Low-pressure steam turbine and method for operating thereof

Abstract

The disclosure relates to a multi-stage low-pressure steam turbine and a method for operating thereof. The steam turbine includes a last stage in which the leading edge of each vane of the last stage is skewed so as to form a W shaped K-distribution across the span of the vanes. This shape allows for efficient last stage operation at low last stage exit velocities. The disclosure includes a method for operating such a steam turbine at a last stage exit velocity between 125 m/s and 150 m/s.

1 . A multi-stage low-pressure steam turbine 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 radially from 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 include a leading edge that is skewed so as to form a W shaped K-distribution across the span of the vanes.   
     
     
         2 . The steam turbine of  claim 1 , wherein the last stage is configured to have a root reaction degree, in operation, of between 15% and 50%. 
     
     
         3 . The steam turbine of  claim 2 , wherein the root reaction degree is between 35% and 45%. 
     
     
         4 . The steam turbine of  claim 2 , wherein the airfoil of each vane comprises:
 a tangential lean angle; and   a trailing edge tilt angle in a meridional plane, wherein the root reaction degree is defined by the lean angle, the tilt angle and a vane pitch.   
     
     
         5 . The steam turbine of  claim 4 , wherein the tangential lean angle is between 16 degrees and 25 degrees. 
     
     
         6 . The steam turbine of  claim 5 , wherein the tangential lean angle is 19 degrees. 
     
     
         7 . The steam turbine of  claim 2 , wherein the tilt angle is between 3 degrees and 13 degrees. 
     
     
         8 . The steam turbine of  claim 7 , wherein the tilt angle is 8 degrees. 
     
     
         9 . The steam turbine of  claim 1 , wherein each vane has a tapering axial width across the span. 
     
     
         10 . The steam turbine of  claim 1 , wherein each vane has a straight trailing edge. 
     
     
         11 . The steam turbine of  claim 3 , wherein the airfoil of each vane comprises:
 a tangential lean angle; and   a trailing edge tilt angle in a meridional plane, wherein the root reaction degree is defined by the lean angle, the tilt angle and a vane pitch.   
     
     
         12 . The steam turbine of  claim 3 , wherein the tilt angle is between 3 degrees and 13 degrees. 
     
     
         13 . The steam turbine of  claim 4 , wherein the tilt angle is between 3 degrees and 13 degrees. 
     
     
         14 . The steam turbine of  claim 8 , wherein each vane further has a tapering axial width across the span. 
     
     
         15 . The steam turbine of  claim 2 , wherein each vane further has a straight trailing edge. 
     
     
         16 . The steam turbine of  claim 3 , wherein each vane further has a straight trailing edge. 
     
     
         17 . The steam turbine of  claim 4 , wherein each vane further has a straight trailing edge. 
     
     
         18 . The steam turbine of  claim 5 , wherein each vane further has a straight trailing edge. 
     
     
         19 . The steam turbine of  claim 6 , wherein each vane further has a straight trailing edge. 
     
     
         20 . A method for operating a steam turbine which includes a last stage having a plurality of stationary vanes circumferentially distributed to form a vane row wherein each vane has an airfoil with a span extending radially from 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 an area of an exit region of the last stage for operation at an exit velocity of between 125 m/s and 150 m/s; and   adjusting a feed flow rate through the steam turbine such that the exit velocity is between 125 m/s and 150 m/s.