Steam turbine and method for extracting moisture from a steam turbine

Abstract

A steam turbine is disclosed which includes a casing containing a plurality of expansion stages, each including stator and rotor airfoils. Downstream of the stator airfoils of one or more expansion stages, the casing includes a slot arranged to receive moisture that concentrates on an end wall of the stator airfoils and discharge it to the outside. Each end wall has a side portion facing the slot having a variable radial obstruction in a tangential direction, in order to define with the slot a passage through which the moisture may pass that is smaller or closed at the suction side and larger at the pressure side of the stator blades, to at least partially prevent the dry steam from entering the slot at the suction side of the blade.

1 . Steam turbine comprising:
 a casing having plural expansion stages, each including stator and rotor airfoils,   a slot arranged in the casing downstream of the stator air foils of at least one expansion stage for receiving moisture that concentrates on an end wall of the stator airfoils and for discharging the moisture, wherein each end wall includes a side portion facing the slot, which has a variable radial obstruction in a tangential direction to define, with the slot, a passage for the moisture that is smaller or closed at a suction side of stator blade and larger at a pressure side of the stator blade, to at least partially prevent the dry steam from entering the slot at the suction side.   
   
   
       2 . Steam turbine as claimed in  claim 1 , wherein said slot is a circumferential slot. 
   
   
       3 . Steam turbine as claimed in  claim 1 , wherein the side portion facing the slot of the end wall comprises:
 at the suction side, a thick zone located adjacent to the stator blade and at least partially obstructs the slot.   
   
   
       4 . Steam turbine as claimed in  claim 3 , wherein said thick portion completely obstructs the slot. 
   
   
       5 . Steam turbine as claimed in  claim 3 , wherein said thick portion is adjacent to each of plural stator blades at the suction side. 
   
   
       6 . Steam turbine as claimed in  claim 3 , wherein, the side portion facing the slot of the end wall comprises:
 a thin zone adjacent to the thick zone in a tangential direction.   
   
   
       7 . Steam turbine as claimed in  claim 6 , wherein, in a tangential direction, said thin zone has a thickness that continuously decreases from that of the thick portion toward the side of the end wall. 
   
   
       8 . Steam turbine as claimed in  claim 6 , wherein, in an axial direction, the thin zone has a thickness that continuously decreases in a direction from opposite the slot towards the slot. 
   
   
       9 . Steam turbine as claimed in  claim 1 , wherein the variable radial obstruction comprises
 a slope at the end wall.   
   
   
       10 . Steam turbine as claimed in  claim 6 , comprising:
 a step between the thick zone and the thin zone.   
   
   
       11 . Steam turbine as claimed in  claim 10 , wherein said step comprises:
 a wall perpendicular to surfaces of the thick zone and thin zone.   
   
   
       12 . Steam turbine as claimed in  claim 10 , wherein said step comprises:
 a wall tilted with respect to surfaces of at least one of the thick zone and thin zone.   
   
   
       13 . Method for extracting moisture from a steam turbine including a casing containing plural expansion stages each including stator and rotor airfoils, the method comprising:
 providing, downstream of the stator airfoils of at least one of the plural expansion stages, the casing with a slot arranged to receive moisture that concentrates on an end wall of the stator airfoils;   discharging said moisture; and   at least partially preventing dry steam from entering the slot at a suction side of a blade of the steam turbine by using any end wall of the stator airfoils.   
   
   
       14 . Method as claimed in  claim 13 , comprising:
 arranging the end wall of the stator airfoils to drive the moisture away from the suction side.