Vibration of Steam Generator Tubes in Two-Phase Cross-Flow: Performance of Flat-Bar Restraints

Abstract

Two-phase cross flow exists in the U-bend region of recirculating-type steam generators. High flows or inadequate tube support can result in excessive flow-induced vibration. To allow for thermal expansion, a support system consisting of vertical flat bars is now favoured. In PWR steam generators, these are called anti-vibration bars, or AVBs. With this system, tubes are relatively free to move in the in-plane direction. The only resistance to motion is due to sliding friction if line contact between tube and flat bar is maintained. In this study, a small U-tube bundle test section with flat-bar restraints was subjected to water and air-water cross flow to determine the effect of tube-to-restraint diametral clearance on U-tube vibration response. The test program was initially designed so that the diametral clearance could be gradually reduced until the expected vibration mechanism (amplitude-limited fluid elastic instability) no longer existed. It was a surprise to find that, even at the largest diametral clearance (0.06 in. or 1.524 mm), the instrumented tubes did not attempt to go unstable at the mode associated with an ineffective support. Significant turbulence-induced excitation was found in both the out-of-plane and in-plane directions. This mechanism may be contributing to the fretting-wear damage that has occurred in U-bend regions of nuclear steam generators.