PSI - Issue 60

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Rahul Mittal/ Structural Integrity Procedia 00 (2019) 000 – 000

Rahul Mittal et al. / Procedia Structural Integrity 60 (2024) 604–613

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4. Methodology for Fatigue evaluation: Fatigue analysis is divided in two parts viz. Thermal Transient Analysis and Static Structural Analysis. In thermal transient analysis, all thermal boundary viz. bulk mean temperature with respect to time, heat transfer coefficient and reference temperature are applied on the component. These analyses give temperature mapping with respect to time within the component or across the thickness/sections. This analysis was carried out for all applicable process transients. In static structure analysis, temperature distribution as obtained from the thermal transient analysis and pressure along with nozzle loads are applied along with the structural boundary conditions and stress intensity have been evaluated for selected sections for every time steps. Based on this, alternating stress and corresponding allowable numbers of cycles are calculated which are further used in calculating fatigue usage factor. Detailed fatigue analysis has been carried out as per ASME Section III Div-1 subsection NB requirements. The alternating stress range and stress amplitude have been calculated for different operational transients. The alternating stress amplitude due to each operational transient is used with the fatigue curve for obtaining maximum number of permitted stress cycles. Fatigue life usage factor is calculated for the given number of cycles for each transient. Thus, Cumulative Fatigue Usage Factor (CUF) has been calculated for each component considering various operational transients. For a typical Stress Classification Line procedure given in NB3216.2 is used for calculating the Usage Factor and Cumulative Usage Factor. Which require evaluation of six stress components for a SCL versus time for the complete stress cycle. If calculated CUF is found to be less than 1 for a component, fatigue failure under cyclic loading considering all operating transient loops is not envisaged as per NB 3222.4. 5. Model Description and Material properties: Feed water assembly along with Steam generator shells is considered for analysis. Refer Figure 3. Meshing is performed using 20 node brick element Solid186, minimum 8 elements have been taken across the thickness shell and nozzle crotch region. Refer figure 4 for the Meshed FE model. The mesh convergence study is performed to arrive at final mesh size and nos. of elements across the thickness. Refer Figure 4 & Table 2 for mesh convergence study. The FE model consists of 366376 elements and 417641 nodes. Temperature dependent properties of Low alloy steel for Shell and FWN are considered for analysis. Properties are referred from ASME Section II Part D and considered as homogenous & isotropic.

Fig. 3(a). CAD geometry with shell- Cross section Fig. 3(b). CAD geometry front view Fig. 3(c) 2D-Cross section of Nozzle