Method of varying turbine output of a supercritical-pressure steam generator-turbine installation

A method of varying turbine output from approximately 85% rated load down to approximately 65% rated output or alternatively from 100% down to approximately 85% where the turbine forms part of a supercritical installation having a steam generator for generating steam to a predetermined amount in excess of critical pressure. The installation includes a feed water pump or maintaining pressures in the steam generator to the predetermined amount in excess of critical pressure, a division valve and a turbine control valve. Output is varied by maintaining the control valve in a fully or partially open position, the division valve in a fully open position and varying the delivery pressure of the feed water pump from the predetermined supercritical pressure to a pressure slightly above critical.

FIELD OF THE INVENTION 
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 
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 subcritical 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 (4) hybrid operation 
which combines methods (1) and (3) or (2) and (3) at different ranges of 
turbine output. 
Each method has its advantages and disadvantages when 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 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 steam generators commonly used 
in the United States is never allowed to fall below the critical pressure 
whenever the steam generator is being fired. 
It is therefore an object of my invention to provide for 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 (1) temperature change and 
consequential thermal stresses imparted to the high pressure turbine may 
be minimized and whereby (2) net heat rate may be improved in lower load 
ranges of operation. The latter advantage can result in substantial fuel 
savings in the operation of a steam power plant. 
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 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 position, maintaining the division valve in a fully open 
position 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 control 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 critical 
pressure at approximately 65% of rated 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. 
In a further embodiment of the invention, the control valve is maintained 
fully opened and turbine output in the range of 100% rated output down to 
approximately 65% rated output is achieved by varying the delivery rate of 
the feed water 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 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 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 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 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 critical pressure is 
determined by the steam generator design.

BEST MODE FOR CARRYING OUT THE INVENTION 
Referring to FIG. 1, there is illustrated a flow diagram of a typical 
supercritical steam generator-turbine installation. As shown, steam is 
generated in the steam generator and then passes through one or more 
division valves and on through a superheater. The steam then passes 
through a steam line to a turbine control valve which controls entry of 
steam into the first stage of a high pressure 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. 
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. 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 in the 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. The 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 FIG. 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%-100% of 
rated output is achieved by varying the opening of the turbine control 
valve between a fully open position (100% rated output) and the partially 
open position (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 opened 
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 approximately 65% is achieved by varying 
the opening of the division valve. By 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 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 FIG. 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 65%-100% of rated output is achieved by opening the turbine 
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 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 FIG. 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 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% output. 
A feature of all three methods of operation is that a part of the range of 
variation of turbine output is achieved by varying the rate of delivery of 
the feed 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. 
Referring to FIG. 2, there is illustrated a graph 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 
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 turbine control valve or valves. 
Control of turbine output in the 100%-85% range is the same as with 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 for 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 critical pressure. As explained 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 pressure to 
remain slightly above critical pressure. 
The advantage of the Dolan Cycle as compared with a conventional cycle is 
illustrated in FIGS. 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 (FIGS. 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 FIGS. 4a and 4b. 
This is important because thermal stresses on the turbine are reduced. 
The higher principal steam temperature over large load ranges improves the 
part load heat rate (thermal efficiency) of the installation. 
Referring to FIG. 5, the increased efficiency of using the Dolan Cycle of 
operation according to the invention as 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 control valve the opening of 
which is varied.