Bucket tip clearance control system

This invention relates generally to land-based, i.e., industrial gas turbines and, more particularly, to a gas turbine bucket tip clearance control system including a flow circuit within a turbine outer shroud that controls a temperature of the outer shroud via a thermal medium.

Hot gas path components in gas turbines typically employ air convection and air film techniques for cooling surfaces exposed to high temperatures. High pressure air is conventionally bled from the compressor, and the energy of compressing the air is lost after the air is used for cooling. In current heavy duty gas turbines for electric power generation applications, the stationary hot gas path turbine components are attached directly to massive turbine housing structures, and the shrouds are susceptible to bucket tip clearance rubs as the turbine casing thermally distorts. That is, the thermal growth of the turbine casing during steady state and transient operations is not actively controlled, and bucket tip clearance is therefore subject to the thermal characteristics of the turbine. Bucket tip clearance in these heavy duty industrial gas turbines is typically determined by a maximum closure between the shrouds and the bucket tips (which usually occurs during a transient) and all tolerances and unknowns associated with steady state operation of the rotor and stator.

In some turbine designs, the stage 1 bucket is unshrouded because of complex aerodynamic loading and the stress carrying capability of the bucket. That is, the stage 1 bucket tip has no sealing mechanisms to prevent hot gas from flowing over the bucket tip. It is desirable to maintain a minimum clearance between the bucket tip and the turbine inner shroud so that an amount of hot gas flow that bypasses the turbine (and therefore is not expanded for work) is minimized.

In an exemplary embodiment of the invention, a bucket tip clearance control system forms part of a turbomachinery apparatus including a casing, an outer shroud in a slip fit configuration with the casing, and an inner shroud coupled to the outer shroud. The tip clearance control system includes a flow circuit for a thermal medium, wherein the flow circuit defines a flow path within the outer shroud. A thermal medium source is provided in fluid communication with the flow circuit and delivers the thermal medium to the flow circuit in a predefined condition according to operating parameters of the turbomachinery apparatus, such as steady state operation and transient state operation. The temperature of the outer shroud is controlled according to the predefined temperature conditioning of the thermal medium.

Preferably, the outer shroud of the turbomachinery apparatus includes an upper half secured to a lower half at the horizontal engine split line. In this context, the flow circuit may include at least two cavities in the outer shroud, one of the cavities being disposed adjacent the split line. The flow circuit may include a first flow path within the upper half of the outer shroud and a second flow path within the lower half of the outer shroud. In this context, the flow circuit preferably includes at least two cavities in each of the first flow path and the second flow path, one of the cavities in each of the first and second flow paths being disposed adjacent the split line. In one arrangement, the flow circuit includes four cavities in the outer shroud. These cavities preferably communicate via at least one hole from cavity to cavity or via an array of metering holes from one cavity to another cavity.

FIG. 1 is a cross-sectional view through a portion of a gas turbine, showing the turbine outer casing, outer shroud, inner shroud and first stage bucket tip;

FIG. 2 is a schematic illustration of the tip clearance control system of the invention;

FIG. 3 is a schematic illustration of an upper half flow circuit; and

FIGS. 4 and 5 illustrate the upper half flow circuit shaped corresponding to an upper half of the outer shroud.

