This invention relates generally to steam turbine technology, and specifically, to an integrally covered bucket blade with a torque zone in the solid shank area radially between the dovetail mounting portion and the airfoil portion of the bucket.
Turbine blades, often referred to as buckets, are subject to vibrational stresses that can impact engine efficiency and part life. To reduce these stresses, a number of ways of damping or limiting bucket vibrations have been devised. One approach is to frictionally dampen certain modes of vibrations by interlocking the tips of covered or tip-shrouded buckets. To dampen vibratory stimuli and control natural frequencies, the integral covers or shrouds of the buckets must maintain contact from bucket to bucket within an annular row. To create the requisite interlock, the airfoil or blade portions are twisted during assembly. This pre-twist is in a circumferential direction as viewed along the long axis of the respective bucket. During operation, centrifugal forces will cause radial growth and twisting of the bucket blade portions, tending to open circumferential gaps between the blade tip covers. Thus, the covers must be assembled with enough compressive contact force between the respective adjacent buckets to provide residual force during operation despite the effects of centrifugal forces. The greater the interference required, the greater the required angle of rotation.
In other words, the present method of assembling integrally covered buckets is to twist the airfoil portion of each bucket so that the pitch of the tip cover (or simply, “cover”) decreases, allowing an entire row of buckets to be placed on the rotor. The inherent torque of the airfoil portion then causes the cover to untwist which produces a residual interference that keeps the row of buckets coupled during operation.
The torque characteristics of the airfoil portion of the bucket may preclude the use of an integral tip cover, however, if the torque characteristics of the airfoil portion do not provide for the desired coupling face pressure at the integral bucket tip covers.
The present invention seeks to disassociate the torque characteristics of the airfoil portion of the bucket from the determination of sufficient bucket cover coupling. In the exemplary embodiment, this is achieved by designing the shank area of the bucket with a specific cross-sectional shape that will achieve a desired torque characteristic for the bucket as a whole in order to obtain the desired contact pressure at the cover coupling facings. In other words, the degree of pre-twist needed to achieve the desired tip cover interference is applied in a torque zone spaced from the airfoil portion rather than in the airfoil portion proper. Various suitable geometrical cross sections that may be utilized to achieve the desired end result can be obtained by machining material away from the solid shank area above the dovetail mounting portion of the bucket.
For example, the torque zone may take the form of a reduced cross-sectional area of circular shape. Other cross-sectional shapes disclosed herein include substantially N-shaped; H-shaped; elongated rectangle-shaped arranged parallel to, at an angle to, or perpendicular to the fore and aft bucket platform edges; and other more complex shapes described further herein. The invention is not limited, however, to the specific shapes disclosed, but also includes other reduced cross-sectional configurations that create a torque zone that allows the desired pre-twist for tip cover coupling to be applied in the torque zone, without having to separately pre-twist the airfoil portion of the bucket.
Accordingly, in one aspect, the present invention relates to a turbine bucket comprising a shank portion, an adjacent and radially inner dovetail mounting portion, an adjacent and radial outer airfoil portion, with a platform at a radially inner end of said airfoil portion adjacent the shank portion, and an integral cover at a radially outer tip of the airfoil portion, wherein the shank portion is shaped to provide a torque zone establishing a desired torque characteristic for the bucket to create, upon insertion into a dovetail groove on a turbine wheel, a desired contact pressure between the integral cover and adjacent covers in a row of similar buckets.
In another aspect, the present invention relates to method of disassociating torque characteristics of an airfoil portion of a turbine bucket from contact pressure at coupling faces of adjacent integral bucket tip covers comprising (a) determining a desired degree of contact pressure at coupling faces of tip covers of adjacent buckets; (b) forming a reduced cross-sectional area torque zone in a solid shank portion of each bucket, located radially between a bucket dovetail mounting portion and a bucket platform adjacent the airfoil portion; and (c) during assembly of a plurality of the turbine buckets on a rotor wheel, applying torque only in the torque zone to achieve the desired contact pressure at the coupling faces of the tip covers.
The invention will now be described in connection with the drawings identified below.
With reference initially to
It will be appreciated that various other geometrical cross sections may be applied to the torque zone. For example,
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As indicated above, other cross-sectional shapes for the torque zone in the bucket shank portion are also contemplated by the invention, so long as the torque characteristics of the bucket as a whole provide the desired coupling of adjacent integral tip covers without having to apply torque to the respective airfoil portions.
While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.
Number | Name | Date | Kind |
---|---|---|---|
2315610 | Flanders | Apr 1943 | A |
4497613 | Carreno | Feb 1985 | A |
5509784 | Caruso et al. | Apr 1996 | A |
6158104 | Roberts et al. | Dec 2000 | A |
20040165989 | Caruso et al. | Aug 2004 | A1 |
20060127221 | Yamashita et al. | Jun 2006 | A1 |
Number | Date | Country | |
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20060193726 A1 | Aug 2006 | US |