Hydrodynamics of High Specific Power Pumps for Off-Design Operating Conditions

Part load operation of high energy centrifugal pumps is associated with increased vibration levels which may adversely affect the machines operational safety. The main sources of vibrations in centrifugal pumps are of hydrodynamic nature. The interaction between rotating and stationary flow fields yields spatially distributed pressure fluctuation patterns, which excites mechanical vibrations of the rotating and stationary pump components. As virtually all modern, high-energy machines are operated using variable speed drives, an operation, where the excitation by rotor-stator interaction pressure fluctuations matches one of the natural frequencies of the impeller or the pumps stationary components, appears to be unavoidable during the impeller life span. This may yield, provided these vibrations are not sufficiently damped, structural damage to the centrifugal pump components. Moreover, at off-design operation of centrifugal pumps, further phenomena are superposed to the Rotor-Stator interaction effects. Local flow separations affect the flow structure in the hydraulic components, modifying existing and creating supplementary hydraulic forces as excitations for deformations and vibrations at additional frequencies. In the present work, experimental investigations of part-load hydrodynamic phenomena as sources of mechanical excitations in a conventional, high-energy centrifugal pump stage are presented. These investigations include unsteady pressure fluctuation measurements in the rotating and stationary elements of the investigated pump stage, performed at different rotational speeds and operating points. The unsteady pressure measurements were accompanied by measurements of impeller deformations using strain gauges embedded in the impeller shroud wall. The acquired data have been completed by measurements of shaft and bearing housing vibrations. The measurements at part-load operation unveiled the existence of stationary and rotating instabilities in the diffuser of the pump stage, referred as stall. Stationary stall, which expresses itself as a non-rotating high pressure zone in the Rotor-Stator interface, yields additional pressure fluctuations at the rotational frequency and its harmonics. At specific relative flow rates, this high pressure pattern begins to rotate around the impeller circumference. This slowly rotating high pressure zone has a dramatic impact on the mechanical behavior of the pump stage. On the one hand, it forms the highest contribution to the impeller shroud strain, generating deformations several times higher than the ones generated by rotor-stator interaction pressure fluctuations. The perturbations in the circumferential pressure distribution yield further a radial net force, slowly rotating around the impeller circumference. This effect has been identified using the shaft vibration measurements, where it can be shown, that the shaft centerline displacement directly follows the radial force direction. This can negatively affect the rotor system dynamic stability but can also be used to detect rotating stall during the operation of the pump, as external pressure fluctuation measurements not always allow the detection of rotating stall. The impeller blade passage through the stall zone strongly affects the flow in the impeller side rooms. The entrainment of fluid with low circumferential velocity reduces strongly the rotation of the flow in the side rooms and thus affects the steady pressure distribution and the axial net force. The blade passage through the detachment zone adds periodic variations of the axial force acting on the impeller, which can be identified in the axial shaft vibration signature, allowing a detection of stationary stall by a thorough analysis of the axial shaft vibrations.

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