Low pressure section steam turbine bucket

Abstract

A bucket for use in the low-pressure section of a steam turbine engine is provided. The bucket has a vane length of at least about 45 inches. The bucket is comprised of a dovetail section disposed near an inner radial position of the bucket, a tip shroud disposed near an outer radial position of the bucket and a part span shroud disposed at an intermediate radial position. The intermediate radial position is located between the inner and outer radial positions. The bucket is comprised of a titanium-based alloy having between about 2% and about 6.25% by weight aluminum, up to about 3.5% vanadium, up to about 2.25% tin, up to about 2.25% zirconium, between about 1.75% and about 5.0% molybdenum, up to about 2.25% chromium, up to about 0.7% silicon and up to about 2.3% iron, with the balance being titanium.

1 . A bucket for use in the low pressure section of a steam turbine, said bucket being formed with a vane length of at least about 45 inches and comprising:
 a dovetail section disposed near an inner radial position of said bucket;   a tip shroud disposed near an outer radial position of said bucket;   a part span shroud disposed at an intermediate radial position, said intermediate radial position located between said inner and outer radial positions; and   wherein said bucket is comprised of a titanium-based alloy having between about 2% and about 6.25% by weight aluminum, up to about 3.5% vanadium, up to about 2.25% tin, up to about 2.25% zirconium, between about 1.75% and about 5.0% molybdenum, up to about 2.25% chromium, up to about 0.7% silicon and up to about 2.3% iron, with the balance being titanium.   
   
   
       2 . A bucket according to  claim 1 , wherein said dovetail section is comprised of a curved, axial-entry dovetail. 
   
   
       3 . A bucket according to  claim 1 , wherein said bucket comprises a last stage bucket. 
   
   
       4 . A steam turbine comprising a low pressure turbine section, said low pressure turbine section comprising:
 a plurality of last stage buckets arranged about a turbine wheel, said plurality of last stage buckets having a vane length of about 45 inches or greater, at least one last stage bucket comprising;
 a dovetail section disposed near an inner radial position of said at least one last stage bucket; 
 a tip shroud disposed near an outer radial position of said at least one last stage bucket; 
 a part span shroud disposed at an intermediate radial position, said intermediate radial position located between said inner and outer radial positions; and 
   wherein each of said plurality of last stage buckets are comprised of a titanium-based alloy having between about 2% and about 6.25% by weight aluminum, up to about 3.5% vanadium, up to about 2.25% tin, up to about 2.25% zirconium, between about 1.75% and about 5.0% molybdenum, up to about 2.25% chromium, up to about 0.7% silicon and up to about 2.3% iron, with the balance being titanium.   
   
   
       5 . The steam turbine according to  claim 4 , wherein said plurality of last stage buckets comprise an exit annulus area of about 112 ft 2  or more. 
   
   
       6 . The steam turbine according to  claim 4 , wherein a said plurality of last stage buckets rotate at an operating speed of about 1,800 rpm to about 3,600 rpm. 
   
   
       7 . The steam turbine according to  claim 4 , wherein the tip shrouds of said plurality of last stage buckets are configured to have a gap between a tip shroud of an adjacent last stage bucket, and wherein said gap is closed as said turbine wheel rotates above a predetermined speed and said plurality of last stage buckets untwist due to the rotation of said turbine wheel. 
   
   
       8 . The steam turbine according to  claim 4 , wherein the part span shrouds of said plurality of last stage buckets are configured to have a gap between a part span shroud of an adjacent last stage bucket, and wherein said gap is closed as said turbine wheel rotates above a predetermined speed and said last stage buckets untwist due to the rotation of said turbine wheel. 
   
   
       9 . The steam turbine according to  claim 4 , wherein said dovetail section is comprised of a curved, axial-entry dovetail.