Note: Descriptions are shown in the official language in which they were submitted.
2~6~
56, 937
INPROVED ~;TATIONARY l~JRBINE BLAD~3
HAVING DIAPHRAGM CO~ISTRUCTIOM
This application is a continuation in-part of co-
pending application S.N. 07/603,332 filed October 24, 1990
and co-pending application 5.N. 07/624,367 filed December 6,
1990, both assigned to the assignee of the present
application.
BACKGROVND OF THE INVENTION
The present invention pertains to steam turbines
for utility power application and, more particularly, to a
stationary blade for use in a low pressure steam turbine.
Steam turbine rotor and stationary blades are
arranged in a plurality of rows or stages. The rotor blades
of a given row are identical to each other and mounted in a
mounting groove provided in the turbine rotor. Stationary
blades, on the other hand, are mounted on a cylinder or
blade ring which surrounds the rotor.
Turbine rotor blades typically share the same
basic components. Each has a root receivable in the
mounting groove of the rotor, a platform which overlies the
outer surface of the rotor at the upper terminus of the
rootr and an airfoil which extends outwardly from the
platform.
Stationary blades also have airfoils, except that
the airfoils of the stationary blades extend downwardly
towards the rotor. The airfoils include a leading edge, a
; trailing edge, a concave surface, and a convex surface. In
~lB9~
2 56,937
most turbines, the airfoil shape common to a particular row
of blades generally differs from the airfoil shape in other
rows within a particular turbine. In general, no two
turbines of different designs share airfoils of the same
shape. The structural differences in airfoil shape result
in significant variations in aerodynamic characteristics,
stress patterns, operating temperature, and natural
frequency of the blade. These variations, in turn,
determine the expected life of the turbine blade under the
operating conditions (turbine inlet temperature, pressure
ratio, and rotational speed), which are generally determined
prior to airfoil shape development.
Development of a turbine for a new commercial
power generation steam turbine may require several years to
complete. When designing rotor blades for a new steam
turbine, a profile developer is given a certain flow field
with which to work. The flvw field determines the inlet
angles (for steam passing between adjacent blades of a row),
gauging, and the force applied on each blade, among other
; 20 things. "Gauging" is the ratio of throat to pitch; "throat"
is the straight line distance between the trailing edge of
one blade and the suction surface of an adjacent blade, and
~pitch" is the distance in the tangential direction between
the trailing edges of the adjacent blades, each measurement
being determined at a specific radial distance from the
turbine axis.
These flow field parameters are dependent on a
number of factors, including the length of the blades of a
particular row. The length of the blades is established
early in the design states of the steam turbine and is
essentially a function of the overall power output of the
steam turbine and the power output for that particular
stage.
When working with the flow field of a particular
turbine, it is important to consider the interaction of
adjacent rows of blades. The preceding row affects the
following row by potentially creating a mass flow rate near
the base which cannot pass through the following row. Thus,
3 2 ~ 56,937
it is important to design a blade with proper flow
distribution up and down the blade length.
The pressure distribution along the concave and
convex surfaces of the blade can result in secondary flow
which results in blading inefficiency. These secondary flow
losses result from differences in steam pressure between the
suction and the pressure surfaces of the blades near the end
walls.
Regardless of the shape of the airfoil as dictated
by the flow field parameters, the blade designer must also
consider the cost of manufacturing the optimum blade shape.
Flow field parameters may dictate a profile which cannot be
produced economically, and inversely the optimum blade shape
may otherwise be economically impractical. Thus, the
optimum blade shape should also take into account
manufacturability.
SUMMARY OF T~E INVENTION
In the present invention, a steam turbine blade
has inner and outer rings which are integrally formed with
~0 an airfoil. A plurality of such blades are joined by
welding the inner and outer rings to form an annular nozzle
assembly. The diaphragm structure of the inventive blades
offers improved performance over blades of similar length in
that performance is improved due to smoother transition
between the airfoil and the inner and outer rings as
compared to conventional segmental assemblies in which a
forged airfoil is welded to inner and outer rings.
Furthermore, the present invention allows a thinner trailing
edge and reduced manufacturing tolerances than is found in
prior art segmental assemblies having forged airfoils with
thick trailing edges and relatively large tolerance
requirements. The particular design of the inventive blade
also controls steam separation by control of the rate of
change of convergence and turning angle. Steam velocity
increases substantially continuously over the major extent
of the suction surface of the blade to avoid flow
separation.
