Patent Application
20080089789
In operation, high pressure fluid enters an inlet end (not shown) of turbine 10 and moves through turbine 10 generally parallel to an axis 19 of rotor assembly 12. The steam strikes a row of nozzles 18 and is directed downstream against buckets 16. The hot gas then passes through the remaining stages, thus forcing buckets 16 and rotor assembly 12 to rotate. The term “axial” as used herein is intended to be in a direction generally parallel to axis 19.
A maximum load for each bucket 16 is partially determined by its natural frequency. Thus, raising the natural frequency of each bucket 16 generally increases the maximum tolerable load for that bucket 16. Continuously coupling bucket tips 34 together facilitates increasing the natural frequency of each bucket 16. Consequently, in the exemplary embodiment, and as described in more detail below, a continuous bucket cover (not shown in
Links 44 are arranged in an alternating overlapping pattern that forms a continuous overlapping chain link bucket cover 42. Generally, links 44 are oriented such that first end 54 of an “over” link 66, 72 overlaps second end 56 of an “under” link 68, 70, and such that the second end 56 of each over link 66, 72 overlaps with the first end 54 of an under link 68, 70. More specifically, in the exemplary embodiment, when fully assembled, the bottom surface 48 of the first end 54 of an over link 66 is positioned substantially flush against the top surface 46 of the second end 56 of an under link 68. Moreover, over link 66 and under link 68 are aligned with respect to each other such that opening 58 of over link 66 aligns with opening 62 of under link 68, and such that opening 60 is aligned with opening 64. Similarly, the bottom surface 48 of second end 56 of over link 66 is positioned substantially flush against the top surface 46 of first end 54 of under link 70. In addition, over link 66 and under link 70 are aligned so that opening 62 of over link 66, aligns with opening 58 of under link 70, and such that opening 64 is aligned with opening 60 of under link 70. When links 42 are aligned, buckets 16 are coupled to bucket cover 42 by inserting tenons 38 and 40 into complementary-configured openings defined by openings 60 and 64, and/or openings 58 and 62. Tenons 38 and 40 are riveted over the “over” links 66 and 72 creating a rigid connection. The “under” links 68 and 70, by comparison, are assembled with a slip fit to allow for expansion. It should be appreciated that although the bucket cover 42 in the exemplary embodiment is an over and under overlapping chain link cover, other various exemplary embodiments may use any other type of continuous bucket cover that enables bucket cover 42 to function as herein described.
Because bucket cover 42 is coupled to each bucket 16, as opposed to extending only over groups of buckets 16, bucket cover 42 continuously circumscribes the periphery of rotor wheel 14 and rotor assembly 12. Continuously coupling the bucket cover 42 to each bucket tip 34 results in a stiffer assembly having higher natural frequency and greater load capability, thus facilitating preventing harmful vibrations and related stresses from developing in buckets 16 during periods of increased turbine output. Moreover, because the cover 42 is not integral with bucket tip 34, bucket 16 is more cost effective. That is, because a shroud isn't required, each bucket 16 is less expensive to manufacture and repair. Furthermore, the exemplary embodiment described herein is easier to assemble, is better suited for variable speed applications than integrally covered axial entry designs, and may be used at all stages of a turbine, not only the last stage.
In contrast, due to the gap inherent in tangential entry dovetail designs, a bucket cover, such as chain link bucket cover 42, cannot be continuously coupled to a complete row tangential-entry of buckets 16 and at the same time continuously circumscribe rotor wheel 14. The axial entry dovetail 22 of the exemplary embodiment is more efficient than tangential entry dovetail designs because a complete row of tangential entry buckets 16 is installed about rotor wheel 14 without a gap. This geometry enables bucket cover 42 to be coupled to each bucket tip 34 in a complete row of buckets 16 while continuously circumscribing rotor wheel 14. As a result, the complete row of axial entry dovetails 22 yields less unbalance.
In each embodiment, the above-described combination of axial entry dovetail bucket 16 and chain link bucket cover 42 facilitates increasing the tolerable loads of buckets 16. More specifically, in each embodiment, the combination of axial entry dovetail bucket 16 and chain link bucket cover 42 results in a significant increase in bucket 16 natural frequencies, fewer modes of bucket 16 vibration and increased overall mechanical reliability. Furthermore, each embodiment eliminates cover lock-up at off design speeds. As a result, turbine operation with higher output is facilitated. Accordingly, steam turbine performance and component useful life are each facilitated to be enhanced in a cost effective and reliable means.
Exemplary embodiments of axial entry dovetail buckets and bucket covers are described above in detail. The axial entry dovetail buckets and bucket covers are not limited to use with the specific steam turbine embodiments described herein, but rather, the axial entry dovetail and buckets can be utilized independently and separately from other components described herein. For example, the axial entry dovetail with continuous chain link bucket cover may be used with any utility, industrial or mechanical drive steam turbine. Moreover, the invention is not limited to the embodiments of the axial entry dovetail and bucket cover described above in detail. Rather, other variations of axial entry dovetail and bucket cover embodiments may be utilized within the spirit and scope of the claims.
While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.
