Note: Descriptions are shown in the official language in which they were submitted.
,Z~59
1 This invention relates to a method of and a
system for controlling the drain water level of a feed
water heater in a steam-power plant or a nuclear power
plant.
In the steam-power plant or nuclear power plant,
generally a plurality of feed water heaters are provided
for elevating the heat efficiency of the plant, and
steam extracted from a turbine is used as a heat source
for preheating the feed water supplied to a steam generator.
As the extracted steam is sub~ected to heat exchange with
the feed water, it is condensed into drain, and drains
from high pressure feed water heaters progressively
~oin together and are directed to low pressure feed
water heaters or an air separator. In the high
pressure feed water heaters, constant drain level control
is effected in order to prevent damage to the turbine
due to reverse drain flow or prevent reduction of the
heat exchange efficiency.
Generally, the drain path from each feed
water heater is changed according to the load of the
plant, and with the prior art control system in which
the switching of drain paths (hereinafter referred to as
drain switching) i3 effected through continuous operation
of individual water level controi valves with continuous
load change, great variations of the drain level are
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liable to result from the delay in the flow i.n the
individual drain ducts and also differences in the control.
signal level. Also, in case where the load decreases,
abnormal drain level increase is liable. Therefore, with
the prior art drain level control method it has been
difficult to obtain steady and stable drain level control.
This invention is contemplated in the light of
the afore-mentioned problems in the prior art drain level
control techniques, and it has for its object to provide a
method of and a system for controlling the feed water
heater drain level, with which water level control valves
are properly operated to obtain steady and reliable control
of the high pressure feed water heater drain level free
from abnormal variations of the level even in the case of
the drain switching or a sudden load change.
In accordance with an aspect of the invention
there is provided a method of controlling the drain level
of a high pressure feed water heater for heating the feed
water with steam extracted from a turbine comprising the
steps of detecting said drain level, generating a control
signal to be applied to a control valve in such a manner
that the detected value is coincident with a predetermined
value, and controlling the openings of first and second water
leve; control valves respectively provided in a first drain
duct leading from said high pressure feed water heater to a
deaerator and a second drain duct leading from said high
pressure feed water heater to a low pressure feed water heater
according to the detected water level, wherein said method
further comprises the steps of detecting a first state of
drain in the
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1 neighborhood of a drain outlet of the high pressure feed
water heater and a second state of drain in the neighbor-
hood of a drain inlet of the deaerator, and selectively
directing one of the drain to be led to the high pres-
sure feed water heater from a preceding state and thedrain from the high pressure feed r~ater heater toward
the deaerator prior to the occurrence of flashing
in the neighborhood of the inlet of the first water
level control valve are provided, thereby ensuring steady
and reliable drain level control at the time of the
flashing.
In another aspect of the inventian, there is
provided a system for controlling the drain water level
of a high pressure feed water heater, which comprises a
high pressure feed water heater for heating feed water
with steam extracted from a turbine, first and second
water level control valves respectively provided in
a first drain duct leading from the high pressure feed
water heater to a deaerator and a second drain
duct leading from the high pressure feed water heater
to a low pressure feed water heater, a detector for
detectlng the openings of the first and second water
level control valves, and a detector for detecting the
drain level of the high pressure feed water heater, and
in which the drain level is controlled through the
control of the openings of the first and second water
level control valves in accordance with the detected
drain water level and openings of the first and second
s9
1 water level control valves. The system mentioned further
comprises a detectof for detecting a first state of drain
in the neighborhood of the drain outlet of the high
pressure feed water heater, a detector for detecting a
second state of drain in the neighborhood of the drain
inlet of the deaerator, and a selector for selectively
directing either the drain led to the high pressure feed
water heater from a preceding stage or the drain from
the high pressure feed water heater prior to the
occurrence of the flashing in the neighborhood of the
inlet of the first water level control valve, thereby
ensuring steady and reliable drain water level control.
The invention will become apparent from the
preceding description of the prior art and the
description of the preferred embodiments of the invention
with reference to the accompanying drawings, in which:
Fig. l is a schematic representation of a
prior art feed water heater drain level control system;
Fig. 2 is a graph showing the load character-
istics of the same system;
Fig. 3 is a graph showing the drain level
control charactersitics of the same system;
Fig. 4 is a graph showing a control valve
characteristic;
Fig. 5 is a schematic representation of a feed
water heater drain level control system embodying the
invention;
Figs. 6 and 7 are block diagram showing the
759
control valve operation signal generator and in-advance
control valve operation signal generator in the embodiment
of Fig. 5;
Figs. 8 and 9 are flow charts illustrating the
embodiment of Fig. 5;
Figs. 10 and 11 are graphs showing the control
characteristics of t'ne embodiment of Fig. 5;
Fig. 12 is a schematic representation of another
embodiment of the invention applied to a digital water
level control system;
Fig. 13 is a view illustrating the function of a
study control system incorporated in the embodiment of
Fig. 12;
Figs. 14A, 14B, 14C and 14D illustrate an example
lS of producing basic data from fetched data;
Fig. 15 is a schematic representation of a
further embodiment of the feed water heater drain level
control system according to the invention;
Fig. 16 is a block diagram showing the con-
struction of a feed forward water level adjuster in the
embodiment of Fig. lS;
Fig. 17 is a block diagram showing the con-
struction of an opening difficiency compensation mechanism
in the feed forward water level adjuster shown in Fig. 16;
Fig. 18 is a flow chart showing the method of
calculation of the flash factor;
Fig. 19 is a block diagram showing the con-
struction of an in-advance drain switching valve gradual
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`3~7J59
1 opening operation signal generator;
Fig. 20 is a flow chart showing the construction
of a drain switching signal selector;
Fig. 21 is a flow chart showing the construction
of gradual opening signal starter;
Fig. 22 is a flow chart showing the construction
of a drain switch circuit; and
Fig. 23 is a graph showing the drain level
control characteristics of the embodiment of Fig. 15.
For the sake of facilitating the understanding
of the invention, a prior art drain level control method
will first be described with reference to Fig. 1 prior to
the description of the preferred embodiments of the inven-
tion.
