Abstract: In a centrifugal separator for instance for purifying oil from water and solids there is a central outlet for separated water, comprising an overflow outlet maintaining a free water surface at a certain level in the door. According to the present invention this central outlet may be closed by means of an axially movable slide (23), which is rotatable together with the rotor. The rotor also has a further overflow outlet (22a) situated radially inside said predetermined level (20) such that upon supply of a so called displacement liquid to the rotor--in connection with the opening of periphery outlets (21) from the separation chamber for discharge of separated solids--separated water cannot leave the rotor through its outlet (12, 15) for separated oil.

Abstract: In a centrifuge rotor provided with sludge channels (25) for conveying sludge radially inwards to a sludge receiving chamber (15), from which the sludge is discharged by means of a central sludge discharge means, the openings of the sludge channels (25) into the sludge receiving chamber (15) are controlled by a slide member (14) being actuated to move in one axial direction by spring means (28). To cause the slide member (14) to move in the other axial direction, a pressure chamber (18), sealed against the sludge receiving chamber (15), is arranged between the slide member (14) and the rotor, and a fluid channel (33) extends through the rotor to the pressure chamber (18), connecting the pressure chamber with a pressure source (35) located outside the rotor. A position regulator (32) is situated in a cavity of the rotor, and is arranged to operate directly on said slide member (14) for regulation of its position.