Overfill of Aluminum Tank
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FIXXERVI6
Contributor
ClayJar:
I meant Yoke, I know how to spell it but I'm multi taskin screens, it was a typeo good catch.
hell I even spelled typo wrong, oh well
I think you might be hungry, too. (I know *I* am... now. 
)
Incidentally, while I cringe to apply Hanlon's razor to a place at which I'm buying life-support, the only two options I can see are either a scrambled version of "a little knowledge is a dangerous thing" or intentionally trying to sell extra stuff.
Hmm... I guess it may be possible that the OP misheard or has mistranslated, which would be the best scenario (as that would absolve the LDS).
)Incidentally, while I cringe to apply Hanlon's razor to a place at which I'm buying life-support, the only two options I can see are either a scrambled version of "a little knowledge is a dangerous thing" or intentionally trying to sell extra stuff.
Hmm... I guess it may be possible that the OP misheard or has mistranslated, which would be the best scenario (as that would absolve the LDS).
That I have to remember.FIXXERVI6:
Here we use 200bar(about 3000psi) and 300bar(4400psi) tanks. Compressors have separate security valves for both pressures, 230bar(3350psi) and 330bar(4800psi) and it's the pressure the tanks are filled everytime. (After cooling they set down about 30bar) If topped after cooling they are "overfilled".
xiSkiGuy
Contributor
My understanding is that at 3500psi you have to go with a different neck to valve thread (7/8 UNF), and that valve (being 300 bar) cannot accept yoke. The 200/230 bar convertibles (can use yoke) are under the 3500psi threshold and use the more common neck thread (3/4 NPS).FIXXERVI6:
So yes, being yoke adaptable is the major part of the calculus behind the sub 3500psi tanks.
Some types of steel tanks can be qualified for a 10% overfill with each hydrostatic test. This is what the "+" mark following the hydrostatic test stamp is for.
This does not mean that a 3000 psi tank can be filled to 3300 (for everyday use). A compact AL80 (which is actually 77cf, like all 80s), that is rated at 3300, HAS to be filled to 3300 in order to get the full 77cf. You're not getting more air, because of the 3300 rating. The volume rating is what matters in terms of total gas, not the pressure rating. In other words, you'll get the advertised volume, if you fill to the rated pressure.
An extra step in the hydro test process has to be performed in order to qualify the tank for an overfill "+" mark. It's always performed on the first factory hydrostatic test, but might be overlooked by whomever does the next hydro. Your LDS might have been referring to this failure of running the extra hydro test, and therefore losing the 10% overfill rating.
However... two things:
1-The higher the pressure in a tank, the more stress you're putting on your 1st stage. 3442s will result in higher 1st stage component failures, like O-rings and seats, than 3000psi fills. (Don't take my word for it... ask PST, Luxfer, or Catalina...actually, just ask PST... the others do aluminum).
2-Why overfill an AL80? To get more gas? Get a higher volume 3442psi HP100 steel tank, and underfill it to 3000psi, which will yield 89 cu.ft. You'll get a tank with better negative buoyancy characteristics, that's easy on your regs and valves, that is physically more durable, is 2" shorter, and the same diameter as an AL80, it's the same weight as the AL80, and will give you 12 cubic feet more of gas, despite the underfill. And, you can still fill it to capacity if you're going deep, and need the extra air (or EANx).
Dive safe...
EDIT: oh... just re-read your post, and see you've bought an AL100 3300psi. Your LDS is... ummm... mistaken. You do NOT overfill aluminum to 3600psi... It is tested at higher pressures, however, isn't meant to hold those pressures for any length of time. Additionally, many, if not most, fill stations will be hard pressed to fill beyond 3500 anyway...
This does not mean that a 3000 psi tank can be filled to 3300 (for everyday use). A compact AL80 (which is actually 77cf, like all 80s), that is rated at 3300, HAS to be filled to 3300 in order to get the full 77cf. You're not getting more air, because of the 3300 rating. The volume rating is what matters in terms of total gas, not the pressure rating. In other words, you'll get the advertised volume, if you fill to the rated pressure.
