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.
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.
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.
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?
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?
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.
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.
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.”
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.
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.
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.
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! :yikes:
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.
That’s PVC pipe. I believe Tristan’s point was that PVC the plastic could be made in a non-shattering formulation with added plasticizers. After all, I can get high grip gloves that are coated in PVC. I’m pretty sure that formulation of PVC isn’t going to shatter at anything above dry-ice temperatures.
In other words, banning “PVC” from compressed air usage is about the same as banning “steel” from being used for something. Both words encompass many materials with a broad range of physical properties.
I expect your gloves will not hold 90 PSI either The warning is specifically about PVC & compressed air systems. I suppose a mfr could find a formulation, test it, test, certify it. But then it would not be the commonly available PVC that the warning applies to. The warning also allows covering the PVC with a shatter proof material covering it. That would be an engineered solution I suppose.
BTW I have several applications that I don’t use “steel” for as well.
Scott,
I hope you kept the pieces. I suspect Andy and Mark would like to take a look at these and contact the manufacturer. I certainly would like to see them.
The OSHA statement comes from an industrial accident in Texas at a chemical plant or refinery, I can’t remember which. The plant was plumbed for pressure lines between buildings and process positions. As an employee was walking past one of the pipes, it burst sending shards into his arm. With a little research you can find the original report and findings of the resulting investigation.
From the pieces shown, it is possible that the first failure occurred at the fitting but the resulting failure of the rest of the tank is unusual. Was the tank chucked up in a vise at some point to assist with fitting insertion? Is it possible that the two tanks were close enough that the failure of one caused a failure in the other? What compressor were you using? The large Thomas compressor can get up to 150-160 psi if the pressure relief valve is not calibrated but that should be well below the failure point for these tanks.
This reminds me of an experience I once had with some lexan sheet. Lexan is normally a very resilient material very resistant to a brittle fracture. On one of my projects I had used lexan panels as body parts on a vehicle. A friend of mine who was helping out decided to clean them using aerosol brake cleaner. After wiping them down with the brake cleaner and a rag, approximately one hour later the entire panel shattered into tiny pieces. We deduced that the chemical agents in the brake cleaner had been absorbed into the plastic causing it to become incredibly brittle. It is possible that a similar process may have affected these two tanks.
That’s just the thing: apparently some state OSHA offices prohibited PVC pipe outright (e.g. Arizona, from which the quotation at issue was sourced) without specifying whether they were only talking about the conventional PVC water pipe. Unless they are confident that no safer PVC pipe can be made, they should only restrict the specific products that pose the hazard.
Exactly: that better-engineered solution conflicts with the prohibitions described by the Arizona, Washington and possibly Nevada OSHA offices. The main OSHA memorandum is better, because it recognizes appropriate ways to use the pipe.
The warning is specifically about PVC & compressed air systems. I suppose a mfr could find a formulation, test it, test, certify it. But then it would not be the commonly available PVC that the warning applies to. The warning also allows covering the PVC with a shatter proof material covering it. That would be an engineered solution I suppose.