89912310 boiler-purge-burner-management-system


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89912310 boiler-purge-burner-management-system

  1. 1. Boiler Protection System and Burner Management System The Boiler Protection System in combination with the Burner Management System protects the boiler (including furnace explosion and implosion) from an unforeseen process fault or an incorrect action at the Distributed Control System. The BPS / BMS in combination ensures all the necessary interlocks for a safe start up and shut down of burner equipment. This narrative contains provisions required for compliance with the NFPA 8501 Standard for Single Burner Operation. The Burner Management System is designed to ensure a safe, orderly operating sequence in the start up and shut down of fuel firing equipment and to reduce possible errors by following the operating procedure. In some phases of operation, the BMS shall provide permissive interlocks only to insure safe start-up of equipment. Once the equipment is in service, the operator must follow acceptable safe operating practices. It is essential that all parts of the BMS are in good working order and in service whenever the burner is in service if the system is to provide the protection for which it is designed. Regular maintenance and inspection of the system and its associated hardware is essential for its continued safe operation. Furnace purge : The purge system assures that the boiler furnace, the associated air and flue gas paths, machineries like FD, ID and APH and equipments containing sources of ignition energy like ESP are purged with air to remove any explosive mixture before ignition. The BPS during the purge verifies that the air and flue gas paths are open, fuel shut off valves are closed and no flame is detected. Master fuel trip : While monitoring the fuel system and the furnace operating condition, a fault is detected, the BPS shall trip the MFT valve and individual burners shut off valves, main and ignitor shut off valves, de energise sparks and other ignition sources. When a MFT is caused by a loss of FD Fan and ID Fan trip signal, all the air and flue gas passages of the boiler should be opened to fully opened condition to provide as much Natural draft as possible. This condition shall be maintained for a 15 minute period. At the end of this period, fans shall be started to perform a post firing purge. Flame sensors: Each ignitor and burner is provided with proper flame sensor (fail safe trip) to continuously monitor the flame presence at any boiler condition. Each ignitor is equipped with IFM flame proving system using flame ionisation principle.
  2. 2. BMS Functions The BMS shall be designed to perform the following functions Prevent firing unless a satisfactory furnace purge has first been completed Prohibit start up of the equipment unless certain permissive interlocks have first been completed Monitor and Control the correct component sequence during start up and shut down of the equipment Conditionally allow the continued operation of the equipment only while certain safety interlocks remaining satisfied Provide component condition feed back to the operator and, if so equipped, to the plant control system and or data loggers Provide automatic supervision when the equipment is in service and provide means to make a Master Fuel trip (MFT) should certain unacceptable firing conditions occur Execute a MFT up on certain adverse unit operating conditions National Fire Protection Association As per NFPA 8501, a common cause of furnace explosions is “Fuel leakage into an idle furnace and the ignition of the accumulations by a spark or other source of ignition.” In order to initiate and complete a satisfactory furnace purge, the operator must verify that all of the following conditions are satisfied. All system power supplies on A minimum of 30% of full load air flow is established All of the auxiliary air dampers modulating is established. Ignitor header fuel trip valve is proven closed No natural draft and waiting time for purge requirement are present Oil header (MFT) trip valves are proven closed All flame scanners “No flame” The drum level is adequate between minimum and maximum of the dangerous condition. No Boiler trip command is present Instrument air header pressure within operating range A “No master Fuel trip condition” is established Flame scanners cooling air not very low Oil header pressure not very low Furnace pressure is between minimum and maximum Atomising steam/air pressure not very low
  3. 3. As each of the above conditions is reached, their respective indicator lights shall illuminate as “PURGE PERMISSIVES” on the operator control console. Once all of the above conditions are satisfied, the “PURGE AVAILABLE” light shall illuminate indicating that the START PURGE switch is enabled by the system logic. Activating the START PURGE control, when the “PURGE AVAILBLE” light is illuminated, will begin a timed furnace purge cycle as indicated by a “PURGE IN PROGRESS” light illuminating. If the operator is starting the boiler from the cold condition or restarting after a Master Fuel trip event, the procedure outlined in the following paragraphs is required. If the boiler is recycling or other wise starting from a warm condition, the MFT circuit will reset automatically and the procedure will skip directly to the main flame light off sequence. Provided that the start up permissive remain satisfied, the system shall initiate a burner start up period timer set for no more than 10 minutes. If the operator does not initiate a start up within this time, the boiler control system will exit the “PURGE COMPLETE” condition and the operator must begin again with the pre purge cycle. If the MFT has been reset, the boiler control system will trip the MFT when leaving the “PURGE COMPLETE” condition. Upon completing a satisfactory purge, the operator must press the “RESET MFT” push button. The boiler control system will indicate that the MFT has been reset with a steady on “MFT RESET” light. At this point, the boiler control system is ready to allow the main flame start up sequence. Main Flame Start-Up Sequence The main flame start up sequence from the light- off the pilot flame through main flame light – off is an automated sequence. Once the start up sequence has begun, only the “BOILER STOP” switch and the “EMERGENCY STOP” will interrupt the start up sequence. Any interruption of the start up sequence requires a post fire purge prior to attempting to start the boiler again. To initiate the start up sequence, the operator activates the “START BOILER” switch. Failure to meet any of these conditions shall prevent the burner light-off operation. To light the pilot flame, the pilot header vent valve shall be closed by the boiler control system. Then, sequentially, the ignitor transformer is energised, the pilot gas block valves are open and a 10 second pilot ignition timer starts counting down. When ignition timer cycle is completed, the ignitor transformer is de-energised and the pilot flame scanner is checked by the control system. If the pilot flame is present, the main flame light-off sequence continues.