Referring to the figure, the feed water to be
heated is forced by a feed water pump 2 through a feed
water duct 1, and first and second high pressure feed
water heaters (hereinafter referred to as first and
second heaters) 3 and 4 and further a prestage high
pressure feed water heater (not shown) are provided on
the feed water duct 1 in the mentioned order from the
upstream side with respect to the pump. First and
second extracted steam ducts 5 and 6, to which steam
extracted from a turbine (not shown) is led, are
connected to the respective first and second heaters
3 and 4 for supplying the steam thereto for heat
exchange with the feed water. The extracted steam ducts
5 and 6 are provided with respect to the turbine such
759
1 that the pressure of the extracted steam is higher for
the downstream side with respect to the feed water. In
other words, the pressure of steam supplied to the down-
steam side second heater 4 is higher than that supplied
to the first heater 3. In these heaters 3 and 4, the
extracted steam is condenced to drain as it effects
heat exchange with the feed water, so that the individual
hèaters 3 and 4 are provided with respective drain ducts.
To the downstream side second heater 4 connected is a
drain duct 7 leading from the prestage heater (not shown),
and a drain duct 8 for leading the combination of the
drain from the prestage heater and the drain produced
in the second heater 4 to the first heater 3 is connected
to the second heater 4. To the last stage first heater
3 is connected, in addition to the afore-mentioned
drain duct 8, a drain duct 9 for leading the combination
of the drain led from the second heater 4 and the drain
produced in the first heater 4. The drain duct 9 is
connected to an air separator 10, which is provided at
a position several ten meters higher in level than the
first heater 3, and through which the drain is converted
to feed water for reuse.
In this system, in case when the operation
condltion gets out of the normal high pressure load
state so that the turbine load is extremely reduced,
the drain from the first heater 3 can no longer be
led to the deaerator 10. In this case, the drain
level in the first and second heaters 3 and 4 is increased,
-- 7 --
759
l thus posing the afore-mentioned problems. Accordingly,
the system is provided with a drain level controller.
More particularly, a drain duct 12 branching from the
drain duct 9 is provided so that in case oP the
reduction of the drain exhausting capacity of the first
heater 3 the drain therefrom can be led to a low pressure
feed water heater (hereinafter referred to as low
pressure heater) which is provided downstream the heater
3 with respect to the drain, and water level control
valves 13 and 14 are provided on the respective drain
ducts 9 and 12. Also, a drain duct 15 branching from
the drain duct 8 leading from the second heater 4
is connected to the deaerator lO, and water level
control valves 16 and 17 are provided on the respective
drain ducts 8 and 15. When the water level of the first
heater 3 is increased at the time of the low load
condition, it is controlled by operating the valves
13, 14, 16 and 17 such that the drain from the second
heater 4 is led to the deaerator 10 while the drain
from the first heater 3 is led to the low pressure
heater 11. More specifically, the control of the water
level of the first heater 3 is effected by operating the
valve 13 through a water level setter 19 for producing
a signal representing the difference between t~e detected
water level detected by a water level detector 18 provided
in the first heater 3 and a reference water level
(hereinafter abbreviated as NWL) and a water level
ad~uster 20 operated according to the difference signal
^l.~l~Z~59
1 or operating the valve 14 through a water level setter 21
for generating a signal representing the difference
between the detected water level detected by the water
level detector 18 and a high reference water level
(hereinafter abbreviated as HNWL) and a water level
adjuster 22 operated by the difference signal. ~he
control of the water level of the second heater 4 is
effected by operating the valve 16 through a water level
setter 24 for generating a signal representing the
difference of the detected water level detected by a
water level detector 23 providedlin the second heater 4
and the NWL and a water level adjuster 25 operated by
the difference signal or operating the valve 17 through
a water level setter 26 for generating a deviation
signal representing the difference between the detected
water level detected by the detector 23 and the HNWL and
a water level adjuster 27 actuated by this deviation
signal.
Thus, in case when the turbine load is at the
rated load level (i.e., high load), the drain G2 from the
second heater 4 is led through the duct 8 to the first
heater 3, and the combination drain Gl therefrom is
led through the duct 9 to the deaerator 10, and the
individual drain levels are controlled to the rererence
level NWL by the valves 16 and 13. In this case, the
valves 17 and 14 are held fully closed. Since the valves
13 and 16 are normally operated to control the drain
level of each heater, they are respectively called first
7S~
1 and second N valves here for the sake of facilitating
the description.
When the turbine load is low, the difference
E between the internal pressure in the first heater
3 and deaerator 10 is substantially equal to or
less than the static head F of these units, and the
discharge capacity is thus lost. In this case, the
drain G2 from the second heater 4 is led through the
duct 15 to the deaerator 10, while the drain Gl from
the first heater 3 is led through the duct 12 to the next
stage low pressure heater 11. The water levels at this
time are controlled by the valves 14 and 17 for maintaining
the HNWL. Also at this time the other valves 16 and 13
are held fully closed. Since the valves 14 and 17 are
operated for the water level control in the case where the
load is low, they are respectively called first and
second X valves here for the sake of facilltating the
description.
As is shown, the switching of the drain paths
is done with respect to a predetermined load level X as
shown in Fig. 2. More particularly, in the case when the
load is increased from a low level to a high level, upon
reaching of the drain switching point X a forced full
closure signal (hereinafter referred to as full closure
signal) which has been given from the ad~usters 20 and
25 to the first and second N valves 13 and 16 is switched
to a wa~er level control signal for permitting the N valves
13 and 16 to effect the water level control while the fu;l
-- 10 -
'7S9
1 closure signal is given from the ad~usters 22 and 27 to
the first and second X valves 14 and 17. On the other
hand, in the case when the load is reduced from a high
level to a low level, the full closure signal is given
from the adjusters 20 and 25 to the N valves 13 and 16,
while the water level control signal is given from the
water level adjusters 22 and 27 to the X valves 14
and 17 to permit these valves to control the water levels
of the heaters 3 and 4.
With the prior art water level control system
described above, however, at the time of the drain
switching great water level variations resulted from the
delay in the control air signals for the valves 13,
14, 16 and 17 and also the differences in the water
level settings; particularly in case of the load reduc-
tion it is liable that an abnormal increase of the drain
level of the first heater 3 is caused due to the phenomenon
of flashing in the neighborhood of the inlet of the
first N valve 13 (which will be discussed later in detail).
When this occurs, steady and reliable water level control
can no longer be obtained.
In Fig. 1, desi~nated at 28 and 29 are electro-
magnetic valves for cutting off steam from boilers
(not shown), and at 32 and 33 reverse drain flow check
valves.
