exploding plastic storage tanks

[Lil' Lavery]

This isn't about whether or not these tanks are quality products, it's about risk. Even if the failures are 100% caused by "user error," the tanks' failure mode is an explosion that sends shard traveling at high speeds. There are two attributes to risk, probability and severity. While we obviously don't have access to FIRST's safety risk matrix, I know that regardless of how low we drive the probability of the risk, the severity is still to high for these tanks to be acceptable for use on 1712 (and I would hope other teams). I feel much more comfortable using a tank with a failure mode that doesn't involve high speed, sharp projectiles.

[AndreaV]

Having the failure point be at the push in fittings instead of at the sealed thread should overcome the explosiveness of the plastic tanks. I am not sure but the 125psi rating is likely given to the push in connector, and not the body of the tank itself.

Just like fuses are meant to to open an electrical circuit when current flow exceeds a predetermined range, this fail safe connection should the weakest link in the pneumatic system. This should make for a much safer alternative to the white pvc tanks.

It will be interesting to see how the black polypropylene tank's safety record will fare in comparison to the diy fitting pvc tanks. Any air tank with a crack will explode sooner or later, In my opinion any sort of plastic air tank with sharp interior corners (threads) should be banned from use in FIRST. All of the boom'ed air tanks are to my knowledge were a result of user error. Whether it be over tightened fittings or hose claps, these pressure vessels demand respect.

This might sound harsh but when you actually witness one go, I bet you would have a change of heart. Keep in mind this is an event geared towards children.

FIRST should step it up and add the mounting and placement of all air tanks to the robot inspector check list. I have inspected at two events so far, and am amazed at how many inspectors just ask the questions in a manner that all a team has to say is "yes" or "no". I understand that there are a lot of robots to inspect, but the job of the robot inspector is to ensure that the robot is legal and above all safe to compete.

This is a problem that Teachers, Mentors, and Officials have to look at more closely. You can't expect a bunch of high school kids to be the responsible ones here.

[Bill_B]

This thread is quite interesting and explanatory in several ways. I am reminded of the recent incident involving the painting of one of the tanks. As I recall, the painted tanks were declared illegal as modifications presumably due to chemical alteration of the tank by the paint.

The other think of interest is the operating temperature range for the tanks. I know certain plastics become very brittle by exposure to cold temperatures. Mailboxes, plastic fencing and furniture suffer adversely from sub-freezing temperatures. I do not know if the manufacturer has advised about how durable their products are in cold weather, but it seems irresponsible to sell an obvious outdoor object for use in a northern climate.

Back to the tanks. If the temperature range pertains to operating conditions and not "storage" then we may be OK with outdoor transport of the tanks. There's a robot with tanks in my unheated garage right now. Maybe I should be concerned if merely getting cold will alter the properties of those tanks.

On the other end of the heat range, there was some concern about heated air being stored. I hope someone can give some observations about this. I suppose the black color of the newer tanks will allow the re-radiation of heated air's temperature more readily than the white ones. That will help keep the system within operating temperature range. I wonder if there are some 1/4 inch i.d. finned heat sinks I can thread onto the exit tubing from the compressor. Will it be worth dealing with a metal tube as the direct output from the compressor? I know I can get finned tubing somewhere.

In any event, our robots will not go back into competition with the white tanks. Another reason to thank CD posters for the heads up.

[yash101]

Originally Posted by Tytus Gerrish

this makes me want to test one to failure just to see that it takes to bust em

You wish
That seems very fun [and dangerous]. Do not try this at home!


I think the best sleeve would be a kit of the tank and sleeve so the strongest parts of the tank are the places uncovered. The rest should be contained. We are using mostly metal tanks, but we might (I can't properly recollect) have 2 plastic tanks!

What are these tanks made of? "Polymer" is quite wide of a material definition. It feels like PVC, but I doubt PVC is that strong!

[AndreaV]

Originally Posted by yash101

I think the best sleeve would be a kit of the tank and sleeve so the strongest parts of the tank are the places uncovered. The rest should be contained. We are using mostly metal tanks, but we might (I can't properly recollect) have 2 plastic tanks!

What are these tanks made of? "Polymer" is quite wide of a material definition. It feels like PVC, but I doubt PVC is that strong!

