Process and apparatus for continuously producing olefin polymers in a slurry phase reactor in a hydrocarbon diluent or liquid monomer. The process comprises continuously withdrawing from the slurry phase reactor a polymer slurry containing polymer and a fluid phase, which contains hydrocarbons and optionally hydrogen, and concentrating the slurry with a self-cleaning screen by removing a part of the fluid phase to provide a concentrated slurry. The openings of the screen, which can have a planar or cylindrical configuration, are smallest at the inflow surface of the screen and increases towards the outflow surface. By the present invention, the need for service and maintenance is significantly reduced.