Fsss

Furnace Safeguard Supervisory System

K.C. Yadav, AVP, Noida Technical Training Centre

Learning Agenda Objective of FSSS » Inter facing of field equipments and instrumentation » Operational features of FSSS » Role of FSSS in starting/shutdown » Role of FSSS in maintaining the design prescribed values of the parameters »

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It must be recognized that no amount of written instruction can replace intelligent thinking and reasoning on the part of boiler operators, especially when coping with unforeseen operating conditions. It is operators’ responsibility to become thoroughly familiar, not only with immediate steam generating equipment but also with all pertinent control equipment. Satisfactory performance and safety depend to a great extent on proper functioning of controls and auxiliary equipment. 3 of 27

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Purpose The FSSS is designed to ensure the execution of safe, orderly operating sequence in the start up and shutdown of fuel firing equipment and to prevent errors of omission and commission in following such a safe operating procedure.

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Purpose  

  

  

Furnace purge supervision. Secondary air damper modulation, on/off control and supervision. Light oil on-off control and supervision. Heavy oil on-off control and supervision. Pulveriser and feeder on-off control and supervision. Flame scanner intelligence and checking. Overall boiler flame failure protection. Boiler trip protection. 5 of 27

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Purpose 







Prevent any fuel firing unless a satisfactory furnace purge sequence has first been completed. Prevent start-up of individual fuel firing equipment unless certain permissive, interlocks have been satisfied. Monitor and control proper component sequencing during start-up and shut-down of fuel firing equipment. Subject continued operation of fuel firing equipment to ascertain safety interlocks remaining satisfied. 6 of 27

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Purpose 





Provide component status feedback to the operator and, in some cases, to the unit control system and / or to the data logger. Provide flame supervision when fuel firing equipment is in service and effect an Elevation Fuel Trip or Master Fuel Trip upon certain condition of unacceptable firing / load combination. Effect a Master Fuel Trip upon certain adverse Unit operating conditions. 7 of 27

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Subsystem of FSSS 

Purge Permissive (s)



HOTV Open/Close Interlocks



HORV Open/Close Interlocks



Oil Guns Start/Stop Cycle



Boiler Protection Logics 8 of 27

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Subsystem of FSSS (Cont.) 

PA Fan Interlocks



Coal Mill Start/Stop Interlocks



Coal Feeder Start/Stop Interlocks



Scanner Air Fans



Seal Air Fans



Secondary Air Damper Interlocks 9 of 27

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Purge Permissives 

All HFO Nozzle Valves Closed



HFO Trip Valve closed



No Boiler Trip From MFR A & B



All Pulverizers Off.



All Feeders Off



All Flame Scanners Sense No Flame



Both PA fans Off 10 of 27

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Purge Permissives (Cont.) 

All Auxiliary Air Dampers Modulating and furnace to wind box P OK



All Hot Air Gates Closed



Air Flow > 30 %



Mode Permit Proven (Air Flow < 40%



Burner Tilt Horizontal)



Drum Level Satisfactory



FSSS AC / DC Power Available. 11 of 27

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HOTV Interlocks To Open HOTV : Permissives SET I 

No. Boiler Trip persisting



HFO Header temperature satisfactory >950 C



HFO supply pressure > 15 Kg / cm2 All HONV’s closed No close / Trip command Open PB depressed

  

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HOTV Interlocks To Open HOTV : Permissives SET II    

All HONV’s closed MFT acted HORV open Open PB depressed

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HOTV Interlocks 1. HOTV closes automatically under the following conditions: Any HONV not closed AND a) HFO pressure low < 3 Kg/cm2 OR b) Atomizing steam pressure low < 3.5 Kg/cm2 OR c) HFO Header Temperature Lo-Lo for > 2 secs. < 95 0C 2. Any HONV not closed and MFT acted 3. Loss of critical power > 2 Seconds 4. HOTV can be closed manually 14 of 27

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HORV Interlocks 

HORV can be opened by pressing the OPEN Push button (PB) from Console if All the HONV s are closed.

