Furnace Safeguard Supervisory System (FSSS)

Introduction :-

The Furnace Safeguard Supervisory System (FSSS) is designed for 

• To assure the execution of a safe, orderly operating sequence in the start up & shut down of fuel firing equipment.
• To prevent errors of omission and commission in following such a safe operating procedure.
• The system provides protection, should there be a malfunction of fuel firing equipment and associated air systems.
• The safety features of the system are designed for protection in most common emergency situations.

Basic Functions of FSSS:-

1. Prevent any fuel firing unless a satisfactory furnace purge sequence has first been completed. 
2. Prevent start-up of individual fuel firing equipment unless certain permissive interlocks have first been satisfied. 
3. Monitor and control the proper component sequence (manual or automatic) during start-up and shutdown of fuel firing equipment.
4. Make continued operation of fuel firing equipment subject to certain safety interlocks remaining satisfied. 
5. Provide components status feedback to the operator. 
6. Provide flame supervision when fuel-firing equipment is in service and initiate fuel trip upon certain adverse operating conditions. 
7. Provide master fuel trip when certain adverse operating conditions exists.

• FSSS is provided for a natural circulation, HO fired boiler with coal as primary fuel and light oil (LO) as the start-up fuel.
• Heavy oil (HO) is used as fuel for warm up and stabilization.
• The system provides three elevations (AB, CD, EF) of high-energy arc (HEA) igniters, oil guns with atomizing steam in all four corners.
• Only AB elevation has LO firing guns with atomizing air.
• CD & EF elevation
• This boiler has six elevations (B,B,C,D,E,F) of coal firing with 24 coal nozzles.
• Mill system includes bowl mill, seal air fan, primary air fan, coal feeder, hot air gate, mill inlet / outlet gates hot /cold PA control dampers.
• Discriminating scanners are provided for sensing flame of both coal and oil.

Boiler Trip Conditions : 

1. Both ID fans OFF.
2. Both FD fans OFF.
3. Total Air flow (secondary + primary) less than 30% of MCR.
4. Furnace pressure very high (+150 mmwc).
5. Furnace pressure very low (-175mmwc). 
6. Drum level very high (+225mm) 10secs delay.
7. Drum level very low (-225mm) 5 secs delay.
8. Loss of all fuel trip.* 
• {*Loss of fuel trip generates when all coal feeders OFF and (all LONV closed or LOTV closed) and (all HONV closed or HOTV closed). Logic is armed from any oil nozzle valve not open.
9.  Loss of reheat protection 10 secs delay. **
• (**Loss of Reheat protection generates when any coal feeder proven and generator Circuit Breaker open and (any HPBP or LPBP valve < 2%). When turbine is in service either of the  HPCV or HPBP and either of the IPCV or LPBP both valves should be open > 2% and any feeder proven.)
10. Loss of 220V dc > 2secs. 
11. Loss of unit critical supply (24V dc). 
12. Loss of 110V AC > 2secs & loss of fireball. *** 
• {***Loss of 110vAC & loss of  fireball happens when all feeders OFF and 110vAC fails in an oil elevation in service or in a coal elevation with mill ON condition.}
13. Generator class-A trip. 
14. Flame failure trip. **** 
• ****Flame failure logic exists in coal firing condition only. Coal elevation not in service votes for flame failure. If running coal elevation monitoring fireball scanner not showing 2/4 flame and is not supported by ¾ oil guns then it will also vote. If any feeder running and all coal elevation vote for flame failure then flame failure logic acts and boiler trips on flame failure.)
15. Both emergency push buttons pressed. 

Purge Permissive :

1. No boiler trip condition persists.
2. All HAGs closed.
3. All PA fans OFF.
4. All feeders OFF. 
5. All HONV closed.
6. All LONV closed. 
7. HOTV closed.
8. LOTV closed. 
9. All scanners sensing no flame. 
10. Aux air dampers modulating. 
11. Air flow > 30% & < 40% MCR. 

If all these conditions are fulfilled then ‘PUSH TO PURGE’ light glows. Press ‘PURGE START’ button. 300 secs purge timer starts. Boiler trip circuit resets after purge complete provided no permissive missed in that period of 300 secs.

Selection Of Light Oil / Heavy Oil Firing : 

Oil elevation AB is provided with HO / LO firing. Permissive for changeover are follows: 

1. Both pairs of oil gun stopped.
2. All HO/LO nozzle valves closed.
3. HO/LO start permit available. 

LOTV LOGICS: 

If MFR reset and LO header pressure adequate then LOTV can be operated. 

