Governor valve positioning to overcome partial-arc admission limits

Method for improving the heat rate of a steam turbine operated in a partial-arc mode includes sequential closing of control valves to establish a first arc of admission at which pressure drop on a first control stage reaches a predetermined level. Steam pressure to the turbine may then be reduced in combination with valve closing to maintain first stage pressure at or below the predetermined level. In a further method, low power operation is achieved by maintaining a constant arc of admission while simultaneously moving all open valves toward a closed position.

This invention relates to steam turbines and, more particularly, to a 
method and apparatus for improving the heat rate (efficiency) of a 
partial-arc admission steam 1 
BACKGROUND OF THE INVENTION 
The power output of many multi-stage steam turbine systems is controlled by 
throttling the main flow of steam from a steam generator in order to 
reduce the pressure of steam at the high pressure turbine inlet. Steam 
turbines which utilize this throttling method are often referred to as 
full arc turbines because all steam inlet nozzle chambers are active at 
all load conditions. Full arc turbines are usually designed to accept 
exact steam conditions at a rated load in order to maximize efficiency. By 
admitting steam through all of the inlet nozzles, the pressure ratio 
across the inlet stage, e.g., the first control stage, in a full arc 
turbine remains essentially constant irrespective of the steam inlet 
pressure. As a result, the mechanical efficiency of power generation 
across the control stage may be optimized. However, as power is decreased 
in a full arc turbine, there is an overall decline in efficiency, i.e., 
the ideal efficiency of the steam work cycle between the steam generator 
and the turbine exhaust, because throttling reduces the energy available 
for performing work. Generally, the overall turbine efficiency, i.e., the 
actual efficiency, is a product of the ideal and the mechanical efficiency 
of the turbine. 
More efficient control of turbine output than is achievable by the 
throttling method has been realized by the technique of dividing steam 
which enters the turbine inlet into isolated and individually controllable 
arcs of admission. In this method, known as partial-arc admission, the 
number of active first stage nozzles is varied in response to load 
changes. Partial arc admission turbines have been favored over full arc 
turbines because a relatively high ideal efficiency is attainable by 
sequentially admitting steam through individual nozzle chambers with a 
minimum of throttling, rather than by throttling the entire arc of 
admission. The benefits of this higher ideal efficiency are generally more 
advantageous than the optimum mechanical efficiency achievable across the 
control stage of full arc turbine designs. Overall, multi-stage steam 
turbine systems which use partial-arc admission to vary power output 
operate with a higher actual efficiency than systems which throttle steam 
across a full arc of admission. However, partial-arc admission systems in 
the past have been known to have certain disadvantages which limit the 
efficiency of work output across the control stage. Some of these 
limitations are due to unavoidable mechanical constraints, such as, for 
example, an unavoidable amount of windage and turbulence which occurs as 
rotating blades pass nozzle blade groups which are not admitting steam. 
Furthermore, in partial-arc admission systems the pressure drop (and 
therefore the pressure ratio) across the nozzle blade groups varies as 
steam is sequentially admitted through a greater number of valve chambers, 
the largest pressure drop occurring at the minimum valve point (fewest 
possible number of governor or control valves open) and the smallest 
pressure drop occurring at full admission. The thermodynamic efficiency, 
which is inversely proportional to the pressure differential across the 
control stage, is lowest at the minimum valve point and highest at full 
admission. Thus, the control stage efficiency for partial-arc turbines as 
well as full arc turbines decreases when power output drops below the 
rated load. 
Sliding or variable throttle pressure operation of partial-arc turbines 
also results in improved turbine efficiency and additionally reduces low 
cycle fatigue. The usual procedure is to initiate sliding pressure 
operation on a partial-arc admission turbine at flows below the value 
corresponding to the point where half the control valves are wide open and 
half are fully closed, i.e., 50% first stage admission on a turbine in 
which the maximum admission is practically 100%. If sliding pressure is 
initiated at a higher flow (larger value of first stage admission), there 
is a loss in performance. However, in a turbine having eight valves, 
sliding from 75% admission eliminates a considerable portion of the valve 
loop (valve throttling) on the sixth valve which would occur with constant 
throttle 1 pressure operation. A similar situation occurs when sliding 
from 62.5% admission, i.e., a considerable portion of the valve loop of 
the fifth valve is eliminated. Elimination of such portions of valve loops 
improves the turbine heat rate and its efficiency. 
