PURPOSE: To provide an equipment for preventing fractionnement, which is fitted with a rotor for pressurized hydrodynamic purification of pulp generated from old papers in a purification machine in a well-known shape including a cylindrical screen.

CONSTITUTION: The rotor for pressurized hydrodynamic purification of pulp generated from old papers comprises a generally a longitudinal axis 6 of rotation and comprising a sequence of n pieces of identical cylindrical elements 8 arrayed side-by-side along the longitudinal axis 6. A wall of one side of the screen has a shoulder 14 on either side of which the wall winds spirally around the longitudinal axis 6 over at least 360 degree. The base and the top of the shoulder subtend a very slight angular gap Ω relative to the axis in such manner that the shoulder side 14 slopes toward the screen surface. The slope angle αrelative to the radial plane through the shoulder base is between 5 and 80 degree and the elements 8 are mutually shifted in the direction of rotation around the axis 6 by an angle β with a magnitude of at least 360 degree/n°, where (n) is at least 3.