Turbine
5.1 Turbine Section
The turbine section is the area in which
energy in the form of high temperature pressurized gas, produced by the
compressor and combustion sections, is converted to mechanical energy. The 4th
stage axial flow turbine consist of the rotor, casing, exhaust frame, exhaust
diffuser, buckets, nozzles and shrouds.
5.2 Turbine Rotor
A) Structure
The turbine rotor assembly consists of the
aft turbine shaft, the first-, second-, third-, and fourth-stage turbine wheel
assemblies with spacers and turbine buckets.
Concentricity control is achieved with
mating rabbets on the turbine wheels, wheel shaft, and spacers. The wheels,
spacers and aft shaft are held together with 5 sets of bolts that pass through
each of the wheels and mating up with bolting flanges on the shafts and
spacers. Selective positioning of rotor members is performed to improve balance
the assembly.
B) Wheel Shafts
The aft shaft of the turbine rotor includes
the NO.2 bearing journal.
C) Wheel assemblies
Spacers between the first and second, the
second and third and between the third and fourth-stage turbine wheels
determine the axial position of the individual wheels. These spacers carry the
flow path seals. Near flow path seals are attached to the spacers using
circumferential dovetails and serve to protect the rotor surface from hot gas
path temperatures. The 1-2 spacers forward and aft faces include radial slots
for cooling air passages.
Turbine buckets are assembled in the wheels
with fir-tree-shaped dovetails that fit into matching cut-outs in the turbine
wheel rims. All three turbine stage buckets are precision investment-cast. The
shank on these buckets effectively shields the wheel rims and bucket dovetails
from the hot gas path temperatures while mechanically damping bucket
vibrations. Stage three and four buckets are further aided in damping vibration
with interlocking shrouds at the buckets tips. These shrouds increase the
turbine efficiency by minimizing tip leakage. Radial teeth located on the
bucket shrouds mate with stepped surfaces on the stator hardware create
labyrinth seals against gas path leakage around the bucket tips.
The increase in size of the buckets from
the first to the fourth stage is necessitated by the pressure reduction
resulting from energy conversion in each stage, requiring an increased annulus
area to accommodate the gas flow.
D) Cooling
The turbine rotor is cooled to maintain
reasonable operating temperatures and, therefore, assure a longer turbine
service life. Cooling is accomplished by means of a positive flow of cool air
extracted from the compressor and discharged radially outward through a space
between turbine wheel and the stator, into the main gas stream. This area is
called wheelspace.
E) Wheel space
Each turbine wheel has a forward and aft
wheelspace that needs to be purged to prevent the hot gas air path from
permeating these cavities. By maintaining these cavities purged, the integrity
of the turbine structure will be maintained.
The first-stage forward wheelspace is
cooled by compressor discharge air. An inducer at the inner flowpath
efficiently pre-swirls the extraction air, which is then passed through holes
in the midshaft and 0-stage spacer. Inducer air then flows through the
first-stage forward wheelspace and is routed through the first stage bucket, is
discharged into the main gas stream aft of the first-stage nozzle.
All other wheelspaces are purges with 10th
stage compressor extraction air (taken form the inner diameter flowpath at
stage10), which flows through the rotor bore, up through passages in the
turbine wheels, and into the turbine flow path.
5.3 Buckets
Air is introduced into each first-stage and
second-stage bucket through a plenum at the base of the bucket dovetail. it
flow through precision cast serpentine passages and is introduced into the flow
path through a series of cooling holes on the airfoil surface, tip and trailing
edge.
Unlike the first-stage buckets, the
third-stage buckets are cooled with machined internal air passages that travel
the entire length of the foil. This cooling air enters cavity in tip shroud
before exiting into the main gas stream. Air is introduced, like the first and
second-stage, with a plenum at the base of the bucket dovetail.
The holes in the first, second, and third
stage buckets are spaced and sized to obtain optimum bucket cooling while
minimizing the compressor extraction air.
The Fourth-stage buckets are not internally
air cooled. The tips of these buckets, like third stage buckets, are enclosed
with interlocking by tip shrouds that are designed to minimize tip leakage and
dampen the mechanical vibration of these long arifoils.
5.4 Near Flow path seals
a. Near flow path seals are installed using
dovetail mounting in the 1-2, 2-3, and 3-4 spacer. These replaceable seals
provide protect the turbine rotor wheelspaces from hot gas path temperature.
Sealing teeth on this part mate with honeycomb attached to the power nozzles to
isolate turbine stages.
5.5 Structure
The casing area of the turbine section is
composed of six major elements. These are the:
a.
Inner turbine shell
b.
Outer turbine shell
c.
Nozzles
d.
Diaphragms
e.
Shrouds
f.
Exhaust Frame
The Inner turbine shell makes up a portion
of the gas path annulus and supports the power nozzle assemblies and shrouds.
The inner turbine shell is encased and supported by the outer turbine shell.
The outer turbine shell also provides a pressure barrier structural strength to
the gas turbine. Cooling air extracted from the compressor flows to static hot
gas path components though the outer turbine shell. The inner turbine shell is
allowed to ‘float ‘slightly within outer turbine shell for improved performance
from bucket tip clearance control. The inner and outer turbine shells are split
horizontally to provide access for servicing internal components
The exhaust frame supports the rotor at the
aft bearing, makes up the outer wall of the gas-path annulus, and supports the
exhaust diffuser. It is split horizontally to facilitate servicing.
Inner Turbine
Shell
The inner turbine shell
controls the axial and radial positions of the shrouds and nozzles. It
determines turbine clearances and the relative positions of the nozzles to the
turbine buckets. This positioning is critical to gas turbine performance.
