Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
-1- Case No. 4401
CONTROL SYSTEM FOR VARIABLE PRESSURE
ONCE-T~IROUG~ BOILERS
TECHNICAL FIELD
The present inven~ion relates generally to the operation
of the boilers~ and more particularly to a control system for
operating variable pressure once-through unit~-
BACKGROUND ART
In a variable pressure boiler system, the throttle pressure
varies with the load. In its ideal form, the throttle valves on
the turbine are left wide open and the throttle pressure varies
directly with the load. Such variable pressure operation is
desirable since it can increase the efficiency of -the turbine. How-
ever, the primary incentive fsr variable pressure operation is that
it can increase the number of times that the turbine can be loaded
and unloaded. This is because with variable pressure operation, the
change in the first stage steam exit temperature in the turbine is
relatively ~inor, thus minimizing thermal stress in the metal com~
prising -the turbine. In contrast, in a collstant pressure type of
opera-tion, the first stage steam exit temperature is load dependent.
This can result in a greater change in temperature for the turbine
which, in turn, can cause excessive metal fatigue.
Because of the foregoing, it has become desirable to develop
a control system for the operaticn of a once-through boiler so that
?5 a variable throttle pressure type of operation can be utilized over
a very wide load range.
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-2- Case NoO 4401
SUMMARY OF THE INVEN ION
The present invention solves the aforementioned problems
assoc;ated with the prior art as well as other problems by provid-
ing a control system so that variable throttle pressure operation
can be lntroduoed at as low a load as possible and can be utilized
for mos~ of the load operating range. This is accomplished by
opening the turbine valve to approximately 70 percent of its full
open position as soon as possible as the system is being loaded,`
utillzing a flash tank while this i5 occurring until the load demand
exceeds the minimum feedwater flow requirements, and then allowirg
the system to assume the variable ~hrot~le pressure mode o~ operation
as load is increased until throttle pressure approximates designed
operating pressure, at which ~ime the ~urbine valve is regulated to
meet load requiremen~s. In essence, the system provides for variable
pressure operation from approximately 20 percent to 75 percent of
load and also provides ~or smooth transistion ~rom lo~ load operation
tc the variable pressure mode of operation, and from the variable
pressure mode of operation to the full pressure mode of operation. In
addition, w~;le in the variable pressure mode of operation, a control
coordinator is provided to monitor and correct steam flow, firing rate
and feedwater flow. In this manner, the system can automatically adjust
and compensate for deviations in these parameters from that which is
desired~
In view of the foregoing, it will be seen that one aspect of
the present invention is to provide a control system which permits
variable thr~ttle pressure operation of a once thrnugh boiler.
Another aspect of the present inven~ion is to provide a control
system in which a onoe-through boiler can be operated in a variable
pressure mode of operation over a wide load range.
Still ano~her aspect of the present invention is to provide
a control system for a once-through boiler in which there is a smooth
transistion from ~he low load type of operation to the variable
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--3--
pressure mode of operation, and from -the varlable pressure
mode of operation -to -the full pressure mode of operation.
In accordance with one aspect of -the presen-t inven-tion
there is provided a con-trol system for a boiler supplyincJ
steam to a load under various load conditions, compris:iny
means for genera-ting steam; means for connectinq said steam
genera-ting means -to -the load, said connecting means inc]uding
a first valve controlling the flow of steam from said steam
generating means to the load under all load conditions; a
flash tank selectively interconnected between said steam
genera-ting means and said first valve; a second valve
operable to connect said flash tank to said steam generating
means and said first valve during low load phase operation of
the system; a third valve operable to connect said steam
generating means to said first valve bypassing said second
valve during once-through variable pressure phase and full
pressure phase operation of the system; and a fourth valve
connected fluidically in pa.rallel with said third valve and
operable to connect said steam generating means to said first
valve during the full pressure phase of operation of the system.
In accordance with a further aspect of the present
invention there i.s provided a method for controlling a once-
through boiler assembly under low load operation, once-through
variable pressure operation, and full pressure operation,
comprising directing the flow of steam from a means for gener-
ating steam to a turbine via a flash -tank only during the low
load phase of operation of the boiler assembly; allowing the
output pressure of said steam generating means to vary in
response to the load requirements during the once-through
variable pressure phase of operation of the boiler assembly;
and directing the flow of steam from said steam qenerating
means -to a turbine and bypassing said flash tank during the
once-through variable pressure phase and the full pressure
phase of operation of the boiler assembly.
