Cored steam turbine bucket

Abstract

Steam turbine buckets are formed by investment casting to be hollow or to have a lightweight lattice-work internal core. The process is also used to integrally form steam turbine buckets having a variety of dovetail configurations, including a tangential fir-tree dovetail, a tangential slot dovetail and an axial gas turbine style dovetail. The investment casting process can also be used to integrally cast a cover at one end of the steam turbine bucket.

Description

FIELD OF THE INVENTION

[0001] The present invention is directed to the investment casting of steam turbine buckets. More particularly, the present invention provides a method for investment casting lightweight yet structurally sound steam turbine buckets.

BACKGROUND OF THE INVENTION

[0002] Current steam turbine buckets are machined from plate stock and as such are solid buckets. This includes steam turbine buckets with traditional tangential fir-tree dovetails, tangential slot dovetails, and axial entry dovetails.

[0003] A critical design element of steam turbine design is rotor stresses driven by bucket weight. With the given manufacturing processes, however, it is very difficult to reduce bucket weight without compromising mechanical integrity.

[0004] The prior art technology for high pressure and intermediate pressure steam turbine rotating bucket airfoils is primarily custom components machined from plate stock. These solid airfoils are necessarily heavy and necessitate significant rotor structures to support them.

SUMMARY OF THE INVENTION

[0005] It is desirable to replace the conventionally machined solid buckets with lighter parts manufactured by investment casting without compromising structural integrity. The present invention accomplishes the goal of providing a lightweight yet strong, steam turbine airfoil.

[0006] The present invention proposes a investment casting process for hollow steam turbine buckets, which may also include a lattice-work of crossing channels internal to the airfoil to reduce weight while maintaining mechanical integrity. The new structural design uses a robust investment casting, facilitates an integral cover (or bucket shroud) design and provides the potential for steam turbine bucket cooling.

[0007] The lattice-work of channels is placed at opposing angles to create a cris-crossed rib structure internal to the airfoil with good stress distribution. Preferably, there are no internal main ribs in the design, but could be if mechanical design considerations required such main ribs.

[0008] The lattice-work of channels is formed from a highly connected ceramic core piece, which leads to very high yield rates for investment castings. The new structural design results in an airfoil having a lower overall weight, which serves to reduce corresponding rotor structures and overall unit size.

[0009] Advantages of the new structural design include weight reduction and an improved rotor thermal transient response due to reduced weight. In addition, investment cast procedures facilitate design features such as integral covers. Moreover, the inventive process provides the ability to tailor design response by altering internal geometry (lattice density).

[0010] The use of investment casting provides a high casting yield relative to traditional gas turbine designs. Finally, the use of investment casting provides the potential for steam turbine bucket cooling.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] FIG. 1 shows a cut-away section of a steam turbine bucket airfoil to have a lattice-work internal core and with the bucket having a tangential fir-tree dovetail;

[0012] FIG. 2 shows a cut-away section of a steam turbine bucket airfoil to have a lattice-work internal core and with the bucket having a tangential slot dovetail;

[0013] FIG. 3 shows a cut-away section of a steam turbine bucket airfoil to have a lattice-work internal core and with the bucket having an axial entry dovetail; and

DETAILED DESCRIPTION OF THE INVENTION

[0014] Due to the custom nature of conventional steam turbine designs, steam turbine buckets are traditionally manufactured from plate stock using flexible machining processes. The present inventors were the first to realize that as steam turbine products become more structured or pre-engineered, as are gas turbine products, the re-use of steam turbine bucket designs made an investment casting manufacturing process more feasible.

[0015] They were also the first to realize that while investment casting provided an opportunity for significant bucket weight reduction, the fragile nature of traditional gas turbine cores with a few internal radial ribs, often lead to a high rate of ceramic core breakage and variability in cast wall thickness. The proposed investment cast process addresses these problems and issues.

[0016] FIGS. 1-3 show in cut-away section, the use of lattice-works 10, 20, 30 that respectively provide high solidity ceramic cores which are much more durable for manufacturing. Alternatively, these steam turbine buckets could be formed by investment casting without the lattice-work internal cores so as to be hollow.

[0017] The lattice-works 10, 20, 30, however, allow greater control of wall thickness in the end product, and improve the mechanical integrity relative to the traditional alternative with large internal cavities. FIG. 1 shows a steam turbine bucket having a tangential fir-tree dovetail 11, FIG. 2 shows a steam turbine bucket having a tangential slot dovetail 21, FIG. 3 shows a steam turbine bucket having an axial entry dovetail.