 HORV closes automatically when any HONV is not closed OR by pressing close PB.

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FROM AUXILIARY STEAM HEADER

HFO Supply Scheme HFO PUMP-1

HFO HTR-1

s

s

HFO PUMP-2 HFO HTR-2

s

HFO HTR-3

s

HFO PUMP-3

s

FROM HFO DAY TANK

s

TO BOILER FRONT

TO HFO DAY TANK 16 of 27

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HFO Scheme At Boiler Front HOTV

HOFCV

FT AB ELEVATION

CD ELEVATION M

SHORT RECIRCULATION

FT

EF ELEVATION

HORV HFO SUPPLY LINE

HFO RETURN LINE

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Atomizing Steam Scheme

AB ELEVATION

CD ELEVATION

EF ELEVATION

Oil Gun Connection

HFO NOZZLE VALVE

SCAVENGE VALVE

OIL GUN

ATOMISING STEAM VALVE 18 of 27

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Pair Firing Mode (Start Up) 





Pair firing mode is selected when no feeder is in service. Pairs are made for the opposite corners. (Pair 1-3 and Pair 2-4) When pair 1-3 start push button is pressed; A 30 second counting period is started A 70 second counting period is started Oil Gun No-1 advances from retracted position if, it is engaged, corresponding scavenge valve is closed, atomizing steam and HFO isolation valves are open. 19 of 27

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Pair Firing Mode (Start Up)       



HEA igniter advances. Atomizing steam valve opens. Igniter starts sparking. HFO Nozzle valve for corner 1 opens HFO catches fire and flame indication comes. After 25 seconds igniter retracts. When the 30 second counting period expires command will go to corner 3 and same events will take place. Corner 2-4 can also be placed exactly in the same manner. 20 of 27

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Elevation Firing Mode (Start Up) 

Elevation firing mode is selected when any feeder is proven.



Oil elevation is placed in service when Pair 1-3 or Pair 2-4 start Push Button is pressed. In elevation firing mode also the oil guns are proven on pair basis.



First command goes for placing Pair 1-3 into service and Pair 1-3 is placed into service exactly in the same way as in the pair mode. 21 of 27

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Elevation Firing Mode (Start Up) 

First start command goes to corner-1 then after 30 second start command goes to corner-3.



After the start command has gone to corner 3, a 30 second counting period is started. When 30 seconds counting period expires then start command goes for corner 2-4 and corner

2-4 are placed into service exactly in the same

manner as in the pair mode.

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PAIR FIRING MODE (SHUT DOWN) 



  

Pair firing mode is selected when no feeder is in service. Pairs are made for the opposite corners. (Pair 1-3 and Pair 2-4) When pair 1-3 stop push button is pressed the following events take place A 30 second counting period is started A 340 second counting period is started Oil Gun No-1 HFO nozzle valve closes. Scavenge and atomizing steam valve opens. HEA igniter advances and spark remains for 15 seconds. 23 of 27

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Pair Firing Mode (Shut Down) 



 

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When atomizing steam valve is proven fully open, a 5 minutes counting period starts. When 5 minutes counting period expires scavenge valve and atomizing steam valve closes and further closing command goes HFO nozzle valves to reinsure that they are fully closed. Oil Gun corner no-1 retracts to its home position. When the 30 second counting period expires stop command will go to corner 3 and same events will take place. Corner 2-4 can be shut-down exactly in the same manner. 24 of 27

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Elevation Firing Mode (Shutdown) 

Elevation firing mode is selected when any feeder is proven.



By pressing stop push-button of Pair 1-3 or Pair 2-4 the following events will take place.



Corner 1-3 will be shut-down in the similar way as in the pair mode.



When stop command has gone to corner 3, a 30 second counting period is started.



