POINTS OF DISCUSSION
SUB CRITICAL & SUPER CRITICAL BOILER
SIPAT BOILER DESIGN
BOILER DESIGN D ESIGN PARAMETERS PARAMETERS
CHEMICAL TREATMENT SYSTEM
OPERATION FEED
WATER SYSTEM
BOILER
CONTROL
BOILER
LIGHT UP
START
UP CURVES
POINTS OF DISCUSSION
SUB CRITICAL & SUPER CRITICAL BOILER
SIPAT BOILER DESIGN
BOILER DESIGN D ESIGN PARAMETERS PARAMETERS
CHEMICAL TREATMENT SYSTEM
OPERATION FEED
WATER SYSTEM
BOILER
CONTROL
BOILER
LIGHT UP
START
UP CURVES
WHY SUPER CRITICAL TECHNOLOGY
To Reduce emission for each Kwh of electricity generated : Superior Environmental 1% rise in efficiency reduce the CO2 emission by 2-3%
The Most Economical way to enhance efficiency
To Achieve Fuel cost saving : Economical
Operating Flexibility
Reduces the Boiler size / MW
To Reduce Start-Up Time
UNDERSTANDING UNDERST ANDING SUB CRITICAL TECHNOLOGY
Water when heated to sub critical pressure, Temperature increases until it starts boiling
This temperature remain constant till all the water converted c onverted to steam
When all liquid converted to steam than again temperature starts rising.
Sub critical boiler typically have a mean ( Boiler Drum) to separate Steam And Water
The mass of this boiler drum, which limits the rate at which the sub critical boiler responds to the load changes
Too great a firing rate will result in high thermal stresses in the boiler drum
Role of SG in Rankine Cycle
Perform Using Natural resources of
UNDERSTANDING SUPER CRITICAL TECHNOLOGY
When Water is heated at constant pressure above the critical pressure, its temperature will never be constant
No distinction between the Liquid and Gas, the mass density of the two phases remain same
No Stage where the water exist as two phases and require separation : No Drum
The actual location of the transition from liquid to steam in a once through super critical boiler is free to move with different condition : Sliding Pressure Operation
For changing boiler loads and pressure, the process is able to optimize the amount of liquid and gas regions for effective heat transfer.
Circulation Vs Once Through
No Religious Attitude
540 C, 255 Ksc °
568 C, 47 Ksc °
492 C, 260 Ksc °
457 C, 49 Ksc °
FUR ROOF I/L HDR
ECO HGR O/L HDR
HRH LINE MS LINE 411 C, 277Ksc °
411 C, 275 Ksc °
SEPARATOR S T O R A G E T A N K
G LPT C O N D E N S E R
LPT
IPT HPT
FINAL SH FINAL RH DIV PANELS SH
LTRH
PLATEN SH
VERTICAL WW c s K 9 4 , C 5 0 3 °
ECO JUNCTION HDR
ECONOMISER
ECO I/L
FEED WATER
BWRP
290 C, 302 KSC °
Steam
Partial Steam Generation Steam
Complete or Once-through Generation
Heat Input Water
t u p n I t a e H
Water Water
SEPARATOR TANK
PENTHOUSE
Eco. O/L hdr (E7) LTRH O/L hdr (R8) 2nd pass top hdrs (S11)
Back pass Roof o/l hdr (S5) SH final I/L hdr (S34) 1st
SH final O/L hdr (S36) F19
pass top hdrs
RH O/L hdr (R12)
RH I/L hdr (R10) Platen O/L hdr (S30)
F28 Platen I/L hdr (S28)
F28 Div. Pan. O/L hdrs (S24)
Div. Pan. I/L hdrs (S20) Back pass Roof i/l hdr Separator (F31)
F8
1st pass top hdrs
S2 Storage Tank (F33)
SIPAT SUPER CRITICAL BOILER
BOILER DESIGN PARAMETER
DRUM LESS BOILER : START-UP SYSTEM
TYPE OF TUBE
Vertical Spiral
SPIRAL WATER WALL TUBING
Advantage Disadvantage over Vertical water wall
Vertical Tube Furnace
To provide sufficient flow per tube, constant pressure furnaces employ vertically oriented tubes.
