System design and cooling method for LP steam turbines using last stage hybrid bucket

Abstract

A steam turbine system for use in conjunction with hybrid last stage(s) LP buckets. The system is adapted to cool the bucket tip region during low VAN windage conditions whereby the beneficial design and efficiency outcomes of the use of hybrid blades can be realized.

1 . An axial flow steam turbine including:
 a rotor;   a last stage including a diaphragm comprising an inner ring, an outer ring, and a plurality of nozzles extending therebetween, and a row of buckets secured to said rotor downstream of said diaphragm for rotation with respect to said last stage diaphragm; and   at least one injection assembly for injecting cooling media toward a vicinity of said last stage.   
   
   
       2 . An axial flow steam turbine as in  claim 1 , wherein the injection assembly comprises water sprays in an exhaust area downstream of the last stage buckets. 
   
   
       3 . An axial flow steam turbine as in  claim 1 , wherein said injection assembly comprises a steam or water injection cavity defined in said outer ring and a steam or water injection opening directed towards said nozzles of said diaphragm. 
   
   
       4 . An axial flow steam turbine as in  claim 3 , wherein said injection opening is disposed upstream of the nozzles with respect to a steam flow path through the last stage. 
   
   
       5 . An axial flow steam turbine as in  claim 3 , wherein the injection cavity is downstream of the nozzles with respect to a steam flow path through the last stage. 
   
   
       6 . An axial flow steam turbine as in  claim 1 , comprising a moisture extraction assembly for extracting moisture from a steam flow path through the last stage, downstream of the nozzles and upstream of the buckets with respect to said steam flow path. 
   
   
       7 . An axial flow steam turbine as in  claim 6 , wherein moisture extraction assembly comprises a moisture extraction cavity defined in said outer ring and a moisture extraction groove having an opening having a scoop directed towards said nozzles of said diaphragm. 
   
   
       8 . An axial flow steam turbine as in  claim 1 , wherein at least one said bucket comprises a) a shank portion; and b) an airfoil portion having an operating temperature range, a design rotational speed, a blade root attached to said shank portion, a blade tip, and a radial axis extending outward toward said blade tip and inward toward said blade root, and wherein said airfoil portion also includes: (1) a metallic section consisting essentially of metal and having a first mass density, wherein said metallic section radially extends from generally said blade root to generally said blade tip; and (2) at least one fiber composite section, having a second mass density less than said first mass density. 
   
   
       9 . An axial flow steam turbine as in  claim 8 , wherein said metallic section and said at least one fiber composite section together define a generally airfoil shape at said design rotational speed. 
   
   
       10 . An axial flow steam turbine as in  claim 8 , wherein said fiber composite section is disposed in a pocket defined in a pressure side of said metallic section. 
   
   
       11 . In an axial flow steam turbine including a rotor and a last stage including a diaphragm comprising an inner ring, an outer ring, and a plurality of nozzles extending therebetween, and a row of buckets secured to said rotor downstream of said diaphragm for rotation with respect to said last stage diaphragm, a method of cooling said last stage, comprising:
 injecting cooling media toward a vicinity of said last stage.   
   
   
       12 . A method as in  claim 11 , wherein said injecting comprises spraying water in an exhaust area downstream of the last stage buckets. 
   
   
       13 . A method as in  claim 11 , wherein said injection comprises injecting steam or water from a steam or water injection cavity defined in said outer ring through a steam or water injection opening directed towards said nozzles of said diaphragm. 
   
   
       14 . A method as in  claim 13 , wherein said injection opening is disposed upstream of the nozzles with respect to a steam flow path through the last stage. 
   
   
       15 . A method as in  claim 13 , wherein said injection opening is disposed downstream of the nozzles with respect to a steam flow path through the last stage. 
   
   
       16 . A method as in  claim 11 , further comprising extracting moisture from a steam flow path through the last stage, downstream of the nozzles and upstream of the buckets with respect to said steam flow path. 
   
   
       17 . A method as in  claim 16 , wherein moisture is extracted through a moisture extraction groove having an opening including a scoop directed towards said nozzles of said diaphragm, and into a moisture extraction cavity defined in said outer ring. 
   
   
       18 . A method as in  claim 11 , wherein at least one said bucket comprises a) a shank portion; and b) an airfoil portion having an operating temperature range, a design rotational speed, a blade root attached to said shank portion, a blade tip, and a radial axis extending outward toward said blade tip and inward toward said blade root, and wherein said airfoil portion also includes: (1) a metallic section consisting essentially of metal and having a first mass density, wherein said metallic section radially extends from generally said blade root to generally said blade tip; and (2) at least one fiber composite section, having a second mass density less than said first mass density. 
   
   
       19 . A method as in  claim 18 , wherein said metallic section and said at least one fiber composite section together define a generally airfoil shape at said design rotational speed. 
   
   
       20 . A method as in  claim 18 , wherein said fiber composite section is disposed in a pocket defined in a pressure side of said metallic section.