Surge Control of Active Magnetic Bearing Suspended Centrifugal Compressors

Stability is a critical factor that limits the performance of compressors. The safe operating region of these machines is limited by the compressor instability known as surge. Surge is a system-wide instability that is characterized by large amplitude oscillations in the output pressure and mass flow. These oscillations can cause extensive damage to the compressor casing and internal components. In this thesis, we present an active surge control scheme that modulates the impeller axial position to stabilize the flow in active magnetic bearing (AMB) supported centrifugal compressors. With the AMB acting as a high bandwidth actuator to regulate the axial displacement of an unshrouded impeller, the compressor flow states are restored to the equilibrium operating point during the early stages of the surge instability without any loss in performance. The work presented in this thesis can be divided into three main parts. First, an industrialsize AMB suspended compressor test rig was commissioned for this study. The test rig allowed us to develop the theory in a physical context and to provide supporting experimental data. The design and implementation of the AMB rotor levitation controllers were important steps in the commissioning of the test rig. These controllers were designed to satisfy the corresponding API and ISO rotordynamic standards. The second part of this study involved the derivation of an accurate surge model. The effect of the impeller axial motion and the acoustic resonance of the piping were included in the final compression system model. The parameters of the mathematical model were ii Abstract iii corrected based on the experimental observations in the test rig. The results from the experimental validation demonstrated that the model accurately captures the essence of the surge dynamics. Finally, the active surge controller was designed and tested in the compressor test rig. The performance degradation of the surge controller due to the dynamic limitations in the AMB levitation system was considered in the design. The experimental surge control test demonstrated that the AMBs are effective in the suppression of surge. Experimental measurements showed that the controller is able to extend the stable flow region of the system by over 21%. This allows the compressor to operate at the highest efficiency point, while maintaining a very conservative margin to the instability region.

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