Referring now to Fig. 3, when the turbine
load is at the rated level (high load), the drain
level (curve B) of the first heater 3 is controlled to
-- 11 --
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, . ' .
7S9
1 NWL by the flrst N valve 13, and the first X valve 14,
which is set to HNWL, is held fully closed by the
deviation signal representing the difference between the
reference level HNWL and detected water level detected
by the detector 18. With gradual decrease of the load in
this state (as shown by curve A), the drain flow rates
in the heaters 3 and 4-are reduced, so that the first N
valve 13 is controlled such that its opening is reduced
(as shown by curve C). With further decrease of the load,
the pressure at the inlet of the first M valve 13
(hereinafter referred to as inlet pressure) becomes
lower than the steam pressure in the drain. As a result,
the phenomenon of flashing is caused in the neighborhood
of the inlet of the valve 13 with the boiling of the
drain under reduced pressure in the duct 9. When this
phenomenon occurs, the drain flows as a two-phase
stream consisting of gas and liquid streams, so that the
volume of the stream is increased. Consequenlty~ although
the first N valve 13 is opened (at an instant tl) in
order to maintain the drain level of the first heater
3 (curve B) at NWL, depending upon the range of decrease
of the load the drain level cannot be held at NWL even
with the first N valve 13 being fully opened. In such
a case, the drain level of the first heater 3 is extra-
ordinarily increased. When the drain level exceedsNHWL, the first X valve 14 provided on the duct 12 begins
to be opened (at an instant t2) as shown by the curve
D and acts to suppress the extraordinary drain level
- 12 -
~a~7ss
1 increase. However, the response of the first X valve 14
is not good because of the delay in the valve control
air signal and of the presence of an insensitive
region in the control valve opening versus valve
operating air pressure characteristic as shown in Fig.
4, and great water level increases are casued a~ the
instants tl and t2 in Fig. 3. ~his phenomenon is
pronounced the more the greater the rate of change of
the load.
When the load (shown as the curve A) reaches
a preset level (X) in this state, the switching of drain
paths is effected. With this prior art methos of the
drain switching, however, great water level increase is
liable depending upon the timing of closing the N
valves 13 and 16. Accordingly, at a drain switching
point t3 the full closure signal is given from the
ad~uster 25 to the second N valve 16 to fully close it,
and then at an instant t4 slightly after the drain switch-
ing point t3 the full closure signal is given from the
adJuster 20 to the first N valve 13 to fully close it.
By this method of drain switching, the increase of the
drain level of the first heater 3 is prevented. At
this time, the rate of drain flow into the first heater
3 is first reduced to reduce the water level thereof.
Besides, although the first X valve 14 is fully closed
by the water level control signal provided with the
reduction of the water level, the first N valve 13 is
still fully open at this moment, so that the water level
- 13 -
.
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.
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p~s9
1 tends to be further reduced. However, after the instantt4, at which the first N valve 13 is fully closed, at an
instant t5 of taking measures for recovering the drain
level the flrst X valve 14 is in the fully closed state,
and also the response of the first X valve 14 is deterio-
rated due to such causes as the delay in the valve
control air signals and the presence of the control
valve action insensitive element. Therefore, great
water level variations occur after the drain switching.
At this time, the greater the reduction of the water
level immediately after the drain switching point t3,
the greater is the water level increase immediately
after the water level recovery point t5, and also the
longer is the period of the subsequent instable state.
In this case, if the proportion gain of the ad~uster
20 and 22 is increased or the integration period is
reduced to improve the control response in order to
prevent the water level increase at the time points tl
and t2, the water level becomes unstable in the low flow
rate region after the drain switching. Further, there
is such a tendency that the larger the difference
between the value NWL before the drain switching and the
value NHWL after the drain switching is the more the
unstable state increases.
Further, while in the heaters 3 and 4 an alarm
is produced when the drain level departs by more than
several 100 mm above or below NW~, the electromagnetic
valves 28 and 29 provided on the extracted steam ducts
- 14 -
~5~'~59
l 5 and 6 are forcibly fully closed when the water level
exceeds the alarm level and reaches a certain level at
the time of the water level increase. This is done so
for the purpose of preventing the damage to the turbine
due to reverse drain flow. ~herefore, when the water
level is extraordinarily increased due to the water level
variation caused by the valve inlet flashing or drain
switching as mentioned above, the electromagnetic valve
28 is closed, and the supply of steam is cut off to
reduce the internal pressure in the first heater.
Therefore, it becomes still further difficult to provide
for the drain flow. Furhter, the drain from the second
heater 4 cannot be led to the first heater 3. ~hus,
the drain levels of the individual heaters are abnormally
increased in a fashion like the chain reaction.
~ he problems discussed above are present in
the prior art water level control.
Now, a preferred embodiment of the invention
will be described in detail with reference to Figs. 5
to ll.
Fig. 5 shows a heater drain level control
system embodying the invention. In the Figure, the same
parts as those in the prior art example described above
are deslgnated by like reference numerals, and their
description is omitted. Designated at 34 and 35 in
Fig. 5 are control valve operation signal generators.
The generator 34 receives the signal 36 representing the
water level of the first heater 3 detected by the
15 -
J?~7~5~
1 detector 18 and compares the detected water level with
NWL and HNWL to generate valve operation signals 37 and
38 for controlling the openings of the valves 13 and 14
as mentioned previously in connection with the prior art
example shown in Fig. 1. The generator 35 receives the
signal 39 representing the water level of the second
heater 4 detected by the detector 23 and compares the
detected water level with NWL and HNWL to gençrate
valve operation signals 40 and 41 for controlling
the openings of the valves 16 and 17 as mentioned
previously in connection with the prior art example shown
in Fig. 1.
According to the embodiment, the following
control is provided in addition to the water level
control valve control in the prior art techniques
described above. The pressure of the extracted steam
flowing through the extracted steam duct 5 into the
first heater 3 is detected as the plant load state by
a plant load detector 24 to generate a detected load
signal 43 which is coupled to the generators 34 and 35.
The detector 42 m~y not necessarily detect the extracted
steam pressure, but it may detect turbine load output,
turbine flrst stage pressure, heater pressure, etc. as well.