4 2~91~9~ 56,937
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a ~ectional view of two adjacent blades
illustrating typical blade features;
FIG. 2 is a vertical sectional view o~ a portion
of a steam turbine incorporating a row of blades a~cording
to the present invention;
FIG. 3 is an enlarged view of a portion of the
turbine of FIG. 2 illustrating a blade of the present
invention;
10FIG. 4 is a graph of cross-sectional area as a
function of radius for the blade of the present invention;
FIG. 5 is a graph of minimum moment of inertia
(I_MIN) as a function of radius for the blade of the present
invention;
15FIGS. 6A-6E are an overlay of cross-sections of
the blade of the present invention; and
FIG. 7 is a graph of steam velocity at the suction
surface and pressure surface of the blade of the present
invention.
20DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to Fig. 1, two adjacent blades of a row
are illustrated in sectional views to demonstrate some of
the features of a typical blade. The two blades are
referred to by the numerals 10 and 12. The blades have
convex, suction-side surfaces 14 and ~6, concave pressure-
side surfaces 18 and 20, leading edges 22 and 24, and
trailing edges 26 and 28.
The throat is indicated in Fig. 1 by the letter
"O", which is the shortest straight line distance between
the trailing edge of blade 10 and the suction-side surface
of blade 12. The pitch is indicated by the letter "S",
which represents the straight line distance between the
trailing edges of the two adjacent blades.
The width of the blade is indicated by the
distance Wml while the blade inlet flow angle is al, and the
outlet flow angle is a2.
"B~' is the leading edge included metal angle, and
the letter "s" refers to the stagger angle.
~ 9 ~ 56, g37
Referring to YIG. 2, a low pressure fossil fuel
steam tuxbine 30 includes a rotor 32 carrying several rows
or stages of rotary blades 34. An inner cylinder 36 carries
plural rows of stationary blades, including a last row of
stationary blades 38, next to last row of stationary blades
40 and a second from last row of stationary blades 42. Each
row of blades has a row designation. Blade 38 is in row 7C,
while the last row of rotary blades is designated 7R. The
immediately upstream stationary blade 40 is in blade row 6C
and the next stationary blade 42 is in blade row 5C. The
present invention is particularly intended for use in row
5C.
As shown in ~IG. 3, the blade 42 includes an
airfoil portion 44, an outer ring 46 for connecting the
blade to the inner cylinder 36, and an inner ring 48
connected to an "inner diameter'l distal end of the airfoil
portion 44. The "outer diameter~ end of the airfoil portion
4~ is formed integrally with the outer ring 46 in a
diaphragm structure process. In a diaphragm structure, the
airfoil, outer ring and inner ring are machinéd from an
integral casting. Normally, a blade used in the 5C row,
typically about 8.65 inches (219.71 MM), would be formed as
a segmental assembly in which the inner and outer rings are
welded to a separately formed airfoil. The diaphragm
structure offers improved performance due to smoother
airfoil to ring transitions, a thinner trailing edge on the
airfoil and reduced manufacturing tolerances. A diaphragm
asseMbly or nozzle assembly is formed by welding the inner
and outer rings to adjacent rings to create an annular ar~ay
of blades. The inner ring 48 is spaced from rotor 32 by a
clearance gap. Seals 50 are positioned in the clearance gap
to limit steam leakage under the stationary blades.
The inner ring 48 and airfoil 44 have been
constructed to tune the fundamental mode of the entire
assembly between the multiples of turbine running speed,
i.e., with respect to harmonic excitation frequencies, thus
minimizing the risk of high cycle fatigue and failure.
Specifically, the blade mass/stiffness is distributed in the
6 2~9~ ~9~3 56,937
radial direction to produce the characteristics shown in
FIGS. 4 and 5. The fundamental harmonic frequency is then
fine-tuned by optimiæing the inner ring shape, i.e., by
adjusting mass and stiffness.
In order to reduce the opportunity for high-cycle
fatigue failure, the diaphragm blade structure is preferably
analyzed by assuming full steam loading of th~ blade acting
as in phase excitation. Such analysis can be done using a
Goodman diagram technique normally reserved for rotating
la blade analysis. The vibratory stresses obtained from this
analysis are then compared to empirically derived allowable
stresses. If necessary, the blade structure can then be re-
tuned and the analysis and comparison repeated until
acceptable results are obtained. This technique has only
been used for a blade of this type. Historically, diaphragm
structures have only been tested for frequency response.
