Note: Descriptions are shown in the official language in which they were submitted.
63E~4
METHOD OF VARYING TURBINE OUTPUT OF A SUPERCRITICAL-PRESSURE
STEAM GENERATOR-TURBINE INSTALLATION
FIELD OF THE INVENTION
5 The invention relates to a method of varying turbine output
in a steam power plant with a supercritical steam
generator-turbine installation.
BACKGROUND OF THE INVENTION
10 Various methods for controlling the output of steam
turbines have been utilized in the past where the steam
turbine forms part of a power installation including
a steam generator for generating steam pressure and a
superheater positioned between the steam generator and ~
the turbine. The turbine itself is usually furnished with
one or more control valves for varying the amount of
steam supplied to the turbine to control its output.
Among the methods of controlling turbine output in sub-
20 critical steam pressure installations have been:
1) constant or fixed throttle pressure-sequential valve
operation in which the active nozzle area in the first
stage of the turbine is varied by sequential operation of
control valves to control flow of steam to the turbine;
2) throttling control-single point valve operation in
which all of the control valves are operated simultaneously
and turbine output is varied by controlling the amount
of throttling across the valves in order to control the
flow of steam to the turbine;
3) sliding pressure operation in which all the control
valves are held in a fixed position and the throttle and
nozzle inlet pressure are varied by varying the steam
generator outlet pressure; and
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4) hybrid operation which combines methods l) and 3) or
2) and 3) at different ranges of turbine output.
Each method has its advantages and disadvantages when
5 applied to commercial installations and their uses are
dependent in part on steam generator and steam turbine
design.
While the above methods of operation are applicable for use
10 in subcritical installations, they may not entirely be
used in supercritical installations utilizing steam pressures
in excess of the critical pressure of steam, which is
3208.2 p.s.i. This is due in part to steam generator
design considerations. In subcritical installations,
sliding pressure operation is achieved by varying the
firing rate of the steam generator which in turn varies the
pressure of the steam delivered to the turbine. This
method is not feasible for use in all supercritical
operations because the pressure in the supercritical
20 steam generators commonly used in the United States is
never allowed to fall below the critical pressure
whenever the steam generator is being f~ired.
It is therefore an object of my invention to provide for
25 a method of varying turbine output in a supercritical
steam generator-turbine installation which includes a
sliding pressure control step over a portion of the
range of turbine output whereby (l) temperature change
and consequential thermal stresses imparted to the high
30 pressure turbine may be minimized and whereby (2) net
heat rate may be improved in lower load ranges of opera-
tion. The latter advantage can result in substantial fuel
savings in the operation of a steam power plant.
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DESCRIPTION OF THE INVENTION
Broadly a method according to the invention for varying
turbine output is applicable for use in an installation
having a steam generator for generating steam, a feed
5 water pump for maintaining pressure in the steam
generator to a predetermined amount in excess of the critical
pressure, a superheater, a pressure reducing division valve
or apparatus located between the steam generator and the
superheater and a control valve associated with the turbine
for regulating entry of steam to the turbine. The operating
steps include setting the turbine control valve to a
predetermined fully open position, maintaining the division
valve in a ~u~ y~opentposition and varying the rate of
delivery of the feed water pump in order to vary steam
pressure in the steam generator within a range, the high
pressure point of which is at a predetermined amount in
excess of the critical pressure and the low pressure point
of which is slightly above the critical pressure.
In one embodiment of the invention, the controi valve is
partially opened at approximately 85% of rated output
and fully opened at 100% rated output to vary turbine
output in the range of 85%-100% of rated output. Variation
of the output in the range of approximately 65~=85% of rated
output is achieved by varying the rate of delivery of
the feed water pump from the point at which it produces
steam pressure at the predetermined amount in excess of
critical pressure at approximately 85% of rated capacity
down to the point where the steam pressure is slightly in
excess of cr-itical pressure at approximately 65% of rated
30 capacity. Turbine output in the range of approximately
65~ of rated output down to 0% output is achieved by
varying the opening of the division valve from the fully
open position at approximately 65% rated output to the
fully closed position at 0% rated output.
