Operation of a Floating Ring Oil Seal in Supercritical CO2 Turbomachinery

Abstract The sealing of pressure in any high-speed rotating machinery has always posed a unique problem. Recent successful turbomachinery designs in the sCO2 field have been based around the use of commercial Dry Gas Seal (DGS) technology. While these units have great operational performance, they also provide limitations to some types of machinery designs. Due to the small clearances and inherent operating principles, DGS units are susceptible to very small foreign object debris (FOD) and require extreme cleanliness. According to industry sources, there are several instances of these seals failing in CO2 machinery, leading to emergency shutdowns that sometimes cause damage to other components. To address these risks and design impacts, the engineering design team investigated the use of another type of rotating seal technology: a floating ring oil seal. This style of seal has been around for decades with great success, but excluded in many instances based on leakage rates, system infrastructure, and rotordynamic effects. Until now, there is no published evidence of oil seal technology used in CO2 turbomachinery. As part of the robust design, an oil seal was selected as the main sealing method used for a new 3 MW single stage CO2 turbine, designed to operate up to 22,000 RPM in mid-enthalpy geothermal conditions (150°C and 160 bar). This paper will discuss the design philosophy of the oil seal unit, and the external equipment needed to support the system. Knowledge gained throughout the fabrication and commissioning efforts will be shared. Further operational techniques and the unique effects on rotordynamics will be discussed based on machinery testing completed in 2024.