Scuba Tank Storage

[Phrenzy]

Ok, I have a question. I would like to ask your advice on storing scuba tanks. Ideally, I would like to take my scuba tanks to my LDS and get them filled with air. Then, on a fairly regular basis, I would like to take these same tanks to a local location and slowly release the air contained inside while I’m underwater.

Unfortunately things do not go as planned and I do not get to release the air pressure contained inside in the manner and frequency that I would prefer. Because of this, I have found myself on one occasion storing my tanks fully pressurized for several months. I asked some more experienced divers their opinion on this and I got some mixed recommendations. So in order to increase my confusion and further muddy my understanding of the situation, I am asking the Scubaboard community at large.

I live in the Seattle area without air conditioning. Where I store my tanks, the temperature hovers at around 66 to 70 Fahrenheit. During the summer, it will see 70 to 95 degrees. I have 2 steel high pressure 120 tanks. What would the long term consequences be if I were to store these tanks full of air? Right now I only fill the tanks just before I use them. Would there be any risks if I were to store them full and then fill them up again right after my dive(s). Would anything change if I were to store Nitrox in them?

 

[mike_s]

you'll find some folks that say store them empty. (almost empty.... like 50psi or more in them to keep moisture out).

but I've never had a problem storing the full, even longer periods and hotter weather than you've got.

Some folks also will worry about nitrox in steel tanks stored long term promoting rust a little more. You can always put air in them for storage if this is a concern, but I've never had a problem with it.

The only time I would say that you definately shouldn't store them "full" is if you'e got the older alloy 6351 tanks. And that's only because of substained load cracking that could occur with this alloy.

 

[Teamcasa]

I asked that same question to the factory rep for Catalina tanks. He said that the factory recommends that if you store them long term (several months), keep the pressure below 600psi. When pressed, he also said their tanks can safely hold full pressure for years.

Scuba tanks are incredibly strong. I’m sure that storing them for months on end with full pressure is not an issue, Nitrox or otherwise.

 

[Buoyant1]

Hmmm...let's think...I had mine viz'd in January, had them filled with air, (only because I knew tht the next dives I'd be doing would be in a pool) and didn't use them until probably March or April (and kept them in the basement) No problems...

Last summer I had them filled, (EAN32) probably the last week of June, and picked up some cruddy upper respitory thing at the beginning of July, and didn't use them until (checking log) 8/4/08. I kept them in the garage, and no ill effects.

If you're worried about a "blow out" in the summer then the price of the burst disc or an O-ring (well, and the fill) is what you'd probably have to worry about. This summer I had some O-ring related issues early on, (on extreme HOT days) and it was just a leak. Only cost me the fill! (LDS covered the O-ring)

You should be fine...

 

[Calle Roo]

Being we're on the tank discussion, what about transport and temporary storage?

This is what Ive been doing, but dont want to risk a tank accident/damage.

What should I be doing instead in each of these situations?

**I pick up my tanks, and I try to hold them in the back of the station wagon with some soft weights, but the do occasionally move around.

**When I get home, they sit outside laying down under the awning of the house.

**They then often sit in my car for up to 8 hrs while Im at work.

 

[spectrum]

Since they are steel tanks you can forget about the risk of sustained load cracking and they will store just fine a full pressure.

From a fire standpoint I'm reading that full storage is also recommended since that will promote early release buy the burst disc before the cylinder is compromised and a destructive failure may occur.

The elevated O2 of a recreational mix should not be significant within the intervals of sporadic use and annual VIP events.

Pete

 

[Doc Harry]

The military did some extreme tests with scuba tanks. They put water in pressurized cylinders and put them in wet, hot conditions. From those extreme conditions we can draw some conclusions about how to store scuba cylinders:

1. A clyinder expands everytime it is filled and contracts when emptied. Repeated expansion and contraction fatigues the metal and eventually it will fail. (This is why we do hydro testing every five years, to detect cylinders that no longer tolerate repeated expansion/retraction.) So, it is best to store cylinders nearly empty to reduce metal fatigue.

2. Small amounts of mositure inside of the cylinder cause corrosion. If the tank is stored on its side, the corrosion is spread across the thinner sidewall. If the tank is stored upright, the corrosion is focused on the thicker base. Thus cylinders should be stored upright.

3. Oxidation of the metal cylinder is directly proportional to oxygen content, or the partial pressure of oxygen. Therefore cylinders should be stored almost empty to minimize the partial pressure of oxygen and corrosion.

