Destructive Air Tank Testing

Technical Technical Discussion
1

Disclaimer
Don’t try this at home. I have experience in destructive testing and I have a safe location to perform these tests.
Don’t construe any results as approved methods for shrapnel mitigation.
Don’t construe any results as suggestions for any action.
Don’t construe any results as anything other than the results of an experiment.
Don’t try this at home.
No students were present during these tests. Experienced adults only.
/disclaimer

[EDIT]
Video!


[/EDIT]

We are experimentalists here on 95. Given the recent chatter on CD about failing plastic air tanks, and several proposed (or implemented) mitigation techniques, we decided to test some of these solutions. We have a video coming, but I thought I would share our preliminary results.

Most of this year’s discussion, and several proposed or implemented solutions, is in this thread: http://www.chiefdelphi.com/forums/showthread.php?t=127695

Some background information about using plastic to transport compressed gasses from The Plastic Pipe Institute: http://plasticpipe.org/pdf/recommendation-b_transport_compressed_gas.pdf

Safety during a destructive test is paramount, so we did a little research and found this paper detailing the radii for various threats at different amounts of stored energy. On Page 4.7 the thread radii are listed. We calculated that the energy stored in a single plastic storage tank at 120psi was around 370lbft of energy. To be safe we used the shrapnel radius for a 500lbft radius and gave ourselves a factor of safety of at least 10, and wore eye and hearing protection.

Paper: http://xpda.com/junkmail/junk215/PNNL-18696.pdf

Results summary:

No mitigation - we found shrapnel of appreciable size up to 45ft away lots of small shrapnel pieces, and some larger ones

2-3 layers of Gorilla tape - as some hypothesized the ends of the tank shot out at high velocity, we found one in a snow bank about 25ft away. Virtually all of the small shrapnel was eliminated, but the tank broke into 4 large pieces. However, the larger pieces didn’t travel as far, 20-30ft or so.

Bumper fabric zip-tied around the tank - the zip-ties failed almost immediately and the fabric simply un-rolled from around the tank. Shrapnel was slightly mitigated, but large and small pieces were still found 20-30ft away.

Completely Assembled Bumper - we essentially replaced the pool noodles with an air tank. This actually contained virtually all of the shrapnel. It was a completely assembled bumper (3/4in plywood, screwed-in angle aluminum, stapled nylon fabric*). The angle aluminum ripped off of the plywood, and many of the staples failed, but it held together long enough to keep basically all of the shrapnel inside.

*This fabric is considerably stronger than your average bumper fabric. It is heavy-duty nylon, double rip-stop, and impossible to tear by hand along any direction.

Pictures!

Un-mitigated. Not bad for finding white plastic on a snow-covered driveway!
http://i.imgur.com/0mvwpkR.jpg

Gorilla Tape
http://i.imgur.com/0Am8Z3h.jpg

Buper fabric and zip-ties.
http://i.imgur.com/8XSY8fh.jpg

Whole bumper.
http://i.imgur.com/d0Nm274.jpg

2

A thick sock tie-wrapped or wired closed around the pneumatic lines.

Few of those methods (safe for the bumper) would “contain” anything. They merely redirect the energy. You want something that will expand and stretch to absorb the energy while containing the fragments and venting the gases.

When it comes to explosions, trying to contain them rigidly (with tie wraps and tape) rarely works.

3

Bumper material covers with reinforcements at the ends (double stitching and double material) and pleated material around the main cylinder of the bottle with sparse stitching that will break away, allow the bag to expand and catch the expanding pieces.

4

Well, if the sock solution has issues with pieces of tank cutting through it some kind of poly-carbonate mesh could be used.

5

Did you have any “before” pictures?

6

Just as a quick note: We had a fixed number of tanks to work with and so we tested the mitigation techniques we saw proposed most often and used materials we had on hand.

We would have tested additional containment methods but we sort of ‘expended’ all our plastic tanks. If anyone wants to send us more we’d be more than happy to do additional testing.

The aforementioned video will go into some depth about the hows and whats of the testing. For now we thought it was worth letting the community know that, at least in our tests, containment of these tanks failed more often then it succeeded, and that it requires substantially more robust solutions than many expect.

7

Here is an air tank ready to test.

8

What’s the diameter on these tanks? I bet something like McMaster p/n 55545K96 would work wonders on containing shrapnel.

9

I was looking at that stuff too, I was wondering how to cap the ends. Maybe I could stitch it down with some heavy duty nylon string?

I am nervous about the location of my air tanks too.

10

What method are you using to cause the failure? Simple over-pressurization or are you intentionally over-tightening the end fittings? It would be brittle failure either way, but it may make a difference in how much energy is in the system at the time of failure. An over-tightened fitting would likely fail at a fair lower pressure (eg, potentially below 120psi) whereas a properly assembled tank would not.

On that same note, did you monitor the pressure that the tanks burst at, and if so how consistent was it?

11

Roughly 2.5" looking at the pictures and from first hand recollection

12

Instead of looking at more wonderful creative ways of containing shrapnel, I’m looking for ways to make sure all of these tanks get replaced with something more reliable at the regionals I attend…

13

I wish clippard made a metal air volume tank the same size as their biggest cylinder. I would love to have a 3" ID tank that is 6" long. hmmm I am gonna email them right now.

14

At what pressure did the tanks actually fail? I would expect them to have a safety factor of at least 3X.

15

I would have to agree with you.

These tanks were discontinued for a reason…

I would also like to know how you made them fail?

Threading the fitting in to far or some other method?

16

All the tanks were pressurised to 120psi (regulated output of a shop compressor) then struck with an air rifle pellet to induce the failure. It turned out to be remarkably consistent and reliable (well, as reliable as my aim!). The video of the testing will get into detail about the methods, but we think it’s a reasonable simulation of a tank failing in FRC use due to a sudden impact

My subjective opinion is that each tank burst with the same sort of force. The shrapnel, noise and effect on containment all seem to back that up.

17

I agree entirely. 95 only uses metal tanks on our robots. Always have, always will.

As hinted at earlier, our goal is not to provide actual shrapnel containment solutions, but rather to demonstrate how difficult it would be to make plastic tanks as safe as metal tanks.

18

Broken laptop in homage to 1310’s driver station? :stuck_out_tongue:

19

Ok so Clippard can make air volume tanks the same dimensions as their cylinders. I went through their distributor and asked for a 3" bore 12" long storage tank (84 ish cubic inches). I will see what the quote is.

20

Maaaaybe…

Also it made the white tank stand out against the snow-covered driveway for Andy to aim at.