FIG. 1 illustrates the effect of sliding pressure control in a partial-arc 
steam turbine having eight control valves. The abscissa represents values 
of steam flow while the ordinate values are heat rate. Line 10 represents 
constant pressure with throttling control while line 12 represents sliding 
pressure on a full arc admission turbine. Line 14 represents constant 
pressure with sequential valve control (partial-arc admission) and dotted 
lines 16, 18, 20 and 22 represent the valve loops. The valve loops result 
from gradual throttling of each of a sequence of control or governor 
valves. Sliding pressure operation from 75% admission is indicated by line 
24. Note that much of the valve loop 20 is eliminated by sliding pressure 
along line 24 but that heat rate (the reciprocal of efficiency) increases 
disproportionately below the 62.5% admission point. Line 26, showing 
sliding pressure from the 62.5% admission point, provides some improvement 
but does not affect valve loops 16, 18 and 20. Similarly, sliding from 50% 
admission, line 28, helps at the low end but does not affect valve loops 
16-22. Each of these valve loops represent higher heat rates and reduced 
efficiency from the ideal curve represented by line 14. 
FIGS. 2, 3 and 4 illustrate the operation of an exemplary steam turbine 
using one prior art control. FIG. 2 shows the locus of full valve points, 
line 30, with constant pressure operation at 2535 psia. The valve points 
are at 50%, 75%, 87.5% and 100% admission with the valve loops identified 
by the lines 32, 34 and 36. Sliding pressure is indicated by lines 38, 40 
and 42. Starting at 100% admission, about 806 MW for the exemplary turbine 
system, load is initially reduced by keeping all eight control valves wide 
open and sliding throttle pressure by controlling the steam producing 
boiler. When the throttle pressure, line 38, reaches the intersection 
point with the valve loop 32, the throttle pressure is increased to 2535 
psia while closing the eighth control valve. The control valve would 
continue to close as load is further reduced while maintaining the 2535 
psia throttle pressure until this valve is completely closed at which 
point the turbine is operating at 87.5% admission. To further reduce load, 
valve position is again held constant, seven valves fully open, and 
throttle pressure is again reduced until the throttle pressure corresponds 
to the intersection of the sliding pressure line 40 and the valve loop 34 
for the seventh valve. To reduce load below this point, the pressure is 
increased to 2535 psia and the seventh valve is progressively closed 
(riding down the valve loop) until it is completely closed. The admission 
is now 75%. To reduce load still further, the pressure is again reduced 
with six valves wide open and two fully closed until the throttle pressure 
line 42 reaches the intersection with the valve loop 36 where the fifth 
and sixth valves move simultaneously with constant throttle pressure 
operation. Then the operation of raising throttle pressure and closing of 
the valves is repeated for any number of valves desired. The variation in 
throttle pressure is illustrated in FIG. 3. The sloped portions 44 of line 
46 relates to the sliding pressure regime with constant valve position. 
The vertical portions 48 relate to the termination of sliding pressure 
with no valve throttling and the uppermost point relates to operation at 
full pressure with valve throttling. The horizontal portions 50 relate to 
the riding down of the valve loop while reducing load at constant 
pressure. FIG. 4 shows the improvement in heat rate as a function of load. 
The line 52 illustrates the difference between valve loop performance at 
constant pressure and the performance with variable pressure between valve 
points. 
The performance improvements shown in FIGS. 2 and 4 are based on the 
assumption that the boiler feed pump discharge is reduced as the throttle 
pressure is reduced. If it is not reduced proportionally, the improvement 
is reduced since the energy required to maintain discharge pressure 
remains high. In the prior art system, a signal is sent to the feed 
pump/feed pump drive system to reduce pressure. In reality, however, the 
feed pump is followed by a pressure regulator in order to eliminate the 
need for constant adjustment of pump speed and the occurrence of control 
instability and hunting because of small variations in inlet water 
pressure to the boiler, resulting from perturbations in flow demand. The 
regulator, then, does more or less throttling which changes pump discharge 
pressure and therefore the flow that the pump will deliver. The pump speed 
is held constant over a desired range of travel of the regulator valve. 
