Patent Application
20100278652
The present invention relates generally to a steam turbine and more particularly to a dovetail design for attaching a steam turbine bucket to a steam turbine rotor wheel.
Generally steam turbine buckets and steam turbine rotor wheels in the latter stages of a low pressure turbine are usually highly stressed during operation due to large centrifugal loads applied by the rotation of longer and heavier latter stage buckets. In particular, large centrifugal loads are placed on the buckets due to the high rotational speed of the rotor wheels which in turn stress the blades. These loads induce higher average and local stresses in the connective dovetails that attach the buckets to the rotor wheels. These stresses along with moisture from the steam flow path of the steam turbine drive stress corrosion cracking. Both the higher average and local stress concentrations can lead to lower fatigue life and stress corrosion of turbine rotor wheels and dovetails. Reducing stress concentrations and stress corrosion cracking in the dovetails under large centrifugal loads is a design challenge for steam turbine manufacturers, especially as the demand for longer blades increases.
In one aspect of the present invention, a bucket dovetail for securing a bucket to a rotor wheel is provided. The dovetail comprises a plurality of hooks that connect the bucket with the rotor wheel. The plurality of hooks are shaped to distribute centrifugal loads placed on the bucket in a substantially even distribution among each of the plurality of hooks.
In another aspect of the present invention, there is a turbine rotor that includes a plurality of turbine buckets coupled with a rotor wheel. Each turbine bucket comprises a bucket dovetail having a plurality of bucket dovetail hooks. The rotor wheel comprises a wheel post dovetail shaped with wheel post dovetail hooks to engage with the bucket dovetail hooks of each bucket dovetail. The connection of the plurality of bucket dovetail hooks from each bucket dovetail with the wheel post dovetail hooks forms at least one pocket therebetween to facilitate substantial sharing of centrifugal loads placed on the turbine bucket by the plurality of bucket dovetail hooks.
In a third aspect of the present invention, there is a bucket dovetail for securing a bucket to a rotor wheel. In this embodiment, the dovetail comprises a plurality hooks that connect the bucket with the rotor wheel. The plurality of hooks are shaped to distribute centrifugal loads placed on the bucket in a desired distribution among each of the plurality of hooks, wherein the desired distribution subjects each of the plurality of hooks to a predetermined amount of centrifugal load and stress.
Referring to the drawings,
A rotor shaft 115 extends through LP sections 105 and 110. Each LP section 105 and 110 includes a nozzle 120 and 125, respectively. A single outer shell or casing 130 is divided along a horizontal plane and axially into upper and lower half sections 135 and 140, respectively, and spans both LP sections 105 and 110. A central section 145 of shell 130 includes a low pressure steam inlet 150. Within outer shell or casing 130, LP sections 105 and 110 are arranged in a single bearing span supported by journal bearings 155 and 160. A flow splitter 165 extends between LP sections 105 and 110.
During operation, low pressure steam inlet 150 receives low pressure/intermediate temperature steam 170 from a source, such as, but not limited to, a high pressure (HP) turbine or an intermediate (IP) turbine through a cross-over pipe (not shown). Steam 170 is channeled through inlet 150 wherein flow splitter 165 splits the steam flow into two opposite flow paths 175 and 180. More specifically, in the exemplary embodiment, steam 170 is routed through LP sections 105 and 110 wherein work is extracted from the steam to rotate rotor shaft 115. The steam exits LP sections 105 and 110 where it is routed for further processing (e.g., to a condenser).
It should be noted that although
In one embodiment, hooks 250T and 250M are shaped to form a cantilever to distribute centrifugal pull loads that are placed on bucket 200 as a result of high rotational speeds obtained by rotor wheel 300. These centrifugal loads induce stresses in bucket 200 which minimize its life expectancy of use. A top portion of hooks 250T and 250M is defined by a radius 260 and a flat depth 270. The distribution of the centrifugal loads by hooks 250T and 250M is a function of the radius and/or flat depth. In particular, an increase in radius 260 and/or an increase in the flat depth section 270 will be proportional to a decrease in the centrifugal load that is placed on hooks 250T and 250M. Likewise, the stress that the centrifugal loads will have on the cantilever is a function of the radius 260 and/or flat depth 270. In particular, an increase in radius 260 and/or an increase in flat depth 270 will be proportional to a decrease in the stress that is experienced by hooks 250T and 250M. In one embodiment, radius 260 ranges from about 2 millimeters (0.079 inches) to about 8 millimeters (0.31 inches), while flat depth 270 ranges from about 2 millimeters (0.079 inches) to about 9 millimeters (0.35 inches). Those skilled in the art will recognize that the values selected for radius 260 and flat depth 270 will depend on how much centrifugal load and stress is desired for a particular hook, and thus values may vary from one dovetail design to the next.
