Filling LP tanks to high pressure

[Superlyte27]

That there + is 10% over the 2400psi or 2640psi. If ya'll are takem to 4000psi stand back some.

Sigh!

---------- Post added January 4th, 2014 at 06:56 PM ----------

So many people "cave fill" LP 2400psi tanks in the 3500-4000 psi range. Due to the extra stress of the extra pressure when frequently used it sounds like most fail hydro on their first or second hydro.

Your post was pretty accurate right up to this point.
I have tanks that are 30+ years old. They've ONLY been filled to 3600+. They pass hydro every time. I've not lost a tank to failed hydro yet. And I've owned over 100 tanks. I currently own more than 20 LP tanks.

---------- Post added January 4th, 2014 at 06:57 PM ----------

If you need 100 cubic foot tanks, then buy them. Don't try to make them out of 72's.

But what if you want a 150cu' tank. Oh wait, just fill an LP104.

 

[Nwcid]

I "fixed" my post above. I though it was one of the things I had read a couple years ago while looking at different tanks. Good info to know.


So I will follow that up with a question then. Does it depend on the tank? I know most talk about the 2400 psi tanks. I have some LP-72's from the '70's that are the 2250 psi tanks. So 5/3 is 3750 psi. What would be reasonable fill in these? I generally fill to 2500 psi as it is a reasonable fill without what I assumed was extra stress. While we are not overhead divers yet we often travel to dive. With more air I could probably get 2 dives on one set.

 

[Superlyte27]

From an ENGINEERING perspective, there is no inherent danger to filling a 2600 psi tank in Hydro to the typical "cave fill" of 3500 psi. The tanks are DOT tested to withstand 5/3 rds of their rating pressure WITHOUT straining the steel.

The "burst disk" in the valve to prevent overfilling is usually rated to either 4000 or 5000 PSI and will prevent a fill which compromises the metal's integrity.

There is no demonstrable reduction in lifespan of 2400/2600 tanks filled routinely to 3500 because the metal is well within its tolerance (usually 4500psi) pressure per square inch. The DOT sets standards to ensure safety by deliberately under-rating 3AA tanks.

Never overfill an aluminum tank... it lacks the stress qualities of steel..... that said... Cave fills under 4000 psi should not, under normal conditions (not over heated, properly cleaned, not banged up, recently hydroed and VIP'd) cause any safety issues.

Agree completely. Add in the fact that there's never been a fatality in cave country due to a cave filled LP tank, and I figure its pretty safe. OMS used to claim their warranty was good for 10,000 fills at 4000psi. Which is what I believe DOT requires. I'm not worried. Fill'er up.

 

[AfterDark]

When things go wrong.



A scuba tank left in a car on a hot day.

 

[Scared Silly]

So I will follow that up with a question then. Does it depend on the tank? I know most talk about the 2400 psi tanks. I have some LP-72's from the '70's that are the 2250 psi tanks. So 5/3 is 3750 psi. What would be reasonable fill in these? I generally fill to 2500 psi as it is a reasonable fill without what I assumed was extra stress.

I would advise against over filling any older LP 72 cylinders. They have different design specs than more modern LP 2400/2640psi cylinders. 2475psi is the working pressure (with the plus rating). As such, when cooled to 72F that is what the pressure should be (i.e. the pressure can be higher if the temperature is higher).

Then I ran across a thread that said LP tanks are made with a different material than HP tanks.

For the OP, what you have found is true. LP cylinders are made from a different material than HP cylinders. The only cylinders that I know are regularly get cave fills are LP cylinders. I am sure there are folks that overfill HP cylinders but I think it is few and far between.

 

[eelnoraa]

When things go wrong.

View attachment 174841

A scuba tank left in a car on a hot day.

In a very hot day, tank pressure goes up by about 10% only. If a tank exploded in a car in a hot day, it is because the tank it self has issue to begin with, maybe someone fake a hydro/viz test result. Not an average cave fill

 

[Luis H]

I would advise against over filling any older LP 72 cylinders. They have different design specs than more modern LP 2400/2640psi cylinders. 2475psi is the working pressure (with the plus rating). As such, when cooled to 72F that is what the pressure should be (i.e. the pressure can be higher if the temperature is higher).



For the OP, what you have found is true. LP cylinders are made from a different material than HP cylinders. The only cylinders that I know are regularly get cave fills are LP cylinders. I am sure there are folks that overfill HP cylinders but I think it is few and far between.

They are both 3AA tanks. That is the exact same spec stamped on both tanks.
There is a lot of miss-information spread about thanks, but this one there is just no room for debate... the spec is stamped right on the tanks. And you can look up CFR49 and get all the details that applies to all 3AA tanks.



---------- Post added January 4th, 2014 at 10:46 PM ----------

From an ENGINEERING perspective, there is no inherent danger to filling a 2600 psi tank in Hydro to the typical "cave fill" of 3500 psi. The tanks are DOT tested to withstand 5/3 rds of their rating pressure WITHOUT straining the steel.

The bold sentence is totally incorrect.

During hydro testing it is very common to have some permanent deformation measured in cc. The pass/ fail criteria is if the permanent deformation exceeds 10% of the maximum deformation. This determines how elastic is the material.

Anyone that knows anything about metallurgy would immediately recognized that the material is taken to the lower end of its elastic limit. Any permanent deformation, by definition means that the metal is starting to deform into its plastic range.

Please do not try to pass yourself as an engineer, if you don't understand basic technical principles.

You should probably look up the definition of strain. That is a basic technical term that you are totally miss using.
Strain is deformation. During any fill there always is elastic strain (which returns when pressure is released). During hydro testing pressure some of the strain is often permanent.

 

[Omisson]

YAWN.

The "bold" sentence - uses common language to explain the underlying principle - but it is perfectly sound... If not technically precise.

Do you disagree that a modern steel LP 2640 tank in current vis and hydro poses any danger with a 3500 psi fill? No. Is there any record with DOT of any such failures meeting those criteria, no. So what exactly is the issue?

 

[BRT]

They are both 3AA tanks. That is the exact same spec stamped on both tanks.
There is a lot of miss-information spread about thanks, but this one there is just no room for debate... the spec is stamped right on the tanks. And you can look up CFR49 and get all the details that applies to all 3AA tanks.





---------- Post added January 4th, 2014 at 10:46 PM ----------



The bold sentence is totally incorrect.

During hydro testing it is very common to have some permanent deformation measured in cc. The pass/ fail criteria is if the permanent deformation exceeds 10% of the maximum deformation. This determines how elastic is the material.

Anyone that knows anything about metallurgy would immediately recognized that the material is taken to the lower end of its elastic limit. Any permanent deformation, by definition means that the metal is starting to deform into its plastic range.

Please do not try to pass yourself as an engineer, if you don't understand basic technical principles.

You should probably look up the definition of strain. That is a basic technical term that you are totally miss using.
Strain is deformation. During any fill there always is elastic strain (which returns when pressure is released). During hydro testing pressure some of the strain is often permanent.

So if the hydro strain is permanent...we gain some tank capacity with each hydro? Some of tanks I've seen should be quite a bit bigger than they started out.

 

[AfterDark]

In a very hot day, tank pressure goes up by about 10% only. If a tank exploded in a car in a hot day, it is because the tank it self has issue to begin with, maybe someone fake a hydro/viz test result. Not an average cave fill

For this thread that would be 10% increase of a cave fill. How hot would it need to be?