Chief Delphi - exploding plastic storage tanks

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- - exploding plastic storage tanks (http://www.chiefdelphi.com/forums/showthread.php?t=127695)

Re: exploding plastic storage tanks

We are having an LRI conference call this evening. I will make sure that it is brought up. I know that Al is aware of these threads so he may already be working behind the scenes. I will post back tomorrow.

Re: exploding plastic storage tanks

Quote:

Originally Posted by Daniel_LaFleur (Post 1357204)

Isn't this the purpose of FIRST? To teach, read inspire, students about the forces and how to use the tanks properly? Each team should find a Fluid Power Engineer or a mentor properly trained in the safe use of pneumatics, or at least follow the manufacturer's instructions for safe installation and use of the parts.

Of course, I don't want any person injured from mis-use or improper installation, so there must be some adult involved in the process. A key tenant of FIRST - students need mentors!

Re: exploding plastic storage tanks

Here is statement from the Plastic Pipe Institute: http://plasticpipe.org/pdf/recommend...ressed_gas.pdf

Notably:

Quote:

The Plastics Pipe Institute recommends that thermoplastics piping intended for the transport of compressed air or other compressed gases should be installed by burial, encasement in shatter resistant material, or other appropriate means, to prevent or minimize the possibility of mechanical damage.

PP is a thermoplastic. Seems like a bad plan to use PP tanks from the get-go.

Quote:

Originally Posted by Daniel_LaFleur (Post 1356851)

Allen, I understand where you are coming from but as someone who has had a 1 liter PVC pneumatic volume explode within 1 foot of me (thank goodness it blew out away from me) I will respectfully disagree here.

The dangers to anyone near one of these if it explodes, especially to bystanders, some of which may be children, is too high for my liking. Thus I say 'ensure it is safe and then we can use it'.

To the best of my knowledge PVC is not rated by any organization to contain compressed gases. Any use of PVC to do so is at your own risk and against industry practices.

Quote:

Originally Posted by Lil' Lavery (Post 1356964)

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.

Exactly!

Quote:

Originally Posted by cgmv123 (Post 1357066)

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.

The plastic tanks are not what I would call "perfectly safe." They are safe when used in exactly their prescribed manor with little room for error. They are not robust or fault-tolerant. Some of these tanks will go to teams where no mentors or students know how to properly handle them, and FIRST has no way to stop this until the robot is at an event.

Quote:

Originally Posted by Dave Campbell (Post 1357215)

Isn't this the purpose of FIRST? To teach, read inspire, students about the forces and how to use the tanks properly? Each team should find a Fluid Power Engineer or a mentor properly trained in the safe use of pneumatics, or at least follow the manufacturer's instructions for safe installation and use of the parts.

Of course, I don't want any person injured from mis-use or improper installation, so there must be some adult involved in the process. A key tenant of FIRST - students need mentors!

They should, but not every team will.

When inspecting our robot's pneumatic system this year our ERF was slightly out of adjustment and only started venting at ~140psi. The plastic tanks are rated for 125psi (the average Clippard metallic tank is rated to 250psi). If our robot had these tanks, they would have been subjected to pressures outside of their rated range. I wonder how many teams this has happened to...

Re: exploding plastic storage tanks

Quote:

Originally Posted by Dave Campbell (Post 1357215)

I agree with the idea of finding a mentor to help with this.

However pneumatics and hydraulics can kill even when properly engineered and suffering from damage from wear and tear.

Sure these are smaller systems but even a syringe can kill you with just air in it.

If we are going to allow plastic tanks with several examples of unpleasant failures FIRST ought to think carefully about enforcing the safety element through inspection and published recommendations about safety sleeves.

Accidents are preventable - all it takes is recognizing the risk.
A few explosions are a demonstrable risk hard to ignore.

I also want to say as a CSA - I am around all of your fielded robot all of the time.
If I get hurt because of something like this don't think you have legal protection.
FIRST you know you have an issue - address it and don't be negligent.

What I wrote here might seem quite blunt - realize I've worked on hydraulic systems that deform building steel.
These things are unforgiving.

Re: exploding plastic storage tanks

Quote:

Originally Posted by Daniel_LaFleur (Post 1357204)

... 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.

