FSSS (Furnace Safeguard Supervisory System)

• A fully digital control system which governs mandatory requirements related to safe startup and safe shutdown of the boiler.
• Includes main boiler permissive and protections.
• Includes heavy oil and light oil firing interlocks.
• Includes permissive & protections related to mills and feeders.

FSSS LOGIC SYSTEM

• Furnace Safeguard Supervisory System (also named Burner Management System; BMS).
• Satisfactory Boiler start up.
• Startup of individual oil system AB/CD/EF.
• Operation of fuel firing subject to certain conditions A/B/C/D/F.
• Protection and interlock of oil /Coal System.

BOILER START- UP SEQUENCE

The boiler can be placed in service, initially or after a “Master Fuel Trip,” in the following sequence through FSSS.

• Furnace Purge:- Before any fuel firing can takes place, initially or following a boiler trip, a satisfactory “Purge Cycle” must be completed.
• Initially, the Heavy Oil must be recirculated and heated to obtain the proper temperature and viscosity for efficient combustion. After this is accomplished, the Heavy Oil Trip Valve (HOTV) can be closed and then the ‘furnace purge cycle’ can be started.
• After the furnace purge cycle is completed, the heavy oil elevation (AB, CD, EF, & GH) can be taken into service. An oil elevation is said to be “In Service,” when at least three out of four heavy oil nozzle valves are open and the flame is sensed by the scanners.
• After the heavy oil elevation is ‘in service’, either of the adjacent coal elevation can be taken into service because “Ignition Permit” is now available for starting the pulverizer.
• A Coal elevation is said to be “In Service” after the Pulverizer is ON, its Hot Air Gate (HAG) is open and the associated Feeder is proven (i.e., feeder ON for more than 50 seconds). A “fireball” signal is established when the associated fireball flame scanner senses flame and “ELEVATION FLAME” indicator is illuminated on the FSSS console in control room.

UNIT START UP SEQUENCE

1. 1. Boiler purge operation
   2. Startup of oil firing(AB,CD,FH & GH)
   3. Adequate Ignition permit
   4. Firing of coal elevation(A,B,C,D,E,F,G & H)
   5. Flame sensing by scanners.

FSSS LOGIC

FSSS logics can be broadly divided into 3 sections.

1. 1. Unit Logic.
   2. Oil Elevation Logic.
   3. Coal Elevation Logic

UNIT LOGIC

1. 1. Boiler purge logic
   2. Master fuel trip logic
   3. Oil trip valve operation logic
   4. PA Fans logic
   5. Scanner air Fans logic
   6. Seal air Fan logic

FURNACE PURGE

Purpose: – Complete Removal of unburnt fuel/residues from entire furnace. Before every start or say restart, a furnace purge is required to be performed.

• Furnace purging refers to the clearing of the entire flue gas path for any combustibles that are leftover in the furnace.
• This clearing function is accomplished by ambient air.
• This is a mandatory safety requirement before ignition of fuel and is an interlock in boiler startup.
• When the boiler trips, the last bit of fuel that made entry in the furnace remains inside.
• There is a gaseous mixture left in the furnace. When an attempt is made to re-ignite the furnace, there is a reaction with the leftover gases, which could possibly result in furnace explosion.
• It applies during startup of the boiler, when there are sometimes multiple attempts required to start.
• Purging is a requirement as warranted by the NFPA (National fire protection agency) norms.
• The time required for purging is determined by the volume of the furnace and requires 5-7 cycles to displace the combustibles out of the boiler.

Procedure:-

• Ensure that all fuel to furnace is cut off and all flame is out.
• Provide adequate airflow through furnace (30 to 40%).
• Initiate a MANDATORY purge time cycle (5, 8, or 10 minutes).
• A 5 or 8 or 10 minutes mandatory cycles in which at least one set of FD fan & ID fan are running with 30 to 40 % air flow with no fuel input to the furnace, to ensure the removal of the un-burnt fuel left over in the furnace area after the last master fuel trip.

Consequence:-

• Master Fuel Trip Relays are reset.
• Firing of fuel into the furnace is allowed.

UNIT LOGIC

• Boiler purge logic, which ensures a five minutes purge cycle is completed before any fuel firing can take place.
• A “Purge Ready” signal at control room will be available when all the following conditions are satisfied:

Boiler Purge Logic

• No MFT condition present / No boiler trip.
• HOTV closed (HFO trip valve is closed)
• LOTV closed
• All HFO Nozzle Valves are closed (all HONV closed)
• All LO Nozzle Valves are closed (All LONV closed)
• All Pulverizers are off
• All feeders are off
• All scanners sensing no flame.
• Both PA fans off
• All HAGs closed
• Air flow more than 30% and less than 40%
• All auxiliary air dampers modulating
• Wind box to furnace DP is adequate.
• MFT DPUS unfreeze

PURGE PERMISSIVE

MASTER FUEL TRIP (MFT)

• • MFT or Boiler Trip means:
    • • Complete isolation of all fuel to furnace under certain conditions.
  • Automatic Procedure:
    • • All oil valves are given closing command.
      • All mills and feeders are given trip command.
      • All Primary Air (PA) Fans are given trip command.
      • All Hot Air Gates (to mills) are given closing command.
  • Further Consequences:
    • • Turbine protection is initiated (to trip turbine).
      • All SH and RH spray valves are given closing command.
      • All elevation SADC are given full open command.
      • Furnace pressure & Airflow controls are set to manual.

