Tanks Left in Sun

[mccabejc]

Is it really a risk of explosion if you leave aluminum tanks in a hot car, or is that just a superstition? I would think that since most diving is probably done in very hot locations, and tanks are always left in the hot sun in some form or another, and there seem to be very few reports of tanks actually exploding, the conditions would have to be just right and extreme before you had a problem.

Though I found out that Aluminum has about 5 times the thermal conductivity as Steel, so maybe AL is more susceptible to transferring the heat into the tank than steel. Although the walls of the AL tank are thicker than the steel tank, so maybe not. Or maybe a reflective paint on the outside of the tank keeps the metal from heating up in the first place.

Geesh, this is giving me a headache...

 

[MechDiver]

Thats why valves have burst disks.

And, yes, they do blow (the burst disks).

 

[teknitroxdiver]

Well, the tank isn't going to blow, but if it gets hot enough the burst disk could pop. I imagine you could figure out the temperature for that, but I'm too lazy to.......

 

[mccabejc]

After a little internet research, I found this useful tidbit:

"When scuba tanks are heated the pressure rises. Would the tanks explode if they were full? The answer is "No." In the valve of the tank there is a safety plug that is designed to split when the pressure climbs near the hydrostatic test pressure. If a 3000 psi tank were heated to the point the pressure climbed to 5000 psi (about 400 degrees F) the safety plug would split and the air would quickly leave the tank. However, there is no danger of tanks getting that hot in the trunk or in the interior of a car!"

 

[redacted]

Pressure is directly proportional to temperature on the absolute scale. So pressure increases from thermal load should stay well under 10%.

 

[jhelmuth]

I beleive that I am correct in stating that the "rated working pressure" for a cylinder is some multiple of it's actual threshold for explosive destruction due to excessive pressure. I'm not speciffically aware of what that multiple/factor is (maybe from 2.0 and up).
Let's assume that to be true (2.0). Then a typical 3000psi/200bar AL3 cylinder would be a likely threat to explode @ 6000 psi/400bar. Now go do the math to calculate the required rise in temperature for a fixed volume of gas (consider a std. 80 cuft cylinder) to the pressure threshold. pV = n R T (ideal gas). If V is constant, the divide both sides by V to get p = (n R T) / V. You'll need more info here than I'm provideing to actually do the calculations, but I recall (from class) that a single degree in temperature raises the pressure in this circumstance by 25 psi. Assuming that is true, you'll have to raise the temerature by 120 degrees! If ambient temperature was 76, the the threshold would probably be ~ 196 degrees. Pretty hot! I doubt that you'd see those kind of temperatures, but if its possible, then you scenario/nightmare could come true...

 

[jhelmuth]

just a thought...

Your burst disk is gonna blow before you get to critical mass!

 

[mccabejc]

As my Thermo professor used to say when he didn't feel like going thru the calculations, "I'll leave it to you guys to crunch the numbers".

The writeup I found seems to indicate that a 67% increase in pressure would require a 533% increase in temperature. So as jhelmuth says, delta P = (nR/V) * delta T.

Which means nR/V must be .67/5.3.

I'll leave it to you guys to crunch the numbers.

 

[mweitz]

I have a book (I'll look it up when I get home) that said that the plastic in your car would melt before your tank blew. Certified Divers Handbook maybe, yeah, thats it.

Mark

 

[rab]

OK, that doesn't make sense. I won't try to quote who's law it is, but:

PV=nRT with constant Volume (V) and the nR terms are constant, too, so:

P1/T1 = P2/T2

P1 = 3442psi (I'm using my PST E-Series tank for reference)
P2 = 5250psi (the test pressure)
T1 = 70 F (what PST says is the temperature at which the tank is "rated")
but on Kelvin scale: T1 = 294.1 K K=C+273 C=(F-32)*5/9

therefore:

T2 ~= 448.6 K ~= 348 F

Don't sit in that car with shorts on!

-Rob