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In a further embodiment of the invention, the control valve
is maintained fully opened and turbine ouput in the range
of 100% rated output down to approximately 65~ rated output
is achieved by varying the delivery rate of the feed water
5 pump from the point at which it produces steam pressure
at the predetermined amount in excess of critical pressure
at 100~ rated output down to the point where it produces steam
pressure slightly in excess of critical pressure at
approximately 65~ of rated output. Variation of output in
10 the range of approximately 65~ rated output down to 0%
rated output is accomplished by varying the opening of the
division valve.
In a still further embodiment of the invention, the control
valve is maintained fully open and turbine output is
varied in the range of 100% of rated output down to
approximately 85% of rated output by varying the delivery
rate of the feed water pump from the point at which it
produces steam pressure at the predetermined amount in
20 excess of critical pressure at 100~ rated output down
to the point where it produces steam slightly in excess of
critical pressure at approximately 85~ of rated output.
Turbine output in the range of 85% of rated output down to
0% of rated output is varied by varying the opening of
25 the division valve from the fully opened position at
approximately 85% of rated output to the fully closed
position at 0% of rated output.
Preferably the components are designed for operating at
30 a predetermined amount of pressure in excess of the
critical pressure such that variance of turbine output
due to varying feed water pump pressure may occur between
the said predetermined pressure down to a point sufficiently
above the critical pressure. The sufficiency above the
35critical pressure is determined by the steam generator
design.
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BRIEF DESCRIPTION OF THE DRAWINGS
Figure l is a flow diagram of a typical supercritical steam
power installation and to which a method of varying
turbine output according to the invention is applicable;
Figure 2 is a graph illustrating change in throttle pressure
at the turbine control valve as a function of turbine
output when using an operating cycle according to the
invention as compared with a conventional operating cycle;
Figures 3a, 3b and 3c are graphs illustrating the principal
steam temperatures as a function of turbine output in an
installation of the type illustrated in Figure l utilizing
'andjoperating cycle according to the invention as
15 compared with a conventional operating cycle;
Figures 4a and 4b are graphs illustrating temperatures at
the inlet and exhaust of the high pressure turbine as a
function of turbine output in an installation of the type
20 illustrated in Figure l utilizing an operating cycle
according to the invention as compared with a conventional
operating cycle; and
Figure 5 is a graph illustrating unit net heat rate as
25 a function of turbine output utilizing an operating
s cycle according to the invention as compared with
conventional operating cycles.
BEST MODE FOR CARRYING OUT THE INVENTION
30Referring to Figure l, there is illustrated a flow diagram
of a typical supercritical steam generator-turbine
installation. As shown, steam is generated in the steam
generator and ~hen passes through one or more division
valves and on through a superheater. The steam then passes
35through a steam line to a turbine control valve which controls
entry of steam into the first stage of a high pressure
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turbine. While only one control valve is shown, a
plurality of valves may be used as previously explained
with method 1) as pertains to a subcritical installation.
5 The steam then passes from the high pressure turbine
through a first reheater into a first reheat turbine.
The steam may then pass from the first reheat turbine
through a second reheater into the entry sides of a
second reheat turbine and then on to low pressure turbines.
10 The high pressure turbine, the first reheat turbine and
second reheat and low pressure turbines are all connected
to one or two electric generators depending on the
compounding of the turbines. The use of a second
reheater is optional and, if desired, steam could flow
directly from the first reheat turbine to the low
pressure turbine.
Steam passes from the low pressure turbines to a steam
condenser. The condensate formed inthe condenser is pumped
by means of a condensate pump through low pressure
condensate heaters to the entry side of a feed water
pump. The feed water pump then pumps feed water through
high pressure feed water heaters and into the steam
generator. Thé feed water pump maintains pressure between
the pump and the division valve at a predetermined amount
in excess of the critical pressure of steam. When the
division valve is fully open, the predetermined pressure
will also be extended to the turbine control valve.
The method of operation of the installation illustrated
in Figure 1 and according to a first embodiment of the
invention is as follows. Control of the output of the
high pressure turbine between the ranges of 85%-lOO~
of rated output is achieved by varying the opening of
the turbine control valve between a fully open position
(lOO~ rated output) and the partially open position
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(85~ rated output).
In order to vary output of the turbine within the ranges
of approximately 65% and 85~ of rated output, the
turbine control valve is partially opèned to a predetermined
position and the division valve is kept fully open.
Pressure within the steam generator and at the turbine
control valve is then varied by varying the delivery
pressure of the feed water pump such that the pressure at
the control valve will vary between the predetermined amount
in excess of the critical pressure down to a point slightly
in excess of the critical pressure.
