Remaining lifetime assessment of power plant steam boilers

The energy producing power plants are designed for operational period of 20, 30 years. During this period, inspections are realized to investigate the operational capability of the respective components and the plant as a whole, and when the designed time is approaching its limit, the crucial questions are raised with respect to the following possible operation, its safety and risks that stem from the fact that the continuous degradation of material properties occurred during the longtime service as a result of service conditions, e.g. high temperatures, fatigue loading etc. In opposite to the non-destructive techniques, accelerated creep to rupture tests of high temperature boiler components, e.g. high temperature headers, can give quantitative results as far as the remaining lifetime of the component is concerned. Several steam turbine boilers were inspected according to the customer's demand to evaluate the remaining lifetime of the boilers that were operated more than 160 000 and 200 000 hours respectively. The evaluation was based on an extensive NDT expection and the measurement of mechanical properties (including creep test data) of high temperature components. Making use of the Larson-Miller parameter in comparison with replica testing made it possible to evaluate quantitatively the lifetime exhaustion, to make an assessment of the remaining lifetime and to make a recommendation as far as the future inspection intervals of the boilers are concerned. On the basis of accelerated creep test data performed on the degraded materials, the remaining lifetime hours were calculated for the three "safety" situations : 1. "ZERO SAFETY" (neither recommended k = 1,5 safety coefficient for working stress nor +70 degrees C increase of working temperature were taken into consideration). 2. "STRESS SAFETY" (1,5 safety coefficient for working stress and real working temperature were taken into consideration). 3. "FULL SAFETY" (both 1,5 safety coefficient for working stress and working temperature + 70 degrees C were taken into consideration). At the Baltica VII conference, an example of this procedure was presented [1]. This paper summarizes the results of several more boilers and the results of the remaining lifetime calculation are discussed.