Scuba Tanks vs. Heat???

[SC_Hoaty]

The "flowering" of the tank indicates multiple failure points, which is usually indicative of rapid pressure changes. The split line is indicative of a single failure point. The initial failure is caused by pressure, not potential energy.

I suppose, put another way, it is easy to gradually increase the tank pressure with a gas - this is likely what happened. It is similarly difficult to gradually increase the tank pressure with a liquid, at least with most hydraulic setups.

Again - good observations. With the hydrotest, it was filled with hyraualic fluid, connected to the pump, and the pressure in the system was slowly increased. When the ultimate strength of the metal was reached, the cylinder failed with a split down its length. The split was slightly wider at the midpoint (probably where the failure initiated). When the failure occurred, the fluid escaped and the pressure dropped to ambient. Lets say the cylinder was about a liter in volume (it was actually less) and the line and pump may have had a small fraction of a liter between them. Since the fluid is virtually incompressible, there was just not that much fluid to release once the cylinder failed. Much of it remained in the cylinder and the fluid in the line and pump all remained where it started.

Pneumatic test - fill with gas (let's pretend it behaves as an ideal gas the entire time) and connect to line and gas compressor. Start increasing the pressure by adding more gas. PV=nRT We can take our time and let the system cool to ambient temperature, the cylinder is of a known, fixed volume (excluding high pressure creep) thus pressure in the cylinder is directly related to moles of gas (or mass of gas if your prefer) in the cylinder. When the ultimate pressure is reached, a small failure initiates, similar to the hydro case. However, there is soooo much more gas that has to escape, because all those moles of gas were compressed into the small volume. As the energy is released, the metal is "pushed out of the way" by the expanding gas. The "flower" effect comes as the metal continues to fail along grain boundaries as it gets displaced further from its original cylindrical shape. It tears into ragged edges. Some high speed photography would be really cool, but I don't have any.

In both cases the failure is caused by over pressure, and initiates at the same point, by the same mechanism. With the hydro, the failure relieves all the pressure quickly, as the fluid does not expand when the failure occurs. The pressure drops to ambient and the event is over. With the pneumatic test, all the added gas must leave the cylinder through the failure point before the pressure will be at ambient. Thus the event lasts a bit longer (still very fast). The escaping gas ripping the metal apart creates a larger opening for the remaining gas to vent through.

How am I doing? Is this getting clearer? Thanks!

 

[SC_Hoaty]

as the tank/temperature has been answered already, I think I see what SC is saying .. with the incompressable fluid, when the tank fails, the pressure goes to 0 rapidly , With the compressed gas, when the tank fails, the gas pressure does not drop as fast and continues to expand through the opening and has more time to work on it making it bigger

?

Yup! You got it.

 

[String]

For those civilised places without burst discs (dont even think they're legal in europe) for a good old steel 232 bar tank the test pressure is 348 bar. It take a LOT to heat it up to that level.

Tanks are fine in a car, in a garage and so on. The rubber parts melting or moulding are more of a problem.

 

[GrumpyOldGuy]

Maybe on planet Mercury, not on planet Earth

No need for space travel. All you need to do is walk barefoot on an asphalt road on a hot afternoon and the significant increase in temperature due to radiant exposure above ambient is understood. The same applies to tanks left in the sun.

 

[captain]

No need for space travel. All you need to do is walk barefoot on an asphalt road on a hot afternoon and the significant increase in temperature due to radiant exposure above ambient is understood. The same applies to tanks left in the sun.

But it's not going to stress them or shorten their life.

 

[GrumpyOldGuy]

But it's not going to stress them or shorten their life.

Then why does Catalina state this on their web site?

"[FONT=Verdana,Tahoma,Arial,Helvetica][FONT=Verdana,Tahoma,Arial,Helvetica]Great care must be taken never to expose charged cylinders to heat in excess of 125F (52C)."



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[String]

No need for space travel. All you need to do is walk barefoot on an asphalt road on a hot afternoon and the significant increase in temperature due to radiant exposure above ambient is understood. The same applies to tanks left in the sun.

Asphalt roads dont get up to 200 degrees even in the sun.

 

[GrumpyOldGuy]

Asphalt roads dont get up to 200 degrees even in the sun.

Not sure where you got 200 degrees from, I only said it would get hotter than ambient.

In the case of Catalina Cylinders, they warn you about exposing charged cylinders to temps >125F which is possible in some climates when exposed for long periods in direct sun.

 

[captain]

No need for space travel. All you need to do is walk barefoot on an asphalt road on a hot afternoon and the significant increase in temperature due to radiant exposure above ambient is understood. The same applies to tanks left in the sun.

The Catalina web site says, "Extreme heat can lead to activation of the PRD on the valve of charged cylinders. Great care must be taken never to expose charged cylinders to heat in excess of 1250F (520C)."

Translation, the burst disc (PRD pressure relief device) could rupture, no mention of cylinder danage or reduced service life.




The rest of the story,
Aluminum cylinders known to have been exposed to overheating or showing evidence of having been overheated, with overheating being a cylinder metal temperature in excess of 350oF, must be condemned per 49 CFR 180.205 (i) (viii). No testing for acceptance or re-heat treatment is authorized.

In CGA pamphlet C-6.1; Standards For Visual Inspection of High Pressure Aluminum Gas Cylinders, it states that cylinders heated to cylinder metal temperatures in excess of 350oF (176oC) must be condemned.

Common evidence of exposure to overheating that may heat the metal temperature to 350oF (176oC) include:


charring, blistering, or discoloration of the cylinder paint or protective coatings;


distortion of the cylinder;


melting of non-metallic (i.e. plastic, etc.) valve components;


charring or burning of labels;


activation of valve pressure relief device; or


activation of the heat indication system.
Catalina Cylinders recommends that if there is evidence, or it is believed, that a cylinder has been exposed to overheating but the temperature of metal of the cylinder is not believed to have reached 350oF (176oC) for any duration of time, the cylinder must still be subjected to hydrostatic testing or condemned. A cylinder that has been exposed to temperatures great enough to change the temper of the cylinder could show an increase in total or permanent expansion as measured during hydrostatic testing. Cylinders showing unusually high total expansion or exceeding the regulatory limits for the relationship of permanent expansion to total expansion (i.e. DOT = 10% and TC = 6%) should be condemned.

No cylinder left in the sun or trunk of a car in the sun will get anywhere near 350 degrees.

 

[GrumpyOldGuy]

I hope you meant 125F, not 1250F.

My logic is a bit different. The over pressure device is there for a reason, specifically to prevent catastrophic failure. Excess stress occurs BEFORE catastrophic failure.

If you are even approaching the trigger point for these disk (like you can by heating a very full tank to over 125F) then you have significantly exceeded the design pressure, even if you don't actually rupture the disk. This causes excess stress and excess stress, repeated many times will shorten the life of a structural object made of AL. I would suggest this is very similar to the stress from fast fills or over filling AL tanks. Not the end of the world, but something that I choose to avoid if possible.

Confession: Last week I left a tank in the back of my SUV and my son closed the hatch while I was diving. When he hooked up the tank for the next dive he noted it was at 3400PSI. If was not even a hot day by Texas standards.