Abstract:
An embodiment providing a seal assembly for turbomachinery for controlling flow of a fluid medium in a fluid path comprises a casing having an internal pressure load path and a seal carrier chamber. A movable seal carrier is disposed within the seal carrier chamber and interposed between the pressure load path and an external flow path. In addition, a pressure selecting apparatus is coupled to the pressure load path to control flow therethrough. Moreover, the position of the movable seal carrier moves in response to a pressure differential determined by the pressure selecting apparatus between the pressure load path and the external flow path.

Description:
BACKGROUND OF INVENTION 
     The present invention relates generally to rotary machines, and more particularly to actuated seals for rotary machines such as steam and gas turbines. 
     Rotary machines include, without limitation, turbines for steam turbines and compressors and turbines for gas turbines. A steam turbine has a steam path that typically includes, in serial-flow relationship, a steam inlet, a turbine, and a steam outlet. A gas turbine has a gas path which typically includes, in serial-flow relationship, an air intake (or inlet), a compressor, a combustor, a turbine, and a gas outlet (or exhaust nozzle). Gas or steam leakage, either out of the gas or steam path or into the gas or steam path, from an area of higher pressure to an area of lower pressure, is generally undesirable. For example, a gas path leakage in the turbine or compressor area of a gas turbine, between the rotor of the turbine or compressor and the circumferentially surrounding turbine or compressor casing, will lower the efficiency of the gas turbine leading to increased fuel costs. Also, steam-path leakage in the turbine area of a steam turbine, between the rotor of the turbine and the circumferentially surrounding casing, will lower the efficiency of the steam turbine leading to increased fuel costs. 
     It is known in the art of steam turbines to position, singly or in combination, variable clearance labyrinth-seal segments and brush seals in a circumferential array between the rotor of the turbine and the circumferentially surrounding casing to minimize steam-path leakage. Springs hold the segments radially inward against surfaces on the casing that establish radial clearance between seal and rotor but allow segments to move radially outward in the event of rotor contact. While labyrinth seals, singly or in combination with brush seals, have proved to be quite reliable, labyrinth seal performance degrades over time as a result of transient events in which the stationary and rotating components interfere, rubbing the labyrinth teeth into a “mushroom” profile and opening the seal clearance. 
     Accordingly, there is a need in the art for a rotary machine having improved leakage control between stationary and rotating components. 
     SUMMARY OF INVENTION 
     One embodiment of the present invention provides a seal assembly for turbomachinery for controlling flow of a fluid medium in a fluid path comprising a casing having an internal pressure load path and a seal carrier chamber. A movable seal carrier is disposed within the seal carrier chamber and interposed between the pressure load path and an external flow path. In addition, a pressure selecting apparatus is coupled to the pressure load path to control flow therethrough. Moreover, the position of the movable seal carrier moves in response to a pressure differential determined by the pressure selecting apparatus between the pressure load path and the external flow path. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     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: 
     FIG. 1 is a schematic, cross-sectional exploded view of a pressure actuated seal comprising a labyrinth seal having a seal carrier disposed therein; 
     FIG. 2 is a schematic, cross-sectional exploded view of another mode of FIG. 1; 
     FIG. 3 is a schematic, cross-sectional exploded view of another embodiment of the instant invention; 
     FIG. 4 is a schematic, cross-sectional exploded view of another mode of FIG. 3; 
     FIG. 5 is a schematic, cross-sectional exploded view of another embodiment of the instant invention; and 
     FIG. 6 is a schematic, cross-sectional exploded view of another mode of FIG.  5 . 
