Effect of Oxide Scale Deposition on the Residual Life of Service Exposed Reheater Tubes in Boiler of a Thermal Power Plant

Abstract

Boiler components in power plants have finite life because of prolonged exposure to high temperature, stress and aggressive environment. However past experience has showed that for a variety of reasons these may have significant remaining life beyond the design specification. This is best estimated by conducting a systematic life assessment exercise during a planned shutdown. In view of the incre-asing cost of setting up a new plant, there is now consi-derable interest in life extension of the existing units. In order to arrive at a quantitative estimate of the remaining life of such ageing components, it is necessary to have some hot tensile, creep and stress rupture data. This paper presents the high temperature tensile and the stress rupture properties of 150,000 hours service-exposed reheater tubes made of 2.25Cr-1 Mo steels in a 120 MW boiler of a thermal power plant. These were used to estimate the remaining life for safety. Experimentally determined yield strength and ultimate tensile strength as well as estimated 10,000 hours - 100000 hours rupture strength as obtained from experimental data in the temp-erature range of 520°C to 580°C exhibit a decreasing trend with increasing temperature. Effect of oxide scale deposition on the inner surface of the platen superheater tubes on the remaining life of the tubes has been disc-ussed. Microstructural study did not reveal any signifi-cant degradation in terms of creep cavities, cracks, graphitization etc. In general, analysis of tensile and stress rupture data reveal that the service exposed reheater tubes can remain in service for a length of more than ten years at the operating hoop stress level of 40 MPa /540°C, provided no localised damage in the form of cracks or dents have been developed. It is recommended that a similar health check should be carried out after 50,000 hours of service exposure at 540°C.