Rotordynamic Force Coefficients for Three Types of Annular Gas Seals With Inlet Preswirl and High Differential Pressure Ratio

Abstract

The following paper presents and compares rotordynamic force coefficients for three types of non-contact annular gas seals, which include a labyrinth (LABY), honeycomb (HC), and a fully partitioned damper seal (EPDS). These three annular seals represent the typical seal types used in process gas centrifugal compressors at the balance piston location or center seal location to limit internal leakage and ensure a robust rotordynamic design. Tests were conducted on 170.6 mm (6.716 in) diameter seals for rotor speeds up to 15 kprm, inlet air pressure of 6.9 bar (100psi), ambient back pressure, and with inlet gas preswiri. The three seals were designed to have the same nominal clearance and similar axial lengths. Testing was conducted on a controlled motion test rig possessing non-synchronous excitation capability up to 250 Hz. Three different test methods were employed to give confldence in the rotordynamic coefflcients, which include static force deflection tests, mechanical impedance tests, and dynamic cavity pressure tests. Results from experiments compare force coefflcients for all seal conflgurations while paying special attention to the crossover frequencies of the effective damping term. All seals possessed negative effective damping at lower excitation frequencies with inlet preswiri, where the straight-through EPDS possessed the lowest cross over frequency of 40 Hz at 15 krpm. The testing also revealed that the preswiri parameter had significantly more influence on effective damping levels and crossover frequencies when compared to rotor speed.

Numerical Investigations on the Leakage and Rotordynamic Characteristics of Pocket Damper Seals—Part I: Effects of Pressure Ratio, Rotational Speed, and Inlet Preswirl

“…In this paper, the FPDS utilized as the computational model are based on the experimental results published by Ertas et al [20]. Figure 1 shows the experimental seal geometry, and Table 1 gives the detailed geometrical parameters and operational conditions chosen for the CFD analyses.…”

“…The high resolu tion scheme and the second order backward Euler scheme were applied for the spatial discretization and the transient term discre tization, respectively. Based on the experimental conditions [20] and the past CFD analyses [27], total pressure, total temperature, velocity direction, and turbulence quantities (turbulence intensity = 5%) were defined for the inlet boundary, while the averaged static pressure was specified for the seal exit. The inlet preswirl velocity is directly determined by the inlet velocity direction.…”

“…Unfortunately, Ertas did not give the conclusions for the effects of pressure ratios and rota tional speeds on the cross-coupling force coefficients and effective damping. Ertas et al [20] also experimentally investigated the rotordynamic characteristics of an eight-bladed FPDS using the above three measurement methods, and compared force coeffi cients to labyrinth and honeycomb seals. Experimental results show preswirls and rotational speeds have a strong influence on the cross-coupling stiffness and effective damping.…”

“…The test results show that the FPDS has destabilizing cross coupling stiffness (same magnitude opposite sign) with preswirl or rotational speed. The latest experiments measuring frequency dependent rotordynamic coefficients for PDS were carried out by Ertas et al [20][21][22]. Ertas et al [21] experimentally investigated the effects of pressure ratios, rotational speeds, and excitation fre quency on rotordynamic coefficients of eight-bladed and twelvebladed PDS using mechanical impedance method and dynamic pressure method.…”

“…Experimental results reveal the FPDS possesses the same-sign cross-coupling stiffness at Oipm and no preswirl. The direct and cross-coupling stiffness of an eight-bladed FPDS with preswirl and rotational speed were measured by Ertas et al [20], using the static force deflection tests. The test results show that the FPDS has destabilizing cross coupling stiffness (same magnitude opposite sign) with preswirl or rotational speed.…”