Over filling LP tanks, what's the real deal?

[oxyhacker]

Polite, candid, and wrong.

European cylinders in general, steel at least, do not have finite lifetimes. They are good as long as they pass the requalification tests, just like in the US. Composite, that's another story.

He was polite and candid. His replies:

• The cylinder sold in Europe is not the same construction as the North America cylinder.
• European cylinders have a finite lifespan.

 

[oxyhacker]

Yes, that is exactly the way the engineers at the DOT like to explain it. But I just don't think it is entirely a coincidence that the only specification under which a tank can be approved for manufacture on the basis of calculations alone is one which long experience in the field has shown to incorporate a massive safety margin. If predicting stress and long term effects of it were really so straightforward and simple, then you'd expect that the DOT would be willing to approve aluminum and exemption tanks on the basis of calculations alone. Which of course they are not.

Actually, it has a lot more to do with being predictable by calculations than being conservative.

The stress calculation for a simple cylinder and the end caps is very straight forward. As long as there are no defects, the geometry is very simple and the math can predict the stress very precisely. calculations.

 

[Packhorse]

European cylinders in general, steel at least, do not have finite lifetimes. They are good as long as they pass the requalification tests, just like in the US. Composite, that's another story.

Cant really speak for EU but in NZ our most common steel tanks are Faber 232 bar. From what I can tell they weight more than 184bar tanks ( dont know if we have any in NZ unless the classic 72cf tank is one) and weight less than the 300bar fabers.
So not all Fabers are created equal. But AFAIK none have a finite life span. We do have yearly vis and 2 yearly hydros though.

 

[Luis H]

Yes, that is exactly the way the engineers at the DOT like to explain it. But I just don't think it is entirely a coincidence that the only specification under which a tank can be approved for manufacture on the basis of calculations alone is one which long experience in the field has shown to incorporate a massive safety margin. If predicting stress and long term effects of it were really so straightforward and simple, then you'd expect that the DOT would be willing to approve aluminum and exemption tanks on the basis of calculations alone. Which of course they are not.

As I said, stress calculations on a cylindrical geometry are very predictable and extremely easy to calculate.

What are not always predictable are the properties of some materials. As I mentioned above, I don’t think we have a lot of experience with the 6351 aluminum (as one example).

We obviously would have very little experience with some new proprietary high strength steel alloy. Therefore, our confidence on it will be lower than for the alloys used on 3AA or 3A steel cylinders.

The alloys approved to be used in 3AA seamless cylinders have been around for a long time and therefore we have a high confidence factor on their long term performance.

Don’t confuse safety factor with confidence factor. Safety factor is often defined and calculated in a very specific manner for the application.

We can have a lot of confidence on old steel 72 because they are very predictable (both geometry and material) and they have a long history, but their design safety factor is the same as most other cylinders.


I have done the engineering "stamped calculations" on several ASME code cylindrical pressure vessels. The calculations are very specific to account for fabrication deviations, corrosion allowance, etc., but the code allows fabrication based on calculations alone… once the shop and the engineering facility is certified by ASME.
We were only certified to fabricate pressure cylinders (and other pressure vessels) with steel alloys per ASME standards.

 

[Leadking]

it took some searching but my posts from 2005 may help
http://www.scubaboard.com/forums/tanks-valves-bands/121138-300-bar-european-cylinders-vs-dot.html

more info from a 2005 post

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Quote:
Originally Posted by divematt
I've heard they fill them to 4500 in Europe. I can get over 130 cubic feet filling the tanks as I do now but just wondered how foolish or not it was.

Thanks,
Matt

Every tank design by every manufacturer must pass a minimum of 10,000 hydro cycles without fail (6-10 times a minute until fail, new cylinder without corrosion or fatigue issues) as required by DOT
Faber makes no tank greater than 7 inches that can take 4000 psi

U.S.tensile strength (105,000-125,000 psi) is not the same as European tensile strength (135,000-155,000 as required by EN 1964 part 1 and ISO 9809 part 1) and I have test data that shows Faber cylinders delivered to us show a tensile strength of 115,000-123,000 psi (as required by DOT)

 

[Tom Winters]

This thread is too depressing for me, especially during the holiday season. My favorite tank guy stopped working in a dive shop to go back to repairing auto transmissions.
Now I only see "4600" on my LP tanks in my dreams, and that's not that often.

