Creep Behaviour and Stress Analysis of a Seventeen-Year Service-Exposed Primary Super Heater Tube in a Thermal Power Plant

Abstract

The elastic, inelastic deformation and stress distribution pattern of a seventeen year service-exposed primary super heater tube in a 120 MW boiler of a thermal power plant have been evaluated by using both analytical and numerical techniques. The commercial finite element computer code ANSYS^® was used for stress analysis. The methodology is a valuable design tool for development of new boiler tubes as well as for considering the effects of numerous operating variables on creep life. The health of the tube was also assessed based on microstructure, hardness and a few conventional creep tests carried out at 500 °C at various stress levels (40 to 177 MPa). Results revealed that there was not much variation in the microstructure and hardness of the service-exposed tubes compared to the virgin material. Creep deformation behaviour of the service-exposed and virgin tubes of the same material at 40 MPa revealed that deterioration of the creep properties fell within the 20% scatter band, which is well within the specified limits of ASTM standard. The service-exposed primary super heater tube is thus in a good state of health.

The elastic, inelastic deformation and stress distribution pattern of a seventeen year service-exposed primary super heater tube in a 120 MW boiler of a thermal power plant have been evaluated by using both analytical and numerical techniques. The commercial finite element computer code ANSYS^® was used for stress analysis. The methodology is a valuable design tool for development of new boiler tubes as well as for considering the effects of numerous operating variables on creep life. The health of the tube was also assessed based on microstructure, hardness and a few conventional creep tests carried out at 500 °C at various stress levels (40 to 177 MPa). Results revealed that there was not much variation in the microstructure and hardness of the service-exposed tubes compared to the virgin material. Creep deformation behaviour of the service-exposed and virgin tubes of the same material at 40 MPa revealed that deterioration of the creep properties fell within the 20% scatter band, which is well within the specified limits of ASTM standard. The service-exposed primary super heater tube is thus in a good state of health.

On a évalué la déformation élastique, inélastique et le patron de distribution des contraintes d'un tube primaire pour surchauffeurs de dix-sept ans d'usage d'une chaudière de 120 MW d'une centrale thermique en utilisant des techniques analytiques et numériques. On a utilisé le code d'ordinateur commercial d'analyse par éléments finis ANSYS^® pour l'analyse des contraintes. La méthodologie est un outil valable de conception pour le développement de nouveaux tubes de chaudière ainsi que pour considérer l'effet de nombreuses variables d'opération sur la vie de fluage. On a également évalué la santé du tube en se basant sur la microstructure, sur la dureté et sur quelques essais conventionnels de fluage effectués à 500 °C à des niveaux variés de contrainte (40 à 177 MPa). Les résultats ont révélé qu'il n'y avait pas beaucoup de variation dans la microstructure et la dureté des tubes usagés en comparaison avec le matériau vierge. Le comportement de déformation de fluage des tubes usagé et vierge du même matériau à 40 MPa révélait que la détérioration des propriétés de fluage tombait à l'intérieur de la bande de dispersion de 20%, ce qui est bien dans les limites spécifiées de la norme ASTM. Le tube primaire usagé pour surchauffeurs est donc en bonne santé.