Abstract: The sleeve is inserted into the passage of the valve casing and fastened to the valve casing by means of a bayonet connection at one end and by means of a support arrangement at the other end. The fastening collar embedded in the sleeve body has projecting lugs. The latter can be moved through grooves on insertion with the sleeve into the valve casing. The holding collar has a flange which on the insertion of the sleeve comes to bear against the support surface of the valve casing. When the sleeve is then turned, the lugs engage behind the holding surface of the valve casing. This permits extremely simple installation and replaceability of the sleeve.