Safety Alert please read!

[ftc3799]

I am a mentor for FRC team 3799 on Sat. 2/26 we had a serious situation occur while pressurizing our Clipper PVC air cylinders. Please read the below letter written by our program leader to our Regional Director.

Glen,
During robot testing today we had a catastrophic failure of two of the new Clippard Air Tanks simultaneously. Luckily no one was seriously hurt. We had them pressurized to approx. 110 psi and during leak checking they blew apart into small shards (see photo). I sent a note to Andy Mark (the supplier we purchased them from) earlier today. If there is someone at FIRST that should be aware of this potential safety issue please forward this to them. We were excited about using the new lightweight tanks but as a result of this have now decided to convert to an aluminum tank.

Scott



We don't really know what caused this but we just want to make sure everyone is aware of this and are taking all proper safety cautions while testing and using.

[sanddrag]

Wow! We haven't had a problem with our plastic tanks from Pneuaire. This just serves as a reminder of the power of a compressed gas. Hopefully everyone was okay.

[Nirvash]

Glad to hear that no one was hurt.
Wondering, did you use exercise force in screwing in the fittings on the tanks or mount them in a way that was putting more then need force on the tank?

[Botwoon]

The tank looks pretty marked up. Was it dropped beforehand?

[thefro526]

This is an interesting failure.

Just out of curiosity, with the recent cold weather, what were the odds that the tank was exposed to temperatures outside of it's operating range before the failure?

[Tristan Lall]

Most ordinary industrial pneumatic parts are designed to burst at over 300 lb/in²; the working ratings (here 150 lb/in²) are lower so that you never approach the burst pressure, even in a dynamic load situation, and are consequently very conservative in an essentially static case.

Were you running any liquid in them, or running them in a liquid?

Did the tanks ever get exposed to any chemical agents whatsoever? Markers/paint/organic solvents/adhesives/etc.? (The only thing¹ you should clean them with is water: not even isopropanol is guaranteed to be safe.)

¹ Edit: Unless you've properly researched the cleaning agent's compatibility with polypropylene, and particularly its tendency to either cause crazing or to dissolve the plastic.

[Kevin Sevcik]

I'm interested in the fact that one of the fittings on the end has been split in two and the other is completely intact. The fitting is a pretty beefed up area that shouldn't be under a lot of stress from the pressure. I'm leaning towards someone overtightening the fitting on that end and starting a crack or creating a highly stressed area where a crack is likely to form. NPT threads are tapered, so it's pretty easy to crack a fitting by overtightening. I've seen guys at work crack brass fittings sometimes, after all.

[Mr. Lim]

We installed fittings on these tanks yesterday, and I can absolutely see how this happened.

As a student and I were tightening the NPT fittings, we wondered "when should we stop?"

The plastic ports didn't offer increasing resistance as you tightened fittings into them, like metal ports do. They just spun and spun and spun with the exact same resistance as we went.

We stopped when we still saw 1-2 threads exposed on the fittings.

They still felt loose in comparison to when you put fittings on metal components, but they did not leak.

WE VERY EASILY could have tightened them so much that the fitting bottomed out, and only then would it feel like the turning resistance increased. By then, you're pre-loading the thread, and could fracture the end of the cylinder...

This is speculation of course, but for any teams using these tanks:

Tighten your fittings until you see 1-2 threads exposed. That is tight enough to NOT LEAK, given you've properly applied teflon tape/have sealant already applied to the fitting. Most fittings in the pneumatic KoP bag have white sealant already applied to the threads.

Don't risk tightening them any more, if this is "tight enough."

[G Fawkes]

There are numerous OSHA references on the internet that say;

"It is our position that PVC pipe shall not be used as a means of transporting compressed air. "
http://www.osha.gov/dts/hib/hib_data/hib19880520.html

The danger is that it shatters upon failure. There are types of plastic pipe rated for compressed gas use but I do not think the standard PVC pipe should be used for compressed air.

Checked AM and the Clippard air tank is listed as made of polypropylene plastic -not PVC but given how it shattered I would be wary of using.

[billbo911]

I have to ask this question. Do we know for certain that these tanks are PVC?
The failure definitely looks like a PVC failure . Is there any chance that these are made of another material?

I would be very surprised if these actually were PVC! Providing those cylinders as FIRST CHOICE items, if they are PVC, is an egregious failure in itself.

[Mr. Lim]

They are polyprolyene...

http://www.clippard.com/part/AVT-PP-41

[Tristan Lall]

Originally Posted by G Fawkes

"It is our position that PVC pipe shall not be used as a means of transporting compressed air. "
http://www.osha.gov/dts/hib/hib_data/hib19880520.html

The danger is that it shatters upon failure. There are types of plastic pipe rated for compressed gas use but I do not think the standard PVC pipe should be used for compressed air.

I suspect that with the inclusion of plasticizers, a PVC pipe could be produced that did not exhibit this failure behaviour.

It's an overreach to take the position that "PVC pipe shall not be used as a means of transporting compressed air" without qualifying the pipe's composition, manufacturing process, installation and operating conditions. Despite their strongly-worded statements, I hope that OSHA is recognizing competent engineering as the appropriate way to circumvent their prohibition.

[Ivan_Yuri]

The Cy-Ranch Cy-Borgs have had several issues with the PVC-esque tubes for pressure, one had a hairline crack in it that ran down a good length of the piece of tubing, thankfully we found it and marked it out of the crowd, we have quite a few pieces of that stuff labeled to go on our robot, but I'm going to send our lead mentor this thread just to be safe! So sorry for your setback, best of luck on recovery!

[Tristan Lall]

Originally Posted by Ivan_Yuri

The Cy-Ranch Cy-Borgs have had several issues with the PVC-esque tubes for pressure, one had a hairline crack in it that ran down a good length of the piece of tubing, thankfully we found it and marked it out of the crowd, we have quite a few pieces of that stuff labeled to go on our robot, but I'm going to send our lead mentor this thread just to be safe! So sorry for your setback, best of luck on recovery!

What's the part number of the component? (Are you talking about the plastic air reservoirs or the plastic pneumatic tubing?)

[FrankJ]

Originally Posted by Tristan Lall

I suspect that with the inclusion of plasticizers, a PVC pipe could be produced that did not exhibit this failure behaviour.

It's an overreach to take the position that "PVC pipe shall not be used as a means of transporting compressed air" without qualifying the pipe's composition, manufacturing process, installation and operating conditions. Despite their strongly-worded statements, I hope that OSHA is recognizing competent engineering as the appropriate way to circumvent their prohibition.

PVC pipe manufacturers also say do not use PVC pipe for compressed air systems. The big problem is that it shatters on failure. That warning along with the OSHA statement is enough for me not to use on compressed air systems. For OSHA competent engineering means following their warnings.

One possibility is the fitting was overtightened, starting the failure.