Maintenance platform built upon retractable support beams

A vapor generating unit having waterwalls that enclose an upright combustion chamber, a nose projection in one of said waterwalls inclined to extend obliquely into said chamber to form a restriction between upper and lower parts thereof, a superheater in the upper part of the combustion chamber, an opening in said waterwall forming the nose projection, a platform adapted to traverse the opening and extend across the restricted portion of the combustion chamber, said platform including an elongate beam, and support means on the inclined waterwall adapted to support said beam for lateral movement through said opening from a position outside the combustion chamber to a position that bridges the restricted portion thereof.