Question regarding tank fills

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CT-Rich

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Anecdotal evidence does not convince anyone of the validity of your arguments.
Have you ever seen an HP aluminum tank? Why not? Have ever seen aluminum springs? Why are springs usually made of steel and not aluminum. What sort of evidence do you not consider “anecdotal”?

All tanks are designed to be safe. If you buy a tank tomorrow, it may well out last your dive career, that is why there is such an active market for used cylinders. Prior history does impact future expectations, though.

Early steel tanks were often given the “+” rating. With the +, they were good for an extra 10% fill. The idea they can get a “cave fill” probably evolved from those early “+” cylinders. Early aluminum tanks were subjected to more rigorous testing because of the wide spread use of the 6351 alloy. Lots of shops decided it was easier to ban older aluminum tanks than to teach fill tech to determine how to identify the 6351 cylinders correctly (they aren’t necessarily stamped with 6351). They are subject to load cracking and aluminum is much softer than steel. With proper care, both materials will last a long time (10,000 fills for aluminum, infinite for steel).

Is over filling cylinders going to lead to catastrophic failure? Probably not, but the consequences will be catastrophic. A failed cylinder can pretty much level small building. If you have a policy of allowing cave fills, the family of they guy who was standing next to the cylinder will be looking for you.

LIFE EXPECTANCY OF A SCUBA TANK - Redland Sport Divers
 

Charles2

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Don't know why I'm bothering...

Have you ever seen an HP aluminum tank?...

Sorry that this post seems to have devolved into a measuring contest. Please remember that the original question in Post #53 was “why aren't 3AL tanks as tolerant of overfilling as 3AA tanks?” and in Post #56 “It is my contention that those differences in properties were taken into consideration before the final design of 3AA and 3AL cylinders and that result is that both cylinders will respond similarly to overfilling.”

I do understand the differences between steel, aluminum, and other metals for that matter. I also understand the characteristics and causes of metal fatigue both in aluminum as well as steel.

I based my comments using the government specifications for 3AA and 3AL cylinders as found at this link: SUBPART - Specifications for Cylinders (govregs.com). If you will read these specifications carefully, you will discover that the stress limits for both cylinders are designed using the same design protocols and limits but using different materials. I have copied excerpts from the wall thickness designs for each below.

SUBPART - Specifications for Cylinders (govregs.com)

§ 178.37 - Specification 3AA and 3AAX seamless steel cylinders. – For cylinders with service pressure of 900 psig or more the minimum wall must be such that the wall stress at the minimum specified test pressure may not exceed 67 percent of the minimum tensile strength of the steel as determined from the physical tests required in paragraphs (k) and (l) of this section and must be not over 70,000 psi.

§ 178.46 - Specification 3AL seamless aluminum cylinders – Wall thickness. The minimum wall thickness must be such that the wall stress at the minimum specified test pressure will not exceed 80 percent of the minimum yield strength nor exceed 67 percent of the minimum ultimate tensile strength as verified by physical tests in paragraph (i) of this section.

Again, my original question is: Why aren't 3AL tanks as tolerant of overfilling as 3AA tanks? It seems to me that both specifications are equally tolerant of overfilling – and both will catastrophically explode if overfilled beyond their ultimate values. What am I missing here?
 

scubadada

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I was recently on a liveaboard that offered AL80s and AL100s, they were both filled to the same pressure, an average of 3084 psi. I dived an AL80 and had an average fill of 79.6 cu ft. Divers with the AL100s has an average fill of 93.4 cu ft, not quite the differential they were expecting. The larger cylinders cost $10/day.

Keeping us in suspense?

Sorry, must have gotten distracted before I finished, fixed it :)
 

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Sorry that this post seems to have devolved into a measuring contest. Please remember that the original question in Post #53 was “why aren't 3AL tanks as tolerant of overfilling as 3AA tanks?” and in Post #56 “It is my contention that those differences in properties were taken into consideration before the final design of 3AA and 3AL cylinders and that result is that both cylinders will respond similarly to overfilling.”