Further, a pressure detector 44 for detecting
the pressure in the neighborhood of the inlet to the
first N valve 13 is provided upstream the valve 13,
and a temperature detector 45 for detecting the
temperature of the drain in the neighborhood of the
- 16 -
27~9
1 drain outlet of the first heater 3 is provided downstream
the heater 3. The control valve operation signal
generator 34 is so constructed as to produce a valve
operation signal for making up for the difficiency of
the opening of the first N valve 13 due to the flashing
at the inlet thereof from a pressure signal 46 provided
from the pressure detector 44, a temperature signal 47
provided from the temperature detector 45, a pressure
signal 47 ~ provided from the pressure detector 42, and
opening signals 50 and 51 provided from opening
detectors 48 and 49 which are provided on the respective
valves 13 and 14. The construction, operation and
, effects of the control valve operation signal generator
will be described hereinunder.
Fig. 6 shows the construction of the generator
34. The generator 34 includes a water level control
signal generator 52, a full closure signal generator
53 and a gradual opening signal generator 54. For the
gradual opening signal generator 54, a gradual
20 opening signal start setter 55, an opening setter 56
and a valve operation period setter 57 are provided.
The generator 34 further includes a high reference load
level setter 58, a low reference load level setter 59,
a control valve operation signal selector 60. The signal
25 generator 52 receives the water level signal 36 and
provides a water level control signal 63 for controlling
the water level control valve opening according to the
detected water level. The signal generator 53 provides
- 17 -
2'7S9
1 a full closure signal 64 for forcibly closing the water
level control valve. The setter 55 determines the timing
of generation of the gradual opening signal, and its
output is supplied to the signal generator 54 for
operating it. The signal generator 54 receives the
output signals from the opening setter 56 and valve
operation period setter 57 and also the plant load
signal 43 and generate the gradual opening signal
for permitting smooth drain switching according to the
input signals. The selector 60 receives the water
level control signal 63, full closure signal 64, gradual
opening signal 68, load signal 43, HL signal 69 provided
from the setter 58 and representing a high reference
load level (hereinafter referred to as HL) during the
drain swltching operation period (see Fig. 3)~ LL
signal 70 provided from the setter 59 and representir.g
a low reference load level (hereinafter referred to as
LL) (see Fig. 3) during the drain switching operation
period and valve opening signals 50 and 50, and it
selectively couples either the water level control
signal 63, full closure signal 64 or gradual opening
signal 68 as its output signal 71 to the first N valve
13 and also selectively couples one of the remaining
two signals as its output signal 72 to the first X
valve 14 in accordance with a method to be described
hereinafter. ~he output signals 71 and 72 are coupled
to the first N and X valves 13 and 14 as their operation
signals 37 and 38. In practical, there is additionally
- 18 -
s~
1 provided a device for producing a control valve in-
advance operation signal (shown in Fig. 7) so that in
a first embodiment a sum signal obtained by adding
the control valve in-advance operation signal to
the signal 72 is applied to the second X valve 17
and alterna~ively in a second embodiment a sum si~nal
obtained by adding the control valve in-advance opera-
tion signal to the signal 71 is applied to the
first N valve 13. Description will be made hereunder
about only the latter or second embodiment by way of
example, while eliminating the description about the
former embodiment. Fig. 7 shows the control valve in-
advance operation signal generator 61, which includes
an enthalpy calculator 75, a forecast inlet flash
factor calculator 76 and an opening difficiency
calculator 78. The enthalpy calculator 75 receives the
pressure signal 47' from the pressure detector 42 and the
temperature signal 47 from the temperature detector 45
and calculates the enthalpy of the drain at the inlet
Of the valve 13. The forecast inlet flash factor
forecasting calculator 76 calculates the forecast inlet
flash factor at the inlet of the valve 13. The calculation
methods will be described later in detail with reference
to Fig. 18. The opening difficiency calculator 78
calculates the opening of the first N valve 13 from the
calculated forecast inlet flash factor and derives the
opening difficiency through comparison of the calculated
opening and the actual opening represented by the opening
- 19 -
~R~'7S9
1 signal 50 for providing the derived opening difficiency
as the first N valve in-advance operation signal 73.
The method of the opening difficiency derivation will be
described later with reference to Fig. 17.
Now, an example of the afore-mentioned
generator 34, which is realized with a mini-computor
or a microcomputor, will now be described with reference
to Figs. 5 through 11.
First, the method of drain switching in the
case where the load increases from low to high level
will be described. In this case, while the load is
still lower than LL, in steps 80 to 88 in the flow diagram
of Fig. 8, the step 80 yields a decision "yes" while
the step 81 yields a decision "no". Thus, the start
signal, i.e., a key-on signal, is led through signal
routes 89 and 90 to select the full closure slgnal 64
for the first N valve 13, while it is also led through
slgnal routes 89 and 91 to select the water le~el control
signal 63 for the first X valve 14. In other words,
although the drain switching is effected during the
increase of the load, before the reaching of LL by the
load the water level control signal 63 is given to the
first X valve 14 to permit passage of the drain through
the valve 14, while the full closure signal 68 is given
to the first N valve 13 to hold the valve 13 fully
clo~ed so as to prevent the drain from entering the
deaerator 10. In respect of the flow chart of Fig. 8,
the water level control signal 63, full closure signal
,
13L~;.;~'759
1 64 and gradual opening signal 68 for the system involving
the first X valve 14 are respectively labeled A, B and C,
and their presence is shown by a logic level "1" and
their absence by a level "0". Thus, the instant situa-
tion is expressed as A = 1, B = 0 and C = 0. Likewise,by label~ng the signals 68, 64 and 63 for the system
- involving the first N valve 13 respectively A, B and C,
the instant situation is expressed as A = 0, B = 1 and
C = 0. Further, for the first X valve 14 the state A = 1,
B = 0 and C = 0 is expressed as D = 1, the state A = 0,
B = 1 and C = 0 as D = 3, and the state A = 0, B = 0
and C = 1 as D = 5. Likewise, for the first N valve
13 the state A = 0, B = 0 and C = 1 is expressed as D = 2,
the state A = 0, B = 1 and C = 0 is expressed as D = 4,
and the state A = 1, B = 0 and C = 0 as D = 6. With
the water level control signal 63 selected for the first
X valve 14, a signal representing the selection state
D = 1 is provided as the selection signal 72. Also,
wlth the full closure signal 64 selected for the ~irst
N valve 13, a signal representing the selection state
D = 4 is provided as the selection signal 71. Thus, in
Fig. 9 only steps D = 1 and D = 4 of steps D = 1 to D = 6
yield a "yes" while all the other steps yield a "no".
