Wrecking a tank by storing it too long with too much pressure?

[Osiris]

I recently came into possession of a rather old aluminum 80 which I took to get hydro'd today. It was manufactured in 1979, and last hydro'd in 1984. I don't have high hopes that it'll pass, although it looks to be in pretty good shape. When I got the tank I was told that it had been stored with air in it, and I casually mentioned that as I handed it off at the dive shop. I have no idea what pressure it was stored at (when I got it, it was about 25-50psi as the previous owner had used up the air in a swimming pool this summer), but the individual who was helping me at the dive shop remarked that if it had been stored at or close to 3000psi, the tank could be damaged to the point of being unserviceable from holding that pressure for so many years.

Now I'm no expert, but from what I understand what wears out a tank is primarily cycling it, filling it and draining it. And of course getting it hydro'd. I would think that storing it full would not significantly reduce it's lifespan, even if left full for 26 years.

Can anyone enlighten me on this? Can you damage a tank by leaving it at or near max rated pressure for extended periods of time, or was what I was told at the dive shop a bit off the mark?

 

[miketsp]

I recently came into possession of a rather old aluminum 80 which I took to get hydro'd today. It was manufactured in 1979, and last hydro'd in 1984. I don't have high hopes that it'll pass, although it looks to be in pretty good shape. When I got the tank I was told that it had been stored with air in it, and I casually mentioned that as I handed it off at the dive shop. I have no idea what pressure it was stored at (when I got it, it was about 25-50psi as the previous owner had used up the air in a swimming pool this summer), but the individual who was helping me at the dive shop remarked that if it had been stored at or close to 3000psi, the tank could be damaged to the point of being unserviceable from holding that pressure for so many years.

Now I'm no expert, but from what I understand what wears out a tank is primarily cycling it, filling it and draining it. And of course getting it hydro'd. I would think that storing it full would not significantly reduce it's lifespan, even if left full for 26 years.

Can anyone enlighten me on this? Can you damage a tank by leaving it at or near max rated pressure for extended periods of time, or was what I was told at the dive shop a bit off the mark?

By the logic, as you say, cycles would be much worse than a constant strain if the material is within its elastic limit.
Your dive shop individual is probably thinking of the old Luxfer 6351 alloy that suffered from sustained load cracking:
Luxfer: Sustained-load Cracking FAQ

I don't remember reading anything about the newer alloys suffering from this effect.

The reason you don't normally store a tank at full pressure long-term is purely a safety issue. It isn't going through regular VIPs or hydrostatic tests. It may be in contact with something corrosive, internally or externally. It may have had a bad fill before being stored - Murphy is always out to catch the unwary.
Why do you want so much energy stored in your house if you don't intend to use it soon?

 

[Osiris]

Thanks for the link, very informative.

The tank was made by US divers, and based on it's 1979 manufacture date I'm guessing it's 6351 aluminum. Looks like I'll have to have it eddie current tested.

 

[Colliam7]

I recently came into possession of a rather old aluminum 80 which I took to get hydro'd today. ... I was told that it had been stored with air in it, and I casually mentioned that as I handed it off at the dive shop. ... the individual who was helping me at the dive shop remarked that if it had been stored at or close to 3000psi, the tank could be damaged to the point of being unserviceable from holding that pressure for so many years. Now I'm no expert, but from what I understand what wears out a tank is primarily cycling it, filling it and draining it.

Yes. The cycling would be more likely to contribute to failure than long term storage at a constant pressure.

By the logic, as you say, cycles would be much worse than a constant strain if the material is within its elastic limit. Your dive shop individual is probably thinking of the old Luxfer 6351 alloy that suffered from sustained load cracking.

I agree, that is quite possibly what the shop person was thinking of.

 

[DennisW]

If it passes hydro, the tester will also visually inspect the tank and will perform an eddy current test to look for cracks in the threads. Well, if they know what they are doing, they will. The only DOT requirement for visually inspecting tanks are the 5 year inspection at the time of hydro. The annual requirement is an industry (non-written) standard.

 

[Scared Silly]

The above post regarding storage at 3000psi and SLC are correct.

Because the cylinder is made from 6351 before spending money on a hydro ask your local shops if they will fill it. Because of the risk of SLC many shops will no longer fill these cylinders. As such, you could spend $40+ on getting a hydro and still not be able to use it.

Also, I see that you are in Canada. It is only the US DOT that requires the eddy current test at the time of hydro. I have no idea the TC standards are at this point in time for 6351 cylinders. As such, it may be the dive shop that will perform the eddy current test to satisfy the Canadian industry standards. If TC is following DOT lead and requiring the eddy current test at hydro make sure it comes back with "VE" stamped on the cylinder next to hydro date. Many hydro shops are doing the eddy current test but not stamping "VE" as such the cylinder is not properly requalified.

Without the eddy current test being done as part of the hydro as far as DOT is concerned the cylinder would be not be properly requalified as could not be legally filled in the USA by a dive shop.

 

[Nemrod]

Most dive shops will no longer fill those 6351 alloy tanks, especially in Florida, eddy current or not, they just say NO!

I would not waste my time on it, aluminum 80s brand new are under 200 dollars.

Aluminum tanks should be stored long term at 500 psi. Steel tanks it does not matter within a few (human) lifetimes.

N

 

[rstofer]

But it is worth noting that Luxfer rates their current Al 80 tanks at 100,000 cycles. At 2 dives per day, every day of the year, that's about 137 years!

The thing about 6351 is that not only is it not worth the aggravation, it is probably better to just drill a hole or two in the tank, destroy the threads (a rat-tail file comes to mind) and scrap the thing.

Richard

 

[mksmith713]

I've never heard of a tank going bad due to storage under pressure.
Logic tells me that the valve (burst disk) would fail before the tank would.

 

[Scared Silly]

I've never heard of a tank going bad due to storage under pressure.
Logic tells me that the valve (burst disk) would fail before the tank would.

Read this article, it might help:

http://www.hawaii.edu/ehso/diving/Cracking and Ruptures of SCBA and SCUBA Aluminum Cylinders.pdf

Then do search ...