Note: Descriptions are shown in the official language in which they were submitted.
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CON~ROL OF FRBQUENCY AND WEIGHT DISTRIB~TION
IN HIGH EFFICIENT BLaDING
BACKGROUND OF THE INVENTION
The present invention relates to steam turbine
blading and, more particularly, to a method and
apparatus for controlling resonant frequency and
weight distribution in rotating turbine blades.
Highly loaded rotating steam turbine blades as
well as long, low pressure steam turbine blades having
low hub-to-tip ratios are generally twisted and
tapered. Twisting is necessary to accommodate the
pressure gradient occurring in front o~ the rotating
blade along with the change in wheel speed occurring
in the same region. As a resuit, blade inlet angles
change dramatically between blade base and tip.
Blade tapering is employed to controI centrifugal
stres~. Control of such stress requires an
appreciably lower cross-sectional area at the blade
tip than at the bladP base. In order to maintain a
finite thickness at the blade tip, the blade width
must be reduced.
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Twisting and tapering of steam turbine blading
reduces the flow control region near the blade tips.
There is also a resultant larger inlet flow angle and
a smaller magnitude of flow turning at the blade tip.
As a consequence, blading efficiency is reduced.
Moreo~er, in highly loaded blading, particularly last
row low pressure blading, steam velocity can be highly
supersonic. For such blading, it is desirable to use
convergent-divergent flow passages to improve
efficiency. However, such flow passage configuration
is difficult to achieve where pitch-to width ratios
are large such as at the tips of long, low pressure
blading.
SUMMARY OF THE INVENTION
; :
It is an object of the present invention to
provide a method and apparatus which overcomes the
disadvantages of prior art steam turbine blading as
set forth above.
It is a further object of the present invention to
provide a method and apparatus for forming a steam
turbine blade which allows the tip end of the blade to
have the same degree of flow control passage area as a
base end of a blade and allow for the development of
convergent-divergent flow passages.
In accordance with one form of the present
invention, there is provided a method for forming a
long, low pressure rotating blade for a steam turbine
in which the blade has a base end and a tip end with
the base end being attached to the rotor of the
turbine in a blade row which includes a plurality of
such blades. Each adjacent pair of blades defines a
steam flow passage from base to tip of the blades.
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The method includes forming a blade having an airfoil
configuration which provides an optimum blade width
distribution from base end to tip end corresponding to
a selected optimum steam flow control passage. A
hollow is machined within the blade beginning at the
blade tip and extending toward the blade base. The
hollow is centered with respect to the blade axis and
has a cross-sectional area so as to maintain blade
wall thickness within structural design limits while
removing su-fficient material from the blade to
maintain centrifugal stress loading within design
limits. The hollow is configured and sufficient
material removed so as to tune each individual blade
in a manner that avoids the blade having any natural
resonant frequencies which coincide with excitation
frequencies to be experienced within the steam
turbine. In one form, the material is removed from
the blade using electro-discharge machining or
electro-chemical machin;ng.
~ILE DESCRIPTION OF THE DRAWINGS
For a better understanding of the present
invention, reference may be had to the following
detailed description taken in conjunction with the
accompanying drawings in which:
FIG. 1 is an elevation view of a turbine blad~ of
the prior art;
FIGS. 2 and 3 are cross-sectional views taken
through the blade of FIG. 1 at th~ lashing wires 12
and 14;
FIG. 4 is a radial cross-sectional view of an
adjacent pair of stea~ turbine blades such as that
illustrated in FIG. 1 showing the flow control area;
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FIG. 5 is a cross-sectional view near the tip end
of the pair of steam turbine blades o~ FIG. 4 showing
the reduced steam flow control area;
FIG. 6 is an elevation view of a steam turbine
blade utilizing the teaching of the present invention;
FIG. 7 is a partial cross-sectional view of the
blade of FIG. 6 showing the method of forming a hollow
tip portion; and
FIG. 8 is a view similar to that of FIG. 5 but
lf~ utilizing the blades of FIG. 6.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is an elevation view of a steam turbine
blade exemplary of the prior art taken transvexse to
the normal plane of rotation of the blade. In this
plane, the blade 10 is essentially a tapered blade
having a pair of connecting points 12 and 14 for
atkaching the blade to adjacent blades. Preferably,
the blades are grouped and tuned in such groups to
avoid resonance in the tangential, axial, and
torsional modes with multiple harmonics. The tuning
also is designed to avoid excitation of frequencies at
multiples of the turbine speed. The connecting point
12, 14 are referred to as inner and outer lashing
wires. The blade may be formed with a zero taper
angle at the base to simplify the manufacturing
process but with the foil portion tapered and twisted
and varying from, ~or example, 4.25 inches at the base
end 16 to an axial width at the blade tip end 18 of
about 1.22 inches. The blades are attached at the
base end 16 to a rotor (not shown) of a steam turbine
and are coupled together by means of the lashing wires
12 and 14 in selecked groups. A blade row comprises a
plurality of the blades 10 attach~ed to a rotor in a
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common circumferential row. A typical blade row may
contain 120 of the blades of FIG. 1. FIGS. 2 and 3
illustrate cross-sections of the blades taken through
the lashing wires 12 and 14 and illustrate the
reduction in blade width at the two different radial
distances ~rom the rotor and also the variation in
blade curvature. FIGS. 4 and 5 are radial views of a
pair of adjacent blades showing the difference in
steam flow control area at a location adjacent the
base of a blade as compared to an area adjacent the
tip end of a pair of blades. In FIG. 4, the flow
control area 15 extends throughout the concave surface
of one blade and the convex surface of the ad;acent
blade whereas in FIG. 5, the flow control area 17
extends only over a small percentage of the blade
surfaces. In a blade of the type illustrated in FIG.
