Below are pictures of an air tank that was purchased from an auction.
The tank was not built to ASME code and not registered with the National Board. The drain valve on the tank was removed, probably because it was leaking, and replaced with a plug. The accumulation of water in the vessel caused severe corrosion and thinning of the metal on the bottom of the tank. Ultrasonic thickness testing revealed a thickness of .070 inches. At this thickness the tank should never have been allowed to be pressurized. The safety valve was set at 140 psi and the pressure controls were assumed to be working properly. The purchaser brought home his new treasure, put it in his garage and plugged it in. When the pressure in the tank reached about 100 psi, the tank ruptured and flew across the garage. Luckily, nobody was injured.
Make sure you drain your air tanks on a regular basis!