The paper focuses on an operational gas expander being used in a natural gas plant for over 10 years, whose recent realtime monitoring shows that the impeller back-side gap pressure is excessively low. To ensure the safe operation, an insight into the complex internal flow of the expander is demanded. The reverse engineering is firstly conducted to reconstruct the flow passage data from the used impeller and nozzle. The physical model includes the main flow domain components (nozzle ring, impeller, and diffuser duct), and the leakages and seal chambers (the impeller front and back-side toothed gaps, shaft seal chamber, and seal gas inlet). Two-phase flow simulation is conducted with the homogeneous multiphase mixture equilibrium model, which is used to allow for the phase change in terms of condensation. Flow analysis is performed based on the obtained numerical results. At the concerned operating point, the expander outlet wetness fraction is about 16.0%, and evident condensation is encountered in the main flow domain and its back-side gap around the pressure tap, which is thought to be responsible for the abnormal pressure reading. The condensed small droplets may grow to block the pressure tap leading to a lower gauge reading. At the operating speed and different flow rates, the flow simulation is conducted for the expander: condensation in the expander is encountered locally at all flow rates and the overall isentropic efficiency closely associated with the overall wetness fraction.

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