Steel tank overfill damage?

[duckbill]

My goal is to keep my steel tanks in service as long as possible. I expect them to outlast me.

I used to pick my tanks up and find that they were being filled about 10% PAST the rated 10% overfill (i.e. 20% overfilled) until I asked the LDS to please only fill my tanks to the marked pressure. When I would receive the tanks over filled like that, I would bleed the air down with the idea that the less time at that higher pressure would cause less long-term damage. However, I don't think that 20% would normally exceed the pressure necessary to cause permanent expansion. The fact is, I don't know where that limit is, or whether duration at a pressure above or below that limit would increase the amount of damage done or not.

I have some 1980 psi (1800 + 10%) 50s which I just recently picked up having been pumped to 2500 psi- nearly a 40% overfill, and nearly the pressure which could burst the disc. Obviously, the person forgot to change the pressure setting after having filled my 72s.

I realize that 40% overfill is not close to hydrotest pressure on these tanks, but I'm curious what it takes to exceed the elastic limits. I bled them down to 2000 psi because they will be stored for several days before being used again, but I don't know if that really helped to prevent the possibility of further damage or not (assuming the elastic limits were exceeded). Is time a factor?

Any thoughts on this?

 

[paulwlee]

A huge proportion of cave divers in Florida regularly fill 2640psi steel tanks (LP80's, LP104's, etc.) to well over 3000psi, sometimes even up to 4000psi without any apparent ill effect.
This probably depends on the make of the tanks, and overfilling aluminum tanks is considered dangerous, but I wouldn't really sweat a 10% overfill.
Now that 40% overfill MIGHT be something to worry about, but I have no idea about those tanks.

 

[AXL72]

yeah...the tanks are probably tested to 125% of the 3000 psig

yielding would probably start to occur at about 125% to 130%...roughly....but maybe more since the grade of steel or aluminum is probably greater than the minimuim specified.

However, it is not a good idea to continue to pressure up to 120% to 125% or greater. Fatigue damage, I suspect


unless the test pressure is 150% of the rated pressure.

 

[Luis H]

Most tanks are tested at 5/3 (167 %) of the working pressure. Some (mostly new HP tanks) are stamped with a test pressure of 150% of the working pressure.

The hydro testing is conducted to test for initial yielding (or permanent deformation). Permanent deformation is was they record and measured against the amount of elastic deformation.

In other words, if it is in good condition (the metal has not been exposed to fire, etc.), the tank would only see elastic expansion when exposed to pressures below the test pressure.

A cylinder with a working pressure 2250 +10% is intended to be filled to 2475 psi at 70F. That same cylinder can have a higher pressure at a higher temperature (up to 130F, according to CFR & CGA codes). If you do the math you will see that due to temperature that can can be filled to another 11% higher to account for the air temperature.

In any case it is not a good idea to overfill because you are cutting into safety margins, but (again if the cylinder is in good condition) the metal is still working in the elastic range.

And, time is irrelevant.

 

[DA Aquamaster]

The hydro test pressure for an 1800 psi service pressure steel tank is 3000 psi and the burst disc is supposed to burst between 2700 and 3000 psi - when it is new. In practice they tend to fatigue with use and will burst below the minus 10% of test pressure to test pressure range.

In theory then, a steel tank cannot receive a fill that will exceed the elastic limit and for a steel tank fatigue or sustained load cracking is not an issue as is the case with aluminum tanks.

Practically, stressing the burst disc with a large overfill can lead to it failung during a dive with rapid gas loss resulting. What often happens is that divers who do 20% overfills will replace the burst disc with one rated for a much higher pressure and/or will double up the burst discs. This prevents the problem of bust discs failing but also then sets up the scenario to exceed the test pressure on a poorly monitored fill.

I am sure that 3AA steel tanks will last nearly indefinitely with 10% overfills (I have seen pre-1920 welding tanks that still meet the criteria for a "+" rating) but there is not adequate data out there yet to support whether this is true with 20% or greater overfills, particularly with tanks certified under an exemption rather than 3AA standards. 3AA tanks are over engineered with very generous safety margins, exempt tanks do not necessarily have that same level of safety margin. Personally, I will not buy a used tank from north Florida's cave country given the filling practices there and the unknowns involved.

