Yunk,
Idaho makes a good suggestion about pressure testing the tanks. Especially if you are gonna resell them as operating compressors. Test the tank and safety relief valve setting. Old tanks can get pitting inside from water condensing out of the air and cause rust pitting in the bottom of the tanks. Regarding the relief valve, the testing should at least demonstrate that it works-or not!
The tank can be pretty easily tested by filling it with water and then bringing the pressure up to 1.5 times (this may not be the exact number, I seem to remember it can be from 1.3 to 1.8 times) the operating pressure with compressed air or nitrogen. The water is there so you only get wet if the tank ruptures. It is safer than using only air because the air can act as a massive spring, causing all sorts of bad things if the tank lets go. The little bit of air you use on top of a tankful of water doesn't have anywhere near as much energy stored in it.
Sometimes the tank has a nameplate tack welded to it, showing the rated operating pressure and who made the tank, the date, etc. You can use that to see what you need to pressurize the tank to. I don't think the small commercial compressor tanks are required to have this plate, but anything 12" in diameter and over need this plate.
The usual disclaimer...I'm not a pressure tank design expert, having only done projects where I requested help from said experts!
Have fun,
Jon.
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