Abstract:
A bucket tip clearance control system forms part of a turbomachinery apparatus including a casing, an outer shroud coupled with the casing, and an inner shroud coupled with the outer shroud. The tip clearance control system includes a flow circuit for a thermal medium defining a flow path within the outer shroud. A thermal medium source delivers the thermal medium to the flow circuit in a predefined condition according to operating parameters of the turbomachinery apparatus. The temperature of the outer shroud is controlled according to the predefined condition of the thermal medium. By accurately controlling the temperature of the outer shroud, bucket tip clearance can be controlled and optimized during all of the various operation stages of turbomachinery.

Description:
BACKGROUND OF THE INVENTION 
     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. 
     BRIEF SUMMARY OF THE INVENTION 
     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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     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.