Failure Analysis of High Strength T91 Boiler Tubes used in Thermal Power Plant: A Case Study

A power generating utility of 600 MW has reported the failure of boiler tubes after a service life of 265 hours against the design life of 25-30 years. The failure of tubes was reported in Re-heater Inlet Coil tubes which were made up of high strength material alloy of SA 213 T91 grade. These boiler tubes were operating under a high temperature and pressure. The material used for re-heater and super-heater boiler tubes should have enough creep strength to sustain at high temperature and pressure. Even though T91 grade steel has superior quality of metallurgical properties in comparison to low alloy existing material like T11, T12 and T22, T91tube was failed in a short duration. This paper describesthe investigation of the failure of high strength T91 gradereheater tubes in a coal fired thermal power plant. The failure investigation covers visual inspection, dimensional measurement, hardness mapping, oxide scale measurement, microstructural studies, and SEM and EDX analysis of the failed tubes. The analysis reveals that Tube No-3 of re-heater boiler tube failed due to short term overheating and other tubes failed due to secondary damage caused by steam impingement.