 

[NAM001]

First...To those that are providing real information...Thank you.

Valhalla and Deepstops have somewhat divergent viewpoints.

Deepstops...what is your rationale?

Valhalla...Im going to check the Faber website, but can you give me a source for your info on the 4000 psi x 10,000 cycles?

And again...thank you for the polite answers. Im a new tech diver with all of the toys, and the cards, but I'm really just getting started. I am using this website as an educational tool.

valhalla is correct in his post. these tanks (lp95's) are pumped overseas to 4000 reoutinly. there is a big difference however. this i found out when trying to figure bouyancy on my tanks. the valve overseas is a much heavier valve. 1-2 lbs heavier. so with the lighter valve i would not try to push these to 4000. i have pumped mine to 3200-3400 at times hot and they cool to 3000 or 3200. cave dive areas over fill at request. not oncommon to have double lp 120's filled to 3500 and packing 300+ cubes. my lp95 at 3000 is just shy of 110 cu ft. at 3500 i get just over 125. now keep on mind you should have a real god reason to pump that high. if you are diving with those with 80's on it is a wast of time to pack 110. just fill to spec and you are fine. if this answers the question of tank strengh and duribilty great. i personally wont push an aluminum tank by the way. all steel tanks are not the same. wall thickness varies as does the weight. so the elasticity of the metal will also vary. ie dont store a 2400 # tank with 3500 in it for the winter. vent it sown to a couple humdred.

i hope this answers what you were getting at.

 

[Lone Frogman]

Wow, 3500psi or more in a LP tank. I have never had a 3500psi fill in a HP tank. All the dive shops I use are really good at short fills. Fortunately most the dive shops I use can't read, so most the time I get a LP with about 3000-3150.

 

[tomfcrist]

Wow, 3500psi or more in a LP tank. I have never had a 3500psi fill in a HP tank. All the dive shops I use are really good at short fills. Fortunately most the dive shops I use can't read, so most the time I get a LP with about 3000-3150.

Aint that the truth......The only option where i live is to fill your own tanks if you want a full fill.
..I've noticed alot of shops in Florida that cant read, but in VA there are some hardcore fill stations....If you want an overfill you need to do it yourself around here.

 

[Leadking]

valhalla is correct in his post. these tanks (lp95's) are pumped overseas to 4000 reoutinly. there is a big difference however. this i found out when trying to figure bouyancy on my tanks. the valve overseas is a much heavier valve. 1-2 lbs heavier. so with the lighter valve i would not try to push these to 4000. i have pumped mine to 3200-3400 at times hot and they cool to 3000 or 3200. cave dive areas over fill at request. not oncommon to have double lp 120's filled to 3500 and packing 300+ cubes. my lp95 at 3000 is just shy of 110 cu ft. at 3500 i get just over 125. now keep on mind you should have a real god reason to pump that high. if you are diving with those with 80's on it is a wast of time to pack 110. just fill to spec and you are fine. if this answers the question of tank strengh and duribilty great. i personally wont push an aluminum tank by the way. all steel tanks are not the same. wall thickness varies as does the weight. so the elasticity of the metal will also vary. ie dont store a 2400 # tank with 3500 in it for the winter. vent it sown to a couple humdred.

i hope this answers what you were getting at.

I call B.S.

I will repeat for the umteenth time that the overseas tanks are a different steel alloy than the 3AA cylinders sold in the U.S.

To say the tanks are heavier because of the valve is pure ignorance and lacks any common sense. Yeah the valves are 5 lbs heavier overseas. It's the thicker walls of the cylinder. Stupid advice like this can kill someone!