BOILER COOLING METHODS

Master Fuel Trip

• When a master fuel trip occurs automatically through normal interlocks, or is initiated manually by an operator, all fuel should be tripped instantaneously.
• If coal is being fired, the pulverizers should be tripped immediately.
• Tripping the pulverizers will automatically trip the feeders and close the hot air shutoff gates.
• If oil was being fired, the fuel trip valves and individual nozzle shutoff valves should be closed immediately.

The following steps should be taken immediately following an emergency fuel trip:

• Maintain the unit airflow at the pre-trip value for at least 5 minutes to purge the system. If a hot restart is anticipated, the airflow may be gradually reduced to a light-up airflow (30% of BMCR load airflow) during the purge. The boiler circulation pumps should be started to maintain uniform temperatures throughout the boiler circulation system and to facilitate lighting off.

Procedure after master fuel trip when all ID & FD fans are off:-

• The ID and FD fans discharge dampers shall be opened to allow natural ventilation of the unit.
• Opening of fan(s) dampers shall be timed or controlled to avoid excessive furnace pressure excursions during fan(s) coast-down.
• The fans discharge dampers shall remain open for a period of at least 15 minutes.
• During this 15-minute period, the ID and FD fans shall not be started.
• If the unit is not possible to restart for an extended time, a flow of air through the unit shall be maintained.

Tube Failures

• If a leak or tube failure does not involve a serious drain on the feed water supply, the water level should be maintained and the unit taken out of service in the normal manner.
• If there is a serious water loss and the water level cannot be maintained with the feed water supply available. Use the following method.

Boiler forced cooling procedure

• Trip all fuel.(Reduce the fuel/feed rate and stop the coal mill one by one, Turbine should kept valve wide open to minimize boiler pressure to minimum, )
• (Before Unit tripping is possible please identify the leakage so once  boiler pressure drop sometime leakage not traceable) The ID and FD fans discharge dampers shall be opened to allow natural ventilation of the unit. Opening of fan(s) dampers shall be timed or controlled to avoid excessive furnace pressure excursions during fan(s) coast-down. The fans discharge dampers shall remain open for a period of at least 15 minutes.
• During this 15-minute period, the ID and FD fans shall not be started.
• At the end of the 15-minute period, the ID/FD fans may be started in accordance with ID/FD fan(s) start-up procedures.
• If the unit is not possible to restart for an extended time, a flow of air through the unit shall be maintained.
• MDBFP started & maintains feed water flow at least 30% BMCR & BCW pump started.
• Drum metal Temp.  top & bottom differences should not be Exceed up to 50*c.
• Maintain only enough airflow 30% to carry the escaping steam up the stack.
• Start the ID & FD fans and gas air heaters in service until pressure is off the unit.
• Turbine inlet valve drain is closed HP-LP by pass valve is also closed if required.
• Drum metal temp. reached to 90*c  then stop BCW.
• Dearator supplied steam valve is stopped.and try to avoid take cold water in Boiler Drum level should be maintain only by Dearator level.
• Air vent & Drain valve is opened carry out Boiler Drain out.
• Air heater inlet gas Temp. less than 90 degree Celsius Decrease the air flow and keep in zero.

Technical point

Turbine Inlet  steam Temperature

• Turbine inlet steam temperature decreases due to cooling steam coming from boiler under boiler forced condition if only Boiler side want to decreases the temp. Turbine side drain should be closed & pressure reducing through HP-LP bypass

Air Flow

• Increase the air flow due to improve the convective heat transfer and accelerate decrease the SH/RH temperature rapidly increasing the air flow make the black smoke so please take care stack outlet under increase the air flow.

Water Hammer

• Under boiler forced cooling please confirm the local condition sometimes water hammer is occurred.

Drain valve closed

• Prevent the air interfusion Main steam pressure reached 3.0 kg/cm2, Drain valve should be closed

Air heater stopped

• Stop Air heater at inlet gas temperature <100*c to prevent the heat transform.

NORMAL SHUTDOWN TO COLD

The following procedures for normal shutdown assume that the unit is operating at full load on automatic control, and the intention is to reduce the unit pressure to zero and cool the boiler completely.

CAUTION: Exercise care to prevent water from entering the turbine through the main steam line(s), the hot reheat line(s), or the cold reheat line(s) during and after the shutdown process. This prevents water damage to the turbine during a subsequent restart.

Possible water sources are:-

• Overflow from the Boiler Drum into the superheater when a high water level exists.
• Water entering the superheater or reheater through the desuperheaters due to open desuperheater control valves or valve leakage with the feed pumps running.

