Originally posted by devjr
Fred, no SCUBA tank is going to withstand 3-4 times the test pressure. A tank with 2450 psi working pressure is rated for about 6500 psi burst.
That "rated" burst is the _minimum_ burst with all manufacturing variables in the worst case condition. This means miminum average wall with maximum concentricity runout and all "allowable" defects all stacked up in the same point on the wall. Calculated design burst of a "perfect" tank to nominal dimensions will be considerably higher. Often 1.5x higher if there is considerable physical variation possible due to the manufacturing process.
As a case in point steel 72s rated at 2250psi working pressure in good shape have been pressurized (with water) to over 9,000 psi without bursting. The tanks yielded, but did not burst. There is a big difference in the strength of steel between the ultimate and the yield point. It's not unusual to have annealed alloy steel with yield strength roughly 60% of the ultimate. As an example the"yield" on A500 steel tube is at least 46,000 psi, while ulitmate is over 58,000 PSI to failure, with a 23% enlongation. A500 is minimally "heat treated" to get the higher yield strength. Once the steel passes out of the elastic range it can absorb an amazing amount of tensile energy before it breaks, but it won't look the same after the pressure is released either.
Aluminum is a different critter. Differences between yield and ultimate get smaller, and the strength factors change a lot.
6061-T6 (The "GOOD" Al tank metal) has a yield/ultimate of 40,000/45000 psi. Heat that tank metal to 350°F and the numbers change to about 8000/18000 psi. That is roughly a 5:1 reduction in tank strength. The "endurance limit" for 6061-T6 is 14,000 psi. As a side note the endurance limit is the maximum load where the fatigue loading properties of Aluminum do not come into play.
BTW heat a low alloy steel tank to 350°F and it actually gets between 2% and 4% STRONGER, and no, _I_ will NOT put 9000psi of AIR in a scuba tank!
FT
