STATIC DESTABILIZING BEHAVIOR FOR GAS ANNULAR SEALS AT HIGH ECCENTRICITY RATIOS
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The centering force of a plain, annular, liquid seal in a centered position has a pronounced effect on the rotordynamic behavior of pumps due to the Lomakin Effect. In centrifugal injection compressors, the centering force for a hole-pattern-stator/smooth-rotor for a balance-piston seal in a straight-through design or a division-wall seal in a back-to-back design has an impact on the rotordynamic behavior of high-pressure injection compressors. Annular seals for pumps and compressors are designed to operate in a nominally centered position. Previous test results show that the direct (centering) stiffness for liquid seals and hole-pattern-stator seals remain relatively constant out to eccentricity ratios of 0.5. For liquid seals, tests have shown positive centering capability at eccentricity ratios on the order of 0.7.New test results are presented for annular gas seals with smooth and hole-pattern stators, moving from centered to fully eccentric (contact) positions. The test fluid is air. The tests are for non-rotating conditions with a supply pressure up to 70 bars and pressure rations (PRs) of 0.4, 0.5, and 0.6. The exit Mach numbers for the tests are on the order of 0.33, so compressibility effects are not pronounced.Test results are presented for the net reaction force for a smooth seal and a hole-pattern-stator gas annular seal in moving from centered to fully eccentric positions. Comparisons are made between measurements and predictions from a bulk-flow code.The smooth seal creates significantly larger de-centering forces than the HP seal. Its measured de-centering force increases steadily as the stator approaches the wall. The negative slope of the reaction force also increases, indicating a steadily increasing negative stiffness. The de-centering force increased with decreasing PR, increasing P, and is much larger than predicted.The HP seal de-centering reaction force is bi-linear with an approximate constant slope out to 0 0.5, and then a much steeper slope out to wall contact. The model does not predict the bilinear behavior but does a reasonable job of predicting the force magnitude at contact.The de-centering forces measured here have not previously been reported and emphasize the importance of concentric assembly of annular seals in pumps and compressors. Unanticipated eccentricities that create negative stiffness will lead to over prediction of critical speeds and overly optimistic stability calculations.
