Note: Descriptions are shown in the official language in which they were submitted.
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"
COMPRESSOR DIAPHRAGM BOX SEAL
BACKGROUND OF THE INVENTION
This invention relates generally to compressor
diaphragms for combustion turbines and, more particularly,
to an improved compressor diaphragm having a more effi-
client sealing structure.
In general terms the combustion turbine come
poses sQmpreSs~r ses-tlQn, a cQm.hustor sect On, and a
turbine section. The compressor section provides a steady
; flow of compressed air to the combustor section, where the
compressed air is heated. From the combustor section, the
hot pressurized gas is delivered to the turbine section,
where gaseous expansion results in rotation of a turbine
rotor, which in turn may drive a generator to generate
electric power.
Thea compressor section of a combustion turbine
typically comprises a plurality of compressor diaphragms
arranged alternately between rows of blades affixed to the
compressor rotor. Air drawn through the compressor sect
lion may typically undergo an increase in pressure to
approximately 15 atmospheres. A single compressor die-
from may be a disc-shaped structure arranged in two
semicircular, or 180, sections. The diaphragm comprises
an outer ring, or shroud, and an inner ring, or shroud,
with a plurality of airfoil-shaped vanes affixed there-
I between. Each compressor diaphragm is assembled around the compressor rotor and between two rows of compressor
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rotor blades, except at the extreme ends of the compressor
rotor.
The inner and outer shrouds typically comprise a
metallic plate of approximately .25 inch thickness,
rolled to the appropriate diameter. Contoured holes are
punched in each shroud at predetermined intervals to
receive as inserts extensions from the compressor vanes,
which are then welded to each shroud. The thickness of
the two shrouds is chosen so as to permit the punching of
holes therein, as opposed to machining the holes. In
determining the thickness of the plate comprising each
shroud, consideration is also given to the life of a
punching tool at a given shroud thickness and the increase
in structural rigidity, and thereby improved turbine
performance, which may be expected from increasing that
thickness.
A compressor diaphragm seal is arranged along
toe inner circumference of the inner shroud so as to
inhibit a leakage flow of compressed air through the
annular space between the inner shroud and the turbine
rotor. The seal typically comprises a pair of axially
spaced angular seal points, having a horizontal arm welded
to the inner face of the plate comprising the inner shroud
so that a radial warm of each seal point protrudes in
warmly, away from the inner shroud to create a small
radial gap between itself and a disc seal arm. This
arrangement provides a pair of seal points positioned at
the opposite axial ends of the inner shroud of each die-
from.
A lack of sufficient structural rigidity in the
rolled plate comprising the inner shroud results in de
election of the inner shroud and a resultant discontinuity
at the horizontal joint between the two semicircular
structures comprising a single diaphragm. The discontinue-
unity at thy horizontal joint causes a flow disturbance
which reduces the overall efficiency of the combustion
turbine. The use of a rolled plate having a maximum
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thickness of .25 inch to construct the inner shroud con-
tributes to the shroud's lack of rigidity. As set forth
above, the .25 inch thickness was selected as a balance
between fabrication costs and performance efficiency.
A problem which occasionally accompanies turbine
compressor operation is the phenomenon of compressor
stall. Compressor stall is a condition associated with
single compressor stage, which is the combination of a
blade row and a vane diaphragm, or a single blade row, or
I a single vane diaphragm. A stall condition typically
occurs when air flow vectors normally impacting an airfoil
surface vary, as a result, for example, of chanting pros-
sure and thermal ratios, so as to produce a splitting of
the air flow at the leading edge of the airfoil. Stall
condition denotes the failure of the blade or vane row to
produce its normal increase in pressure. A turbine may
have one or more stages or rows operating in the stall
condition and yet malnt2in successful operation.
The phenomenon of compressor surge occurs when
too many compressor stages or rows are operating in the
stall condition. Surge denotes a failure of the entire
compressor section ox the turbine and is accompanied by an
immediate system shutdown, or turbine trip, and the no-
sultan loss of power generation capacity.
