Filling LP tanks to high pressure

[Luis H]

Thank you. This is exactly the information that I was looking for, but I do have a few follow up questions. Suppose a tank has been overfilled to the point of plastic expansion enough to produce a volume of, say, 50cc more than it's baseline capacity. The tank then goes in for hydro which adds, say, another 5cc (but still passes) and it's service is continued.

As far as I can tell, there is no baseline established for a particular cylinder's volume, and the hydrotest will measure only the initial, loaded, and rebound volumes of a particular tank. In theory, an overfilled LP tank could see any number of plastic expansions before it's deformation is measured, and even then it's an isolated measurement. In this case, the tank that left the hydro facility with 55cc of expansion from it's born date might return next round with 100cc of expansion, and nobody would know it.

Obviously very few keep records like you have, but I am curious...how much expansion is too much when looking at the life of a cylinder? It would seem to me that, for a cylinder operating continuously at close to or above yield strength, a simple hydrostatic test may be limited in it's usefulness. Is this a valid point to consider?

I am sorry I didn't make it clear that the elastic expansion is the portion of the expansion that returns back to zero, it is purely elastic (the approximately 55 cc is elastic).

The magnitude of the plastic expansion has to stay less than or equal to 10% of the elastic to pass hydro. The hydro is only checking if the material is still ductile (elastic).

To answer your question is yes there would be a very small increase in volume every time the cylinder is pressurized into it yield stress, but look at the relative numbers.
We are talking about a 5 cc increase (of max permanent expansion, or it fails) for a cylinder that has a total volume of 11,960 cc. That is a permanent volume increase of 0.04%. You will never notice.

The hydro test does not care about any permanent expansion from the previous hydro test. All it is measuring is if the material at this instant is still elastic enough or has it become somewhat brittle.

The issue with filling to 4000 psi or higher (into the lower end of the yield stress) is that you are work hardening the material.

It is similar to bending a metal coat hanger. Sure you can bend it a number of times and it can still be used to temporarily hold your car muffler, but if you bend it once too many it fractures because it has become very brittle in the spot that it has been work hardened (by bending).

The coat hanger example is obviously a lot more extreme since the level of deformation, but anyone that has done it knows that it doesn't take that many cycles.

A scuba cylinder that is taken to the lower level of the yield stress will be able to tolerate a lot more cycles, but enough of those cycles and it will shorten its life. The amount that it will shorten its life is probably not an issue if it is cough during a regular hydro test. The material will show as being more brittle.

Most of my steel 72 are 40 years old (some are older) and I expect that they will last me for a very long time.


I hope that answers your question.

Wow, I am a slow typist and it takes me a long time to write all this stuff.

 

[dumpsterDiver]

I thought if the tank is retaining a certain level of elasticity, then it will pass hydro. If it is "all stetched out" then it will be harder and less able to contract after the hydro and will therefore fail..So the test is measuring the remaining elastic properties rather than how much it has expanded during its prior service life. I too would like to hear the real answer..

Oh it looks like he did answer it while I was reading and writing..

 

[lv2dive]

I don't know a lot of people who would encourage others to fill to 4k on regular basis or who have done that for "years and years." That's a bit much IMO... Now, a solid 3600 cool, yeah.

Anyway it is all about personal comfort, and no one should fill their tanks or encourage anyone else to fill their tanks, to a pressure higher than that at which they are comfortable. On the other hand, I don't need to see an engineering thesis either (bc I'm on around cave fills on a regular basis.)

What's going to be the difference between a tank exploding at 2600 compared to 3500? When a tank goes she is gonna make on heck of a mess no matter what the pressure is. As I stated in my first post, I was questioning the material the LP tanks were made out of, not talking about a catastrophic failure or some freak accident that would cause any tank to blow. As I said before, people in the "cave" country encouraged me to over fill the tank 4000, which I said was not my intention but 3400-3500 is good enough for me. They claim that they have dove the tanks for years and years and have never suffered any problem what so ever. My curiosity was if there is a material difference in the manufacturing of the tank that causes it to be rated to low pressure. I was also told that LP tanks exist because some dive shops or compressors cannot pump past 2500.

 

[pocky21]

Crystal clear. Thus, just like bending a coat hanger, it gets more difficult to "bend" at the stress point as the process of work hardening continues, up to the point of failure. (I think, anyway...brain is a little fried at the moment)

Would it be prudent, then, to consider advancing a hydrotest schedule for tanks that are overfilled regularly as a risk mitigation tool? Or do you feel that the current 5 year schedule is sufficient?

 

[Luis H]

Crystal clear. Thus, just like bending a coat hanger, it gets more difficult to "bend" at the stress point as the process of work hardening continues, up to the point of failure. (I think, anyway...brain is a little fried at the moment)

Would it be prudent, then, to consider advancing a hydrotest schedule for tanks that are overfilled regularly as a risk mitigation tool? Or do you feel that the current 5 year schedule is sufficient?

It would not hurt to do hydros more frequent, but mostly on cylinder that have seen a lot of cycles, or that the previous hydro had a relatively high permanent expansion, or that the elastic expansion was close to the REE number.

I would definitely keep track of the hydro numbers, but I realize that most people don't know how to interpret them. It would be good to know how it did the last time.


Very important could be to do more frequent visual inspections. Moisture can cause corrosion at an accelerated rate at higher pressures.

