Tank Storage Question

[Mitten Diver]

I had also read some where on this board that it would be okay to store my steel tanks in my unheated garage here in Michigan. I have two HP 100 and one HP 80 two of the tanks are completely full about 2350psi and I burned about a 1000psi on the turkey dive at the end of my season at least for this year. Can these tanks be stored strapped to the wall so they don't tip in my garage? I will be taking these tanks for hydro and visuals some time in the next month or so.

Mitten Diver

 

[Genesis]

Sure.

 

[IndigoBlue]

I probably won't be diving anymore until the water temps warm up in the spring. I have a steel tank that is filled to 3400 PSI. Should I vent it down to 500PSI or so? Or doesn't it matter what the pressure is for a long length of time?

I do not fill my tanks until I am ready to go diving.

I normally keep about 250 psi in them between dives.

Metals are elastic. When you fill tanks, they stretch. When you keep them filled, you keep them stretched.

When a tank gets hydrostatically tested every 5 years, it gets stretched and the ability of the metal to contract to its original size is tested. If the metal cannot return to within a stated tolerance, the tank fails hydro and must be condemned from further scuba use.

The scuba literature recommends that tanks that are being stored be bled until they have about 150 to 200 psi in them.

OK, all the above statements are facts. Now for some conjecture.

I cannot believe that a tank that is stored full, to its working pressure, will successfully pass hydrostatic testing as often as a tank that has been stored almost empty.

But I see that some of you here believe that it will, or that it does not make any difference. I find that hard to believe. But I do not know the answer, because I have not experimented with full and empty stored tanks over a 15 to 20 year period and then compared them.

My tanks are just going through their first rounds of hydrostatic testing, and that is quite young, as tanks go.

 

[Genesis]

With Steels it probably does not matter. Steel welding and storage tanks are routinely kept full (indeed, they're full to some extent EXCEPT when being refilled!) and the O2 one currently in my garage (which has to go back for swap this week as its empty) has a first hydro date in the early 1960s.

I strongly suspect that my steel tanks will outlast ME.

 

[medical1]

I strongly suspect that my steel tanks will outlast ME.

That may be why I have 8 steel tanks and only 2 aluminum ones. Plus the steel give me better bouyancy.

 

[Boogie711]

IB - a Faber is designed to last for 10,000 hydro CYCLES.

You could fill a steel Faber to hydro test pressure every single day, and conceivably, it will last 27 years.

Even if you only hydro every 2 years instead of 5, and it only lasts 1,000 cycles before it fails, that steel tank is still going to last. You could be using the same tank that Jesus breathed out of.

Why go through the hassle???

 

[IndigoBlue]

IB - a Faber is designed to last for 10,000 hydro CYCLES.

You could fill a steel Faber to hydro test pressure every single day, and conceivably, it will last 27 years.

Even if you only hydro every 2 years instead of 5, and it only lasts 1,000 cycles before it fails, that steel tank is still going to last... .

Why go through the hassle???

Thanks for the info on Faber tanks, Boogie. I have one Faber 72, which I bought for my wife. She loves it because of its beautiful white enamel paint job. The rest of my steel tanks are PSTs which are metallic rather than painted.

So with steel tanks, I suppose that fire danger is the only reason then, to store them with 150 to 200 psi in them? Yes, the burst disc would blow if there were a fire, but the tank would still be hissing air while the firemen were putting out the blaze. That would also make nitrox a greater fire hazard than air alone, as well, and more necessariy to store bled, I would think?

We watched the fire dept put out a real blaze in the building across the street from us over the holidays. Someone had loaded up a gas fireplace with wooden logs for a nicer fire, and the chimney system was not meant to cope with the excess heat, so the whole building caught fire. It is the closest I have ever been to a real fire. Fires happen.

 

[Big-t-2538]

Metals are elastic. When you fill tanks, they stretch. When you keep them filled, you keep them stretched.

When a tank gets hydrostatically tested every 5 years, it gets stretched and the ability of the metal to contract to its original size is tested. If the metal cannot return to within a stated tolerance, the tank fails hydro and must be condemned from further scuba use.

OK, all the above statements are facts. Now for some conjecture.

I cannot believe that a tank that is stored full, to its working pressure, will successfully pass hydrostatic testing as often as a tank that has been stored almost empty.

O.K...I'm pretty sure Fred T knows way more about this than I do...hopefully he'll chime in with a response...

from my limited experience in engineering, when you stretch a metal, so long as you don't go beyond the upper limit of the modulus of elasticity, you are fine. There is also something called a lifecycle calculation which takes this modulus into account...but it does not take into account how much time something stays stretched...it counts the cycles. Thus to me, you should be able to leave a tank filled and it (in theory) should not affect the lifespan.

Now the annealing issue with Aluminum tanks (i.e.being in a fire) is one to think about...again...FredT probably knows more than I ever will...but in general, I beleive you shold leave them either fully filled or almost empty so that either the burst disk goes, or the tank never reaches it's Ultimate Tensile Strength and has the opportunity to burst....in the event of a fire have the tank hydro'd

 

[CBulla]

....in the event of a fire have the tank hydro'd

I can just picture the look on the persons face at the LDS when you tell them that you would like to have your tank hyrdro'd because it was in a fire... something like saying "I'd like 10,000 marbles please!"

 

[Big-t-2538]

I can just picture the look on the persons face at the LDS when you tell them that you would like to have your tank hyrdro'd because it was in a fire... something like saying "I'd like 10,000 marbles please!"

Better yet...just take it to the hydro station itself and ask if you can watch them blow up your tank....if nothing else that should be really cool.