storing a full tank

[SSIdiver02]

Does anyone know the physics on storing a full tank of air? Would there be any ill effects of storing a full aluminum 80cf tank (3000psi luxfer) for a week? Came up at a advanced dive class i was "speaking" at. A student asked the question, and the instructor said...."uhhhh i dont think so?" I have not studied the effects of having max pressure on a tank for a period of time, but i would like to answer this kids question tomorrow, seeings as he didn't get a real definitive one.

 

[Scubaroo]

http://www.scubaboard.com/showthread.php?s=&threadid=10155

Executive summary: either full or at or below ~100 psi

 

[roakey]

The WKPP will leave full emergency bottles in a cave for months or more.

Roak

 

[dc4bs]

As I understand it, if you use the tank on a regular basis (once a month or so) go ahead and store it full.

If you are going to store it long term (for the winter, etc.) then store it with low pressure in it.

100 PSI should be enough but bleed off the pressure slowly to prevent any condensation from forming. Another one of the reasons for planning to end a dive with 500 PSI or more in the tank as well.

 

[dmdalton]

Store AL tanks full, steel tanks with 100 psi. Storing steel tanks full can accelerate rusting due to increased PP of O2 if there is moisture inside. AL this is not an issue. That being said, if you take care of your steel tanks and don't let water get in them, it is not really an issue.

The military did a test of this accelerated rusting process and found it could be problematic. However, they put a considerable amount of water in them and stored them in a high temperature enviornment. I think it is cited in Bill High's book "Inspecting Cylinders".

Dave D

 

[pescador775]

There are some pretty good answers here but don't address the physics that you requested. Firstly, the relative humidity in a tank increases with air pressure and decreases with temperature rise. Depending on the quality of air, the relative humidity inside a full tank can be close to 100%. When the air is drained some moisture is expelled and moreover, the rel humidity decreases due to the drop in air pressure. Therefore, when a tank is stored full and when low atmospheric temperatures are expected, condensation is possible and, logically, it is a good idea to maintain only a few hundred psi in the tank but not less than 100 psi. Secondly, there is the controversial subject of stress on a tank when stored at high pressure. This seems to affect steel tanks more than aluminum. The steel actually stretches when pressurized and some testers I have talked to report failing more steel tanks which have been stored full. For some reason, aluminum tanks seem to be less affected by storage conditions but are stressed by repeated fast fills. This is a complicated metallugical area and mostly involves older aluminum tanks made before 1989 (SP6498). In summary, as others have said, long term storage should preferably be at low pressure but for normal use max pressure is OK.

 

[SSIdiver02]

Thanks everyone. I told him that it was my undersanding that storing an aluminum 80, at full pressure, for only a week would not be detrimental to the tank. Also, he uses it often.

Thanks again.

 

[The Chairman]

SLC (Sustained Load Cracking) was found in the earlier aluminum tanks. It is a result of high pressures over a long time, causing a phenomenon known as grain creep. While this has NOT been found in the more recent tanks, it is still a possibility with aluminum. It is always recommend reducing AL tanks to 50-100 psi for long term storage... and I would consider 3 months or more as long term. BTW, these cracks can be found BEFORE they become dangerous with a competent Visual Inspection if PSI's protocol has been followed. Shops that rely solely on a Vis+ device will usually miss many of a tank’s flaws that are not part of the threaded area. These are just as dangerous, but are seldom uncovered when a proper inspection is not done.

As for moisture, MOST compressed air has been hyper filtered and this includes moisture. Air from your tank is quite dry and there is little chance for moisture to form on the inside UNLESS you vent all of the pressure and allow ambient air inside.

BTW in case of a fire, partially filled tanks are far more dangerous than filled ones. The change in heat and pressure on a full tank will cause the burst disc to go BEFORE the tank. In a partially full tank, the aluminum has a much greater chance to weaken to a point BELOW that of your burst disc rating, causing a full cylinder explosion. You pay the same for a fill from 50 psi as you do from 1000 psi, so play it safe and vent the tanks if you are going to store them.

 

[The Chairman]

and Mike, I apologise publicly if it seems that I have singled you out. I did not intend that at all. As I told you in a PM, I had just finished deleting a tonne of posts in an unrelated thread due to their crossing the line in harrassment. As an aside, my mom would never use the word "Friggin'".

My biggest grief is in anyone getting "nailed" for asking a question... here they are, exposing their jugular by actually admitting that they don't understand something, and then people start to question their legitimacy. Well, you can be an instructor and STILL not understand all that there is to know about diving.

If there are other areas that bring their legitimacy into question, well fine. But please don't harangue someone for asking a question... or have it even appear that it is because they asked a question. Keep the forum open and the questions coming... be gentle with those that admit to not knowing it all.

As a caveat (and I like caveats), let me point out that readers should take the time to read (and re-read) their TOS. As a board, we do NOT try to validate who is and who is not an instructor. I've got witnesses who can vouch for me... and quite a few of them are even regulators. You need to read many of the posts with a grain of salt... some may need the entire shaker! Be careful who you listen to... it could mean your life, Chester!

Now as a caveat to my caveat (did I tell you that I liked caveats?), I will be the first to admit how MUCH I have learned ON THIS BOARD. I have even espoused erroneous information, only to have my head handed to me on a platter. That can be good, but it can also be destructive. So learn a lot, be gentle with each other and try not to provide any errata!

 

[Uncle Pug]

Pete brings up a good point... don't bash folks for asking the honest question... (and I know most of you would never do that even if it really was a stewpid question.)

However.... I can't stand the trolls and sockpuppets that infest the forums. They set everyone on edge and that can lead to the innocent & ignorant getting their chops busted... sorta our version of collateral damage.

So... use discernment... and be gentle... but be firm... nail the imposters... and set the captives free... hmmm... nope that's another speach... ummmm.... ahhh... nail the imposters.... but inform the honest inquirers with all humbleness.