Study Of Damping And Dynamic Stress For Semi-open Centrifugal Compressor Impellers

Centrifugal compressors are important equipments in metallurgy, chemical electric power, petroleum and power engineering and widely used in other industries. With the development of science and technology, the performance of centrifugal compressors has been improved a lot during the last decades. The continuous improvement of the aerodynamic performance leads to higher system load. In addition to the increased steady load, centrifugal compressors are subject to vibration that may lead to high cycle fatigue failures, especially for the impellers. If a machine experiences a impeller failure, it not only causes significant damage to the machine, it also causes down time to the process that the machine is supporting.These repairs and down time may cost a lot.The impeller is the core component of centrifugal compressors. It usually endures complicated load, including centrifugal force and aerodynamic force. High dynamic stress caused by the load may lead to high cycle fatigue failures. Therefore, predicting dynamic stress is an important step for both design reviews and root cause failure analysis. To predict dynamic stress, it is necessary to estimate the load and damping property of structure.The dynamic stress is estimated for a semi-open centrifugal impeller which has experienced high cycle fatigue failure. The aerodynamic load on blade is obtained by transient CFD analysis. The frequency domain of the load obtained from Fast Fourier Transformation indicates that the blade is subject to the load with different frequencies. For centrifugal impellers, damping contains material and aerodynamic damping. Material damping ratio of the impeller is predicted based on an empirical equation and expressed as the dynamic stress or the vibration amplitude. Very limited data on aerodynamic damping are available for rotating blades under operation conditions. In this paper,the aerodynamic damping of the failure impeller with various vibration amplitudes, modes and operation conditions is estimated by numerical calculation. A nonlinear total damping model is then proposed,including both material and aerodynamic damping. With this model, dynamic stress of the impeller under forced vibration and resonance is calculated by the equivalent static response method. The volume flow rate decreases with the decrease of temperature at inlet for the same mass flow rate and the machine is operating at off design point. The components of the load for two operation conditions are found to be similar while the amplitude of each exciting force as well as the dynamic stress for off design point is significantly larger than that of design point. The result of fatigue strength calculation indicates that the blade root at leading edge may experience fatigue failure which is consistent with the actual position of impeller fatigue failure.