PL08 - Torsional Rotordynamic Excitation, Natural Frequency and Damping Characteristics in a Viscous Centrifugal Pump Application

Abstract

This lecture is concerned with the torsional vibration characteristics of a double-suction centrifugal pump handling a highly viscous product. This test case serves to verify engineering assumptions about torsional excitation and damping that are commonly applied when performing torsional analyses, but on which the margins of uncertainty are known to be significant. The experimental characterization is conducted in the test laboratory using oil at varying viscosities and measuring static and dynamic torque with a torquemeter. The steps of extracting information about the relevant sources of torsional excitation and the modal parameters (natural frequency and damping), starting from the frequency spectra of the dynamic torque, are illustrated. A torsional model is developed to correlate the measured torsional vibration response to the excitation source. The damped torsional model is used to back-calculate the torsional excitations existing at the impeller, based on the dynamic torque measured on the coupling. Finally, the estimated magnitudes of the hydraulic torque components at the blade passing frequency (BPF) and at two-times the blade passing frequency (2xBPF) are compared to the transient CFD prediction. The authors make recommendations on how to enhance the numerical predictions by properly including the added fluid inertia and by estimating torsional damping.