Results of examination of the TGMP-314 boiler superheaters of the power-generating units at Kashira state district power plant using a magnetic ferrite meter

Abstract

The results of investigating the elemental composition of the scale and the metal of a tube circuit from the austenitic steel grade 12Cr18Ni12Ti are presented. The superheater is part of the high-pressure convection steam superheater of a TGMP-314 supercritical-pressure gas-and-oil-fired once-through boiler that had been operated for a long period of time. A structurally transformed layer, poor in iron, manganese, and chromium and rich in nickel was detected on the outer surface. The layer consisted predominantly of the FeNi3 phase with ferromagnetic properties. The heat-resistance test of the steel in contact with ash that simulated the combustion products of fuel oil showed that the higher the temperature and the longer the test time, the higher the content of the ferritic phase in the layer was. The established pattern of the structural transformations underlies a method for nondestructive control of the thermal nonuniformity and detection of “worst” tube circuits of superheaters from austenitic steel. The magnetic ferritometry complements the conventional selective thickness gauging that does not characterize the condition of the heating surfaces of hightemperature steel grades to the full extend. Data on damageability of high-pressure convection superheaters and low-pressure second-stage convection superheaters with rarefied tube banks of TGMP-314 boilers are presented. The damage is caused by overheating resulting from the nonuniform temperature field at the inlet and by the nonuniform flue gas velocity field in rarefied superheater banks. Sections of the tube circuits from the steel grade 12Cr18Ni12Ti, outlet superheater stages of the TGMP-314 boiler of the power-generating units at Kashira SDPP were examined using an MF-51NTs AKASKAN magnetic ferrite meter. Thermal nonuniformity was established and the “worst” superheater tube circuits were detected. It was shown that the zones with the “worst” and damaged tube circuits coincide. The results of examining the superheaters with the help of a magnetic ferrite meter, of dimensional control, and of metallographic investigation of the samples cut out of the tube circuits are in agreement with each other.