500 MW Unit Shut Down Procedure

Readiness Check for Unit shutdown

• Information on Unit shut down is known to all concerned HODs .
• Inform NRLDC regarding the approximate time of shutdown, the reason and duration.
• Inform switchyard control room /coal handling plant about Unit Shutdown .
• Inform shift maintenance (Mech/Elect/C&I) /Shift chemist/ MTP .
• Shut down programme is normal and not an emergency.
• Consolidate defects and make a list available in control room.
• Unit load is 500 MW with all LP/HP heaters and both TDBFPs in service.
• MDBFP # C is available on auto .
• Atomizing steam and HFO is available at requisite pressure & temp . ( oil 100 Deg C/17-18 ksc , Steam 200 – 210 Deg C & 6.5 ksc )
• Open atomizing steam line drains at all four corners in AB elevation .
• Keep LDO pump running for the corresponding unit .
• Turbine on limit pr. control and Boiler master on manual.
• Make Aux.. Steam available from other unit through HT/LT station header interconnection and ensure it is on auto .
• Isolate Aux. PRDS. ( both MS and CRH ) of this unit. . Close LT/HT interconnection valve (manual valve) & isolate spray water from CEP discharge .Close spray isolating valves.
• Maintain HT PRDS temp 280 - 300 Deg C by maintaining the HT PRDS in adjoining units > 300 Deg C . ( By maintaining MS temp > 537 Deg C)
• Plan WB/LRSB operation such that it is completed well before shut down activity starts.
• CRH/ Aux. Steam to D/A pr. controller on auto.
• If it is planned for ATT for protection device /valves , then inform C&I maintenance department persons while conducting the test.
• Open the GS supply line drain to Atmosphere at 8.5 mtr . Ensure MAW11 drain is already in open condition .
• Open the unit APRDS HT hdr drain(both trap & bypass ) as well as the drains in the loops from other adjacent units . Charge APH SB steam line .

Shutdown Procedure Step by Step of the Unit

STATUS-

Unit Load 500 MW with 6-7 Mills in service.

• Reduce load in one top mill & stop it after emptying the feeder and the mill .

(Reduce MS/ HRH temp set point)

• Reduce load in remaining Mills to 70% bring down the load to 350 -360 MW with 5-6 mills 

(Ensure HAG , CAG & dampers are closed)

• Reduce MS pressure set point to 150 ksc from 170 ksc slowly during the process of load reduction .

(Open The seal steam supply bypass valve to 

establish flow from APRDS HT Hdr .)

(HT Hdr temp > 280 -300 DegC)

STATUS-

Stable load of 350 - 360 mw with 5-6 mills in service

• Cut in one elevation oil support in EF elevation.

(Start APH soot blowing & note down thepressure & temp).

• Start MDBFP after taking to manual and load by unloading any one TDBFP.

(Check for 20 mins bearing temp. and oil temp of MDBFP.)

(Ensure MOV in CW line is full opened)

• Keep FW master on Auto and. Put MDBFP # C onAuto with one TDBFP.
• Reduce the speed of unloaded TDBFP to 3500 RPM (Disch pr =100 ksc)
• Reduce load on one bottom mill & stop it after emptying the feeder.
• Reduce MS pressure set point to 130 ksc
• Gradually reduce load to 300 MW .
• Open TDBFP recirculation valve

STATUS-

Load 300 MW , 4-5 Mills in service, One elevation oil support

• Cut in 2nd elevation oil guns in CD

(Monitor all 4 APH SB are operating continuously & 

it is continued till 1 hr after BMFT)

(Check for any oil leakage at corners.)

• Maintain oil pr. 7.0 KSC.
• After cutting in oil reduce load in available top mill .
• Stop the mill after emptying .

(Check the total air flow is around 1250 -1300 tons)

• Reduce load to 225 MW with 3 mills & 2 elevation Oil support .

(Maintain APH outlet temperatures by adjusting the dampers)

• Reduce MS pressure to 120 ksc

STATUS-

Load 225 MW , 3 Mills and 2 elevation oil

• Unload running TDBFP , keep the load on MDBFP.

(Isolate CRH steam to TDBFP by closing theBlock valve .

To avoid steam side SV lifting)

(Take care of the SH /RH spray & if required close the Block valves)

• Trip the other TDBFP & put it on barring gear (450 RPM)
• Reduce the bottom most mill loading, & reduce unit load to 180 MW .

