stationary part preferably cylindrical and provided with at least one pulse means 14 the pulse means 14 are arranged upon rotation of the rotary screening means 4 to create pressure pulses for clearing the first screening means 4 the screening is performed from the outside in which is preferable due to the centrifugal force preventing large and heavy particles from being in close contact with the screening means this first screening stage can preferably perform the task of separating mainly larger impurities e g knots such a screening stage being commonly known as a knotter the reject portion i e the particles not passing through the screen is taken out through a reject outlet 15 this reject portion is also denoted the coarse reject in case the first screening means is a coarse screen the accept fraction is further fed to a second screening chamber 16 to form an inject to the second screening means 8 the pulp passing the second screening means 8 is taken out through an accept outlet 26 while the reject portion is taken out through at least one reject outlet 27 a secondary pulp inlet 17 is arranged so that pulp flowing through the secondary inlet is mixed with the accept portion from the first screening means 4 before entering the second screening means 8 the pulp entering the secondary inlet 17 consists of an accept fraction from a subsequent screening arrangement not illustrated in this figure in which the reject from the second screening means 8 has been screened again the accept portion of the subsequent screening arrangement together with the accept portion from the first screening means 4 in the screening arrangement 100 forms the inject to the second screening means 8 in this way the u201c good fibers.