Offseason storage -- did I screw up?

[scottm]

This afternoon I got around to admitting the local dive season is over for a few months. I went ahead and drained most of our tanks, and remembering how much longer it took last year then I expected, was a little more aggressive. I drained them a little faster and let them run longer... I took an SPG down this evening and realized I'd gone too far. All the tanks are 3442 "HP"s. My set of 120s is down to about 200 psi, which is just a tad lower then I planned to store them.

Our two single 100s though are nearly empty; the needle does not move. There is still pressure (purged regs to remove first stage), but very very little. All the tanks are due for a viz this spring anyways (before their first use in all liklihood).

Am I risking moisture in the tanks storing them this low? I have access to a transfill whip (though not a lot of gas to transfill from ) and could bump the singles up a little bit and even all 4 tanks out. If so, have I already potentially let moisture in that would necessitate a viz+cleaning now?

 

[Bubbletrubble]

What's the harm in leaving the tanks full for a few months? (I'm an ignorant West Coast diver who can dive year-round.)

There's still some pressurized air inside the tanks, the valves are closed, and the tank neck o-ring is still holding. How can any moisture get inside (that isn't already there in the first place)?

 

[drdaddy]

You do not need to drain your tanks for storage. For the short time they will be stored, your air will not go stale and there will be virtually no dangerous of over-expansion or explosion with your steel HP 3442. Get a "tank to tank equalizer" Tank to Tank Equalizer with Pressure Gauge , fill up one of your tanks and then put @500psi in each of your tanks to prevent moisture. Get all of your tanks checked/vis'd before you dive next year. ... and don't do this again.

 

[scottm]

What's the harm in leaving the tanks full for a few months? (I'm an ignorant West Coast diver who can dive year-round.)

There's still some pressurized air inside the tanks, the valves are closed, and the tank neck o-ring is still holding. How can any moisture get inside (that isn't already there in the first place)?

Thanks for the reply! My understanding is full would have been no problem, but midway between is a potential fire/explosion hazard. They're too empty for the burst disk to go before the tank walls give out in a fire. Empty I think is generally ok, I just don't know if I let them get _too_ empty as there is very little pressure in the 100s...

 

[JBRES1]

As noted before, find a buddy with a fill whip and put a little pressure into the 100's.
Even 100 psi is enough pressure to keep the cylinders safe.

If you can't get a fill whip, take the cylinders into your shop and have the VIP done and ask for a short storage fill.

You could also have the cylinders o2 cleaned at this time so they are ready for next season.

Jim Breslin

 

[Scared Silly]

If you open the valves and hear hissing there is positive pressure. Relax and enjoy the winter.

Otherwise, just take them to the shop, tell them you just drained then a bit more than you wanted and would they put a 100 psi in them. Then relax and enjoy the winter.

 

[McKay]

"They're too empty for the burst disk to go before the tank walls give out in a fire."

This is not correct

 

[BlueWaterDiving]

PSI says that there should be 10 pounds or full for a fire. Just saying.

 

[Spimon]

They're too empty for the burst disk to go before the tank walls give out in a fire.

The burst disks in the valve are made of copper and will soften and fail at a relatively low temperature (compared to the cylinder itself), regardless of internal pressure. There is no way the cylinder wall can fail due to internal pressure before the burst disk fails.

but midway between is a potential fire/explosion hazard.

You have (almost*) nothing to worry about :cool2:....except the fire that's engulfing your house...

Even if there were no burst disks, scuba tanks (and generally all non-destructive pressure vessels) are designed according to leak before burst (LBB) failure theory.

This basically means that there's no way a tank can explode as a result of internal pressure. They are designed in such a way that under extreme pressure a small leak in the material will develop and slowly propagate and thus reduce pressure before fast brittle failure ('exploding') occurs. Without going into detail, this is achieved by carefully selecting the tank's material and wall thickness.

If you're interested, try googling 'Leak before burst failure theory' or consult any Strength of Engineering Materials text book.

*Tanks have properly exploded before but in all of the cases I've studied this has been due to exceptional circumstances, such as something changing the material properties, poor manufacturing or sustained load cracking (as in pre-1991 aluminum cylinders). This post was to shed some light on the myth that pressure vessels in a fire will generally explode.

 

[captain]

If a closed valve can keep 3500 psi in the tank it can keep atmospheric pressure out of an empty tank. That said there is no reason not to just keep them full.