Cave Fills on LP tanks

[rjack321]

They may very well be made of different steels using an otherwise identical manufacturing process in the same facility. They also may be made of nominally identical steel but subject to more careful testing regarding the composition and properties of the steel and the finished cylinders.

May this, may that

so much speculation in the face of a complete absence of actual failures.

 

[Lorenzoid]

They may very well be made of different steels using an otherwise identical manufacturing process in the same facility. They also may be made of nominally identical steel but subject to more careful testing regarding the composition and properties of the steel and the finished cylinders.

I recall you saying this in another thread, and it certainly seems plausible. But why has no one in the diving community apparently ever been able to respond in one of these threads with documented evidence one way or the other? Even if it were a trade secret, you'd think it would have surfaced at some point over the decades.

 

[Charles2]

I was told it has to do with regulations. In the US a high pressure vessel can only have a working pressure that is ¼ of the burst pressure. In the EU it's something like ⅓. Don't quote me on that, I don't remember where I got that info and did not recently research it.

US government DOT 3AA specifications -- 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. Since the minimum test pressure for 3AA tanks is 5/3rds of the working pressure, then the burst pressure can be calculated by multiplying 5/3 times the reciprocal of 67%. Simplified, 5/3 times 1/0.67 = 2.48 or approximately 2½ times the working pressure. Therefore, the calculated design burst pressure for an LP (2400 psi) tank designed to 3AA specifications is 2400 times 2½ or about 6000 psi. There are other design criteria that improve those numbers; however, in answer to your comment, the US regulation is 2½ times working pressure, not 4.

3AL specifications -- Three samples must be pressurized to destruction and failure may not occur at less than 2.5 times the marked cylinder service pressure. Each cylinder must remain in one piece. Failure must initiate in the cylinder sidewall in a longitudinal direction. Rate of pressurization may not exceed 200 psig per second. The wall thickness specification is almost identical to the 3AA specification; however, in the 3AL specification this burst pressure test was actually directly included in the specification.

There are similar requirements for HP cylinders if you care to look them up.

 

[2airishuman]

I recall you saying this in another thread, and it certainly seems plausible. But why has no one in the diving community apparently ever been able to respond in one of these threads with documented evidence one way or the other? Even if it were a trade secret, you'd think it would have surfaced at some point over the decades.

The 3AA specification is well known and all current manufacturers use 4130X steel (which is one of several permitted). I don't know what kind of steel is used in other markets and don't have sources for that information.

I will point out that toward the beginning of the HP cylinder era, some posters -- on scubaboard and elsewhere -- claimed that E9791 HP cylinders were also just 3AA cylinders with different markings, noting the similar sizes, shapes, and wall thicknesses to the 3AA cylinders then produced by PST. This claim was false, because the steel in E9791 cylinders, while within the composition requirements of 4130X, does not meet the non-brittleness requirements for the 3AA specification, and the steels allowed for 3AA cylinders cannot meet the yield strength requirements of the E9791 specification. Later exemption/special permit authorizations for other manufacturers are largely identical to E9791.

The evolution of the steels used in cylinders, from 3 to 3A to 3AA to E9791, has all been based on the development of metals that have higher yield strength without being excessively brittle. This has allowed progressively thinner walls resulting in lighter cylinders that will withstand the same pressure. Except for Canada, which follows U.S. practice in this area, I don't know where along that progression other countries' specifications sit.

 

[2airishuman]

May this, may that
so much speculation in the face of a complete absence of actual failures.

Let's be clear what we're talking about.

If you, or any other posters here, have made an informed decision to pressurize your cylinders beyond the pressures permitted in commerce by the CFRs, I'm really OK with that. You have your reasons which have been outlined throughout this thread. I understand that for reasons of cost, availability of cylinders that trim out well for you or in the preferred size and shape for your situation, and minor differences in weight and buoyancy, you prefer to run 3AA cylinders at these elevated pressures rather than using HP cylinders. You're all adults and you all know what you're doing. Go forth and have a great dive, and if you're going to blow up your cylinders to 3600 or 3900 PSI, well, that's your choice. You have assessed the risks and assessed how any federal and state regulations affect your activities, and made your choices.

I, obviously, have various reservations about this, and choose to utilize HP cylinders for those dives where I need to bring larger amounts of gas. Whatever cylinders I'm using, I choose to fill them to their rated capacity. That, I believe, is the best choice for the kinds of dives that I make.

I hope that all of you respect that choice just as I respect your choices.

I do not believe that it is prudent to promulgate or institutionalize the practice of overfilling 3AA cylinders as a routine measure for routine recreational dives, which is why I post in this thread. I believe there is greater risk and consider this greater risk to be based on sound science and engineering and not mere speculation. I believe that, should widespread adoption of the practice of filling to 90% of test pressure occur, there will be accidents as the very small risk of a rupture per cylinder is taken with an aging and growing population of cylinders. I believe there are better alternatives and that recreational divers asking for advice on cylinder selection in 2018 should be informed of these alternatives.

 

[JohnnyC]

Has anybody bothered to ask Faber if their Euro rated cylinders are the same as the US LP rated cylinders, simply stamped with a different rating? Somebody who speaks the language and can understand the nuances of the conversation?

For simple supply chain management I would have a hard time believing Faber sources different steel because they can get away with different steel for the US market. The cost differences between those steel prices would be pretty insignificant. I'd be willing to bet that other cost difference would more than make up for any insignificant savings by sourcing a similar, certified, steel supply.

 

[PfcAJ]

Belief in the absence of evidence is a strange thing.

 

[2airishuman]

Belief in the absence of evidence is a strange thing.

Take a couple of probability and statistics courses, study some materials science, spend some time in the lab, build some things based on what you know and find out if you're always right, then come back and tell me if you still feel the same way.

 

[tomfcrist]

Belief in the absence of evidence is a strange thing.

Yet the vast majority of people believe e in things they have zero evidence of. Thats concerning in my opinion.

 

[2airishuman]

Has anybody bothered to ask Faber if their Euro rated cylinders are the same as the US LP rated cylinders, simply stamped with a different rating? Somebody who speaks the language and can understand the nuances of the conversation?

I think that's a really good question.

For simple supply chain management I would have a hard time believing Faber sources different steel because they can get away with different steel for the US market. The cost differences between those steel prices would be pretty insignificant. I'd be willing to bet that other cost difference would more than make up for any insignificant savings by sourcing a similar, certified, steel supply.

I agree but suspect that they may be dealing with non-overlapping requirements. It isn't always possible to build a product that is compliant with multiple regulatory frameworks. Sometimes that's by accident, and sometimes its designed in by the bureaucrats as a protectionist move. I have a hard time believing it's just a matter of marking because of the amount of effort that went into coming up with the new UN standard for cylinders to be used worldwide.