Different gas turbine models incorporate different components for desired results, operation and the like. One design includes inner and outer shells with four stages of the inner shell mounting the first and second stage nozzles as well as the first and second stage shrouds, while the outer shell mounts the third and fourth stage nozzles and shrouds. An example of such a turbine design is described in U.S. Pat. No. 6,082,963. An alternative turbine design, which is the subject of the present invention, does not include inner and outer shells, but rather includes an outer casing, an outer stator shroud, and an inner stator shroud disposed adjacent a first stage bucket, which in this design is unshrouded. With reference to FIG. 1, the unshrouded first stage bucket is shown at 12 . The gas turbine 10 includes an inner stator shroud 14 disposed adjacent the first stage bucket 12 defining a bucket tip clearance 16 between the inner stator shroud 14 and the first stage bucket 12 . An outer stator shroud 18 supports the inner stator shroud 14 radially and axially by hooks 24 and circumferentially by pins 20 or the like. An outer casing 22 is coupled with the outer stator shroud 18 . One method of coupling the outer shroud 18 to the turbine casing 22 is a pin scheme similar to that of the inner/outer shell design noted in the patent referenced above. Using this method, the first stage turbine nozzle and shroud can be removed and replaced without removing the entire rotor structure. Another method of attaching the outer shroud 18 to the turbine casing uses transverse hooks 24 in the turbine case 22 and the outer shroud 18 . These hooks 24 have ample clearance to accommodate the radial and circumferential relative motion between the casing 22 and the shroud 18 . This method allows radial expansion with ease of assembly and attachment. Small spring-loaded pins 26 can be installed through the turbine casing 22 to hold down the outer shroud 18 and reduce vibrations. The assembly process would be to install a stage 2 nozzle hanger 28 into the turbine casing 22 , then lower an outer shroud ring assembly of the outer shroud 18 over the transverse hooks 24 until it rests on the nozzle hanger 28 . Of course, the turbine casing can be coupled with the outer shroud, and similarly the outer shroud coupled with the inner shroud, in any known manner accommodating relative radial and circumferential motion between the casing 22 and the shroud 18 . Since the specific coupling between these components does not form part of the present invention, additional details thereof will not be further described.

The outer shroud 18 of the invention is modified from its known construction to accept externally conditioned air (or other suitable fluid medium) flow. As shown in FIG. 2, the external source of air flow comprises a clearance control skid 30 that includes heat exchange components and the like to effect temperature conditioned fluid flow. In this context, the heat exchange components of the clearance control skid 30 can supply cooled air flow or heated air flow according to turbine operating conditions (discussed below). The air flow is conditioned to control the temperature of the outer shroud 18 and thus its radial growth. When the radial position of the outer shroud 18 and thus its attached inner shroud 14 can be externally controlled independent of gas turbine operation, the resulting tip clearance 16 can be chosen to provide optimum turbine efficiency and power generation with minimum risk of rubbing during transient operation (start-up, cool-down, hot restart, etc.).

The outer shroud 18 is preferably formed of two half ring pieces that are bolted together at each horizontal joint and include cloth seals or the like for preventing leakage to form a complete ring encircling the bucket tip circumference. The outer shroud 18 may be fabricated from machined forged plates that are welded together. As an alternative, the outer shroud can be cast, which would minimize machining costs. The size, material and ease of core access makes the outer shroud 18 suitable for a casting process.

High pressure air bled from the compressor existing above the stage 1 nozzle inlets provides flow into tubes 32 via scallops 34 machined into the side of the outer shroud 18 . A metering orifice (not shown) may be disposed at the bottom of the supply holes just prior to entering the inner shroud supply plenum 36 . Preferably, the size and number of scallops 34 , flow tubes 32 and the subsequent metering orifice diameter are optimized to closely match design requirements. An upper leaf seal 38 covers most of the circumference of the outer shroud 18 , except locally at the horizontal engine split line joint, where bolting of the two halves of the outer shroud 18 occurs, thus sealing compressor discharged air from leaking aft.

Externally supplied flow from the clearance control skid 30 provides temperature conditioned air into the outer shroud 18 from suitable connectors that enable fluid flow between components. One such suitable connector is a so-called “spoolie” that is described in, for example, commonly owned U.S. Pat. No. 5,593,274, the contents of which are hereby incorporated by reference. The spoolies 40 or like connectors penetrate the turbine casing 22 at or near a top dead center (TDC) position and a bottom dead center (BDC) position of the engine. In a preferred configuration, four spoolies 40 are included, one at each inlet and exit at both TDC and BDC.