Also if you live in cold climates do not store a cylinder with high pressure air in the cold. It can condense even with the so cold dry air from a compressor. The higher the pressure the more likely for condensation. I store all my full cylinders in my basement, not in the garage.

Before filling any steel cylinder do what is called a hammer test. You lightly hit the side of the mostly empty cylinder with a hammer, a wrench, or any flat metal. It should sound clear as a bell. If it doesn't sound like a bell, you have rust or moisture inside. This should always be done before filling any steel tank (according to CGA codes).

 

[lv2dive]

...
Before filling any steel cylinder do what is called a hammer test. You lightly hit the side of the mostly empty cylinder with a hammer, a wrench, or any flat metal. It should sound clear as a bell. If it doesn't sound like a bell, you have rust or moisture inside. This should always be done before filling any steel tank (according to CGA codes).

You are saying hitting the side of a mostly empty cylinder sounds clear as a bell...

So how would that work on a set of LP tanks that an intro cave diver brought in for fills after they only got one dive in, before they left last weekend? What it is supposed to sound like if it has 2400k PSI?

Ok never mind that - ANY cave diver should never be bringing in "mostly empty" tanks so some other description that does not involve hitting a mostly empty cylinder may be better.

And I guess I am either extremely unobservant or I need to get busy looking up those CGA codes and sending them to my friends who are filling tanks because I don't want them to get in trouble and I have never noticed this hammer test before.

---------- Post added January 15th, 2014 at 12:13 AM ----------

Except that, most of us have seen a lot coat hangers bend and bend and finally break... and can, in fact, make it happen at will without much effort. The same can not be said for steel tanks.

I could go look up several of LP104's born on dates but I am pretty sure I just sold some from the 80's that had a recent hydro. Now - in all fairness - they were not getting that constant "bendy" action of which we have been speaking, which is why I sold them in the first place. But I believe that they have seen several of cycles where they were.

Crystal clear. Thus, just like bending a coat hanger, it gets more difficult to "bend" at the stress point as the process of work hardening continues, up to the point of failure. (I think, anyway...brain is a little fried at the moment)

Would it be prudent, then, to consider advancing a hydrotest schedule for tanks that are overfilled regularly as a risk mitigation tool? Or do you feel that the current 5 year schedule is sufficient?

 

[Luis H]

Most dive shops are not even aware of the hammer test and there no one enforcing it. It is actually just a code.

It actually works even with a full tank, but I would not do it intentionally on an over filled tank. It is just a very mild hit and I am sure you must have heard how your tanks sound when they get hit.

I just went to my shop and tried it on several full and empty tanks (singles and doubles). It works on all of them, but the bands on double tanks does change the tone some.

Note: this is not an early warning. A visual is by far the better test. This will give you a good indication if rust has been forming since your last visual.




You can bend a metal coat hanger very easily and break it. How far you bend it will determine how many cycle it takes until it fractures. If you want to break it quickly, you fully bend it as tight as possible. If you just bend it a little it takes many more cycles.

If you read my post, you will see that I mentioned that the amount of flexing (and corresponding plastic deformation) happening with an overfill is very small. Just don't expect to get 40 or 50 years out of a cylinder that has been filled close to (or to) the test pressure. I would be guessing if tried to estimate how many actual cycle can it can go through before it fails a hydro.


By staying at about 66% of the yield stress you are almost guarantied to never to exceed the fatigue limit (for a pressure cylinder application). The 2/3 of yield (or 66% or 2640psi/4000psi) is a common structural safety factor applied to pressure vessels, etc.

If you are doing 3500psi/ 4000psi you are doing 87.5% of yield. If life is perfect, that should be OK for a while. One key factor is to try to make sure that life continues to be perfect.

 

[captain]

I own a set of the built specifically for the Navy aluminum 90 cu/ft cylinders. What is nice about them is all the original hydro data, elastic expansion, permanent expansion, cylinder volume, test pressure and REE are plainly stamped on them.

 

[victorzamora]

That's fantastic! I mean, I don't want that crap stamped on my tanks.....but I'd love to get that information. It's awesome to see the plastic and elastic expansion. It'd be great to be able to note it, file it, and track it. Can you imagine if they stampd that on every tank every time? There would be LP72s with size 0.1 font just to get the whole stamp to fit

 

[Luis H]

I own a set of the built specifically for the Navy aluminum 90 cu/ft cylinders. What is nice about them is all the original hydro data, elastic expansion, permanent expansion, cylinder volume, test pressure and REE are plainly stamped on them.



View attachment 175587

I would like to see that much information from the manufacturer, but we will never see that.

With that size lettering it does take up a lot of room. I have seen old tanks that start to run out of room for hydro dates.



BTW Captain,
The 704 cu in (0.407 cu ft) works out to 83 cu ft of air at 3000 psi.

I was going to kid that the Navy got short-changed, but that is actually what the mil-specs calls for ( Mil-C-24316A). The spec calls out for an internal volume of 700 +/- cubic inches.

It is interesting that they called it a 90 cu ft when in reality the volume could only range from 79 to 86 cu ft (and still meet the spec).

If anyone is interested, the Mil specs for this cylinder and the steel 72 are available from VDH. Obviously the specs have been long time ago approved for public release.

My steel 72 range in volume from 70.2 to 72.2 cu ft at the rated pressure (2475 psi).

The more surprising was that my wife's HP 80 from PST are actually 85 cu ft at 3442 psi.