Over filling LP tanks, what's the real deal?
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adm3745
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At one of the dive shows last year - I can't recall which one, maybe DEMA - I asked the Faber rep point-blank about overfills, and how if the pressure was ok in Europe, then why not here?
He was polite and candid. His replies:
His last reply came in the form of a question: would you buy a steel tank, one of an identical pair, in great (to the eye) condition, that has it's first hydro coming up in about a year or less, from central Florida? If your answer is "No", as is for many, then what does that tell you about the practice?
All the best, James
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.
His last reply came in the form of a question: would you buy a steel tank, one of an identical pair, in great (to the eye) condition, that has it's first hydro coming up in about a year or less, from central Florida? If your answer is "No", as is for many, then what does that tell you about the practice?
All the best, James
irishsquid
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Scott L
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<scratches head> I'll be danged if I know. I just remember walking away with the impression that high pressure cylinders in Europe had a separate certification from the norm (kind of like our "Exemption" for HP cylinders), that had a finite lifespan or something.
Maybe someone from the EU can clarify this, I know I'd appreciate it.
When looking up cylinders on Faber's website, I came across this page:
Faber Cylinder specs
It's interesting to look through this. When comparing similar cylinder sizes, the DOT 3AA spec cylinder (184 Bar) is always a different weight than the European cylinders of similar size. So that supports the "different construction" assertion.
Also, when you compare the Euro (232 Bar) vs 3AA (184 Bar), the Euro cylinders are quite a bit heavier, presumably because of more steel. That to me sounds like the US LP cylinders are inferior in strength.
As you know, 3AA cylinders - as long as they pass hydro - are good forever, so that'd be lifetime. And Exemption cylinders are good as long as they pass hydro and someone renews the Exemption, so that'd be sorta lifetime, I'd think.
Anyway, I know that cave country has a pretty good track record for not having cylinders blowing up from overfills. But, these overfilled LP cylinders also have a reputation for not passing hydro, and to me that implies damage to the steel.
All the best, James
Scott L
Contributor
halocline
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It is very important to keep in perspective that the live history of tanks in northern Florida is very short.
Even if a tank has been in service for 20 years and it is over-filled lets say 3 times a week… every week of all those 20 years, we are only talking about 3120 cycles. How many cave divers actually have been using the same tanks 3 times a week (every week) for 20 years?
Even if a few of these tanks had 3000 cycles, that is not a high number of cycles on fatigue curves for steel, as long as it is working below the yield strength. There are a few other factors, but I don’t need to get into that.
It would be very interesting to know if there is a higher hydro failure rate even with this very small sample of cylinders. What I mean is that the sample of cylinders that have between 1000 and 3000 cycles is probably very small (I think most cave divers still have to work for a living and of course, there is also the time spent on ScubaBoard
).
The bigger issue with overfills is that (depending on how much of an overfill) it may not have much if any safety factor allowance for possible defects or other loads (like being in a car accident with an overfilled tank in the trunk).
If you overfill near the hydro test, you are basically not allowing any safety factor based on yield (there is still some safety factor based on ruptured, but you can’t count on that).
BTW, a good thing is that there are very few tank defects on steel tanks. In almost 40 years I have only seen two steel tanks with cracks. They both passed hydro, but a VIP caught the crack.
Even if a tank has been in service for 20 years and it is over-filled lets say 3 times a week… every week of all those 20 years, we are only talking about 3120 cycles. How many cave divers actually have been using the same tanks 3 times a week (every week) for 20 years?
Even if a few of these tanks had 3000 cycles, that is not a high number of cycles on fatigue curves for steel, as long as it is working below the yield strength. There are a few other factors, but I don’t need to get into that.
It would be very interesting to know if there is a higher hydro failure rate even with this very small sample of cylinders. What I mean is that the sample of cylinders that have between 1000 and 3000 cycles is probably very small (I think most cave divers still have to work for a living and of course, there is also the time spent on ScubaBoard
).The bigger issue with overfills is that (depending on how much of an overfill) it may not have much if any safety factor allowance for possible defects or other loads (like being in a car accident with an overfilled tank in the trunk).
If you overfill near the hydro test, you are basically not allowing any safety factor based on yield (there is still some safety factor based on ruptured, but you can’t count on that).
BTW, a good thing is that there are very few tank defects on steel tanks. In almost 40 years I have only seen two steel tanks with cracks. They both passed hydro, but a VIP caught the crack.
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.
It is also very easy to instrument a cylinder and test it to confirm the stress calculations.
Hydraulically or pneumatically, a cylinder doesn’t care, it is pressure.
The difference is that cycle testing and sustain load cracking are two totally different phenomenon’s. Fatigue life of aluminum has been studied extensively for the aircraft and aerospace industries. But, (as far as I know) sustained loads for several years at 60% of yield (or 60% of hydro load) is not something that many other aluminum structures have experienced in the past or present.
Some aluminum structures are designed for a maximum loads of 60% of yield, but not a sustained load (steel is not really affected just because a load is sustained for long periods).
I personally have also not seen the 6351 Al alloy used in any of the other industries that I have worked in. Therefore, I don’t believe that we had extensive experience with that alloy, but I don’t know for sure.
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