Dive tank permanent expiration?

[fjpatrum]

It's always best to compare apples with oranges. The discussion is about aluminum tanks. The AL80 is often used as a loss leader to get people in the door. Compare the prices on those. They aren't that far off.

That makes sense. I didn't compare AL80s because Leisurepro doesn't list the brands my LDS says they "carry" (they don't actually keep any in stock, usually) so I didn't want to be disengenuous about comparing different brands and assuming they would all be the same. It didn't occur to me there would be that much difference in profit margin between steel tanks and aluminum tanks. Looks like the AL80s are all about the same price (roughly) whether buying online or at the LDS. Thanks for the clarification.

 

[elmer fudd]

I have two of the so said can't be hydroed/used Navy 6061 80 cu/ft aluminum tanks made of 6061. All the original hydro number are stamped on these tank so you can compare the results of any hydro to the original and the last hydro I had done on them 3 years ago yielded the same results as the original hydro. Also the eddy current test, though not required on 6061 tanks was also negative. These tanks were made in 1967, before any aluminum tanks were offered for sale on the civilian market.. Also there is no DOT restriction on the use of bushings in tanks provided they are rated for use at the tank's working pressure.

The thing about old aluminum tanks that would give me pause would be how many fills they have had more than the age.

Maybe, I'm wrong on this, (resort tanks certainly get used hard and they don't seem to be exploding regularly), but there's no denying that aluminum is a metal that doesn't stand up well to repeated flexing.


I think that shops refusal to fill tanks with bushings probably stemmed from early divers using dissimilar metals and soft metals like brass. My guess is that more than a few of those bushings came from the plumbing aisle of the local hardware store.

 

[captain]

The thing about old aluminum tanks that would give me pause would be how many fills they have had more than the age.

Maybe, I'm wrong on this, (resort tanks certainly get used hard and they don't seem to be exploding regularly), but there's no denying that aluminum is a metal that doesn't stand up well to repeated flexing.


I think that shops refusal to fill tanks with bushings probably stemmed from early divers using dissimilar metals and soft metals like brass. My guess is that more than a few of those bushings came from the plumbing aisle of the local hardware store.

I believe cylinder design criteria is 10,000 cycles to hydro pressure before failure. Failure being the failure to pass hydro not a catastrophic failure. In any event that would be one a day for 27 years. I am pretty comfortable with that.

In the case of the bad alloy it wasn't a cyclic failure it was a sustained load failure. I know of no other cases of a sustained load failure in cylinders made of other materials other I believe some composite hoop rapped cylinders used for SCBA's.

 

[The Chairman]

Maybe, I'm wrong on this, (resort tanks certainly get used hard and they don't seem to be exploding regularly), but there's no denying that aluminum is a metal that doesn't stand up well to repeated flexing.

Just because steel is heavier (denser), does not make it superior. Most metals have two types of stretch (deformation): Elastic and Plastic. The former is temporary like a rubber band where the metal expands and contracts easily with negligible work hardening. The latter is permanent like putty and causes both hardening and fatigue. Both steel and aluminum tanks do this and Hydro-static Testing actually gauges their elasticity by comparing their displacement while at ambient pressure, 5/3s of their working pressure and then back at ambient pressure.

If you take a look at the difference in tank thicknesses, you'll notice that the aluminum is a lot thicker. This has a lot to do with the strength needed, but it also affects the structural properties of the vessel. Either way, the designs are more than adequate to withstand thousands of cycles of pressurization. So why should we hydro them? Incidents like over filling, dropping or dinging them have deleterious effects and of course there is also the problem of corrosion. There is simply no reliable way to quantify these, so we need to make sure that they are still elastic. Which will fail first? The abused tank, by a long margin. It doesn't matter whether it's steel or aluminum (or aluminium for Doppler).

 

[elmer fudd]

Just because steel is heavier (denser), does not make it superior. Most metals have two types of stretch (deformation): Elastic and Plastic. The former is temporary like a rubber band where the metal expands and contracts easily with negligible work hardening. The latter is permanent like putty and causes both hardening and fatigue. Both steel and aluminum tanks do this and Hydro-static Testing actually gauges their elasticity by comparing their displacement while at ambient pressure, 5/3s of their working pressure and then back at ambient pressure.

If you take a look at the difference in tank thicknesses, you'll notice that the aluminum is a lot thicker. This has a lot to do with the strength needed, but it also affects the structural properties of the vessel. Either way, the designs are more than adequate to withstand thousands of cycles of pressurization. So why should we hydro them? Incidents like over filling, dropping or dinging them have deleterious effects and of course there is also the problem of corrosion. There is simply no reliable way to quantify these, so we need to make sure that they are still elastic. Which will fail first? The abused tank, by a long margin. It doesn't matter whether it's steel or aluminum (or aluminium for Doppler).

It's not due to it's weight, but steel is superior in this regard. It's why they use steel for springs and not aluminum. Titanium is supposed to make a good spring as well. It has to do with the fatigue life of the metal. Steel can flex a certain amount with it causing no damage at all, regardless of how many times you do it. It's not the same with aluminum. Every time you flex aluminum it breaks down it's structure a little.

Fatigue limit - Wikipedia, the free encyclopedia

 

[The Chairman]

Again, I would suggest that simple abuse causes far more failures than the actual material used. I have seen a significant number of steel 72s retired over the years. I am almost certain that they were victims of serial over filling.

 

[elmer fudd]

Again, I would suggest that simple abuse causes far more failures than the actual material used. I have seen a significant number of steel 72s retired over the years. I am almost certain that they were victims of serial over filling.

I wouldn't doubt that at all. They're a 2250 tank in an era where shop compressors often go up to 4000 or higher. I've got to plead guilty to topping mine off at 2800 occasionally though.

 

[captain]

Again, I would suggest that simple abuse causes far more failures than the actual material used. I have seen a significant number of steel 72s retired over the years. I am almost certain that they were victims of serial over filling.

A more likely reason is for failing to follow the recommended practice of doing the round out procedure prior to the hydro on hot dipped galvanized steel cylinders.

http://www.ctcseminars.com/files/technical/worthington_hrp.pdf

http://www.ctcseminars.com/files/technical/PST Tecnical bulletin D100.pdf

 

[The Chairman]

I don't know captain... most of the hydro facilities I have used, pride themselves on following the written procedures for each tank. I have picked up my 72s with 3600 in them way too often and that's after they had cooled down. Twice I have picked them up with over 4,000 psi. You can't exceed rated pressure with impunity. The shop isn't doing me any favors with such a hot fill.

 

[PfcAJ]

These shops what won't fill a tank made before 1990 because they're scared of old tanks...next year will the cut off be 1991? Then the following year 1992? Etc?

I bet you a doughnut that its been '1990' for years.

It becomes not about age of the tank, but some misinformed nonsense about the old alloy tanks. Politely inform and attempt to educate (documents in hand). If that fails, just move along.