Tubes are appropriately sized and arranged in multiple passes in the lower furnace where the burners are located and the heat input is high.
By passing the flow twice through the lower furnace periphery (two passes), the mass flow per tube can be kept high enough to ensure sufficient cooling.
In addition, the fluid is mixed between passes to reduce the upset fluid temperature.
Spiral Tube Furnace
The spiral design, on the other hand, utilizes fewer tubes to obtain the desired flow per tube by wrapping them around the furnace to create the enclosure.
This also has the benefit of passing all tubes through all heat zones to maintain a nearly even fluid temperature at the outlet of the lower portion of the furnace.
Because the tubes are “wrapped” around the furnace to form the enclosure, fabrication and erection are considerably more complicated and costly.
SPIRAL WATER WALL ADVANTAGE
Benefits from averaging of heat absorption variation : Less tube leakages
Simplified inlet header arrangement
Use of smooth bore tubing
No individual tube orifice
Reduced Number of evaporator wall tubes & Ensures minimum water flow
Minimizes Peak Tube Metal Temperature
Minimizes Tube to Tube Metal Temperature difference
DISADVANTAGE
Complex wind-box opening
Complex water wall support system
tube leakage identification : a tough task
More the water wall pressure drop : increases Boiler Feed Pump Power
Adherence of Ash on the shelf of tube fin
BOILER OPERATING PARAMETER FD FAN
2 No‟S ( AXIAL )
11 kv / 1950 KW
228 mmwc 1732 T / Hr
PA FAN
2 No‟s ( AXIAL)
11 KV / 3920 KW
884 mmwc 947 T / Hr
ID FAN
2 No‟s ( AXIAL)
11 KV / 5820 KW
TOTAL AIR
2535 T / Hr
SH OUT LET PRESSURE / TEMPERATURE / FLOW
256 Ksc / 540 C
RH OUTLET PRESSURE/ TEMPERATURE / FLOW
46 Ksc / 568 C
SEPARATOR OUT LET PRESSURE/ TEMPERATURE
277 Ksc / 412 C
ECONOMISER INLET
304 Ksc / 270 C
MILL OPERATION
7 / 10
COAL REQUIREMENT
471 T / Hr
SH / RH SPRAY
89 / 0.0 T / Hr
BOILER EFFICIENCY
87 %
2225 T / Hr 1742 T / Hr
3020 T / Hr
Coal Analysis Unit
Design Coal
Worst Coal
Best Coal
Young Hung #1,2(800MW)
Tangjin #5,6(500MW)
kcal/kg
3,300
3,000
3,750
6,020
6,080
Total Moisture
%
12.0
15.0
11.0
10.0
10.0
Volatile Matter
%
21.0
20.0
24.0
23.20
26.53
Fixed Carbon
%
24.0
20.0
29.0
52.89
49.26
Ash
%
43.0
45.0
36.0
13.92
14.21
Fuel Ratio (FC/VM)
-
1.14
1.00
1.21
2.28
1.86
Combustibility Index
-
2,067
2,353
2,476
2,781
3,492
Carbon
%
39.53
31.35
40.24
63.03
62.15
Hydrogen
%
2.43
2.30
2.68
3.60
3.87
Nitrogen
%
0.69
0.60
0.83
1.53
1.29
Oxygen
%
6.64
5.35
8.65
7.20
7.80
Sulfur
%
0.45
0.40
0.60
0.72
0.68
Ash
%
43.00
45.00
36.00
13.92
14.21
Moisture
%
12.00
15.00
11.00
10.00
10.00
HGI
50
47
52
45
48
-
Hi –Vol. „C‟ Bituminous
Hi –Vol. „C‟ Bituminous
Hi –Vol. „C‟ Bituminous
Midium Vol. Bituminous
Hi –Vol. „C‟ Bituminous
Parameter High Heating Value
Proximate Analysis
Ultimate Analysis
Grindability ASTM Coal Classification
1.