4. In steel cylinders, oxidation of the steel consumes oxygen in the breathing gas and has been shown to dramatically reduce the oxygen content of the gas in certain situations. Therefore, steel clyinders should be stored nearly empty so that must be filled with fresh gas before use. Alternatively, the gas in a steel cylinder must be tested for oxygen content after long-term storage.

 

[SparticleBrane]

The military did some extreme tests with scuba tanks. They put water in pressurized cylinders and put them in wet, hot conditions. From those extreme conditions we can draw some conclusions about how to store scuba cylinders:

1. A clyinder expands everytime it is filled and contracts when emptied. Repeated expansion and contraction fatigues the metal and eventually it will fail. (This is why we do hydro testing every five years, to detect cylinders that no longer tolerate repeated expansion/retraction.) So, it is best to store cylinders nearly empty to reduce metal fatigue.

2. Small amounts of mositure inside of the cylinder cause corrosion. If the tank is stored on its side, the corrosion is spread across the thinner sidewall. If the tank is stored upright, the corrosion is focused on the thicker base. Thus cylinders should be stored upright.

3. Oxidation of the metal cylinder is directly proportional to oxygen content, or the partial pressure of oxygen. Therefore cylinders should be stored almost empty to minimize the partial pressure of oxygen and corrosion.

4. In steel cylinders, oxidation of the steel consumes oxygen in the breathing gas and has been shown to dramatically reduce the oxygen content of the gas in certain situations. Therefore, steel clyinders should be stored nearly empty so that must be filled with fresh gas before use. Alternatively, the gas in a steel cylinder must be tested for oxygen content after long-term storage.

Find me some references for any/all of these, if you would please. Why? Because honestly, in the long run, they don't matter.

1: Cylinders have a fill cycle life of tens, if not hundreds of thousands of fill cycles -- more than any of us will ever use them for.

2: Cylinders shouldn't have moisture/gunk in them in the first place. So, forget about 'how to store' them (upright vs on their side) and deal with the apparent problem of gunk in the cylinders.

3: ...who cares?

4: To the best of my knowledge, there was one test done for this and it involved a bunch of seawater, so the oxygen in the gas was consumed while producing rust in the cylinders....over a period of months. Also, there was something like a liter of water in the cylinder -- that's a LOT.



So really -- most of what you just said doesn't apply for our purposes here.

 

[Doc Harry]

..in the long run, they don't matter...

I am sorry that these issues don't matter to you. They should.

1. Cylinder can and do fail hydro long before they have had "hundreds of thousands of fill cycles."

2. True, clyinders "shouldn't" have gunk in them. But they do. You don't need a liter of seawater in a cylinder to cause enough corrosion to condemn a clyinder. Even small amounts of seawater can get into a tank from improperly changing your first stage. Leave that sea water sitting in your tank for months and it can cause significant amounts of corrosion, enough to condemn a cylinder. A corrision pit on a sidewall that condemns a cylinder may not condemn the cylinder if it's on the base.

3. Cylinder corrosion can and does cause tanks to explode. People get maimed and killed. Proper storage and inspection programs have reduced the incidence of explosions to a very small number.

4. Steel cylinders corrode quickly. If I pay hundreds of dollars for a steel tank I don't want it condemned or sent off for tumbling after just a few years. Simple steps to ensure proper storage can save you hundreds of dollars in the long run.

5. Personally I am concerned about the amount of oxygen in my tank, especially when I am breathing gas under pressure. Take an air fill, reduce the oxygen content through oxidation over long-term storage, take it depth, and you may have a hypoxic mixture.

 

[Charlie99]

5. Personally I am concerned about the amount of oxygen in my tank, especially when I am breathing gas under pressure. Take an air fill, reduce the oxygen content through oxidation over long-term storage, take it depth, and you may have a hypoxic mixture.

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Please provide a link to your source of info. My back of the envelope calculations say that before you use up enough oxygen to be a problem, you have rusted through the tank wall and exploded the tank.

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Calculations to show why I think the tank will blow up before the FO2 gets too low:

Lets assume HP100 with air. 8 pounds of air, about 1-2/3 pound of O2. If the O2 percentage drops from 21% to hypoxic 16%, then the oxygen consumed in rusting is 5% of 8 pounds, or 0.4 pounds O2 used up.

Rust is mostly Fe2O3. Molecular weight around 160, with Oxygen making up 48 of that. So the total weight of rust is 0.4 * 160 / 48 = 1.34 pounds.

1.3 pounds of rust is an awful lot of rust. The iron content will be almost 1 pound! That's an awful lot of tank material to remove.

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As a side note, the danger point is not at depth. A hypoxic mix is most dangerous on the surface.