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For those of you who like to store your tanks with just a little air left in them (100psi or so), how do you empty them to the level you want after a dive? Is it ok to just throw the valves open and wait until they're low? I noticed that when I did this with mine they got very cold, does that present any problem or is it ok?
Maybe I am being dense, but why would you reduce the pressure in them to store them?
Not surprised that they get cold - releasing compressed air is endothermic from the expansion of the air (for the same reason tanks heat up when you fill them with compressed air). I used to work for a dive store filling tanks, and when the burst disks would blow (as they would from time to time), even in the hot Caribbean sun, the entire tank valve would completely ice over.
For long term storage, I open the valve a little and drain it down to +/- 500 psig , then close it. I drain it down slow, so that I don't frost up the valve.
For short term storage, I let the tank have what ever it has in it.
As long as you tank is not left empty its fine. If its full leave it full.
During a psi inspector course I took the instructor stated that in the case of a fire its better to have a full tank, the burst disk will blow. If the tank only has a small amount of pressure in it by the time the pressure gets high enough to blow the valve the tank has already been severly weekend and the tank itself could blow
Store them full or so low that a heat related failure would not be catastrophic.
For both Al and steel tanks, if stored full, the burst disc will rupture in a fire before the tank fails.
If stored at low pressure (100-300 psi) an steel tank will most likely will not catastrophically fail as the pressure will not get that high in a fire. If stored about half full, I suspect the busrt disc would still fail before the tank as it takes a great deal of heat to cause the steel tank to fail.
An AL tank stored at low pressure may fail as the metal starts to lose its heat treatment at 350 degrees F and gets downright mushy around 450-600 degrees F depending on the alloy, but the damage is minimal as the pressure is still comparatively low.
AL tanks stored more or less half full are problematic in a fire as the pressure does not build up enough to rupture the burst disc but is enough to cause the tank to fail and does a great deal of damage when the tank fails due to heat.
From just the tank perspective, it does not care if it is stored full or empty, time spent at high pressure will not wear one out any faster.
So to be safe, steel or aluminum, store them full or store them near empty.
Rapid depressurization to a low pressure will cause no harm in of itself. However, the tank will become wet including the valve. After the tank warms up, the valve should be vented for a few more seconds to expel any water.
Store them full or so low that a heat related failure would not be catastrophic.
For both Al and steel tanks, if stored full, the burst disc will rupture in a fire before the tank fails.
If stored at low pressure (100-300 psi) an steel tank will most likely will not catastrophically fail as the pressure will not get that high in a fire. If stored about half full, I suspect the busrt disc would still fail before the tank as it takes a great deal of heat to cause the steel tank to fail.
An AL tank stored at low pressure may fail as the metal starts to lose its heat treatment at 350 degrees F and gets downright mushy around 450-600 degrees F depending on the alloy, but the damage is minimal as the pressure is still comparatively low.
AL tanks stored more or less half full are problematic in a fire as the pressure does not build up enough to rupture the burst disc but is enough to cause the tank to fail and does a great deal of damage when the tank fails due to heat.
From just the tank perspective, it does not care if it is stored full or empty, time spent at high pressure will not wear one out any faster.
So to be safe, steel or aluminum, store them full or store them near empty.
I agree with what you are saying, but in the case of a steel tank, it really doesn't mater what pressure it is stored.
Keep in mind that the same temperature that is degrading the structural properties of the cylinder is also degrading the burst disc. The chrome-molybdenum steel used in Scuba tanks is less susceptible to heat degradation than the burst disc. Therefore in a fire the burst disc will still do its job even if the tank is half way full.
The bold statement is correct.
In the case of aluminum, all bets are off. I would do as you suggest, either mostly full or mostly empty.
The above conclusion are based the best simplified analysis I could perform with some of the material data I have available.
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Originally Posted by DA Aquamaster
