Patent Publication Number: US-2012027568-A1

Title: Low-pressure steam turbine and method for operating thereof

Description:
RELATED APPLICATION 
     This application claims priority under 35 U.S.C. §119 to Italian Patent Application No. MI2010A001447 filed in Italy on Jul. 30, 2010, the entire content of which is hereby incorporated by reference in its entirety. 
     FIELD 
     The present disclosure relates to low-pressure steam turbines for addressing exhaust losses through last stage design. 
     BACKGROUND INFORMATION 
     In the field of low-pressure steam turbines there is a desire to reduce exhaust losses in order increase turbine net efficiency. One way of achieving this is to increase the efficiency of the last turbine stage. 
     As described in http://www.powermag.com/issues/cover stories/The-long-and-short-of-last-stage-blades 483 p3.html (14 Jul. 2010) it is known that above last stage exit velocities of approximately 190 m/s (600 ft/sec), exhaust losses in low-pressure steam turbine can increase in a squared relationship with velocity. Although there may seem to be a general benefit in reducing exit velocities, when reduced to below approximately 170 m/s (550 ft/sec), net efficiency can decrease due, for example, to the increasing influence of reverse vortices at the blade root. This conclusion is further supported by, for example, K. Kavney et al “Steam turbine 34.5 Inch Low-Pressure Section Upgrades” GE Energy GER-4269 (08/06), where, on page 7, it concludes that generally exhaust losses are at their minimum when the exhaust velocity is about 600 ft/s (190 m/s). 
     As described in EP 1260674 (A1) turbine stages can be classified as being either impulse or reaction stages depending on the blade root reaction degree. In this context, blade root reaction degree can be defined as the ratio of the heat drop (variation of enthalpy) across a moving blade to the total heat drop across a turbine stage. Impulse blade stages, for example, can have a blade root reaction degree of between 0 to 10%. For reaction blade stages, the blade root reaction degree can rise to about 50%. Stages with blade root degrees of between 10% and 50% can be classified as low-reaction type stages. Methods of achieving a particular blade root reaction degree are both varied and known and can be achieved, for example, by modifying the blades lean or sweep. 
     Depending on the design of a stage, the blade root reaction degree may influence overall blade efficiency. For example, U.S. Application Publication No. 2004/0071544 describes an impulse type stage that has improved efficiency through adaptation of the reaction degree. 
     SUMMARY 
     A multi-stage low-pressure steam turbine according to the disclosure having a last stage, comprising a plurality of stationary vanes circumferentially distributed to form a vane row wherein each vane has an airfoil with a span extending from a radially extending a base to a tip of the airfoil; a plurality of blades circumferentially mounted and distributed on a rotatable rotor of the steam turbine; and a K ratio defined as a ratio of vane throat to pitch, where each of the vanes includes a leading edge that is skewed so as to form a W shaped K-distribution across the span of the vanes. 
     A method is disclosed for operating a steam turbine including a stage having a plurality of stationary vanes circumferentially distributed to form a vane row wherein each vane has an airfoil with a span extending from a radially extending a base to a tip of the airfoil; a plurality of blades circumferentially mounted and distributed on a rotatable rotor of the steam turbine; and a K ratio defined as the ratio of vane throat to pitch, where each of the vanes includes a leading edge that is skewed so as to form a W shaped K-distribution across the span of the vanes; the method comprising configuring the area of the exit region of the last stage for operation at an exit velocity of between 125 m/s and 150 m/s; and adjusting the feed flow rate through the steam turbine such that the exit velocity is between 125 m/s and 150 m/s. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       By way of example, exemplary embodiments of the disclosure are described more fully hereinafter with reference to the accompanying drawings, in which: 
         FIG. 1  is a sectional view of part of an exemplary embodiment of low-pressure steam turbine; 
         FIGS. 2   a  and  2   b  show perspective views of an exemplary embodiment of last stage vane of the steam turbine of  FIG. 1 ; 
         FIG. 3  shows a top view of two last stage vanes of an exemplary embodiment of vane row of the steam turbine of  FIG. 2 ; and 
         FIG. 4  is a graph showing the K-distribution across the span of an exemplary embodiment of vane. 
     
