Tank explosion kills one - Cozumel

[DandyDon]

Hydro testing is every 5 years, I believe. Last time I had mine done, I took them to the fire extinguisher shop that does them in Lubbock - and while there, asked to show me around some. Very impressive to this old farm boy.

 

[grf88]

Is it possible there are still old alloy cylinders in the Cozumel inventory.

 

[boulderjohn]

When I took my PSI tank inspection class, the instructor said that when a tank fails hydro it likely had previously passed several visual inspections that it should have failed. A common place for cracking is in the threads in the neck, and that can be hard to detect. You really have to take your time and use a good light to check it out carefully. If a shop owns a lot of tanks, it will usually be any one of several employees who do the visual inspection, and they might give the OK to a whole bunch of tanks in a single session. It is really pretty unusual for a tank to fail a visual inspection. I don't do that many, but the only ones I have done that did not pass (outside of my training) were a couple of steel tanks that had gotten too much rust inside and had to be tumbled, after which they passed. I imagine that if you are doing many, many tanks in a hurry after not ever failing one in your inspecting career, it can be very easy to get careless and not give the process the attention it requires.

 

[DandyDon]

Is it possible there are still old alloy cylinders in the Cozumel inventory.

Sure, it's possible. Divers have been going to Coz since before Cousteau, and some materials get traded up and down the coast as far as Panama. SCC has an excellent reputation, but controls in Mexico are generally casual. Bribes work well.

 

[dumpsterDiver]

I think you misunderstand the general concept. While you're correct that a burst disc is set to blow at a SLIGHTLY higher pressure than the working pressure of the tank, the working pressure of a tank is not also the pressure at which it will rupture/fail. Tanks are hydroed at 5/3 their stamped/working pressure. An aluminum tank would be hydrostatically tested at 5000psi. So the order of march would be working pressure, burst disc pressure, hydro test pressure, tank failure pressure.

The whole idea behind a burst disc is that it blows BEFORE the tank explodes, not the other way around. But in some areas, burst discs may not be replaced regulaly and have corroded shut, and they've been deliberately disabled, or discs of significantrly higher pressure are instaklkled to allow for higher over-fills without triggering the burst disc.

No one knows the history of this particular tank. It may have been repeatedly over-filled which can induce metal fatigue. It may have been out of hydro. It may have had undetected or known-but-ignored hairline fractures. But simply put, tanks that are properly inspected and meet certain standards will not rupture at pressures less than that of a burst disc.

- Ken

What geographic area have burst discs "corroded shut". Have you worked with them much? I have a hard time understanding how corrosion of the disc would strength it and make it "shut"????

Sound like other people like your comments, but I am just confused.

 

[chillyinCanada]

"Have you worked with them much" heehee can't wait to see Ken's reply.

 

[scubafanatic]

About 10 years ago, when I was still a very new diver, I was taking a tech diving class at the Blue Hole in New Mexico. While there our instructor gave us a tour of the tank fill area, and I found myself standing next to a tank being filled that was beginning to burst. I heard what sounded like escaping air and saw a tank losing air through a crack in the crown area of the tank. (the curved dome shaped upper part of the tank) I quickly moved away and alerted my instructor who had the filling stopped !

Looking back it's hard to believe I may have been THIS CLOSE to meeting the same fate as the unfortunate victim !

 

[ZombieZombie]

dumpsterDiver- although my experience is with the chemical process industry, I can't imagine that bursting disks are designed any differently for the scuba industry. They are normally a thin piece of precision weakened metal held between 2 flanges and designed to fail at a defined differential pressure. The weakening is made with deep groves etched into the metal. I agree with you that it would be difficult for corrosion to strengthen a bursting disk. I have heard of them not bursting because of a large amount of solids build upstream or liquids downstream but it''s hard to imagine that SCC would be so sloppy as to allow such an amount of salt or dirt to build up.

In any case, didn't someone on here confirm that the tank burst after filling was complete and when the tank was sitting upright in a storage area? That would suggest that a fatigue failure - with a crack in the threads or elsewhere in the body finally reaching its critical length and rapid failure occurred. It would make sense that this would happen shortly after filling to 3000 psi g. Stress corrosion cracking can also occur in both aluminum and stainless steel tanks where salt (chloride ions) gets inside the tank. Stress corrosion cracking is like a slow shattering of the material and gradually weakens it. Eventually it can't take 3000 psi and it goes. Of course, we assume that the tank in question was not over-pressurized- and that brings us back to the use of bursting disks.

I haven't found data yet, but I would guess that the infrequency of tank failure globally means that properly inspected and periodic hydraulically tested tanks do not fail in normal use. (If they do, the frequency and pressure of hydraulic testing needs to be increased until the failure rate drops to zero). At least Al 6061-T6 or 316SS materials shouldn't.

In the process industry we have much bigger issues with much larger tanks in much more corrosive environments that contain many more flaws introduced through welding. Rigorous visual inspection and pressure testing on a strict schedule does have reduced catastrophic tank failures to close to zero. It is always dangerous to draw conclusions without access to all of the facts and we should not do so here. However, if I was a betting man, I would place money on a breakdown of the tank hydro-testing regime in some way.

I am skeptical about the value of visual inspection in an industry where people unlikely to have training in materials failure or metallurgy are expected to spot micro-flaws reliably. In my time in the jet engine industry we once asked a group of graduate mechanical engineers with at least 2 years of work experience to identify cracking in the roots of engine blades and the diversity of responses indicated that people could not distinguish between real cracks and superficial surface discoloration. Accurate inspection requires a lot of training, practice, and testing. This just isn't feasible in a leisure industry like diving.


update- I'm having a rummage through the internet and I am amazed at how much material there is on tank failure!

 

[Ken Kurtis]

I have a hard time understanding how corrosion of the disc would strength it and make it "shut"????

Bad phrasing. I have seen burst discs that were caked with salt corrosion on top of the burst disc. Not often, but I've seen it. Although I did not test any of these discs to see if the corrosion strengthened or weakened them, my sense of it is that salt corrosion on top of a burst disc would perhaps make the burst disc hold in some cirucmstances where it should have failed. (BTW, you forgot to take me to task for misspelling "regularly".)

 

[Peter69_56]

I had a set of doubles where one bursting disc had been replaced with a solid disc, and the other side had 2 discs. The previous owner was an instructor. Naturally enough I had the whole lot fixed and tested. Don't own them anymore but I wonder if that was done to get 300 BAR into 235 BAR tanks as they were twin 7 litre.

Some people just should NOT be born and make the world a safer place I think!