In other words, the full closure signal 64 from the
signal generator 53 is transmitted through routes 92
and 93 to the first X valve 14, while the water level
control signal 63 is transmitted from the signal generator
52 through a route 94 to the first N valve 13. If the
~z~s9
1 control signal generator 35 is constructed such that
it gives the full closure signal 64 to the second N
valve 16 and the water level control signal 63 to the
second X valve 17, the drain of the second heater 4 at this
time is all led to the deaerator 10 while the drain of
~ the first heater 3 is all led to the low pressure heater
:; 11.
When the load is increased to exceed LL, the
steps 81, 86, 87 and 88 of the steps 80 to 88 in Fig. 8
yield a l'yes" while the steps 82 and 84 yield a
"no", so that i~ the step 83 becomes to yield a
"no" the key-on signal is led through routes 89, 95, 96
and 97 to select the gradual opening signal 68 for the
first N valve 13 while it is also led through routes
15 89, 95, 96, 98 and 91 to select the water level control
signal 63 for the first X valve 14. In other words, at
this time the first X valve operation signal 72 represents
the state D - 1 while the first N valve operation signal
71 represents the state D = 6. ~his means that only the
steps D = 1 and D = 6 among the steps D = 1 to D = 6
in Fig. 9 provide a "yes" while the other steps all
provide a "no". In other words, the water level control
signal 63 from the signal generator 52 is given through
a route 94 to the first X valve 14 while the gradual
opening signal 65 from the signal generator 57 is given
through routes 99 and 100 to the first N valve 13. At
this time, the signal generator 57 produces the gradual
opening signal 65 in response to a start signal coupled
- 22 -
~ , , .
. ~
2759
1 to it through a route 101, and the signal 65 dictates the
control of the first N valve 13 such that it is gradually
opened according to output signals 66 and 67 from signal
generators 55 and 56. On the other hand, since the
- 5 opening of the first N valve 14 is controlled so as to
cause the heater water level to be constant by the
water level control signal 63, the opening is gradually
closed by this control signal 63 with the gradual opening
of the first N valve 13. In the load state higher than
LL but lower than HL, i.e., in the state before the
completion of the drain switching, if the first X valve
14 becomes its full closed state or reaches its lower
limit value as shown in Fig. 10, the step 83 in Fig. 8
yields a "yes", and as a result the key-on signal is
led through routes 89, 95, 96, 102 and 103 to select the
full closure signal 64 for the first X valve 14 while it
is led through routes 102, 103 and 104 to select the
water level control signal for the N valve 13. Thus, the
first X valve operation signal 72 represents the state
D = 3 while the first N valve operation signal 71
represents the state D = 2. It is to be understood that
the water level of the first heater 3 is controlled
either by the first X valve 14 or first N valve 13 alone,
and thus it is possible to prevent the reduction of the
control performance due to the interaction of the two
control valves with each other. Reviewing this state
with reference to Fig. 9, the steps D = 2 and D = 3
are providing a "yes" while the steps D = 1, D = 4,
- 23 -
.
Z'7S9
1 D = 5 and D = 6 are pvoding "no", and a reset signal is
thus given through routes 105, 106 and 107 to the signal
generator 57 to reset the generator 57, thus bringing the
gradual opening signal 65 to a "0" level. Also, it will
be readily understood that the full closure signal 64
is given from the signal generator 53 through routes 108
and 94 to the first X valve 14 and the water level control
signal 63 is given from the signal generator 52 through
routes 109 and 110 to the first N valve 13.
When the load is increased to exceed HL, all
the steps except for the step 87 in Fig. 8 yield a "yes"
~hus, the key-on signal is led through routes 89, 95 and
103 to select the full closure signal 64 for the first
X valve 14 while it is led through routes 89, 95 and 104
for giving the water level control signal 63 to the first
N valve 13. This means that the operation signals 71
and 72 represent the respective states D = 3 and D = 2,
that is, the previous control state mentioned above is
maintained. In the above way, the drain switching in the
case when the load is increasing is completed. It will
be understood that the drain switching for the second
heater by the generator 35 is effected in the same
manner as described above.
Fig. 11 shows the changes in load and
changes in valve openings in the case when the load
decreases from high to low level. In this case, the
drain switching is effected in a manner similar to that
in the case of the increasing load. Further, even in case
- 24 -
7~;g
1 when the valve inlet flashing phenomenon occurs the output
signal 73 from the control valve in-advance operation
signal generator 61 in the signal generator 34 is
additionally applied to the second X valve 17 in advance
through the signal generator 35, so that the opening of
the first N valve 13 is not so increased compared to the
case of the prior art, and it is possible to alleviate
the sudden change of the water legel caused by the
reduction of the drain exhaust capacity of the first
N valve due to the valve inlet flashing phenomenon.
Now, a different preferred embodiment of the
invention will be described with reference to Figs. 12
to 14. In the figures, like component parts as those
already described are designated by like reference
numerals, and their detailed description is omitted.
In the system shown in Fig. 12, a digital
water level control unit 120 of a study control system
ls used for the water level control of the first heater
3. The control unit 120 receives, as signals having
dlrect bearing upon the water level of the first
heater 3, opening signals 50, 51, 121 and 123 respectively
provided from opening detectors 48, 49, 31 and 122
for detecting the openings of water level control valves
13, 14 and 16 and an extracted steam check valve 30, a
water level signal 124 provi~ed as a plant state signal
from a water level detector 18, a signal 43 representing
the plant load state provided from a plant load detector
42, a load change rate signal 125 representing the
- 25 -
t,~Z 7S9
1 transient running state of the plant and a pressure
signal 127 provided from a pressure detector 126 for
detecting the pressure in the deaerator 10. ~hese
input signals are processed in a manner as will be
described hereinafter with reference to Fig. 13 to
generate a water level control signal 63 which is coupled
to the first N valve 13 and first X valve 14 for the
opening control thereof.
Now, the study control system of the digital
water level control unit 120 will be descri~ed with
reference to Fig. 13. The control unit 120 may be
realized as a computor, and it fetches the signals
from the individual detectors as mentioned previously
with reference to Fig. 12 for each predetermined sampling
cycle period ~T. The water level signal 124 is compared
with a preset level, and the difference is sub~ect to
a proportlonal integrating differentiating (PID) proces-
slng to provide a water level control valve feedback
signal 128. When the initial basic data for determining
the control valve operation signals from the fetched
data are not prepared yet, these data are prepared through
a data logger. That is, before the preparation of the
initial basic data is completed, the signal 128 is
provided as the water level control signal 63, and the
water level control depends only upon the feedback
control through the PIC control. However, where the
plant characteristics are forecastable and it is
possible to prepare the initial basic data in advance,
' - 2~ -
., .