With continued reference to FIG. 1 and with reference to FIGS. 3 and 4, a closed circuit 42 for conditioned air from the clearance control skid 30 is defined by a plurality of cavities within the outer shroud 18 . The flow circuit 42 defines a flow path within the outer shroud for the conditioned flow from the clearance control skid 30 . As discussed above, since the outer shroud 18 includes an upper half secured to a lower half at a split line, each half of the outer shroud 18 includes a separate inlet and outlet for conditioned flow and separate flow paths, respectively. Although the inlets to the upper and lower halves of the outer shroud 18 are separate, all conditioned flow is preferably provided by a single clearance control skid 30 , ensuring that uniform temperature conditioned flow is supplied to both halves of the shroud 18 . This prevents detrimental distortion of the shroud 18 due to non-uniform temperature conditioning fluid medium. Alternatively, multiple clearance control skids 30 could be used to supply each of the upper or lower halves of the shroud 18 . Since the respective flow circuits of the upper and lower halves of the outer shroud 18 are substantially identical, the flow circuit 42 in the upper half of the outer shroud 18 only will be described.

The conditioned flow from the clearance control skid 30 enters the flow circuit through the spoolie 40 at TDC (and BDC). The flow is split at the inlet 50 (FIGS. 4 and 5) by a component 51 that extends from the inlet 50 locally to the bottom inlet cavity of 52 . The conditioning flow is then sent circumferentially via 52 nearly to each horizontal joint within each outer shroud half. The flow is ported through one or more holes from a first end cavity 54 to a second end cavity 56 . More than one hole may be used for porting flow between cavities along with other small diameter holes farther circumferentially back in the flow path to accommodate casting core support. Alternatively, a large slot may connect the two end cavities. The flow in the second end cavity 56 is then directed circumferentially back toward TDC via 58 to a third cavity 60 at TDC again through one or more large holes or series of smaller holes. The flow path continues from TDC back to the horizontal split line of the engine within the third cavity 60 via 62 and passes from the third cavity 60 to a fourth cavity 64 . The flow travels back up to TDC in the fourth cavity 64 via 66 , which acts as a heat exchanger to the first cavity 54 , the second cavity 56 and the third cavity 60 to minimize thermal gradients and overall fluid heat up. Thermal gradients would cause detrimental distortions in the shroud 18 and defeat the purpose of creating a uniformly round static structure to encircle the rotating blades or buckets, and provide an optimized, performance enhancing tip clearance. Finally, the flow exits the outer shroud 18 through a slot outlet 68 that is circumferentially out of plane with the inlet spoolies at TDC, i.e., at the same radial diameter and axial station, just moved circumferentially (e.g., 15 degrees) from TDC. The flow is collected in an outlet spoolie and then piped back to the clearance control skid 30 where the closed loop flow circuit starts over. When the flow in the second cavity 56 follows circumferentially back to TDC, the flow acts as a log mean temperature difference heat exchanger within the outer shroud 18 . That is, the small higher velocity center cavities act as buffering cavities between the large low velocity cold cavity at the back and the low velocity hot cavity at the front, which if adjacent each other could create large thermal gradients within the shroud structure. In flowing back and forth (i.e., top to horizontal) and back and differing velocities the heat of the internal flow in each cavity will conduct to the adjacent cavity creating a heat exchanger between the two cavities and minimizing the given heat up in any one cavity. The method of calculating these fluid heat ups is known as log mean temperature difference.

With the structure of the present invention, internal passages within the outer shroud define a flow path of a flow circuit that condition the outer shroud for minimum thermal gradients (stress) and optimum uniform growth. By assembling the outer shroud in halves, the occurrences of leakage is reduced as compared to existing components while allowing the inner shroud to be positioned optimal to the bucket tip. The clearance control skid communicating with the flow circuit can provide heated flow during transients to move the inner shroud away from the rotor. Subsequently, during steady state operation, the clearance control skid can controllably supply cooling flow to shrink the tip clearance thereby improving efficiency and output.

While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications. and equivalent arrangements included within the spirit and scope of the appended claims.