Abstract:
A system for removing moisture from a steam/water mixture engaging a stationary component of a steam turbine. The system includes an airfoil located within a flow path of a steam turbine. The airfoil is configured for removing moisture from a steam/water mixture traveling in the flow path. To this end, the airfoil includes a cavity in flow communication with the steam path through at least one inlet and outlet opening, near the leading and trailing edge of the airfoil, respectively. Moisture and steam are extracted from the surface through the inlet openings, the steam and water are separated in the cavity, the separated water flows towards the bottom end, and the dry steam flows through the outlet opening and returns to the steam path. The dry steam blowing out of the trailing edge reduces the size of secondary droplets, and thereby prevents erosion.

Description:
BACKGROUND OF THE INVENTION 
     The present application relates generally to steam turbines, and more particularly, to systems for reducing the level of erosion experienced by steam turbine components. 
     Low-pressure steam turbines are typically driven by wet steam, the moisture content of which may have the form of water film or water droplets. This moisture causes efficiency losses and potential erosion of steam turbine components. This erosion is most prominent in steam turbine airfoils/blades as the moisture content of the steam impacts the nozzles (stationary airfoils) or buckets (rotating airfoils). The erosion is even more exaggerated in some last stages of steam turbines, where speed and local wetness values are highest. 
     Several solutions have been proposed to reduce the amount and/or size of water droplets accumulated on steam turbine components. One solution adds radial grooves close to the leading edge of rotating airfoils to remove the deposited moisture. These grooves, however, only remove moisture that has already caused significant efficiency losses to the rotating airfoils and upstream stationary airfoils. Other solutions rely on protective measures, which include water removal through water drainage arrangements in outer sidewalls (end walls) of the nozzle; or through suction slots made in hollow stator airfoils. This moisture is then collected in circumferential cavities between the diaphragm and the casing and drained to a condenser. 
     These moisture removal concepts are based on extraction of moisture film from blade surfaces, through slots, driven by the pressure drop between the steam path and the hollow blade inner space. This pressure drop causes a significant amount of steam to pass through the hollow stator blades and into the condenser. This decreases the steam turbine efficiency. 
     Another recently developed technique extracts moisture from blade surfaces through multiple extraction bores in the airfoils. There, the extracted moisture is led to an external steam/moisture separator, the separated water is drained, and the steam is returned back to the main steam path through a steam injection bore located in the center of the pressure side. This technique provides moisture removal as well as steam reinsertion into the steam path, thus improving steam turbine efficiency. There remains, however, room for improvement in providing further structures aimed at reducing blade erosion. 
     As a result, there is a desire for improved systems for efficiently and cost effectively reducing moisture-related issues in steam turbine components, such as efficiency losses and potential erosion. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The present application describes a system for removing moisture from a steam/water mixture engaging a stationary component of a steam turbine. The system includes an airfoil, which is disposed in a group of airfoils located within a flow path of a steam turbine. The airfoil is configured for removing moisture from a steam/water mixture traveling in the flow path. Here, the airfoil includes a first and second longitudinal ends and an outer peripheral wall that integrates the first and second longitudinal ends. The first and second longitudinal ends and the outer peripheral wall collectively define a leading edge, a trailing edge, a suction-side face, and a pressure-side face of the airfoil. The airfoil further includes an extraction cavity laterally extending between a portion of the leading edge and a portion of the trailing edge; the extraction cavity comprising an inlet opening in flow communication with the flow path, and an outlet opening in flow communication with the flow path. Moreover, the airfoil includes a cavity configured for separating the steam/water mixture into steam and water, which extends longitudinally within at least a portion of the airfoil. The cavity comprises a top end integrated with the extraction cavity, and a bottom end configured for allowing water to exit the airfoil. As the steam/water mixture travels in the flow path, the inlet opening draws in a portion of the steam/water mixture. A pressure drop across the leading edge and the trailing edge then allows for the portion of the steam/water mixture to enter the cavity. Density differences of the steam and water allow the water to separate from the steam. The separated water flows towards the bottom end, and the steam flows through the outlet opening and returns to the steam path. 
     These and other features of the present application will become apparent upon review of the following detailed description of the preferred embodiments when taken in conjunction with the drawings and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic cross-sectional view of a portion of a steam turbine stage illustrating steam and moisture flow there through. 
         FIG. 2  is a schematic illustrating an isometric view of an airfoil, in accordance with an embodiment of the present invention. 
         FIG. 3  is a top sectional view of the airfoil of  FIG. 2 , illustrating the flow path through the airfoil, in accordance with an embodiment of the present invention. 