; ,
.
s9
1 the initial basic data may be coupled as input data as
well. The data logger is progressively renewed.
When the preparation of the initial basic data
is completed, the fetched data are compared with the
basic data before the calculation processing to determine
a water level control valve forecast control signal 129.
Concurrently with this signal processing, whether the
water level is within a predetermined control range is
checked. If it is not within the control range, it
means that the water level control valve control signal
determined on the basis of the data fetched at an instant
(T - ~T) one sampling cycle before the instant time (T)
has been unsatisfactory. In this case, the correction of
the basic data is made by using values fetched at the
present instant (time T) as the opening slgnals 50 and
51 while using the values at the instant (T - ~T)
one cycle before as the other fetched data. If the water
level is within the control range, no correction of the
basic data is made.
In the above way, when the preparation of the
initial basic data is completed, the water level control
valve forecast operation control signal 129 obtained
through the calculation with the fetched data as mentioned
; serves as the basic valve control signal, and to this
signal the afore mentioned water level control valve
i feedback control signal 128 is added to produce the
water level control signal 63 for controlling the valves
13 and 14.
- 27 -
,~,
., .
759
1 Now, an example of the method o~ preparing the
basic data ~ill be briefly explained referring to Fig.
14. ~he fetched data are put in order with respect to
the load as shown in Figs. 14A, 14B and 14C, and the
basic charactersitics of the individual fetched data
corresponding to the load are grasped as respective
patterns. On the basis of these patterns the openings
of the water level control valves 13, 14 and 16 are
determined according to the load while also making it
possible to make correction of the water level control
valve openings by using deviations of the individual
fetched data from the basic characteristics as shown in
Fig. 14D. As is seen from Fig.14D, this correction of
the opening of the first N valve 13 is made by fore-
casting the inlet flashing phenomenon that occurs whenthe laod change rate takes a negative value as is apparent
from Fig. 3.
As has been shown, by carrying out the feed
water heater water level control with the study control
system as according to the embodiment, it is possible
to prevent abnormal variations of the drain level of the
feed water heater at the time of the afore-mentioned
drain switching and sudden load change.
A further preferred embodiment of the invention
will now be described with reference to Fig. 15 and
following figures. In the figures, like component parts
as those already described are designated by like reference
numerals, and their detailed description is omitted.
- 28 -
,.
'7S~
1 In this embodiment, a feed forward water level adjuster
129 is provided. The water level adjuster 129 receives
opening signals 50 and 51, a water level signal 36, a
plant load signal 43, a drain temperature signal 132
provided from a drain temperature detector 130 connected
to the drain duct 9, a drain pressure signal 133 provided
from a drain pressure detector 131 also connected to the
drain duct 9, an inlet drain pressure signal 46 provided
from the drain pressure detector 44 connected to the drain
duct 9 and a flow rate signal 135 provided from a flow
rate detector 134 for detecting the flow rate of drain
through the drain duct 9, and it provides valve operation
signals 37 and 38 to the water level control valves 13
and 14 respectively.
Fig. 16 shows the construction of the
feecback water level adjuster 129. It includes an
opening dlfficiency compensation mechanism, an in-
advance drain switching valve gradual opening mechanism
and a drain switching mechanism.
The opening difficiency compensation mechanism
includes an inlet flash factor forecaster 136, which
forecasts the flash factor at the inlet of the first N
valve 13 by fetching the drain temperature signal 132,
a drain pressure signal 133, an inlet drain pressure signal
46 and a drain flow rate signal 135, and an opening
difficiency in-advance compensator 138, which calculates
the opening difficiency corresponding to the inlet
flashing from the output signal 137 of the forecaster
- 29 -
~tljz~7s9
1 136 and the valve opening signal 50 and providing an
output signal 139 representing the result of the calcula-
tion. Fig. 17 shows the detailed construction of the
forecaster 136 and compensator 138.
The iniet flash factor forecaster 136 includes
an enthalpy claculator 140, which calculates the
enthalpy of the drain at the outlet of the first heater
3 from the drain temperature signal 132 and drain
pressure signal 133. The forecaster 136 also includes
a drain flow delay calculator 141, which receives the
drain flow rate signal 135 and calculates the delay of
the drain from the outlet of the heater 3 before reaching
the inlet of the first N valve 13 from the drain flow
rate, the length and diameter of the duct 9, etc. The
outputs of the enthalpy calculator 140 and drain flow
delay calculator 141 are coupled to an inlet enthalpy
calculator 142, whlch calculates the enthalpy of the drain
at the inlet of the first N valve 13. The output of the
inlet enthalpy calculator 142 is coupled together with
the output of the inlet drain pressure signal 46 to an
inlet flash factor calculator 143. The inlet flash
factor calculator 143 calculates the dryness and volume
factor which are physical properties of the drain and
also calculates by using these parameters the inlet flash
factor and bu~ble ratio (i.e., void factor Vr), and
the calculated data are provided as an output signal 137
of the forecaster 136.
The construction of the inlet flash factor
- 3 -
.' , .
' ' ,
~3~ ~59
1 calculator 143 will now be described with reference to
the flow chart of Fig. 18.
The calculator 143 fetches the inlet drain
enthalpy iH calculated in the calculator 142 and derives
the saturation pressure PlV corresponding to the enthalpy
iH. The saturation pressure PlV is compared with the
inlet drain pressure Pl represented by the input signal
46, and if P1 > PlV it means the flash state, so that the
following calculation is made. If P1 < PlV, it means
the single phase stream, so that no correction of the
valve opening is made. If P1 > PlV,
decided that the inlet flash state is in force, the
enthalpy ilg and volume ratio Vlg of the saturated steam
with respect to the inlet pressure Pl and the enthalpy
ilQ and volume ratio VlQ of the saturated water are
derived from approximate functions stored. The void
factor Vr is calculated form these values and the
afore-mentioned inlet enthalpy iH by using an equation
Vr V r V (l~X~ ~~~~~~~~~~ (1)
where X is the flash factor given as
~'
X = H iQ ~~~~~~~~~~~~~~~~ (2)
, lg lQ
,j 20 When the pressure of the saturated water of the
' drain at the inlet of the first N valve 13 becomes lower
$:
than the saturated steam pressure, the flashing occurs
- 31 -
~;
.', ~ -
.. . .
