The invention relates to an attachment ring for attaching an object in a flow channel. The attachment ring has a first surface for contacting the object and a second surface for contacting a surface of a shoulder. An expansion member expands the attachment ring by pushing the ends away from each other when the expansion member is moved into the opening. At least one of the second surface of the attachment ring and the surface of the shoulder is inclined in order to wedge the attachment ring between the surface of the shoulder and the object when the attachment ring is expanded by the expansion member.