MASTER FUEL TRIP LOGIC

• This ensures a safe shut down of the boiler in case of any abnormal conditions in boiler/ furnace.
• If some abnormal condition occurs, Master Fuel Trip Relay acts and ensures the closure of Heavy Oil Trip valve (HOTV) and all Oil Nozzle Valves (ONV), thus cutting off total oil supply.
• It also ensures the stopping of all pulverizer systems and Primary Air (PA) fans, cutting off total coal supply to boiler.
• The various abnormal operating conditions under which Master Fuel Trip occurs are as follows:-
  • 1. Both FD fans OFF
    2. Both ID fans OFF.
    3. Inadequate water wall circulation for more than 5 seconds.
    4. Loss of Reheat Protection for greater than 10 seconds.
    5. All BFPs tripped.
    6. Drum level very high for more than 10 seconds (2/3 logic)
    7. Drum level very low for more than 10 seconds (2/3 logic)
    8. Furnace pressure very high (2/3 logic)
    9. Furnace pressure very low (2/3 logic)
    10. Loss of all fuel.
    11. All feeders OFF and loss of AC Supply to the elevation in service.
    12. Unit flame failure.
    13. Loss of Unit Critical Power for more than 2 seconds
    14. Both Secondary APHs OFF.
    15. Loss of 220 V DC Supply for more than 2 seconds.
    16. Air flow less than 30% and boiler load not greater than 30%.
    17. Manual Emergency trip.

Unit Logic MFT consequences

• Trips all mills.
• Trips all feeders.
• Trips PA fans.
• Closes HFO trip valve.
• Closes all elevation nozzle  valves.
• Close all HAGs.

OIL TRIP VALVE OPERATION LOGIC

• This logic ensures that oil trip valve can be opened after the purge cycle is completed and no MFT condition is present, if the following conditions are also satisfied.
• All nozzle valves are closed.
• HFO supply pressure is satisfactory.
• HFO header temperature is satisfactory.
• Similarly, this logic ensures the closure of oil trip valve in case of MFT.

SCANNER AIR FANS LOGIC

• It monitors the scanner air duct to furnace DP and ensures starting of scanner air fan – A or in case it fails to start, scanner air fan – B. This logic gives an open command to scanner fan outlet damper after starting the fan.
• Also, in case of both FD fans not running, it ensures the scanner air emergency damper is open.

SEAL AIR FANS LOGIC

• Monitors seal air header to cold air duct DP and ensures the starting of seal air fan – A or B (in case A does not start), provided at least one PA fan is running.
• Gives an open command to seal air fan discharge damper once fan starts.

OIL ELEVATION LOGIC

Start of oil elevations AB, CD, EF and GH

• Sequential operation with start command.
• Corner-1,corner-3,corner-2 and corner-4.
• Gun advance, igniter advance.
• Spark, atomizing valve open, oil valve open.
• Sensing of discriminatory flame.
• Retract of igniter.

Stop of oil elevation

• Closure of oil valve, opening of scavenge valve.
• Igniter advance and sparking.
• Closure of atomizing valve and scavenge valve.
• Retract of igniter and gun.

Oil Elevation Logic

• There are four heavy oil elevations AB, CD, EF & GH.
• For all these elevations, logics are same.
• Oil elevation can be placed in service auto/manual.
• For a particular elevation, only one command is given for all the four corners together. (E.g. One common start command for elevation AB-1, 2, 3 and 4 guns).
• However, start command goes to corners 1 to 4 sequentially. First command goes to corner 1; after 15 seconds start commend goes to corner 3; again after a lapse of 15 seconds start commend goes to corner 2; and again after a gap of 15 seconds start command goes to corner 4.
• A particular corner takes the start command only if the following conditions are satisfied:-
  • The particular corner oil gun engaged feedback is available.
  • The steam scavenge valve is closed for the particular corner.

Sequence of operation

The sequence of operation taking place for a particular oil gun once the command is given is as follows:-

• First, the oil gun is advanced.
• Once gun advanced feedback is available the HEA igniter is advanced.
• Then atomizing steam valve is opened for the particular corner.
• Then HFO nozzle valve gets open command.
• ‘Spark’ command is given to igniter transformer, thus producing a spark at the spark tip.
• Then for this particular corner ‘HFO flow satisfactory’ feedback comes from the flow switch in the oil line.
• The corner oil gun proves when the DISCR FLAME SCANNER senses FLAME.
• When 25 seconds elapses after oil valve open, igniter spark signal is removed and spark rod is retracted.
• If the corner oil gun did not prove FLAME by this time, ‘Oil nozzle valve’ and atomizing steam valve closes. Gun and igniter are retracted.

