Abstract: An apparatus for sealing an orifice is disclosed. The apparatus includes a shutter and an annular seat. The shutter includes a shutter body and a soft peripheral gasket. The shutter has a closed position in which the gasket is laid against the annular seat, in order to ensure sealing between a medium located on the same side of the seat at a given pressure and a medium located on the opposite side at a higher pressure. The seat is associated with the pressure compensator acting on the shutter in its opening direction under the action of a hydraulic fluid, the thrust of which is adjusted so as to correspond approximately to the force acting on the shutter in its closed position and resulting from the pressure differences prevailing either side of the shutter.