High erosion potential for pulverizer and backpass tube is expected due to high ash content.
2. Combustibility Index is relatively low but combustion characteristic is good owing to high volatile content.
Ash Analysis Unit
Design Coal
Worst Coal
Best Coal
SiO2
%
61.85
62.40
61.20
57.40
57.40
Al2O3
%
27.36
27.31
27.32
29.20
29.20
Fe2O3
%
5.18
4.96
5.40
4.40
4.40
CaO
%
1.47
1.42
1.52
2.70
2.70
Ash
MgO
%
1.00
1.03
0.97
0.90
0.90
Analysis
Na2O
%
0.08
0.08
0.08
0.30
0.30
K2O
%
0.63
0.32
1.22
0.70
0.70
TiO2
%
1.84
1.88
1.80
1.30
1.30
P2O5
%
0.54
0.55
0.44
-
-
SO3
%
0.05
0.05
0.05
-
-
%
-
-
-
3.10
3.10
1200
1200
Parameter
Others
Young Hung Tangjin #1,2(800MW) #5,6(500MW)
o
C
1150
1100
1250
Temp. ( C) Softening
o
C
-
-
-
(Reducing Hemispheric Atmos.) Flow
o
C
1400
1280
1400
o
C
1400
1280
1400
Ash Content
kg/Gcal
130.3
150.0
96.0
23.12
23.37
Basic / Acid
B/A
0.09
0.09
0.10
1.63
1.63
Ash Fusion Initial Deformation o
1.
Lower slagging potential is expected due to low ash fusion temp. and low basic / acid ratio.
2. Lower fouling potential is expected due to low Na2O and CaO content.
AIR AND FLUE GAS SYSTEM AIR PATH
: Similar as 500 MW Unit
FLUE GAS PATH : No Of ESP Passes
:
6 Pass
No Of Fields / Pass
:
18
1-7 fields
70 KV.
8&9 field
90 KV
No Of Hopper / Pass
:
36
Flue Gas Flow / Pass
:
1058 T/ Hr
M
M
M
TO PULVERISER SYSTEM
M
M
PA FAN # A
HOT PRIMARY AIR DUCT
PAPH # A
M
M
AIR MOTOR
M
AIR MOTOR
M
M
M
M
FD FAN # A
M
M
M
SAPH # A
M
M M
M M
SAPH # B M
M
FD FAN # B
M
AIR MOTOR
M M
M
M
M
M
PA FAN # B
M
AIR MOTOR
PAPH # B
HOT PRIMARY AIR DUCT
M
M
TO PULVERISER SYSTEM
LHS WIND BOX
FURNACE L E N A P L A N O I S I V I D
S L I O C N E T A L P
R E T A E H E R L A N I F
R E T A E H R E P U S L A N I F
BACK PASS H R T L
R E S I M O N O C E
RHS WIND BOX
AIR PATH
FUEL OIL SYSTEM Type Of Oil
:
LDO / HFO
Boiler Load Attainable With All Oil Burner In Service
:
30 %
Oil Consumption / Burner
:
2123 Kg / Hr
Capacity Of HFO / Coal
:
42.1 %
Capacity Of LDO / Coal
:
52.5 %
HFO Temperature
:
192 C
All Data Are At 30 % BMCR
DESIGN BASIS FOR SAFETY VALVES : 1. Minimum Discharge Capacities. Safety valves on Separator and SH
Combined capacity 105%BMCR
(excluding power operated impulse safety valve)
Safety valves on RH system
Combined capacity 105% of Reheat flow at BMCR
(excluding power operated impulse safety valve)
Power operated impulse safety valve
40%BMCR at super-heater outlet 60% of Reheat flow at BMCR at RH
outlet
2. Blow down
4% (max.)b
BOILER FILL WATER REQUIREMENT Main Feed Water Pipe ( FW Shut Off Valve Valve to ECO I/L HDR)
28.8 m3
Economizer
253.2 m3
Furnace ( Eco Check Valve Valve to Separator Link)
41.5 m3
Separators & Link
13.8 m3
OXYGENATED TREATMENT OF FEED WATER
“WATER CHEMISTRY CONTROL MAINTAINS PLANT HEALTH.”