The above and other aspects of the present invention
will become more clearly unders-tood after a review of the
-3a-
following descrip-tion of the preferred embodimen-t when
considered with the following drawings.
BRIEF DESCRIPTlON OF THE D~WlN~,S
Figure 1 is a schematic diagram of -the systern which
u-tilizes -the invention of -this dlsclosure.
Figure 2 is a graph of percen-t pressure or valve
opening verses percent load and illus-tra-tes flash tank
pressure, furnace pressure, turbine valve position and
throttle pressure.
Figure 3 is a schema-tic diagram which illustra-tes the
overall sys-tem utilized by the invention of this disclosure.
DESCRIPTION OF THE PREFERRED E~BODIMENT
. _
Referring now to -the drawings where the illustrations
are for the purpose of describing the presently known preferred
embodiment of the present invention and are not intended to
limit the invention hereto, Eigure 1 is a schematic drawing of
the system 10 used by the apparatus of the present invention.
System 10 is comprised primarily of a furnace 12 whose output
is connected to the input to a primary superheater 14, a flash
tank 16, a secondary superheater 18 whose output is connec-ted
to the input to a turbine 20 via a turbine valve 22, a generator
23, and a condenser 24. The condenser 24 is connected to the
input to the furnace 12, via a low pressure heater 26, a
deaerator 28, a boiler feed pump 30, and a high pressure
heater 32.
The primary superheater 14 is connected to the input
to the flash tank 16 via a valve 34 and the flash tank 16 is
connected to the secondary superheater 18 via a valve 36. A
pair of valves 38 and 40 are connected in parallel across the
input to valve 34 and
~ 3~
-4- C~ S' No. ~10
the output of valve 36. A valve 42 is provided between the flash
tank 16 and the condenser 24 and controls the flow of water from
the flash tank 16 to the condenser. A superheate~ steam attemperator
valve 44 is provided between the output of the secondary superheater
18 and the flash tank 16.
As for the principle of operation for this system, refer to
Figune 2. In Figure 2, percent pressure or valve opening is shown
verses percent load, and illustrates flash tank pressure, furnace
pressure, turbine valve position and throttle pressure. The objec-
tive is t~ obtain variable throttJe pre~sure at as low a load as
possible, provide a smooth transistion from low load operation to
once-through operation~ and incorporate the capabilities of a con-
trol coordinator 50~ shown on Figure 3, during the variable throttle
pressure phase of operation. This is accomplished by opening the
turbine valve 22 as soon as possible, by operating the flash tank
16 until i~ is dry and by using valves 38 and 40 between the primary
superheater 14 and the secondary supenheater 18 as throttle valves,
- as will be hereinafter described. The unique feature of this control
strategy is that throttle pressure is not directly controlled, except
at minimum pressure, but is permitted to float to whatever level is
required for the desired load. Thus~ variable pressure operation is
achieved over a very substantial portion of the load range.
In order to accomplish the foregoing, a syst~m organization
schematic is illustrated in Figure 3. In this Figure, an lncoming
control signal is applied to the unit load demand development function
52, the output of which is directed to the control coordinator 50 and
to a turbine valve program 54, a steam flow modifier 56~ a firing rate
modifier 58, a feedwater modifier 60, and controls for valve 36. The
turbine valve program 54 controls the operation of a t~rbine valve 22,
the steam flow ~odifier 56 controls ~he operation of valves 38 and 40,
the firing rate modifier 50 controls the fuel and air mixture in the
system, and the feedwater modiFier 60 regulates the flow of feedwater
-5- Case ~o. 4401
throuyhollt the system. A pressure transmitter 62 is connected to
both control coordinator 50 and to the control valve 349 and an
electrical transmitter 64, a feedwater temperature transrnitter
66 and a superheater temperature transmitter 68 are also con-
nected as inputs to the control coordinator 50 which~ in turn,
regulates the steam flow modifier 56, the firing rate modifier
58 and the feedwater modifier 60 by means of control signals
gerlerated therein.
