CFD Analysis of the Forces on a Steam Turbine Balance Seal Packing Ring

Abstract

Abstract In high-pressure single flow Steam Turbines, sealing of the radial gap between rotating and stationary parts in the front end of the turbine is a known design challenge. A traditional solution to control and reduce the leakages between the stationary and rotating parts in that area is to implement spring-backed labyrinth packings. The operating position of such packings, which affects the effective clearance and in turn the leakage flow, is determined by the equilibrium of the forces exerted by the steam and the spring load applied on the back. In order to select the adequate spring and guarantee the desired clearance, it is hence of paramount importance to precisely predict the forces acting on the packing ring during the design stage. This paper describes a computational analysis aimed at characterizing the forces on the packing ring. A full 3D model of a single packing ring, assuming periodic flow in the tangential direction, including both the labyrinth seal on the bottom side, the packing ring chamber on top and the gap between consecutive packing rings was developed. Different clearances between the packing ring and the surrounding casing as a result of manufacturing tolerances are considered both in the axial and tangential direction. The analysis showed that the radial force varies quite significantly with such clearances. The analysis also permitted to highlight that 2D modelling of the labyrinth seals, being intrinsically insensitive to such clearances, is not capable to estimate with sufficient accuracy the radial force on the packing ring.