Transcript Document

CONTROLS OF SUPER CRITICAL BOILERS
Presentation By
AJAY SHUKLA
Sr.Faculty PMI,
Controls of Super Critical
3 rd Feb 2009
Boiler Turbine Control
Controls of Super Critical
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Boiler Following Mode
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Turbine Following Mode
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Coordinate Mode Control
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Coordinate Mode Control
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Drum and OT Control Comparison
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Drum and OT Control Comparison
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OT Control Overview
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Cycle of Supercritical Power Plant
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Controls of Super Critical
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Supercritical Power Plant
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OT Control Overview
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OT Start up Control mode
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Super Critical Units Controls Mode
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Flushing Mode
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Flushing Mode
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Start-Up System with Recirculation
Separator
SH
WW
ECO
C
Flash
Tank
Start-Up System
Recirculation Pump in Main Bypass Line
HWL
C
To Condenser
Deaerator
C
HPH
BFP
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Start UP System
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START-UP
If the water system of the boiler is empty (economizer, furnace walls,
separators), then the system is filled with approximately 10% TMCR
feed water flow.
 When the level in the separator reaches set-point, the WR valve will
begin to open.
When the WR valve reaches >30% open for approximately one minute,
then increase feed water flow set-point to 30% TMCR. As the flow
increases, WR valve will reach full open and ZR valve will begin to
open.
The water system is considered full when:
The separator water level remains stable for two(2) minutes
and
The WR valve is fully opened and ZR valve is >15% open for two(2)
minutes.
Controls of Super Critical
3 rd Feb 2009
Controls of Super Critical
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SEPARATOR STORAGE TANK LEVEL
CONTROL
• Separator level is maintained by the combined action of a separator storage
tank level feed water demand and the positioning of WR and ZR drain
valves.
• Feed water demand is developed in response to separator storage tank level
error and total fuel flow so as to prevent tank level from dropping too low.
• The WR and ZR valves are controlled in a split range manner to maintain
the liquid level once the level reaches a high limit.
• The WR valve will respond first and then the ZR when the WR exceeds its
linear operating range.
• Tank geometry is such that fluctuations in tank level are very dynamic, for
this reason, only proportional control action established through the WR/ZR
function curves is used to position these valves in response to level error.
Controls of Super Critical
3 rd Feb 2009
UG VALVE CONTROL
Control objective:
Maintain minimum economizer inlet flow.
Control action:
•
The boiler circulating pump is started following the start of a
turbine-driven feed water pump and the final clean-up cycle. This
pump circulates feed water from the evaporator outlet back to the
economizer inlet.
•
Located at the outlet of this pump is the UG valve which
controls economizer inlet flow during the start-up phase of operation.
Demand for this recirculation control valve is established based on
measured economizer inlet flow compared to a minimum boiler
flow set point.
Controls of Super Critical
3 rd Feb 2009
Separator water circuit of Super Critical
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FEEDWATER CONTROL LOOP
•
•
•
•
•
.
Control objective:
Develop total unit feed water demand as required to support unit
load.
Adjust feed water demand to maintain desired separator outlet
temperature.
Adjust separator outlet temperature set point as required to maintain
acceptable platen superheat spray control range.
Incorporate separator storage tank level (wet mode) feed water
demand.
Maintain minimum required economizer inlet flow.
Controls of Super Critical
3 rd Feb 2009
Feedwater Control
Natural circulation boiler
•
m
Once-through boiler
Steam flow
T
Enthalpy
T
P
T
Level
Feedwater
control
Eco
Eva
Eva
Feedwater
control
Eco
Feedwater
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Feedwater
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Feed water firing rate ratio control
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Feed water firing rate ratio control
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Feed Forward with model
Process
Setpoint
f(x)
Feed
Forward
MODEL
S
P
+/-
C
-/+
D
d
K dt

C
+/-
P
+/-
X
Controls of Super Critical
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Feed Forward
FF
PROCESS
process
setpoint
f(x)
P
+/-
C
-/+
D
d
K dt 
C
+/-
Controls of Super Critical
f(x)
P
+/-
C
-/+
D
d
K dt 
P
+/-
S
FF
C
+/-
P
+/-
X
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Model predictive control
+
MW
_
Fuel with model
Without model
Turbine setpoint
Water wall Temperature
C
0
60
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120
180
240
300
t[sec]
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Firing Rate Master
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Fuel Rate Master
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Feed Forward Firing Rate Control
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Feed Forward Firing Rate Control
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Yuhuan 4x1000MW Preparation of light off
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Multi-combusting Nozzles with
Separated Overfire Air Damper
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START-UP contd..
Water flows through the economizer and evaporator, and discharges the boiler through
the WR valve to the flash tank and via connecting pipe to the condenser.
 Start BCP and open the UG valve to establish minimum water wall flow
at 30% TMCR.
 As the pressure is raised, first the WR and then the ZR valves will open when
•separator water level increases due to boiler water swell. As pressure further
•increases, the WR and ZR valves will start to close as the water level decreases.

•The steam temperature at the separator inlet will reach a stable superheated
•condition at app. 30% TMCR, causing the level in the separator to decrease and
•eventually disappear. The boiler is now in once-through mode (dry mode). The
•BCP (Boiler Circulating Pump) will be stopped automatically.
•It is extremely important that minimum water wall flow be maintained at all times when
firing the boiler to prevent tube damage due to overheating.
Controls of Super Critical
3 rd Feb 2009
FEEDWATER CONTROL LOOP contd..
 Demand for feed water is established predominately by the Boiler Master demand.
 This signal, processed though a “boiler transfer function” provides the feed
forward component of the total feed water demand.
 The “boiler transfer function” is a tunable dynamic element providing a means to
dynamically match the feed water feed forward demand to actual evaporator heat
transfer.
 Optimization of the feed forward in this manner minimizes temperature
fluctuations that may otherwise result from varying dynamic response between the
firing and feed water control systems (as they relate to evaporator heat transfer)
thereby lessening the dependence on feedback correction.
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FEEDWATER CONTROL LOOP contd..
 The first controller acts on a load dependent average platen spray differential
temperature.
 Its output represents the required adjustment to evaporator heat transfer/steam
generation to maintain both the steam conditions and flue gas temperatures
entering the platen superheat section so as to ensure adequate platen spray control
range.
 A second controller acts on a load dependent separator outlet temperature set point
corrected by the platen spray differential temperature controllers output.
 This controller acts to adjust feed water in response to firing system disturbances
and the relatively fast effect they have on separator outlet steam temperatures.
 The overall combined feed water feedback control action is such that feed water
demand is responsive to changes in the overall unit heat transfer profile.
Controls of Super Critical
3 rd Feb 2009
FEEDWATER CONTROL LOOP contd..
 The combined feed forward/feedback demand signal is subject to a minimum
economizer inlet flow set point (wet mode) activated if the boiler circulation pump
is not in service and the unit is being fired.
 This ensures the minimum economizer inlet cooling flow is maintained by the
feed water supply system in the event the start-up system is not available.
 The feed forward/feedback demand signal is subject to a second “wet mode” feed
water demand developed to support separator storage tank level control.
 The resulting demand provides the set point to a feed water master controller.
The fuel/feed water ratio protection logic provides overriding control of individual
feeder speed demands in the event of an excessively high fuel to feed water ratio.
Controls of Super Critical
3 rd Feb 2009
THANK YOU
Controls of Super Critical
3 rd Feb 2009