A method and a rapper device for cleaning a heat exchange surface in a pressure vessel. The rapper device comprises a striker rod which has one end contacting the heat exchange surface in the pressure vessel and one end projecting out of the pressure vessel via a passage opening. The rapper device also comprises an impacting device for impacting the projecting end of the striker rod. A section of the striker rod is slideably supported in a pressure chamber adjacent the passage opening. The pressure chamber is connected to a sealing gas supply maintaining the pressure in the pressure chamber at a higher level than the pressure in the pressure vessel. A bellows defines a pressure chamber around at least a section of the striker rod.