This invention relates generally to steam turbines and, more specifically, to attaching steam turbine buckets to steam turbine wheels.
At least some known steam turbine buckets are subjected to high centrifugal loads. Specifically, buckets located in the last few stages of low pressure wheels may be more stressed than buckets in other stages due to centrifugal loads caused by rotation of steam turbine wheels. Such loads induce higher average and local stresses in the connective dovetails. Stress corrosion cracking (SCC) in the low pressure buckets is a serious concern and is driven largely by local stresses. As such, higher local stresses can lead to lower fatigue life of wheel and bucket dovetails. With an increasing demand for longer and longer buckets, the dovetails are required to operate under higher loads.
For at least some known low pressure turbines, the rotor wheel may be more limiting than the bucket. Specifically, the material used to manufacture at least some known buckets is more resistant to SCC than the material used for wheels. An effective means of avoiding SCC failure in low pressure wheels may be to reduce the local stresses in the wheel dovetail.
In one aspect, a method of assembling a steam turbine including a rotor assembly is provided. The method includes providing at least one turbine bucket including a dovetail that includes a plurality of crush surfaces, a plurality of non-contact surfaces, and at least one neck defined between one of the crush surfaces and one of the non-contact surfaces. The method also includes providing a turbine wheel that includes at least one dovetail slot defined therein that is defined by a plurality of crush surfaces and a plurality of non-contact surfaces, and coupling the dovetail of the at least one turbine bucket within the turbine wheel slot such that a slant angle of the at least one neck facilitates a substantially uniform distribution of load between the dovetail and the at least one slot.
In another aspect, a dovetail assembly for a turbine is provided. The dovetail assembly includes a bucket dovetail and a wheel dovetail slot sized to receive the bucket dovetail. The bucket dovetail and wheel dovetail slot each include a plurality of crush surfaces, a plurality of non-contact surfaces, and a plurality of necks defined by a transition from a crush surface to a non-contact surface. Each neck includes a slant angle that facilitates distributing a substantially uniform load between the bucket dovetail and the wheel dovetail slot.
In another aspect, a steam turbine includes a rotor assembly having a plurality of turbine buckets coupled to a turbine wheel. Each turbine bucket includes an airfoil and a dovetail, and each turbine wheel includes a plurality of dovetail slots sized to receive the plurality of turbine bucket dovetails. Each bucket dovetail and each dovetail slot includes a plurality of crush surfaces, a plurality of non-contact surfaces, and a plurality of necks defined by a transition from a crush surface to a non-contact surface, and each neck includes a slant angle that facilitates distributing a substantially uniform load between a bucket dovetail and a respective wheel dovetail slot.
At least one embodiment of the present invention is described below in reference to its application in connection with and operation of a steam turbine engine. Further, at least one embodiment of the present invention is described below in reference to a nominal size and including a set of nominal dimensions. However, it should be apparent to those skilled in the art and guided by the teachings herein provided that the invention is likewise applicable to any suitable turbine and/or engine. Further, it should be apparent to those skilled in the art and guided by the teachings herein provided that the invention is likewise applicable to various scales of the nominal size and/or nominal dimensions.
During operation, low pressure steam inlet 30 receives low pressure/intermediate temperature steam 50 from a source, such as, but not limited to, an HP turbine or IP turbine through a cross-over pipe (not shown). Steam 50 is channeled through inlet 30 wherein flow splitter 40 splits the steam flow into two opposite flow paths 52 and 54. More specifically, in the exemplary embodiment, the steam 50 is routed through LP sections 12 and 14 wherein work is extracted from the steam to rotate rotor shaft 16. The steam exits LP sections 12 and 14 and is routed to a condenser, for example.
It should be noted that although
In the exemplary embodiment, dovetail 400 also includes a plurality of hook fillets 418, 420, and 422. Specifically, dovetail 400 includes a top hook fillet 418, a middle hook fillet 420, and a bottom hook fillet 422. Top hook 418 is formed with two identical radii 424 and a flat surface 426 extending therebetween. Middle hook 420 is also formed with two identical radii 428 and a flat surface 430 extending therebetween. In the exemplary embodiment, radii 424 and 428 are identical and each measures between 0.425 millimeters (mm) and 1.441 mm or, more specifically, approximately 0.933 mm. Alternative embodiments may vary the radius of each hook, either individually or in common. In the exemplary embodiment, flat surfaces 426 and 430 each measure between 1.000 millimeters (mm) and 3.952 mm or, more specifically, approximately 1.412 mm. Alternative embodiments may use one or more flat surfaces that each have a different length.
Bottom hook 422 is formed with a compound radius 432 and a flat surface 434 that defines the bottom surface of dovetail 400. In the exemplary embodiment, compound radius 432 includes two radii 436 and 438. In the exemplary embodiment, radius 436 measures between 1.344 millimeters (mm) and 2.36 mm or, more specifically, approximately 1.852 mm. Radius 438 measures between 3.617 millimeters (mm) and 8.189 mm or, more specifically, approximately 5.903 mm. Alternative embodiments may include different radius measurements and/or may include bottom hook 422 including only a single radius. In the exemplary embodiment, flat surface 434 measures between 2.974 millimeters (mm) and 8.054 mm or, more specifically, approximately 5.514 mm. Alternative embodiments may include a flat surface having a different length.
