Flow analysis of an operational natural gas turbo expander

The paper focuses on an operational gas expander beingused in a natural gas plant for over 10 years, whose recent realtime monitoring shows that the impeller back-side gap pressureis excessively low. To ensure the safe operation, an insight intothe complex internal flow of the expander is demanded. Thereverse engineering is firstly conducted to reconstruct the flowpassage data from the used impeller and nozzle. The physicalmodel 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 sealchamber, and seal gas inlet). Two-phase flow simulation isconducted with the homogeneous multiphase mixtureequilibrium model, which is used to allow for the phase changein terms of condensation. Flow analysis is performed based onthe obtained numerical results. At the concerned operating point,the expander outlet wetness fraction is about 16.0%, andevident condensation is encountered in the main flow domainand its back-side gap around the pressure tap, which is thoughtto be responsible for the abnormal pressure reading. Thecondensed small droplets may grow to block the pressure tapleading to a lower gauge reading. At the operating speed anddifferent flow rates, the flow simulation is conducted for theexpander: condensation in the expander is encountered locallyat all flow rates and the overall isentropic efficiency closelyassociated with the overall wetness fraction.