The device comprises a housing (1) with a curved side wall, said housing forming a vortex chamber and having an inlet opening (2) and an outlet opening (3), means (15) being provided at the inlet opening (2) for fastening a flow controlling element (7), and which device comprises a discharge pipe (5) connected with the outlet opening (3) of the vortex chamber. In the discharge pipe (5) a by-pass is provided which may be closed by means of a closing member (11) and which has a flow capacity so dimensioned that it does not at expected maximum liquid pressure substantially exceed the flow capacity of the vortex chamber.