(If required unload one PA fan , otherwise maintain air flow 

through at least 3 mills to avoid stalling )

• Close the LLCV bypass valve FDV-16 .
• Take drum level control on LLCV (FDV-14).
• Stop the unloaded mill . reduce unit load to 150 MW

( If pressure is high then HPCV opening will reduce very much )

• Trip the 2nd TDBFP & put it on barring gear
• Reduce MS pressure set point to 90 - 100 ksc

STATUS-

 Load 140- 150 MW , 2 Mills with 6 oil guns are in service .

(Observe the seal steam supply has changed over to supply from self sealing.

Observe the temperature , it should be around 250 Deg C)

• Only MDBFP in service
• Take out HP heaters one by one

(Match the HPBP pressure set point with actual pressure . 

Open HP/LP bypass by reducing the pr set point by 10-15 % .

 Do not allow MS/HRH temp to fall below 450 Deg C)

• Slowly reduce load & mill loading also .
• Ensure MS pressure is not increasing .
• If valve testing /ATT for safety devices are to be carried out then proceed for the activity.

(Switch OFF ESP fields )

STATUS-

 Load 120- 130 MW , 2 Mills with 6 oil gunsare in service, 

• Only MDBFP is in service
• Bring down Unit load to 80 MW by opening HPBP further .
• Stop one mill .
• Maintain drum level at least + 50 to +100 before hand tripping the turbine

 ( To avoid boiler trip on drum level LOLO )

• Hand trip the turbine .
• After turbine has tripped check the following.

1. HP BP will go to fast opening & take care of set point so that it remain in open position & maintain drum level .
2. Check that GCB & FCB have opened.
3. HP/IP stop and control valves all are closed.
4. Ensure closing of all extraction block valves and NRVs.
5. AOP took start at 2850 rpm. Lub oil pr 3.8 ksc / Temp 45 Deg C.
6. Increase Lub Oil temp by bypassing the cooler ( TCV)
7. JOP starts at 510 RPM
8. It takes approximately 45 minutes for barring gear valve to open (210 RPM)
9. Another 45 min to reach 110-120 RPM
10. Isolate steam line to D/A from APRDS header if it was charged .
11. Ensure the TG has come on barring gear.
12. Inform chemist to isolate CPU.

STATUS-

Turbine tripped , 1 Mills , with 6 oil guns are in service , Only MDBFP in service

• Stop the mill , stop oil guns , check boiler tripping on loss of all fuel .
• After boiler has tripped check the followings

1. PA fans tripped .
2. On BMFT ensure the closing of HOTV/ROTV,LOTV.
3. SH/RH spray block valves are closed if not closed earlier.
4. Keep open economizer recirculation valve
5. Isolate boiler HFO steam tracing line.
6. Isolate HFO/LDO supply & HFO return oil line manual valves at control station.
7. Isolate individual corners oil valves and steam/air isolating valves.
8. Ensure AC Flame scanner cooling air fan in service.
9. All SAPH & PAPH remain in service.
10. Monitor turbine expansions, casing temps.At every shift and record in log book.
11. Monitor hot well level, D/A Level , CEP disch Hdr pressure & flow .
12. TG barring gear parameters monitoring sheet to be filled

• Depressurise Boiler to 45-50 ksc before closing Stop valves through HP/LP Bypass .
• Close Boiler Stop Valves; depressurise MS / HRH lines through HPLP Bypass. After  depressurisation, close HPLP Bypass. 
• CLOSE HPBP SPRAY ISOLATION MANUAL V/V. ALSO ENSURE FULL CLOSING OF BD / BPE1 / BPE2 V/Vs, BOTH BY MONITORING REMOTE ‘CLOSED’ INDICATION & LOCAL V/V POSITIONS.
• Purge the boiler for 10 mins.
• Stop one set of ID/FD fans after purging.
• Follow the Boiler Cooling procedure
• Draft -30-40 mmwc
• Monitor rate of cooling by following BCW pump suction ,manifold temp drop(< 87 Deg C/Hr)

Unit Condition

• Unit Under Shut Down . BMFT has taken place
• TG on barring gear
• TDBFP A/B on barring gear
• Vaccum Killed after stable barring gear speed
• APRDS is in charged condition from other Unit .

(One set ID /FD fan in service air flow of 700 T/Hr

 with one set fans if force cooling of boiler is required .)

(If vacuum is to be killed then follow the 

standard vacuum killing procedure after depressurizing MS/HRH )