Development of Large-Diameter Hybrid Film-Riding Seals for sCO2 Turbines
Authors: Bidkar Rahul A. , Kumar Uttara , Trivedi Deepak , Zhang Xiaohua , Rambacher Collin , Rimpel Aaron , Kerr Thomas , Johnson Jeremy 
Published: August 2024

Abstract
Compared to traditional labyrinth seals, large-diameter (about 24 in or 609.6 mm) film-riding face seals are worth 0.55–0.65% points efficiency for utility-scale supercritical carbon dioxide (sCO2) turbines based on Recompression Closed Brayton power cycles. Conventional dry gas seals (DGS) that operate with aerodynamic spiral grooves and 0.0002–0.0005 in (0.005–0.013 mm) physical clearances cannot be used at such large diameters due to manufacturability limitations as well as mechanical deformations of the seal-rotor interface at challenging operating conditions of 1087 psia (75 bar) differential pressures and 200–400°F (93–205 °C) inlet temperatures. Hybrid film-riding face seals operating at slightly larger gaps are excellent compromise candidates that can operate at large diameters with slightly increased albeit leakier films. Hybrid film-riding face seals use aerodynamic spiral grooves as well as pressurized fluid to levitate the seal and provide film stiffness needed for the seal to dynamically track the rotor incursions. This paper presents the development and test data for a hybrid face seal developed at GE Research for the utility-scale sCO2 turbine application. Specifically, the seal working principle and a high-level overview of the seal design considerations — including fluid/thermal/mechanical analyses are presented followed by test data on three seals — a small-size seal (5.7 in or 144.8 mm diameter), an intermediate-size seal (14-in or 355.6 mm diameter) and two full-size seals (24 in or 609.6 mm diameter) tested over a range of pressures and temperatures with both air and CO2 as the working fluid. The small- and intermediate-size seals were tested with air on the existing Cold5 rig and the Advanced Seals Test Rig (ASTR) at GE Research, while the full-size seal was tested at Southwest Research Institute (SwRI) with both air and CO2 on a new GE-SwRI seals test rig designed and commissioned for this seal development effort. This paper presents non-dimensional seal film thickness and seal leakage (effective clearance) data as a function of the pressure ratios, speed and temperature. The film thickness measurements presented in this paper agree well with the CFD-based film thickness predictions. The measured seal effective clearance for the seals (across all three sizes) was around 0.001 in (0.0254 mm) effective clearance, which is better than published data on aerostatic face seals and less risky compared to traditional DGS. Rotating test data across all three length scales presented in this work show successful laboratory-scale seal operation during these early stages of technology development for the newly-design hybrid face seals at surface speeds up to 375 ft/s (115 m/s), seal inlet temperatures up to 287°C and low-medium differential pressures ranging from 72 to 232 psi (5 to 16 bar). Overall, the rotating full-size air-based seal tests on the new GE-SwRI seals rig, together with the successful high-temperature demonstrations of the intermediate-size seal on the ASTR at GE Research, show promising results for the split-segment, hybrid face seal technology at the large-diameter scale.