In 2013, Atomic Energy of Canada Limited (now Canadian Nuclear Laboratories) took vibration damping measurements to examine low-frequency damping associated with vibration of a tube parallel to Anti-Vibration Bar (AVB) supports.&nbsp; These tests were performed to better understand and control in-plane fluidelastic instability of tubes in the U-bend region of &nbsp;recirculating steam generators.AVBs are also referred to as flat-bar U-bend restraints or simply flat bars.&nbsp; These supports are used to restrain tubes in the U-bend regions of many recirculating steam generators.&nbsp; The bars are aligned in the plane of the U-tubes to allow for thermal expansion of the tubes.&nbsp; As a result of their alignment, these bars restrict out-of-plane tube movement but do not absolutely prevent in-plane movement.&nbsp; If static friction between tubes and AVBs is insufficient to prevent in-plane tube vibration, then the outer U‑tubes in large steam generators can vibrate at natural frequencies of less than 5 Hz.&nbsp; Under these conditions, AVBs only control in-plane vibration by adding support damping.&nbsp; A correlation commonly used to predict steam-generator tube damping indicates that low-frequency damping varies as the inverse of frequency and, therefore, becomes quite high at low frequencies.&nbsp; This low-frequency behaviour is attributed to the squeeze-film damping that is associated with drilled-hole and other types of tube supports.&nbsp; The tests preformed by AECL were largely designed to examine the low-frequency in-plane damping behaviour associated with AVBs. &nbsp;&nbsp;In the tests, the damping ratios of a single steam generator tube vibrating parallel to a single pair of AVBs were measured using both a log-decrement and a power-based method. &nbsp;&nbsp;Non-contacting excitation and position-sensing techniques were employed to improve accuracy.&nbsp; The tests examined the effects of fluid (water or air), natural frequency, gap width, preload, and vibration normal to the bars.&nbsp; Additional tests were also done using a drilled hole support and without supports to allow validation against previously published work.&nbsp;This paper describes the tests including the test apparatus, test methods, and analysis techniques. &nbsp;A summary of the results is presented.&nbsp; These results show that the damping results measured with a drilled-hole support and without any supports are consistent with previously published work and with the damping correlation under considered.&nbsp;&nbsp; However, the AVBs resulted in no additional viscous or squeeze-film damping for vibration parallel to the flat bars.&nbsp; Therefore, the in-plane viscous-type support damping caused by AVBs would be significantly over-predicted by the damping correlation.&nbsp; However, the results also showed that AVBs can introduce significant in-plane Coulomb-type damping as a result of friction and impacting. &nbsp;

Measurement of Steam-Generator Tube Damping Caused by Anti-Vibration-Bar Supports

Paper Proceedings: https://asmedigitalcollection.asme.org/PVP/PVP2021/volume/85314

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