Fly Ash System

 INTRODUCTION : 

In coal fired boilers ash is collected in two forms, bottom ash and fly ash. BA
constitutes around 20% of the total ash and remaining 80% is collected in the form of fly ash. Fly
ash evacuation is done online using vacuum pumps either in dry ash mode or in wet mode. In
wet mode we use collecting towers which sprays water on ash going to vacuum pump suction
and makes it slurry which will be discharged in to seal box and then to slurry trench. In dry mode
we collect dry ash in buffer hoppers there by with the help of transport air it will be carried to
hoppers in the ash dyke and from there it is stored and distributed to different users. 

ISOLATION PROCEDURE FOR FLY ASH SYSTEM :

SHUT DOWN CHECKS:

1. Before shut down of fly ash system, ensure all hoppers are empty by checking their vacuum in MHV open condition, if not. Pock the hoppers and empty them 
2. After ensuring emptiness of all hoppers and no ash slurry is observed in CT flash box, stop all the vacuum pumps and isolate their cooling sealing water line and isolate the unit CSW line v/v to ensure adequate pressure to other running units.
3. Check all the MHV’s and line header valves are closed.
4. Stop the fluidizing blower and isolate the heater supply.
5. Stop the fly ash pump and isolate its makeup valve from BALP discharge.
6. Empty the settling bin by sludge pump and stop the sludge pump after level becomes low.

START-UP CHECKS FOR FLY ASH SYSTEM: 

1. After getting all the clearances from AHD and EMD for running the fly ash system line up the FA water pump, seal water pump, BALP water pumps, vacuum pumps, cooling sealing water pumps.
2. Check for availability of at least one collecting tank in each pass with all mechanical fittings.
3. Check the hopper plate valve of each hopper is in open condition.
4. Check all the drains of settling bin are closed and open manual isolation v/v’s of settling bin make up v/v from BALP discharge.
5. Check the ash water tank level is adequate and start minimum number of BALP pumps for filling the settling bin and check ash water tank is getting makeup from at least one of its sources (ash water re circulation, raw water st-1, raw water st-2, CW blowdown)
6. Ensure sludge p/p is mechanically normalized and start it after ensuring settling bin is in filled condition.
7. Check the availability of at least one pair of vacuum pump and collecting tank in each pass and ensure ash intake v/v, water v/v, vacuum line v/v and vacuum pump suction v/v are in open condition.
8. Check both clean water tank levels are high and ensure they are getting makeup either from CW blow down or from BALP discharge.
9. Start at least one CSW pump for one unit and one seal water pump.

Starting Permissive for CSW pump:

1. Emergency stop push button at field is in released condition.
2. Module is healthy.
3. O/L of MCC in normal condition (RESET).
4. Emergency stop button from MCD not pressed.
5. Overhead tank – 1 (Stage-1 side) level high.
6. Selected in operation mode. (Oprn/maint).
7. Start command from MMI given.
8. Emergency stop push button at field is in released condition.

Auto Start Condition:

1.  The pump selected on auto will take start if any of the running pump trips subject availability of the permissive as stated above.

Tripping Conditions: -

1. Pump will tripped automatically if any one of the following conditions is satisfied 
2. Emergency stop push button at field is in pressed condition
3. Emergency stop button at MCD pressed
4. Module is not healthy
5. MCC module is overloaded
6. Overhead Tank level – 1 level low

Starting Permissive for FAW pump: -Pump will take start from MMI page if all of the following permissive are available

1. Pump selected in manual mode. (Auto/man)
2. Emergency stop push button at field is in released condition
3. Module is healthy
4. 4 O/L of MCC in normal condition
5. Emergency stop button from MCD not pressed
6. BA settling tank level high
7. Any one seal water pump is running
8. Seal water pressure is normal (1.5 ksc)
9. Start command from MMI given

Tripping conditions:

Pump will trip automatically if any one of the following conditions is satisfied

1. Emergency stop push button at field is in pressed condition
2. Emergency stop button at MCD pressed
3. Module is not healthy
4. MCC module is overloaded
5. Seal water pressure is low (1.5 ksc)
6. Seal water pump is not running 10 sec time delay
7. BA settling tank level DANGER LOW, 5 sec time dela

Starting Permissive for SEAL WATER pump:

Pump will take start from MMI page if all of the following permissive are available

1. Pump selected in manual mode. (Auto/man)
2. Emergency stop push button at field is in released condition
3. Module is healthy
4. O/L of MCC in normal condition
5. Emergency stop button from MCD not pressed
6. Clean water tank level is high

