Spalling of steam-grown oxide from superheater and reheater tube steels. Technical planning study 76-655. Final report

This technical planning study examines world experience outside the U.S. on the spalling of steam-side oxide scale from austenitic and ferritic superheater and reheater tube materials with special emphasis on the study of this problem being conducted in the U.K. The physical properties of the oxide scales are reviewed and expansion coefficient data are used to estimate the cooling strains in the oxide for various alloys. Other contributory sources of strain are discussed and models are presented for the failure behavior of oxide layers both in tension and in compression. A method of predicting quantities of debris release is described. The hydrodynamic behavior of spalled oxide flakes in steam circuits is considered analytically and the conclusions are used to construct a method for assessing the risk of tube blockages. Attention is drawn to the importance of the kinetic energy of the debris particles in affecting wear rates on the eroded circuit components. The concluding sections of the study examine the various palliative operational and design solutions to prevent turbine erosion and tube blocking, and highlight the areas where further study of the problem are required.