Power Plant Boiler

1. Boiler P & I
Boiler protections:
Boiler will trip if any of the conditions are met

2. Both FD fans OFF
Both ID fans OFF
MFT
RELAY
Air flow less than 30% and boiler load not greater than 30%
Loss of 220 V DC Supply for more than 2 seconds
Manual Emergency PB trip
Both Secondary APHs OFF
Loss of Unit Critical Power for more than 2 seconds
Unit flame failure
All feeders OFF and loss of AC Supply to the elevation in service
Loss of all fuel
Furnace pressure very low (2/3 logic)
Furnace pressure very high (2/3 logic)
Drum level very low for more than 10 seconds (2/3 logic)
Drum level very high for more than 10 seconds (2/3 logic)
All BFPs tripped
Loss of Reheat Protection for greater than 10 seconds
Inadequate water wall circulation for more than 5 seconds

3. ACTION TO BE INITIATED ON UNIT SHUT
DOWN
- Stop all fuel by tripping feeders/ mills in service, close
HOTV, and trip PA fans.
- To trip the turbine and generator
- To close SH and RH spray isolating valves with a time delay
of 0-3 minutes
- To bring burner tilt mechanism to horizontal position
- To trip one BFP if two BFP’s are in operation

4. Purge permissive
Furnace purging can be started by pressing
‘Purge start’ PB after ensuring that AC and DC
supplies to FSSS is ON, Drum level is
adequate, one ID fan and one FD fan have
started and atleast 30% air flow is established
and provided following permissives have
been satisfied:

5. PURGE PERMISSIVES
TO
FSSS
All HFO nozzle valves closed
All pulverisers OFF
All feeders OFF
No MFT condition present
All scanners senses no flame
All Auxiliary Air dampers modulating
Both PA fans OFF
All Mill HAGs closed
Air flow more than 30% and less than 50%
HFO trip valve is closed
Wind box to furnace DP is adequate

6. Purge ready indication will glow if above
permissives are fulfilled. On pressing ‘purge start’
PB furnace is purged for 5 min. After that purge
complete indication will come if none of the
boiler trip condition exists.
After the purge complete indication comes
operator can open the HOTV.

7. HOTV
-Press open PB provided following conditions are satisfied:
HO pressure is adequate,
All HONV’s are closed,
No boiler trip condition exist,
No valve close command is there,
Purge complete.
-This valve will close automatically if any of the following
condition occurs:
Boiler trip occurs, HO header pr is low for more than 2 secs
when any HONV is not closed, HO temp low, DP between HO
and atomising steam is low.
-HO recirculation valve can be opened provided HOTV and all
HONV are closed. This valve will close automatically when any
of the HONV’s is not closed.

8. INTERLOCKS FOR MILLS
Pulveriser –A
A) Pulveriser A motor shall be prohibited from starting
unless all the following conditions are fulfilled:
All discharge valves are open, Seal air valve is open, cold
air damper is open, tramp iron hopper valve is open, PA
header pr is adequate, DC power to FSSS is available,
burner tilt is in horizontal position and air flow adjusted
between 30- 40% if no feeder is ON., mill O/L temp is less
than 90 degree C, either ¾ HONV’s on elevation AB are
open or mill B is working at more than 50% load and
boiler load is greater than 30%.
If all conditions are met ‘mill A ready light for mill A’ will
come up.

9. Mill - A
B) When mill A is started, command will be given for the
following:
Permission to open HAG, permission to start R C Feeder
–A, permission for controllers impulse to act on HAD
C) Mill A shall trip under the following conditions:
Boiler trip, Loss of D C supply, Discharge valve are not
open, PA header pr less than 450 mmwcl or when one
PA fan is tripped and four or more mills are in service, if
the permissive given above disappear before 180 secs
from feeder start or less than ¾ associated oil guns are
in service.

10. Mill - A
D) When mill A is tripped command will be given for
following:
To trip R C Feeder- A, close HAG, close HAD, to permit
closing of MDV’s, to bring CAD to minimum.
E) HAG is prohibited from opening unless mill is ON.
F) If HAG is open, impulses shall be given to release HAD
and CAD to regulate provided mill is still ON and there is
no closing command to HAG.
G) HAG shall close automatically after 30 sec if any of the
following conditions are met:
Mill is tripped, mill O/L temp is high, feeder stops iwthin
5 sec of its starting

11. Mill - A
H) Mill air and temp control loop shall be released to auto
only after R C feeder speed is more than 50%.
MILL – B
- A) Mill B starting requirements are same as for Mill A
except for the last condition which should be read as:
either a min. of ¾ HONV’s are open in elevation AB or
boiler loading is more than 30% and either mill A or
mill C is working at 50% loading.
- B) Similar to mill A
- C) Similar to mill A except for if PA hdr pr is low or both
PA fans trip or one of the operating PA fans trip when 4
or more mills are in service and mill A is OFF.
- D) Similar to mill A.

