Annular Clearance Gas Seals In The 21st Century: A REVIEW OF THE EXPERIMENTAL RECORD ON LEAKAGE AND DYNAMIC FORCE COEFFICIENTS, INCLUDING COMPARISONS OF MODEL PREDICTIONS TO TEST DATA

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2022

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Turbomachinery Laboratory, Texas A&M Engineering Experiment Station

Abstract

Turbomachinery seals are engineered to maintain efficiency and power delivery by minimizing leakage. Seals also appreciably affect the system rotordynamic behavior due to their relative position within a turbomachine. The tutorial reviews the experimental record on gas seals as published in the 21st century, and gives insight on the physical models predicting leakage and dynamic force coefficients. Simple leakage and transport equations are the basis for bulk flow models (BFMs), whereas computational fluid dynamics (CFD) software relies on the solution of the Reynolds-Averaged Navier-Stokes equations (RANS) with an appropriate turbulence flow model in a multi-million node mesh count. Leakage data for seals with nominal clearance (Cr) include labyrinth and interlocking labyrinth seals, damper seals such as honeycomb (HS), pocket damper seals (PDS), conventional and hybrid brush seals, and the advanced (clearance control) HALO seal. A flow factor characterizes the seals as a single-knife (restriction) with an effective clearance (Ce). The extensive comparisons of leakage, measured and predicted, show that engineered seals have an effective clearance Ce/Cr = 0.30 - 0.40, and which is not a function of either inlet pressure, exit pressure, rotor speed, or even actual clearance. The finding attests to the well-understood nature of the fluid flow through the seals. Both BFM and CFD models accurately predict seal leakage. Seals produce reaction forces due to shaft lateral displacements, and with stiffness and damping coefficients characterizing a seal dynamic response. A direct stiffness (K) produces a centering action whereas a direct damping (C) dissipates mechanical energy. A cross-coupled stiffness (k) arises due to shaft rotation dragging the gas around the seal circumference. In general, the force coefficients of a gas seal are frequency (�) dependent. A positive k (>0) decreases the seal effective damping Ceff = (C-k/�), hence degrading the stability of the mechanical element. k is proportional to the gas circumferential speed entering the seal; hence, adding either a swirl-brake or shunt-injection with (excess) gas flowing in the direction opposite to shaft rotation helps to diminish k ( 0 or negative). The tutorial reviews multiple examples of normalized stiffness and damping coefficients for various seal types including uniform clearance seals, LS, and damper seals (HS and PDS). LS with teeth on the rotor surface are notorious for producing large k. Poorly designed LS, installed as either balance piston seals or impeller eye seals, are the cause of many rotordynamic instability fiascos. Damper seals produce direct K and C orders of magnitude larger than those from conventional LS. Damper seals in conjunction with a swirl brake also produce very small k; hence, effectively removing a concern on rotordynamic instability. Past are the days when LS, known bad actors, were the only choice effectively sealing the stages in a turbomachine. Incidentally, damper seals, honeycomb and hole-pattern seals in particular, can produce a large centering stiffness (K>>0 ) that makes a balance piston seal in a back-to-back centrifugal compressor act as a third bearing, hence rising the first natural frequency of the rotor system. Although both the BFM and CFD models are good at predicting leakage, they fall short of replicating the experimental force coefficients of damper seals. At times, C is accurately predicted while K or k are not, or vice versa. The predictive methods still need improvement, hence the need of constant and continuous experimental verification. Differences between predictions and measured force coefficients are likely due to the lack of fidelity in reproducing complex unsteady (highly turbulent) flows whose dynamic pressure acting on the rotor produces the seal reaction force. Derived from the comprehensive experimental results, the review concludes by advancing rules of thumb to estimate the range of expected direct stiffness (K) and effective damping (Ceff) applicable to damper seals. Unlike experiences in the past century, damper seals offer a remarkable opportunity to control the leakage and tailor the rotordynamic performance and stability of modern rotating machinery.

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