When 30 seconds counting period expires then stop command goes for corner 2-4 and corner 2-4 are shut-down exactly in the same manner as in the pair mod 25 of 27

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Boiler Trip Conditions 

Both FD Fans Off



All ID Fans Off



Inadequate Water Wall Circulation



Loss Of Reheater Protection



Deaerator level Lo-Lo ( -2134 mm 2 / 3 logic)



All Boiler Feed Pumps Off



Drum Level High-High (> + 250 mm, delay of 10 seconds)



Drum Level Lo-Lo (< - 375 mm, delay 10 seconds)



Elevation Power Failure For > 2 seconds.



Furnace Pressure High-High (> + 325 mmWCl, 2 / 3 logic) 26 of 27

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Boiler Trip Conditions 

Furnace Pressure Lo-Lo (< -250 mmWC, 2 / 3 logic)



Loss Of All Fuel



Unit Flame Failure



Loss Of Critical Power For > 2 seconds



Loss Of 220 V DC For > 2 seconds



Loss Of ACS Power > 2 seconds



Simulated Trip



Air Flow < 30 %



Both Emergency Push Buttons Pressed. 27 of 27

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Boiler Trip When the boiler trips the following events takes place 

Boiler trip red lamp comes on.



MFT A & B trip lamp comes on and reset lamp goes off. Cause of trip memory can not be reset till the furnace purge is completed. F D Fans Control is transferred to manual. I D fans vane position is transferred to manual. Pulverizers are tripped. Coal feeders are tripped. P A fans are tripped.



    

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Boiler Trip (Cont.) When the boiler trips …….     

 



Cold air dampers opens 100 %. HFO trip valve closes. All HFO Nozzle valves closes. Upper and Lower Fuel air damper opens. Auxiliary air damper opens and control is transferred to manual. Loss of all fuel trip protection disarms. S/H and R/H spray block valves S-82 & R-31 closes and can not be opened unless furnace purge is completed. Turbine Trips 29 of 27

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Inadequate Water Wall Circulation 1. Boiler load > 60 % and less than two pumps or pump on and differential pressure < 10 psi. 2. Boiler load < 60 % and no BCW pump on or pump on and differential pressure < 10 psi.

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Reheat Protection 1. Turbine trip or generator circuit breaker open and hp/lp bypass opening < 2 %. 2. Turbine working (hp & ip cvs opening > 2 % and load shedding relay operated and hp/lp bypass opening < 2 %. 3. Turbine not working and boiler working and hp/lp bypass opening < 2 %.

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MFT

5 SEC

RESET Loss of all fuel arming SET

Any elevation ¾ Nozzle valve proven All HFO Elevation Trip All feeders off

2 SEC

All HFO Nozzle valve closed HOTV NOT OPEN

All feeders off

Loss of all fuel trip 2 SEC

Loss of All Fuel Trip 32 of 27

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Any Feeder Proven

2 Sec

Elevation GH No Flame Vote

Elevation FG No Flame Vote

Flame Failure Trip

Elevation DE No Flame Vote Elevation BC No Flame Vote Elevation AB No Flame Vote

Flame Failure Trip 33 of 27

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Flame Failure Vote Logics Elevation DE Scanners No Flame Feeder E Off Feeder D Off

2 Sec

Elevation DE No Flame Vote

Elevation BC Scanners No Flame

Feeder C Off Feeder B Off

2 Sec

Elevation BC No Flame Vote

Elevation AB Any HFO Nozzle Valve Not Closed Elevation AB 2/4 Nozzle Valve Not Proven Elevation AB Scanners No Flame

Feeder A Off

Elevation AB No Flame Vote

2 Sec

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ELEVATION GH ANY HFO NOZZLE VALVE NOT CLOSED

Flame Failure Vote Logics

ELEVATION GH 2/4 NOZZLE VALVE NOT PROVEN

ELEVATION GH SCANNERS NO FLAME

ELEVATION GH NO FLAME VOTE

FEEDER H OFF 2 SEC ELEVATION FG SCANNERS NO FLAME

FEEDER G OFF

ELEVATION FG NO FLAME VOTE

FEEDER F OFF

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Mill Interlocks Start Permissive (s): Mill starts after the ‘ON’ push button is pressed if following conditions are fulfilled. Pulveriser Ready available signal will be available if following conditions are fulfilled  M.D.V. is open (all four)  Mill outlet temp < 93 0 C  Feeder local switch in Remote 36 of 27

