Storage vessel
(a) Storage vessel means each tank, reservoir, or container used for the storage of petroleum liquids, but does not include: (1) Pressure vessels which are designed to operate in excess of 204.9 kPa (15 psig) without emissions to the atmosphere except under emergency conditions. (2) Subsurface caverns or porous rock reservoirs, or (3) Underground tanks if the total volume of petroleum liquids added to and taken from a tank annually does not exceed twice the volume of the tank.