A certain amount of flow leakage from the high
pressure side of a diaphragm through the diaphragm seal to
the low pressure side of the diaphragm seal to the low
pressure side of the diaphragm is inherent in typical
prior art combustion turbine compressors. The inherence
of this leakage arises from the practicality of completely
sealing the space between two metallic surfaces, one of
which is rotating. Any Leakage through the diaphragm seal
results in a distrains of the flow pattern near the
inner shroud of the compressor diaphragm. An air flow
disturbance may in turn increase the siesta ability of the
stage or row to the stall condition and thereby contribute
to a compressor surge. The flow disturbance resulting
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from leakage through a diaphragm seal also contributes to
a loss of turbine operating efficiency. Hence, improved
efficiency of a diaphragm seal translates into improved
turbine operating efficiency and diminished susceptibility
to compressor stall and surge.
In summary, typical prior art compressor die-
phragms have been structured to operate in a way that
limits operating efficiency and to some extent contributes
to the possible development of stall and surge conditions
during operation. Although these inadequacies do not
prevent the combustion turbine from operating satisfactory
fly, they do tend to inhibit to some extent the overall
operational efficiency of the combustion turbine.
SUMMARY OF THE INVENTION
Accordingly, an improved combustion turbine come
presser diaphragm comprises an outer shroud, an inner
shroud, a plurality of airfoils supported between the
inner and outer shrouds and a boy sell. The compressor
diaphragm is preferably structured in two semicircular
sections. Affixed to an inner surface of the inner
shroud, the box seal provides improved sealing efficiency
and increased structural sufficiency for the diaphragm.
The box seal comprises an arcuate seal support base fad-
tally spaced from the inner surface of inner shroud and a
pair of arcuate legs generally perpendicular to the seal
support base and supporting the base in spaced relation-
ship from the inner shroud. An inner surface of the seal
support base defines a plurality of grooves for receiving
a plurality of seal strips.
The box seal arrangement permits the inner
shroud to be maintained at a thickness which may be easily
punched, while providing a seal support structure of
sup fishnet thickness to accommodate grooving and thereby
enable attachment of the plurality of seal strips, in
contrast to the two seal points available in typical prior
art arrangements. The seal strips are mechanically sup-
ported in the grooves, rather than welded as in prior
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arrangements. In addition, the mechanically inserted seal
strips may be more easily removed and replaced than the
welded seal strips of the typical prior art seals. Final-
lye the box or channel-shaped structure of the seal sign
nificantly increases the structural rigidity of the inner shroud and thereby diminishes the discontinuity typically
found at the horizontal joint of the prior art compressor
diaphragms. This in turn decreases flow disturbance at
the horizontal joint and thereby increases overall operate
in efficiency, while diminishing susceptibility of the
compressor to stall and surge conditions.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 shows a sectional view of a portion of
a compressor section of a combustion turbine;
Figure 2 shows a 180 section of a compressor
diaphragm; and
Figure 3 shows a sectional view of a compressor
diaphragm structured cording to the prinrl~les of. the
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Figure 1 shows a portion of a combustion turbine
compressor 10 in cross-section. The compressor lo coy
proses a rotor 12, supporting a plurality of rotor blades
14, and an outer compressor casing 16, supporting a plower
amity of compressor diaphragms 18. Each diaphragm 18
supports a plurality of compressor vanes 20, only one of
which for each diaphragm I may be seen in the sectional
view of Figure 1. When operating, the rotor 12 and rotor
blades 14 rotate with respect to the compressor casing 16
and the diaphragms 18, drawing air, represented at refer-
once character I through the narrowing, annular passage-
way 24 to a combustor section (not shown) of the combs-
lion turbine. As air is drawn through the compressor
section 10, the air pressure gradually increases to apt
proximately 15 atmospheres.
Compressed air from the compressor section of the turbine is then heated by a plurality of combustors
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(not shown) in the combustor section of the turbine. From
the combustor section the hot compressed gases are direct-
Ed to the turbine section of the combustion turbine,
wherein the energy released upon expansion and cooling of
the gases is translated into rotation of the combustion
turbine rotor. In large combustion turbines used in power
generation, the turbine rotor rotation is connected to a
generator which thereby generates electric power.
Each compressor diaphragm 18 is constructed in
lo two 180, semicircular sections, one of which is depicted
in Figure 2. Each compressor diaphragm 18 comprises an
outer arcuate shroud 30, an inner arcuate shroud 32, and a
plurality of compressor vanes 20. The compressor die-
from I shown in Figure 2 with a few vanes 20 for pun-
poses of illustration, normally supports a plurality of vanes 20 at regular intervals about the entire annular
space between the inner shroud 32 and the outer shroud 30.
