An aerodynamic and rotordynamic performance test stand has been developed to test a 105,000 rpm compressor-expander which is intended for subsurface natural gas reinjection. The machine consists of a two-stage centrifugal compressor which is driven by a single-stage expansion turbine, and the rotor is supported by foil gas journal bearings and a spiral-groove gas thrust bearing. The test stand is configured for open-loop testing of the overall compressor and turbine performance with air as the working fluid and atmospheric pressure at the compressor suction and turbine discharge locations. The dynamic performance of the test rig was monitored with proximity probes (relative rotor displacements) and accelerometers (housing acceleration) during tests. An instability of the conical rigid rotor mode was observed at or near the designed operating speed of 105,000 rpm which prevented testing of the machine at the maximum speeds required by the aerodynamic performance test matrix (110% of the designed operating speed). Various rotordynamic analyses investigated the effects of hole-pattern and labyrinth seals on stability, including a hole-pattern seal taper sensitivity study and the effect of higher compressor discharge densities. Based on the conclusions of the low-pressure condition rotordynamic analyses, the hole-pattern seals were replaced with labyrinth seals in the open-loop test rig, and the stability issue was no longer present up to the maximum speed.

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