Protections (OR logics):-

1. MFT (2/3).
2. Light oil header pressure very low. 
3. Atomizing air header pressure very low. 
4. NO light oil elevation in start or stop. 
5. LOTV is commanded to close by hard wire circuit on loss of +24V DC. 

HOTV LOGICS : 

Once MFR resets and all HONV close and HORV open and HO header pressure and temperature is adequate then HOTV can be open / close. 

Protections: 

1. MFT.
2. HONV not closed and atomizing steam pr very low and no oil elevation in start / stop > 2 secs.
3. HONV not closed and HO header pressure very low >2 secs.
4. HO header temp very low and any HONV nozzle valve not closed >2 secs and HORV valve not open.

LIGHT OIL ELEVATION START:

Select light oil firing. Then check following conditions: 

1. No master fuel trip. 
2. LOTV open.
3. Pulverizer / feeder start permit available. i.e. nozzle tilt horizontal and air flow >30% and <40% or any feeder ON 

PAIR MODE/ELEVATION MODE LOGIC:- 

For both LO and HO oil guns are put into service by sequence control logic.

Pair Mode:-

Pressing pair start (1-3 or 2-4) PB, provided the above conditions are satisfied will initiate a 70 secs start time to place the associated light oil guns in service. Second oil gun gets start command after 15 secs. 

Elevation Mode:-

When associated coal feeder is in service, oil guns acts under elevation mode. I.e. all four guns put in or out by single command. 

When any of the pair ON PB pressed two things happens a) Elevation start command is issued for corner 1&3 b) A 40 secs counting period is started to established an elevation start command for corner 2&4. After 70 secs if ¾ nozzle valves are not proven then “unsuccessful elevation start” alarm is initiated. This will stop corner 1&3 and after 40 secs stop corner 2&4. 

This pair mode & elevation mode logic is same for both HO & LO elevations. 

CORNER TRIP LOGIC (HO):-

1. Oil gun not engaged.
2. Lo selected .
3. Ho man isolation valve not open.
4. Local maintenance switch.
5. Steam atomizing man isolation valve not open.
6. Corner start time / scavenge time expired & (corner no flame or HONV not open or steam valve not open).

Similar logic will be there for light oil guns. 

START-UP LOGIC: 

STEP 1: Open steam / air atomizing valve 

STEP 2: If atomizing valve opens then advance HEA igniter 

STEP 3. If HEA rod advanced then open HONV / LONV and spark HEA igniter for 15 secs 

STEP 4: If HEA rod retracted and oil valve open in condition and disc scanner shows Flame then end step sequence.  

If flame does not stable within 15 secs from oil valve open then corner trip Initiated. All the valves of that corner closed and HEA igniter withdrawn. 

SHUT-DOWN LOGIC: 

STEP 1: Close HONV/LONV.

STEP 2: After oil valve closes, open steam / air atomizing valve.

STEP 3: After atomizing valve opens, HEA advances & scavenge valve opens. After advancement of HEA sparks for 15 secs.

STEP 4: After scavenge time of 300 secs atomizing and scavenge valve close. 

HORV OPERATION:-

Any HONV not close condition is the protection close for HORV.

All HONV close is Open permissive.

MILL LOGICS:- 

Seal Air Fan:

    MFT and electrical trip are the protections for this drive. 

    This can be run when no protection present. 

    SAF outlet damper can be opened by operator after 40 sec of SAF ON.

    Seal air to Pulverizer inlet damper is automatically open when SAF ON and SAF outlet damper open. 

    It closes by Pulverizer trip command (30secs pulse ). 

HAG Logics:-

    Mill outlet temp >110o C for >=30 secs and MFT are the protection close. 

    When no protection is present it can be opened and if PA fan OFF then it can be closed also. 

Mill Discharge Gate:-

    This can be opened any time by open command. 

    But it can be closed when PA fan OFF & PA general shut off damper closed. 

Mill Lube Oil Pump:-

    When one pump running and lube oil pressure low or flow low occurs then 2nd pump will takes start. 

When Mill running and running pump for any reason gets OFF, 2nd pump will take start. 

Mill Inlet Gate:-

    This can be open any time by open command. Off permissive is Feeder OFF. 

PA General Shut Off Damper:-

    If PA fan ON and mill discharge valve open then it can be opened. Pa fan OFF closes this damper automatically. 

