Note: Descriptions are shown in the official language in which they were submitted.
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IN~R1;~ OU11 .J . ON FUI~-ARC
AnMT.C:.~ION IMPULSE TIJRRTNl~S
RA"KrDOUND OF THE INVENTION
The present invention relates to steam turbines
and, more particularly, to a method for increasing the
output power of nuclear powered steam turbines using
throttling to limit the steam supply system.
5Most of the nuclear turbines sold or designed
before 1967 were found to have considerably higher steam
flow capacity than the designers had intended. Because of
NRC (Nuclear Regulatory Commission) mandated power output
limits on the NSSS (Nuclear Steam Supply System), it was
necessary to incur excessive valve throttling to limit
cycle heat input to the licensed or warranted level. This
excess throttling reduced turbine output. The power loss
was less severe on partial-arc admission turbines as only
one active control valve was throttling as compared to
15 full-arc admission designs in which all of the control
-valves would throttle. In the former case, only a fraction
of the total flow was throttled while all of the flow was
throttled with the latter design.
Most nuclear turbines have four control valves
and in the case of partial-arc designs, each control valve
supplies steam to a 25% admission arc of the first or
control stage. The first stage in this instance must be an
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impulse (low reaction) stage. It was found that in many
instances the fourth control valve (supplying the 75-100%
admission arc) was practically closed and so this admission
arc operated at considerably lower inlet pressure than the
other three active arcs. In some instances, not only was
the fourth valve completely closed but the third valve
(supplying the 50-75% admission arc) was also throttling.
The cause of the higher flow capacity is lower
than expected exit pressure of the first stage as well as
lower pressure for most of the other stages in the high
pressure (HP) turbine section. On the first stage the
result was poorer conversion efficiency because of off-
design pressure ratio (poorer stage velocity ratio). In
addition, the first stage did a greater proportion of the
HP section work and the downstream stages did less work.
Since the first stage has lower efficiency than the
downstream stages, this further reduced the HP section
output.
The reason for the lower stage pressure is two-
phase or non-equilibrium effects. The term
"supersaturation" has been applied to this phenomenon when
it initially occurs. For example, if dry or slightly
superheated steam is expanded rapidly in a turbine blade
passage, moisture droplets are not formed until a condition
is reached which is the equivalent to 2~ to 3% moisture, at
which condition the moisture droplets suddenly appear.
This 2% to 3% range is called the Wilson zone. Flow
capacity increases up to about 3% compared to the level
that would occur if the fluid had been in thermal
equilibrium.
Turbine designers considered the effects of the
Wilson zone in their first stage passage area calculations.
However, the conventional wisdom was that once moisture
appeared, the fluid would be in thermal equilibrium. In
reality, the fluid (steam-water mixture) continued to
exhibit non-equilibrium effects at moisture levels much
larger than 3~. Consequently, all of the stages in the HP
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section operated at different pressure levels than they
were designed for (when thermal equilibrium was believed to
exist). As a result, the accumulation of reduced pressure
drops on the downstream stages reduced the first stage exit
pressure. The reduced first stage exit pressure increased
the first stage flow coefficient and so the turbine had
excess flow capacity. On full-arc admission turbines, the
accumulation of reduced pressure drops also reduced first
stage inlet pressure.
In the case of partial-arc designs, the flow
capacity is reduced by effectively reducing the active
first stage nozzle area by closing a valve and/or reducing
the nozzle inlet pressure (by control valve activation) on
one of the active arcs with a concurrent output loss. If
the first HP stage is a full-arc design, it could be either
a reaction or impulse type first stage. In the case of
partial-arc designs, only impulse stages are a viable and
efficient choice. Partial-arc reaction stages would incur
much higher efficiency degradation than impulse stages
during operation at flows lower than the maximum value.
A number of reaction turbines have been
retrofitted with new blading to restore the balance between
the work done by the first stage and the downstream stages.
