jim T.
Guest
So...even in a desert at 100=120degrees F. no problem having tanks in a car?
Thinking about the next road trip...
Thinking about the next road trip...
Temperatures?
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hey
PV = NRT
assuming V (volume) remains constant in a tank (good assumption cause it does remain constant)
N & R = constants (ie remains unchanged)
T = variable delta T but on rankine scale.
Resulting pressure thus becomes equal to the ratio of the ending temperature to the starting temperature.
if you fill an air tank to 3000 psig at 60F = 520 degrees rankine
if you allow the temperature in your car to reach 200 F = 660 R
then the pressure will increase from 3000 psig to 660/520*3000 = 3807 psig
it all has to do with delta (change in) temperature as opposed to the maximum temperature.
depending on the preesure at which the relief rings will blow.....you can calculate the temperature required to make them blow based on the starting (fill) temperature and the ratio above
PV = NRT
assuming V (volume) remains constant in a tank (good assumption cause it does remain constant)
N & R = constants (ie remains unchanged)
T = variable delta T but on rankine scale.
Resulting pressure thus becomes equal to the ratio of the ending temperature to the starting temperature.
if you fill an air tank to 3000 psig at 60F = 520 degrees rankine
if you allow the temperature in your car to reach 200 F = 660 R
then the pressure will increase from 3000 psig to 660/520*3000 = 3807 psig
it all has to do with delta (change in) temperature as opposed to the maximum temperature.
depending on the preesure at which the relief rings will blow.....you can calculate the temperature required to make them blow based on the starting (fill) temperature and the ratio above
A burst disc is required to fail within a range of no more than the hydro test pressure to no less than 10% below the hydro test pressure. So depending on where the burst disc blows in this range, the temperature required for a 3000 psi service tank to reach a pressure where the burst disc would fail would be 335 to 425 degrees F. Actually it will probably fail at a bit lower temperature, as the disc itself is probably going to be affected by the heat as well, but either way, it's not likely to happen in a car trunk, the back of a pickup or standing in full sunlight.
That's a good thing too as temperatures in that range will exceed the temperature at which the heat treated aluminum in an aluminum tank becomes scrap metal and will be not all that far from the temperature where the tank itself will fail. (It's a good idea to store aluminun tanks either near empty (a few hundred psi) or to store them full - storing them partially full creates a scenario where the tank could fail before the burst disc in a fire.)
But...bursts discs blow fairly often (comparatively speaking) in car trunks or wherever and they do so because they fatigue over time with numerous fill cycles. So in effect if your burst disc has been recently replaced, the odds are good it will withstand pressures very close to the rated range. If it has been in use a few years, seen a couple hundred cycles and perhaps even endured some corrosion, it may fail at not much more than the service pressure. If left in the valve long enough, it will eventually fail at the service pressure or even below the service pressure during a fill.
So there really is no good way to tell when your burst disc will fail other than to replace it at regular intervals. For the average diver, replacing it at each hydro test is probably a good rule of thumb.
That's a good thing too as temperatures in that range will exceed the temperature at which the heat treated aluminum in an aluminum tank becomes scrap metal and will be not all that far from the temperature where the tank itself will fail. (It's a good idea to store aluminun tanks either near empty (a few hundred psi) or to store them full - storing them partially full creates a scenario where the tank could fail before the burst disc in a fire.)
But...bursts discs blow fairly often (comparatively speaking) in car trunks or wherever and they do so because they fatigue over time with numerous fill cycles. So in effect if your burst disc has been recently replaced, the odds are good it will withstand pressures very close to the rated range. If it has been in use a few years, seen a couple hundred cycles and perhaps even endured some corrosion, it may fail at not much more than the service pressure. If left in the valve long enough, it will eventually fail at the service pressure or even below the service pressure during a fill.
So there really is no good way to tell when your burst disc will fail other than to replace it at regular intervals. For the average diver, replacing it at each hydro test is probably a good rule of thumb.
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