A plant was doing a pneumatic pressure test on a pipe connected to a tank. There was no blind flange between the piping being tested and the tank. The tank was isolated from the pressurized piping with a closed block valve. The block valve leaked, allowing the pressure from the pneumatic test to leak into the tank. The tank (which either did not have a pressure relief device installed, or the pressure relief device was too small) was overpressured, and it failed at the bottom. The tank lifted into the air and came to rest on the top of the plant.