Turbine bucket tip shroud reinforcement

Details Turbine bucket tip shroud reinforcement Turbine bucket tip shroud reinforcement

1999-08-26

2001-06-05

Look, Edward K. (Department: 3745)

Rotary kinetic fluid motors or pumps

Working fluid passage or distributing means associated with...

Plural distributing means immediately upstream of runner

C416S191000, C416S24100B, C416S500000, C415S173100

Reexamination Certificate

active

06241471

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to tip shrouded turbine buckets and, in particular, to the reinforcement of the tip shroud to stiffen the shroud and enhance creep life.
Gas turbine buckets or blades are airfoil-shaped components designed to convert the thermal and kinetic energy of flowpath gases into mechanical rotation of the rotor. Turbine performance may be enhanced by providing a seal at the tip of the airfoil to block the flow of air over the top of the airfoil which would otherwise bypass the airfoil and thus not perform any work on the rotor. Thus, such tip seals close the gap between the bucket and the surrounding stationary casings. A variety of seal designs have been developed, all of which require the introduction of a tip shroud to the tip of the airfoil.
A typical tip shrouded turbine bucket configuration is illustrated by way of example in
FIGS. 1-5
. With reference thereto, a typical gas turbine bucket
10
includes a bucket airfoil
12
. This is the active component that intercepts the flow of gases, acting as a windmill vane to convert the energy of the gases into tangential motion, which in turn rotates the rotor to which the buckets are attached. At the top of the airfoil, a seal rail
14
is provided to prevent the passage of flowpath gases through the gap between the bucket tip and the inner surface of the surrounding stationary components (not shown). The seal rail
14
extends circumferentially around the bucket row, beyond the airfoil
12
sufficiently to match up with the seal rails provided at the tip of adjacent buckets, effectively blocking flow from bypassing the bucket row so that airflow must be directed to the working length of the bucket airfoil
12
. The tip shroud
16
is added to provide lateral support to the connection also provides a vibrational constraint to the buckets, raising the buckets' natural frequencies and helping to prevent resonance failures.
The tip shroud
16
is essentially a flat plate supported towards its center by the airfoil
12
but subject to high temperatures and centrifugal loads. As a result, material creep becomes a concern. As noted above, the placement of the tip shroud
16
over the airfoil tip allows the airfoil
12
itself to support some of the shroud directly. However, as can be seen from the schematic illustration of
FIG. 6
, the corners of the shroud are relatively unsupported. Some designs allow for the positioning of the shroud to allow corners corresponding to corners
18
and
26
to be essentially directly over the airfoil
12
but with the consequence that corners
20
and
24
are left quite unsupported. The stiffness of the shroud plate (
16
) is enhanced by the presence of the seal rail
14
. As mentioned above, the shroud is shaped such that the aerodynamic loading on the airfoil causes adjacent shrouds to lock together providing a built in boundary condition for the bucket tip and raising a number of bucket vibrational modes out of the machine operating range. However, creep deformation of the corners of the shroud may result in this locking being lost, lowering the bucket response frequencies, potentially resulting in airfoil high cycle fatigue (HCF) failures.
There have been a number of prior approaches to reduce the problems of shroud creep. For example, the use of directionally solidified or single crystal alloy to enhance creep resistance of the airfoil has been proposed. The complex geometry and tight radii associated with the tip shroud region of the design, however, has prevented bucket casting technology from successfully introducing these features into the tip shroud. Another approach is cooling the shroud. Airfoils themselves may be cooled with, e.g., cooling air introduced at the bucket inner attachment and circulated through the airfoil either through radial passages that are open to the tip of the airfoil or through cast serpentine passages in the airfoil. Again, however, the complexity and size of the tip shroud generally precludes manufacture of such holes in the shrouds, thus preventing active cooling of the shrouds. Finally, modifications to the mechanical design of the shrouds have been considered. Specifically, the shrouds may be shaped or scalloped to reduce centrifugal loads on the corners. This approach has met with significant success but there are limits to its effectiveness, and growth engines will require further enhancements.
BRIEF SUMMARY OF THE INVENTION
The present invention is intended to minimize creep deformation particularly at the unsupported corners of the tip shroud of a turbine bucket. More specifically, in accordance with an exemplary embodiment of the invention, the tip shroud is reinforced by introducing reinforcing bars or rods into the casting to stiffen the shroud and enhance creep life by having the reinforcing bars or rods carry some of the load, reducing the tendency of the shroud to creep. In a presently preferred embodiment of the invention, the reinforcing rods or bars are a ceramic or ceramic-matrix composite. In an exemplary embodiment, the reinforcing rods or bars extend from the relatively unsupported corners of the tip shroud to the region of the seal rail structure.
Thus, the invention is embodied in a tip shrouded blade that comprises an airfoil, a tip shroud provided at a tip of the airfoil, the tip shroud having first and second axial end edges and first and second side edges, and a reinforcing structure integrally formed with the tip shroud, and having a longitudinal axis, wherein the longitudinal axis of the reinforcing structure extends at an angle with respect to a longitudinal extent of the first axial end edge of the shroud.
The invention is also embodied in a turbomachine that includes at least one row of circumferentially engaged tip shrouded blades, each tip shrouded blade including an airfoil, a generally circumferentially extending tip shroud provided at a tip of the airfoil and carried for rotation with the airfoil about an axis, and a reinforcing structure integrated with the tip shroud, each of the tip shrouds being configured for interlock with the tip shrouds of adjacent blades in the row, wherein each reinforcing structure extends at an angle with respect to a circumferential extent of the respective tip shroud.


REFERENCES:
patent: 4776764 (1988-10-01), Ortolano
patent: 4884820 (1989-12-01), Jackson et al.
patent: 5083903 (1992-01-01), Erdmann
patent: 5120197 (1992-06-01), Brooks et al.
patent: 5238364 (1993-08-01), Kreitmeier
patent: 5531568 (1996-07-01), Broadhead

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