Mass transfer in back-to-back elbows arranged in an out of plane configuration

Flow-Accelerated Corrosion (FAC) is a major degradation mechanism affecting carbon steel piping systems in nuclear power plants (NPPs). Flow and mass transfer conditions determine the local distribution of wall thinning, even though chemistry and materials determine the overall potential for FAC. Different localized thinning rates in back-to-back elbow configurations between the first and second elbows have been noted at NPPs, and this difference depends on the distance between elbows, flow conditions, and the configuration of the back-to-back elbows (S-, C-, or out of plane). This paper will focus on mass transfer measurements for back-to-back elbows arranged in an out of plane configuration for different elbow separation distances under single-phase flow conditions. The mass transfer measurements were performed using a mass dissolution technique of gypsum test sections in water. The experiments were performed at a Reynolds number of 70,000 and a resulting Schmidt number of 1280, which is similar to that for the diffusion of the iron magnetite layer of carbon steel piping in water, providing a mass transfer environment analogous to that in NPPs. Experiments were performed with 0, 1, 2 and 5 pipe diameters in length between the elbows. The mass transfer results show regions of higher mass transfer in the second elbow in comparison to the first elbow. The maximum mass transfer enhancement factor decreased from 2.7 to 2.1 as the separation distance increased from 0 to 5 pipe diameters. Flow streaks on the second elbow surface indicated swirling flow and its strength decreased with increasing separation distances. The relative roughness in the upstream pipe was found to be 0.003–0.004. The roughness level in the second elbow is 1.5 times higher than the upstream pipe and decreases with increasing bend separation distance.