Safety Alert please read!

[Kevin Sevcik]

Originally Posted by FrankJ

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.

[FrankJ]

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.

[Al Skierkiewicz]

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.

[fox46]

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.

[Tristan Lall]

Originally Posted by FrankJ

But then it would not be the commonly available PVC that the warning applies to.

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.

Originally Posted by FrankJ

The warning also allows covering the PVC with a shatter proof material covering it. That would be an engineered solution I suppose.

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.

[Al Skierkiewicz]

Fox,
The same thing happens if Loctite gets on the Lexan.

[G Fawkes]

There is plastic pipe suitable for compressed gas use and they are typically labeled as such. They are designed to split open upon failure and not throw fragments.

I mentioned this issue for 2 reasons;

1) A lot of folks do not realize that it is not a good idea to use the rigid PVC water pipe from the hardware store for compressed gases (unless buried or encased) because it shatters upon failure.
2) The air tank in question is listed as being made of polypropylene but the way it shattered upon failure is cause for concern.

[ftc3799]

Update on the exploded tanks, I had time today to do some analysis of the fragments, first of all this is of my own opinion and I have reviewed this with some of my colleagues to confirm my findings. This had nothing to do with the handling of tanks all marks on the tanks came from the parts being scattered across a machine shop floor. It also had nothing to do with temperature the tanks had been at room temperature for a few days. Like most before looking we were thinking damage by over tightening but this was not the case. What I found was a elongated air bubble in the material. The bubble looked like a worm hole around part if the threaded openings base expanding in size and thinning out the threaded wall. My assumption is the fitting stressed the wall enough to open a crack to the bubble that ripped apart when under pressure. When the tank exploded the fragments caused the tank next to it to blow apart.
The one thing I would like to know would be does the manufacturer do any type of vision inspection either by ultrasound or x-ray of the tanks. Because if they don't how many more of these tanks could have hidden rupture points.

[Dave Campbell]

I will make sure the folks at Clippard see this thread first thing Monday morning. They are one of our team sponsors.

[Andy Baker]

Originally Posted by Dave Campbell

I will make sure the folks at Clippard see this thread first thing Monday morning. They are one of our team sponsors.

We at AndyMark have contacted the manufacturer about this issue, as has FIRST. More news will come from this today or tomorrow. Hopefully, we will have explanations and a direction of what to do.

Andy Baker

[mikegrundvig]

That failure really surprises me, I would have not expected it to shatter. I've seen burst PVC pipe (schd 40) where people used it to make pneumatic air cannons. They charged and then dropped the cylinder and it exploded. This failure looks EXACTLY like that.

My guess is that this cylinder had been dropped at some point on a corner or edge which can stress fracture and weaken it a lot. Never EVER drop a charged cylinder - very bad things happen. Another possibility is that it's been in the sun for a long while at some point in it's life, that can weaken most plastics a lot too. Some weakening combined with over-tightening and stressing the threads == kablowie!

-Mike

[ttedrow]

What type and part number fittings did you use? Did you inspect the tanks after screwing the fittings?

[JDL]

When we first got those things the first things that came to my mind were, 'someone is gonna over tighten a fitting and crack it or strip the treads out' and 'I'd hate to get one with a defect'.

[Seth Mallory]

Thank you 3799 for bringing this to our attention. After reading about this Sunday morning we decided to change out our 2012 plastic tank and install a metal one in it's place. We felt that for safety reasons it was better to wait and see what the final determination of the problem is before going back to plastic tanks. Our tank is a different production style but why take any risks with the students.

[fox46]

I've been mulling this around in my head after seeing the kids had been experimenting with pneumatics this past weekend. In the interim, if teams insist on using the plastic tanks rather than switching to metal, what about placing some sort of containment device around the tanks? I was thinking a heavy sock with tie wraps around where the pneumatic tube connects to the fitting. At the very least it should help contain any shrapnel or at least reduce its kinetic energy should one rupture.