I do understand the differences between steel, aluminum, and other metals for that matter. I also understand the characteristics and causes of metal fatigue both in aluminum as well as steel.

I based my comments using the government specifications for 3AA and 3AL cylinders as found at this link: SUBPART - Specifications for Cylinders (govregs.com). If you will read these specifications carefully, you will discover that the stress limits for both cylinders are designed using the same design protocols and limits but using different materials. I have copied excerpts from the wall thickness designs for each below.

SUBPART - Specifications for Cylinders (govregs.com)

§ 178.37 - Specification 3AA and 3AAX seamless steel cylinders. – For cylinders with service pressure of 900 psig or more the minimum wall must be such that the wall stress at the minimum specified test pressure may not exceed 67 percent of the minimum tensile strength of the steel as determined from the physical tests required in paragraphs (k) and (l) of this section and must be not over 70,000 psi.

§ 178.46 - Specification 3AL seamless aluminum cylinders – Wall thickness. The minimum wall thickness must be such that the wall stress at the minimum specified test pressure will not exceed 80 percent of the minimum yield strength nor exceed 67 percent of the minimum ultimate tensile strength as verified by physical tests in paragraph (i) of this section.

Again, my original question is: Why aren't 3AL tanks as tolerant of overfilling as 3AA tanks? It seems to me that both specifications are equally tolerant of overfilling – and both will catastrophically explode if overfilled beyond their ultimate values. What am I missing here?
Just the fact that a piece of aluminum that is bent back and forth will break before a piece of steel that is bent back and forth. It doesn't matter how thick you make the aluminum. If you stretch it and it returns to shape it will fail before a piece of steel that you do the same thing to. As others have said, aluminum springs don't work well. The tanks are not made so strong that they don't stretch. They would be too heavy. Steel stands up to this stretching and returning process better than aluminum. Remember, the hydro process measure permanent deformation. They measure the amount of water the tank will hold, then put it under hydro pressure, then release the pressure and measure how much the tank will hold. If the number is close, it is acting like a spring and returning to it's original size. If it is not acting as a spring, the tank gets bigger under the overload. Which means it will eventually expand far enough to fail.
 

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Sorry that this post seems to have devolved into a measuring contest. Please remember that the original question in Post #53 was “why aren't 3AL tanks as tolerant of overfilling as 3AA tanks?” and in Post #56 “It is my contention that those differences in properties were taken into consideration before the final design of 3AA and 3AL cylinders and that result is that both cylinders will respond similarly to overfilling.”

I do understand the differences between steel, aluminum, and other metals for that matter. I also understand the characteristics and causes of metal fatigue both in aluminum as well as steel.

I based my comments using the government specifications for 3AA and 3AL cylinders as found at this link: SUBPART - Specifications for Cylinders (govregs.com). If you will read these specifications carefully, you will discover that the stress limits for both cylinders are designed using the same design protocols and limits but using different materials. I have copied excerpts from the wall thickness designs for each below.

SUBPART - Specifications for Cylinders (govregs.com)

§ 178.37 - Specification 3AA and 3AAX seamless steel cylinders. – For cylinders with service pressure of 900 psig or more the minimum wall must be such that the wall stress at the minimum specified test pressure may not exceed 67 percent of the minimum tensile strength of the steel as determined from the physical tests required in paragraphs (k) and (l) of this section and must be not over 70,000 psi.

§ 178.46 - Specification 3AL seamless aluminum cylinders – Wall thickness. The minimum wall thickness must be such that the wall stress at the minimum specified test pressure will not exceed 80 percent of the minimum yield strength nor exceed 67 percent of the minimum ultimate tensile strength as verified by physical tests in paragraph (i) of this section.

Again, my original question is: Why aren't 3AL tanks as tolerant of overfilling as 3AA tanks? It seems to me that both specifications are equally tolerant of overfilling – and both will catastrophically explode if overfilled beyond their ultimate values. What am I missing here?

