Turbomachinery and Pump Symposia

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    Dry Gas Seal Material Investigation for Enhanced Slow Roll & Wind-Milling Reliability
    (Turbomachinery Laboratory, Texas A&M Engineering Experiment Station, 2020) Twist, Christina; Yang, Jiao; Amanullah, Kanza; , Clive; Vickers, Tim
    Dry gas seals, while ideally non-contacting, must reliably perform under all operating conditions encountered by a compressor, including those conditions which induce contact such as slow roll, turning gear, ratcheting, and windmilling. Windmilling is of concern particularly for compressors driven by aero-derivative turbines, in which the shaft may rotate freely at a relatively low speed, presumably lower than the lift off speed of the gas seal. The critical sealing interface in a dry gas seal must therefore withstand contact without experiencing excessive wear and without degrading the performance of the seal. In an effort to improve the friction and wear-resistance of this interface while in contact, a new material combination was experimentally evaluated: a silicon carbide rotating ring against a stationary ring made of a solid lubricant interspersed within a carbon graphite matrix. The tribological performance of this material combination was assessed via ball-on-disk and full-scale seal tests. The full-scale seal tests were designed to mimic the operating conditions encountered over the life of the seal, comprised of maximum pressures and speeds, numerous start-ups and shut-downs, 20 hours of windmilling, 1000 hours of slow roll, and 37 hours at turbine wash speed. The windmilling speed was run at 90% of the lift off speed of the seal, established through acoustic emission measurements during seal start-ups. This speed presents a challenging scenario for the gas seal as it is running at a relatively high speed while still in contact. After testing, the seal faces exhibited minor wear, with the solid lubricant-doped carbon graphite rings exhibiting the majority of the wear while the harder silicon carbide mating rings remained in excellent condition. Residue matching the carbon graphite material composition was observed on the surface of the silicon carbide ring surface. This material transfer was likely providing a lubricious, protective layer during contacting conditions. Seal leakages were also tracked throughout the test program and found to remain consistent from the beginning to the end of the test program. Dynamic leakages even decreased from the first to the last performance test, indicating that the seal is capable of continued service despite the challenging test program designed to create an “end-of-life” seal condition. Actual leakage values were compared to predicted leakages from a combined CFD and FE model; correlation between the two was quite good for dynamic testing but was off by as much as a factor of ten for static testing. The static leakage discrepancy between the model and measurements is likely caused by manufacturing deviations, whose effect is magnified by the very small film thickness and consequently low leakage during static conditions.