| dc.description.abstract | Balance piston seals restrict leakage at the discharge side of single shaft centrifugal compressors. There are multiple types of annular seals including labyrinth seals, textured surface seals such as honeycomb seals and hole-pattern seals, conventional pocket damper seals (PDS), and fully-partitioned damper seals (FPDS). These seals face high pressure differentials with large changes in gas density to develop large radial forces impacting the compressor rotordynamic performance. There is an extensive experimental database available quantifying the dynamic forced performance of labyrinth seals and textured surface seals. On the other hand, an experimental database for PDS and FPDS is limited and prevents a fair and direct comparison of their dynamic force performance against textured surface seals. This lecture presents experimentally derived rotordynamic force coefficients for a FPDS obtained at relevant operating conditions in shaft speed and pressure ratio (exit/inlet) along with a direct comparison to published test data available for a honeycomb seal, similar in size and in operating conditions. The dynamic load tests with the FPDS, 4.5 inch in diameter × 3.37 inch in length, include operation at three shaft speeds (10, 15 and 20 krpm: max surface speed of 120 m/s), a supply pressure of 70 bar(a), three exit/inlet pressure ratios (25%, 50%, 65%), and with circumferential inlet flow pre-swirl conditions ranging from 10% to 160% of shaft surface speed.Of interest to a damper seal performance is the cross-over frequency, which determines the transition from a negative to a positive effective damping coefficient. The FPDS produces effective damping magnitudes near 10 kN.s/m above the cross-over frequency, which ranges from 28-62Hz and above 70 Hz for the lowest (below 0.1) and highest inlet pre-swirl (above 0.6) conditions, respectively. On the other hand, the FPDS generates small direct stiffness coefficients, varying with frequency though contained within +/- 2 MN/m band. Therefore, the FPDS can significantly improve damping ratio of a compressor rotor-bearing system without shifting the natural frequencies. Comparisons of the force coefficients for the FPDS versus a honeycomb (HC) seal demonstrate that both seals generate comparable effective damping coefficients and similar cross-over frequencies with some differences highlighted in the discussion of the results. As expected from a textured seal, the HC produces much larger direct stiffness coefficients when compared to the FPDS. In terms of leakage performance, the FPDS leaks roughly 20%-25% more than the HC. Experimental results are also compared against predictions obtained from CFD and bulk flow models. The CFD and bulk flow predictions yield a good to moderate correlation to the experimental data depending on the operating conditions of rotor speed and pressure ratio. Both methods represent a viable approach for designing FPDSs and predicting their rotordynamic performance. | |
