Scuba Tanks vs. Heat???

[SC_Hoaty]

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.

This may be a minor point, but the terminology is not quite correct. There is a difference between the working pressure and the design pressure. The hydro is done to 150% of working pressure (3000 psi working, 4500 psi hydro). Design pressure is higher than either of these, since the cylinder is intended to have a periodic hydro.

You are correct that the cylinder is pressurized above its working pressure. I'm not sure if this stress actually changes the life, or if it is so far below the yield strength that it is insignificant. Certainly operating frequently above the working pressure is something the manufacturer would advise against.

 

[captain]

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.

Actually that is Catalina's typo. I copied and pasted from their site. Didn't notice it.

Your logic is not proven engineering pratices.

A tank's working pressure is determined at what is called standard temperature which is 70 degrees F. In the case of an AL80 that was filled to 3000PSI on a 90 degree day it is under filled. Cool it to 70 degrees and the pressure is below 3000 PSI. If it was filled on a 70 degree day to 3000 PSI and it is now a 100 degree day it is not now overfilled even though the pressure is now above 3000 PSI.

 

[GrumpyOldGuy]

Actually that is Catalina's typo. I copied and pasted from their site. Didn't notice it.

Your logic is not proven engineering pratices.

A tank's working pressure is determined at what is called standard temperature which is 70 degrees F. In the case of an AL80 that was filled to 3000PSI on a 90 degree day it is under filled. Cool it to 70 degrees and the pressure is below 3000 PSI. If it was filled on a 70 degree day to 3000 PSI and it is now a 100 degree day it is not now overfilled even though the pressure is now above 3000 PSI.

No, the Catalina site is correct, 125F, you screwed it up when you cut n paste and did not notice the problem, but that error is besides the point.

The post that started this lovely conversation stated filling the tank to 3000PSI at 50F and they then did the math (correctly) showing that it only increased the pressure to 3300PSI (110% of the initial pressure, not a big deal) at 100F ambient. I pointed out that when ambient temp is 100F, the tank can be much hotter and the pressure higher when left in direct sun, even to a point where it might be of concern.

Your definition of temperature compensated working pressure is fine and accepted practice, right up until you exceed the the operational envelope of the charged tank. I think it is 125F (per Catalina site), for some reason you think it is 350F (that is most likely a value for an empty tank). If I take your definition of working pressure to the extreme I should be able to fill my tank to 3000 at 70F and then heat it up to lets say 300F and still be at your definition of working pressure (well over 4000PSI indicated).

Care to try it?

 

[GrumpyOldGuy]

This may be a minor point, but the terminology is not quite correct. There is a difference between the working pressure and the design pressure. The hydro is done to 150% of working pressure (3000 psi working, 4500 psi hydro). Design pressure is higher than either of these, since the cylinder is intended to have a periodic hydro.

You are correct that the cylinder is pressurized above its working pressure. I'm not sure if this stress actually changes the life, or if it is so far below the yield strength that it is insignificant. Certainly operating frequently above the working pressure is something the manufacturer would advise against.

Yup, you are right, design pressure was the wrong term.

 

[captain]

The end

No, the Catalina site is correct, 125F, you screwed it up when you cut n paste and did not notice the problem, but that error is besides the point.

The post that started this lovely conversation stated filling the tank to 3000PSI at 50F and they then did the math (correctly) showing that it only increased the pressure to 3300PSI (110% of the initial pressure, not a big deal) at 100F ambient. I pointed out that when ambient temp is 100F, the tank can be much hotter and the pressure higher when left in direct sun, even to a point where it might be of concern.

Your definition of temperature compensated working pressure is fine and accepted practice, right up until you exceed the the operational envelope of the charged tank. I think it is 125F (per Catalina site), for some reason you think it is 350F (that is most likely a value for an empty tank). If I take your definition of working pressure to the extreme I should be able to fill my tank to 3000 at 70F and then heat it up to lets say 300F and still be at your definition of working pressure (well over 4000PSI indicated).

Care to try it?

The point I am trying to make is simple. The higest recorded temperature on earth was 136 degrees F in Libya. Lets say in a black car trunk it would be 50 degrees hotter, still half that required to affect an aluminum tank. Ambient natural temperature from the sun in a car trunk anywhere will not heat a, tank full or empty
to a temperature anywhere near the point of causing metallurgical structural damage that will affect the tank to the point of weakening it or causing it to fail a hydro.

Yes the possibility exists that the PRD may blow but this is a hundred or so PSI less than the hydro pressure and several thousand PSI below the catastrophic rupture pressure and Catalina's point is if you definitely don't know why the PRD blew then treat the tank as though it may have been damaged by whatever.

 

[GrumpyOldGuy]

One more tidbit, from the Luxfer site.......

Prevent the cylinder from being exposed to direct sunlight or where the sun is
directed through windows or clear glass roofing.

Keep your cylinder cool. Filled cylinders that become warm (up to 60°C) could result in
breathing difficulties due to the temperature of the contained gas. Never expose your
cylinder to temperatures in excess of 60°C.

My underline, 60C is about 140F.

My point is simply I do not know at what point the combined heat and pressure of a tank will start affecting it. The manufactures (who know a heck of a lot more about this than you and combined) suggest 125F or 140F is the safe limit for a charged tank. The actual values is probably padded for liability, but in the absence of better information, I will buy this.

 

[Tigerman]

One more tidbit, from the Luxfer site.......

Prevent the cylinder from being exposed to direct sunlight or where the sun is
directed through windows or clear glass roofing.

Keep your cylinder cool. Filled cylinders that become warm (up to 60°C) could result in
breathing difficulties due to the temperature of the contained gas. Never expose your
cylinder to temperatures in excess of 60°C.

My underline, 60C is about 140F.

My point is simply I do not know at what point the combined heat and pressure of a tank will start affecting it. The manufactures (who know a heck of a lot more about this than you and combined) suggest 125F or 140F is the safe limit for a charged tank. The actual values is probably padded for liability, but in the absence of better information, I will buy this.

I suspect padded quite a bit for liability, but if they see the need to pad it that much to be safe, why risk it?

 

[chokdeekap]

i live in the middle east we get temps of 48-54 degrees centigrade in the summer my tanks are in the back of my jeep all year round apart from Fridays and Saturdays when they are where they should be under the water.