Tank Storage Question

[quonniediver]

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?

 

[LUBOLD8431]

Lots of different feelings on this. If its only for a few months, and the cylinder is not subjected to a temperature above 70 deg F, I dont see a problem with keeping it at that pressure...

 

[DA Aquamaster]

Well...it depends. Is it a 3500 psi tank or it a low pressue tank that has been badly overfilled? It it is an HP tank, I would not worry about it.

 

[Genesis]

For a steel, if its at or below working pressure, leave it alone.

If its an AL tank, blow it down to 100-200 psi. Why so low? Because in a fire AL tank alloys will change properties at about 350F, and the result is about 1/8th as strong as the original! The result will be an instant explosion.

If the tank is full, the burst disk will PROBABLY blow first. However, that's bad for AL tanks, as they are succeptable to sustained-load cracking. If the tank is PARTIALLY full, however (e.g. 2000 psi), the disk will NOT blow off in time, and the tank is extremely likely to explode first.

This does not apply to steels.

 

[ShoalDiverSA]

What is the general feeling about accelerated corrosion due to higher oxygen content at higher pressure? Is this effect noticeable or negligible?

Cheers,
Andrew

 

[DA Aquamaster]

Negligible to non existant.

With steel tanks if the air is dry enough to meet an E grade standard there will be no rust as there is not enough moisture to support the oxidation process. With aluminum tanks, the process is self limiting as long as there is no water in the tank. The tank oxidzes until there is an even coating over the surface and then it stops as the coating protects the metal underneath from further oxidation.

 

[ShoalDiverSA]

Negligible to non existant.

With steel tanks if the air is dry enough to meet an E grade standard there will be no rust as there is not enough moisture to support the oxidation process.

Thanks Aquamaster,

I was getting a bit twitchy about storing my steel Fabers in a filled state. It would be very inconvenient to have to fill the cylinders before the first dive of the morning, especially considering that we launch these days at 6am! Unfortunately, I cannot answer on the dryness of the air pumped into my cylinders. I am trying to fill at reputable air stations and dive shops as far as possible. However, the odd fill up in Mozambique probably doesn't help much.

Thanks again for your comments.

Cheers,

Andrew

 

[medical1]

For a steel, if its at or below working pressure, leave it alone.

If its an AL tank, blow it down to 100-200 psi. Why so low? Because in a fire AL tank alloys will change properties at about 350F, and the result is about 1/8th as strong as the original! The result will be an instant explosion.

If the tank is full, the burst disk will PROBABLY blow first. However, that's bad for AL tanks, as they are succeptable to sustained-load cracking. If the tank is PARTIALLY full, however (e.g. 2000 psi), the disk will NOT blow off in time, and the tank is extremely likely to explode first.

This does not apply to steels.

I was getting worried about my Faber 108's in the garage which I won't use till the gulf warms up a little. Too big for the springs . Marked on the tanks 36.7% when I had them filled. I will check them with a O2 analyzer before I dive them again in the warmer weather. If the O2% drops a few points from what I hear - rust may be the cause and it would be wise to crack then and do a VIP otherwise they are pretty safe. Also wise to crack the valve open and sniff the gas as well as feel if the air is dry.

Correct me if I'm wrong - My G/F always does

 

[Genesis]

LOL!

Naw, you should be fine

 

[FredT]

Thanks Aquamaster,
However, the odd fill up in Mozambique probably doesn't help much.

If you question the quality of fills deep in the "back woods" it may be worth while to carry a portable filter tower with you. Use it as an in line filter between the shop air and your tank, along with a hopcalite layer to handle any CO issues. Since you maintain that filter you know the condition of it and what the minimum standards of the air gooing into the tank is. One side benefit is that if you are doing PP nitrox mixing at least the air going into your tanks is properly "polished" to avoid unnecessary O2 cleaning rituals.

FT