When operational, bucket 200 and the other buckets that form the turbine rotor are subject to large centrifugal pull loads due to the high rotational speed of rotor wheel 300 which in turn stresses the buckets. In a conventional bucket assembly, it has been determined herein that most of the load is concentrated on the top bucket dovetail hooks of the dovetail as compared to the hooks of the other sections of the dovetail. As a result, the bucket will be subject to higher stresses at the top section of bucket dovetail hooks which limit how much load the bucket dovetail can actually carry. Sometimes, the actual load may be less than what the bucket dovetail was designed to carry. Further, such higher stresses will diminish the overall life of the turbine bucket.
Embodiments of the present invention overcome these load problems by ensuring that the centrifugal loads are shared by all of the bucket dovetail hooks in the dovetail and not concentrated on only one section of bucket dovetail hooks. In particular, by having hooks 250T and 250M form a cantilever, the centrifugal loads are distributed substantially even among all of the hooks (i.e., 250T, 250M, 250B). As used herein, centrifugal loads that are distributed substantially even mean that the load on any hook is within about 15% of the load placed on any of the other bucket dovetail hooks. By using radius and flat depth values in the ranges identified above, hooks 250T and 250M become longer, which makes them less stiff and more flexible. Furthermore, as will be shown below, such values will result in the formation of pockets at locations where bucket dovetail hooks 250T and 250M of dovetail 210 engage with wheel post dovetail hooks 320T and 320M, respectively. These pockets form because the values of the radius and flat depth of bucket dovetail hooks 250T and 250M change with respect to the values of the radius and flat depth of wheel post dovetail hooks 320T and 320M. A conventional assembly of bucket dovetails to the wheel post dovetail will not have pockets as herein because the dimensions of the bucket dovetail hooks and wheel post dovetail hooks are substantially similar. As a result of the configuration described herein, embodiments of the present invention are able to distribute the loads substantially even among all of the bucket dovetail hooks (e.g., 250T, 250M, 250B) as compared to the conventional assembly which has the loads concentrated on the top hooks of the bucket dovetail.
A comparison of the difference between the assembly of the bucket dovetail hooks to the wheel post dovetail hooks is illustrated in
The below table shows an example of results comparing the load sharing and stresses associated with the bucket dovetail 410 shown in
As shown in the table, the load on the hooks of the prior art bucket dovetail shown in
The benefits of this load sharing are felt in the stresses that are incurred by the hooks. For example, the table indicates that the stress at the top hooks of the bucket dovetail is 144,900 psi, while the stress at the middle hooks and bottom hooks of the bucket dovetail is 127,700 psi and 155,100 psi, respectively. The stress at the wheel post dovetail hooks of the rotor wheel is 134,400 psi for the top hooks, 140,600 psi for the middle hooks and 159,100 psi for the bottom hooks. As one can see, the maximum stress on the hooks of the bucket dovetail for the designs of the embodiments of the present invention is 155,100 psi, which is significantly less than 198,700 psi for the prior art design. And this maximum stress is on the bottom bucket dovetail hooks and not the top bucket dovetail hooks. Similarly, the maximum stress on the hooks of the wheel post dovetail for the designs of the embodiments of the present invention is 159,100 psi, which is less than 173,100 psi for the prior art design. Reducing stresses on the hooks for both the turbine bucket dovetail and the wheel post dovetail will lead to an increased life for both.
Note that although the results shown in the table were obtained by making modifications to bucket dovetail hooks 250T and 250M in the ranges described above, those skilled in the art will recognize that this is only an example and that one may obtain a desirable load distribution by modifying only the bucket dovetail hooks in the top hook section or the hooks in the middle hook section. Moreover, those skilled in the art will recognize that modifications to bucket dovetails that comprise more or less than three hooks will result in different possibilities for making modifications to the bucket dovetail hooks. Selection of such modifications will ultimately depend on the desired load distribution and the number of hooks in the bucket dovetail.
While the disclosure has been particularly shown and described in conjunction with a preferred embodiment thereof, it will be appreciated that variations and modifications will occur to those skilled in the art. Therefore, it is to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the disclosure.
