This invention relates to turbine rotors and specifically to a new bucket design for the last or xe2x80x9cclosurexe2x80x9d bucket assembled in a row of tangential entry type buckets on a turbine rotor wheel.
Typically, turbine buckets, or blades, are assembled onto a rotor wheel either individually in axial directions, or tangentially through an opening to a circumferential dovetail. More specifically, tangential entry buckets each have dovetail hooks formed at its base. The buckets are assembled by inserting them, one at a time, axially into an opening at a tangent to the wheel surface so that the bucket dovetail hooks are aligned with the dovetail on the wheel, and then sliding the buckets circumferentially about the wheel along the dovetail, until all but one bucket has been assembled. The final or closure bucket is then inserted axially into the opening and secured to adjacent already in-place buckets.
In general, the purpose of the closure bucket is to complete the assembly of a row of tangential entry buckets on the rotor wheel, and, thus, it is the last bucket mounted on the wheel. Since the closure bucket cannot be attached directly to the rotor wheel dovetail like the remaining buckets, there is no need for the dovetail hooks found on all of the other buckets in the row.
Centrifugal stresses in the bucket/wheel dovetail are greatest near the closure bucket, and therefore, it is desirable to minimize the weight of the closure bucket. Current steam turbine closure buckets are designed such that the unnecessary interior hook elements are substantially removed, but material is added for strength. On the other hand, the overall weight is reduced by: 1) externally, removing material from the sides of the dovetail hooks; 2) removing material from the closure bucket sealing platform; and 3) removing material by forming cutouts on the front and back faces of the dovetail hooks. Material removal from these areas, however, increases performance losses. Specifically, this approach results in a discontinuity in the otherwise circumferentially continuous external dovetail hook surfaces when the entire row of buckets is assembled on the rotor wheel. This surface discontinuity contributes to performance losses first by windage heating of the steam flowing near the bucket dovetail, and second by allowing increased leakage flow through the stationary seal.
This invention provides a steam turbine closure bucket that maintains the external shape of the neighboring buckets, thereby minimizing windage losses, while maintaining sealing surfaces on the bucket dovetail. In accordance with an exemplary embodiment of the invention, the weight of the closure bucket is reduced by removing pockets of material internally, from the front and back faces of the dovetail hooks. This removal of material creates internal cavities that do not alter the external shape of the bucket. The cavity geometry has been designed to remove the maximum amount of material while maintaining sufficient strength for assembly and operation.
In its broader aspects, therefore, the invention provides a turbine closure bucket adapted to complete assembly of a row of tangential entry buckets on a rotor wheel comprising an airfoil portion and a mounting portion for mounting the bucket to a turbine wheel, the mounting portion having front and back faces, and wherein weight reduction cavities are formed internally in the front and back faces of the mounting portion, but wherein the front and back faces of the closure bucket are substantially identical to front and back faces of adjacent buckets in the row.
In another aspect, the invention relates to a turbine closure bucket adapted to complete assembly of a row of tangential entry buckets on a rotor wheel comprising a blade portion and a dovetail portion for mounting the bucket to a turbine wheel, the dovetail portion having front and back faces and a sealing platform, and wherein weight reduction cavities are formed internally in the front and back faces of the dovetail portion, but wherein the front and back faces of the closure bucket are substantially identical to front and back faces of adjacent buckets in the row.
In still another aspect, the invention relates to a turbine rotor wheel having a dovetail formed about its periphery, with an axial opening therein, the rotor wheel having a plurality of turbine buckets received on the dovetail, each bucket having front and back faces; and a closure bucket that is received in the opening, the closure bucket having external front and back faces substantially identical to corresponding front and back faces of the plurality of buckets.