Abstract: A combination liquid petroleum storage tank and emergency drainage system for draining rain water accumulated on the floating roof of the storage tank, including a drain duct having an inlet adapted to receive rain water accumulated on the roof and extending downward about a central axis, and a petroleum vapor barrier valve in the drain duct at an axial location downstream of the inlet, the valve formed as a multi-blade rotor that is rotatably mounted in the drain duct for rotation about a central rotor axis extending generally perpendicular to the drain duct central axis at the axial location, the rotor having a plurality of generally semi-circular blades.

Abstract: A combination liquid petroleum storage tank and emergency drainage system for draining rain water accumulated on the roof of the storage tank, including: a. a storage tank including a roof, side walls and a base which define an interior space, b. at least one rainwater drain duct having an inlet adapted to receive rain water accumulated on the roof, the drain duct having a predetermined bore diameter and extending downward about a central axis and communicating with the interior space, c. a petroleum vapor barrier valve in the drain duct at an axial location downstream of the inlet, the valve formed as a multi-blade rotor that is rotatably mounted in the drain duct for rotation about a central rotor axis extending generally perpendicular to the drain duct central axis at the axial location, d.