Tank overfill = mini hydro?

[halocline]

Do you know what the hydrostatic testing pressure of a HP steel tank is? It's 5/3 the rated service pressure. So, for your 3442 psi tank, a hydrostatic test would be conducted at 5737 psi.

3442 PSI HP 100 are special permit tanks, and as such I believe the hydro pressure is 150%, not 5/3, of service pressure.

 

[BarryTX]

Interesting discussion - on the concept of metal memory, I think things are being stretched (pun intended) in the discussion. A couple of points -

Any closed tank or vessel that is being pressured up is being stressed, and there is a resulting strain (or, in simple language, the vessel wall is stretching). The classic stress-strain diagram for any material shows from zero pressure to some higher pressure the strain change is linear with increasing pressure - this defines the elastic region. At that specific higher pressure (which is unique for each material, wall thickness, etc.) the slope of the strain change breaks over dramatically and basically the material is stretching fast enough that continuing to pump into the vessel does not result in any more pressure - this defines the plastic region. Of course at some point it falls apart as it stretches too thin. Within the elastic region as pressure is released the material returns to its original shape / size (strain is reduced with decreasing stress). But within the plastic region the material has been permanently deformed and it does not return to its original shape / size when pressure is released. I believe the comment about metal memory is referring to the elastic region of the stress-strain relationship.

Back to the tank fill and hydro test, these pressures are obviously well within the elastic region. So why do we need to hydro the tank more than once if we are never going to have a working pressure that high? Because, even ignoring any potential negative effects of corrosion, the stress-strain relationship is different when the material undergoes cyclic loading than static loading. That difference is the material is weakened due to being subjected to stress cycles - and when we are filling (pressurizing) the tank, using the air (releasing pressure), and repeating over and over we are subjecting the tank wall to cyclic loading. So the stress-strain relationship for the tank material is changing the more we use the tank. This is a fundamental reason for the need to hydro the tank, although corrosion (if present) will certainly worsen the weakening process in and of itself.

Back to the original question - does overfilling harm? Well, the fact that the pressure is higher on an overfill means that for that cycle the stress is a bit higher, even if it is only for the period that the air in the tank is hot. So IN THEORY it is speeding up the weakening process a little bit. But in practice I don't believe it makes any real difference, the big effect is the stress cycling that occurs from as tanks go from near-empty pressures to full pressures over and over.

 

[JahJahwarrior]

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.

Terry, this quote and post seem to go against what you say.

PV=nrT. Government regulations only concern pressure and temperature, but only the pressure is stamped on the tank, so fill stations only worry about the pressure.


OP- if you can, here is my suggestion. Get a cooler large enough to hold your tank, and several bags of ice. Wheel the contraption in, and have them fill the tank while it resides in your ice bath. You'll get a good fill.

 

[WD8CDH]

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.

This is the key for legal "hot fills"

The pressure MUST NOT exceed 5/4 of the stamped service pressure.

AND

The temperature of the tank MUST NOT exceed 131F.

AND

When cooled to to 70F the pressure in the tank MUST NOT exceed the service pressure (or 10% over for + rated tanks).

----

So a fill in a 3442 psi tank to less than 4302.5 psi that doesn't heat the tank up to over 130 degrees is legal as long as the pressure doesn't exceed 3442 psi once it cools to 70F.

But, to the OP, since your tank only cooled down to 3600 psi (not 3442 psi), that was not a legal fill.

 

[captain]

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. QUOTE]

I think you are referring to the round out procedure that should be done on hot dipped galvanized tanks prior to hydro and is not metal memory as such.

