A drain valve for a portable tank has a flange securable externally to a wall of the tank and formed with a hole centered on a flange axis perpendicular to a face of the flange. A housing extends from the flange and forms a housing flow passage extending along a passage axis opening into the tank at the hole of the flange and forming an angle of less than 90° with the flange axis. A ball valve element in the housing at the housing flow passage is movable between an open position permitting flow through the housing flow passage and a closed position blocking such flow. A shaft journaled in the housing is fixed to the ball valve element and extends along a shaft axis substantially perpendicular to the passage axis. An actuator connected to the shaft outside the housing is operable to pivot the ball valve element about the shaft axis between the open and closed positions.