The old white tanks are made of pvc, a quick google search reveals the new tanks are made of polypropylene.

Rupturing tanks at 120 psi is not fun.. not in the slightest. While there have been (luckily) no serious injuries from exploding tanks, it can leave you quite shaken. The sound is nothing to sneeze at either, at GTRW many students had momentary hearing loss and ear pain that lasted into the next day just from being in close proximity of it going off, and this was in a large open area.

[MrBasse]

Polypropylene, not PVC.

From the FIRST blog last year:
Over the weekend, it was brought to our attention that a failure of the white polypropylene air tank, donated by Clippard (Clippard PN: AVT-PP-41, AndyMark PN: am-2007, FIRST Choice PN: fc13-100), had occurred.

[Trent B]

Originally Posted by AndreaV

The old white tanks are made of pvc, a quick google search reveals the new tanks are made of polypropylene.

The old tanks were also made of polypropylene: http://www.chiefdelphi.com/forums/sh...d.php?t=112396

As are Pneuaire air tanks.

[Jon Stratis]

Originally Posted by AndreaV

The old white tanks are made of pvc, a quick google search reveals the new tanks are made of polypropylene.

Rupturing tanks at 120 psi is not fun.. not in the slightest. While there have been (luckily) no serious injuries from exploding tanks, it can leave you quite shaken. The sound is nothing to sneeze at either, at GTRW many students had momentary hearing loss and ear pain that lasted into the next day just from being in close proximity of it going off, and this was in a large open area.

Correction. The old white tanks, Clippard PN AVT-PP-41, are polypropylene as well, not PVC. I am not aware of a single manufacturer that rates PVC for use in gas storage - PVC is generally only rated for liquid pressure.

[AndreaV]

wow I was corrected 4 times over super quick! thanks!

[Jon Stratis]

Originally Posted by AndreaV

wow I was corrected 4 times over super quick! thanks!

Everyone makes mistakes

[Moon2020]

Additionally, plastic becomes brittle with age.
It is why infant seats for cars have an expiration date.

We use Consequence vs. Likelihood in our 5x5 risk matrices.
All components that see 125 psi should be rated for 125 psi or above. There is a factor of safety already built into their rating. Make sure the PRV is calibrated properly. Is the gage calibrated properly?

The hazard is from the shrapnel created. The cause can be tank rupture due to: brittle plastic, over pressurization (not properly calibrated PRV), manufacturing flaw, design flaw, workmanship issues (assembly), damage from improper handling, mistreatment, environmental factors (temp, humidity, etc.), etc.

[yash101]

So that makes me think, if these tanks can easily become such a hazard, why are they legal? We have our tanks outside, right under our catching mechanism so if something fails, the tanks are in risk of being damaged. However, those are the metal tanks

[Trent B]

Just doing some rudimentary comparisons with CES EduPack (materials database).

PP seems to generally have higher impact toughness (absorbs more energy when fracturing) than PVC but is slightly lower yield strength. Tensile strength differences are pretty marginal.

Probably the biggest draw for PP over PVC is the comparative glass transition temperatures, PP is below freezing while PVC is nearly the boiling point of water. This is a reversible transition that is accompanied by a change in elastic modulus (stiffness), addition when above this temperature the polymer should be more ductile which correlates with PP taking more energy to fracture.

[cgmv123]

Originally Posted by yash101

So that makes me think, if these tanks can easily become such a hazard, why are they legal? We have our tanks outside, right under our catching mechanism so if something fails, the tanks are in risk of being damaged. However, those are the metal tanks

The tanks themselves are perfectly safe. It's using them properly that's the issue. Putting stress on the tank increases the likelihood of it bursting.

[Daniel_LaFleur]

Originally Posted by cgmv123

The tanks themselves are perfectly safe. It's using them properly that's the issue. Putting stress on the tank increases the likelihood of it bursting.

... and this is the core of my issue with the tanks.

You have students that have very little understanding of the forces and failure modes and 'how to use the tanks properly' in an environment where the public is being exposed to that risk.

All it takes is one serious injury. Safety (for our students, mentors and the general public) needs to be the top priority, always.

If it's possible that through ignorance of safe protocol the tanks can injure the public, then we must look at ways to mitigate that danger, including possibly banning their use.