         FIG. 4  is a schematic top view of an airfoil having multiple inlet openings, in accordance with an alternate embodiment of the present invention. 
         FIG. 5  is a schematic isometric view of an airfoil having multiple openings, in accordance with another alternate embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following terms used in the description are defined as follows. The terms “downstream” and “upstream” indicate a direction relative to the flow of working fluid through the steam turbine. As such, the term “downstream” means the direction of the flow, and the term “upstream” means in the opposite direction of the flow through the steam turbine. Related to these terms, the terms “aft” and/or “trailing edge” refer to the downstream direction, the downstream end and/or in the direction of the downstream end of the component being described. Moreover, the terms “forward” or “leading edge” refer to the upstream direction, the upstream end and/or in the direction of the upstream end of the component being described. 
       FIG. 1  is a schematic cross-sectional view of a portion of a steam turbine stage illustrating steam and moisture flow there through.  FIG. 1  illustrates a portion of a steam turbine stage illustrating the steam and moisture flow through the various stage components. A steam turbine stage generally include two rows of interspersed airfoils—one row of stationary airfoils  102  and the other of rotating airfoils  104 , with the rotating airfoils  104  disposed downstream of the stationary airfoils  102 . The stationary airfoils  102  (sometimes referred to as nozzles) can direct the steam onto the rotating airfoils  104  (sometimes referred to as buckets) to cause the rotating airfoils  104  to rotate with a speed corresponding to the steam pressure. Together, a set of stationary airfoils  102  and a set of rotating airfoils  104  form a steam turbine stage, and the steam turbine may include multiple such stages. 
     In low-pressure steam turbines, some of the steam may nucleate to form moisture droplets, referred to as primary droplets  106 , which can be very small (typically less than 0.2 micron). As illustrated in  FIG. 1 , these primary droplets  106  generally follow the main steam path (depicted generally at  108 ); However, due to inertial and turbulent deposition, some primary droplets  106  can deposit on the nozzle surfaces in the form of water films or rivulets and may travel downstream to the trailing edge  112  of the nozzle. Additionally, since the main steam path  108  is turning inside the airfoil channel, the centrifugal force will push the droplets towards the pressure side face  114  of the airfoil. These droplets will also accumulate near the trailing edge  112  of the pressure side face of the airfoil; forming water films and rivulets that travel downstream to the trailing edge  112 . On reaching the trailing edge  112 , these water films or rivulets tend to liberate from the stationary airfoil  102  and may form relatively larger secondary droplets  116  (as large as 100-300 microns). 
     Secondary droplets  116  may be accelerated by the main steam path  108 , and due to size, may lag behind the main steam path  108 . The secondary droplets  116 , moving slower than the surrounding steam, may reach the downstream rotating airfoils  104  and impact the suction side (convex side) of the leading edge  118 . This moisture impact may cause potential erosion and efficiency losses in the steam turbine. 
     To reduce the erosion effects on the rotating blades and to improve steam turbine efficiency, an embodiment of the present invention provides an improved airfoil  200 .  FIG. 2  is a schematic illustrating an isometric view of an airfoil  200 , in accordance with an embodiment of the present invention. In one embodiment, the airfoil  200  may be a stationary airfoil, which may be interspersed in a set of airfoils, or the airfoil  200  may be a first stage stationary airfoil. The airfoil  200  may be located within a low-pressure steam turbine, as seen in  FIG. 2 , in which the main steam path  202  is indicated by dotted lines, and hashed lines indicate the moisture path  204 . The airfoil  200  may generally be described as having two longitudinal ends and a peripheral wall, defining a leading edge  206 , a trailing edge  208 , a pressure-side face  210 , and a suction-side face  212 . 
     An embodiment of the airfoil  200  may include at least one opening  218  to draw in moisture from the airfoil  200  surface. Some steam may also escape with the moisture; to return this steam to the main steam path  202 , the airfoil  200  may include a cavity  214  that separates the moisture from the steam, drains the moisture, and returns dry steam to the main steam path  202 . This feature of the cavity  214  may increase the steam turbine efficiency. The cavity  214  may extend longitudinally through at least a portion of the length of the airfoil  200 . The top end of the cavity  214  may be integrated with the top end surface of the airfoil  200 , while the bottom end of the cavity  214  may include a moisture draining facility  216 . The moisture draining facility  216  may be connected to an external condenser. This may allow the drained water to flow to the condenser for further use. The moisture draining facility  216  from each airfoil  200  may be connected to a circumferential cavity in the diaphragm outer ring, or the inner ring, that provides water collected from the airfoil  200  to the external condenser. In an alternate embodiment, the airfoil  200  may be hollow and not integrated with condenser. In an alternate embodiment of the present invention, the moisture draining facility  216  may discharge to a common receiver  500 , as illustrated in  FIG. 5 . 