The inner turbine shell is
cooled during operation by air flowing from the 8th and 11th
stage compressor extraction air. After cooling the inner turbine shell, 8th
and 11th stage air is directed to the third and second-stage nozzles
respectively for cooling.
The center-line of the
inner turbine shell is aligned to the rotor center-line during assembly and is
supported by ledges in the outer turbine shell.
Outer Turbine
Shell
The outer turbine shell is
bolted to and aft end of the compressor discharge casing. It supports the inner
turbine shell, provides structural strength to the gas turbine, makes up the
outer pressure boundary, and provides a connection point for compressor
extraction piping.
Nozzles
In the turbine section there
are four stages of stationary nozzles which direct the high-velocity flow of
the expanded hot combustion gas against the turbine buckets causing the turbine
rotor to rotate. Because of the high pressure drop across these nozzles, there
are seals at both the inside and the outside diameters to prevent loss of
system energy by leakages. Since these nozzles operate in the hot combustion
gas flow, they are subjected to thermal stresses in addition to gas pressure
loading.
First-Stage Nozzle
The first-nozzle receives the hot combustion gases from
the combustion system via the transition pieces. The transition pieces are
sealed to both the outer and inner sidewalls on the entrance side of the
nozzle.
The 9HA.01 gas turbine first-stage nozzle contains a
forward and aft cavity in the vane and is cooled by a combination of film,
impingement and convection techinques in both the vane and sidewall regions.
The nozzle segments, each with a single airfoil, are
supported at the inner diameter by a horizontally split retaining ring which is
supported by the aft end of the compressor discharge casing. They are supported
at the outer diameter by the first stage shroud.
Second-Stage Nozzle
Air exiting from the first stage buckets is again
expanded and redirected against the second-stage turbine buckets by the
second-stage nozzle. This nozzle is made of cast segments, each with two
airfoil. The male hooks on the entrance and exit sides of the outer sidewall
fit into female grooves on the aft side of the first-stage shrouds and on the
forward side of the second-stage shrouds to maintain the nozzle concentric with
the turbine shell and rotor. This close fitting tongue-and-groove fit between
nozzle and shrouds acts as an outside diameter air seal. The second-stage
nozzle is cooled with 11th stage extraction air.
Third-Stage Nozzle
The third-stage nozzle receives the hot gases as it
leaves the second-stage buckets, increase its velocity by pressure drop, and
directs this flow against the third-stage buckets. The nozzle consists of cast
segments, each with one airfoil. It is held at the outer sidewall forward and
aft sides in grooves in the turbine shrouds in a manner similar to that used on
the second-stage nozzle. The third stage nozzle is cooled by 8th
stage compressor extraction air.
Fourth-Stage Nozzle
The fourth-stage nozzle receives the hot gas as it leaves the
third-stage buckets, increases its velocity by pressure drop, direct this flow
against the fourth-stage buckets. The nozzle consists of cast segments, each
with three airfoils. It is held at the outer sidewall forward and aft sides in
grooves in the turbine shrouds in a manner similar to that used on the
second-stage nozzle. The fourth-stage nozzle is uncooled.
Diaphragm
Attached to the inside diameter of the second third, and
fourth –stage nozzle segments are the nozzle diaphragms. These diaphragms deter
air leakage past the inner sideall of the nozzles and the turbine rotor The
high/low, labyrinth seal teeth are machined into the inside diameter of the
diaphragm. They mate with opposing sealing lands on the turbine rotor. Minimal
radial clearances between stationary parts (diaphragms and nozzles) and the
moving rotor are essential for maintaining low inter-stage leakage, this result
in higher turbine efficiency.
Shrouds
Unlike the compressor blading, the turbine buckets tips do
not run directly against an integral machined surface of the casing but against
thin walled segments mounted female grooves located in the turbine shell. The
shrouds primary function is to provide a cylindrical surface to minimize bucket
tip clearance leakage.
The turbine shrouds’ secondary function is to provide a high
thermal resistance between the hot gases and the comparatively cool turbine
casing. By accomplishing this function, the turbine casing cooling load is
drastically reduced, the turbine casing diameter is controlled, the turbine
casing roundness is maintained, and important turbine clearances are assured.
The first stage stationary shroud segments are in low
pieces. The gas-side inner shrouds is separated from the supporting outer
shroud to allow for expansion and contraction and thereby improve low-cycle
fatigue life. The inner shroud is cooled b impingement, film, and convection.
He shroud segment are maintained in the circumferential
position by radial pins from the inner turbine shell. Joints between shroud
segments are sealed by fixable metal seals.
Exhaust Frame
Gases exhausted from the fourth turbine stage enter the
diffuser where velocity is reduced by diffusion and pressure is recovered. The
exhaust frame is bolted to the aft end of the turbine casing. Structurally, the
frame consists of an outer cylinder and an inner cylinder interconnected by the
radial struts. The No. 2 bearing is supported from the inner cylinder.
Exhaust frame radial struts cross the exhaust gas stream.
These Struts position the inner cylinder and No. 2 bearing in relation to the
outer casing of the gas turbine. The struts must be maintained at a constant
temperature in order to control the center position of the rotor in relation to
the stator. This temperature stabilization is accomplished by protecting the
struts from exhaust gas with an airfoil shaped metal firing that forms an air
space around each strut. Off-base blowers provide cooling air flow through the No.
2 bearing tunnel and then to the fourth-stage aft wheelspace and air space of
the struts.
Removable trunnions on the sides of the exhaust frame are
used with similar trunnions on the inlet casing to lift the gas turbine when it
is separated from its base.
The exhaust diffuser located at the aft end of the turbine
is bolted to the exhaust frame. At the exit of the diffuser, the gases are
directed into the exhaust plenum.
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