When the valve travel gets outside these limits, the pump speed is 
adjusted to move the valve to some desired mean position. As a 
consequence, the pump discharge pressure does not equal the minimum 
allowable value (throttle pressure plus system head losses) and so the 
performance improvement is not as large as shown by FIGS. 2 and 4. In 
addition, in order to achieve quicker load response, the regulator valve 
is usually operated with some pressure drop so that if there is a sudden 
increase in load demand, the valve can open quickly and increase flow. The 
response of the pump and its drive is slower than the response of the 
regulator valve. 
While sliding throttle pressure operation improves part load performance of 
steam power plants, studies have demonstrated that the highest performance 
levels are achieved by partial-arc admission turbines which initially 
reduce load from the maximum value by successively closing governor or 
control valves (sequential valve operation) while holding throttle 
pressure constant. When half the control valves are wide open and half are 
closed (50% admission on the first stage), valve position is held constant 
and further load reductions are achieved by varying or sliding throttle 
pressure. This combined method of operation has been referred to as hybrid 
operation. Hybrid operation with the transition point at 50% admission is 
believed to be the most efficient operation. However, a partial-arc 
admission turbine is subjected to shock loading at part load as the 
rotating blades pass in and out of the active steam arc. As a result, the 
blades must be stronger, which affects the aspect ratio and consequently 
the efficiency. Blade material or blade root damping is desirable to 
reduce the vibration stresses associated with partial-arc admission. In 
addition, the kilowatt loading (bending forces) on the individual rotating 
blades increases as the arc of admission is decreased. Sliding pressure 
operation (hybrid operation, more particularly) reduces the shock loading 
on the turbine first stage because the optimum values of minimum admission 
are higher than with constant throttle pressure operation. 
Obtaining a first stage blade material or design with the required damping 
and strength for partial-arc operation is more difficult at elevated steam 
pressures and temperatures, for example, 4500 psia and 1100.degree. F., of 
today's turbines. This limitation forces such high pressure, high 
temperature turbines to be operated with full-arc admission first stages 
because suitable materials for partial-arc admission are not available. If 
a material cannot be found that will allow partial-arc admission at 50% 
admission, the minimum admission arc could be increased to 62.5% or 75% 
admission, for example, with some loss in performance. The performance 
level would still be better than a full-arc admission design operating 
with sliding throttle pressure. However, with minimum arcs of admission 
much above 75%, there is little benefit to hybrid operation. In other 
cases, older turbines of more conventional type, such as those operating 
at 1000.degree. F. or 1050.degree. F., have been stressed such that 
partial-arc operation is limited. For such turbines, it is desirable to 
provide a method for improving performance without exceeding minimum 
allowable stress conditions. 
If, because of first stage distress during constant pressure operation, it 
is necessary to increase the primary admission arc above 50%, the power 
plant owner and operator will experience a reduction in plant efficiency 
(higher heat rate). If the turbine is now operated in the hybrid mode, the 
reduction in plant efficiency will be much less. There will however, still 
be some decrease in plant efficiency compared to the original operating 
procedure. This is illustrated by the attached table. These data were 
developed for a 500 MW rated output turbine with valve points at 50%, 
62.5%, 75% and 100% admission. FIG. 7 is a schematic of the nozzle 
chambers (admission area) for the 8 valve 500 MW unit. At 50% admission, 
chambers A, BC and D are active. AT 62.5% admission, chamber E is also 
active. 
Suppose that the turbine could operate with hybrid variable pressure 
starting at 50% admission. At loads below 334.9 MW, Table III, the 
throttle pressure would be varied to control load while above 334.9 MW, 
control valves would be modulated to control load. In this instance, for 
the power range between 334.9 MW and 405.8 MW, there would be throttling 
on one control valve of the 50% admission design, in this instance chamber 
E. 
The design limited to 62.5% minimum admission would vary throttle pressure 
on all five active arcs of admission for loads below 405.8 MW (Table III). 
Conventional practice has been to limit the admission to the nearest valve 
point where control stage reliability is assured. In actual turbines, the 
partial-arc (first) stage usually has some additional design margin at 
this admission where reliability is assured. 