With respee~ to this control system~ there are basically
three modes of operation - low load operation, once-through var-
iable pressure operation, and full pressure operation. Low load
operation occurs when the boiler feedwater flow ls limited to its
minimum Flow rate. Once-through varlable pressure operation occurs
when the feedwater flow rate exceeds its minimum flow ra~e and con-
tinues until throttle pressure reaches full design pressure, i.e.,
furnace pressure. Full pressure operation occurs when the throttle
pressure has reached full design pressure and continues until full
load is achieved.
During the low load phase of operation, i.e., between O a~d
approximately 25 percent luad, the throttle pressure is maintained
constant and the turbine val~e 22 is rapidly opened to approximately
70 percent of its full open position~ as shown in F~gure ~. In
this mode of operation, valves 38 and 40 are closed and valves 34
Z5 and 36, along with the turbine valve 22, are openedO Valve 34 con-
trols furnace pressure, whereas valve 36 controls throttle pressure.
Valve 42 is also opened and regulates the water level in the flash
tank 16. During this phase of operation, all flow from the furnace
12 is directed to the flash tank 16 and starts as water, and as
firing is increased, becomes steam. The flash tank 16 acts as a
steam and water separator and directs the water to the condenser 24
and the steam to the turbine 20. By the time steam flow to the
turbine 20 equals the minimum feedwater flow rate of approximately
25 percent, the flash tank 16 has dried up. At that time9 valve 40
6- Case No. 4401
opens and valves 34 and 36 start closing, stopping the f10w to
the flash tank l6. This occurs at approximately 25 percent oF
load and starts the next phase of operation~ i.e., the variable
throttle pressure phase or once through variable pressure phase
of operation.
In the once-through variable pressure phase or variable
throttle pressure phase of operation, the turbine valve 22 is
maintained at approximately 70 percent of its full open position
by the turbine valve program 54. During this phase of operation,
steam flow control is regulated by valve 40 and this valve, in
essence, acts as a remote throttle valveO In this phase of
operation, ~he feedwater flow is given the responsibility of con-
trolling steam temperature, whereas the firing rate controls the
load, and throttle pressure is permitted to float to whatever value
is necessary to satisfy the load requirements. The control coordi-
nator 50 assumes an important function in this phase oF operation
since it produces error or correction signals to the steam flow
modifier 56, the firing rate modifer 58 and the feedwater modifier
60. These error or correction signals are as follows: A megawatt
error minus a furnace pressure error control signal which is
directed to the steam flow modifier 56, a megawatt error plus a
~urnace pressure error control signal which is directed to the
Firing rate modifier 58, and a superheat temperature error plus a
feedwater temperature control signal which is directed to the feed-
water modifier 60. In this manner, the feedwater flow can be
adjusted to maintain steam temperature while the steam flow and the
firing rate can be corrected to maintain proper furr1ace pressure
and megawatts.
When valve 40 approaches its full open position, valve 38
starts opening and turbine valve 22 is permitted to start opening
further from its 70 percent open position. This commences the next
phase of operation, i.e., the full pressure phase of operation.
In the Full pressure phase of operation, throttle pressure
-7- Case No. ~401
reaches Full design pressure and -the turbine valve 22 is al'lowed to
open still further to m~int,ain the load as is necessary. In this
mode of operation, the steam flow is contro'lled b,y ~he turbine
valve 22 rather Lhan by valve 40, and the comb;ned capacity of
valves 38 and 40 is sufficient to provide the steam flow required
by turbine valve 22 as it further opens from its 70 percent open
position. It should be noted that with respect to non-variable
pressure once-through boiler systems, as in the prior art, this is
the nornlal mode of operation once above the m;nimum feedwater flow
rate and thus, a variable pressure phase is never introduced
therein.
The foregoing control system produces a number of benefits~
For example, by using variable pressure the first stage steam tem-
perature can be closely controlled which permits the rapid loadingof the turbine without creatin`g excessive thermal stress~ In addi-
tion, the foregoing system provides for quickly achieving variable
pressure operation, turbine metal temperature matching, and the
smooth transistion to once-through operation. And lastly, the
control~oordinator regulates and controls the overall operation
of the system in the variable pressure phase of operation.and
adjusts the system components to compensate for various operational
deviati'ons .
Certain modifications and improvements will occur to those
ski11ed in the art upon reading the foregoing. It should be under-
stood that all such modifications and improvements have been
deleted herein for the sake of conciseness and readability but are
properly within the scope of the following claims.
