Transcript POINTS OF DISCUSSION  SUB CRITICAL & SUPER CRITICAL BOILER  SIPAT BOILER DESIGN  BOILER DESIGN PARAMETERS  CHEMICAL TREATMENT SYSTEM  OPERATION  FEED.

POINTS OF DISCUSSION
 SUB CRITICAL & SUPER CRITICAL BOILER
 SIPAT BOILER DESIGN
 BOILER DESIGN 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 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 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 energy …….
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
G
LPT
C
O
N
D
E
N
S
E
R
LPT
FINAL SH
FINAL
RH
DIV PANELS SH
LTRH
PLATEN
SH
VERTICAL WW
ECO
JUNCTION
HDR
305°C, 49 Ksc
S
T
O
R
A
G
E
T
A
N
K
IPT
HPT
ECONOMISER
ECO I/L
FEED WATER
BWRP
290°C, 302 KSC
FUR LOWER HDR
FRS
Steam
Partial Steam Generation
Steam
Complete or Once-through
Generation
Water
Heat Input
Heat Input
Water
Water
Boiling process in Tubular Geometries
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
S2
1st pass top hdrs
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
2225 T / Hr
RH OUTLET PRESSURE/ TEMPERATURE /
FLOW
46 Ksc / 568 C
1742 T / Hr
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 %
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
Proximate Volatile Matter
Analysis Fixed Carbon
%
21.0
20.0
24.0
23.20
26.53
%
24.0
20.0
29.0
52.89
49.26
%
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
Ash
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
MgO
%
1.00
1.03
0.97
0.90
0.90
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
-
-
Others
%
-
-
-
3.10
3.10
Initial Deformation
o
C
1150
1100
1250
1200
1200
Softening
o
C
-
-
-
Hemispheric
o
C
1400
1280
1400
Flow
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
Parameter
Ash
Analysis
Ash Fusion
Temp. (oC)
(Reducing
Atmos.)
Young Hung
Tangjin
#1,2(800MW) #5,6(500MW)
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
M
AIR MOTOR
M
AIR MOTOR
M
PA FAN # A
HOT PRIMARY AIR DUCT
PAPH # A
M
M
M
M
M
M
M
SAPH # A
M
FD FAN # A
M
M
M
M
M
M
AIR MOTOR
M
AIR MOTOR
M
M
M
FD FAN # B
SAPH # B
M
M
M
PA FAN # B
M
PAPH # B
HOT PRIMARY AIR DUCT
M
M
TO PULVERISER SYSTEM
LHS WIND BOX
ECONOMISER
BACK PASS
LTRH
FINAL SUPERHEATER
FINAL REHEATER
PLATEN COILS
DIVISIONAL PANEL
FURNACE
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 to ECO I/L HDR)
28.8 m3
Economizer
253.2 m3
Furnace ( Eco Check 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
WATER LINE
N2 FILL LINE
N2 FILL LINE
N2 FILLING LINE
VENT LINE
SAMPLE COOLER
SAMPLE COOLER
SEPRATOR #1
1
2
DRAIN LINE
SAMPLE COOLER LINE
SEPRATOR #2
1
2
1
2
1
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
TO DRAIN HDR
WATER CIRCULATION SYSTEM
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
Turbine inlet pressure Mpa
 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
24.1 Mpa
20
15
10
9.0 Mpa
5
0
0
25
50
Turbine load (%)
75
100
CONSTANT PRESSURE Vs VARIABLE PRESSURE BOILER CHARACTERSTIC
Boiler Load %
20
40
Efficiency Change %
+1
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℃ above
100℃ above
100℃ below
Separator Tank pr
120～200㎏/㎠
30～120㎏/㎠
30㎏/㎠ below
0㎏/㎠
Starting Time
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
 Horizontal and vertical buck stay with tension strip
• Vertical wall  Horizontal buck stay