     One or more inlet openings  218  and outlet openings  220  connecting the airfoil surface to the cavity  214  may extract moisture from the surface of the airfoil  200  and re-introduce the dry steam into the main steam path  202 , respectively. Moreover, the inlet openings  218  and outlet openings  220  may include multiple openings or a single longitudinally extending cavity, depending on the application.  FIG. 2  depicts one embodiment of the inlet openings where the inlet opening  218  may connect the cavity  214  to the outer surface of the leading edge  206 . The inlet opening  218  may extend longitudinally along at least a portion of the leading edge  206 . The inlet opening may be in flow communication with the main steam path  202 . This inlet opening  218  may extend from the outer surface of the leading edge  206  to the cavity  214 . 
     The location of the inlet openings  218  may be based on pressure distribution across the airfoil  200 . A single inlet opening  218  may be located at any position on the airfoil  200  that allows moisture extraction, such as the leading edge  206 , the pressure-side face  210 , or the suction-side face  212 . If the airfoil  200  includes multiple inlet openings  218 , the location of the inlet openings  218  on the airfoil surface may be selected to minimize the pressure difference between the multiple inlet openings  218 . Maintaining a minimum pressure difference between the inlet openings  218  may ensure that steam entering from one inlet opening  218  does not exit from another inlet opening  218 . For example, but not limiting of, the inlet openings  218  may be located on the airfoil surface in regions of maximum moisture impact having similar pressure values. 
     The outlet openings  220 , similarly, may be positioned based on the pressure distribution across the airfoil  200 . The outlet opening  220  may be at a lower pressure level than that of the inlet openings  218 , so that steam moves toward the low-pressure area and exits the airfoil  200 . The trailing edge  208  typically has the lowest pressure value on the airfoil  200 ; and in one embodiment, the outlet opening  220  may be positioned near the trailing edge  208 . The outlet opening  220  may extend from the cavity  214  to the surface of the trailing edge  208 . The outlet opening  220  may also extend longitudinally along at least a portion of the trailing edge  208 . The outlet opening  220  may also be in flow communication with the main steam path  202 . In other embodiments, the outlet opening  220  may be positioned at a relatively lower pressure region than the inlet openings  218 . In  FIG. 2 , embodiments of the inlet opening  218  and outlet opening  220  are illustrated as single elongated slots extending along the airfoil edges. 
     The inlet opening  218 , which may be located on the leading edge  206 , may draw in the water film/droplets due to a pressure difference between the main steam path  202  and the cavity  214 . The structure of the passage between the inlet opening  218  and the outlet opening  220  may induce a negative pressure at the trailing edge  208  of the airfoil  200 . That effect, combined with the relatively high pressure at the inlet opening  218 , may produce a net pressure drop across the airfoil  200 , inducing a general flow towards the trailing edge  208 . Consequently, steam (from the main steam path  202 ) may also be drawn into the cavity  214  through the inlet opening  218 . After the steam-water mixture enters the cavity  214 , water may naturally separate from the mixture. This effect may occur because of the velocity decrease associated with the effect of relatively larger cavity size  214 . 
     Gravity acts on the low-velocity steam-water mixture; and the denser water, naturally separates from the mixture, and may be collected at the bottom of the cavity  214 . The remaining steam may flow towards the trailing edge  208  (as the pressure at the trailing edge  208  may be the lower). This steam may be re-introduced to the main steam path  202  via the outlet opening  220 . Here, the outlet opening  220  may be relatively narrower than the cavity  214 , and thus the velocity of the dry steam may increase prior to reentering the main steam path  202 . The dry steam exiting the trailing edge  208  may reduce the size of secondary droplets  116 , accumulated near the trailing edge  208 . The dry exiting steam may energize the moisture film accumulated on the surface of the airfoil  200 , reducing the size of the droplets, thus reducing the effect of the secondary droplets  116  on the steam turbine blades. As moisture may be substantially removed in upstream stationary airfoils  102  and droplet size of the remaining moisture may be reduced, the downstream rotating airfoils  104  may be less impacted by erosion. 
