Tank overfill = mini hydro?

[GrumpyOldGuy]

You can't fix stupid (Sports Chalet and some other places)!

Invest in a hand-held tank checker gauge. Then drop your tanks them off for a fill and come back when they are cool and check the pressure BEFORE you pay. When you get a short fill, show it to them and have them top it off on the spot.

Yup, they might think you are a pain in the tocus, but since you are paying for a full tank, you might as well get it. Some of them will even learn and top them off for you without asking after a few go rounds.

 

[JahJahwarrior]

I don't know why he was dancing around the issue. If he doesn't want to overfill a tank, he's not required to.

Terry

The issue here is defining "overfilled." I would argue that a tank that is filled above the rated pressure, which is a pressure and temperature "combination," but which is filled to a safe temperature with approximately the same ratio of pressure to temperature, is not really "overfilled" the same as a cave fill. IE: if the tank is not too hot during the fill process, and if it cools to the correct pressure, then the tank was never really "overfilled" in the traditional, "cave fill" sense. It was merely correctly filled given the increased temperature.

Ask him how accurate his guage is, and when it was last calibrated. He can be all worried about hitting 3442 precisely when in reality, the pressure in the tank is 3300 because his gauge is off. If he's really splitting hairs over the gauge reading, you could bring this point up to try to get him to relax and realize that a pressure reading of 3500 on the gauge might not be a violation of company policy to not "overfill", then you can start working on making him realize that "overfill" is a term that depends very much on temperature, and not just on pressure.

 

[vladimir]

1. tanks rarely fail hydro, and over-pressurized tanks (ie overfilled) fail even less. This is true because the metal is contracting and expanding, gaining a metal memory and able to acceptably expand within limits. If you leave an empty tank empty for a long time and take it to hydro without filling it and letting it regain its memory, it will probably fail. It is also true because water molecules are much larger than air molecules, allowing water to be retained in a tank with a very small hole that air can trap in. This is one of the reasons more tanks fail visual than a hydro...

If there's a mechanical engineer out there who can explain this concept of "metal memory" to me, I'd appreciate it. I am skeptical.

 

[imwright1985]

If there's a mechanical engineer out there who can explain this concept of "metal memory" to me, I'd appreciate it. I am skeptical.

umm i think its the oposite of cyclidic loading and strain hardening but my engineering materials class never touched on it. the only thing i can think about that would be a "metal memory" would be loading the tanks surface with a force that pushes it beyond the modulous of elasticity which is where plastic deformation starts and the tank would become plasticaly/perminantly deformed (failing a hydro based on tanks external dimensions changing and increasing the point for the modulous of elasticity, causing it to take up a slightly different rate of change on the expansion)

Just Guess though

But an average overfill to compensate the cooling of air causing a pressure drop doesnt push it near that point so you really dont have to worry much, If you use that tank once a week for about 15 years it might fail at that point, but i havent been diving that long so am not positive on it

 

[adelman]

Filling above the rated service pressure isn't overfilling.

Excerpted from the actual regulation:

49 CFR Part 173 Subpart G Section 173.301:

173.301a:

(c) Cylinder pressure at 21C (70F). The pressure in a cylinder at 21C
(70F) may not exceed the service pressure for which the cylinder
is marked or designated, except as provided in Section
173.302a(b). [...]

(d) Cylinder pressure at 55C (131F). The pressure in a cylinder at
55C (131F) may not exceed 5/4 times the service pressure, except:

[...]

2) For a cylinder filled in accordance with Section 173.302a(b), the
pressure in the cylinder at 55C (131F) may not exceed 5/4 times
the filling pressure.

Section 173.302a(b) refers to filling "+" rated cylinders 10% over the
marked pressure.

 

[tep]

umm i think its the oposite of cyclidic loading and strain hardening but my engineering materials class never touched on it. the only thing i can think about that would be a "metal memory" would be loading the tanks surface with a force that pushes it beyond the modulous of elasticity which is where plastic deformation starts and the tank would become plasticaly/perminantly deformed (failing a hydro based on tanks external dimensions changing and increasing the point for the modulous of elasticity, causing it to take up a slightly different rate of change on the expansion)

I was wondering if this was a reference to annealing... Heat/cooling (properly) can really strengthen metal. The SR-71 Titanium skin annealed to the point that the skin material was about 15-20% stronger after the 30 years of flight ops than when it was built.

 

[vladimir]

I was wondering if this was a reference to annealing... Heat/cooling (properly) can really strengthen metal. The SR-71 Titanium skin annealed to the point that the skin material was about 15-20% stronger after the 30 years of flight ops than when it was built.

Thanks. A quick search on annealing turned up some explanations that seemed somewhat plausible to me, even at the low temperatures we are talking about.

 

[zaberman1]

tep is correct...I was quoting the material from PSI, he provides a much more technical answer

 

[DA Aquamaster]

It's nice to use extemes in examples.

Ask your shop the following:

If you fill my 100 cu ft 3442 psi service pressure tank to exactly 3442 psi at room temperature and then set it in the sun where the temp rises to 130 degrees, the pressure will be over 3442 psi, but is it over filled? (The answer should be "No". In case they don't get it, point out that it still has the same 100 cu ft of gas in it.)

If you filled the same tank in the dead of winter at -10 degrees F with no heat in the building to exactly 3442 psi, and then it warms back up to room temp when the heat comes back on, is it overfilled - even though the fill pressure never exceeded 3442 psi? (The answer here should be "yes" as it always had more than 100 cu ft of gas in it.)

If your LDS can't answer those questions correctly, they don't know enough to understand what a "legal" fill really is.

As long as it does not exceed the service pressure at 70 degrees F, it is not overfilled, regardless of the pressure you reached during the fill process prior to the tank cooling back to room temperature. The average tank may be close to 90-100 degrees F after a "normal" fill and overshooting the fill pressure by 200-300 psi will generally result in the tank being very close to the actual service pressure at room temperature - it's a full fill, not an overfill and when you consider that 39 psi in an AL 80 equals 1 cubic foot, the 2800 psi fill that often results from a hot fill to only 3000 psi is robbing you of 5 cu ft of gas.

 

[Web Monkey]

The issue here is defining "overfilled." I would argue that a tank that is filled above the rated pressure, which is a pressure and temperature "combination," but which is filled to a safe temperature with approximately the same ratio of pressure to temperature, is not really "overfilled" the same as a cave fill. IE: if the tank is not too hot during the fill process, and if it cools to the correct pressure, then the tank was never really "overfilled" in the traditional, "cave fill" sense. It was merely correctly filled given the increased temperature.

Ask him how accurate his guage is, and when it was last calibrated. He can be all worried about hitting 3442 precisely when in reality, the pressure in the tank is 3300 because his gauge is off. If he's really splitting hairs over the gauge reading, you could bring this point up to try to get him to relax and realize that a pressure reading of 3500 on the gauge might not be a violation of company policy to not "overfill", then you can start working on making him realize that "overfill" is a term that depends very much on temperature, and not just on pressure.

This is still dancing around the issue. If the tank has a DOT rating of 3442Lbs, that means that nobody is supposed to fill it to more than that regardless of what they think it's going to cool down to.

All the fill guy needs to do is his job, which is fill the tank slowly until it contains the rated pressure, then stop. There is no reason for him to try to come up with explanations for not exceeding the rated pressure.

Terry