rx7diver wrote:
I was taught a long time ago that if a scuba cylinder were stored at maximum pressure (i.e., at its working pressure) for long periods of time, then it is more likely to fail its hydrostatic test.
Safe Diving,
rx7diver
You may be thinking of “sustain load cracking” on the older aluminum alloy cylinders (AL- 6351). That is kind of a unique phenomena that occurs to the old aluminum alloy cylinders that are left pressurized for extended periods of time.
Steel structures should not experience degradation for any constant stress, as long as the stress is not close to the material yield strength and corrosion doesn't degrade the structure. Extreme heat can also degrade the material strength.
Many structures are design to operate at around 60% of its yield strength (if fatigue cyclic loading is not a big concern). The 60% of yield is normally considered enough of a safety margin when the expected loads are well understood. The safety margin is supposed to account for unexpected loads (like handling or other accidental loads) and for some geometry imperfections.
The two things that normally kill a steel cylinder is rust or excessive heat (like the heat from a fire). Exposure to heat can ruin the heat treatment on the alloy, but in the case of steel cylinders we are talking temperature somewhere above 800 degrees F (I am not a metallurgist and my memory is not the most reliable so don’t rely on that number).
A tank that was exposed to a fire (even just to the radiant heat of a fire) needs to be hydro tested before it is used again, even if it has a current hydro stamp.
BTW, aluminum cylinders will start losing their heat treatment (start annealing) at very low temperatures, as low as 300 to 350 degrees F. That is why aluminum cylinders cannot be painted with any process that involves any kind of heat.
Some dive shops will not fill any aluminum cylinder that has a paint job that doesn’t look like it came from the factory, like custom paint jobs, etc
I remember two accidents in Florida were two freshly painted cylinders blew up in the same day (I think it was in the 70’s). The owner of the cylinders used a baked on paint or some hot flame sprayed paint (I can’t remember the details). The first one blew up without killing anyone. He just got pissed and went to a second dive shop to fill his second tank. The second ne was not so lucky for the shop owner. At least, that is what I remember.