In the exemplary embodiment, slot 500 also includes a plurality of hook fillets 520, 522, and 524. Specifically, in the exemplary embodiment, slot 500 includes a top hook 520, a middle hook 522, and a bottom hook 524. Middle hook 522 is formed with two identical radii 526 and a flat surface 528 extending therebetween. In the exemplary embodiment, each radius 526 measures between 1.604 millimeters (mm) and 2.62 mm or, more specifically, approximately 2.112 mm. Flat surface 528 measures between 0.250 millimeters (mm) and 3.393 mm or, more specifically, approximately 0.853 mm. Alternative embodiments may use one or more flat surfaces having a different length. Further, alternative embodiments may use a different radius or may use two different radii.
Bottom hook 524 is formed with two identical radii 530 and a flat surface 532 extending therebetween. In the exemplary embodiment, each radius 530 measures between 0.425 millimeters (mm) and 1.441 mm or, more specifically, approximately 0.933 mm. Flat surface 532 measures between 0.500 millimeters (mm) and 3.707 mm or, more specifically, approximately 0.663 mm. Alternative embodiments may use one or more flat surfaces having a different length. Further, alternative embodiments may use a different radius or may use two different radii. Each of middle hook 522 and bottom hook 524 are shaped to facilitate carrying load approximately equally. Top hook 520 includes a radius 534 which, in the exemplary embodiment, measures between 1.255 millimeters (mm) and 5.827 mm or, more specifically, approximately 3.541 mm. Alternative embodiments may use a different radius for top hook 520. Radius 534 is selected to facilitate a smooth transition between slot 500 a top wheel surface 536.
In the exemplary embodiment, and as shown in
During operation, rotation of wheel 300 causes centrifugal forces to develop in buckets 200, which are then transferred to each dovetail assembly 600 through crush surfaces 440 and 538. Such forces induce stresses in each dovetail assembly 600. Concentrated stress loading results when load paths are forced to change direction. As such, with a slanted crush surface, such as crush surfaces 440 and 538, the change in direction is less severe and, as such, the resulting stress concentration is reduced. Additionally, a slant angle, such as slant angle 444 and 542, induces a component of the forces in an axial direction, giving rise to bending of bucket platform 416, further reducing stress concentration. Predetermined radius values in the hook fillets 418, 420, 422, 520, 522, and/or 524 and neck fillets 404, 406, 408, 502, 504, and/or 506 further mitigate stresses caused by the centrifugal forces generated by wheel 300 by allocating in a more equal fashion the stresses on each of the hook and neck fillets.
The above-described methods and apparatus facilitate minimizing local stresses in bucket and wheel neck fillets caused by the high centrifugal force induced to buckets. An optimized slant angle and optimized fillet radii facilitate uniformly distributing the load on the dovetail assembly, thereby resulting in low local and average stresses in both the bucket dovetail and the wheel dovetail slot. Such a reduction in stress concentration facilitates carrying higher centrifugal loads giving improved power output.
Exemplary embodiments of methods and apparatus that facilitate minimizing local stresses in a dovetail assembly are described above. The methods and apparatus are not limited to the specific embodiments described herein, but rather, components of the methods and apparatus may be utilized independently and separately from the other components described herein. For example, the dovetail assembly described herein for use in a power plant may also be fabricated and/or used in combination with other industrial plant or component design and/or monitoring systems and methods, and is not limited to practice with only power plants generically or to steam turbine engines specifically, as described herein. Rather, the present invention can be implemented and utilized in connection with many other component or plant designs and/or systems.
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.
Number | Name | Date | Kind |
---|---|---|---|
2753149 | Alexander | Jul 1956 | A |
4191509 | Leonardi | Mar 1980 | A |
4260331 | Goodwin | Apr 1981 | A |
4824328 | Pisz et al. | Apr 1989 | A |
5110262 | Evans | May 1992 | A |
5147180 | Johnson | Sep 1992 | A |
5174720 | Gradl | Dec 1992 | A |
5494408 | Seeley et al. | Feb 1996 | A |
5531569 | Seeley | Jul 1996 | A |
5554005 | Nguyen | Sep 1996 | A |
6142737 | Seeley et al. | Nov 2000 | A |
6435833 | Reluzco et al. | Aug 2002 | B1 |
6435834 | Reluzco et al. | Aug 2002 | B1 |
6652237 | Yehle et al. | Nov 2003 | B2 |
20050175462 | Lagrange et al. | Aug 2005 | A1 |
20090022591 | Mujezinovic et al. | Jan 2009 | A1 |
Number | Date | Country | |
---|---|---|---|
20090129932 A1 | May 2009 | US |