Torsional Vibration In Pump/Driver Shaft Trains - The Role Of External Damping From Pump Impellers

Abstract

Consideration of torsional vibration in pump/driver shaft trains has become of increasing importance with the development of electronically controlled variable speed electric motors. Such motors not only provide a static driving toque, but develop superimposed continuous torque pulsations. The frequency of these torque pulsations varies with speed, such that typically resonance situations with a torsional natural mode at distinct speed cannot be avoided. Whether such resonance situations can be sustained during extended time periods without endangering the shafts by fatigue depends primarily on the total amount of damping available in the shaft train. Besides material and structural damping, which are internal damping sources, pump impellers are expected to provide external damping by convective energy dissipation into the pumped fluid. From literature, predictions of impeller damping on a theoretical basis are known, but almost no information about an experimental identification of torsional damping coefficients is available. The authors describe the development of a test rig for the identification of torsional damping and added mass moment of inertia of pump impellers. To find a suitable method to excite the test shaft train within a sufficiently large frequency band was the main difficulty to solve when developing the test rig. Test results are shown and compared with theoretical predictions. Application on an installation with a variable speed motor is described and conclusions with respect to shaft safety against fatigue are drawn.