Patent Publication Number: US-2013243565-A1

Title: Steam turbine and method for removing moisture from the steam turbine

Description:
FIELD OF THE INVENTION 
     The present invention generally involves a steam turbine and method for operating the steam turbine. In particular, the invention involves a system and a method for removing moisture from a steam flow path. 
     BACKGROUND OF THE INVENTION 
     The steam flow path of a steam turbine is generally formed by a stationary casing and a rotor. In this configuration, a number of stationary vanes are attached to the casing in a circumferential array and extend inward into a steam flow path. Similarly, a number of rotating blades are attached to the rotor in a circumferential array and extend outward into the steam flow path. The stationary vanes and rotating blades are arranged in alternating rows so that a row of vanes and the immediately downstream row of blades form a stage of the turbine. The vanes serve to direct the flow of steam so that it enters the downstream row of blades at the correct angle. The steam imparts kinetic energy to the blades and causes the rotor to rotate, thereby creating mechanical work to drive the turbine and/or any other loads, such as a generator, attached to the rotor. 
     During particular operating conditions of the steam turbine, such as startup or shut down, the steam may condense on the casing, the vanes and/or the blades, thus forming water droplets within the steam flow path. Water droplets moving through the steam flow path create at least two significant issues. First, the presence of water droplets in the steam flow path reduces stage efficiency. Second, such moisture causes premature erosion of the vanes and/or the blades. Many methods exist for removing moisture from the steam flow path. For example, particular methods include providing drain orifices in the casing of the steam turbine to allow the moisture to drain away from the steam flow path. Although generally effective, the drain orifice remains open during steady state operation of the steam turbine, thereby providing a leakage path for the steam to escape. The escaping steam during steady state operation results in decreased turbine efficiency and increased operating costs. Therefore, there is a need for improved systems for removing water in steam turbine engines. 
     BRIEF DESCRIPTION OF THE INVENTION 
     Aspects and advantages of the invention are set forth below in the following description, or may be obvious from the description, or may be learned through practice of the invention. 
     One embodiment of the present invention is a steam turbine that includes a system for removing moisture from a steam flow path of the steam turbine. The steam turbine includes an inner casing that at least partially defines the steam flow path, a passage extending through the inner casing to at least partially define a moisture flow path out of the steam flow path, and a seal operably connected to the passage. The seal has a first position associated with a first set of operating conditions within the steam flow path and a second position associated with a second set of operating conditions within the steam flow path. Another embodiment of the present invention is a method for removing moisture from a steam flow path based on operating conditions within the steam flow path of a steam turbine. The method includes flowing the moisture from the steam flow path through a passage defined through an inner casing of the steam turbine when a first set of operating conditions within the steam flow path exists, and actuating a seal when a second set of operating conditions exists to reduce the moisture flow from the steam flow path. 
     The present invention may also include a steam turbine including an inner casing that at least partially defines a steam flow path, a passage extending through the inner casing to at least partially define a moisture flow path out of the steam flow path, and means for reducing moisture flow from the steam flow path. 
     Those of ordinary skill in the art will better appreciate the features and aspects of such embodiments, and others, upon review of the specification. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A full and enabling disclosure of the present invention, including the best mode thereof to one skilled in the art, is set forth more particularly in the remainder of the specification, including reference to the accompanying figures, in which: 
         FIG. 1  is a cross-sectional view of a steam turbine according to one embodiment of the present invention; and 
         FIG. 2  is an enlarged cross-sectional view of the steam turbine as shown in  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference will now be made in detail to present embodiments of the invention, one or more examples of which are illustrated in the accompanying drawings. The detailed description uses numerical and letter designations to refer to features in the drawings. Like or similar designations in the drawings and description have been used to refer to like or similar parts of the invention. As used herein, the terms “first”, “second”, and “third” may be used interchangeably to distinguish one component from another and are not intended to signify location or importance of the individual components. In addition, the terms “upstream” and “downstream” refer to the relative location of components in a fluid pathway. For example, component A is upstream from component B if a fluid flows from component A to component B. Conversely, component B is downstream from component A if component B receives a fluid flow from component A. 
     Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that modifications and variations can be made in the present invention without departing from the scope or spirit thereof. For instance, features illustrated or described as part of one embodiment may be used on another embodiment to yield a still further embodiment. Thus, it is intended that the present invention covers such modifications and variations as come within the scope of the appended claims and their equivalents. 