The solutions have included (1) replacing the stationary
blades or nozzles with blades having a smaller throat area
(reduced gaging), (2) replacing the entire blade path,
rotating and stationary row with blades with smaller
heights and/or reduced gaging, (3) reducing the height of
the first stage nozzles on partial-arc admission designs
and (4) various combinations of these changes. The heat
rate/power output improvement as well as the capital cost
will increase as a greater number of blade rows are
replaced. In addition, the changes which involve rotating
blades result in longer duration outages and have added
costs associated with replacement energy expenditures.
Stationary blade replacements are an alternative that
realizes a major fraction of the potential output
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improvement at minimal outage time. In other instances,
the additional kilowatts achieved by a total HP blade path
redesign may warrant the purchase of a new rotor and
associated stationary parts. However, each of thése
solutions represent a major capital cost investment and
substantial turbine downtime.
S~nnM~ OF THE INVENTION
It is an object of the present invention to
provide a method and apparatus for improving output of a
nuclear powered steam turbine having excess flow capacity
above the NSSS limits without incurring the expense and
downtime common to prior methods. The present invention
incorporates closing off of a selected number of nozzle
passages in each stage of the turbine to reduce flow area.
The method increases stage pressure ratio to the level
intended by the original design but not achieved due to
non-equilibrium effects. In an exemplary form, blocks are
placed in the flow passages between some of the nozzle
blades. The blocks are distributed about each nozzle
assembly but are varied in spacing to avoid in-phase
disturbances of the flow. The number of blocks is
determined as a function of the percent of excess flow
margin and the nozzle area to be blocked.
BRIEF n~CRTPTION 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 a simplified cross-sectional view of one
form of steam turbine;
FIG. 2 is an axial view of a steam turbine nozzle
assembly; and
FIG. 3 is a radial view showing a partial section of
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nozzle and rotating blade assembly with the present
invention.
DETATT~n DESCRIPTION OF THE PREFERRED EMBODIMENT
The present invention improves turbine power
output and improves efficiency by eliminating throttling of
the steam supply to a nuclear powered turbine with impulse
blading. Starting with the NRC mandated NSSS cycle heat
power input, applicant determines the excess flow capacity
of the turbine either by recalculation of turbine flow
using present day modeling which accounts for non-
equilibrium effects or by actual measurement of pressure
drop at each stage. Excess flow capacity as a percentage
of total turbine flow capacity establishes the amount by
which each nozzle flow area should be reduced to meet the
mandated power input.
Before turning to the present invention,
reference is first made to FIG. 1 which illustrates one
form of a steam turbine 10 having a plurality of axially
spaced blade stages comprising nozzle blade assemblies 21
and rotating blade assemblies 23. The turbine 10 includes
an outer casing or housing 17 and a rotor 19. The rotating
blade assemblies 21 are coupled to the rotor 19 while the
nozzle blade assemblies are coupled to an inner casing 27
which is coupled to outer casing 17. Steam enters turbine
10 through inlet snout 25 and flows axially outward through
the stages.
Referring now to FIGS. 1 and 2, there are shown
an axial view of a nozzle assembly 21 and a partial radial
view of nozzle assembIy 21 and rotating blade assembly 23,
respectively. Each nozzle assembly 21 is made up of a
plurality of circumferentially spaced, fixed nozzle blades
29 having a configuration designed to turn steam entering
the assembly 21 in the direction of arrows 31 into an
optimum path for reacting against rotating blades 33 of
assembly 23. Arrows 32 show steam direction exiting the
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rotating blades 33.
The circumferential area defined by the blades 29
establishes the turbine flow capacity. Applicant proposes
to adjust this flow capacity by blocking selected flow
paths between some of the blades 29. By way of example, if
the nozzle assembly 21 comprises 120 nozzle passages 35 and
the excess flow capacity is about 10 per cent, the flow
capacity can be reduced by 10 per cent by blocking twelve
of the nozzle passages. FIG. 2 illustrates a blocking
device 37 inserted between a pair of adjacent blades 29A
and 29B in a nozzle passage.
