The purpose of this study is to evaluate the design verification of the welded type 45° lateral tee for the steam pipe in power plants. For it, first, the stress analysis was carried out under design condition in accordance with ASME Sec. VIII Div. 2 in order to evaluate the possible occurrence of plastic collapse and local failure. And next, the creep-fatigue damage analysis was performed under the normal operating condition in accordance with ASME Sec. III Subsection NH considering the service temperature of 566°C. From the results, it was found that the welded type 45° lateral tee satisfies the design criteria corresponding to the plastic collapse and the local failure. However, it has a probability of creep rupture during the design life due to the high stress localized in the crotch region. Therefore, a welded type 90° lateral tee was also evaluated with the same analysis procedures to consider the influence of the geometry at the crotch region. Based on the results, the welded type 90° lateral tee satisfies the design criteria of the plastic collapse, local failure and the creep-fatigue strength. This result indicated that an optimal shape design of the crotch region shall be required in order to secure the creep strength of the welded type 45° lateral tee having high service temperature.

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