12. Mill - C
- A) Mill C starting requirements are same as for Mill
A except for the last condition which should be read
as: either a min. of ¾ HONV’s are open in elevation
CD or boiler loading is more than 30% and either
mill B or mill D is working at 50% loading, OR a min.
of ¾ HONV’s are open in elevation AB and mill B is
loaded for more than 50%.
- B) Similar to mill A
- C) Similar to mill A except for if PA hdr pr is low or
both PA fans trip or one of the operating PA fans trip
when 4 or more mills are in service and mill A and
mill B are OFF.
- D) Similar to mill A.

13. R C FEEDERS
A) Feeder can be started provided following conditions are met:
No boiler trip condition prevails, Elevation DC and AC is
available, ignition energy is available, mill is ON, HAG is open, No
stop signal and fuel air dampers closed.
B) When feeder is started the following command will be given:
to release feeder speed control to auto after 50 sec, to
modulate fuel air dampers according to feeder speed after 50 sec,
to permit feeder speed variator to move.
C) Feeder motor shall trip automatically under any of the
following conditions:
Boiler trips, elevation DC fails after 2 sec, elevation AC fails,
ignition energy disappears before 180 sec from starting of the
feeder, mill trips, loss of coal flow and mill motor current low (this
impulse will act only after 5 sec from feeder start).

14. R C FEEDER
D) When the R C feeder motor is tripped the command will go to
close the HAG.
E) When mill motor current / load is above max set point or mill
bowl DP is high 250 mmwcl, a command will be given to bring
the R C feeder speed to min
SADC
AUXILIARY AIR DAMPERS
A) During the purge period and unit operation upto 30% loading, all
elevations of auxiliary air dampers modulate to maintain appx 40
mmwcl DP between furnace and WB.
B) When load exceeds 30% , the DP set point is changed and ramps
up to a higher setting. Simultaneously, the aux air dampers
associated with the coal or oil elevations not in service close in
timed sequence starting with the upper elevations of dampers and
progressing to lowest elevation.

15. AUX AIR DAMPERS
C) When load is reduced below 30%, the aux air dampers open in a
timed sequence starting with lowest elevation of dampers.
Simultaneously the DP set point is changed to lower setting.
D) All the aux air dampers maintain status quo upon a boiler trip
and will open fully when both FD fans are OFF.
E) Aux air dampers associated with the oil elevations open to a
preset position whenever oil is being fired or modulate as a
function of oil hdr pr when H O is being fired. Otherwise they
open and close with the balance of aux air dampers.
FUEL AIR DAMPERS
All fuel air dampers are normally closed. They open after 50 sec
after the associated feeder is started, and modulate as a function of
feeder speed.
50 sec after the feeder is removed, associated fuel air damper close.
The fuel air dampers will open fully when both FD fans are OFF.

16. I D FANS
- Conditions existing when unit is shutdown ie; prior to starting ID fans:
Both ID Fans are off, Regulating vanes, outlet dampers and inlet dampers
are fully open. Interconnecting damper is closed.
- STARTING ID FAN ‘A’ (IF ID FAN B IS OFF)
- Operator shall give command to bring the regulating vane to min
position and close the outlet damper. ID fan A shall be prohibited from
starting till following conditions are met:
- Regulating vanes in min position, outlet damper closed, lub oil pr
adequate, fan/ motor bearing temp not very high.
- When ID fan A is started (ID B is off) impulses shall be given for
opening the O/L damper after a time delay of 45 sec, opening the
interconnecting dampers. Closing the inlet and outlet dampers of ID B and
bringing the inlet vanes to min position. Permission is given for starting of
FD A or FD B.
- STRTING ID FAN ‘B’ ( IF ID FN A IS OFF)
- Similar to above.

17. I D FANS
Tripping of ID A (ID B is OFF):
ID fan A shall trip automatically under following conditions:
Motor/ Fan bearing temp too high, lub oil pr low, AH- A and AH-B is
OFF.
When ID fan A trips (ID B is OFF) command will be given for:
Opening of I/L dampers, O/L dampers and regulating vanes of ID B.
opening of regulating vane of ID A, closing of the interconnecting
dampers. Trip the working FD fan and working PA fan.
Tripping of ID B (ID A is OFF):
Interlocks are similar to above.