     Various embodiments of the present invention provide a steam turbine including a system and a method for removing moisture from a steam flow path defined within the steam turbine. A passage for removing moisture from the steam flow path may be defined through an inner casing that at least partially surrounds a rotor shaft and alternating stages of stationary vanes and rotating blades. A seal may be operably connected to the passage. The seal has a first position associated with a first set of operating conditions within the steam flow path and a second position associated with a second set of operating conditions within the steam flow path. For example, the seal may be configured to open when the first set of operating conditions exist and close when the second set of operating conditions exist within the steam flow path. In particular embodiments, the seal may include a temperature activated valve and/or a pressure activated valve. In other embodiments, the seal may include an electromechanical valve configured to actuate based on at least one operating condition including temperature, pressure or flow rate. In operation, steam is supplied through an inlet, flows into the steam flow path and across the alternating stages of stationary vanes and blades. During certain operating conditions, such as startup and shut down, the steam may condense on the casing, the stationary vanes and/or the blades, thereby creating water droplets within the steam path. The water droplets may flow from the steam flow path and through the passage. Once the steam turbine reaches the second set of operating conditions within the steam flow path, the seal may actuate to close the passage and reduce the moisture flowing from the steam path and/or leakage of the steam from the steam flow path. In this manner, turbine efficiency may be increased and wear on the stationary vanes and the blades may be decreased. Although exemplary embodiments of the present invention will be described generally in the context of an industrial steam turbine steam flow path for purposes of illustration, one of ordinary skill in the art will readily appreciate that embodiments of the present invention may be applied to any steam turbine and are not limited to an industrial steam turbine unless specifically recited in the claims. 
       FIG. 1  shows a perspective partial cut-away illustration of a steam turbine  10 . The steam turbine  10  includes a rotor shaft  12  that includes a plurality of axially spaced rotor wheels  14 . A plurality of rotating blades  16  are mechanically coupled to each rotor wheel  14 . More specifically, the blades  16  are arranged in rows that extend circumferentially around each rotor wheel  14 . A plurality of stationary vanes  18  extend circumferentially around the rotor shaft  12 , and is axially positioned between adjacent rows of the blades  16 . The stationary vanes  18  cooperate with the blades  16  to form a turbine stage. An inner casing  20  at least partially surrounds the rotor shaft  12 , the rotating blades  16  and the stationary vanes  18  and at least partially defines a steam flow path through the turbine  10 . 
     In operation, steam  22  enters an inlet  24  of the steam turbine  10  and is channeled through the steam flow path. The vanes  18  direct the steam  20  downstream against the blades  16 . The steam  22  passes through the remaining stages imparting a force on the blades  16  causing the rotor shaft  12  to rotate. At least one end of the turbine  10  may extend axially away from the rotor shaft  12  and may be attached to a load or machinery (not shown) such as, but not limited to, a generator, and/or another turbine. Accordingly, a large steam turbine unit may actually include several turbines that are all co-axially coupled to a common shaft. Such a unit may, for example, include a high pressure turbine coupled to an intermediate-pressure turbine, which is coupled to a low pressure turbine. 
       FIG. 2  provides an enlarged cross-sectional view of the steam turbine  10  as shown in  FIG. 1  according to one embodiment of the present invention. As shown in  FIGS. 1 and 2 , the steam turbine  10  may include a system  30  for removing moisture from the steam flow path of the steam turbine  10 . As shown in  FIG. 2 , the system  30  may generally include at least one passage  32  extending through the inner casing  20  and at least one seal  36  positioned to seal the passage  32 . The passage  32  may at least partially define at least one moisture flow path  34  through the inner casing  20 , thereby allowing the moisture to flow through the passage  32  and away from the steam flow path. The passage  30  may be conical, oval, circular, scooped, slotted, raised and/or any other shape that may allow the moisture to flow towards and/or through the passage  32 . In particular embodiments, the passage  32  may be positioned at one or more locations along a bottom portion of the inner casing  20  and/or radially around the inner casing  20  in one or more locations along the steam flow path. In particular embodiments, the passage  32  may be radially aligned with the stationary vanes  18  and/or the rotating blades  16 . In addition or in the alternative, the passage  32  may be positioned anywhere within the inner casing  20  where moisture may collect within the steam flow path. 
     The various embodiments of the present invention include a means for reducing the moisture flow from the steam flow path. As used herein, the function “reducing the moisture flow from the steam flow path” includes reducing moisture, such as water droplets or condensate flowing from the steam flow path and/or steam flow leakage from the steam flow path. The structure for “reducing the moisture flow from the steam flow path” includes the at least one seal  36 . The seal  36  may be operably connected to the passage  32 . For example, in particular embodiments, the seal  36  may be inserted and/or press fit into the passage. In other embodiments, the seal  36  and the passage  32  may be complementary threaded to allow the seal  36  to be threaded into the passage  32 . In addition or in the alternative, the seal  36  may be mechanically coupled to an inner surface  40  and/or an outer surface  42  of the inner casing  20 . For example, the seal  36  may be coupled to the inner casing  20  by a hinge, within a slot, within a groove, by welding and/or by brazing. The seal  36  may be configured to transition between an open and/or closed position as various operating conditions occur within the steam flow path. As used herein, the term “operating conditions” may include but is not limited to at least one of steam temperature, steam pressure, steam moisture content, steam path moisture content, steam path flow rate or turbine metal temperature. The seal  36  may include any seal known in the art which may be configured to seal the passage  32 . In at least one embodiment, the seal may include at least one valve  36 . According to particular embodiments, the seal  36  may include at least one of a temperature activated valve, a bimetallic actuator, a liquid-filled bellows, a pressure activated valve, a variable clearance pressure activated seal, a pressure activated seal, or an electromechanical valve. 