    
    
     DETAILED DESCRIPTION 
     A pressure actuated seal assembly, generally designated  100 , is disposed between a rotating member  110 , for example a rotor, and a casing  115 , where such casing  115  comprises an arcuate seal carrier  130  disposed adjacent to rotating member  110  separating pressure regions on axially opposite sides of arcuate seal carrier  130  (see FIG.  1 ). “Adjacent,” as used herein, means that arcuate seal carrier  130  typically does not touch rotating member  110 . FIG. 1 shows casing  15  having an arcuate seal carrier  130  disposed in a seal carrier chamber  145 . Seal carrier  130  typically comprises, but is not limited to, at least one seal  140 , such as, for example, at least one brush seal, coupled to seal carrier  130 . It will be appreciated that other seals  140  may alternatively comprise abradable seals, honeycomb seals, leaf seals, finger seals, ceramic seals, aramid seals, aspirating seals and combinations thereof. It will be appreciated that in FIGS. 5 and 6, seal carrier  130  is disposed in a labyrinth seal  135  having the aforementioned components, singly or in any combination thereof. 
     A pressure selecting apparatus  165 , used as a pressure source to radially displace seal carrier  130 , is in fluid communication with seal carrier  130 . In one embodiment, by way of example not limitation, as shown in FIGS. 1 and 2, pressure selecting apparatus  165  comprises, but is not limited to, a bellows  160 , a valve  170 , or combinations thereof (see FIGS.  1  and  2 ). In an alternative embodiment, pressure selecting apparatus  165  comprises, but is not limited to, a tubing valve  190 , atmospheric valve  200  or combinations thereof (see FIGS.  3  and  4 ). A drive system (not shown), for example, is used to power pressure selecting apparatus  165 . It will be appreciated that the drive system typically comprises a motor, a pump, an electric power supply, for example. The pump may comprise a gas pump, water pump, or hydraulic pump, for example. The drive system may be manually or electrically actuated and the position of the drive system may be internal or external to pressure actuated seal assembly  100 . 
     These components form part of a turbine in which a flowing fluid medium in a fluid path  105 , for example, gas or steam, is passed between the rotating member  110  and casing  115  (see FIG.  1 ). It will be appreciated, however, that fluid medium in fluid path  105  flows from the high pressure side, designated “Phi”, towards the low pressure side, designated “Plow”, i.e., from the left to right of drawing FIG.  1 . For illustrative purposes, FIG. 1 shows seal carrier  130  and accompanying seal  140  in the closed position. “Closed” as used herein means that seal carrier  130 , in conjunction with seal  140 , restricts the flow of pressure from Phi to Plow. It will be appreciated that seal carrier  130  may comprise, for example, at least one gasket  125  comprising, for example, an o-ring, c-seal or w-seal so as to provide a seal between seal carrier  130  and labyrinth seal  135  (see FIGS. 5 and 6) or alternatively seal carrier  130  and casing  115  (see FIG.  1 ). 
     When pressure selecting apparatus  165  is not displaced, Phi generates a large radial pressure load, designated “F”, on top of seal carrier  130 . As used herein, directional words such as, for example, “on”, “in”, “over”, “above”, and “under” are used to refer to the relative location of elements of pressure actuated seal assembly  100  as illustrated in the Figures and is not meant to be a limitation in any manner with respect to the orientation or operation of pressure actuated seal assembly  100 . Pressure load “F” keeps seal  140  disposed against rotating member  110 . 
     Alternatively, when pressure selecting apparatus  165  is displaced, Plow is disposed in pressure load path  150  so as to create a pressure differential between the top section of seal carrier  130  exposed to pressure load path  150  and the underside of seal carrier  130  exposed to external flow path  155  (see FIG.  2 ). Consequently, the difference in pressures causes Phi to force seal carrier  130  radially outward and open a fluid path gap defined between seal  140  and rotating member  110 . 
