Minimizing unbalance response of the CRBRP sodium pumps

The unbalance response characteristics of the vertical pumps for the Clinch River Breeder Reactor Plant are investigated. Finite-element shell and beam models representative of the pump-motor structure including the rotating assembly are developed to assess structural stiffnesses of dominant joints as well as the foundation support stiffness so as to exclude the danger of resonant excitation during normal operation. Less than four mils peak-to-peak vibration amplitude at the pump tank discharge nozzle results from just 10% frequency separation between the first rocking mode and the maximum operating speed of 1116 RPM, based on 0.5% modal damping ratio and balance quality grade of ISO/ANSI G2.5 for the rotating components: motor rotor, pump shaft, Bendix diaphragm-type flexible coupling, and centrifugal double-suction impeller. Several design options are explored for raising shaft critical speed beyond 125% of maximum operating speed. NASTRAN complex eigenvalue solution is performed to take credit for damping in bearings to help meet this requirement. The mitigating effect of increasing shaft critical speed on both unbalance response and seismic response is evaluated. The influence of horizontal foundation stiffness on shaft whip is demonstrated, apart from staggering lateral modes in an effort to minimize dynamic coupling between them; higher the stiffness, less the shaft whip expressed as the peak lateral response along the vertical shaft. Parametric curves are presented illustrating the extent to which the fundamental rocking mode frequency is influenced by the combined rotational stiffness of the pump foundation and closure flange assembly. The fundamental rocking mode is raised high enough to become less sensitive to the foundation stiffness. (orig.)