Turbulent Buffeting of Helical Coil Steam Generator Tubes

Abstract

Turbulent buffeting is the most common flow-induced-vibrating mechanism in the nuclear power industry since turbulent flow is present in virtually all power plant components and will always apply a random pressure to the surface of the structures wherever there is a flow over, along, inside or cross a structure. For each specific component subjected to turbulent buffeting, the applicable potential degradation mechanisms, such as wear, fretting, or fatigue must be identified so that appropriate evaluations can be performed. For SG tubing, the two degradation mechanisms associated with turbulent buffeting response are tube-to-support wear and tube fatigue, both of which can be evaluated using the root mean square (RMS) response of turbulence induced vibration. In this paper, a methodology is proposed and implemented using ANSYS APDL command objects to estimate the RMS response for multiply supported SG tubes, such as the helical coil SG tubes in the NuScale design. As expected, the random vibrations due to the lower velocities associated with the natural circulation design result in lower RMS responses, which help prevent wear and fatigue failures in reactor module components, such as SG tubes. The number of modes required to adequately capture the RMS response is examined. The cross-modal contributions to RMS response are also examined to provide a justification to exclude the cross-modal terms in future evaluations. In order to capture the dynamic behavior accurately, the hydrodynamic mass and mass inside the helical tubes are fully accounted for in the total effective mass along the SG tubes.