The subject of considerations is a spherical bifurcation pipe of a live steam made of steel P91, which is an element of a block of coal-fired power plant working with a 18K370 turbine. As experience shows, it is a very sensitive element of the boiler pipelines. An extreme work scenario for such a block has been adopted, in which the turbine is shutting down to a warm state three times in 24 hours. This is an action dictated by new challenges in the field of electricity network regulation, caused by increasing share of renewable energy sources. A one-sided numerical thermal-FSI analysis was performed. The focus was on hoop stresses as the most significant for the bifurcation pipe durability. The daily runs of these stresses at three points of the thickness of the bifurcation pipe sphere have been presented. Mechanical stresses derived from pressure and thermal stresses derived from temperature changes have been isolated. It has been shown that depending on the thermal load, some areas of the cross section are compressed while adjacent ones are stretched and vice versa. Thus, the mechanical stresses can be reduced under thermal conditions by thermal stresses. It has been proven that the bifurcation pipe is able to withstand the given extreme loads with stresses more than twice smaller than the yield point at a given operating temperature.
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