Dosing of oxygen(O2) or Hydrogen peroxide
(H2O2)
in to feed water system. Concentration in the range of 50 to 300 µg/L. Formation of a thin, tightly adherent ferric oxide
(FeOOH) hydrate layer. This layer is much more dense and tight than
that of
Magnetite layer.
39
All Volatile Treatment
Oxygenated Water Treatment
40
DOSING POINTS
41
“AVT” Dosing Auto Control
42
“OWT” Dosing Auto Control
43
U#1
FUR ROOF I/L HDR VENT HDR
VENT HDR N2 FILL LINE
N2 FILL LINE
SAMPLE COOLER
SAMPLE COOLER
SEPRATOR #1
1
2
WATER LINE N2 FILLING LINE VENT LINE DRAIN LINE
SEPRATOR #2
1
2
1
2
1
SAMPLE COOLER LINE
2 VENT HDR
VENT HDR
FUR WW HDR
FUR INTERMITTENT HDR STORAGE TANK DRAIN HDR
FUR BOTTOM RING HDR FLASH TANK
DRAIN HDR
MIXING PIECE WR
VENT HDR
ZR
BACK PASS ECO O/L HDR N2 FILL LINE
ECO JUNCTION HDR
BRP
ECO MIXING LINK
BACK PASS ECO I/L HDR BLR FILL PUMP FROM FEED WATER
FEED WATER SYSTEM MODES OF OPERATION 1.
BOILER FILLING
2.
CLEAN UP CYCLE
3.
WET MODE OPERATION (LOAD < 30 % )
4.
DRY MODE OPERATION (LOAD > 30 %)
5.
DRY TO WET MODE OPERATION ( WHEN START UP SYSTEM NOT AVAILABLE)
BOILER FILLING LOGIC
If the water system of the boiler is empty (economizer, furnace walls, separators), then the system is filled with approximately 10% TMCR ( 223 T/Hr) 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 ( approx 660 T/Hr).
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
After completion of Filling, the feed water flow is again adjusted to 10 % TMCR for Clean up cycle operation
BOILER INITIAL WATER LEVEL CONTROL (UG VALVE)
The boiler circulating pump is started following the start of a 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.
Boiler Clean-up When
the feedwater quality at the outlet of deaerator and separator is not within the specified limits, a feedwater clean-up recirculation via the boiler is necessary. During
this time, constant feedwater flow of 10% TMCR ( 223 T/Hr) or more is maintained. 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. From
the condenser, the water flows through the condensate polishing plant, which is in service to remove impurities ( Like Iron & its Oxide, Silica, Sodium and its salts ), then returns to the feed water tank. The
recirculation is continued until the water quality is within the specified limits.
FEED WATER QUALITY PARAMETER FOR START UP
MODE OF OPERATION WET MODE :
Initial Operation Of Boiler Light Up. When Economizer Flow is maintained by BCP.
Boiler Will Operate till 30 % TMCR on Wet Mode .