.~.
7S~
1 to reduce the drain exhausting capacity of the first N
valve 13. The reduction is pronounced the more the
higher the flash factor. In the neighborhood of the
inlet of the N valve 13, the flash factor is about 80%
even when the dryness of the drain saturated water is
about 0.5%, and the corresponding control valve Cv
(velocity coefficient) value correction coefficient
which has close bearing upon the flash factor is large.
This means that under the ~lashing phenomenon the drain
in the case when there is no flashing cannot be e~hausted
with the same valve opening.
Accordingly, the compensator 138 includes
calculators 144, 145 and 146 and a subtractor 147 and
has the following function. The calculator 144 receives
the output signal 137 of the inlet flash factor forecaster
136 and calculates the opening Cv value correction
coefficient which is proportlonal to the inlet flash
factor Vr. The calculator 145 receives the first N
valve opening signal 50 and calculates the present Cv
value of the first N valve 13 from the input signal.
The outputs of the calculators 144 and 145 are coupled
to the calculator 146, which calculates the necessary
opening of the first N valve 13 compensated for the
1, reductlon of the valve capacity due to the inlet
flashing. The output signal of the calculator 146 is
coupled to the subtractor 147, which also receives the
opening signal 50 and subtracts the present opening
from the necessary opening to derive the opening
- 32 -
/
!
., .
~ 7S9
l difficiency and provide a signal 139 representing the
difficiency as the output of the compensator 138. The
output signal 139 is coupled to an adder 149 shown in
Fig. 16, which provides the operation signal 37 to the
5 first N valve 13.
The in-advance drain switching valve gradual
opening mechanism, which serves to cause gradual
opening of the first X valve 14 in advance to make up
for the transient capacity difficiency of the first N
lO valve 13, includes an in-advance drain switching valve
gradual opening operation signal generator 150 which
receives the first N valve opening signal 50 and
forecast inlet flash factor signal 137. Fig. 19 shows
the detailed construction of the signal generator 150.
15 The inlet flash factor signal 137 from the inlet flash
factor forecaster 136 and a reference valve set in a
drain flash factor setter 125 are coupled to a subtractor
151, and when the signal 137 exceeds the reference
value, i.e., when the forecast flash factor exceeds the
20 preset reference flash factor, the difference signal
is provided from the subtractor 151. Also, the first
N valve opening signal 50 and a reference value set in
an opening setter 154 are coupled to a subtractor 153,
and when the signal 50 exceeds the reference value,
s 25 i.e., when the opening of the first N valve 13 exceeds
the preset opening, the difference signal is provided
, from the subtractor 153. The difference signals from
, the subtractors 151 and 153 are coupled to an in-advance
- 33 -
..
,, ~
.,
.
:
,,
7S9
1 gradual opening start signal generator 55, which generates
a start signal according to the input difference signals.
The start signal is coupled together with gradual opening
period and gradual opening upper limit data preset in
respective setters 156 and 157 to a gradual opening
signal program setter 158, and the output signal of the
setter 158 is coupled to a non-linear signal converter
159, which converts the input signal into a non-
s linear signal. Thus, when the opening of the first N
', 10 valve 13 exceeds a predetermined value or when the iniet
flash factor exceeds a preset flash factor, the signal
generator 150 generates a gradual opening signal 160
based upon the program in accordance with the opening
, signal 50 and forecast inlet flash factor signal 137.
s 15 This gradual opening signal 160 is coupled to an adder
161, whlch provides the operation signal 38 for the first
X valve 14, whereby the first X valve 14 is gradually
~, opened in advance so as to make up for the difficiency
of the opening of the first N valve 13 and prevent the
20 loss of control due to otherwise liable full opening of
the first N valve 13.
The drain switching mechanism in the feed
forward water level adjuster 129 has the following
construction. The signal provided from the load detector
25 42 provided on the extracted steam duct 5 is converted
'~ by a load converter 163 into a turbine load signal. The
output signal of the load converter 163 is coupled to a .
load change rate calculator 164, which converts the rate
- 34 -
:
,
. .
., ~
1 of change of the turbine load. A load conversion signal
165 provided from the converter 163 and a load change rate
signal 166 provided from the calculator 164 are coupled
together with the opening signals 50 and 51 to a drain
switching signal selector 167. The selector 167 compares
the preset value of load with HL and LL according to the
input signals mentioned above for switching the paths,
, through which the full closure signal, gradual opening
s signl and water level control signal for controlling
10 the water level according to the drain level are
transmitted to the first N valve 13 and first X valve 14.
s Fig. 20 shows the construction of the signal
~ selector 167 as a flow chart. In the drain switching
i~ signal selector 167, the selection of switching as to
15 which one of the valves 13 and 14 the water level control
signal 63 is to be given to is effected by providing
a signal D which has either content "1" or "2" depending
upon the load conversion signal 165 and opening signals
, 50 and 51 coupled to a signal receiving section 168.
20 The signal D has the same meaning as described earlier
in connection with Fig. 8. That is, like the case of
Fig. 8, if D = 1 the coupling of the water level control
s signal to the first X valve 14 is selected, and if D = 2
'si the coupling of the water level control signal to the
25 first N valve 13 is selected. The selection of the
coupling path of the gradual opening signal 68 is
effected by providing a signal D which has either content
, "5" or "6" depending upon the load conversion signal 165
s
~ ~ 35 -
:.
,,.