     In operation, pressure selecting apparatus  165  causes the seal carrier  130  to move radially in seal carrier chamber  145  during operation or during transient events, for example, during startup and shutdown. In one embodiment, by way of example and not limitation, pressure selecting apparatus  165  comprises a bellows  160  coupled to a valve  170  as shown in FIGS. 1 and 2. When the bellows  160  is not displaced or pressurized, the valve  170  allows Phi to enter pressure load path  150  and subsequently force seal carrier  130  radially inward and thereby causing a restriction of flow of the fluid path  105  between the seal  140  and the rotating member  110  (see FIG.  1 ). However, when the bellows  160  is displaced or pressurized, the valve  170  stops the flow of Phi into pressure load path  150  and exposes pressure load path  150  to Plow (see FIG.  2 ). As a result, seal carrier  130  is forced radially outward due to the pressure difference between the top section of seal carrier  130  exposed to pressure load path  150  and the underside of seal carrier  130  exposed to external flow path  155 . It will be appreciated that Phi may be obtained from internal system pressures or from an external pressure source. For example, Phi may comprise internal pressures generated by the turbine where such pressures are disposed between a turbine housing  120  and casing  115 . It will be appreciated that at least one pressure selecting apparatus  165 , for example, may be disposed in fluid communication with at least one seal carrier  130  (meaning a plurality of seal carriers  130  or a single seal carrier  130 ) in pressure actuated seal assembly  100 . By being able to radially move pressure actuated seal assembly  100  during operation, the life of the seal  140  (e.g., bristles) is extended by moving the position of the seat  140  and seal carrier  130  so as to reduce wear against rotating member When the seal  140  comprises a brush seal, the life and performance of the brush seal bristles, singly or in combination with other seals, for example, is increased due to the fact that the bristles do not continuously rub rotating member  110  at a high speeds in cases when rotating member  110  is off-balance. Continuous rubbing causes the bristles of seal  140  to reach a melting temperature or to cause excessive wear on the bristles. When labyrinth seals are used, the performance of labyrinth teeth seals, singly or in combination with other seals, for example, degrade less rapidly over time because the teeth are less prone to break off or be rubbed into a “mushroom” profile thus maintaining an optimal seal clearance between labyrinth seal  135  and rotating member  110  (see FIGS.  5  and  6 ). 
     In another embodiment, the pressure, designated “F”, may be obtained from internal or external pressure sources by means of a tubing  180  connected internally or externally to pressure actuated seal assembly  100  (see FIG.  3 ). In one embodiment, tubing  180  comprises a manifold-type tubing which comprises a tubing valve  190  disposed internally to the turbine. During steady state operation, tubing valve  190  is positioned to allow Phi to flow therethrough to maintain a radially inward pressure on seal carrier  130  and thereby cause a restriction of flow of the fluid medium in the fluid path  105  between the seal  140  and the rotating member  110  as shown in FIG.  3 . In an alternative embodiment, the pressure “Phi”, may also be obtained from an external system pressure source which uses a drive system (not shown) to supply radially inward pressure “F” to seal carrier  130  (see FIG.  4 ). In this embodiment, tubing valve  190  is closed so as to restrict the flow of Phi into pressure load path  150 , and an atmospheric valve  200  is open so as to allow Plow to be disposed in pressure load path  150  and create a pressure differential between the top section of seal carrier  130 , exposed to pressure load path  150 , and the underside of seal carrier  130  which is exposed to external flow path  155 . As a result, the difference in pressures causes Phi to force seal carrier  130  radially outward and open a fluid path gap defined between seal  140  and rotating member  110 . It will be appreciated that valve  170  (see FIG.  1 ), tubing valve  190  and atmospheric valve  200  (see FIG.  3 ), singly or in combination, are configured so as to allow a fluid medium from either internal or external pressure sources therethrough and create a throttling effect on said seal carrier. “Throttling,” as used herein, means using valve  170  (see FIG.  1 ), tubing valve  190  and atmospheric valve  200  (see FIG.  3 ), singly or in combination, to obstruct the internal or external pressure flow so as to selectively control the force “F” disposed on the seal  140  of the seal carrier  130 . The number and the position of tubing valve  190  and atmospheric valve  200  may be varied depending on the application and the number and position of these valves are used only for illustrative purposes herein. It will be apparent to those skilled in the art that, while the invention has been illustrated and described herein in accordance with the patent statutes, modification and changes may be made in the disclosed embodiments without departing from the true spirit and scope of the invention. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.