 

[Davidstealey3]

this is one of those " it is all in how you interpert the statment " question.

if you do not know you just do not know.

points can be made all around for and against the subject, and even if you have the DOT, CFR and other document support numbers to back you up not everyone will beleive you. i know from experience.

so is this overfilling? technicaly yes and no. granted you go over the stamped pressure but you do have to consider the heat exchange of the tank.

da aquamaster makes a good point. if it is filled to 3500@ 70degrees it is full and legal, but you take it out in the sun on a hot summer day and now it say 3650@ 100degrees ( about 5psi for evey 1 degree of temp gain ) it is not over filled. then you hop in a body of water that is 60degrees. well now the tank is 3450psi. so did you get an underfill? no the temp of the water has lowered the pressure in the tank.

so to help with this dilema, instead of filling tanks at 300-600psi, fill alittle slower. i like 200-400 psi. takes a little longer, but the tank does not get as hot.

for the water bath fill, if you were to have a tank failure not only do you have to worry about the metal and air from the rupture, you also have to worry about the water particles flying at a high rate of speed. and as we know from flow jet machines, water under pressure is not good for humans.

 

[SCUBA482]

So it is sounding like the group generally believes a slightly overfilled tank at a warm temp that cools down is fine. Even if the tank PSI reaches nearly 4000 (i think once it hit 3900) and cools down to the 3600 that the chance of damage is remote considering what happens during hydros. What about the 1st stage yoke failure issue? Has anybody seen one of those fail causing obvious damage to the diver? I do like the post about bringing my tanks into the shop in an ice chest full of ice for my fill... Funny thing about all this is that I went there yesterday, different guy, and he was cool about it all. I told him what I wanted, gave him 4 hours to get it done and when I went back they were at 3400 cold...he told me his compressor gage must be off because all the tanks were at the exact same temp and he then kindly touched them up a bit.

 

[Web Monkey]

Terry, this quote and post seem to go against what you say.

PV=nrT. Government regulations only concern pressure and temperature, but only the pressure is stamped on the tank, so fill stations only worry about the pressure.

I must be missing your point. If the fill station observes the tank pressure limit, it won't be overfilled.

Terry

 

[JahJahwarrior]

I must be missing your point. If the fill station observes the tank pressure limit, it won't be overfilled.

Terry

You are missing my point. It is incorrect to say "If the fill station observes the tank pressure limit, it won't be overfilled." What we SHOULD say is "If the fill station observes the tank pressure limit in its relation to tank temperature, the proper and legal fill pressures can be reached without under or overfilling a tank." We might should tack on something about "within a 5% error range given the inherent inaccuracies of Bourdon tube gauges and the possibility of incorrect calibration."

Tank pressure is a 3 fold problem: pressure, temperature and gauge calibration/errors. Pressure and temperature both need to be quoted when talking about fill pressures, both in terms of desired fill pressure and legal fill pressure, and possibly, safe fill pressure. We need to say that the "legal fill pressure of this tank is 3442 psi at 70 degrees fahrenheit." To leave off the 70 degrees farenheit bit leaves room for error. For example, if we drop the temperature of the tank to absolute 0, I'm not certain I would want it filled at all, same as if we heated the tank up enough that the metal melted. 3442 is a very unsafe pressure at both absolute 0 temperature and at the melting point temperature. 3442 is the legal limit at 70 degrees farenheit. What is the legal limit at 130 degrees farenheit? 5/4 of the limit at 70 degrees fahrenheit.

All the tank, divers quote pressures, when it is really only correct to quote a pressure/temperatre coordinate.

Anyone with tons of spare time want to draw a graph or make a chart relating temperature to fill pressure in various tanks, so we can use a digital thermometer to read tank temperature and ensure we are getting the proper fill pressure for that temperature?

 

[Web Monkey]

All the tank, divers quote pressures, when it is really only correct to quote a pressure/temperatre coordinate.

Anyone with tons of spare time want to draw a graph or make a chart relating temperature to fill pressure in various tanks, so we can use a digital thermometer to read tank temperature and ensure we are getting the proper fill pressure for that temperature?

Except that because of the mass of the tank and the time required for thermal transfer, you can't know what the temperature is inside during a fill, by measuring the outside.

Terry