On the Influence of Gas Content on the Rotordynamic Force Coefficients of a Three-Wave (Air in Oil) Annular Seal for Multiple Phase Pumps

Abstract

Subsea multiphase pumps must operate reliably with a process fluid transitioning from a pure liquid, to a mixture of gas and liquid, and to eventually just gas; and pressure seals controlling secondary leakage paths are important for both pump efficiency and dynamic stability considerations, in particular, for flows with a sizable gas volume fraction (GVF). Early work advances a three-wave shape seal that generates significant direct stiffness and hence aids to increase the natural frequency of a vertical pump. This paper details dynamic load tests and produces rotordynamic force coefficients for the wavy-seal operating with a gas in liquid mixture made of air in light ISO VG 10 oil. For operation with pure liquid, GVF = 0, a stiffness (K), damping (C), and added mass (M) model fully characterizes the test article. For operation with a mixture, the seal dynamic stiffness coefficients vary greatly with excitation frequency; the direct dynamic stiffness hardens while the cross coupled stiffness decreases as the whirl frequency approaches shaft speed and then increases for super synchronous frequency. For operation with GVF from 0.1 to 0.8, the seal produces a large positive centering dynamic stiffness. Notably, the seal direct damping coefficient does not depend on frequency though reduces continuously as the inlet GVF increases from 0 to 1 (all gas). The current research adds relevant test data and unique dynamic force coefficients to the design selection of seals in multiphase pumps.