~ Z ~ 5 9
and opening signals 50 and 51 coupled to a signal receiv-
ing section 169. If D = 5 the coupling of the gradual
opening signal to the first X valve 14 is selected,
and if D = 6 the coupling of the gradual opening signal
to the first N valve 13 is selected. The selection
of the coupling path of the full closure signal 64 is
effected by providing a signal D which has either content
"3" or "4" depending upon the load conversion signal
165 and oepning signals 50 and 51 coupled to a signal
receiving section 170. If D = 3 the route to the valve
14 is selected, and if D = 4 the route to the valve 13
is selected. Thus, in the normal turbine load state in
excess of the preset value HL, the output signal D of
the drain switching signal selector 167 has the content
(D = 3) of dictating the coupling of the full closure
signal to the first X valve 14 and also the content
(D = 2) of dictating the coupling of the water level
control signal for the feedback control of the water
level according to the detected drain level to the first N
s, 20 valve 13. In the case where the turbine load is lower than
the preset level LL, the output signal D has contents
(D - 4 and D = 1) of dictating the coupling of the full
closure signal to the first N valve 13 and the water
level control slgnal to the first X valve 14. In the case
where the turbine load is between the preset levels HL and
HL, the output signal D has the contetnt (D = 3 or D = 4)
of dictating the continual coupling of full closure
signal to the water control valve in the fully closed
, ~
- 36 -
i ,, :
.;: . .
....
~.~t'.~ S9
1 state to maintain this state and the content (D = 2 or
D = 1) of dictating the coupling of coupling the water
control signal to the water control valve which is
not fully closed. Further, when the rate of change of
5 the load is positive, the signal D has the content (D =
6) of dictating the coupling of the gradual opening signal
to the valve 13 after the confirmation of the fact that
the valve 14 is not fully closed, and in the case of a
negative load change rate the signal D has the content
10 (D = 5) of dictating the coupling of the gradual opening
signal to the valve 14 after the confirmation of the fact
that the valve 13 is not fully closed. These output
signals D of the drain switching signal selector 167
are transmitted to a drain switch circuit 171 (shown
15 ln Fig. 16).
Referring again to Fig. 16, the signals 165
and 166 from the converter 163 and calculator 164 are
also coupled to a gradual opening signal start selector
172. When the load conversion signal 165 and load
change rate signal 166 are coupled to the selector 172,
whether the rate of change of the load is positive or
negative is first checked, and then whether the load is
~ increasing from low to high level or decreasin~ from
s high to low level is checked. If the rate of change of
25 the load is negative, a gradual opening signal starter 155
!, iS rendered operative upon detection of the reaching of
the preset level HL by the load. If the rate of change
of the load is positive, the starter 155 is rendered
- 37 -
;~
~:~ C,~;~'7S9
1 operative upon detection of the reaching of the preset
level LL by the load.
When the starter 155 is rendered operative upon
reaching of the preset level ~L or LL depending upon
5 whether the rate of change of the load is positive or
negative, it provides an output signal to a gradual
opening signal generator 173. As a result, the generator
173 generates the gradual opening signal 68 which is
coupled to the drain swtich circuit 17I mentioned above.
To the drain switch circuit 171 are coupled
the gradual opening signal 68 from the signal generator
173, the signal D from the selector 167, the full closure
signal 64 from the signal generator 174, and the water
level control signal 63 provided from the feedback
15 water level controller 175 according to the water
~; level signal 36. The function of the drain switch circuit
171 will now be described in detail with reference to
Fig. 22. It will be readily seen from the figure that the
water level control signal 63, full closure signal 64
and gradual opening signal 68 are selectively provided as
signal 177 to be coupled to the valve 13 and also signal
177 to be coupled to the valve 14 in accordance with the
content of the signal D. The content of the signal D and
the selection of the valves are as has been described
previously in connection with Fig. 19.
The operation signal 176 for operating the first
N valve 13, provided from the drain switch circuit 171,
is added in an adder 149 to the output signal 139 from
, - 38 -
. . .
. .
-
. ,.
5~
1 the opening difficiency compensation mechanism (i.e.,
the signal representing the difficiency of the opening
of the first N valve 13) to provide the first N valve
operation signal 37. On the other hand, the operation
signal 177 for the first X valve 14 is added in an adder
' 161 to the output signal 160 from the in-advance drain
I switching valve gradual opening mechanism (i.e., the
gradual opening signal for making up for the difficiency
of the capacity of the N valve 13) to provide the first
X valve operation signal 38. ~he signals 37 and 38 are
given as the output of the feed forward water level
ad~uster 129 to the first N valve 13 and first X
valve 14 respectively.
As has been shown, with this embodiment the
gradual opening signal generator 173 is rendered operative
by the load conversion signal 165 and load change rate
~ignal 166, and even during the drain switching state
i the water level control signal 63 based upon the drain
water level is always transmitted to either the first
N valve 13 or the first X valve 14 for the opening
,' control while the forced closure signal 64 or gradual
opening signal 68 is always transmitted to the other
~; water level control valve for operating it in accordance
$ with the signals 165 and 166 and also the opening signals
, 25 50 and 51 representing the openings of the valves 13
and 14. Further, before the drain led out from the
outlet of the first heater 3 with a constant flash factor
reaches the neighborhood of the inlet of the first N valve
,,.
~ - 39 -
.
'$
7S~
1 13 with a flow delay, a sum control instruction is
transmitted to the control valves, to which the afore-
mentioned instructions have been given, in correspon-
, dence to the detected flash factor in order that the
5 difficiency of the exhausting capacity will not result.
More particularly, the flashing phenomenon occurring
due to the decrease o~ the load is preliminarily detected
f at the outlet of the first heater 3, and the first N
valve 13 is opened in advance with respect to the delay
10 in flow rate the difficiency of capacity should not
result in correspondence to the flash factor, while the
, first X valve 14 is gradually opened in advance according
to a program in the case where a difflciency of the capacity
is going to result with the sole first N valve 13 being
15 open. The situation that the dlfflciency of capacity
is going to result with the sole first N valve 13
being open is grasped as a case when the flash factor
exceeds a predetermined level or as a case when the
opening of the first N valve 13 exceeds a predetermined
20 value. Further, through the control of the first N
valve 13 and first X valve 14 in advance in the manner
as has been shown, it is possible to absorb the deteriora-
tion of the response due to the delay in the control air
signals for the indivldual valves and ellminate the
25 likelihood of the delay of the control.
Fig. 23 shows experimental data obtained when
the drain water level control is made by using the
system and method as in the above embodiment. According
l. ~ "
J - ~0 _
'k
,
.,
;:
.,~,
S9
1 to the embodiment, no great water level variation is
caused even in case of occurrence of the inlet flashing,
and also water level variations accompanying the drain
~ switching can be avoided.
As has been described in the foregoing, accord-
ing to the invention it is possible to obtain satis-
factory water level control even in the case of the drain
switchlng or in the case of a sudden load change and avoid
abnormal varlat~ons of the drain level.
~,
i
.
,
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i .
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... .
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