A sealing apparatus adapted for use in a pressure vessel such as a synthesis gas cooler. The sealing apparatus is of a segmented plate construction formed around an outer wall section of a conduit means of the pressure vessel that defines at least part of a passage for receiving effluent from a gasification process. Pressure responsive mechanisms are provided on the plate segments to maintain the pressure difference across the sealing apparatus within the acceptable operating limits as well as to permit instantaneous pressure release to prevent damage to the pressure part assembly or cage. The seal is also maintained continuously through the differential growth movement between the conduit means and the pressure vessel during heat up and cool down cycles.