PA Fan Permissives:-

1. No MFT 
2. PA inlet damper in min position 
3. PA general s/o damper closed 
4. Ignition permit available * 
• {*Ignition permit available when adjacent oil elevation has 3/4 oil gun in service or adjacent coal feeder has feeding >50% (18 tph). }
5. Coal elevation start permit available ** 
• {**One running FD fan can give start permit to 1st three coal elevation. 2nd FD fan is required for 4th coal elevation.}
6. PA lube oil system o.k. 
7. Cold air damper <5% 
8. Mill release available *** 
• {*** Mill release : Available when PA fan ON or (CAD<5% and PA general shut off damper close & burner all manual isolation valves open and feeder inlet / outlet gate open and HAG, CAG open and mill lube oil o.k. and mill gas pressure not low or not very low.}
9. No pulverizer trip 
10. Seal air system o.k **** 
• {**** Seal air system O.K. will be available when seal air fan ON & SAF o/l damper open and SA to pulverizer inlet damper open and seal air to hot PA DP >100 mmwc.}
11. PA breaker in service  

PA Fan Protections : 

1. Pulverizer trip 
2.  2/3 MFT 
3. PA fan electrical trip
4. PA fan lube oil system failed (lube oil pressure 0.4 kgf cm-2 < more than 60 secs) 
5. Loss of FD fan trip (for D, E, F elevation) 

Mill Permissives:-

1. Mill release available.
2. PA fan ON >20 secs.
3. PA general s/o damper open.
4. Ignition permit available.
5. Coal elevation start permit available.
6. Pulverizer / feeder start permit available.
7. Pulverizer outlet temp not low (>65degC).
8. Mill both discharge valves open.
9. Pulverizer breaker in service.

Pulverizer / feeder start permit:-

Burner tilting horizontal and total air flow < 40% or any feeder running > 53 secs. 

Pulv Trip Logics:-

 1.Pulv ON and (seal air to hot PA DP low (<100mmwc) >70 secs or seal air fan OFF >60 secs). 

2. 2/3 MFT 

4. MILL ON and PA fan OFF 

5. Mill outlet temp>110 o C and feeder ON 6. Mill release not available 

7. Electrical trip 

8. Any mill discharge valve not open>3 secs 

9. Ignition permit not available and Ignition permit required. 

10. Mill gas pressure very low 

11. Mill lub oil pressure very low>3 secs or (both lube oil pump OFF or lube oil flow low)>5 secs. 

Feeder Permissives:-

1. Coal on belt 

2. Feeder in remote 

3. Pulverizer / feeder start permit

4. Ignition permit 

5. Coal elevation start permit 

6. Mill outlet temp not high and not low.

Feeder protection:-

1. Ignition permit not available and Ignition permit required 

2. Mill outlet temp high 

3. 2/3 MFT 

4. No coal on belt 

5. Feeder discharge plugged 

6. Pulverizer trip 

“Ignition permit required” signal is generated when feeder status (ON / OFF) changes and it lusts for 3 minutes. 

Flame scanner: 

We have 5 elevations (AB, BC, CD, DE, EF) of optical scanner, which can convert flame frequency and amplitude to a current signal. 

Coal and oil flame is discriminated by different flame frequency i.e. coal flame frequency 3.5 to 10 Hz and oil flame frequency 24 to 30 Hz. 

It thresholds the amplitude and gives flame / no flame switching signal to FSSS. 

Secondary Air Damper Control: 

Coal air damper:- A, B, C, D, E, F modulates as a function of coal feeding in auto mode. 

Oil air damper:- AB, CD, EF Opens 70% when oil gun in service. Else acts as aux air damper. 

Aux air damper:- BC, DE Modulates in auto to maintain wind box to furnace DP. DP set point is a function of boiler load. 

Over fire dampers:- Modulates in auto as a function of boiler load to prevent SOx, NOx. When MFT operates, all these dampers are forced to 100% open and stays there for 5 mins.

Scanner Fan:- 

Two scanner air fans (AC, DC) are provided. Normally AC fan is running if it trips or scanner duct to furnace DP low (150mmwc) comes then DC fan takes start. Scanner fan normally takes suction from FD fan discharge and supply air to the flame scanner duct. One emergency damper is provided in the suction. In case of both FD fan OFF, this damper will provide alternate suction. 

Mill Fire Fighting Inerting System:- 

After mill trip, fire fighting inerting command can be given. It closes Mill inlet / outlet gates, general s/o damper, seal air to mill inlet damper. After that all the steam valves to mill open for 30 secs. After 30 secs steam valve closes and cycle stops. 

***