A STEPPED SHAFT LABYRINTH SEAL VS. A POCKET DAMPER SEAL: LEAKAGE AND DYNAMIC FORCE COEFFICIENTS UNDER WET GAS OPERATION

Centrifugal compressors use annular gap seals, typically labyrinth type, to restrict process gas leakage from high-pressure regions to low-pressure regions. Modern machinery favors pocket damper seals (PDS) which produce significantly more effective damping than labyrinth seals (LS) do. Presently, liquid tolerant compressors enable efficient deep sea oil and gas (O&G) production facilities; and seals supplied with a two-phase flow mixture, liquid in gas, can have a large impact on the dynamic stability and leakage performance of O&G turbomachinery. The paper details experimental results for the leakage and force coefficients procured in a wet gas test rig hosting both a stepped shaft PDS and LS. The two seals have similar geometry and operate with shaft speeds up to 5,250 rpm (surface speed = 35 m/s) and pressure ratios (inlet/exit) = 2.5 to 4.2. The seals are supplied a light oil in air mixture whose liquid volume fraction (LVF) reaches up to 10% (liquid mass fraction > 90%). Both seals feature the same shaft diameter D = 127 mm, axial length L = 0.38D, four blades (and eight 45o pockets). The LS has a 15% larger clearance than the PDS, hence it leaks more. For a pure gas condition (LVF = 0), the PDS is 25% more effective to restrict leakage than is the LS. Supplied with a wet gas, the mass flow through the seals grows since the mixture density increases. The effective clearance (C-r.e) of the PDS tends to decrease as LVF increases whereas that for the LS increases. The direct stiffness (K) for both seals is small and becomes negative as the liquid content increases. The cross-coupled stiffnesses are too small for accurate identification. The PDS produces a greater direct damping ( C) than the LS when operating with a wet gas of identical LVF. The PDS C significantly grows as the LVF increases from 0% to 0.7%. Derived from a computational fluid dynamics model, predictions of leakage agree well with the measurements, whereas force coefficients are roughly 1/2 of the experimental ones. Changes in the LVF produced broadband subsynchronous vibrations (SSV) in the PDS, not so in the LS. The experimental results are a reference for the engineered design of LSs and PDSs operating with two-phase flow.