18. FD FANS
- Conditions prior to starting FD fans:
Fan impeller in max position, O/L dampers fully open. Interconnecting
damper may be in open or close position.
- Starting FD A (FD B is OFF)
Operator shall give command to bring Fan impeller to the min position
and to close the O/L damper.
- FD fan shall be prohibited from starting until the following conditions
are satisfied:
- ID A or ID B is ON, control oil pr adequate, fan impeller in closed
position, O/L damper in min position, FD fan A and motor brg temp not
very high.
- When FD A is started impulse shall be given for:
Opening the O/L damper with a time delay of 30 sec, opening the
interconnecting damper, connecting impeller control drive to the
regulator, closing the O/L damper of FD B and bringing the impeller of FD
B to min position.
- Starting FD B (FD A is OFF):
- Interlocks are similar to above.

19. FD FANS
Tripping of FD A (FD B is OFF):
A) FD A shall trip under following conditions:
FD fan/ motor brg temp very high, both ID fans trip.
B) When FD A trips (FD B Off) impulse shall be given for:
Disconnecting the impeller control drive from the regulator,
bringing the impeller to max position, bringing the impeller of FD B
to the max position, opening the O/L damper of FD B.
To open the emergency scanner air damper.
Tripping of FD B (FD A is OFF):
Interlocks are similar to above.

20. PA FANS
Conditions prior to start up:
PA fan A and PA fan B off.
O/L dampers open, regulating vanes in wide open position,
interconnecting dampers may be in open or close position.
- Starting PA fan A (PA fan B is OFF)
- Operator shall give impulses to close the O/L damper and to bring the
regulating vane to min position.
- PA fan A shall be prohibited from starting until following conditions are
satisfied:
Regulating vane in min position, O/L damper in closed position, lub oil pr
adequate, Pa fan/ motor brg temp not very high, any one FD fan is ON.
- When PA fan A is started (PA fan B is OFF) impulse shall be given for:
Opening the O/L damper with a time delay of 45 sec, connecting the
regulator vane to the regulator, opening the interconnecting damper,
bringing regulating vane of PA fan B to the min position, closing the O/L
damper of PA fan B.

21. PA FANS
- Starting PA fan B (PA fan A OFF)
Interlocks are similar to the one given above.
-Tripping of PA fan A (PA fan B is OFF):
PA fan A shall trip under the following conditions:
PA fan/ motor brg temp too high, lub oil pr low, any boiler trip
condition is present, both FD fans are tripped,.
- When PA fan A trips (PA fan B OFF), impulses shall be given for:
- Disconnecting the regulating vane from the regulator, bringing
the regulating vane to wide open position, opening the outlet
damper, bringing the regulating vane of PA fan B to wide open
position, opening of O/L damper of PA fan B.
- Tripping of PA fan B (PA fan A OFF):
- Interlocks are similar to one given above.

22. SEAL AIR FANS
INTRLOCKS FOR SEAL AIR FAN ‘A’
- Seal air fan A shall be prohibited from starting until PA fan A or PA
fan B is running.
- Seal air fan A can be manually tripped only if all the mills are
tripped or fan B is ON.
- Seal air fan A will be tripped automatically with a time delay of 60
sec when both P fans trip.
- When seal air fan A is working and if seal air pr low alarm comes
operator has to start seal air fan B. Afterwards seal air fan B shall
be tripped manually.
INTERLOCKS FOR SEAL AIR FAN ‘B’
- Interlocks are same as given above.

23. AIR HEATER INTERLOCKS
Air heater A
- AH- A electric motor shall trip automatically when the temp of
support / guide brg of AH A is very high. Under this condition AH-
A air motor shall also trip if in operation.
- Whenever AH- A electric motor trips a command shall be given
for starting AH- A air motor provided temp of support/ guide brg
is less than 70 deg C.
- Whenever both the electric motor and air motor of AH-A trip,
impulse shall be given for
Closing the inlet and outlet dampers. Tripping of ID A provided ID B
is ON and AH-B is ON. Tripping of both the ID fans if AH-B is OFF.
Opening the interconnecting damper if AH-B is ON. This
interconnecting damper shall close when both AHs are started.
Air heater B
Interlocks are same as for AH-A given above