to limit who had access.