"Cave Fills"

[Bobby]

One point that I didn't see mentioned for those that may not know. "Cave" or over fill applies to only steel cylinders. I have a set of 95's with a born on date in 1985 that still pass hydro. Aluminum cylinders are another story though and should not be over filled. There are many incidents of injury and death from over filling AL cylinders. There are also a few "self service" shops in FL so it is good for everyone to know there is a difference in filling steel versus aluminum.

 

[Jax]

One point that I didn't see mentioned for those that may not know. "Cave" or over fill applies to only steel cylinders. I have a set of 95's with a born on date in 1985 that still pass hydro. Aluminum cylinders are another story though and should not be over filled. There are many incidents of injury and death from over filling AL cylinders. There are also a few "self service" shops in FL so it is good for everyone to know there is a difference in filling steel versus aluminum.

Thank you!!!! It needed to be said.

 

[Jim Lapenta]

Bobby is right. The shop I was initially trained through had a habit of overfilling al cylinders. I hated that being that I work in a machine shop and know a little about how metals react.

Saw some burst discs go in the pool as a result of this. Not fun when you have just enough tanks for the number of students you have and suddenly someone has no tank to use.

I own 5 al 80's, an al 40 and, 2 al 30's and while they may get filled to 3200 on a hot day knowing they will cool down to 3000 in the water, I don't overfill them more than that. One never sees more than 2400. That's my O2 bottle.
My LP steels on the other hand generally get "good" fills.

 

[cool_hardware52]

Tobin, I certainly don't understand the process, but I do know that Hydro tests are evaluating work hardening in a tank.

No, not really. A hydro is evaluating if the tank suffers permanent deformation at the test pressure. Permanent deformation does not require work hardening. A a tank that passed a retest or hydro has been stretched, but not permanently deformed, i.e. it is able to return to or very close to it's original volume. A tank that fails stays "stretched" because some portion of it has exceeded it's elastic limit, meaning it permanently yielded. Such would be the case if a portion of the cylinder was thinned by internal rust etc. It does not require "work hardening" for the material to exceed the elastic limit in an area of the tank that has thinner walls due to rust etc.

If Hydros (or "cave fills) were "work hardening" steel cylinders one would expect to hear of routine failures of tanks in Australia where they require annual retests, not every 5 years as is the case in the US.

Those that aren't elastic anymore are condemned. How would you account for this happening?

Those that permanently yield are condemned. Damage to the tank, like internal rust, or mishandling, or heat exposure, or the retester failed to follow the test protocol as is required with some Exemption (E) cylinders.

If one subjected a brand new tank to a *single* cycle of enough pressure, well in excess of the DOT mandated retest pressure, the tank would at some point yield and be permanently deformed, i.e. stretched past the elastic limit. Was "work hardening" involved in this example? If so how? It should be pretty evident that "work hardening" is not required for a tank to yield.

There is a concept of "True Elastic Limit" which suggests that small plastic deformations happen at relatively low pressures and are cumulative.

If so why aren't we seeing routine hydro failures in cave country and Australia?

Such is some of the speculation behind SLC in the 6351 alloy.

Who cave fills al 80's?

I am in no way advocating filling beyond the working pressure for any tank. I am in favor reducing the mythology surrounding pressure vessels, and their care, handling and use.

 

[The Chairman]

If so why aren't we seeing routine hydro failures in cave country and Australia?

Your definition and my definition of 'routine' are different. If tanks never failed, we wouldn't need to test them. Tanks undergoing hydro are to be visually inspected before the test. Rust or impact issues would condemn a tank before it was pressurized, or they are not following protocol. I'm not the one who invented the concept of true elastic limits, but I feel that can account for hardening and fatigue over time. Obviously, I'm also in favor of reducing the mythology concerning tanks but I haven't met the engineer who can adequately describe the failure process for me. So far, you haven't filled in any of the blanks for me.

Who cave fills al 80's?

No one that I know of and yet SLC exists.

 

[oldschoolto]

I think every diver should go to a hydro place and look at the pile of split tanks that fail in the hydro tank under pressure... Now 90% of those are fire extinction bottle but they always have a few " Real tanks " that have split...

Jim..

 

[cool_hardware52]

Your definition and my definition of 'routine' are different. If tanks never failed, we wouldn't need to test them.

When did I say tanks never fail hydro, or that there is no need to test them?

I have limited my comments to your contention that tanks fail hydro due to "work hardening" I don't see the evidence to support this assertion.

I'd ask if "work hardening" is a genuine issue how do you explain the numerous reports of divers using cave fills in 3AA tanks for decades without a failed hydro?

Tanks undergoing hydro are to be visually inspected before the test. Rust or impact issues would condemn a tank before it was pressurized,

Rust and pits can no doubt condemn a tank based on a visual alone, but we both know that's not what I'm referring to.

Tumble a rusty tank and it's no longer rusty, but is it now safe? That's what the hydro determines. If there is enough wall left then the elastic limit will not be exceeded, if not it will. Pretty simple.

It's also possible that tanks have been filled well in excess of the hydro pressure. That could cause portions of the tank to yield and thin.

The reports of hydro stations failing to follow the specific protocols for PST "E" tanks are well known too.

In short there are many well understood mechanisms to explain tanks that fail hydro. One need not invent new unproven theories.

I'd love to hear from divers that had 3AA tanks fail hydro for no apparent reason, and I'd love to hear from Cave divers that had 2400/ 2640 tanks fail after receiving overfills that were in otherwise good condition.

I own over 80 cylinders, mostly industrial gas bottles. I have one O2 supply bottle that was born in 1906. I haven't counted the number of hydros, but there isn't much space left on the crown of the cylinder.

No one that I know of and yet SLC exists.

Throwing in the problems with aluminum tanks of a discontinued alloy not used since 1987 adds what to a discussion of cave fills in steel tanks?

Tobin

 

[The Chairman]

When did I say tanks never fail hydro, or that there is no need to test them?

Why are you misrepresenting what I wrote? I never suggested, intimated, implied or indicated that you said any such thing. This kind of passive aggressive tactic makes discussions with you tiresome.

One need not invent new unproven theories.

Again, I didn't invent this nor is it unproven. That you don't know about it, let alone understand it does not negate it. Your understanding of what's happening seems to suit you. Bully for you. I am not satisfied by that and believe that there is more to the situation that appears to be beyond your ken. I'm not a slave to anyone's ignorance.

 

[cool_hardware52]

Your definition and my definition of 'routine' are different. If tanks never failed, we wouldn't need to test them.

Tell me exactly what you intended with these comments made in direct response to my post?

Tobin

 

[cool_hardware52]

I'm not the one who invented the concept of true elastic limits, but I feel that can account for hardening and fatigue over time.

Besides your "feelings" can you provide *any* evidence?

Tobin