storing a full tank

[pescador775]

What your instructors are not telling you; hydro testing causes the 'sustained load cracking', not storage. I have three aluminum, 80 cu ft Alcan, USD brand tanks which have been stored full since 1972. I had them hydroed and vised with eddy tester (all for first time) last year. They passed easily. These tanks are used for non diving related purpose and are topped off to 3000 psi at all times.

 

[jonnythan]

pescador775 once bubbled...
What your instructors are not telling you; hydro testing causes the 'sustained load cracking', not storage. I have three aluminum, 80 cu ft Alcan, USD brand tanks which have been stored full since 1972. I had them hydroed and vised with eddy tester (all for first time) last year. They passed easily. These tanks are used for non diving related purpose and are topped off to 3000 psi at all times.

Hydro testing lasts, what, an hour max? SUSTAINED load cracking. 3000 psi over a few weeks to a year is a sustained load, 4500 psi for a brief amount of time isn't sustained.

 

[Ladydiver]

Portions of this thread have been deleted due to their harrassing nature and uncalled for personal attacks. Those of you doing the bashing know who you are and I know all of you have been on Scuba Board long enough to know this kind of attack is not allowed. Do not let it happen again.

 

[pescador775]

When mechanical engineers discuss 'SLC' of aluminum tanks, particularly 6351, it is usually in the context of cycling although they sometimes make oblique mention of storage conditions as well. However, the culprit most certainly is cycling as this involves repeated flexing and temperature stress. Luxfer and British ALCAN have cited divers who overfill aluminum tanks. Personally,I have used aluminum tanks since 1963 beginning with Navy 90's and then the new USD aluminum. When I bought the USD aluminum I was aware that they were not made of proven Navy alloy (6061) but some new stuff stamped with a scary special permit from the DOT. I decided not to overfill and particularly not to hydrotest these new tanks which would require 5000 psi pressurization. Call me psychic but I didn't trust them. However, I felt they would be safe in the special static application I had for them, especially if I inspected them regularly. In summary, I wish to restate that the term SLC does not necessarily imply a particular handling or usage or storage per se but that it is often used in context of multiple pressurizations, fast fills, and thermal stress.

Edit: Oh, oh, forgot to mention. At a YMCA instructors conference in 1980, I raised the aluminum tank safety issue but was told that there had been no documented failures. They were pretty adamant. Hoo, boy.

 

[The Chairman]

If it were due to pressure variances, then it would be Cyclical Load Cracking... but it is indeed SUSTAINED Load cracking. Just as sustained gravity has caused the glass from ancient Egypt to be thicker at the bottom than at the top, the sustained pressure on aluminum causes a re-alignment of the grain, called "grain creep". No, that's not some guy out in the corn field, but a phenomenon that generates cracks over time, while an aluminum vessel is under pressure. Some AL alloys resist this better than others, but all have the tendency.

Stress cracking (or hardening) is what you are referring to, and I don't think that there is that much deformation (plastic or permanent) during a normal pressurization process to cause that. I do know that over-pressurization of a vessel causes it to go from an elastic deformation to a plastic deformation (a permanent one). That's what hydrostatic testing measures... the elastic limits of the vessel. Once the elasticity of the vessel has been compromised it is not safe.

As for hydrostatic testing, the heat generated in compressing water is trivial when compared to pressurizing a vessel with a gas. I would rather have 5000 COLD psi applied to my tanks than 3000 HOT psi any day. Heat significantly lowers the threshold at which permanent deformation will take place.

 

[pescador775]

Antique glass was cast intentionally to be thicker at the bottom. The 'glass creep' is an urban legend. So I was told by a Corning glassworks engineer. As to the rest, heh, heh. You really don't have a clue as to the relationship between hydro tests, plasticity, and failure modes of aluminum tanks. BTW, 6061 alloy is not susceptible to SLC, period.

 

[AquaTec]

I used to do Hydrostatic testing when I was about 14 years old, since that was many decades ago i don't remember it all. which also might have to do with the fact that it was mostly a phsycodelic period as well.

But it seems that when we put them in the tank to test them we also measured the presure of the water surrounding the tank which then measured the expantion on the tank.

now we where doing steel 72's then..."those knew fangeled aluminum tanks where way to dangerous. nothing that light could hold up to the pressure." to quote my boss at the time

 

[The Chairman]

maybe you can enlighten us then.

If SLC has never been found in 6061 then why are you so against hydrostatic testing? That you seem to think hydorstatic testing is, in itself, destructive, makes me feel that you do not have a complete grasp of the differences between elastic and plastic deformations and how one affects the catastrophic failure of a vessel.

I never said SLC is common among all aluminum alloys, but grain creep (which is the actual culprit) is. Many factors (heat, alloy type, impurities) affect the grain of any aluminum alloy, just as they can affect steel. Don't you just hate it when stainless steel rusts? While it is possible to ameliorate certain tendencies, eliminating them can be almost impossible.

You can dismiss my posts with a glib "you don't have a clue" as you wish... I am certain that most readers are looking for something more substantial than that. And could you possibly explain why it is called Sustained Load Cracking, if it indeed comes only during the fill portion of a tank?

 

[The Chairman]

Now for an interesting side trip… I had been taught as a youth that glass was a super cooled liquid. I have believed that for some time, but that belief has been challenged. So instead of merely stating that the challenger had no idea what he was talking about, I searched the internet. First, I went to Urban Legends where I found no reference either way. Well, he said it was an “urban legend”, so I looked. Shoot me! Then I did a search on “ancient glass”… nothing definitive, “Egyptian glass”… still nothing, “Ancient Egyptian Glass”… still grasping at straws, until I finally typed in “liquid glass”… that yielded both of these;
Antique windowpanes and the flow of supercooled liquids
Is glass liquid or solid?
Bottom line; they don’t think it is, but they still aren’t %100.00 sure… it is obvious that they still possess that inquisitive nature and open mind. I like that!

As I try to point out from time to time… it doesn’t matter WHO is right, as much as it matters WHAT is right. I want to die knowing the truth, rather than merely protesting to others that I alone possess it! I will never use the window pane analogy again! Don't quit searching until you find the real answer!