Leakage and Rotordynamic Force Coefficients of a Three-Wave (Air in Oil) Wet Annular Seal: Measurements and Predictions

In subsea environments, multiphase pumps and compressors add pressure to the process fluid thus enabling long distance tie back systems that eliminate topside oil and gas separation stations. One challenge to construct a reliable multiphase pump or a reliable wet gas compressor is that the first must handle, without process upset, a mixture whose gas volume fraction (GVF) changes suddenly; while the other must remain stable while working with a liquid volume fraction (LVF) changing over long periods of time. The mixture GVF/LVF affects the static and dynamic forced performance of secondary flow components, namely seals, and which could lead to an increase in both rotor lateral or axial vibrations, thus compromising system reliability and availability. The current research is a planned endeavor towards developing seal configurations amenable to maintain rotor dynamic characteristics during changes in the contents of flow components. This paper extends prior work with uniform clearance annular seals and presents the static and dynamic forced performance of a three-wave surface annular seal designed to deliver a significant centering stiffness. The test element has length L = 43.4 mm, diameter D = 127 mm, and mean radial clearance c_m = 0.191 mm. At a shaft speed of 3.5 krpm (23 m/s surface speed), an air in ISO VG 10 oil mixture with an inlet GVF, 0 to 0.9, feeds the seal at 2.5 bara pressure and 37 °C temperature. The mixture mass flow rate decreases continuously with an increase in inlet GVF; shaft speed has little effect on it. Dynamic load tests serve to identify the seal dynamic force coefficients. The liquid seal (GVF = 0) shows frequency independent force coefficients. However, operation with a mixture produces stiffnesses that vary greatly with excitation frequency, in particular the direct one that hardens. The direct damping coefficients are not functions of frequency albeit dropping rapidly in magnitude as the GVF increases. The work also compares the performance of the wavy seal against those of two other seals; one with clearance equal to the mean clearance of the wavy seal, and the other with a large clearance emulating a fully worn wavy seal. The small clearance seal leaks 20% less than the wavy seal, whereas the leakage of the worn seal is twofold that of the wavy seal. For the three seals, the leakage normalized with respect to a pure liquid condition collapses into a single curve. The wavy seal produces the greatest direct stiffness and damping coefficients whereas the worn seal produces the smallest force coefficients. Derived from a homogeneous mixture bulk flow model, predicted force coefficients for the three-wave seal match well with the test data for operation with a pure oil and an inlet GVF 0.2. For operation with GVF > 0.2, the discrepancy between the prediction and experimental data grows rapidly. The extensive test campaign reveals a wavy-surface seal offers a centering stiffness ability, a much desired feature in vertical submersible pumps that suffer from persistent static and dynamic stability issues.