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Mill Interlocks  Pulverizer start permit (if Nozzle tilt Horizontal & air flow < 40 %) OR Any feeder proven & NO MFT Tramp iron valve open  Cold air gate open  Feeder inlet gate open  Lub oil level & temp. adequate  Lub oil pressure not low for more than 300 sec  No Mill trip existing 37 of 27

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Mill Interlocks P.A. permit available. (i) Hot P.A. duct Pressure. not low (< 625 mmWCl) (ii) P.A. duct pr. not lo-lo ( > 500 mmWCl AND (iii) Both FD s ON and both PA fans ‘ON’ or (Any PA fan ON and any FD Fan ‘ON’ and > 4 mills not ON)  No Auto pulverizer unsuccessful start  Seal air Header to mill under bowl DP satisfactory > 8" WCl  Mill not ‘ON’  Pulverizer Ignition permit available.

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Mill Interlocks FOR Mill A  AB elevation 3/4 HONV’s open OR  Boiler load > 30 % and Feeder - B speed > 50 % ( > 30 T/hr.) For Mill B  AB elevation proven 3/4 HONV’s open OR  Boiler load > 30 % and Feeder A or Feeder C proven.

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Mill Interlocks For Mill C  CD elevation proven

OR

 AB oil elevation and feeder B proven

OR

 Boiler load > 30 % & either feeder B or Feeder D proven

For Mill D  CD elevation proven

OR

Boiler load > 30 % and either feeder C or feeder E proven. For Mill E,F,G & H identical logic

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Pulveriser Trip Conditions  Auto / manual mode selected to manual & stop PB depressed.  Pulv. start command given & not ‘ON’ for > 2 Secs.  Loss of Elevation OR Unit Power for > 2 Secs  Pulv ‘ON’ & seal air / under DP Low > 1 min < 5”wc  Pulv ‘ON’ and lub oil pump OFF > 10 sec.  Pulv ‘ON’ and lub oil pr. low > 10 secs. < 1.76 Kg/cm2  Ignition permit required (> 180 secs) & ignition permit lost (i.e. within 180 sec of feeder start) 41 of 27

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Pulveriser Trip Conditions  Any of the MDV is not fully open  Hot PA duct. pr. Lo < 625 mm of WCL (delay 5 sec)  Hot PA duct. pr. Lo - Lo = < 525mm WC (instantaneous)  MFT  PA / FD Fan trip a) If both PA Fans & both FD fans running & any PA or any FD Fan trips then mills start tripping from top elevations with 2 secs time delay till 4 mills are available. b) If both PA / FD fans trip then all mills will trip. 42 of 27

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Pulverizer Seal Air Valves Interlocks  Valve opens when Pulverizer start command is given OR Millis ON  Valve closes when Pulverizer is OFF

Hot Air Gate Interlocks  Hot air gate (HAG) opens when Pulverizer is ‘ON’ AND Hot air gate open  P.B. depressed or auto HAG open command is there AND NO HAG close command. 43 of 27

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Hot air gate closes when 1. Loss of power > 2 secs 2. Pulverizer off 3. Pulverizer outlet temp > 93 0 C 4. Close P.B. depressed 5. Feeder Trips.(Delay 40 sec) 6. Auto pulverizer unsuccessful start 7. Auto pulverizer run feeder speed to minimum 44 of 27

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Feeder Start Permissive (s)  Pulveriser Ready is available  Feeder speed demand at minimum  Pulveriser manual mode  Pulveriser ignition permit available  No feeder stop command  No MFT  Pulveriser ‘ON’ 45 of 27

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Feeder Trip Condition  Ignition energy removed within 180 seconds of feeder start  Feeder in manual mode & stop PB depressed  Auto Pulverizer Feeder stop command  Pulverizer OFF  MFT  No coal on belt Discharge plugged 46 of 27