     In an alternate embodiment of the present invention, a steam/moisture separator (not illustrated) may be installed in the cavity  214 . The separator may use centrifugal force, or impingement and gravitational forces, to separate the water from the steam-water mixture. For example, but not limiting of, a cylindrical pipe may be introduced in the cavity  214 . Here, the steam-water mixture may be directed into the cylindrical pipe in the tangential direction allowing the water to separate due to the centrifugal force. The separated water may be collected and drained using the moisture draining facility  216 . The moisture draining facility  216  may then discharge the separated water to a common receiver, such as, but not limiting of, a feed water reservoir or a condenser. Alternatively, the moisture draining facility  216  may simply discard the separated water. Alternatively, any conventional mechanism may be employed to separate water from steam within the cavity  214 . 
       FIG. 4  is a schematic top view of an airfoil  200  having multiple inlet openings, in accordance with an alternate embodiment of the present invention. This embodiment may include a first inlet opening  402 , which may be located near the leading edge  206  on the suction-side face  212 ; and a second inlet opening  404 , which may be located along the pressure-side face  210 . The outlet opening  220  may be located near the trailing edge  208 , as illustrated in  FIG. 2 . During operation, secondary droplets  116  may impact the suction side leading edge  206 . The inlet openings  402  and  404  of this alternate embodiment may be provided in this general area. This alternate embodiment seeks to maintain a minimum pressure difference between the inlet openings  402  and  404 . The position of the inlet opening  404 , along the pressure-side face  210 , may be selected to keep the pressure difference between the two inlets at a minimum level for effective operation. 
     The structure of the cavity  214 , including the inlet openings  402  and  404 , and the outlet openings  220 , may be similar to the structure described in connection with  FIG. 2 . The steam-water mixture from both the inlet openings  402  and  404  may enter the cavity  214 . Here, the water may be separated from dry steam and exit via the outlet opening  220 , as described. 
       FIG. 5  is a schematic isometric view of an airfoil  200  having multiple openings, in accordance with another alternate embodiment of the present invention. 
     The outlet opening  220  in this embodiment may be multiple ports that blow dry steam from the cavity  214  into the main steam path  202 . In a similar embodiment, the inlet opening  218  can take the form of multiple ports. Moisture from the leading edge  206  surfaces may be directed into these ports due to the pressure drop. Recessed cavities may be provided around these inlet ports to facilitate moisture collection and to direct the moisture into the inlet ports. It will be understood that the inlet ports and outlet ports may be formed of any shape or number depending on the application and that any variation in inlet or outlet port shape, number, or size does not depart from the scope of the present invention. 
       FIG. 5  illustrates the moisture draining facility  216  discharging to a common receiver  500 , in accordance with an alternate embodiment of the present invention. This embodiment may be applied on a steam turbine employing multiple airfoils  200  each of which having a moisture draining facility  216 . 
       FIG. 5  illustrates a cavity  214  integrated with a swirling mechanism  510 , in accordance with an alternate embodiment of the present invention. The swirling mechanism  510  may assist with separating the water from the steam of the steam/water mixture flowing through the airfoil  200 . The swirling mechanism  510  may comprise the form of a swirler, impeller, or the like. Here, the steam/water mixture flowing through the airfoil  200  moves the swirling mechanism  510 . 
     Whenever possible, common industry terminology has been used and employed in a manner consistent with its accepted meaning in this disclosure. It is intended, however, that any such terminology be given a broad meaning and not narrowly construed such that the meaning intended herein and the scope of the appended claims is unreasonably restricted. Those of ordinary skill in the art will appreciate that often certain components may be referred to with several different names. In addition, what may be described herein as a single part may include and be referenced in another context as consisting of several component parts, or, what may be described herein as including multiple component parts may be fashioned into and, in some cases, referred to as a single part. As such, in understanding the scope of the present invention, attention should not only be paid to the terminology and description provided, but also to the structure, configuration, function, and/or usage of the component as described herein. 
     As one of ordinary skill in the art will appreciate, the many varying features and configurations described above in relation to the several exemplary embodiments may be further selectively applied to form the other possible embodiments of the present invention. For the sake of brevity and taking into account the abilities of one of ordinary skill in the art, all of the possible iterations is not provided or discussed in detail, though all combinations and possible embodiments embraced by the several claims below or otherwise are intended to be part of the instant application. In addition, from the above description of several exemplary embodiments of the invention, those skilled in the art will perceive improvements, changes, and modifications. Such improvements, changes, and modifications within the skill of the art are also intended to be covered by the appended claims. Further, it should be apparent that the foregoing relates only to the described embodiments of the present application and that numerous changes and modifications may be made herein without departing from the spirit and scope of the application as defined by the following claims and the equivalents thereof.