Overfilling LP Steel Tanks -- How bad is it?

[FredT]

Cave Bum,
There have been several instances over the years where a steel bottle let go and caused the death of either the filler or a bystander. Without exception these did not happen as a result of a unique overpressure event, but were due to other severe problems with the tank,most notably line or ring corrosion.

In the great preponderance of incidents the tanks were well out of hydro and had not been properly inspected, or in most cases even opened, in years.

Definitions for the uninitiated:
Line corrosion. is corrosion pits forming a line, or multiple line in the side wall of the tank along the length of the tank. This is caused by water being in the tank, and then storing the tank on it's side. A very bad thing which will cause a bottle to fail visual instantly no matter what the individual pit depth is.

Ring corrosion: No longer common, but it is a ring of pits around the OUTSTDE of the tank, generally either at the top of the boot, or under a permanent stainless band around the tank. Caused by moisture and salt being trapped next to or under a boot or band, and more common when dealing with epoxy coated tanks that weren't galvanized underneath the coating. Most of these bottles have long since been condemned as they have not been legal to hydro with the coating in place for at least 15 years.

Steel tank failures ALL initiate in the sidewalls, as the threads and base of the bottles are their strongest parts. This means the tank unwraps, but only throws one piece of shrapnel, the entire tank. If you are in the way of that one piece really bad things can happen.

Aluminum bottles that catastrophically rupture in the field from pressures that should not have caused failure by design have the rupture initiate at the neck thread. This causes the neck to spread rapidly, leading to brittle failure of the side walls and nasty flying bits.

 

[rjack321]

I'm still waiting for some documented proof that LP72s have repeatedly ruptured due to dive shop overfills.

Most LP72s have been removed from service due to rust. The vinyl coatings trapped salt water and/or made them impossible to VIP. The compressors of yore (and some today) pumped some pretty wet gas. Plastic liners and epoxy coatings were tried for awhile and then the great AL saviors came along and many divers switched. I'm sure one or 2 lp72s have exploded but most were never overfilled by 20-50% and got retired for reasons other than even hydrostatic test failure long before they ruptured.

 

[FredT]

No offense, but there's no way you can use this story as an example that AL tanks explode into shrapnel when over filled. You're talking about a tank filled with 100% O2 (highly flammable) that was destroyed by sudden trauma. Couldn't it be that the valve breaking simply generated a spark and ignited the O2? In any case, it had NOTHING to do with overfilling.

I'm still waiting for some documented proof that LP72s have repeatedly ruptured due to dive shop overfills.

It is not possible to break the valve off of an AL40 by dropping it! The physics of the world will not allow the tank to store enough energy to shear the metal simply by falling from the bench. If it was possible there would be a screw on safety cap on the tank! Making the valve unusable with a fall is one thing, but breaking it off is another thing entirely.

Steel tank failure from overfill happens during hydro, when it is too brittle to pass the test. NOT by methods that result in sudden expansion of gas.

 

[JDMerk]

Thought it might be nice to have some actual pics of ruptured cylinders:

Vintage 83 Walter Kidde

More of same, looks to be in four pieces

Cylinder explosion aftermath

Real life account of a catastrophic cylinder failure

 

[FredT]

Chris was lucky to have survived that one.

 

[halocline]

That's quite a story...of course it has nothing to do with overfilling, but it is an effective reminder of how dangerous a ruptured tank can be!

 

[JDMerk]

I posted it to show how tanks fail...grenade or split.
There seemed to be some friction on that point.

 

[H2Andy]

I had only met the lady, her name was Fran, a couple of times getting fills, but a loss like this effects us all, even divers and fill station employees who never met her.

i only knew her in passing, but lots of people had a real loss with her death. i remember a lot of bummed out divers. everybody felt pretty bad.

 

[rjack321]

That particular AL cylinder failed as designed by splitting down the side. And it broke up a bit further as it wasn't designed to. Whether the 4 peices were due to the water bath or the nearby wall or whatever is unknown. No matter how a tank fails if you are nearby you're gonna be hurt bad or die. From the concussion, from a piece of tank or from a piece of debris are all quite likely. That particular failure 10yrs ago was fatal, and yes tramatic. If anything, failures like that created the shift to 6061 aluminum which appears, so far, to be a superior alloy for HP uses like scuba. Bottom line though, that kind of failure has nothing to do with overfilling 3AA steel tanks beyond the 10% plus rating.

 

[Gombessa]

Just to bring the conversation back into focus, I bought my LP80 (2400 +10%) doubles used, which had been stored for over 12 months at 3,500 psi. The original owner overfilled to this amount regularly.

I analyzed the gas, dove the cylinders, and brought them in for hydro, where they received a renewed plus rating. I'm typically getting fills from 3,200-3,400 psi on the tanks now.

Multiply this same story x3 for my regular buddies, who all got hydros re-plussed last year, and I'm not coming away worried about moderate overfills. After all, if regular overfilling takes 30 years off the life of the tanks, they'll probably still outlast me.

However, I'll admit I won't leave them in the car on a hot day