SUMMARY OF THE INVENTION 
The method of the present invention is described in a system in which a 
combination of control valve closure, sliding pressure and valve 
throttling is utilized to achieve better efficiency. In one embodiment, 
the method is illustrated for use in a turbine system in which the control 
stage can only tolerate the combined stresses of partial-arc shock loading 
and pressure drop corresponding to a 62.5% arc of admission due to 
material and blade root fastening limitations. Initial turbine power 
reduction is achieved by sequentially closing control valves to reduce the 
arc of admission to 62.5% at full operating steam pressure. Further 
reduction is achieved by partially closing the control valve which would 
normally be used to reduce admission to 50%. This last control valve is 
only closed until the pressure drop across the first stage reaches a 
maximum allowable value. At that point, the control valves are essentially 
all locked in place and further power reduction is achieved in one form by 
reducing steam pressure to the turbine. In the event that the system is 
not capable of sliding steam pressure, power reduction is achieved by 
concurrently closing each remaining open control valve in uniform 
increments so that the first stage pressure drop does not exceed the 
maximum allowable value. The process of closing all valves simultaneously 
may also be used when steam pressure in the system has been reduced to its 
minimum allowable value.

DETAILED DESCRIPTION OF THE INVENTION 
Before describing the method of the present invention, reference is first 
made to FIG. 5 which depicts a functional block diagram schematic of a 
typical steam turbine power plant suitable for embodying the principles of 
the present invention. In the plant of FIG. 5, a conventional boiler 54 
which may be of a nuclear fuel or fossil fuel variety produces steam which 
is conducted through a header 56, primary superheater 58, a finishing 
superheater 62 and throttle valve 61 to a set of partial-arc steam 
admission control valves depicted at 63. Associated with the boiler 54 is 
a conventional boiler controller 64 which is used to control various 
boiler parameters such as the steam pressure at the header 56. More 
specifically, the steam pressure at the header 56 is usually controlled by 
a set point controller (not shown) disposed within the boiler controller 
64. Such a set point controller arrangement is well known to all those 
skilled in the pertinent art and therefore, requires no detailed 
description for the present embodiment. Steam is regulated through a high 
pressure section 66 of the steam turbine in accordance with the 
positioning of the steam admission valves 63. Normally, steam exiting the 
high pressure turbine section 66 is reheated in a conventional reheater 
section 68 prior to being supplied to at least one lower pressure turbine 
section shown at 70. Steam exiting the turbine section 70 is conducted 
into a conventional condenser unit 72. 
In most cases, a common shaft 74 mechanically couples the steam turbine 
sections 66 and 70 to an electrical generator unit 76. As steam expands 
through the turbine sections 66 and 70, it imparts most of its energy into 
torque for rotating the shaft 74. During plant startup, the steam 
conducted through the turbine sections 66 and 70 is regulated to bring the 
rotating speed of the turbine shaft to the synchronous speed of the line 
voltage or a subharmonic thereof. Typically, this is accomplished by 
detecting the speed of the turbine shaft 74 by a conventional speed pickup 
transducer 77. A signal 78 generated by transducer 77 is representative of 
the rotating shaft speed and is supplied to a conventional turbine 
controller 80. The controller 80 in turn governs the positioning of the 
steam admission valves using signal lines 82 for regulating the steam 
conducted through the turbine sections 66 and 70 in accordance with a 
desired speed demand and the measured speed signal 78 supplied to the 
turbine controller 80. The throttle valve 61 may be controlled at turbine 
start-up thus allowing the control valves 63 to be fully open until the 
turbine is initially operating at about five percent load. The system then 
transitions to partial- arc operation and the throttle valve 61 fully 
opened. However, the throttle valve 61 is generally an emergency valve 
used for emergency shut-down of the turbine. The line 65 from controller 
80 provides control signals to valve 61. 
A typical main breaker unit 84 is disposed between the electrical generator 
76 and an electrical load 86 which for the purposes of the present 
description may be considered a bulk electrical transmission and 
distribution network. When the turbine controller 80 determines that a 
synchronization condition exists, the main breaker 84 may be closed to 
provide electrical energy to the electrical load 86. The actual power 
output of the plant may be measured by a conventional power measuring 
transducer 88, like a watt transducer, for example, which is coupled to 
the electrical power output lines supplying electrical 1 energy to the 
load 86. A signal which is representative of the actual power output of 
the power plant is provided to the turbine controller 80 over signal line 
90. Once synchronization has taken place, the controller 80 may 
conventionally regulate the steam admission valves 63 to provide steam to 
the turbine sections 66 and 70 commensurate with the desired electrical 
power generation of the power plant. 
