Noise reduction processing for work-rate computation of a vibrating steam generator tube

The objective of this paper is to describe appropriate noise reduction processing to characterize the vibration behavior of U-tubes in steam generators of nuclear power plants. Two-phase cross flow in the U-tubes section of steam generators is a source of flow-induced vibration of U-tubes, causing impacts or rubbing of the tubes against their anti-vibration bars (AVB). The resulting in-plane and out-of-plane motions of the tubes are causing fretting-wear and impact abrasion. Since fretting-wear work-rate is used to predict steam generators U-tubes service life, suitable noise reduction processing is necessary in order to evaluate the work-rate accurately. In this experimental study, a 2,5m tube is simply supported at both ends and interacts with a fixed flat bar at mid-span. Flow-induced excitation forces are generated with narrow band random noise around the first and second modes of the tube. The excitation frequency and magnitude, and the clearance or preload between the tube and the AVB are controlled parameters. The time responses of the tube displacement and the contact force at the AVB are measured and multiplied together to compute the work-rate. Due to poor signal-to-noise ratios, appropriate noise reduction is required to avoid large errors. Filters are used to reduce the electromagnetic and the mechanical noise originating from the measuring system components and the supporting structure. Electromagnetic noise is eliminated from the signals by applying upper and lower noise thresholds, which are estimated from the sensors noise levels. But-terworth band-cut filters are used to attenuate the mechanical noise at specific frequencies. Finally, fretting-wear work-rate is computed from the processed time signals. The results are discussed in this paper.