Abstract
An important issue for a fossil power plant is maintaining its reliability and safety against frequent start-ups and load changes. Unstable states arising during start-ups, shut-downs, and load changes give rise to unsteady temperature distribution with respect to time in steam turbine components. Thermal stresses caused by the rapid increase in temperature make the components susceptible to failure and reduce their remaining life. In particular, HP and IP casings undergo frequent cracking due to thermo-mechanical low-cycle fatigue at the nozzle fit corner radius or other stress concentration shapes. Thus, for effective maintenance, it is necessary to have accurate knowledge of transient thermal stresses at the critical position that is susceptible to failure. In this paper, thermal stress concentration factors for the inner surface of the casing and valve are defined to account for variations in the geometry using three-dimensional finite element analysis. In addition, the total strain-range is obtained to assess the fatigue life according to the life assessment procedure in Korea. Using this study, the life consumption of steam turbine inner-casing was obtained, and a guideline for effective maintenance is proposed.
