The door of a horizontally disposed pressure vessel is adjustably supported by a track mounted trolley for reciprocation between door closed and opened positions. The door is provided with apertures for freely receiving trolley mounted pins, which are employed to support an outer end of a material supporting carriage movable from within the pressure vessel as the door is moved towards its opened position. The apertures are sealed by flexible metal bellows. Independent movements of the carriage and door relative to the trolley permit the door to be properly seated in a closed condition under varying carriage loading and/or pressure vessel thermal distortion conditions.