660 Mw Sipat Boiler

February 4, 2019 | Author: Tochi Krishna Abhishek | Category: Boiler, Steam, Furnace, Hvac, Heat Transfer
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Short Description

ntpc sipat...

Description

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

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