In accordance with the present invention, an optimum turbine efficiency 
controller 92 is disposed as part of the steam turbine power plant. The 
controller 92 monitors thermodynamic conditions of the plant at a desired 
power plant output by measuring various turbine parameters as will be more 
specifically described herebelow and with the benefit of this information 
governs the adjustment of the boiler steam pressure utilizing the signal 
line 94 coupled from the controller 92 to the boiler controller 64. In the 
present embodiment, the boiler pressure adjustment may be accomplished by 
altering the set point of a set point controller (not shown) which is 
generally known to be a part of the boiler controller 64. As may be the 
case in most set point controllers, the feedback measured parameter, like 
steam pressure, for example, is rendered substantially close to the set 
point, the deviation usually being a function of the output/input gain 
characteristics of the pressure set point controller. The controller 92 
also supplies signals via line 46 to superheater 62 to control the final 
steam temperature. 
Turbine parameters like throttle steam pressure and temperature are 
measured respectively by conventional pressure transducer 96 and 
temperature transducer 98. Signals 100 and 102 generated respectively by 
the transducers 96 and 98 may be provided to the optimum turbine 
efficiency controller 92. Another parameter, the turbine reheat steam 
temperature at the reheater 68 is measured by a conventional temperature 
transducer 104 which generates a signal on line 106 to the controller 92 
for use thereby. The signal on line 90 which is generated by the power 
measuring transducer 88 may be additionally provided to the controller 92. 
Moreover, an important turbine parameter is one which reflects the steam 
flow through the turbine sections 66 and 70. For the purposes of the 
present embodiment, the steam pressure at the impulse chamber of the high 
pressure turbine section 68 is suitably chosen for that purpose. A 
conventional pressure transducer 108 is disposed at the impulse chamber 
section for generating and supplying a signal 110, which is representative 
of the steam pressure at the impulse chamber, to the controller 92. 
One embodiment of the turbine efficiency controller 92 sufficient for 
describing the operation of the controller 92 in more specific detail is 
shown in U.S. Pat. No. 4,297,848 assigned to the assignee of the present 
invention, the disclosure of which is hereby incorporated by reference. 
As described in the aforementioned U.S. Pat. No. 4,297,848, the controller 
92 and the controller 80 may include microcomputer based systems for 
computing appropriate set points, e.g., throttle pressure and steam flow, 
for optimum operation of the steam turbine system in response to load 
demands. In the present invention, it is desirable to control throttle 
steam pressure applied to valves 63 in order to optimize system efficiency 
while having the ability to rapidly respond to increased load demand. The 
system of FIG. 5 achieves this result by controlling the boiler 54, 
primary superheater 58 and the finishing superheater 62 in a manner to 
regulate throttle steam pressure and temperature. 
The method of operation of the system of FIG. 5 can best be understood by 
reference to FIG. 6 which illustrates a plurality of steam flow versus 
steam pressure diagrams for various partial-arc admissions of a high 
temperature, high pressure steam turbine. For purposes of discussion, it 
is assumed that the design of this turbine is such that the control stage 
blading is limited to 75% admission at full operating steam pressure, 
i.e., about 4300 psia at the inlet to the control stage nozzles. Line 110 
represents the pressure drop across the control stage (nozzle inlet to 
impulse chamber). Line A, B, C, D, E represents full operating steam 
pressure. For example, the control stage pressure drop at full arc is 
about 850 psia, i.e., the difference between point 110A and 4300 psia. The 
maximum allowable pressure drop occurs at 75% admission and is about 1500 
psia. Lines 122 and 124 bracket a typical minimum pressure zone for most 
utility turbines, i.e., a pressure between 500 and 1000 psia. Control 
valves 63 are sequentially closed to reduce the arc of admission to 75% 
in response to load demands determined by controllers 80 and 92. At point 
B of FIG. 6, representing 75% admission, the controllers hold admission 
constant while reducing throttle steam pressure along line 112 to point G. 
Pressure is then held constant and additional valves are closed to bring 
the turbine operating point to point H on the 50% admission line 114. The 
difference between the pressure at point H and the impulse chamber 
pressure at point K is essentially the same as between points B and 110A 
so that the shock stresses at 50% admission are no greater than the design 
limit at 75% admission and should be lower because of the lower steam 
density. 