OIL FIRING PERMISSIVE-

• Purge complete/MFT Reset and NO FIRST UP FAULT.
• HFO Pr. Ok >25 ksc.
• HFO Temp ok >95c
• Ignitor air pr. >200 mmwg.
• Ignitor oil pr. >10 ksc.
• No light up in progress.
• No burner start inhibit.
• Permissive to fire oil.

OIL BURNER START LOGIC

• Oil burner start push button (no other oil burner start in progress).

OIL BURNER START SEQUENCE

• Oil burner sec. Air damper opened.
• Open igniter oil master trip valve.
• No flame sensed in last 5 sec.
• Energies igniter transformer.
• Open igniter oil solenoid v/v.
• Detect ignition oil flame.
• 4 way v/v to firing position.
• Oil flame detected.
• Close IOTV/SOL V/V

COAL ELEVATION LOGIC

PULVERIZER START LOGIC

• Availability of PA permit
• Availability of ignition permit
• Seal air /bowl differential pressure
• No MFT
• Lube oil pressure adequate
• Pulverizer stop logic

FEEDER START LOGIC

• Pulverizer on
• Outlet temperature adequate
• Speed demand minimum
• No MFT
• Feeder stop logic
• Had/cad logic

PULVERISER LOGIC

• This logic takes the count of total number of pulverisers in service at any point of time, the number of FD Fans & PA fans in service.
• With PA Fan ‘ON’ and PA Header Pressure adequate (>560 mmwcl), it gives “PA permit” to start the Pulverizer.
• In case of PA Header Pressure going low-low or PA/ FD fans trips, it ensures the shut down of Pulverizers from top elevation to bottom elevation in a timed sequence.
• Each mill can be taken in to service manually or automatically.
• Auto manual selection push button is available on FSSS console for each mill.
• Initially, “Pulverizer Ignition Permit” signal must be established for the mill to start.
• Other condition required to be established for starting the mill are as follows:-

MILL PERMISSIVE

• Mill lub oil pressure adequate.
• Start permit from FSSS.
• Mill Discharge valve open.
• Mill outlet temp. > 93^O(200^OF)
• Cold Air Gate (CAG) open.
• Tramp iron vale (Top Reject Gate) open.
• Feeder inlet gate open.
• Seal air DP adequate (200 mmwcl).
• Feeder local switch in remote.

Once the mill is ON, associated HAG can be opened and feeder can be placed in service.

OIL BURNER FIRING ON MILL BEING BROUGHT INTO SERVICE

For Any Mills (A,B,C,D,E,F,G,H)

1. At least three oil burners firing in selected elevation.
2. Boiler load > 30% BMCR & upper or lower adjacent coal burner in operation.

EXAMPLE:

For MILL-A START PERMISSIVE:

Elevation AB-3/4 oil burner firing.

OR

MILL-B firing.

AND

BOILER LOAD >30% BCMR.

FOR B, D, F MILLS

At least 3 OIL BURNERS firing in the burner group above

AND

Adjacent COAL BURNER above in operation

AND

This elevation is highest in the burner group

EXAMPLE

For MILL-B START PERMISSIVE

Elevation AB-3/4 OIL burner are in service

OR

MILL-A firing OR Mill-C Firing

AND

BOILER LOAD > 30% of BCMR

OR

MILL-C firing

AND

ELEVATION CD-3/4 oil burner are  in service

• Note that B-ELEVATION is highest in the burner group AB.

PARTIAL FLAME FAILURE

It will never occur for the following condition-

• If three or more mills firing then partial flame failure will never exists.
• If less than three mills are firing then partial flame failure does not exists if mill, which is firing, have the mill permissive for firing.

It will occur when this logic satisfy-  

VOTING OF ELEVATION

Voting in AB elevation will be available

Case-1

Mill A feeder OR mill B feeder is in service.

AND

Elevation AB 2 / 4 flames in.

Interlocked with (all burner in AB elevation satisfactory S/D or 3 / 4 oil burner in that elevation are is in service).

Case-2

Voting in BC elevation will be available.

Mill B feeder or mill-C feeder is in service.

AND

Elevation B 2 / 4 flames in.

VOTING WIL NOT BE AVAILABLE –
(1) Both feeders on burner elevation are not is in service.
                                          OR
(2) 2/4 flame are not sensed interlocked with 3 / 4 oil   burner  are not firing.
                                           OR
(3) oil gun in that elevation  not satisfactorily shutdown interlocked with 3 / 4 oil burner in that   elevation  not firing.  

FEEDER START LOGIC

Feeder start command is given from FSSS Elevation Logic only. This command is continuously required to keep the feeder in service. Feeder speed demand minimum, Pulverizer ON, pulverizer outlet temp > 130^OF, HAG open and no MFT are the important conditions required to give the feeder start command from FSSS.

In case of pulveriser power high, bowl DP High or feeder OFF command, feeder speed demand is driven to minimum.

Release feeder to Auto Signal is also coming from FSSS. All of the above conditions are present and feeder is not ON for more than 50 seconds, release signal for auto is available.