Damping of Steam Generator Tubes

The flow-induced vibration (FIV) analysis is one of the most critical evaluations needed to ensure long-term operation of steam generator. Original design hardware is usually based on experience and testing. Performance changes, such as an increase in power with increased feedwater flow, or tube repairs including plugs, stabilizers, and sleeves are best evaluated analytically. A critical factor in an FIV analysis is damping. The effective damping for a steam generator tube is a function of the structural support configuration, vibration amplitude, and the void fraction of the shell-side fluid. Numerous tests have been performed in the nuclear industry to determine appropriate damping ratios for the analysis of steam generator tube bundles. As part of the root-cause analysis for a recent plugged tube sever in a Once-Through Steam Generator (OTSG), Framatome ANP performed damping tests on tubes that were swollen due to internal pressure. Damping tests were performed on virgin tubes. The tests were repeated after swelling the tube into the tube supports and again after the swollen tube was severed and stabilized. Tests were performed with air or water inside the tubes. The tests results showed a damping ratio of 2% to 5% for both a virgin tube and a tube that was swollen and locked into the tubes supports. Stabilization increased the damping ratio to greater than 8%.