An extra step in the hydro test process has to be performed in order to qualify the tank for an overfill "+" mark. It's always performed on the first factory hydrostatic test, but might be overlooked by whomever does the next hydro. Your LDS might have been referring to this failure of running the extra hydro test, and therefore losing the 10% overfill rating.
However... two things:
1-The higher the pressure in a tank, the more stress you're putting on your 1st stage. 3442s will result in higher 1st stage component failures, like O-rings and seats, than 3000psi fills. (Don't take my word for it... ask PST, Luxfer, or Catalina...actually, just ask PST... the others do aluminum).
2-Why overfill an AL80? To get more gas? Get a higher volume 3442psi HP100 steel tank, and underfill it to 3000psi, which will yield 89 cu.ft. You'll get a tank with better negative buoyancy characteristics, that's easy on your regs and valves, that is physically more durable, is 2" shorter, and the same diameter as an AL80, it's the same weight as the AL80, and will give you 12 cubic feet more of gas, despite the underfill. And, you can still fill it to capacity if you're going deep, and need the extra air (or EANx).
Dive safe...
EDIT: oh... just re-read your post, and see you've bought an AL100 3300psi. Your LDS is... ummm... mistaken. You do NOT overfill aluminum to 3600psi... It is tested at higher pressures, however, isn't meant to hold those pressures for any length of time. Additionally, many, if not most, fill stations will be hard pressed to fill beyond 3500 anyway...
lamont
Contributor
BKP:
3300 psi at 70F, they can overfill it if it is hot and cools to that or less.
pescador775
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It seems doubtful that I had anything to do with the change over to the 3442 psi tank and elimination of the DIN valve requirement. However, for two years I argued that a DIN connection was unnecessary for an HP tank. I was the only one who described the features of the modern yoke valve invented by Sherwood Selpac in 1977 and who fully described its adaptations to higher pressures and why it was designed to be reliable to use at HP. I was the one who did the experiments of modern yoke clamps describing their huge strength and safety at 3500 psi and explained the fancy footwork by the manufacturers which lead to the various and contradictory ratings embossed on different yokes. Almost all these ratings were/are bogus and result from committee established performance limitations which are mostly outdated or overly conservative and even politically motivated in the sense that they are attempts to reconcile Euro opinions or prejuduces with our own.
It was no secret that PST watched these comments. Several years ago, we had a running dialogue with Mr Ivy of Product Development. He was very helpful in explaining some of the characteristics of PST tanks. However, I don't recall any substantive discussion of the metallurgy of the HP 100, particularly how US industry was able to produce a 3500 psi rated tank with near neutral buoyancy in sea water. I get the impression that not many people are aware of how unique this high pressure vessel is and what are some of the possible limitations. One clue is that this type of tank is not rated for overfill. Another is that the hydro test pressure is set at 3/2 WP. Why is that? One possible explanation is that E9791 super high tensile steel lacks the ductility of 3AA chrome moly steel. This would explain the "rounding out" procedure for hydro testing as well as the test pressure restriction. It would tend to make the tanks trickier to test and explain some failures of relatively new tanks. These would be in the category of trade offs for high performance.
Americans never took to the idea of very high pressure tanks, primarily due to weight and because the dive shops were not prepared to fill them when introduced into the US in the 1970's (Aga Divator). Basically, it was "ho humm". However, it seems to me that the advent of super tensile alloy steel which is used in the 3500 psi tank could be used to produce practical vhp (4400 psi) tanks. Of course, this would require the 300 bar valve and a return to special neck thread size.
In any case, however that may be, PST realized that the HP tank should have more versatility in the market and found their own way to reconcile the performance characteristics of modern yoke valves with the PST line of hp tanks. In short, they restamped their tanks with a lower pressure, a bit of fancy footwork of their own. I presume that PST had confidence in this as long as they could control the type of yoke/DIN valve supplied with the newly rethreaded and restamped tank. This was clever by any measure and is a stick in the eye to the idiot committees. Too bad that their company could not stay afloat in the face of financial realities. Technically driven, PST was a remarkable company.
