On-line stress calculation and life monitoring systems for boiler components

Abstract

The increasing prevalence of two-shifting of power station operation can cause fatigue problems, particularly on boiler thick-wall pressure components such as headers and manifolds. To deal with the problems, a life monitoring system has been developed in which stresses within selected components are computed and displayed on-line to plant operators. Since the stresses affect the fatigue damage in the components, the operator can use this information to maximise the start-up rates on the plant and, provided the displayed stresses are within the prescribed limits, fatigue cracking will not occur. The most serious locations of fatigue cracking in a thick-wall component are at the crotch corners of the inner surfaces. However, the measurement of inner-wall temperature or strain is impractical. On-line methods have been proposed in this paper which calculate the inner surface temperature and thermal stress by using the temperature on the outer surface of the component or its tube legs. This can be easily measured. One is the recursive identification method in which the outer surface tempera tures are used to identify on-line the coefficients of the series solution of thermal diffusivity equation. The other method is based on Laplace transform through which the transfer functions between the fluid, inner and outer wall temperatures are derived. The comparisons of the simula tion results by these methods to the accurate results have shown that they are fast, reasonably accurate and suitable for uses in cost-effective stress monitoring and control systems.