1, the ratio of pitch to-width at the blade tip is
approximately ten times the ratio of pitch-to-width at
the blade base, where blade width and pitch are
measurements of the dimensions indicated in FIGS. 4
and 5.
FIG. 6 is an elevation view of a steam turbine
blade similar to that shown in FIG. l but in which the
teaching of the present invention has been employed to
allow the tip end 20 of the blade to have a greater
width. The blade 22 includes a base end 24 which may
have substantially the same width as the tip end 20.
The base en~ 24 may be mounted to a dovetail root
section 26 in the same manner as was done with respect
to the prior art in FIG. I. FIG. 7 is a partial
cross-sectional view of the tip end of the blade of
FIG. 6 showing the forming of a cutout portion 28 or
hollow within the tip end of the blade 22. The hollow
portion is indicated by the dashed lines 30 and
extends downward towards the base end of the blade a
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predetermined radial distance which varies as a
function of the design constraints placed on the
blade. ~he design constraints may be established by
centrifugal stresses on the blade and by the need to
machine or remove sufficient material from the blad~
to avoid natural resonant frequencies. FIG. 8 is a
radial view of the tip end 20 of a pair of blades 22
showing the extended steam flow control passage 19 at
the tip end of the blades. It should be noted that
the use of the method of hollowing out the tip ends of
the blades 22 allows the outside dimensions of the
blade to be formed in any desired configuration. For
example, in the view of FIG. 8, the blade
configuration will be seen to form a convergent-
divergent flow control passage. Such configuration of
passages has been very difficult with tapered blades
characteristic of the prior art.
In order to obtain the required cross-sectional
foil area of the blade, electro-discharge milling
(EDM) or electro-chemical milling (ECM) may be used to
form the hollows within the tip ends of the blades.
It will be appreciated that the hollow portion can be
extended to any desired depth at any taper angle
required. The precise characteristics of EC~ and EDM
machining allow the manufacturer to maintain any
required thickness of the wall of the blade. The
particular details of ECM and ED~ machine are well
known in the art and are not considered part of the
present invention. By allowing the blads to be
machined with a hollow tip end, the blade itself can
be designed with any blade width distribution re~uired
to give optimum steam flow passage efficiency.
By use of the method of the present invention for
forming a hollow passage within the tip ends of long,
low pressure steam blading, blading can be machined to
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have practically unlimited resonant frequency control
and foil section area conkrol. Furthermore, the flow
passage control can be designed to provide higher
efficiency steam flow passages near the tip end of
steam blading. Additionally, the tip end of the steam
blading can be designed to accommodate convergent or
convergent-divergent flow passages which are not
available in the prior art. Furthermore, the external
configuration of the blading is provided with greater
flexibility since the centrifugal stresses and
frequency tuning can be achieved by hollowing the
blade to any desired cross-sectional area or depth.
Still further, the lashing required in the prior art
to join adjacent blades in order to prevent
undesirable vibrations may be eliminated since each of
the blades may be individually tuned to avoid resonant
frequency vibration. This elimination of lashing
wires may also improve steam flow efficiency. And
further, a costs savings may be achieved by using
fewer blades per row of blades since the width of the
blades at the blade tips may be increased. More
particularly, it is expected that performance gains
utilizing steam flow control in the-blade tip region
will provide a 0.4% heat rate improvement and
elimination of lashing wires may provide as much as
0.3% heat rate improvement. In a typical fossil
fueled turbine generator unit, this heat rate
improvement may be in the order of 52 BTU's/KW hour.
It will be appreciated that what has been
described is a method for forming a long, low pressure
rotating blade for a steam turbine in which the blade
has a base end and a tip end which may be of
substantially the same width and provides
substantially the same pitch/width between adjacent
blades. The base end is att~ched to a rotor o~ a
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turbine in a blade row which includes a plurality of
such blades so that each adjacent pair of blades
defines a steam flow passage which may be convergent-
divergent and provide greater steam flow control. In
the disclosed method, the blade i~ formed with a foil
configuration providing an optimum blade width
distribution from base to tip, which configuration
corresponds to a selected optimum steam passage
efficiency. A hollow is machined within the blade
beginning at the blade tip end and extending toward
the blade base end. The hollow is centered with
respect to an axis of the blade and has a cross-
sectional area so as to maintain blade wall thickness
within structural design limits while removing
sufficient material from the blade to maintain
centrifugal str~ss loading within design limits for
the blade. In one form, the material may be removed
from the blade by EDM or ECM machining. The disclosed
method allows the outer configuration of the blade to
be any selected configuration.
While the invention has been described in what is
presently considered to be a preferred embodiment,
other variations and modifications will become
apparent to those skilled in the art. It i5 intended,
therefore, that the invention not be limited to the
disclosed embodiment but be interpreted within the
full spirit and scope of the appended claims.