If the turbine were designed to withstand shock loading at 62.5% admission 
at full pressure, the initial power reduction can be achieved by closing 
control valves 63 following line A, B, C, D to point C. Steam pressure can 
then be reduced along line 116 to point J. At that point, pressure is held 
constant and additional valves 63 are closed to reach point F. Further 
power reduction is achieved by reducing pressure along line F-L. 
The controllers 80, 92 can also be programmed to adjust steam pressure and 
close valves 63 concurrently so that turbine operation follows line 118 
directly from point B to point H. Such operation may require alternate 
adjustment of pressure and valve closure so that line 118 appears more as 
a stair-step than a linear path. The same approach can be used o 
transition from point C to point F along line 120. In this method, the 
differential pressure 1 is maintained substantially constant, i.e., lines 
110, 118 and 120 are substantially parallel. This method of operation is 
more efficient than the first disclosure method since it maintains the 
control stage at its designed pressure drop. 
In general, both of the above methods of operation follow the same pattern 
once 50% admission is reached, i.e., pressure is allowed to slide until a 
minimum pressure is reached, typically about 600-1000 psia on turbines 
operating at a design throttle pressure of 2400 psia. For loads requiring 
less than this minimum pressure at minimum design admission, throttling of 
the control valves is used to reduce power output. However, as was shown 
in FIG. 1, throttling produces a higher heat rate and is therefore less 
efficient. However, Applicant has found that even though such turbines are 
designed to operate at optimum at some set admission, e.g., 62.5% 
admission, additional improvement in heat rate can be attained by further 
reducing the arc of admission at low or minimum steam pressures. Table I 
illustrates a typical set of heat rates for an exemplary turbine with 
inlet steam conditions of 2400 psia and temperature of 1000.degree. 
operating at low loads and a minimum pressure of 600 psia. Note that there 
is a small improvement between 50% admission and 37.5% admission although 
there is no additional improvement in going to 25% admission. However, 
Table II illustrates that an improvement can be realized at 25% admission 
for a 2400 psia design throttle pressure turbine operating at a minimum 
throttle pressure of 1000 psia. Thus, this method of operation reduces 
heat rates when minimum throttle pressure is used and provides a benefit 
from operation at lower values of admission without detrimental effect on 
the control stage blading. 
The above described methods of turbine operation have been based upon the 
assumption that a preselected group of valves may be closed to bring power 
down. However, as the control stage is stressed by cycling of the turbine, 
the predetermined maximum allowable control stage pressure drop is 
reduced. Table III illustrates data for a turbine in which the original 
design allowed operation at full pressure down to a 50% arc of admission, 
but in which repeated cycling has stressed the blading such that a maximum 
allowable control stage pressure drop is now 1083 psia for reliable 
operation. Note that the control stage pressure drop is 973 psia at 62.5% 
admission and 1231 psia at 50% admission. Thus, reliable operation within 
the stress limits occurs between a valve point at 62.5% and a valve point 
at 50% admission. Note also that at 49% load (243.3 MW), the design 
limited to 62.5% admission incurs a heat rate penalty of 81 BTU/KWH which 
continues to increase for decreasing load. At 29% load (145.4 MW), the 
heat rate penalty at 62.5% admission is 152 BTU/KWH. 
The present invention provides a heat rate improvement by partially closing 
the control valve that supplies the 12.5% admission arc between 50% and 
62.5% admission. This control valve is allowed to close to the point at 
which the first stage pressure drop reaches the predetermined maximum 
allowable drop, i.e., 1083 psia in this example. Table III shows a heat 
rate improvement of 48 BTU/KWH using partial closure as compared to 
operation at 62.5% admission at 243.3 MW. At a load of 145.4 MW, this 
method improves heat rate by 105 BTU/KWH. When the control valve has been 
partially closed such that the pressure drop has reached the maximum 
allowable value, further power reduction is attained by sliding pressure 
in the manner described above, unless the turbine is of a type in which 
pressure is not variable. In that instance, it has been found that 
reliable operation is possible by concurrently closing all open control 
valves by substantially identical increments. This latter technique can 
also be used if steam pressure has been reduced to a minimum value, such 
as that represented by line 122 in FIG. 6. 
