Furnace Safeguard Supervisory System

K.C. Yadav, AVP,

Noida Technical Training Centre

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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.

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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.

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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.

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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.

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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

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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

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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

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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.

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HOTV InterlocksTo 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 InterlocksTo 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

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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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s

s

ss

s

s

HFO PUMP-1

HFO PUMP-2

HFO PUMP-3

HFO HTR-1

HFO HTR-2

HFO HTR-3

FROM HFO DAY TANK

FROM AUXILIARY STEAM HEADERHFO Supply SchemeHFO Supply Scheme

TO BOILERFRONT

TO HFO DAY TANK

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M

FT

FT

AB ELEVATION

CD ELEVATION

EF ELEVATION

HOFCVHOTV

HORV

SHORTRECIRCULATION

HFO SUPPLY

LINE

HFO RETURN

LINE

HFO Scheme At Boiler FrontHFO Scheme At Boiler Front

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OIL GUN

ATOMISINGSTEAM VALVE

HFO NOZZLEVALVE SCAVENGE

VALVE

AB ELEVATION

CD ELEVATION

EF ELEVATION

Oil Gun ConnectionOil Gun Connection

Atomizing Steam Scheme

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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.

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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.

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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.

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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 placeA 30 second counting period is startedA 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.

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Pair Firing Mode (Shut Down) 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.

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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

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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)

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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.

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Boiler TripWhen 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

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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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Loss of all fuel armingSET

RESETMFT 5 SEC

Any elevation¾ Nozzle valve proven

All feeders off 2 SEC

All HFO Nozzle valve closed

All feeders off

HOTV NOT OPEN

2 SEC

All HFO Elevation Trip

Loss of all fuel trip

Loss of All Fuel Trip

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Flame Failure Trip

Elevation AB No Flame Vote

Elevation BC No Flame Vote

Elevation DE No Flame Vote

Elevation FG No Flame Vote

Elevation GH No Flame Vote

Any Feeder Proven 2 Sec

Flame Failure

Trip

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Flame Failure Vote Logics

Feeder A Off2 Sec

Feeder B Off

Elevation AB Any HFO Nozzle Valve Not Closed

Elevation AB 2/4 Nozzle Valve Not Proven

Elevation AB Scanners No Flame

Feeder C Off

2 Sec

2 Sec

Elevation BC Scanners No Flame

Elevation AB No Flame Vote

Elevation BC No Flame Vote

Elevation DE No Flame VoteFeeder D Off

Feeder E Off

Elevation DE Scanners No Flame

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FEEDER H OFF

ELEVATION GH ANY HFO NOZZLE VALVE NOT CLOSED

ELEVATION GH 2/4 NOZZLE VALVE NOT PROVEN

2 SEC

ELEVATION GH NO FLAME VOTE

ELEVATION FG NO FLAME VOTE

Flame Failure Vote Logics

FEEDER F OFF

FEEDER G OFF

ELEVATION FG SCANNERS NO FLAME

ELEVATION GH SCANNERS NO FLAME

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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

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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

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Mill InterlocksP.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 InterlocksFor 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)

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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.

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Pulverizer Seal Air Valves Interlocks Valve opens when Pulverizer start command is given

OR Mill is 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.

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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

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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’

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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

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SADCThe 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 the no. of firing elevations decide the damper position.

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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.

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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.

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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).

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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.

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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)

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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.

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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