| dc.description.abstract | Industrial centrifugal compressors use annular gap seals, typically labyrinth type, to restrict process gas leakage from high-pressure regions to low-pressure regions. Currently liquid tolerant compressors enable efficient deep-sea oil and gas facilities; and seals supplied with a two-phase flow mixture, liquid in gas (wet gas), can have a large impact on the compressor dynamic stability and mechanical efficiency. Prior research shows that pocket damper seals (PDS) provide much more effective damping than labyrinth seals (LS), albeit a one to one comparison between the two seal types for operation with a wet gas is still scarce. This lecture details experimental results for the leakage and dynamic force coefficients for a stepped shaft PDS and a stepped shaft LS. Both seals feature the same journal diameter D =127 mm, seal length L = 0.38D, and four blades (eight 45º pockets), and slightly different clearances. The operating conditions are similar: shaft speed up to 5,250 rpm (surface speed = 35 m/s), pressure ratios (inlet/exit) = 2.5 to 4.2, and a wet gas composition with up to 10% in liquid volume fraction. For an inlet LVF ranging from 0 to 10%, the LS leaks more, as its step clearance is 15% larger than that of the stepped PDS. For operation with just gas (LVF=0), the LS effective clearance (Ce) is ~ 60% larger than that for the PDS. Under wet gas conditions (maximum inlet LVF = 6%), Ce for the PDS decreases as LVF increases, whereas Ce for the LS increases. The LS produces negligible direct stiffness (K) and effective damping (Ceff) compared to the force coefficients of the PDS. The excitation frequency more so than journal speed affects the K and Ceff of the PDS. The PDS direct damping decreases steadily with frequency while slightly increasing as the inlet LVF increases from 0 to 0.7%. For whirl frequencies below 60 Hz, the PDS direct (centering) stiffness is negative, its magnitude increasing with the liquid content. The stepped PDS exhibited subsynchronous vibrations (SSVs) for operation with a wet gas, and which became broadband with more liquid added. 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. | |
