Session: 25-01 Annular Seals 1

Paper Number: 82280

82280 - 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 dynamic force coefficients procured in a unique wet gas test rig hosting both a stepped shaft PDS and LS with similar geometry and operating conditions, namely a shaft speed up to 5,250 rpm (surface speed=35 m/s), a pressure ratio (inlet/exit) = 2.5 to 4.2, and supplied with a light oil in air mixture whose liquid volume fraction (LVF) reaches up to 1.7% (liquid mass fraction ~ 80%). Both seals feature the same shaft diameter D=127 mm, axial length L=0.38D, four blades (and eight 45^o pockets), and slightly differing clearances. The LS has a 15% larger clearance than the PDS, hence it leaks more.  For a pure gas condition (LVF=0), the effective clearance (C_eff) for the LS is ~ 40% larger than that for the PDS.  Supplied with a wet gas, the mass flow through the seals grows since the mixture density increases. The C_eff of the PDS tends to decrease as LVF increases whereas the C_eff of the LS increases. The direct stiffness (K) for both seals is small though turning more 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 slightly grows as the LVF increases from 0% to 0.7%, albeit C<0 at low excitation frequencies. Hence, not unexpectedly, rapid variations in the LVF produced broad band sub synchronous vibrations (SSVs) in the PDS; not so in the LS. The experimental results are a reference for the engineered design of LSs and PDSs, in particular, impeller neck-ring seals in wet gas centrifugal compressors.

Presenting Author: Jose Torres Texas A&amp;M University

Presenting Author Biography: Jose Torres is a graduate research assistant at the Turbomachinery Lab of Texas A&amp;M University. He is currently working on research studying the rotordynamic force coefficients of seals operating with a two-phase flow.

Authors:

Jose Torres Texas A&M University
Luis San Andrés Texas A&M University
Jing Yang Texas A&M University