The valve has two flaps (2) which during opening are moved from their closed position while maintaining a seal at their sealing points (2a) against the inner wall of a central housing section. They are initially coaxial with the pipe and in opposite directions until they form a unit (2) holding the leakage chamber (16). The unit is rotatably fitted directly or indirectly in the housing section (1a) through two coaxial spindle pins (5a) set on opposite sides of the unit and is brought by the bearing into the open position. The flaps can each bear by their circumference on a support ring (5) set between the flaps.
