Air time in Tanks

[CMOST]

What is the time span that air can stay in a tank and still be good? I'm sure there are a lot of variables so I'll set some. A semi new tank, certified by LDS, filled with air to 3000 psi. Does it go "stale"? Can old air harm you? What constitutes old air?

When you store a tank for any length of time how much pressure should you keep the tank at? And why?

 

[adurso]

Air can stay in the tank forever, absent any contamination or rust. Some folks store the tank topped off, others just a couple of hundred pounds. The tank should be stored at a positive pressure to keep moisture and other bad things out.

My practice is to store all tanks topped off so that they may be grabbed on the way out the door to the dive site.

 

[ronbeau]

What is the time span that air can stay in a tank and still be good? I'm sure there are a lot of variables so I'll set some. A semi new tank, certified by LDS, filled with air to 3000 psi. Does it go "stale"? Can old air harm you? What constitutes old air?

When you store a tank for any length of time how much pressure should you keep the tank at? And why?

I agree with the other poster.

The fire departments are now saying that they prefer to have the cylinders totally full so that an increase in heat will cause the burst disk to blow before the heat degrades the material the cylinder is made from so that it fails before the burst disk does.

I am making no judgment on the validity of the fire department position.

As the other poster pointed out keeping them full has the added advantage that they are ready to go whether the dive shop is open or not.

 

[DA Aquamaster]

For steel tanks I agree with storing the tank full as it ensures the tank contents will reach a high enough temperature and pressure to blow the burst disc prior to the tank failing (exploding). Steel tanks are not prone to fatigue or sustained load cracking so there is no real downside to storing them full.

For aluminum tanks it depends on the alloy. For 6351-T6 alloy tanks made by anyone other than Catalina prior to about 1988-89, storing them full is probably bad idea. For newer 67061-T6 alloy tanks, the jusry is probbaly still out in terms of long term effects but they are not believed to be prone to SLC issues. Still, I'd probably store them with around 100-200 psi in the off season.

 

[CMOST]

Aquamaster,

What do you mean by SLC issues. Not sure what that stands for.

 

[adurso]

Sustained Load Cracking

 

[Teamcasa]

Sustained Load Cracking

 

[Land Locked]

What is the time span that air can stay in a tank and still be good? I'm sure there are a lot of variables so I'll set some. A semi new tank, certified by LDS, filled with air to 3000 psi. Does it go "stale"? Can old air harm you? What constitutes old air?

When you store a tank for any length of time how much pressure should you keep the tank at? And why?

If it is my tank, no issue. They don't go stale!

If it is a tank purchased used, current vis or not, valve is pulled and vis'd again.

I've kept my tanks as low as 100psi in non-season, check with your local lds on their requirements.

 

[robint]

hmmm......... I was always told never to store tanks for a long period of time (more than a few months) completely full. It is better to store at 300-500psi. Also, at the LDS where I used to work, tanks left sitting around for over a year were drained and refilled - air did smell stale and customers did complain if renting them. My husband is still an instructor at the dive shop and pulled out some tanks recently to show it to the students - they all agreed that the air in those tanks had a smell. This was true mainly for the 63s which weren't rented much so sat around in closet. The 80s at the LDS are all used regularly so never had a complaint about stale air in them.

 

[mike_s]

Aquamaster,

What do you mean by SLC issues. Not sure what that stands for.

CMOST, Sustained Load Cracking, as the others pointed out is an issue with older Aluminum alloy tanks made by Luxfer before mid 1988. They were made out of an alloy called 6351-T6 that had issues with "Sustained Load Cracking" when left under pressure. Bascially the tank could/would develop small cracks in the alloy. As we all know that under pressure, small cracks can become big cracks, and so on....

because of issues with these tanks, many dive shops don't want to service/fill them anymore.


for more info on it, see Luxfer: Sustained-load Cracking FAQ

Luxfer: Technical Bulletins: October 22, 2007