Patent Application
20100166557
The invention relates generally to steam turbines with drum rotors and more specifically to retention arrangements for buckets of steam turbines with drum rotors.
The current practice for radial loading of steam turbine reaction style buckets involves inserting a dovetailed portion for each bucket into a retaining groove in the steam turbine drum rotor, inserting a loading pin in a tightly controlled radial gap between a base of the retaining groove and a cutout in the bottom of the dovetailed portion, and then hammering the pin into the radial gap such that the pin deforms in the rotor radial direction and loads the bucket radially against a hook in the retaining groove.
For each bucket, there is a loading pin and each loading pin must be hammered manually until the bucket does not move in the rotor groove. This hammering operation, however, introduces an opportunity to damage the bucket as well as the rotor. As an example, for a 30-stage high pressure steam turbine approximately 2600 loading pins must be manually hammered to fasten the buckets radially in place.
The drum rotor 24 is formed with an annular bucket retaining groove configured as a female dovetail slot 26 about the periphery of the wheel with a radially outer wide groove portion 28 for receiving the outer male projection 18, a radially inner wide groove portion 30 for receiving the inner male projection 20, and an intermediate narrow groove portion 32 for receiving the narrow neck 22. An undersurface 33 of the narrow groove portion 32 forms a so-called “hook” that is engaged by the inner projection 20 on the male dovetail 16.
A semicircular retaining groove 35 extends across undersurface 40 of male dovetail 16. When each bucket is loaded into female dovetail slot 26 about the periphery of the wheel, a solid semicircular pin 37 is manually hammered into the semicircular retaining groove 35 to bias the bucket in a radially outward direction, loading the bucket radially against the hook 33.
Traditional methods, and variations thereof, have been to essentially push the bucket outwards, radially, to ensure tight contact between the bucket load surface 33 and the rotor mating surface 39.
Accordingly, there is a need for an improved radial loading technique that provides parts reduction, rotor assembly time reduction, and consistent radial loading of the buckets against the rotor groove hook without danger of damage to the buckets and/or rotor.
The present invention relates to an arrangement and method for positioning and retaining rotor dovetails with a hook-to-hook fit in a drum rotor for a steam turbine.
Briefly in accordance with one aspect of the present invention a loading arrangement adapted for radially loading turbine buckets on a drum rotor of a steam turbine is provided. The loading arrangement provides a drum rotor formed with an annular bucket retaining groove configured as a female dovetail slot about the periphery of a rotor wheel including a radially outer wide groove portion, a radially inner wide groove portion, and an intermediate narrow groove portion. Also provided is a root portion of a turbine bucket with a male dovetail including a radial inner projection and a radial outer projection, spaced by a narrow neck adapted for engaging the female dovetail slot. An undersurface of the narrow groove portion is adapted for engaging with an upper surface of inner projection formed on the male dovetail, adapted for retaining the male dovetail 16 within the female dovetail slot. An undersurface of radial outer projection of male dovetail closely engaging upper surface of radially outer wide groove portion is adapted for loading the bucket radially against the hook.
According to a second aspect of the present invention, a steam turbine drum rotor and bucket assembly is provided. The assembly includes a drum rotor formed with a bucket retaining groove about a periphery thereof and a plurality of buckets, each having a mounting portion including a radially inner face received within the bucket retaining groove. The assembly also includes an upper hook formed at an upper end of a narrowed portion of the bucket retaining groove, and a lower hook formed at a lower end of a narrowed portion of the bucket retaining groove. Also included in the assembly is the mounting portion of each of the plurality of buckets with a male dovetail including a radial inner projection and a radial outer projection, spaced by a narrow neck, adapted for engaging the bucket retaining groove, wherein the upper hook engages the radial outer projection and the lower hook engages the radial inner projection according to a designated loading clearance.
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:
The following embodiments of the present invention have many advantages, including providing the bucket to drum rotor interface with a tighter controlled relationship, providing the benefit of reduced cost and complexity. Rather than utilizing loading pins between the drum rotor and the bucket dovetail, the buckets would be cut such that the clearances were maintained to achieve a minimum of movement from bucket to rotor. Instead of a loose fit, which is then overcome by adding an additional component to fill this gap, the bucket would be produced with a tight “hook-to-hook” fit, which would create the same desired result. Conceptually, this arrangement pulls or lifts the bucket outward radially onto the load surfaces, verses the traditional pushing or wedging outward methods.
This method allows for elimination of loading pins, a significant cost savings and ergonomic improvement to assembly. It also addresses backward compatibility with existing field units as this alternate design can be used on any retrofits or rebucketing. Additionally, it supports ongoing efforts to reduce variation in assembly and maintain consistent compaction of each row of buckets. Further, there is no change to the rotor wheel configuration or to stresses in either the bucket or the wheel. This configuration also allows for easy deviation of the dovetails to support serviceability.
Turbine bucket 10 includes an airfoil portion 12 and a root or base portion 14 that is configured as a male dovetail 16. The male dovetail 16 includes radially outer and inner projections or hooks 18, 20 radially spaced by a narrow neck 22. The drum rotor 24 is formed with an annular bucket retaining groove configured as a female dovetail slot 26 about the periphery of the wheel with a radially outer wide groove portion 28 for receiving the outer male projection 18, a radially inner wide groove portion 30 for receiving the inner male projection 20, and an intermediate narrow groove portion 32 for receiving the narrow neck 22. An undersurface 33 of the narrow groove portion 32 forms a so-called “hook” that is engaged by a top surface 39 of the inner projection 20 on the male dovetail 16.
The underside 25 of outer male projection 18 is cut to establish a tight clearance with upper surface 29 of outer wide groove 28. Such a cut may provide a clearance of about 0.001 to 0.003 inch, eliminating the broad clearance shown in prior art
As an artifact of prior loading on the bucket, semicircular cavity 31 may remain in the male dovetail 16, even though it is no longer required. For new or retrofit buckets, the semicircular cavity need not be provided as it no longer provides the loading function.
While various embodiments are described herein, it will be appreciated from the specification that various combinations of elements, variations or improvements therein may be made, and are within the scope of the invention.
