Abstract
The electric power plant technology for the utilization of heavy fuel oil rich in sulphur, vanadium and asphaltenes poses very particular difficulties for plant design and materials selection. The case study reported in this paper considers the high corrosion rate associated with carburization of SA-213TP347H stainless steel tubes of the platen zone of a secondary superheater bank of a 350 MW power unit. The tubes were in service during 42,700 hours in the environment of combustion products of unusually heavy fuel oil rich in sulphur, vanadium and asphaltenes. Flame enlargement by the high asphaltene content of the fuel oil and the upward tilting of the burners mandated by steam temperature requirements are believed to create conditions for flame-tube contact. Metallographic, microhardness and SEM-WDS analysis are reported of sections of a corroded tube representative of the platen zone. The corrosive process affected mostly the surface facing the flame radiation, even though carburization phenomena was present in both tube faces. The degradation of the tubes involved carburization and sulphidation processes which have been controlled by temperature. A softening process associated with the metal deterioration was detected and it was superimposed to the typical hardening due to carbides precipitation. Carburization in the zone of the tube opposite to the flame was hidden due to this softening process, and was found only after a detailed WDS microanalysis.
