A Geometry-Adapting Methodology for the Numerical Investigation on Flow-Driven Erosion Processes

Erosion can cause failures in piping systems and equipment in all types of facilities, often in fossil or nuclear power plants. These kinds of failures are a dangerous hazard, and additionally, they are an unwanted and unplanned interruption of an operating system. Reducing the risk for everyone involved and, at the same time, reducing the maintenance time, is a typical engineering task. Therefore, reliable tools for predicting erosion locations and their magnitude are necessary to achieve the engineering target.

In this work, a methodology is presented and tested which aim is to predict and model flow-driven erosion processes over time. The prediction over time happens in an iterative workflow, where the geometry is adjusted at each time step in accordance with the simulated erosion rate. Hence, erosion over time can be simulated, and possible self-reinforcing or dilution processes, due to a geometry change, can be investigated. The erosion processes in question are particle erosion and Flow Accelerated Corrosion (FAC). FAC was investigated in water systems but also heavy liquid metals. These fluids are promising working fluids because of their high heat conduction capacity, feasibility for high temperatures at atmospheric pressure, and their favorable neutron physics in the nuclear field, but they have a high erosion potential.
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With the proposed methodology, the modeling of the FAC is able to predict the magnitude and the location of the highest wall loss to a satisfactory degree. Possible reasons for differences of the predicted and measured distribution of the wall loss are discussed.

The heavy liquid metal investigation gave good predictions of the wall loss by using the proposed methodology. It overpredicted the measured wall loss by a factor of only 1.4, whereas prediction from the literature overpredicted the wall loss by a factor of 2.4.

Additionally to the FAC investigation in piping systems, this work also studies FAC minimization in pump impellers. A numerical pump investigation set-up was established and validated. As working fluids, water and the heavy liquid metal lead-bismuth eutectic (LBE) were compared. The Wall Shear Stress (WSS) was taken as an indicator for enhanced FAC. Compared to water, the calculated WSS distribution was shifted by a factor of ten in LBE. The influence of different geometry parameters on the WSS distribution was also discussed. The parameters in question were the number of impeller blades and the outlet angle of the impeller blade. It could be shown that for higher outlet angles and fewer blades, the averaged WSS of a single blade could be reduced significantly. This change of the geometry has to be done carefully because both arrangements lead to a flatter pump characteristic. Impellers with flatter characteristic curves have often problems of recirculations near the design points. Recirculation phenomena were investigated with the given numerical set-up.

Particle erosion was also investigated in a 90° elbow geometry with the proposed methodology. In contrast to the FAC investigations, where the overall erosion is diluted by itself over time, a self-reinforcing process could be depicted. In the geometry adapting process, a pocket is formed at the location of the highest erosion. With this pocket, the magnitude of the erosion rate was predicted accurately in comparison to the measured erosion rate.