Steam Seal Clearance Of Steam Turbines

Radial Clearance
For a preliminary selection of radial clearance, the empirical formula δ = 0.001d + (0.1–0.2) mm (where δ represents the clearance value) may be used. This formula takes into account various factors, including the shaft diameter, the structure and material of the steam seals, the distance of the seals from the support bearings, the type of support bearings used, and the direction of rotor rotation.

During the design phase, values ​​may be selected based on the following guidelines (smaller values ​​are typically chosen for medium- and low-pressure steam turbines): Radial clearance for shaft-end seals and diaphragm seals: For the inserted-segment type, the range is 0.25–0.70 mm (with smaller values ​​selected when brass or German silver is used for the sealing segments); for the integral-ring type, 0.40–0.70 mm; for the thin-plate type, 0.40–0.65 mm; and for the fir-tree type, 0.25–0.50 mm.

When cylindrical or elliptical support bearings are employed and the direction of rotation is clockwise, the radial clearance on the left side should be 0–0.20 mm greater than that on the right side. Furthermore, for the front seal of the high-pressure section and the diaphragm seals within the high-pressure stages, the radial clearance at the bottom of the seal should be 0.2–0.3 mm greater than that on the sides. Radial clearance for shroud seals: 1.5–2 mm. Radial clearance between the shroud rivet heads and the seal body: 2.5–3.5 mm.

Axial Clearance
The selection of axial clearance values ​​for the flow path components and steam seals is guided by the principle that no axial friction should occur between the rotating and stationary parts under both normal and fault conditions. This clearance value can be determined through calculations involving the thermal expansion of the rotor and casing during operation, diaphragm deflection, and estimations of differential expansion caused by maximum temperature gradients during turbine startup and shutdown; alternatively, it may be established by referencing operational experience with steam turbines.

Generally, the arrangement of axial clearances follows a trend of gradually increasing values ​​moving away from the thrust bearing. To enhance the startup and shutdown performance of large-capacity steam turbines-thereby reducing the time required for these procedures-certain manufacturers have adopted specific design strategies. These include increasing the axial clearances within the flow path and steam seals, maintaining relatively smaller radial clearances for the steam seals, and incorporating radial-type seals at the blade root locations.