Steel tank overfill damage?

[in_cavediver]

Yea, I didn't mean to start the whole cave diver overfill debate all over again. In fact, my original question was aimed toward the opposite end of the spectrum- that of being conservative to maximize the life of the tank. I really just was interested to find out how much the life of the tank could be shortened by an occasional and accidental 40% overfill, and if increased time at overfill would increase the damage. To me, permanent expansion is DAMAGE, since I'm aiming at keeping my tanks going for life.

I think I've gotten my answers, though, so just go on with the discussion.

Here's a bit to keep your mind at ease. I quoted a rumor about a tank going 10,000 hydro cycles prior to failure. Lets assume its really 1/10th of that number or 1000 hydro cycles. Given your tanks hydro every 5 years, that 5000 years of life.

Now, lets also make the assumption a hydro is roughly equivalent to an overfill. (its not, hydro is higher pressure). Each time you have a 'very hot' fill, you lost one hydro cycle. You are still looking at a LONG time for tank life. Other factors will likely kill the tank first.

NOTE - This is all anedocatal based on a rumor of tank life (at least rumor until I find the link I once had to confirm it). Take it for what it is and apply your own logic to this to form your own opinion.

 

[lamont]

Yea, I didn't mean to start the whole cave diver overfill debate all over again. In fact, my original question was aimed toward the opposite end of the spectrum- that of being conservative to maximize the life of the tank. I really just was interested to find out how much the life of the tank could be shortened by an occasional and accidental 40% overfill

They're rated for 10,000 cycles at their hydro pressure before failure and must fail in leak-before-burst mode. Personally, that makes me feel okay about the occasional overfill.

I accidentally overfilled one of my al40 deco bottles to 3800 psi the other day by accidentally looking away from the gauge and talking to someone while I was boosting off a high pressure source (boosters work a little bit faster under those circumstances than when i'm trying to take 500 psi of helium into 1000+ psi double-130s... oops...)

I'm not planning on repeating that mistake, but I'm not running out in fear to get the tank hydro'd either...

YMMV.

 

[lamont]

To me, permanent expansion is DAMAGE, since I'm aiming at keeping my tanks going for life.

I don't think it works that way either. I think most of the cracking comes from the cycling and not the time held at the higher pressure.

 

[duckbill]

Sounds good to me!
Thanks, Indiana cavediver et al.

 

[captain]

Like bending a piece of wire back and forth hardens it until it becomes so hard it breaks the expansion and contraction a tank cycles through each time it is filled or hydroed slightly hardens the metal making it less elastic until at some point it reaches failure. The good thing is that the cycles required to do this is so many that it would take 30 years of filling and emptying a tank every day to reach 10,000 cycles. Filling and emptying one once a week for 60 years is only 3100 cycles. Most tanks will out live us.

 

[in_cavediver]

Not to revive a dead thread, but as I said, I was taking in my set of 104's for hydro that had seen numerous cave fills (>150) in there first 5 years. (thier first fill might have been to 3500 even) Both tanks passed - no problems.

 

[pescador775]

Some opinions regarding overfilling aluminum cylinders seem to be based on the lack of a plus rating for aluminum and also related to the widely reported failures of a few 6351 alloy tanks. The plus rating of steel cylinders is an artifact of the second world war and was never applied to aluminum cylinders, probably because they were uncommon at the time and not a factor in commercial transport of gases vital to the war effort. However, aluminum vessels are quite common nowadays, not only with scuba but as components of the compressors used to fill the scuba tanks. The cycle life of the compressor condensators and filters vary widely with fill pressure. However, operating at the maximum pressure, rarely are less than 4,000 cycles and sometimes much more, up to 80,000. Moreover, I suspect that the fill cycle recommendations include a 100 % safety factor for liability reasons. I have always taken a conservative approach to cylinder fill pressures which stems from the knowledge that steel tanks are subject to rust and because aluminum was very controversial in times past. Nobody knew what would happen if they were overfilled. I've come to the opinion that a 10-20% overfill of an aluminum or steel cylinder is not a big deal provided the cylinder is of modern construction and has been inspected regularly.