The invention relates to an apparatus for treating a fluid having at least one treating zone, with a fluid inlet and separation device in it for introducing a separated mixture of liquid and gas into the treating zone. The fluid inlet device has a series of coaxially disposed circumferential vanes, in which each circumferential vane curvedly extends between an intercepting part having a substantially axially directed leading edge and a deflecting part having a substantially radially outwardly extending trailing edge, and wherein the central axis of the inlet device extends parallel or coaxial with an upwardly extending axis of the treating zone. The invention further relates to a fluid inlet device, to a use of the fluid inlet device, and to a method for treating a fluid.