Why not overfill Aluminum cylinders?

[Charles2]

Is the elastic limit of steel tanks defined? What is the limit for the LP Worthington tanks for example?

I had always heard that 3AA tanks were manufactured from 4130-N steel alloy. If so, the elastic limit (yield point) of 4130-N steel is normally specified at 63,100 psi.

On the other hand, I have heard that 3AL tanks were manufactured from 6061-T6 alloy which has an elastic limit of 40,000 psi.

This is a good place to start if you don't want to accept the enormous number of steel overfills that have been done over the years: Vintage Double Hose • View topic - Vintage tank hydros and "+" stamping

I do accept that there have been an enormous number of steel overfills over the years. I did not find anything in the link that related to my original question. Perhaps you could help?

For the record, I won't ever ask that any of my 2250 steel tanks get pumped to over 3K.

Again, if you are willing to pump a steel tank rated at 2250 psi to 3000 psi, why are you unwilling to pump an aluminum tank rated at 3000 psi to 4000 psi? After all, it's the same percentage of overfilling.

 

[Tug]

Steve Lewis (@Doppler) just posted an article about overfilling aluminum tanks

Don’t even think about asking for an overfill in your aluminum cylinder…

 

[rjack321]

I had always heard that 3AA tanks were manufactured from 4130-N steel alloy. If so, the elastic limit (yield point) of 4130-N steel is normally specified at 63,100 psi.

On the other hand, I have heard that 3AL tanks were manufactured from 6061-T6 alloy which has an elastic limit of 40,000 psi.

Those numbers look about right to me. I am not positive what exemption series tanks have. Its not orders of magnitude different than 3AA steel though.

Again, if you are willing to pump a steel tank rated at 2250 psi to 3000 psi, why are you unwilling to pump an aluminum tank rated at 3000 psi to 4000 psi? After all, it's the same percentage of overfilling.

Some people do. But the reality is that very few people cave dive with 72s since they are a relatively large tank for the volume of gas inside. If cave diving and in a really small cave you'll bring lp50s overfilled - a long narrow tank. The next size up wouldn't be 72s because lp85s are the same diameter (just longer) with a lot more gas. Cave fills came out of the cave diving community and the comfort level with overfills is thus in part based on what people have experience with.

 

[Compressor]

I just saw this in a blog I scubscribe.
............

I see after posting that it was mentioned a few threads up.

 

[Charles2]

Steve Lewis (@Doppler) just posted an article about overfilling aluminum tanks

Don’t even think about asking for an overfill in your aluminum cylinder…

Great post.

Are you OK with overfilling a steel cylinder?

 

[Compressor]

It would be educational to hear from a dive operator in a locale with heavy tank use as to see how long their tanks last. Such locale could be a cayman or Florida diveshop where dive boats go out everyday with 40 tanks.

Thanks guys and gals; it's been educational.

 

[rjack321]

Great post.

Are you OK with overfilling a steel cylinder?

Only using even a 6061 aluminum cylinder for 6 or 7 years is ridiculously conservative.

 

[lowviz]

...//... I do accept that there have been an enormous number of steel overfills over the years. I did not find anything in the link that related to my original question. Perhaps you could help? ...

Yes, I said that it was a good place to start.

Few people know more about steel tanks than Luis H. If he says something, I just go with it. So now you have your feet on the ground with respect to steel tanks and the following posts highlight problems with aluminum tanks that steel tanks don't share. You may wish to pursue data on steel overfills, I don't need it although it would be novel if you actually found good data.

...//... Again, if you are willing to pump a steel tank rated at 2250 psi to 3000 psi, why are you unwilling to pump an aluminum tank rated at 3000 psi to 4000 psi? After all, it's the same percentage of overfilling.

Yes, the same percentage on two different metals. It is different.

People do overfill aluminum tanks, though. Their choice, I'm not one to stop them.

Problem is, if I did that I'd be on the other side of the fence as both Luis H and Steve Lewis. That would give me a really creepy feeling...

 

[scubadada]

In another thread, this comment was made:



Since both 3AL and 3AA tanks are both hydro tested to 5/3rds rated pressures, what is the problem with overfilling an aluminum cylinder to the same proportionate amount as a steel cylinder? If a cave filled LP cylinder is filled to 3500 psi, why not fill an AL80 to 4000 psi?

I apologize in advance for my ignorance.

This is an interesting question from Charles2. Many of us have heard that AL cylinders should not be over filled. Many of us also know that LP steel cylinders are not uncommonly, significantly overfilled.

I do much of my diving in SE FL and do not have my own tanks at my home there, so rent AL80s when visiting. I use 5 different operators in 3 different cities. It has been my impression that most of my fills are over the standard operating pressure of AL80s of 3000 psi.

My primary computer is AI, so I have all my tank pressures downloaded. For fun, I checked the starting pressure for my last 50 dives using AL80s. The average fill pressure was 3291 psi with a range of 3032-3552 psi. I found these results very interesting, average overfill of 9.7%, peak overfill of 18.4%. Not exactly cave fills, what do folks think?

 

[Doppler]

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.