A pulper comprises a rotor (5) mounted in a tub adjacent to a screening plate (4) having holes (4a). The rotor (5) includes helical vanes (5a) whose diameter decreases towards the outer end of the rotor. The rotor vanes (5a) carry in the vicinity of the screening plate a radially and outwardly projecting shoulder-like element (10) having a leading surface (10b) which is generally flat and inclined relative to the screening plate (4) at an angle of 55°-85°, and a trailing part having a reversed wing-profile shape as seen in the direction of rotation. The element forms a pressure-generating and subpressure-generating means which exerts an alternating effect on the pulp as the rotor rotates, therewith counteracting clogging of the holes (4a) in the screening plate. The shoulder-like element (10) may comprise a completely or partially separate element in relation to the rotor, and may be detachable therefrom and replaceable, and also optionally reversible.