Ok here goes, IMO I think what you maybe missing is that AL tanks will not tolerate overfilling as long or as many fills as steel. I have steel tanks from the 1960's that I still use and overfill 10% with my own compressor. I only had one AL 80 tank, it was the troublesome alloy type so I scrapped it.
I don't think anyone will have a 50 year old AL tank they're still filling +10% in the future. I don't think it will hold up.

Also it is always a possibility a tank could fail in dramatic fashion and cause damage and death, it has happened. What mostly happens is what happened to my 1st steel tank and for a long time the only tank I had so it had a lot of use. It failed hydro because it would not return to acceptable dimensions post hydro, it lost it's ability to return after being pressurized so now it's gone.

Now someone may take such tank and say heck I can use that for portable air around the property filling tires and such. The hydro is only if I want to transport it on the roads. That person may experience a failure in dramatic fashion.
steel tank.jpg
steel
tanks.png
middle one AL
 

CT-Rich

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Sorry that this post seems to have devolved into a measuring contest. Please remember that the original question in Post #53 was “why aren't 3AL tanks as tolerant of overfilling as 3AA tanks?” and in Post #56 “It is my contention that those differences in properties were taken into consideration before the final design of 3AA and 3AL cylinders and that result is that both cylinders will respond similarly to overfilling.”

I do understand the differences between steel, aluminum, and other metals for that matter. I also understand the characteristics and causes of metal fatigue both in aluminum as well as steel.

I based my comments using the government specifications for 3AA and 3AL cylinders as found at this link: SUBPART - Specifications for Cylinders (govregs.com). If you will read these specifications carefully, you will discover that the stress limits for both cylinders are designed using the same design protocols and limits but using different materials. I have copied excerpts from the wall thickness designs for each below.

SUBPART - Specifications for Cylinders (govregs.com)

§ 178.37 - Specification 3AA and 3AAX seamless steel cylinders. – For cylinders with service pressure of 900 psig or more the minimum wall must be such that the wall stress at the minimum specified test pressure may not exceed 67 percent of the minimum tensile strength of the steel as determined from the physical tests required in paragraphs (k) and (l) of this section and must be not over 70,000 psi.

§ 178.46 - Specification 3AL seamless aluminum cylinders – Wall thickness. The minimum wall thickness must be such that the wall stress at the minimum specified test pressure will not exceed 80 percent of the minimum yield strength nor exceed 67 percent of the minimum ultimate tensile strength as verified by physical tests in paragraph (i) of this section.

Again, my original question is: Why aren't 3AL tanks as tolerant of overfilling as 3AA tanks? It seems to me that both specifications are equally tolerant of overfilling – and both will catastrophically explode if overfilled beyond their ultimate values. What am I missing here?
You see to be asking a question expecting a specific answer. Cylinders are not made to barely exceed their working pressure. They are built with a significant safety margin. Both types are required to undergo periodic inspections to confirm the integrity of the tank. Is there sufficient margin to over fill either cylinder type by 10%? Absolutely.

If you significantly over fill a cylinder, the burst disk is likely to fail long before the cylinder does

over hundreds or thousands of fill will it shorten the working life of the tank? Probably in the case of the Al tanks. Most of the responders here agree on that. As for steel, probably not, based on most of the responders. Dive centers that have hundreds of tanks that are used daily probably care somewhat more because a tank that fails hydro is lost inventory. Even so, the tank jockeys filling them are probably more interested in good enough.

Tank failures are pretty rare. Of the 25 million tanks made with the 6351 alloy only 19 blew up. As someone else pointed out, no manufacturer is going to say tanks can and should be overfilled.

Again, what are looking for in an answer? Steel tanks sometimes get “cave fills”. Generally, aluminum tanks don’t. In either case, catastrophic failure is unlikely (but not impossible).
 
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