• Reduce the unit load at a rate of 2% per minute following the prescribed sliding pressure vs. load curve. Hold pressure constant at app. 125 kg/cm² until the turbine is taken off line. Reduce the firing rate in line with the decreasing steam flow. Reduce the steam temperature at the rate of 220 *c/h  Leave the steam temperature, combustion and feed water controls on automatic until better control can be obtained on manual.
• When the feeder rating on all pulverizers is reduced to 40% put the upper elevation of oil guns in service. Place the pulverizer supplying the upper coal elevation on manual control. Gradually reduce the feeder rating. When minimum rating is reached, close the coal shutoff gate above the feeder, run the feeder until the spout and feeder are empty, and then stop the feeder. Make sure the pulverizer is completely empty by running it for at least Five (5) minutes. Close the hot air shutoff gate. When the coal/air temperature has dropped to approximately 50℃, then, stop the pulverizer. Unless an emergency condition exists, the pulverizer should always be emptied before being stopped.
• Take the second pulverizer supplying the next highest elevation of coal nozzles out of service when the feeder rating on all remaining pulverizers reaches 40%. Continue taking additional pulverizers out of service in the same manner. When only two(2) pulverizers remain in service, the adjacent oil guns must be placed in service prior to reducing the feeder rating of either pulverizer to below 65%. It is recommended that this consecutive elevation shutdown sequence be followed. Firing coal at separated/non-adjacent coal nozzle elevations without auxiliary supporting fuel is not permitted.
• Overfired air will be reduced automatically as the unit load drops per the prescribed control curve. Stop admitting overfire air when the unit load drops below the predetermined starting load. Keep the over fire air dampers at minimum open position to prevent the nozzles from overheating.
• Operate all soot blowers. Refer to the soot blowing philosophy section and to the soot blower manufacturer’s instructions.
• Reduce the airflow in line with the fuel reduction until 30% BMCR airflow is reached. Do not reduce the airflow below this rating until all fires are out and the unit is off the line. To avoid firing instability during this period of high airflow, the auxiliary air dampers at all elevations should open and the windbox-to-furnace differential pressure reduced to its lower setting per the prescribed control curve.
• Check the boiler expansion movements as the load is reduced.
• At app. 35% steam flow, open the HP and LP bypass valves. Remove the turbine from service following the turbine operating instructions. Boiler steam production will be transferred to HP/LP bypass operation.
• At app. 30% BMCR steam flow, the steam temperature at Boiler Drum  will fall below the superheated region, and a water level will develop in the Drum. The boiler must now be returned to recirculation mode, to provide minimum water wall flow
• After the last pulverizer is shut down, remove the oil guns from service. Shut off the cooling water to the pulverizers. All fires should be extinguished when the unit is off the line. With no pulverizers in service, the PA fans and the pulverizer seal air fans may also be shut down. The PA fans should not be shut down if the gas air heater gas inlet temperature is above 400℉ (205℃).
• After all fuel firing is removed, close the HP and LP bypass valves.
• Immediately after closing the HP/LP bypass valves, open all superheater outlet vents and drains and all reheater vent and drains. Open the steam line and turbine drains per the turbine manufacturer’s instructions.
• Close the de-superheater isolating valves.
• Do not exceed the following cool down rates referenced to saturation temperature in the Boiler Drum: For pressure above 35 bar: 83℃per 15 minutes; for pressure less than 35 bar: 42℃ per 15 minutes.
• If the steam generator will not be entered during the shutdown period, a slower rate of pressure and temperature decay may be desirable. This can be accomplished as follows:
  1. Purge the unit by running the ID and FD fans for at least Ten (10) minutes at 30% BMCR airflow after the fires are extinguished.
  2. Reduce the airflow through the unit to minimum. Keep the fans and gas airheaters in service until the unit has cooled to the desired temperature. Gas air heaters and fans should not be taken out of service until the gas air heater gas inlet temperatures have dropped below  (100℃).
  3. Keep the boiler circulating pump in service until the boiler water temperature is reduced to the desired point (100℃ i.e. app. 15℃ less than the saturation temperature corresponding to the SH pressure at 30 % TMCR load). Keep the water level near normal in the Drum, adding makeup water as required.
  4. Keep the boiler circulating pump in service to increase recirculation. Keep the water level near normal in the Drum. Shut down the pump when the boiler water temperature is reduced to the desired level, which is differential temperature, 20°C, between Drum outlet water temperature and economizer inlet water temperature.
• Do not shut down the scanner air fan until the furnace temperature has dropped to below 140⁰F (60°C).
• If the boiler is to be emptied, the boiler water temperature should be reduced to at least 200⁰F (93℃) before draining.

BOILER FORCE COOLING PROCEDURE.

• Once it is ensure that in the boiler BTL happened, please identify the location if possible.
• Coordinate with trading for schedule revise and also inform all departments for unit shutdown so they can also plan accordingly.
• After getting clearance from trading and boiler maintenance , load reducing to be start, empty all coal feeder one by one if possible, Turbine should run valve wide open so boiler pressure reduce to minimum . Once the boiler pressure upto 50-60 kg/cm2 unit to be shutdown.
• Inform WRLDC about unit shutdown and tentative start up plan with reason of shutdown.
• As unit is stopped for BTL, so turbine side all mal drain to be closed after MS and CRH and HRH header depressurized to avoid cooling. (Strainer and atmosphere drain to be kept open).
• HP/lp bypass to be charged and confirm leakage is in first pass or reheater. After that reheater to be drain and vent to be open.
• As our boiler is subcritical, so boiler cooling rate to be depend on drum top bottom temperature difference (it should not more than 50°C).
• After boiler MFT, purging to be done, after purge one ID fan and FD fan to be stopped.

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