In a preferred embodiment, a blocking device 37A
fills the nozzle passage 35 from the leading to trailing
edges of blades 29 as is shown. The device may be held in
a fixed, assembled position by welding the device to the
~ i ng edges of blades 29A, 29B. Alternatively, the
device 37 could extend only between the leading edges of
the blades 29A, 29B, as is shown by device 37B. It is
preferable to position a blocking device 37 at the leading
edge of the blades since steam pressure will tend to urge
such device into engagement with the blades rather than
trying to dislodge such device if attached to the trailing
edge of the blades. However, a device 37 at the trailing
edge does offer the advantage of preventing steam flow off
the trailing edges of adjacent blades 29 from attempting to
flow into the blocked nozzle passage 35 and creating
- undesirable turbulence in the steam flow. Thus, use of the
device 37A filling the entire flow passage satisfies both
desirable features of minimizing turbulence and preventing
separation of the device from the nozzle assembly. The
alternate device 37B could be a flat plate. Attachment can
be by welding, silver solder or mechanical connection.
In a preferred form, the blocking devices 37 are
distributed about the nozzle assembly 21 rather than
grouped together. Grouping would produce undesirable shock
loading on the rotating blades 33 as they pass in and out
of the blocked steam flow area. However, it is also not
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desirable to close off nozzle passages in a uniform
pattern, e.g., every twelve nozzle passage in the
illustrative example. Such an arrangement would produce
twelve in-phase distllrh~nces in every revolution of the
rotating blade assembly 23 and the in-phase excitation
could result in a build-up of vibration in the blades
leading to possible failure. Accordingly, it is desirable
to arrange the blocked nozzle flow passages such that a
varied number of un-blocked passages are between the
blocked passages. This arrangement would then produce a
non-periodic stimulus and any induced vibratory stress on
the blades 33 will be lowered. An example would be to
space the blocks 37 such that some blocks 37 have as few as
eight passages between adjacent blocks while others are
spaced apart by as many as sixteen passages. It is also
desirable to vary the position of the blocked passages from
stage-to-stage to distribute voids in the steam flow and
reduce flow connection at each stage.
A study performed on a 900NW nuclear turbine
designed and built before non-equilibrium effects were
recognized and before a suitable, predictive correlation
was developed, showed significant improvement in power
output. The turbine was a partial-arc design having a
warranted output slightly above 900MW and a 10% excess flow
margin beyond the originally designed 5% level of margin
for a total 15% flow margin.
Turning to Table I, it can be seen that the
increase in power output for this turbine ranges between
10,680 KW and 13,850 RW over a base flow rate power output
of 905,750 RW. For the inventive process, sufficient
nozzle passage ways are sealed off on downstream stages to
reduce the turbine nozzle area by 10%. The first stage
nozzle area is reduced by 10% in one example and by 13.5%
in the other example. The second example actually produced
a higher net power increase. The improvement is believed
best when all stages are adjusted by partial blockage so as
to establish an optimum steam velocity and pressure ratio
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at each stage. Further performance improvement is also
attained from a secondary effect of achieving a higher
final feedwater temperature as a result of the increased
extraction pressure. Distributing the pressure increase
over multiple stages also places the first stage closer to
optimum presæure.
TABLE I
Case Output Increased
KW RW
Base (Current State) 905,750 0
First Stage Equiv.Adm.=0.90 (Alt.1) 916,430 10,680
First Stage Equiv.Adm.=0.865(Alt.1) 918,730 12,980
First Stage Equiv.Adm.=O.90(Alt.2) 917,300 11,550
First Stage Equiv.Adm.=0.865(Alt.2) 919,600 13,850
Since the present invention only proposes closing
or blocking, on an average for a 10% excess flow capacity,
one out of every 12 nozzle passages, the flow from adjacent
nozzle passages is believed to be sufficient to fill the
resulting void in the flow prior to the steam reaching the
rotating blade assembly. Accordingly, shock loading is
avoided. In a control stage, the blocked passage would be
about 0.5 inches in width with about 5.5 inches of open
nozzle passages between the blocked passages.
While the invention has been described in what is
presently considered to be a preferred embodiment, many
variations and modifications will become apparent to those
skilled in the art. Accordingly, it is intended that the
- invention not be limited to the specific illustrative
embodiment but be interpreted within the full spirit and
scope of the appended claims.