Tripping conditions:Pump will trip automatically if any one of the following conditions is satisfied

1. Emergency stop push button at field is in pressed condition
2. Emergency stop button at MCD pressed
3. Module is not healthy
4. MCC module is overloaded
5. Clean water tank level low

Starting Permissive for VACUUM pump: -

Pump will take start from MMI page if all of the following permissive are available:-

1. Pump selected in manual mode. (Auto/man) Suction valve and seal water solenoid valve of selected pump will open when pump is selected in operation mode.
2. Emergency stop push button at field is in released condition
3. Module is healthy
4. O/L of MCC in normal condition
5. Emergency stop button from MCD not pressed
6. Suction valve of pump is open and Suction valve of other pump is close.
7. Instrument air pressure at selected air receiver is adequate (>5.5 ksc)
8. Any one cooling sealing water pump is running
9. Cooling sealing water flow at cooling sealing water station is adequate.
10. Cooling sealing water pressure at cooling sealing water station is adequate.(1.5 ksc)
11. Vacuum breaker of selected tower is open condition.
12. Vacuum line header valve of selected tower is open and Vacuum line header valve of other tower is close.
13. Fly ash intake valve of selected tower is open and Fly ash intake valve of other tower is close.
14. Buffer hopper vacuum line valve of dry ash system is close
15. Any one Fly ash water pump is running and Fly ash water pressure of selected tower WH is adequate (>5.5 ksc).

Tripping conditions:Pump will tripped automatically if any one of the following conditions is satisfied:-

1. Emergency stop push bottom at field is in pressed condition
2. Emergency stop bottom at MCD pressed with accept PB of alarm pressed
3. Module is not healthy
4. MCC module is overloaded
5. Suction valve of pump is close
6. Suction valve of other pump is open.
7. Instrument air pressure is low
8. Any one cooling sealing water pump is not running (5 sec time delay).
9. Cooling sealing water flow at cooling sealing water station is low. (5 sec time delay).
10. Cooling sealing water pressure at cooling sealing water station is not adequate (5 sec time delay).
11. Any one Fly ash water pump is not running (5 sec time delay)

GETTING STARTED:

1. After collecting towers and vacuum pumps are lined up, start the FA water pump, check for its discharge pressure and observe water is coming in each seal box.
2. After all the permissives for vacuum pump are satisfied start the vacuum pumps in each pass and observe water flow in each seal box.
3. Start the fluidizing blower and check for its discharge pressure (>0.4ksc). Check the heater is mechanically normalized and is taking auto cut-on and cut-off.
4. Check the hopper temperature is maintaining between 90-1200C
5. Check for vacuum in vacuum breaker open condition, in better condition it should be <100mmwc.
6. Check for shut off vacuum in each pass (should be >400mmwc) and ensure vacuum breaker is getting opened at vacuum>450mmwc otherwise inform to C&I for adjustment in VB setting.
7. Check for line vacuum in individual line, for better operating conditions it should be with in 70-90 mmwc. If line vacuum is low check for any line leakages and to be tightened.
8. If all the checkings are satisfied then check the individual hopper empty condition by opening MHV (material handling valve) one at a time in each pass and by observing the drop in vacuum.
9. After coal firing was started in boiler and oil guns are removed fields have to be charged and after ash got accumulation we can observe increase in vacuum once ash flow is established by opening individual MHV’s.

ROUTINE CHECKS DURING RUNNINGCONDITION:

1. In each shift check for proper line vacuum, shutoff vacuum and vacuum in VB open conditionwhich ensures cleanness of vacuum suction line.
2. Check each seal box for adequate slurry or water flow, if sufficient water flow was not observed then its water nozzles might got choked and change the CT and go for same procedure.
3. If vacuum in VB open condition is high (>100mmwc) then vacuum suction line might got choked and go for water washing by stopping the vacuum pump in that pass.
4. After ensuring CT performance and vacuum settings, start the ash evacuation by opening one MHV in each pass after opening that line header v/v.
5. Check for steady line vacuum in case no MHV is opened. If the line vacuum is not steady and hunting widely, check for the MHV passing of any hopper otherwise it will choke the vertical line due to sudden rush of ash.
6. Check for any ash carryover to the vacuum pump suction by observing VP discharge. If slurry is observed in the pump discharge because of improper CT function due to its water nozzles choking and go for CT change over.
7. Check each hopper is getting emptied and check for the hopper temperature also. If the hopper temperature is maintaining low (<900C), inform to EMD.