Implosion report: 140m

pressure-proofing

#1

Hi all,

I’m partway through a long build of one of the original Kickstarter units (v2.3 I think). Part of the reason for such a long build is my particular requirement for regular 70-100m deep dives (I’m a technical SCUBA diver and much less than this I’ll just dive it!) - other reasons are not having enough time and too many other projects. Having found the acrylic endcaps I assembled to have several issues (I believe now fixed in later versions) I eventually got around to getting aluminium endcaps machined and anodised.

I have a pressure pot and was able to test the resulting (empty) pressure housing well beyond its rating to 140m - and at this depth the cylinder imploded rather impressively, shattering into literally100 scarily small pieces with a VERY loud bang. Luckily the endcaps survived and I have another tube on the way. I had contacted the openROV team who suggested others on here may be interested.

If anybody does want anything tested in the pot, then feel free to get in touch (it’s about 300mm/12" inside diameter, about 1m/3ft long, and will go to 200m water depth if I change the gauge on it). An OpenROV should just fit. Only thing is that I and it are in Australia…

Cheers,
Stephen Fordyce
Melbourne, Australia


Crush depth of tubing
Manufacturing a shell for deep dives at 1000m+
#2

Did you capture any video or photos of the carnage?


#3

Interesting report Stephen. I’m wondering if the “crush depth” could be improved if rather than using standard acrylic tube, we might use Lexan.


#4

Hi guys, apparently I can’t post photos of the aftermath yet (and the camera inside wasn’t working at the time), but none of the pieces were bigger than my thumb!

I agree with the conclusion of Brian from OpenROV that the best way to improve depth rating is to increase the thickness of the tube. I’ve done other previous research on this, and Lexan doesn’t perform quite as well in the underwater context, and acrylic is pretty well universally used for UW windows.


#5

Ok, here’s a photo:


#6

Nice endcaps. How are you taking the wires through? Could you make less machined solution if you 3D printed a PLA mould and done a GRP one with mat/roven and desolved the PLA with sodium hydroxide?


#7

dang that’s quite the implosion!
For future testing, if you don’t want to have a high energy implosion, I recommend mostly filling the empty space with some sort of potting material (like small plastic beads/pellets), so that the total internal air volume is decreased (you don’t want it packed completely full, otherwise the potting would provide additional structural support, thus making the test inaccurate). We did that when doing deep pressure testing (1500m+) of a vehicle I designed at MBARI’s pressure vessel so as not to scratch up and damage their pressure testing vessel.


#8

That’s a great idea Colin, I will do that (had expected the crush depth to be more - that was a first implosion I’ve had).

Fishingrov - thanks (they look even better after being anodised). One of my issues with the wires on the original OpenROV design was that if you messed it up, the endcap was kind of useless. So you can see the hole in the centre of the endcaps with the O-ring seal, and the blanks that I used for testing. The blanks get replaced with a similar arrangement of stacked/glued acrylic and wires potted in epoxy to what’s on the original design.

A quick calculation indicates about 900kg (1800lb) of force pushing the 2 endcaps towards each other at 100m/300ft. That’s why they are aluminium! (it’s distributed all over which lessens it, but still a lot of forces on the endcap, and flat is not a very strong shape) Also because it makes the machined O-ring grove and blind tapped holes for the blank/penetrator plate possible. Overkill? Yeah probably :).

If anyone’s interested I can post the drawing or get some more made and send to you, but they would be expensive and there’s plenty of good data (I think) that says the standard acrylic caps are fine at 70m.


#9

A note on high pressure endcaps. They should be machined out of plate. They should never be made from bar stock, unless you will be drilling out the center for some sort of fitting.

I learned this the hard way when as a novice engineer I used 5" PVC round stock to make endcaps for a pressure vessel to go to several hundred feet deep. The vessel had a mysterious leak that I could not track down. I checked all the penetrators and seals but none of them would leak. Eventually I found water leaking right through the center of the PVC disk, about 1.5" thick!

Dumbfounded I showed the leak to everybody and asked if they had seen this before. The old machinist said “Of course! What did you expect?” He explained that as the hot extruded PVC billet cools the outside cools and hardens first. As the inner plastic cools it needs to contract but with the now rigid exterior it has nowhere to contract from. As a result it draws a vacuum, thin tendrils of porosity along the center of the billet. This is the center of cooling, probably not the mechanical center if the billet is lying on a table as it cools.

When this round billet is sliced into disks the disks have a good chance of having microscopic wormholes through the middle which can leak water. If you are going to drill out the center to install a connector the problem is solved. In my case we machined out a small cavity and glued in a coin size disk of plate PCV to block the porosity.

This is very common with plastics but also occurs in metals. It does not happen in plates as the contraction of the middle material in a plate just makes the whole plate slightly thinner.


#10

Yup, had the exact same thing happen when I had some endcaps turned from ABS round stock. To fix it I drilled a hole in the middle, tapped it and then plugged it with a screw and hard rubber washer.