GE Gas Turbine - Manual (Page 19)

19 GE Energy Gas Turbines
First Stage Nozzle
The complete first stage nozzle assembly consists of airfoil-shaped vanes which are contained
between an inner and an outer sidewall. The design of the nozzle assembly and the arrangement
for its support within the turbine shell accommodate the effects of thermal expansion caused by
the hot gases and keep the assembly properly aligned in the gas path. Seals in the turbine
shell prevent leakage of combustion gases around the nozzle from the inlet to the
exhaust. Compressor discharge air is fed to these sealing rings through orifices in
the shell. A key feature of the first-stage nozzle is the air-cooled partition which
increases nozzle life substantially. Cooled air from the compressor discharge is
directed through the body of the individual nozzle partitions and out holes near
the trailing edge. This not only cools the metal, but blankets the trailing edge
with a film of air. Additionally, relatively thick nozzle partition trailing edges
provide increased strength and oxidation resistance, again providing longer
nozzle life.
Buckets and Wheels
The long shank bucket design lowers the turbine wheel rim a substantial distance below the hot
gas path. The high thermal resistance of the shank results in a considerable temperature drop
between the hot bucket vane and the wheel dovetail, thus reducing temperature levels and
gradients in the turbine wheel and the dovetail area where rotating stresses are high. Further
wheel protection is provided by radial seals on the first-stage bucket shanks that restrict hot gas
leaks into the wheel cavities. Compressor bleed air is used to cool the wheels and maintain
relatively low temperature levels.