The invention relates to a device for separating a flowing liquid into at least two fractions with differing mass density, comprising: a main tube in the form of a cylinder jacket and having an infeed opening and an outfeed opening; a first swirl element arranged in the main tube for imparting a rotating movement to the fluid flowing through the main tube; a first auxiliary tube placed in the main tube at a distance from the first swirl element; and first discharge means connecting to the jacket of the main tube for discharging a heavier fraction of the fluid. The invention also relates to a method for separating into fractions a fluid comprising several fractions.