Abstract

Supercritical carbon dioxide (sCO2) is the fluid medium for those novel thermodynamic cycles aiming to produce electric power at a reduced environmental impact. CO2 at supercritical conditions is still in the gas phase but very close to the liquid-gas transition, it is a dense fluid allowing to design more compact size turbomachinery delivering same power. The centrifugal compressor which shall be employed into this thermodynamic cycle is supposed managing sCO2 at suction (79.8bar, 33°C). The Authors’ Company has been working in a EU project called sCO2Flex (H2020 founded program, grant agreement #764690) with the aim to design and test a 5MW prototype which has been finally tested at full density and full load in a dedicated test loop at the Authors’ premises.

Main subject of this paper is the prototype rotordynamic design with a special focus on the balance piston seal design. Since the compressor is operating at very high density (600kg/m3 at suction and 800kg/m3 at delivery) the stability aspect is the main concern and seal design is very critical. During the design phase a trade-off between alternative damper seals was performed and a Pocket Damper Seal (PDS) was selected. PDS showed more stability with respect to honeycomb seal, together with a lower stiffness level. PDS predictions are considered reliable enough since they are based on a proprietary numerical code which has been calibrated on high pressure experimental data, nevertheless, the very high-density level of the current application requires an experimental validation. PDS was also selected due to the good performance shown in managing liquid phase (as experienced previously by authors) which might be present in the balance piston seal during transient operation close to the supercritical conditions.

From rotordynamic viewpoint the test was conducted through steps: at the beginning a mechanical running test with nitrogen at 10bara was performed to assess the vibration behaviour of rotor running on journal bearings. After that the CO2 test started, the pressure level was increased by steps and the main compressor operating parameters were continuously monitored. The vibration behaviour during machine startup/shutdown showed a very damped response with no indications of any critical speed. When compressor was running at steady speed the vibration spectrum showed only the synchronous component and relevant minor harmonics. The whole compressor operating curve was explored, from choke to near surge conditions, and no major subsynchronous vibrations was detected. At the same time high frequency vibration data were recorded and postprocessed through OMA approach: no mode was identified due to the very high damping level.

The test campaign finally confirmed all the positive design indications for the PDS technology application in sCO2 service.

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