Normal shutdown of Boiler to Cold / Hot

Normal shutdown of Boiler to Cold :

• Note : In shutting down the boiler, the reduction of firing rate isdetermined by the requirement of the turbine.
• Do soot blowing before reducing load or keeping load more than 75 %
• Check HP / LP bypass operation is O.K.
• After soot Blowing gradually reduce load on the unit by reducing firing in the mills.
• Steam flow will be reduced, Allow the pressure to drop with the reduction in load.
• Reduce the temperature of the rate prescribed by the manufacturer for turbine but never exceeding the forced cooling rate of boiler if required to do some work on turbine or maintain MS and HRH temperature constant.
• Reduce coal firing to 70 % of load .
• Put SADC on manual to maintain wind box to furnace Dp more than 70 mmWCL.
• Take HP / LP bypass into service and reduce steam flow to 300 MW .
• Keep HP bypass atleast 25 % open to facilitate flow through Reheater.
• Take oil support for lower most mill (A, B,C,D). Do APH soot blowing.
• Reduce firing in the top most mill to minimum feeder rate, open cold air damper fully and close HAG after closing hot air damper. When mill outlet temp falls below 60 deg. Cent., stop feeder. Then stop mill after completely emptying it. Close cold air damper and CAD fully.
• Adjust air flow for required O2 %.
• Reduce firing further in top mill by reducing feeder rate. Load 200 MW and throttle pressure to 100 ksc, temp. As per TSE. ( Temp. To be kept more or less constant)
• Stop bottom feeder and mill after emptying it, load 100MW,80ksc.
• Change over FRS from high range to lowrange. ( Ensure mill left is having oil support)
• Open economiser recirculation valves.
• Close CBD if in open condition.
• Start reducing firing in last mill, stop feeder and mill by emptying mill and feeder.
• Reduce load to 50MW and pressure 80ksc. Stop both PA fans.
• Ensure Unit 11 KV bus supply change to stationsource .
• Trip the boiler through E.P.B. Turbine will trip and generator breaker will open.
• PA fan will trip automatically if not stopped earlier.
• Ensure closing of all oil valves, Ignitor valves& trip valveLDO/HFO trip valve and stopping of all feeders and mills.
• Close boiler stop valves, SH/RH spray all isolating valves to avoidpassing.
• Do boiler purging.
• See no flame inside the furnace.
• Keep close watch on HP bypass spray.
• Ensure drain opening before / after stop v/v.
• Close HP bypass, LP bypass close on auto, If not close LP bypass.
• Make ESP fields off & run Rapping motors ( Collecting & Emmitting) for 6 hours then stop rapping & isolate flushing water(after ID fan stopping).
• When APH inlet gas temp. falls below 205 deg.C shut down the FD & ID fans. (ID to FD protection can be checked.)
• Do APH washing.
• Ensure natural circulation
• Close Aux. Steam Control valve, isolating v/v and spray line isolatingv/v and control valve.
• Close atomising and heating steam line.
• Close HFO / HSD station inlet manual isolating valve.
• Open RH startup vents & CRH drains.
• Maintain drum level by BFP.
• Open drum & SH air vents, SH start-up vents, SH header drains when drum pressure falls to 2 ksc.
• Drum vents of 500 MW units can be opened after the stopping of CC pumps only.
• When drum metal temp. Comes to 85 deg. C close FRS control valves and isolating V/V. Stop BFP.
• Open low point drain header drains to blow down tank. Close CC p/ps discharge valves before draining.
• Stop S/C, C/G when no ash is coming through S/C.
• Isolate HP / LP water.

EMERGENCY SHUT DOWN DUE TO SEVERE TUBE LEAKAGE

• Always small tube leakages are detected by hissing sound and increased make-up water requirement. After investigation at the earliest possible time unit should be shutdown in a normal manner.
• A sudden major tube leakage indicate by reduction in load, sudden furnace pressurization, increased ID fan loading and sudden increase in makeup water requirement. D/A and Hot well level could not be maintained after opening normal & emergency make-up.
• The sequence of operation in this case will be as follows.
• Reduce load as fast as possible to 70% of NCR.
• If feed water flow can not be maintained by two BFP’s take third BFP service if available ( pr. 130 ksc ). Check leakage area. Inform SCE & maint. engineer.
• Take oil support for lower most mill.
• Reduce feeder speed of top most mill and ensure mill is empty completely. Reduce load to 40% of NCR. Reduce Pressure gradually.
• Similarly reduce feeder speed of second top most mill, stop mill. Load – 30%of NCR.
• Take HP / LP bypass into service. Try to maintain MS & HRH temp.
• Open economiser recirculation valves. Close CBD if in open condition.
• Stop bottom most mill after emptying mill. Load 50MWpressure 50 ksc.
• Stop second bottom most mill after emptying it.
• Trip boiler . Ensure tripping of PA fans, HFO, Ignitor trip v/v , mills and feeders off , all dampers HAD, CAD , HAG close. All oil valves are closed.
• Close boiler stop v/v. Close HP /LP bypass. Open RH air vent & startup vents
• Open drain after boiler stop valves.
• Maintain drum level.
• Stop one set of ID and FD fans. Stop ESP fields, do the rapping.
• Purge the boiler .
• Run the fans till gas temp. Falls to 205 deg.C at APH inlet.
• Then stop ID and FD fans, only allow natural cooling.
• When drum pressure comes to 2 ksc, open SH, drum air vents and SH startup vents.
• Open SH header drain to blow down tank.
• Do APH’s washing.
• When drum pressure comes to zero or drum metal temp. Is approx. 100 deg.C start one set of ID and FD fan for force cooling. (Cooling rate should not exceed as recommended.)
• Rest follow same procedure as per normal shut down.

** In case tube leakage is major and water level can not be maintained, then trip the boiler from EPB.

• Follow same procedure as per EM shut down.

NATURAL COOLING

• All FG dampers will remain open.
• All air side dampers will remain open.
• All SADC will remain open.
• Keep open IGV, Blade pitch of fans. Air circulation will take place.

NORMAL SHUT DOWN TO HOT STANDBY

It is assumed that the boiler will be shut down for a relative short period of time. It is expected 

that the unit will be restarted in a very short period of time, when the turbine metal temp. will not 

decrease appreciably . 

Here the normal procedure for normal shut down should be followed except 

in reduction in MS pressure to 120 ksc and temp almost constant.

• Mills should be stopped one by one after emptying mill only after HP /LP bypass into service.
• Feeder should not be made empty except particular requirement.
• Boiler stop valve should be closed after tripping the boiler.
• Boiler side drain should not be opened. Purge the Boiler.
• Open only RH vents & RH startup vents after closing HP/ LP bypass.
• One set of ID and FD fan to be kept in service till APH inlet temp. Falls 205 deg.C. After 205 deg.C, stop ID and FD fan. Leave the boiler natural cooling. Efforts to be made to preserve the Boiler heat.
• APH washing may not be done due to short shut down.
• ESP fields may be just made off and keeping rapping motors in normal position.
• Ensure closing of all oil valves, trip valves, Ignitor trip valve and ignitor valves and all mill and feeders off.
• HFO may be kept in short recirculation.
• Ensure CC pumps are running(for 500 MW ).