Aluminum tanks have their own limitations including bulk and weight. Simply line up a 100 cf aluminum against a steel 100. If that doesn't convince you try lifting one in each hand. I guarantee your body will lean heavily to the side carrying the alu 100. Advantages of alu include cost and corrosion resistance. As to O2 and NITROX, the jury is out but industry is leaning to steel for containing oxygen although many of the small medical tanks continue to be made of aluminum. As to the issue of over fills, 6061 T6 aluminum has high ductility and my impression is that over fills would be safer than would be the case with high tensile steel and about the same risk/reward as LP or HP tanks made of 3AA steel. Doubt is cast on aluminum because of the lack of a plus rating. However, the plus rating is more of a traditional exemption for 3AA and 3A which dates to WWII and I am not convinced that that alone means anything for aluminum although it could for high tensile (9791) steel.
It was no secret that PST watched these comments. Several years ago, we had a running dialogue with Mr Ivy of Product Development. He was very helpful in explaining some of the characteristics of PST tanks. However, I don't recall any substantive discussion of the metallurgy of the HP 100, particularly how US industry was able to produce a 3500 psi rated tank with near neutral buoyancy in sea water. I get the impression that not many people are aware of how unique this high pressure vessel is and what are some of the possible limitations. One clue is that this type of tank is not rated for overfill. Another is that the hydro test pressure is set at 3/2 WP. Why is that? One possible explanation is that E9791 super high tensile steel lacks the ductility of 3AA chrome moly steel. This would explain the "rounding out" procedure for hydro testing as well as the test pressure restriction. It would tend to make the tanks trickier to test and explain some failures of relatively new tanks. These would be in the category of trade offs for high performance.
Americans never took to the idea of very high pressure tanks, primarily due to weight and because the dive shops were not prepared to fill them when introduced into the US in the 1970's (Aga Divator). Basically, it was "ho humm". However, it seems to me that the advent of super tensile alloy steel which is used in the 3500 psi tank could be used to produce practical vhp (4400 psi) tanks. Of course, this would require the 300 bar valve and a return to special neck thread size.
In any case, however that may be, PST realized that the HP tank should have more versatility in the market and found their own way to reconcile the performance characteristics of modern yoke valves with the PST line of hp tanks. In short, they restamped their tanks with a lower pressure, a bit of fancy footwork of their own. I presume that PST had confidence in this as long as they could control the type of yoke/DIN valve supplied with the newly rethreaded and restamped tank. This was clever by any measure and is a stick in the eye to the idiot committees. Too bad that their company could not stay afloat in the face of financial realities. Technically driven, PST was a remarkable company.
Aluminum tanks have their own limitations including bulk and weight. Simply line up a 100 cf aluminum against a steel 100. If that doesn't convince you try lifting one in each hand. I guarantee your body will lean heavily to the side carrying the alu 100. Advantages of alu include cost and corrosion resistance. As to O2 and NITROX, the jury is out but industry is leaning to steel for containing oxygen although many of the small medical tanks continue to be made of aluminum. As to the issue of over fills, 6061 T6 aluminum has high ductility and my impression is that over fills would be safer than would be the case with high tensile steel and about the same risk/reward as LP or HP tanks made of 3AA steel. Doubt is cast on aluminum because of the lack of a plus rating. However, the plus rating is more of a traditional exemption for 3AA and 3A which dates to WWII and I am not convinced that that alone means anything for aluminum although it could for high tensile (9791) steel.
lamont:
Absolutely. It's a normal practice for my LDS (which I'm grateful for).
In fact, it's annoying when you get a short fill, because the hot fill went to capacity and they didn't top it off when it cooled...
I think the OP's LDS was inferring that a final 3600psi was acceptable (after cooling).
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