Predicting the preferential sites to liquid droplet erosion of the bellows assemblies by CFD

Bellows assemblies are placed within extraction steam lines to absorb the differential displacement (driven primarily by thermal growth) between the turbine lower casing and the condenser shell in nuclear power plants. The thin wall thickness and service environmental conditions of the bellows predetermine their vulnerable endurance to the liquid droplet erosion (LDE). A three-dimensional computational fluid dynamics (CFD) bellows model proposed in this paper consists of a hydraulic model and a LDE model aiming to investigate the LDE on the external surface of the bellows assemblies. The result shows that the most prone site to LDE locates at the upwind side of the first bellows convolution (USFBC). The droplet diameter has significant effect on the LDE extent, rather than the LDE distribution. Moreover, the addition of the weld seam before the bellows can mitigate the LDE of the bellows downstream. In addition, the protecting cover (PC) effectively prevents the bellows from the LDE, but causes the droplet accumulation on the convolution, which possible leads to corrosive damage. The PC structure was designed optimization according to the requirements of the displacement compensation, the manufacture cost and the LDE resistance.