To accordance iota the principles of the invention, ye
compressor diaphragm 18 is structured with an improved
compressor diaphragm box seal 34 for improved structural
sufficiency and sealing efficiency. The box seal 34 is
shown in greater detail in Figure 3.
Construction of the compressor diaphragm 18
generally progresses as follows. Contoured holes are
punched in the outer shroud 30 and in the inner shroud 32,
as shown by reference characters 36 and 38, respectively,
for receiving extensions of the compressor vane 20. Next,
an outer extension 40 of the compressor vane 20 is in-
sorted through the hole 36 in the outer shroud and the
extension is thereafter welded to the outer shroud as
shown at reference characters 42 and 44. This is done for
each vane 20 for the diaphragm 18. Next, an inner extent
soon of the compressor vane 20 is inserted into the hole
38 and the inner shroud 32 and welded thereto. A semi
circular seal support block 35 of channel-shaped cross
section is then welded to the inner side of the inner
shroud 32. The seal support bloc 35 comprises an arcuate
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support base 50 and a pair of arcuate legs 52. After the
block welding is completed the seal support block 35 is
machined to the final shape shown in Figure 3, including
grooves 46 in the inner surface of the support base 50 for
receiving seal strips 48, which are preferably formed from
sheet metal. The seal strips 48 are thereafter edge-
rolled into the grooves 46 and secured by mechanical
staking, such as by preening.
The number of seal points 48 included on the
support base 50 varies with the position of the diaphragm
within the compressor. Generally, the number of seal
strips 48 increases with the downstream position of the
diaphragm, corresponding to the increasing proximity of
blade rows on the compressor rotor and the increased
pressures at the downstream end. As can be seen in Figure
3, the support base 50 ma be arranged to support more or
fewer seal strips 48, depending upon the physical require-
m~n'CS _
The compressor diaphragm box seal 34 provides a
0 number of advantages over prior art seals described in toes
Background Of The Invention. The I base 50 of the
seal support block 35 is thick enough to support machining
of a plurality of grooves 46; whereas, thy inner shroud 32
which was maintained to a maximum thickness of approx-
irately .25 inch, was not thick enough to support machine
in of the grooves therein. As a result, tune seal support
block 35 can support a plurality of seal points 48 while
the inner shroud 32 was capable of supporting only a pair
of seal points which had to be welded thereto. The come
presser diaphragm box seal I provides for improved effi-
Chinese over the two-point inner shroud seal by increasing
the number of seal strips used. In addition, the mechanic
gaily staked seal strips 48 of the box seal 34 are more
readily removed and replaced than the two seal points of
the prior art which were rigidly welded to the inner
shroud 32.
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A further advantage of the box seal 34 is the
increased structural rigidity provided to the sheet metal
seal strips 48. In prior art designs, the seal strips
were required to extend radially from the inner surface of
the inner shroud 32 to the compressor disc seal arms.
Flow-induced excitation caused the seal strips to vibrate,
resulting in reduced fatigue life of the long seal strips.
In addition to offering a more rigid point of attachment
for the seals, the box seal 34 permits use of seal strips
which are shorter in radial length and, hence, more nests-
lent to fatigue than prior art seal strips.
Still another advantage of the box seal 34 is
the increased structural rigidity which it provides to the
inner shroud 32. Maintaining the inner shroud I to a
thickness which can be mechanically punched results in a
structure subject to deflection, producing discontinuities
at the horizontal joints of the compressor diaphragms.
The box seal 34 effectively increases the thickness of the
inner shroud and correspondingly increases the structural
rigidity of the inner shroud, significantly improving the
continuity at the horizontal joint. This reduces or elm-
inmates flow disturbances which occurred at the horizontal
joint in prior art compressors and thereby increases the
overall efficiency of the combustion turbine.
A final advantage of the box seal 34 is the
increased torsional rigidity provided to the inner shroud
32. In prior art designs, variations in the axial deflect
lion of individual vanes 20 caused a war page or twist in
the inner shroud 32. The resultant distortions in the
exposed surface of the inner shroud 32 caused flow disturb
banes and a decrease in operating efficiency. By erect-
in a "torque box," the box seal 34 and inner shroud 32
combination greatly reduces susceptibility of the inner
shroud 32 to war page and thereby promotes operating effi-
Chinese.
