Uncertainties in the thermal fatigue assessment of pipes under turbulent fluid mixing using an improved spectral loading approach

Abstract

This paper proposes improved thermal fatigue assessment of pipes subjected to turbulent fluid mixing using an improved spectral loading approach. The fluid temperature histories are generated synthetically from the spatially incomplete experimental or very expensive computational data, preserving consistency with the first two statistical moments and power spectral densities of the measured/computed temperatures. This enables a variety of affordable and physically adequate fluid temperature distributions. These have been used in a novel and rather straightforward analysis of the uncertainties involved in the calculated fatigue life times.The proposed thermal fatigue assessment procedure has been fully developed for the equi-biaxial stress fields on the pipe surface. This rather realistic assumption allows for a simple one-dimensional model of the pipe with numerically resolved time-dependent temperatures and analytical expressions for the linear elastic wall thermal stresses varying only in the radial direction. The fatigue lives are predicted for diverse variations in fluid temperatures following the ASME Nuclear Boiler and Pressure Vessels codified rules for varying principal stress direction, Rainflow counting algorithm, linear damage accumulation and the NUREG/CR-6909 design fatigue curve.The results of the proposed method are less conservative than results of similar methods in the literature. This is expected, since the proposed method accounts for a much wider variability and more physical description of the fluid temperatures. Further, the reduction in the conservatism is effectively compensated through the ability to assess the uncertainties inherent in the calculations of fatigue life time. The proposed assessment could be straightforwardly extended to other fatigue design rules and, with the use of finite elements, to possibly nonlinear behavior and discontinuous shapes of the pipes. In this way the proposed improved thermal fatigue assessment of pipes could facilitate further development of currently very scarce and simple design rules for thermal fatigue.