Why not overfill Aluminum cylinders?

[lowviz]

...//... also another nice feature of getting fills like this is you can top off your bailout and still have a full main tank. ...

a US 72 is only 72 at 10% overfill. (for almost all 72's) So 72-7.2 = 64.8 cuft at its service pressure of 2250.

64.8 x 3200/2250 = 92 cuft. Even if my pony is flat, I can fill it and have a full tank (72 cuft) left for me.

Edit
In honor of a special person who would have been all over this thread: What's a cave fill?

 

[scubafanatic]

With respect to AL tanks:
AL 80 listed max fill pressure: 3000 psi
AL 100 listed max fill pressure: 3300 psi

Question: is it REALLY the case that the AL 100 is made from a different alloy such that it's rated to 3300 psi ? I find it hard to believe, given the rarity of the AL 100, that it's produced with a different alloy, but perhaps that's really the case ?

 

[Ghetto Diver]

Same material.
Al 100's are bigger and have thicker walls and are heavy SOB's. Ask me how I know.

 

[Peter69_56]

A buddy told me about this thread very soon after I posted my blog entry (referenced above).

A couple of points NOT expanded on or explained fully in my blog, but relevant to some questions asked here.

First is that aluminum has a completely different response to steel when it is filled... unlike steel tanks which reach a point of reduced elasticity and stays AT THAT POINT for "ever", the reduction in aluminum's ability to expand without failure CONTINUES TO be reduced and will eventually result in fatigue (this is an oversimplification but gets the point across). I am aware the number of fills that an aluminum cylinder can endure is in the order of 10^5 (a million). However, that is absolutely contingent on the manufacturer's rated pressure. A metallurgist explained to me that overfilling an aluminum cylinder reduces that number "exponentially." I didn't ask for specifics but she said it could be several orders of magnitude with constant fills to 250 bar (about 3600 psi). That worries me. I try to make sure my students worry about it too.

I mentioned in the blog that I even Eddy Current new cylinders... I am aware of the false negatives reported and associated with this practice...

I am also aware that my practice of selling cylinders after about six or seven years is conservative... I do so out of concern for the buyer... and because I can afford to be cautious.

When I teach a VIP course, I tell participants that in the final analysis, if they would not be comfortable sitting on a tank as it is being filled, they should not pass it. Ridiculously conservative? Possibly. But I would like to see all cylinders filled in containment stations. Odd perhaps, but I'm okay with that label.

Finally, I have been made aware of a rash of aluminum cylinders in oxygen service going tits-up. I have seen divers throw deco bottle around like sandbags. Not a good practice. Be careful.

Whats the relationship between oxygen use with aluminium vs air with aluminium and tank failures?

 

[Compressor]

The generally accepted numbers for 6061-T6 aluminum are: Yield Strength of 40,000 psi and an Ultimate Strength of 45,000 psi.

Now I am confused. With these numbers so high, who cares if we overfill then by 10% or so. I don't ask for overfilling.

 

[Peter69_56]

So I posted earlier that I commonly get AL cylinders filled 10-20% over working pressure. Are there any implications here or is this below worrisome pressures? These are cool pressures, not immediately after filling.

There was an interesting article written by Steve Burton (rocket and scuba engineer) from Asset regarding overpressurisation of tanks on filling, relating to hot fill temperatures.

DOT3AL3000 tropical scuba tank filling

 

[Ghetto Diver]

Now I am confused. With these numbers so high, who cares if we overfill then by 10% or so. I don't ask for overfilling.

See post #60


ETA
Yield and Strength data pertain to the material itself and does not correspond directly with the completed product/design.

 

[scubafanatic]

Same material.
Al 100's are bigger and have thicker walls and are heavy SOB's. Ask me how I know.

I have used AL 100s on a couple of dive trips, as they're often the biggest tanks available for those ops not offering steels, and I didn't find them to be as bad to dive as most folks appear to think. Once in the water they seemed to dive just fine.

 

[Peter69_56]

I have used AL 100s on a couple of dive trips, as they're often the biggest tanks available for those ops not offering steels, and I didn't find them to be as bad to dive as most folks appear to think. Once in the water they seemed to dive just fine.

I have a mate who has a twin set in steels. Too heavy for me but he handles them fine.

 

[captain]

Now I am confused. With these numbers so high, who cares if we overfill then by 10% or so. I don't ask for overfilling.

Realize that the psi numbers in yield or tensile strength have absolutely no correlation to the pressure in a tank. A tank would fail long before it reached 40,000 psi. The actual failure pressure of a tank is in the 7000 to 10,000 psi range. Any undetected flaws in the tank can substantially reduce what pressure it fails at.
Yield strength testing involves taking a small sample with a fixed cross-section area, and then pulling it with a controlled, gradually increasing force until the sample changes shape or breaks. This is called a Tensile Test.

Had there been air instead of water in the tank the hole it is in would have gotten a lot larger.