     The seal  36  generally includes a first position associated with a first set of the operating conditions within the steam flow path and a second position associated with a second set of the operating conditions within the steam flow path. The first set and the second set of the operating conditions may include one or more of the operating conditions disclosed above. In particular embodiments, the first position may correspond to an instance wherein the seal  36  is fully open, thereby allowing the moisture to flow through the passage and away from the steam flow path. For example, the first position and the first set of operating conditions may occur during startup of the steam turbine  10 . During startup the pressure within the steam flow path may be lower than the pressure of the steam flowing through the inlet. In addition, the temperature of the rotor shaft  12 , the rotating blades  16  and/or the stationary vanes  18  may be lower than the temperature of the steam. As a result, the steam may condense on the cooler surfaces, thus forming water droplets within the steam flow path. Therefore, when the first position corresponds to the seal  36  being in the fully open position, the moisture may flow from the steam path, through the passage  34  and away from the steam flow path. As a result, the potential for erosion of the vanes and/or blades may be significantly reduced. In other embodiments, the seal  36  may actuate to the first position when the first set of operating conditions correspond with the steam turbine  10  shutting down. For example, as the steam turbine  10  shuts down the mass flow rate of the steam entering the steam flow path is suddenly decreased or stopped, thus resulting in a sudden drop in pressure and the formation of water droplets within the steam path. 
     The seal  36  generally includes a second position. For example, in particular embodiments, the second position may correspond to the seal  36  in an at least partially and/or a fully closed position wherein the seal  36  reduce and/or prevent the moisture from flowing and/or the steam from the steam flow path from leaking through the passage  32  when a second set of operating conditions exists within the steam flow path. The second set of operating conditions may include one or more of the operating conditions as defined above. In particular embodiments, the second set of operating conditions may correspond to the instance when the steam turbine  10  is operating at a steady state. Steady state operation generally occurs when the temperature and the pressure within the steam flow path and/or the mass flow rate of the steam flowing through the steam flow path have normalized so that the formation of moisture within the steam flow path is minimal. In alternate embodiments, the seal  36  may be actuated from the first position to the second position or from the second position to the first position after a predetermined temperature and/or pressure threshold has been reached. In other embodiments, the seal  36  may be actuated to the first or second position after a specific period of time has elapsed. For example, after the steam turbine  10  has run at steady state for thirty minutes, the seal  36  may actuate to the second position. 
     In further embodiments, the steam turbine  10  may also include at least one sensor  50  disposed within the steam flow path and communicatively coupled to a controller  52 . In particular embodiments, the sensor  50  may be communicatively coupled to the seal  36 . The sensor  50  may be configured to sense at least one of the operating conditions and transmit the sensed operating condition to the controller  52 . The controller  52  may be configured to provide an output signal to the seal  36  to actuate the seal  36  between the first position and the second position. The controller  52  may be configured to automatically actuate the seal  36  based on the sensed operating conditions. In addition or in the alternative, the controller  52  may be configured to alert an operator of the particular operating conditions within the steam flow path to allow the operator to manually actuate the seal  36 . 
     The various embodiments of the present disclosure may provide a method for removing moisture from a steam flow path based on operating conditions within the steam flow path of the steam turbine  10 . The method includes flowing the moisture from the steam flow path through the passage  32  defined through the inner casing  20  of the steam turbine  10  when the first set of operating conditions within the steam flow path exists, and actuating the seal  36  when the second set of operating conditions exists to reduce the moisture flow from the steam flow path. The method may further include positioning a valve in the seal  36  to reduce the moisture flow from the steam flow path. The method may also include actuating the seal to stop the moisture flow from the steam path. 
     The various embodiments shown and described with respect to  FIG. 2  provide one or more technical advantages to enhance the operation of the steam turbine  10 . For example, the passage  32  allow the moisture formed during particular operating conditions within the steam flow path, to flow away from the steam flow path, thereby reducing the potential for erosion of the stationary vanes and/or the rotating blades. The seal  36  may reduce or prevent the moisture and/or steam from flowing through the passage once the steam flow path reaches a predetermined set of operating conditions, such as those that occur at steady state operation. In this manner, more steam may be directed through the steam flow path to impart kinetic energy to the rotating blades and the rotor shaft in order to enhance turbine efficiently. 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they include structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.