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SADC The secondary air dampers located in wind box apportions the total secondary air between various elevations. How this air is proportioned, depends on which fuel is being burnt, the number of elevations in service and the firing rate at each elevation. The function of Secondary Air Damper Control System (SADC) is to proportion the total secondary air around the firing elevation as per the criteria spelt above. In fact the SADC system is a combination of both analog and digital system, as the amount of fuel fired and confidentia the no. of firing elevations decide the damper 47 of 27

The auxiliary air dampers are positioned to maintain the required wind box to furnace differential pressure according to the boiler load. In the initial periods of operation, the flame condition with reference to ignition stability, ignition point relative to fuel nozzles, overall combustion conditions in the furnace are to be studied for various wind box to furnace differential pressures and the values are to be optimized at site (the values are adjustable). Based on this, the final curve showing the relationship between the wind box to furnace differential pressure versus the boiler load is to be decided for the type of coal used. 48 of 27

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The auxiliary elevation dampers act as fuel (oil) air dampers when oil firing is taking place and is open to a preset position. The fuel (oil) elevation dampers are selected to be closed if there is a back up trip in that elevation and adjacent pulverizers are off and “No Boiler Trip” signal is present. When a HFO Elevation is started (i.e. when the associated “elevation back-up” trip is removed) the associated auxiliary air dampers in that elevation are set to preset position for HFO firing.

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Above 30% MCR boiler load, the auxiliary air elevation associated with the main fuel elevation in service modulate to maintain the varying DP. Those not associated with any elevation in service are closed from top to bottom. The closing signal comes from FSSS. When the unit load is increased above 30% MCR, the Aux. air dampers of elev. AA,AB, BC, CD, DE, EF & FF are closed in a timed sequence (10 sec. interval) provided the associated coal elevations are not in service and the associated oil elevations are not in service. The auxiliary air dampers are closed, starting from the top elevation, progressing towards the lowest elevation. 50 of 27

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   

  

All elevations of the aux. air dampers will be automatically switched to “Manual” control and then commanded to open 100%, if any one of the following conditions exist. Both FD fans are OFF. Both ID fans are OFF. Master fuel trip occurs. Loss of unit critical power . (+24V DC) Also the auxiliary air control loop is tripped to manual if any of the following conditions exist. DP transmitter deviation high. Failure of selected DP transmitter Furnace to Windbox DP very high (> 240mm WCL). 51 of 27

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The fuel air damper control system positions the fuel air damper in proportion to the fuel firing rate. In the initial periods of operation, the flame condition with reference to ignition stability, ignition point relative to fuel nozzles, overall combustion conditions in the furnace are to be studied for various fuel air damper positions and the values are to be optimized at site. Based on this the final curve showing the relationship between the fuel air damper positions versus the fuel feed rate is to be decided for the type of coal used. 52 of 27

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All elevations of fuel air dampers are commanded to open 100% when any one of the following conditions exist.  Both FD fans are OFF.  Both ID fans are OFF.  Master fuel trip occurs.  Loss of unit Critical Power.(+24 VDC) 53 of 27

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On coal fired boilers the characteristics of fuel like, Fixed Carbon, Volatile Matter and Ash composition will determine the optimum wind box to furnace differential pressures and the fuel air damper positions. Accordingly the optimized settings are to be decided as above for the coal used. 54 of 27

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Later, if the coal quality changes or the coal source itself is changed, it shall be necessary to once again carryout optimization studies for the new coal and revised settings are to be finalized.

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Modulation of Auxiliary Air Dampers

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Modulation of Fuel Air Dampers

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Modulation of Over Firing Air Dampers

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The two elevations of the over fire air dampers will be automatically switched to “Manual” control and then commanded to open 100%, if any one of the following conditions exist     

Master fuel trip occurs. Loss of unit critical power .(+24 VDC) Both FD fans are OFF. Both ID fans are OFF. WB To Furnace DP exceeds 240mm WCL.

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Thank you 12th July, 2008