Ref. replies #15 and #18
Rex, and other interested parties--
Working backwards from the data you stated, it appears your tank is 12” in diameter with 0.060” wall aluminum material. The photo doesn’t very well show the 1/2” radius mentioned at the joint of the cylinder and head, but I assumed it was there per your description. While the radius is much improved from a sharp corner, it doesn’t really constitute a spherical or elliptic tank head insofar as nicely accommodating stresses from internal pressure. Taking these dimensions, and assuming a 1/2” crown to the lid, an axisymmetric finite element model was made of the upper quadrant of the tank and was subjected to 20 psi internal pressure. As you can see from the jpeg’s attached (if I can do that correctly), Pd/2t isn’t the last word on the stress distribution of the vessel.
Given the number of commercially available pressure vessels of various sorts (air tanks, scuba tanks, beer kegs, gas transport tanks, etc., etc.) that are necessarily built to ASME or other criteria, it seems much easier and confidence inspiring to re-purpose such a thing as opposed to making one from scratch. Pressure vessels are trickier to do correctly than they might appear to be.