 

[fweber]

yeah...the tanks are probably tested to 125% of the 3000 psig

yielding would probably start to occur at about 125% to 130%...roughly....but maybe more since the grade of steel or aluminum is probably greater than the minimuim specified.

However, it is not a good idea to continue to pressure up to 120% to 125% or greater. Fatigue damage, I suspect


unless the test pressure is 150% of the rated pressure.

Where do you come up with crap like this?

Test pressures for most scuba cyclinders will be 5/3rds or 3/2s dependent on their rating. If permanent expansion/elastic expansion is less than 10% the tanks pass. Most well cared for tanks will show no permanent expansion at test pressure. However, they are only exposed to that pressure for 30 seconds. Don't sweat a %10-20% overfill, especially if the tanks are warm. As has been noted, "full" is determined by pressure at temp.

However, I too am curious about reduced cyclelife when steel cylinders are routinely "cavefilled". Defining "cavefilled" as something like routinely filling a cylinder to test pressure for extended periods of time, compared with the 30 second hydro cycle.

 

[duckbill]

Thankyou for the feedback.
Good point on the temperature effects. The math does work out, so I won't sweat the occasional 20% overfill, especially if time is not a factor.
It also sounds like the accidental 40% overfill probably didn't put my tanks into permanent stretch, but the disc fatigue is another matter, and one which I will bring up with the LDS.
And, from what I am hearing, once a tank is overfilled there is no advantage (related to permanent expansion) to bleeding it down right away, as opposed to just leaving it that way and diving it a week or so later, no matter how high the overfill was (as long as they don't get hot).
Are these the correct conclusions?
Thanks again.

 

[AXL72]

Where do you come up with crap like this?

Test pressures for most scuba cyclinders will be 5/3rds or 3/2s dependent on their rating. If permanent expansion/elastic expansion is less than 10% the tanks pass. Most well cared for tanks will show no permanent expansion at test pressure. However, they are only exposed to that pressure for 30 seconds. Don't sweat a %10-20% overfill, especially if the tanks are warm. As has been noted, "full" is determined by pressure at temp.

However, I too am curious about reduced cyclelife when steel cylinders are routinely "cavefilled". Defining "cavefilled" as something like routinely filling a cylinder to test pressure for extended periods of time, compared with the 30 second hydro cycle.

ahh sorry...I did not have the references in front of me.

I did put a caveat and a hint at the fact I was grabbing from thin air as an example. See the last line? Yeah, I know it was not very clear, and I am sorry.

-It all comes down to what is the allowable stress for the max rated fill pressure (what is the %SMYS)

-and the test pressure ratio (thanks for confirming 5/3 * max rated fill pressure)

I will recrunch the numbers for ya when I use the correct info for the above bullet points


I am willing to plead "nolo contendere" on this one....even though the post should have been viewed as an example as a bare minimum of safety factors.

==> permanent yielding typically occurs at or around 95% of the actual yield strength, but for design, it is considered a possibility at 95% of the specified minimum yield strength


shame on me for applying pipeline codes as an example to scuba cylinders on the backs of humans...safety factors would be greater for pressurized tanks on the backs of humans


==>Please remind me how many times in a tank's life it gets presssure tested to the 5/3 test pressure?

 

[paulwlee]

particularly with tanks certified under an exemption rather than 3AA standards.

Do people overfill exemption tanks to the same extent as 3AA tanks?
I know there are some that do it (heck, there seem to be people grossly overfilling AL tanks!! :11: :shakehead ) but as far as I can tell it's mostly 3AA tanks that people overfill? I mean, exemption tanks are already around the pressure that 3AA fills get overfilled to.

 

[captain]

Can any one come up either the K factor or the elastic expansion rejection limit number used to determine if the old steel 72's can be + rated.