Experimental Results and CFD Simulations of Labyrinth and Pocket Damper Seals for Wet Gas Compression

Abstract

The most recent development in centrifugal compressor technology is towards wet gas operating conditions. This means the centrifugal compressor has to manage a liquid phase which is varying between 0 to 3% Liquid Volume Fraction (LVF) according to the most widely agreed definition. The centrifugal compressor operation is challenged by the liquid presence with respect to all the main aspects (e.g. thermodynamics, material selection, thrust load) and especially from a rotordynamic viewpoint. The main test results of a centrifugal compressor tested in a special wet gas loop [1] show that wet gas compression (without an upstream separation) is a viable technology. In wet gas conditions the rotordynamic behavior could be impacted by the liquid presence both from a critical speed viewpoint and stability wise. Moreover the major rotordynamic results from the previous mentioned test campaign [2] show that both vibrations when crossing the rotor first critical speed and stability (tested through a magnetic exciter) are not critically affected by the liquid phase. Additionally it was found that the liquid may affect the vibration behavior by partially flooding the internal annular seals and causing a sort of forced excitation phenomenon. In order to better understand the wet gas test outcomes, the authors performed an extensive CFD analysis simulating all the different types of balance piston annular seals used (namely a Tooth on Stator Labyrinth Seal and a Pocket Damper Seal). They were simulated in both steady state and transient conditions and finally compared in terms of liquid management capability. CFD simulation after a proper tuning (especially in terms of LVF level) showed interesting results which are mostly consistent with the experimental outcome. The results also provide a physical explanation of the behavior of both seals, which was observed during testing.

Experimental Study of the Static and Dynamic Characteristics of a Long (L/D = 0.75) Labyrinth Annular Seal Operating Under Two-Phase (Liquid/Gas) Conditions

“…to experimentally investigate the mechanical vibration of centrifugal compressors operating under wet-gas conditions. Ransom et al [14] , Bertoneri et al [1] , Vannini et al [11] , and Vannini et al [15] performed different experiments as a part of this collaboration, and the results are discussed below.…”

“…From 2014 to 2015, Vannini et al [11] and Vannini et al [15] experimentally investigated the effects of seal flooding on the compressor's rotordynamic behavior by injecting liquid directly into the seals of a single stage compressor. The seals included shaft end seals, an impeller eye seal, and a balance-piston seal, and all of them were TOS labyrinth seals.…”

“…The amplitude of the 0.45XREV subsynchronous vibration dropped further when the original TOS labyrinth balance-piston seal was replaced with a pocket damper seal (PDS). In 2015, Vannini et al [15] investigated the original TOS labyrinth seal and the new PDS using multiphase CFD. The simulation results showed a greater tendency for liquid to be trapped inside the cavities of a labyrinth seal under wet gas conditions.…”

“…al. [15] reported a subsynchronous vibration at LVF = 3% when liquid was injected directly into the seals. Vannini et.…”

Design and Calibration of a Full Scale Active Magnetic Bearing Based Test Facility for Investigating Rotordynamic Properties of Turbomachinery Seals in Multiphase Flow

“…As indicated above the research on single-phase seal rotordynamics is well established. However, the research on multiphase seal rotordynamics is presently only in its infancy [30][31][32][33][34]. The experimental validation of existing mathematical models is in particular insufficient.…”