DRY MODE :
At 30 % TMCR Separator water level will become disappear and Boiler Operation mode will change to Dry
BCP Will shut at this load
Warm Up system for Boiler Start Up System will get armed
Boiler will turn to once through Boiler
ECO Water flow will be controlled by Feed Water Pump in service
SYSTEM DESCRIPTION ( WET MODE OPERATION) 1. Flow Control Valve ( 30 % Control Valve )
Ensures minimum pressure fluctuation in Feed Water Header
It measures Flow at BFP Booster Pump Discharge and compare it with a calculated flow from its downstream pressure via a function and maintains the difference “ 0 “
2. 100 % Flow Valve To Boiler
Remains Closed
3. BFP Recirculation Valve
It Measures Flow at BFP Booster Pump Discharge
Ensures minimum Flow through BFP Booster Pump
Closes when Flow through BFP Booster Pump discharge > 2.1 Cum Open When Flow through BFP Booster Pump Discharge < 1.05 Cum ( Minimum Flow will be determined by BFP Speed via BFP Set limitation Curve)
4. BFP Scoop
It measures value from Storage tank level Transmitter
Maintain Separator Storage Tank Level
5. UG Valve
Maintain Minimum Economizer Inlet Flow ( 30 % TMCR = Approx 660 T/Hr)
Maintain DP across the BRP ( Approx 4.0 Ksc)
It Measures Flow Value from Economizer Inlet Flow Transmitter
6. WR / ZR Valve
Maintains Separator Storage Tank Level
It Measures value from the Storage tank Level
7. Storage Tank Level
3 No‟s Level Transmitter has been provided for Storage tank level measurement
1 No HH Level Transmitter has been provided
At 17.9 Mtr level it will trip all FW Pumps also MFT will act
1 No LL Level Transmitter has been provided
At 1.1 Mtr level MFT will Act
SYSTEM DESCRIPTION ( DRY MODE OPERATION) 1. Following System will be isolated during Dry Mode Operation
FCV ( 30 % )
Start Up System Of Boiler
WR / ZR Valve Storage Tank BRP BRP Recirculation System
BFP Recirculation Valve
2. Following System will be in service
UG Valve ( Full Open)
100 % FW Valve ( Full Open)
Platen / Final Super-heater spray control
Start Up System Warming Lines
Separator Storage Tank Wet Leg Level Control
SYSTEM OPERATION ( DRY MODE OPERATION) 1. START UP SYSTEM
2.
3.
In Dry Mode Start Up System Of Boiler will become isolated
Warming System for Boiler Start Up system will be charged
Separator Storage Tank level will be monitored by Separator storage tank wet leg level control valve ( 3 Mtr)
TRANSITION PHASE :- Changeover of FW Control valve (30 % to 100 % Control )
100 % FW Flow valve will wide open
During the transition phase system pressure fluctuates
The system pressure fluctuation will be controlled by 30 % FW Valve. After stabilization of system 30 % CV Will become Full Close
FEED WATER CONTROL
It will be controlled in three steps
Feed Water demand to maintain Unit Load Maintain Separator O/L Temperature Maintain acceptable Platen Spray Control Range
FEED WATER DEMAND ( DRY MODE OPERATION) 1.
FINAL SUPER HEATER SPRAY CONTROL
2.
Maintain the Final Steam Outlet Temperature ( 540 C)
PLATEN SUPER HEATER SPRAY CONTROL
Primary purpose is to keep the final super heaer spray control valve in the desired operating range
3.
Measures the final spray control station differential temperature It Compares this difference with Load dependent differential temperature setpoint Output of this is the required temperature entering the Platen Super Heater Section (Approx 450 C)
FEED WATER DEMAND 1.
FEED FORWARD DEMAND
It is established by Boiler Master Demand.
This Demand goes through Boiler Transfer Function where it is matched with the actual Evaporator Heat Transfer to minimize the temperature fluctuations
2.
FEED BACK DEMAND
Work With two controller in cascade mode
FEED WATER DEMAND ( DRY MODE OPERATION) 2.
FEED BACK DEMAND
Work With two controller in cascade mode
FIRST CONTROLLER
One Controller acts on Load dependent average platen spray differential temperature
Its Output represents the desired heat transfer / steam generation to maintain the desired steam parameters and Flue gas parameters entering the Platen section
SECOND CONTROLLER
Second Controller acts on the load dependent Separator Outlet Temperature adjusted by Platen spray differential temperature
This controller adjust the feed water in response to firing disturbances to achieve the separator O/L Temperature
THE RESULTING DEMAND FROM THE COMBINED FEEDFORWARD AND FEEDBACK DEMANDSIGNAL DETERMINED THE SETPOINT TO THE FEED WATER MASTER CONTROL SETPOINT
DRY TO WET MODE OPERATION ( START UP SYSTEM NOT AVAILABLE) 1.