When the blades of the present invention are
assem~led in~o a blade row 5C, the efficiency of the blade
row or stage is optimized by minimizing the steam flow
incidence angle and secondary flow loss. The optimum inlet
angle and ganging distributing are obtained using a qu~si-
three dimensional flow field analysis. The unique shape of
the airfoil 44 influences the flow conditions leaving
rotating blade row 5R and the performance of the last two
stages of the low pressure turbine 30. The inlet angles of
blade row 5C are influenced by the condition of the steam
leaving rotating blade row 4R.
FIGS. 6A-6E show the general shape of the
inventive blade 42 and the convergent configuration of the
steam passage between blade 42 and an adjacent blade
indicated by pressure side profile line 43. The section of
FIG. 6A is taken adjacent the radially inner end of the
blade 42 (the tip end) and the section of FIG. 6E is taken
adjacent the radially outer end (the ~ase end) of blade 42.
Table 1 lists the important characteristics of each of the
sections of FIGS. 6A-6E in corresponding sequence. It will
~e noted that certain characteristics such as stagger angle,
exit opening angle and principal coordinate (alpha) angle
9 ~
7 56,937
remain substantially constant over the extent of the airfoil
44. 5tagger angle is the angle formed between a horizontal
line and a line tangent to leading and trailing edge circles
in a cross-sectional view. The principal coordinate angle
is the angle between a horizontal line and a minimum second
moment of area axis. One measurement not listed in Table 1
is the nominal thickness o the blade trailing edge 44A.
For the inventive blade, this thickne~s can be reduced to
about 80 mils for significantly reducing wake mixing loss
and improving turbine performance. In referring to Table 1,
it is noted that suction surface turning is the change in
the slope of the suction surface from a throat point (the
point where the minimum pa~sage chord intersects the suction
surface) to the exit of the airfoil. Inlet metal angle is
the angle between the vertical direction and a bisecting
line formed between the two tangen~ lines to the suction and
pressure surfaces, respectively, at the leading edge
tangency points. The inlet included angle is the angle
between these two tangent lines. The exit opening is the
shortest distance between adjacent airfoils at the steam
passage exit.
RADIVS (IN) 26.63 28.25 29.70 32.00 35.26
1. WIDTH (IN) 1.71 1.77 1.81 1.89 1.99
2. CHORD (IN) 2.88 3.03 3.16 3.36 3.66
3. PITCH/WIDTH 1.15 1.20 1.23 1.27 1.33
4. PITCH/CHORD 0.69 0.70 0.70 0.71 0.72
5. STAGGER ANGLE52.85 53.66 54.31 55.23 56.39
(DEG~
6. MAXINUM THICKNESS 0.45 0.44 0.47 0.51 0.56
7. MAX THICKNESS/ 0.16 0.15 0.15 0.15 0.15
CHORD
8. TURNING ANGLE 80.31 79.42 74.53 65.38 52.88
9. EXIT OPENING (IN) 0.56 0.61 0.66 0.73 0.85
10. EXIT OPENING 24.36 23.43 24.84 25.59 25.34
ANGLE
11. INLET NETAL 82.57 82.57 87.33 96.27 109.72
ANGLE
12. INLET INCL. 54.98 49.19 60.85 61.05 59. 42
ANGLE
13. GAUGING 0. 2855 .2932 .2991 .3091 .3238
14. SUCTION SURFACE 9. 92 8.11 9.47 10.66 11.08
T~RNING
15. AREA ( IN**2) 0.74 0.75 0.83 0.95 1.12
16. ALPHA (DEG)54.24 55.50 55.56 56.6057.33
17. I MIN (IN**4) 0.02 0.02 0.02 0.02 0.02
20~169~
8 56,937
18. I MAX ~IN**4) 0.32 0.36 0.42 0.55 0.73
FIG. 7 illustrates another important
characteristic of the present invention. As shown in FIG.
7, the velocity ratio of steam flow over the suction surface
(convex surface) of blade airfoil 44 increases continuously
over nearly the full width of the airfoil. This
acceleration characteristic maintains the steam in contact
with or closely spaced to the blade surface. Thus, ~his
characteristic is achieved by the decreasing rate of
convergence of the area between adjacent blades form leading
to trailing edge and by controlling the rate of change of
the turning angle. Turning angle is the amount of angular
turning from inlet to exit of the blade.
The blade 42 provides improved performance and
efficiency in fossil fueled steam turbines. It utilizes
manufacturing and tuning techniques which are not believed
previously applied to stationary blades of this dimension.
'