[0018] Preferably, the range of angles for the lattice work crossing channels or ribs is from 25° to 55° on one wall and then −25° to −55° on the opposite wall. The preferred ratio for size of the channels to the thickness of the ribs is 4:1 to 1:1. For example, for the midpoint of the range, ribs 0.1 inch would be spaced 0.25 inches apart.

[0019] The investment casting process also provides the opportunity to cast-in design features such as integral covers 12, 22, 32 that reduce bucket tip steampath leakage. The reduced weight of the cast buckets and associated rotor improve thermal transient response to be more consistent with the static shell structures, improving transient response, maintaining tighter clearances and improved unit performance.

[0020] In a structured steam turbine product line HP and IP buckets will be re-used from unit to unit as steampaths are duplicated. This re-use makes casting tooling and casting buckets as a manufacturing process feasible. Casting opens the opportunity to significantly reduce bucket weight and the associated rotor mass by the use of cores.

[0021] 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.

Claims

1. A method of forming a steam turbine bucket airfoil by using an investment casting process and forming a hollow steam turbine bucket airfoil.

2. The method as claimed in claim 1, wherein a cover is integrally cast with the hollow steam turbine bucket airfoil.

3. The method as claimed in claim 1, wherein the formed hollow steam turbine bucket airfoil is integrally cast with a tangential fir-tree dovetail.

4. The method as claimed in claim 1, wherein the formed hollow steam turbine bucket airfoil is integrally cast with a tangential slot dovetail.

5. The method as claimed in claim 1, wherein the formed hollow steam turbine bucket airfoil is integrally cast with an axial entry dovetail.

6. The method as claimed in claim 2, wherein the formed hollow steam turbine bucket airfoil is integrally cast with a tangential fir-tree dovetail.

7. The method as claimed in claim 2, wherein the formed hollow steam turbine bucket airfoil is integrally cast with a tangential slot dovetail.

8. The method as claimed in claim 2, wherein the formed hollow steam turbine bucket airfoil is integrally cast with an axial entry dovetail.

9 A method of forming a steam turbine bucket airfoil, said method comprising utilizing an investment casting process to form an internal core of the steam turbine bucket airfoil having a lattice-work.

10. The method as claimed in claim 9, wherein a cover is integrally cast with the steam turbine bucket airfoil having a lattice work.

11. The method as claimed in claim 9, wherein the formed steam turbine bucket airfoil is integrally cast with a tangential fir-tree dovetail.

12. The method as claimed in claim 9, wherein the formed steam turbine bucket airfoil is integrally cast with a tangential slot dovetail.

13. The method as claimed in claim 9, wherein the formed steam turbine bucket airfoil is integrally cast with an axial entry dovetail.

14. The method as claimed in claim 10, wherein the formed steam turbine bucket airfoil is integrally cast with a tangential fir-tree dovetail.

15. The method as claimed in claim 10, wherein the formed steam turbine bucket airfoil is integrally cast with a tangential slot dovetail.

16. The method as claimed in claim 10, wherein the formed steam turbine bucket airfoil is integrally cast with an axial entry dovetail.

17. A steam turbine bucket airfoil for use in a steam turbine comprising a lattice-work internal core.

18. The steam turbine bucket airfoil as claimed in claim 17, further comprising a cover formed at one end.

19. The steam turbine bucket airfoil claimed in claim 17, further comprising a tangential fir-tree dovetail.

20. The steam turbine bucket airfoil claimed in claim 17, further comprising a tangential slot dovetail.

21. The steam turbine bucket airfoil claimed in claim 17, further comprising an axial entry dovetail.

22. The steam turbine bucket airfoil claimed in claim 18, further comprising a tangential fir-tree dovetail.

23. The steam turbine bucket airfoil claimed in claim 18, further comprising a tangential slot dovetail.

24. The steam turbine bucket airfoil claimed in claim 18, further comprising an axial entry dovetail.

25. A steam turbine bucket airfoil for use in a steam turbine comprising a hollow airfoil core.

26. The steam turbine bucket airfoil as claimed in claim 25, further comprising a cover formed at one end.

27. The steam turbine bucket airfoil claimed in claim 25, further comprising a tangential fir-tree dovetail.

28. The steam turbine bucket airfoil claimed in claim 25, further comprising a tangential slot dovetail.

29. The steam turbine bucket airfoil claimed in claim 25, further comprising an axial entry dovetail.

30. The steam turbine bucket airfoil claimed in claim 26, further comprising a tangential fir-tree dovetail.

31. The steam turbine bucket airfoil claimed in claim 26, further comprising a tangential slot dovetail.

32. The steam turbine bucket airfoil claimed in claim 26, further comprising an axial entry dovetail.