The combined Feed Forward and Feed back demand ( as calculated in dry mode operation) will be compared with minimum Economizer Flow This ensures the minimum flow through Economizer during the period when start up system is unavailable
2.
Output of the first controller is subjected to the second controller which monitors the Separator Storage tank level ( Since the system is in Wet Mode now)
3.
The output of the second controller is the set point of Feed water master controller.
4.
The Feed back to this controller is the minimum value measured before the start up system and Economizer inlet.
WATER & STEAM PATH
BLR PATH ( WHEN WET MODE) Separator - Backpass Wall & Extended Wall - SH Division - Platen SH - Final SH HP By-pass - Cold R/H Line - Primary R/H (Lower Temp R/H) - Final R/H - LP Bypass - Condenser BLR Path (When Dry Mode)
Primary Eco - Secondary Eco - Ring HDR - Spiral W/W - W/W Intermediate HDR Vertical W/W - Separator - Backpass Wall & Extended Wall - SH Division - Platen SH - Final SH - HP TBN - Cold R/H Line - Primary R/H (Lower Temp R/H)- Final R/H - IP and LP TBN - Condenser
Wet Mode and Dry Mode of Operation
DIV SH
406
451
PLATEN SH
440
FINAL SH
480
486
DSH1 15%
DSH2 3%
540
BOILER LOAD CONDITION Constant Pressure Control
Above 90% TMCR The MS Pressure remains constant at rated pressure
The Load is controlled by throttling the steam flow
Below 30% TMCR the MS Pressure remains constant at minimum Pressure
Sliding Pressure Control
Boiler Operate at Sliding pressure between 30% and 90% TMCR
The Steam Pressure And Flow rate is controlled by the load directly
CONSTANT PRESSURE VS SLIDING PRESSURE
Valve throttling losses occur because the boiler operates at constant pressure while the turbine doesn't.
The most obvious way to avoid throttling losses therefore is to stop operating the boiler at constant pressure!
Instead, try to match the stop valve pressure to that existing inside the turbine at any given load.
Since the turbine internal pressure varies linearly with load, this means that the boiler pressure must vary with load similarly.
This is called .sliding pressure operation..
If the boiler pressure is matched to the pressure inside the turbine, then there are no valve throttling losses to worry about! While sliding pressure is beneficial for the turbine, it can cause difficulties for the boiler.
ADVERSE AFFECT
As the pressure falls, the boiling temperature (boiling point) changes. The boiler is divided into zones in which the fluid is expected to be entirely water, mixed steam / water or dry steam. A change in the boiling point can change the conditions in each zone.
The heat transfer coefficient in each zone depends upon the pressure. As the pressure falls, the heat transfer coefficient reduces. This means that the steam may not reach the correct temperature. Also, if heat is not carried away by the steam, the boiler tubes will run hotter and may suffer damage.
CHALLANGES
The heat transfer coefficient also depends upon the velocity of the steam in the boiler tubes. Any change in pressure causes a change in steam density and so alters the steam velocities and heat transfer rate in each zone.
Pressure and temperature cause the boiler tubes to expand. If conditions change, the tubes will move. The tube supports must be capable of accommodating this movement. The expansion movements must not lead to adverse stresses.
The ability to use sliding pressure operation is determined by the boiler
Boilers can be designed to accommodate sliding pressure .
When it is used, coal fired boilers in the 500 to 1000 MW class normally restrict sliding pressure to a limited load range, typically 70% to 100% load, to minimize the design challenge. Below this range, the boiler is operated at a fixed pressure.
This achieves an acceptable result because large units are normally operated at high load for economic reasons.