Referring to FIG. 7, the present invention proposes variable pressure 
operation in which control valve position is held constant with four 
valves wide open (feeding steam to chambers A, BC and D) and one partially 
closed valve feeding steam to chamber E for an exemplary eight control 
valve system. If pressure can be reduced while holding valve positions 
constant, an improvement in heat rate over the fixed 62.5% admission can 
be realized. An additional improvement can be obtained by interrupting 
steam pressure reduction at a predetermined point and reducing load by 
completely closing the valve controlling chamber E, i.e, the partially 
open valve. Once the chamber E valve is closed, load is again reduced by 
sliding pressure downward. 
When a turbine is operated in the partial-arc admission mode, the control 
stage rotating blades experience shock loading as they pass in and out of 
the active admission arc. In addition, the blades are subjected to 
vibratory stimulus. The resulting blade loading was investigated by R. P. 
Kroon in the late 1930's and reported in an ASME paper "Turbine-Blade 
Vibration due to Partial Admission", Journal of Applied Mechanics, vol. 7, 
pp. A161-165, Dec. 1940. FIGS. 8, 9 and 10 illustrate the forces and 
vibration that occur during partial-admission. Note that the vibratory 
force is higher when the blades leave the active jet than when they enter 
the active jet. Compare the sum of b and b1 to the sum of d and d' on FIG. 
10. 
If the arc with the partially closed valve (chamber E) is the trailing 
admission arc during operation, it will cushion the rebounding force, d', 
of FIG. 10, there is no steam admission on the side of the admission arc 
that has wide open control valves supplying it. In the case of FIG. 7 and 
with the indicated clockwise rotation, the trailing admission is chamber 
E. If the partially throttled arc leads (is ahead) of the fully active 
arc, it would reduce the magnitude of b1 in FIG. 10. In this instance, 
chamber F of FIG. 7 is the leading admission arc. In this instance, the 
magnitudes of b1 and c of FIG. 10 would be reduced and consequently the 
sum of d and d' would be lower. However, a partially open trailing chamber 
is the preferred embodiment. 
The above described procedure can be used on operating turbines that have 
experienced first stage distress from use or to obtain a more optimum 
transition load when switching from constant to variable pressure 
operation. The procedure can also be used on full-arc admission turbines 
to improve part load performance while still admitting steam to all of the 
admission arcs. 
TABLE I 
______________________________________ 
600 Psia Pressure 
Heat Rate Comparison 
(BTU/KWH) 
% 62.5% 50% 37.5% 25% 
Load Adm. Adm. Adm. Adm. 
______________________________________ 
17 9654 9649 9649 9649 
13.6 10089 9927 9927 9927 
10.3 10781 10593 11492 10492 
7.7 11675 11448 11238 11238 
______________________________________ 
TABLE II 
______________________________________ 
1000 Psia Pressure 
Heat Rate Comparison 
(BTU/KWH) 
% 62.5% 50% 37.5% 25% 
Load Adm. Adm. Adm. Adm. 
______________________________________ 
30.2 8768 8763 8763 8763 
29.8 8935 8874 8874 8873 
23.5 9137 9010 9010 9010 
20.1 9390 9252 9218 9218 
16.8 9710 9563 9426 9426 
13.5 10156 9993 9842 9834 
10.2 10867 10678 10501 10336 
7.6 11792 11563 11352 11154 
______________________________________ 
TABLE III 
______________________________________ 
500 MW Rated Load 
Load Heat Rate, Btu/kwh 
MW 50% Adm. 62.5% Adm. Proposed 
______________________________________ 
405.8 7958 7958.sup.(2) 
7958 
373.3 8019 8013 8019.sup.(3) 
355.3 8043 8054 8048 
339.5 8056 8084 8077 
334.9 8060.sup.(1) 
8096 8086 
323.7 8084 8123 8109 
307.7 8119 8167 8147 
291.7 8158 8214 8188 
275.7 8202 8268 8234 
259.5 8253 8327 8286 
243.3 8312 8393 8345 
227.1 8378 8469 8415 
210.8 8455 8555 8494 
194.5 8543 8653 8584 
178.1 8652 8767 8689 
161.8 8765 8895 8807 
145.4 8907 9059 8954 
______________________________________ 
.sup.(1) Control Stage Pressure Drop = 1231 psia 
.sup.(2) Control Stage Pressure Drop = 973 psia 
.sup.(3) Control Stage Pressure Drop = 1083 psia