Low-Leakage Shaft-End Seals for Utility-Scale Supercritical CO2 Turboexpanders

Abstract

Supercritical carbon dioxide (sCO2) power cycles could be a more efficient alternative to steam Rankine cycles for power generation from coal. Using existing labyrinth seal technology, shaft-end-seal leakage can result in a 0.55–0.65% points efficiency loss for a nominally 500 MWe sCO2 power cycle plant. Low-leakage hydrodynamic face seals are capable of reducing this leakage loss and are considered a key enabling component technology for achieving 50–52% thermodynamic cycle efficiencies with indirect coal-fired sCO2 power cycles. In this paper, a hydrodynamic face seal concept is presented for utility-scale sCO2 turbines. A 3D computational fluid dynamics (CFD) model with real gas CO2 properties is developed for studying the thin-film physics. These CFD results are also compared with the predictions of a Reynolds-equation-based solver. The 3D CFD model results show large viscous shear and the associated windage heating challenge in sCO2 face seals. Following the CFD model, an axisymmetric finite-element analysis (FEA) model is developed for parametric optimization of the face seal cross section with the goal of minimizing the coning of the stationary ring. A preliminary thermal analysis of the seal is also presented. The fluid, structural, and thermal results show that large-diameter (about 24 in.) face seals with small coning (of the order of 0.0005 in.) are possible. The fluid, structural, and thermal results are used to highlight the design challenges in developing face seals for utility-scale sCO2 turbines.

Demonstration of a Dynamic Clearance Seal in a Rotating Test Facility

“…Typical fluid film thickness in such seals are of the range 0.005-0.013 mm [18]. These seals would also require change to current steam turbine construction.…”

“…Currently dynamic seal technologies have yet to make their way into steam turbines, although they are under development for applications such as supercritical carbon dioxide (sCO2) Brayton cycles [18]. Two dynamic seal concepts published in the wider literature are discussed here; the "Hydrostatic Advanced Low-Leakage" (HALO) seal and annular floating ring seal.…”

Simulation and development of dry gas seal for supercritical CO2 carbon dioxide

“…The net deformation for both applied pressure load and thermal load result in the positive-coning (i.e., film thickness decrease with flow direction from outer to inner edge). Despite positive coning or a convergent gap, being desirable to the design of the dry gas seal, as it gives rise to positive film stiffness and fluid film stability, excessive convergence is undesirable as it reduces the film stiffness and increase the inflow area hence the leakage rate [11,18,25,67,69]. The results indicate significant coning of the stationary ring face with a magnitude in excess of ∆ = -2.5 µm.…”

“…Using the labyrinth seal technology as shaft end seal can results in a 0.55% to 0.65% point efficiency loss as shown in Figure 1.3. The low leakage dry gas seal, capable of reducing the leakage flow rate is considered an enabling technology for achieving 50-52% thermodynamic cycle efficiency in the supercritical CO2 power cycle [18]. Due to the substantial gas leakage through the labyrinth seal, the power cycles typically require ejector systems to collect the shaft seal leakage flow and to reinject the fluid back into the loop to minimize mass loss from the system, which reduces leakage and increases the power cycle efficiency [13,19].…”

“…Therefore dry gas seal is the preferred choice for supercritical CO2 power cycle applications when the technology is ready [13,20,21]. Cycle efficiency impact of two labyrinth end seals for a utility-scale supercritical CO2 turbine and desired dry gas seal leakage and expected efficiency [18].…”