Alternatively, the turbine control valves are kept
fully open for outputs in the range of approximately 85%-
100~ and the pressure at the control valve is varied
as described immediately above.
Control of turbine output below approximateiy 65~ is
achieved by varying the opening of the division valve. By
20 partially closing the division valve, pressure to the
partially open turbine control valve is reduced to
subcritical values thus reducing pressure in the entry
side of the high pressure turbine. The use of a division
valve or valves assures that pressure within the steam
25 generator will remain in excess of the critical pressure
at all times thus preventing damage to the steam
generator because of thermal stresses even though pressure
in the superheater and steam line downstream of the
valve may be subcritical.
A further method of operation of the installation
illustrated in Figure 1 according to a second embodiment of
the invention is as follows. Control of the output of
the high pressure turbine between the ranges of approximately
35 65%-100% of rated output is achieved by opening the turbine
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control valve to the fully opened position and varying
the delivery pressure of the feed water pump such that
the pressure at the control valve will vary between the
predetermined amount in excess of critical pressure down to
5 a point slightly in excess of the critical pressure
while keeping the division valve fully open. Control of
turbine output within the range of approximately 65~-0%
of rated output is accomplished by varying the opening of
the division valve from the fully open position to the
closed position.
A still further method of operation of the installation
of Figure 1 according to a third embodiment of the
invention is as follows. Control of the output of the
turbine between 100% and approximately 85% of rated
capacity involves keeping the control valve fully
open, keeping the division valve fully open and
varying the delivery of the feed water pump so as to vary
the steam pressure between a predetermined amount
in excess of critical pressure down to a point slightly
20 in excess of critical pressure. Control of the output
of the turbine between approximately 85~ of rated output
down to 0~ involves varying the opening of the division
valve from a fully open position at approximately 85%
of turbine output to a fully closed position at 0%
25 output.
A feature of all three methods of operation is that a
part of the range of variation of turbine ouput is
achieved by varying the rate of delivery of the feed
30 water pump such that the steam pressures produced will
vary between the predetermined amount in excess of the
critical pressure and an amount slightly in excess of
the critical pressure.
35 Referring to Figure 2, there is illustrated a graph
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depicting turbine output as a function of pressure utilizing
a conventional control cycle as compared with the control
cycle according to a first embodiment of the invention
which for convenience is defined as the Dolan Cycle. The
5 conventional control cycle referred to is one where the
control valve is subjected to a constant pressure above
the critical pressure and output is varied by regulating
opening of the turbinecontrol valve or valves. Control of
turbine output in the 100%-85% range is the same as with
10 the conventional cycle, namely by varying opening of the
control valve with the result that throttle pressure for
both cycles remains supercritical and the steam generator
pressure is the same ror both.
As shown in connection with the Dolan Cycle, turbine
output is reduced from approximately 85% rated capacity
to approximately 65% capacity by reducing throttle
pressure from the normal full load pressure down to
slightly above criti~cal pressure. As explained
20 previously, this is accomplished by varying the delivery
pressure of the feed water pump so as to reduce the steam
generator pressure. Turbine output is reduced below 65%
by closing the division valve, which as shown, further
reduces throttle pressure below the critical pressure
but allows the steam generator press-lre to remain slightly
above critical pressure.
The advantage of the Dolan Cycle as compared with a
conventional cycle is illustrated in Figures 3 and 4.
While temperature changes of the Dolan Cycle as compared
with a conventional cycle may appear insignificant in
connection with the steam temperature entering the turbines
under varying turbine outputs (Figures 3a, 3b and 3c~ use
of the Dolan Cycle results in less temperature change in
the high pressure turbine over a wide load range than when
using a conventional cycle as shown in Figures 4a and 4b.
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This is important because thermal stresses on the turbine
are reduced.
The higher principal steam temperature over large load
5 ranges improves the part load heat rate (thermal
efficiency) of the installation.
Referring to Figure 5, the increased efficiency of using
the Dolan Cycle of operation according to the invention as
10 compared with conventional operation cycles is illustrated.
As shown, the Dolan Cycle, between the output ranges of
85% down to approximately 20%, has a better heat rate than
the conventional cycle of a throttled controlled turbine
utilizing constant pressure imparted onto a turbine
15 control valve the opening of which is varied.
