Fox,
The same thing happens if Loctite gets on the Lexan.
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;
- 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.
- The air tank in question is listed as being made of polypropylene but the way it shattered upon failure is cause for concern.
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.
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
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
What type and part number fittings did you use? Did you inspect the tanks after screwing the fittings?
:eek:
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’.
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.
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.
I wonder whether mere tensile strength is sufficient, or whether the shearing action of those shards in motion is a consideration? And do small shards pose less of a risk due to their small mass (like birdshot compared to buckshot)?
Although your suggestion of a sock sounds pretty good, what about versions of the mesh bags used to contain onions and other similar vegetables? (Those might be easier to incorporate into existing designs.)
This is an opportunity for a good discussion on risk, and what drives prudent decision-making. How much of the hazard and the associated uncertainty are realistically transferable between the specific tank that failed, others in its lot, others of the same model, others in the product line, and other product lines of similar manufacture? Or is this strictly a matter of perception being everything—action is being taken, so outsiders will perceive it as the responsible course of action?
I’m curious about what specific factors are driving your decision—not particularly to criticize you, but to discuss the proposition that a drastic response often isn’t a methodologically valid way to approach a spectacular failure.
After all, you (and everyone else) obviously take plenty of risks with the students. It’s our responsibility to try to manage those risks in a sensible way.
2 tanks burst at the same time? What was the ‘leak checking’ procedure? (tightening fittings or spraying soap?)
Were the tanks connected together with a common brass fitting, or a flexible hose between them?
Guys,
The OP posted that further examination showed what appeared to be a manufacturing defect in the plastic. Please remember that these tanks have been used on FRC robots in the past without failures.
In addition to Al’s statement above: Clippard,* FIRST* and AndyMark take safety VERY seriously and all of those parties are currently evaluating the situation. I believe they will have a statement and course of action very soon.
Al, correct me if I’m wrong, but I was of the impression that these Clippard tanks were new, and not before tested in an FRC environment, but that they were an answer to similar model tanks from another manufacturer used in the last couple FRC seasons by some of the heavy hitters like 217 and 1114.
Yes the Clippard tanks are new and not what we used last year. We will be harvesting the tanks from last years bots until this is resolved.
Would it make it safer to put a wrapping of duct tape around them, at least for testing and until we are given an actual course of action? I realize this would likely render them illegal, but might help to contain any shards since dict tape usually splits on a seam.
The chemicals in the adhesive of duct tape could also potentially weaken the plastic, I would be wary of this as a solution.
Theoretically, same materials, similar specs. I would suspect we are going to have a response soon. Please be patient. Use the tanks from last year in the meantime.
Indeed they should, but all too often people who don’t understand the science behind a decision are the ones who make it, especially when the government is involved…
I’ll never forget teaching in California when a law was passed (unanimously by both houses and signed into law by Governor Davis) that outlawed “chemicals made from dangerous elements” in science classrooms… Further investigation indicated that “dangerous” included anything “toxic” or “flammable”.
Not withstanding the absurd misunderstanding that toxicology is a matter of dosage (and that even water ingested in sufficient amounts is toxic), the ignorance of even middle school level chemistry was quite astounding.
Among the things unintentionally outlawed in California science classrooms: matches, table salt, water, air, humans.