    
    
     DETAILED DESCRIPTION 
     Exemplary embodiments of the present disclosure are now described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purposes of explanation, numerous specific details are set forth to provide a thorough understanding of the disclosure. However, the present disclosure may be practiced without these specific details, and is not limited to the exemplary embodiments disclosed herein. 
     A low-pressure steam turbine last stage is disclosed that can overcome losses due to reverse vortices resulting from low last stage exit velocity. 
     The disclosure is based on providing a low-pressure steam turbine with last stage vanes that have leading edges skewed so as to form a W shaped K-distribution across the span of each of the vanes. This configuration provides a means to adapt the last stage for efficient operation at low exit velocities. 
     In exemplary embodiments, the last stage can be configured as a low reaction stage through&#39;a combination of vane  14  and blade  12  parameters including the tangential lean angle  30 , the tilt angle of the trailing edge in the meridional plane  32  and the vane pitch  26 . A reaction degree configuration between impulse type blading and high root reaction e.g. &gt;50% provides further means of delaying efficiency collapse while operating at low exit velocities. 
     An exemplary method is provided for operating at low exit velocities between 125 m/s and 150 m/s. This can be achieved by adjusting the steam rate through a steam turbine with a configured exit annulus area that has skewed vane leading edges that form a W shaped K-distribution across the span of each vane and can have a last stage root reaction degree of between 15%-50%. 
     Other exemplary embodiments further include vanes with straight trailing edges. 
       FIG. 1  shows an exemplary low-pressure multiple stage turbine steam turbine  10 . Each stage  16  of the low-pressure steam turbine  10  includes a plurality of stationary vanes  14  that are circumferentially distributed on the inner casing  15  to form a vane row, and a plurality of blades  12  that are circumferentially mounted and distributed on a rotating rotor  17 . The pressure of the steam exiting the last stage  18  of the low-pressure steam turbine  10  defines the steam turbine as a low-pressure steam turbine  10 . Typically, at steady state conditions, the last stage  18  exit pressure of a low-pressure steam turbine  10  ranges from atmospheric pressure to a low vacuum. 
     Immediately downstream of the last stage  18 , the inner casing  15  forms an annulus (not shown). The area of the annulus, together with the volumetric flow rate of steam exiting the last stage  18  defines the exit velocity of the steam turbine  10 . 
     As shown in  FIGS. 2   a  and  2   b , each vane  14  has an airfoil  31  with a leading edge  20  and a span  36  defined as a radial extension between a base  38  and a tip  34  of the airfoil  31 . As shown in  FIG. 3 , the distance between adjacent vanes  14  taken from the trailing edge  22  of one of the vanes  14  to the face of an adjacent vane  14  defines a throat  24 . The distance between leading edges  20  of two adjacent vanes  14  defines the pitch  26 . The ratio of the throat to pitch further defines a K-value. 
     In an exemplary embodiment, the leading edge  20  of each vane  14  is skewed so as to form, as shown in  FIG. 2   b  and  FIG. 4 , a W shaped K-distribution across the span  36  of the vanes  14 . 
     In an exemplary embodiment, the skewed vanes  14  can be configured to have a root reaction degree, in operation, between 15% and 50%, for example, between 35% and 45%. As shown in  FIG. 2   a , in a vane  14  of an exemplary embodiment, the trailing edge  22  has a title angle in the meridional plane  32  and, as shown in  FIG. 2   b , a tangential lean angle  30 . The root reaction degree of exemplary embodiments can be defined by a combination of vane  14  and blade  12  parameters including the tangential lean angle  30 , the tilt angle of the trailing edge in the meridional plane  32  and the vane pitch  26 . 
     In an exemplary embodiment, the tangential lean angle  30  can be between 16 degrees and 25 degrees, for example, about 19 degrees. 
     In an exemplary embodiment, the tilt angle  32  can be between 3 degrees and 13 degrees, for example, about 8 degrees. 
     In an exemplary embodiment, as shown in  FIG. 2   b , each vane  14  further has a tapering axial width across the span  36 . 
     In an exemplary embodiment, each vane  14  further has a straight trailing edge  22 . 
     Another exemplary embodiment relates to a method for operating a steam turbine  10  of any of the previously described steam turbines  10 . The method involves configuring the area of the exit region of the last stage  18  for operation at an exit velocity of between 125 m/s and 150 m/s and adjusting the feed flow rate through the steam turbine  10  such that the exit velocity is between 125 m/s and 150 m/s. 
     Thus, it will be appreciated by those skilled in the art that the present invention can be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restricted. The scope of the invention is indicated by the appended claims rather than the foregoing description and all changes that come within the meaning and range and equivalence thereof are intended to be embraced therein. 
     REFERENCE SIGNS 
     
         
           10  Low-pressure steam turbine 
           12  Blade 
           14  Vane 
           15  Inner Casing 
           16  Stage 
           17  Rotor 
           18  Last stage 
         Leading edge 
           22  Trailing edge 
           24  Throat 
           26  Pitch 
           30  Tangential lean angle 
           31  Airfoil 
           32  Tilt angle in the meridional plane 
         Tip 
           36  Span 
           38  Base