In contrast, when sliding pressure is used in combined cycle plant, the steam pressure is varied over a wider load range, typically 50% to 100% load or more
As stated, in coal-fired plant, sliding pressure is normally restricted to a limited load range to reduce design difficulties.
In this range, the boiler pressure is held at a value 5% to 10% above the turbine internal pressure. Consequently, the governor valves throttle slightly.
The offset is provided so that the unit can respond quickly to a sudden increase in load demand simply by pulling the valves wide open.
This produces a faster load response than raising the boiler firing rate alone.The step in load which can be achieved equals the specified margin ie 5% to 10%.
The throttling margin is agreed during the tendering phase and then fixed.
A margin of 5% to 10% is usually satisfactory because most customers rely upon gas turbines, hydroelectric or pumped storage units to meet large peak loads.
The throttling margin means that the full potential gain of sliding pressure is not achieved.
Nevertheless, most of the throttling losses which would otherwise occur are recovered.
ADVANTAGES
Temperature changes occur in the boiler and in the turbine during load changes. These can cause thermal stresses in thick walled components.
These are especially high in the turbine during constant-pressure operation. They therefore limit the maximum load transient for the unit.
By contrast, in sliding pressure operation, the temperature changes are in the evaporator section. However, the resulting thermal stresses are not limiting in the Once through boiler due to its thermo elastic design.
In fixed pressure operation , temperature change in the turbine when load changes, while in sliding-pressure operation ,they change in the boiler
The enthalpy increase in the boiler for preheating, evaporation and superheating changes with pressure.
However, pressure is proportional to output in sliding pressure operation
In a uniformly heated tube, the transitions from preheat to evaporation and from evaporation to superheat shift automatically with load such that the main steam temperature always remains constant.
Sliding Pressure
At loads over 25% of rated load, the water fed by a feed-water pump flows through the high pressure feed-water heater, economizer ,furnace water wall, steam-water separator, rear-wall tubes at the ceiling, and super heaters, The super heaters steam produced is supplied to the turbine.
At rated and relatively high loads the boiler is operated as a purely once through type. At partial loads, however, the boiler is operated by sliding the pressure as a function of load.
25
a p M e 20 r u s s e 15 r p t e l n 10 i e n i b 5 r u T
24.1 Mpa
9.0 Mpa
0
0
25
50
Turbine load (%)
75
100
CONSTANT PRESSURE Vs VARIABLE PRESSURE BOILER CHARACTERSTIC Boiler Load % 20
40
+1 % e g n a h C y c n e i c i f f E
0 -1 -2 -3 -4
Variable Pressure
60
80
100
Benefits Of Sliding Pressure Operation ( S.P.O)
Able to maintain constant first stage turbine temperature
Reducing the thermal stresses on the component : Low Maintenance & Higher Availability
No additional pressure loss between boiler and turbine.
low Boiler Pr. at low loads.
WHY NOT S.P.O. IN NATURAL/CONTROL CIRCULATION BOILERS
Circulation Problem : instabilities in circulation system due to steam formation in down comers.
Drum Level Control : water surface in drum disturbed.
