Active tip clearance control for shrouded gas turbine blades and related method

A turbine bucket tip clearance control system comprising:

• a rotor assembly including a rotor having a plurality of axially spaced wheels, each of said axially-spaced wheels mounting an annular row of buckets, said annular row of buckets on at least one of the plurality of axially-spaced wheels having a radially outer tip shroud provided with at least one seal tooth;
• a stator assembly comprising a section surrounding said tip shroud and formed with radially inwardly facing seal surfaces including at least one axially-oriented surface substantially parallel with the rotor axis and at least one contiguous acutely angled surface wherein said at least one axially-oriented surface defines a maximum clearance gap and said at least one contiguous acutely angled surface defines a range of clearance gaps less than said maximum clearance gap;
• wherein the at least one seal tooth is configured to be moved axially is such a way as to be positioned opposite the section at a location defined between and including the axially-oriented surface provided the maximum clearance gap and any of the clearance gaps defined by the at least one contiguous acutely angled surface.

A method of controlling tip clearances between a tip shroud on an annular row of turbine buckets mounted on a turbine rotor and a substantially concentrically arranged turbine stator, wherein the tip shroud is provided with at least one radially outwardly projecting seal tooth, and wherein said stator includes a section opposite the tip shroud comprising a radially inwardly facing surface including at least a first horizontal seal surface and a second horizontal seal surface, the first seal surface defining a first distance from the tip shroud and the second seal surface defining a second distance from the tip shroud, the second distance being different than the first distance, the method comprising:

• shifting one of said turbine rotor and said turbine stator axially to cause said at least one radially outwardly projecting seal tooth to radially align with said first horizontal seal surface during transient operations of the turbine, the first horizontal seal surface defining a first seal clearance;
• and
• shifting one of said turbine rotor and said turbine stator axially to cause said at least one radially outwardly projecting seal tooth to radially align with said second horizontal seal surface when the turbine is operating at substantial thermal equilibrium, the second horizontal seal surface defining a second seal clearance.