Designs of steam turbine blades
The design of the working blades of steam turbines depends on the conditions of their operation in a multistage turbine and is very diverse
The development of the design of the blades is based on the requirement to ensure high reliability, cost-effectiveness and manufacturability.
The main elements of the turbine working blade are the profile or working part, streamlined by steam, and the root, with which the blade is attached to the disk. The blades are combined into packs by a bandage or wire ties.
The design of the working part of the blade depends on its length, or, more precisely, on the ratio of the average diameter of the stage to its length.
In high-power NPP turbines, these are the blades of the first stages of the HPC. The profile part of the blade is twisted, of a variable cross-section, usually smoothly tapering from the root to the periphery. For the last stages of the low-pressure cylinder of high-power steam turbines, the ratio of the root section area to the peripheral section area reaches 7-10, the profile twist (the difference between the installation angles of the peripheral and root sections) is 65-70°. The development and manufacture of such blades present great difficulties. Therefore, a series of turbines of various capacities and purposes is built on the basis of the created blade of the maximum length.
The choice of the root type is determined by the loads acting on the blade, which are perceived by the root and transmitted by it to the disk. The design shapes of the roots also depend on the manufacturing technology used at the plant.
T-shaped and single-support mushroom-shaped roots are fairly simple in design and manufacturing technology. Shoulders are made on the blades or disk to reduce bending stresses in the root and cheeks of the disk. To fit the blades onto the disk, usually in two diametrically opposite places on the disk rim, locking wells (for blades with a T-shaped shank) or locking cutouts (for blades with a mushroom shank) are made. After the blades are assembled and their shank ends are tightly fitted into the locking wells or cutouts, special locking blades are installed and secured to the disk rim with one or two pins.
The advantages of the forked shank are the absence of special locking blades and the possibility of partial replacement of damaged blades during rotor repair without reblading the entire disk. Another advantage of the forked shank is that the centrifugal forces acting on the blade do not create bending stresses in the disk rim.
The load-bearing capacity of the considered types of shank is relatively small, and they can only be used for blades of moderate length. Long blade roots are made multi-support or with a large number of forks to increase their load-bearing capacity. The load-bearing capacity of a forked root joint can be further increased if the root and disk rim clevis forks are made stepped, i.e., the joint design is brought closer to equal strength.
The highest load-bearing capacity is provided by herringbone root roots with end-on insertion, used for the longest and most heavily loaded blades of the last stages.
These root roots are often made (in plan) along an arc of a circle, which prevents the edges of the root section profile from hanging beyond the root and facilitates insertion of the blades into the disk.
In the overwhelming majority of cases, the working blades of steam turbines are connected into packs with bandages or wire ties. Combining the blades into packs increases the strength and vibration reliability of the blade apparatus. The bandage forms the peripheral wall of the working blade canal and reduces steam leakage. The bandage is made either in the form of a tape riveted to the blade, or in the form of a shelf milled together with the blade. The blades are combined with a solid-milled bandage into packs by welding or riveting an additional overlay tape bandage. For long blades of the last stages, the solid-milled bandage shelves come into contact with each other along the surfaces "protrusion - groove", forming a closed annular connection. Under the action of centrifugal forces, the peripheral sections of the blades tend to rotate, which is prevented by the bandage. Friction on the contact surfaces of the bandage ensures its high damping properties.
Wire ties are often used to improve the vibration reliability of long blades. Wire segments are passed through holes in the blades and are either soldered to the blades or installed freely. In the latter case, the wire, under the action of its own centrifugal force, is pressed against the blades and acts as a damper for blade vibrations (damper connection).