Drum : (most critical thick walled component) under highest thermal stresses
The Basis of Boiler Start-up Mode
Mode Basis
Restart
Hot
Warm
Cold
Stopped time
2Hr Within
6~12Hr
56Hr Within
96Hr Above
SH Outlet Temp
465
above
300
100
100
Separator Tank pr
120
200 /
30
Starting Time
above 120 /
30 /
above below
below
0 /
STARTING TIME
Startup Mode
Light off →TBN Rolling(minutes)
Light off → Full Load(minutes)
Cold
120
420
Except Rotor and Chest Warming Time
Warm
90
180
"
Hot
-
-
․
Restart
30
90
․
PURGE CONDITIONS
No Boiler Trip Condition Exists
All System Power Supply Available
Unit Air Flow > 30 % BMCR
Nozzle Tilt Horizontal and Air Flow < 40 %
Both PA Fans Off
The Following Condition Exist At Oil Firing System The HOTV / LOTV Should Be Closed All Oil Nozzle Valve Closed
The Following Condition Exists at Coal Firing System All Pulverisers are Off All Feeders are Off All Hot Air Gates Of Pulverisers are closed
All Flame Scanner on all elevation shows no Flame
Aux Air Damper At All Elevation should be modulating
After Purging Boiler Light Up activites are same as in 500 MW plant
MFT CONDITIONS
Both ID Fans Off
Both FD Fans Off
Unit Air Flow < 30 % TMCR
All Feed Water Pumps Are Off For More Than 40 Sec
2 / 3 Pressure Transmitter indicate the furnace pressure High / Low for more than 8 sec ( 150 mmwc / -180 mmwc))
2 / 3 Pressure Transmitter indicate the furnace pressure High – High / Low - Low ( 250 mmwc / - 250 mmwc)
Loss Of Re-heater Protection
EPB Pressed
All SAPH Off
Economizer Inlet Flow Low For More Than 10 Sec (223 T/Hr)
Furnace Vertical Wall Temperature High For more than 3 Sec (479 C)
SH Pressure High On Both Side (314 KSc)
SH Temperature High For More Than 20 Sec ( 590 C)
RH O/L Temperature High For More Than 20 Sec ( 590 C)
Separator Level Low-Low During Wet Mode ( 1.1 M)
Separator Level High-High During Wet Mode ( 17.7 M)
MFT Relay Tripped
Loss Of Fuel Trip : It Arms when any oil burner proven. it occurs when all of the following satisfied
All Feeders Are Off
HOTV Not Open or all HONV Closed
LOTV Not Open or all LONV Closed
Unit Flame Failure Trip : It Arms when any Feeder Proves it occurs when all 11 scanner elevation indicates flame failure as listed below ( Example is for only elevation A)
Feeder A & Feeder B is Off with in 2 Sec Time Delay
following condition satisfied
Any oil valve not closed on AB Elevation
3 /4 valves not proven on AB Elevation
Less Than 2 / 4 Scanner Shows Flame
Both Of The Following Condition Satisfied
Less Than 2 / 4 Scanner Flame Shows Flame
2 / 4 Oil Valves not open at AB Elevation
Boiler Light Up Steps
Start the Secondary Air Preheater
Start one ID fan, then the corresponding FD fan and adjust air flow to a min. of 30% TMCR
Start the scanner air fan.
Adjust fan and SADC to permit a purge air flow of atleast 30% of TMCR and furnace draft of approx. -12.7 mmWC.
When fans are started, SADC should modulate the aux. air dampers to maintain WB to furnace DP at 102 mmWC(g).
Check that all other purge permissives are satisfied.
Place FTPs in service.
Check The MFT Conditions
For First Time Boiler Light Up do the Oil Leak Test
Initiate a furnace purge.
SYSTEM / EQUIPMENT REQUIRED FOR BOILER LIGHT UP FURNACE READINESS
PRESSURE PARTS
SCANNER AIR FAN
BOTTOM ASH HOPPER READINESS
FUEL FIRING SYSTEM
START UP SYSTEM
SEC AIR PATH READINESS
FD FAN
SAPH
WIND BOX / SADC
FLUE GAS SYSTEM
ESP PASS A , B
ID FAN
SYSTEM / EQUIPMENT REQUIRED FOR BOILER LIGHT UP CONDENSATE SYSTEM
CONDENSER
CEP
CPU
FEED WATER SYSTEM
D/A
MDBFP # A
VACCUME SYSTEM SEAL STEAM SYSTEM TURBINE ON BARRING
Evaporator – heat absorption
Reduced number of evaporator wall tubes. Ensures minimum water wall flow.
SPIRAL WALL ARRAMGEMENT AT BURNER BLOCK AREA :
Support System for Evaporator Wall
• Spiral wall • Vertical wall
Horizontal and vertical buck stay with tension strip Horizontal buck stay
