Fluid reaction surfaces (i.e. – impellers) – With weight-balancing means
Reexamination Certificate
1999-08-31
2001-02-20
Look, Edward K. (Department: 3745)
Fluid reaction surfaces (i.e., impellers)
With weight-balancing means
C416S221000, C416S19300A, C416S22000A
Reexamination Certificate
active
06190131
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to coverplates axially overlying root portions of turbine buckets and mounting angel wing seals. The invention particularly relates to coverplates having balance weights to minimize stresses under centrifugal loading and centering slots to control the circumferential position of coverplate and bucket axial retention pins.
BRIEF SUMMARY OF THE INVENTION
Coverplates are generally used along the admission and discharge faces of turbine buckets adjacent their connection with the turbine wheel and support seals, generally referred to as angel wings, about the turbine rotor. The coverplates may be integrally cast with the bucket or may comprise non-integral separate parts mechanically assembled along axial faces of the bucket root and the wheel dovetail. In the design of an advanced gas turbine, it has been found necessary to lower the natural frequency of the buckets of in at least one of the turbine wheels to improve frequency margins. To accomplish that objective, non-integral coverplates and non-contoured bucket shanks are used to increase shank flexibility. While non-integral coverplates have been used in the past, the new advanced gas turbine imposes certain additional requirements on coverplate design.
Particularly, axial expansion and clearance requirements of the turbine require the angel wing seals for the coverplates on at least one of the turbine wheels to extend axially a greater distance than angel wing seals in prior coverplate designs employing non-integral coverplates. Because the angel wings extend axially from the outer axial faces of the coverplates, significant bending moments may be developed in the coverplates in response to centrifugal forces on the coverplates at rotor speed. The stresses caused by these bending moments may be increased to unacceptable levels and the coverplates may deflect toward the buckets, creating a large force on the buckets. Further, the interaction between the coverplates and buckets resulting from these stresses can stiffen the bucket shanks and reduce the effectiveness of the design with respect to lowering bucket natural frequencies. Consequently, there is a need for a coverplate design which controls the coverplate bending moment and deflections.
Additionally, the turbine wheel of the new advanced gas turbine has wide dovetail slot bottoms for stress reduction purposes. Wheel slot dovetails used previously in association with non-integral coverplate designs have typically utilized a narrow slot bottom. In prior non-integral coverplate designs, a twist lock cylindrical retainer pin is form-fit in the narrow bottom of the wheel slot. However, with the wider bottom slot configuration, there is a potential for circumferential movement of the retainer pin within the bottom slot which, if unconstrained, could cause the pin to rotate to a position where the bucket and coverplate are not constrained axially relative to the wheel. Thus, there is an additional need for limiting any circumferential motion of the twist lock retention pin.
In accordance with a preferred embodiment of the present invention, non-integral coverplates are provided for axially overlying root portions, i.e., the shanks and dovetail connections between the buckets and the wheel slots. Each coverplate, however, is provided with balancing weights, e.g., ribs, to control bending moments at rotor speed. Particularly, the balance ribs are located on an axial side of each coverplate opposite the side mounting the angel wing seals. The balance ribs of each cover plate project axially, straddle the shank of the bucket and cause the axial center of gravity of the coverplate to be located along the coverplate and a radial line of symmetry of the coverplate. The balance ribs also stiffen the coverplates, which further maintains control of the deflection of the coverplates relative to the buckets at speed.
In an additional preferred aspect of the present invention, the twist lock retainer pin retaining each coverplate in axial overlying relation to the bucket dovetail and shank is maintained centered against circumferential movement in the wide bottom wheel slot by the coverplate. The wide bottom slot, i.e., the non-form-fitting wheel dovetail wide sub-slot provided to reduce stresses does not provide adequate circumferential constraint of the twist lock pin. In conventional arrangements of the twist lock pin, the cylindrical pin is mounted in a form-fitting base of the wheel slot and is captured by the form-fit against circumferential movement. However, in the wide bottom slot arrangement, i.e., a wide sub-slot, the potential for circumferential movement of the pin exists. To preclude circumferential movement, the base of the dovetail of the coverplate has a flat and a centering slot in the flat for limiting the circumferential motion of the twist lock pin. The twist lock pin has half-heads at opposite ends to retain the pin against axial movement in the slot. The centering slot on the coverplate is engaged by the pin, particularly during turbine operation wherein centrifugal forces cause the pin to seat in the apex of the centering slot, constraining the pin from circumferential movement.
In a preferred embodiment according to the present invention, there is provided a coverplate axially overlying portions of a turbine bucket secured to a turbine wheel, comprising a coverplate body having a plurality of bosses and grooves along one end of the body for securing the coverplate in a generally complementary-shaped slot in the turbine wheel, an arcuate flange projecting from an end face of the coverplate body at a location radially outwardly of the body end and in a generally axial direction for forming a seal with flanges of adjacent coverplates about the turbine wheel, at least one balance weight projecting from an opposite end face of the coverplate body and in a generally opposite axial direction from the flange to provide a counterbalance to the flange when the coverplate body is subjected to centrifugal forces upon rotation of the turbine wheel.
In a further preferred embodiment according to the present invention, there is provided in a turbine having a turbine wheel rotatable about an axis and having a plurality of circumferentially spaced slots receiving generally complementary-shaped end portions of turbine buckets, the buckets including shanks intermediate the end portions, a platform, and turbine blades, a circumferential array of coverplates axially overlying shanks and the end portions of the buckets, each coverplate having an end portion thereof generally complementary in shape to the slot for reception in the slot, an arcuate flange projecting from one end face of each coverplate at a location radially outwardly of the coverplate end portion and in a generally axial direction for forming a seal with adjacent coverplates, an opposite end portion of each coverplate engaging the platform to retain the opposite end portion against axial movement relative to the bucket and at least one balance weight projecting from an opposite end face of each coverplate and in a generally opposite axial direction from the flange to provide a counterbalance to the flange when the coverplate is subjected to centrifugal forces upon rotation of the turbine wheel.
In a still further preferred embodiment according to the present invention, there is provided in a turbine wheel rotatable about an axis and having a plurality of circumferentially spaced slots receiving generally complementary-shaped end portions of turbine buckets, a circumferential array of coverplates axially overlying end portions of the buckets received in the slots and one axial face of the wheel, the coverplates having end portions thereof generally complementary in shape to the slots for reception in the slots, each coverplate having an arcuate flange projecting from one end face thereof at a location radially outwardly of the coverplate end portion and in a generally axial direction away from the wheel for forming a seal with flanges of adjacent coverplates, the
General Electric Co.
Look Edward K.
Nixon & Vanderhye
Woo Richard
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