I worked in the petroleum equipment industry for a dozen years and was a certified tank installer and tester. You can not use old fuel storage tanks for the storage of compressed air. Many of these tanks can suffer structural damage or failure at pressures as low as 10 psi or even lower depending upon their condition. This was a regular topic in our safety training and there are plenty of documented fatalities from over pressurizing storage tanks not designed as a pressure vessel. Latent petroleum compounds also pose a serious risk of explosion as it is almost impossible to remove the impregnated petroleum from the tank in a reasonable period of time. A better re-use for these is water storage (after they have been cleaned and epoxy lined). I like the idea of using compressed air for energy storage but a new unused propane tank would be a better solution provided it was designed for the system pressures and had provisions for removing condensation and inspecting the tank for corrosion.
BE SAFE