You had better make some changes for the rest of this season and every season from now on. With that attitude it would be downright crazy to allow a high school student near a welder, drill, table saw, chop saw, band saw, drill press, mill, lathe, plasma cutter, water jet, can of spray paint, battery, rotating component, belt drive, spring, bungee, surgical tubing, sander, robot cart, any heavy item, shearing press, sheet metal brake, or anything else that could cause an injury if someone doesn't follow the directions. Just because there is an issue with an item doesn't mean it should be banned. There are better suited items on the market, and maybe we should all use those. But, I for one, like threaded fittings that let me change my push to connect fittings because I know they wear out. After working at Parker Hannifin Brass Products for a few years after college I am very familiar with the design of those fittings and I know that they don't last forever. I don't necessarily want to buy a new tank when my o-ring gets a tear. I can just be careful when assembling my tanks.

With that said, we only use the stainless tanks. Besides, they look cooler and I like shiny things...

Re: exploding plastic storage tanks

Here's a suggestion, require teams to install shields (e.g. Lexan) around tanks, and for teams tight on weight don't include the shield weight in the weight of the robot. Sleeves might work, but small pieces could still escape - at high velocity.

This is a very dangerous failure and should be addressed by teams and by FIRST as soon as possible.

Re: exploding plastic storage tanks

Quote:

Originally Posted by JCharlton (Post 1357243)

Why would you do that when a metallic tank leaves nothing to chance?

How are teams going to provide engineering-based proof that their blast containment scheme works?

This is still a band-aid fix to mitigate an inherently dangerous failure. The real solution is to use tank material that does not exhibit brittle failure in the first place.

Re: exploding plastic storage tanks

MrBasse has a point. With the mechanisms on the robot for today's game there are just as much if not more chance of people getting hurt. Surgical tubing can cause great hurt when attached to metal parts. On my teams robot there are 4 springs, 2x60lb and 2x90lb. That's 300lb force on the arm. I have seen and inspected even stronger mechanisms.

There are many tools that we use that are dangerous. This does not stop us from using them. We must take precautions and make sure safety is prime in our thoughts, second nature if you will. If we find that a saw blade breaks and has issues we don't continue to use those blades we find better quality ones. The same would go for pneumatics, tanks prove to have problems, find better. I am all for safety and agree we must be doing something but banning is not the issue.

It's Tuesday, maybe we will see something from FIRST today. After all, there are a lot of bright minds at FIRST that want the best for the students.

Re: exploding plastic storage tanks

Quote:

Originally Posted by JVN (Post 1357289)

Has anyone seen a black tank fail?

I cut a black tank in two because it was damaged by being rubbed by a drill chuck. Doesn't hold air now. But that probably not what you were asking. :)

Re: exploding plastic storage tanks

I've never seen or heard of a black tank failing, yet. Perhaps this is a result of the design improvements and the heightened awareness of plastic tank failures.

Re: exploding plastic storage tanks

Quote:

Originally Posted by JVN (Post 1357289)

Has anyone seen a black tank fail?

Here are examples of the black Clippards failing, albeit a different failure mode than the "exploding":

Quote:

Originally Posted by Wren Hensgen (Post 1356698)

Team 225 has managed to make one of the new black plastic tanks fail, but the failure actually occurred on the push-to-connect fitting, which snapped out under pressure. This might be an intentional design, as losing a hose is probably less dangerous than throwing shrapnel around

Quote:

Originally Posted by thefro526 (Post 1356713)

Oddly enough, this may be due in part to a manufacturing defect, as we've had one of the fittings come out as well, and AM has a note about it on their site. http://www.andymark.com/product-p/am-2649.htm

Re: exploding plastic storage tanks

We broke the fitting on one of our black tanks, but that was the fitting. I'm fairly certain the tank is still able to hold air just fine.

Re: exploding plastic storage tanks

Quote:

Originally Posted by cgmv123 (Post 1357066)

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.

What evidence do you have to back your claim? What kind of testing or observational data can you share to support your hypothesis that these tanks are "perfectly safe" and that the only issue is "using them improperly?" What kind of impact or thermal testing can you provide about using these tanks in an FRC environment?

It's not that I disagree with your general point that user error is a factor in the failures (though I also don't think that's an acceptable excuse), but if you're going to make an assertion like that (repeatedly, no less) you need to have something to back it up. Especially when safety is involved.

So, how do you know these tanks are safe?

Re: exploding plastic storage tanks

Quote:

Originally Posted by Lil' Lavery (Post 1357347)

The identified main source of failure is from user error in over tightening the fittings. Maybe since Clippard has discontinued production and sales we should all just call it a day and stop using them. However, it is a product defect as much as a car has a defect when a person claims they thought they could take a nap while driving because they had cruise control on...

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

Re: exploding plastic storage tanks

Quote:

Originally Posted by Steve W (Post 1357214)

I hope it's not too drastic. I just placed an order for 6 of those black tanks.