CREEP-FATIGUE ANALYSIS FOR REMAINING LIFE ASSESSMENT OF STEAM TURBINE BLADE USING NUMERICAL SIMULATION AND EXPERIMENTAL TESTING

Abstract

Due to the rapid growth of the global economy and the rising need for electric energy, there is an urgent requirement to enhance the power output and efficiency of steam turbines. Blades, which serve as the crucial element of a steam turbine, have a direct impact on the efficiency and safety of the equipment. The phenomenon of water droplet erosion (WDE) is a significant factor in the working conditions of turbine blades, and it has a detrimental impact on the lifespan of the blades. Because of the intricate nature of turbine blade operating conditions, which encompass blade vibration, material erosion, and liquid-solid interaction, properly forecasting their useful lifespans using theoretical calculations is challenging. The final stage blades of steam turbines consistently operate inside the wet steam region and are susceptible to the influence of WDE (Water Droplet Erosion) or wet steam erosion, which has two negative effects. Firstly, it increases the losses and decreases the efficiency of the turbine stage. Secondly, it can induce blade breakage, posing a risk to the safety of the equipment. Currently, the utilisation of ultralong blades and the expansion of working conditions in new steam turbines have presented significant challenges to the resilience of blade materials against water droplet erosion (WDE). To guarantee the secure functioning of the steam turbine, it is necessary to halt the equipment for maintenance as per schedule. Observing the worn surface of the blade poses a challenge in determining the blade's potential for continuing use. Due to the economic implications and concerns over operational safety, it is challenging to ascertain the necessity of replacing the deteriorated blades. Simultaneously, due to the blades operating at high speeds, temperatures, pressures, and within a confined environment, it is not feasible to visually examine the surface characteristics and wear debris accumulation of the blades when they are in normal operation. Hence, it is crucial to ascertain the remaining useful life (RUL) of the blades based on the erosion morphology of the blades during maintenance. Based on the literature survey, it is evident that there is limited study on the WDE life of turbine blades, and a comprehensive approach for predicting the WDE life has not yet been established. Previously, researchers predominantly relied on initiation crack life as the measure of useful blade life. However, in real engineering applications, the blade may still be utilised despite the presence of a WDE fault on its surface, as long as it does not pose a significant risk to the equipment's vii safety. This is done in order to reduce costs. Therefore, the fault diagnosis of the remaining useful life (RUL) of the blade is a more significant metric compared to the initiation crack life of the material. Theoretical calculations are conducted to assess the creep-fatigue damage by considering the degraded material characteristics. The failure probability is then determined by calculating the damage using a deterministic model and accounting for the dispersion in material property data. Material testing is conducted to detect both surface and volumetric cracks, as well as to assess micro-structural damage. This is achieved through the use of replica or microscopic tests. Crack presence testing is conducted on all available surfaces of the component, whereas microstructure damage and volumetric crack tests are specifically carried out at the most significant sites determined through theoretical calculations. Damage calculations serve the purpose of identifying essential regions for testing and providing crucial information regarding the following aspects: - The dominant damage mechanism at a certain location. - The extent of damage for the most unfavourable material properties. - The rate at which damage accumulates. This information is valuable for determining suggestions regarding component repair, inspection intervals, or modification of operating conditions. The significance of both creep and fatigue damage in the overall damage is crucial in terms of failure probability. The failure probability is influenced by the contribution of creep and fatigue damage for a given total damage.