  4. 4. If the pilot flame fails, the boiler control system initiates a pilot flame failure shutdown. Additional attempts of the pilot light-off are permissible provided a successful pilot light-off are made with in 10 minutes after the furnace purge. Note, if the pilot flame continues to fail after several attempts, the boiler should be inspected to determine the fault and the condition corrected. Main Flame Light-off Once the pilot flame is made, the boiler control system opens the header block valves for the selected fuel. A main flame light-off burner timer begins a 15 second countdown to establish and stabilise the main flame. At 5 seconds before the time out, the boiler control system closes the pilot block valve and opens the pilot vent valve. The remaining 5 seconds are used to detect the main flame. For the typical dual flame scanner design, a main flame failure shutdown is initiated if both flame scanners return a “no flame” signal to the burner control system. This will generate a boiler trip, and another furnace purge will be required. Once the burner is lit, the system is in the NORMAL RUN CONDITION and combustion controls should be released to modulation control. Shutdown The normal shut down cycle for the boiler shall accomplish the following in the order listed - Shut off fuel supply to the main burner - Interrupt spark and shut off fuel supply to ignitor, if in operation - Where used, open the oil recirculation valve - Shut off atomising medium Boiler Master Fuel Trip Any of the following conditions shall cause a boiler trip to occur. This results in the shutdown of all fuel and requires another furnace purge cycle before any attempt at re-lighting. - Steam drum very low level - Steam drum very high level - Loss of Fans - Total air flow less than 30% - Furnace pressure very high - Furnace pressure very low - Oil header very low pressure - Atomising steam very low pressure - Flame scanner cooling air very low pressure
  5. 5. - Loss of fuels - Loss of flames - DCS not available - Loss of Instrument air AFBC Boilers with Oil burners for initial heating of the fluid bed Boiler General Protection Master Fuel trip condition healthy - Drum level is not very low - Drum level is not very high - ID Fan in service - FD Fan in service - Furnace pressure is not very low - Furnace pressure is not very high - Bed temperature is not below 650deg.C - Bed temperature is not above 980deg.C - Instrument air is within limits Note: The individual coal feeder will trip, if the corresponding temperature falls into the logic and start permissive is applicable once the bed temperature is normalised. Other wise, the MFT will act based on the logic incorporated. Boiler Purge permissive - absence of boiler trip conditions - Oil shut-off valve closed - Drum at start up level - Air flow > 30% - At least one air gas circuit in service - waiting time for purge requested - Natural draught not required Oil Shut-Off Valve condition healthy - boiler purge completed - oil header pressure is within limits - atomising medium is not very low
  6. 6. CFBC Boilers with Oil burners for initial heating of the fluid bed Boiler air protection - Fluidised bed temperature <max.2 - Furnace draught >min.2 - Furnace draught <max.2 - Siphon I exit temperature <max.2 - Siphon II exit temperature <max.2 - Drum level >min.2 - Drum level <max.2 - Primary air flow >min.2 Note: Boiler must be tripped if only one bed temperature measurement is available. Boiler Oil protection / Preconditions to start hot gas generator - Boiler air protection OK - Fluidised bed temperature <min.1 - Primary air flow >min.2 - Fuel oil pressure >6kg/cm2 - Atomising air/steam pressure >7kg/cm2 - HGG out gas temperature <850deg.C - No flame in HGG - Differential pressure across HGG >25mmwg - LPG press >0.4kg/cm2g Preconditions to start Main Fuel - Coal bunker level >min - Boiler air protection OK - Fluidised bed temperature 20deg.c above ignition temperature - Fluidised bed temperature <920deg.C - Coal feeder cut off gate Open - Coal feeder down stream gate Open - Drum level >min.2 - Drum level <max.2 Note: min & max depends on OEM recommendations
  7. 7. General Protections 1. Furnace draught <-150 or >100mmwc 2. Drum level (2 out of 3) <-320 or >150mm 3. Bed temperature>940deg.C 4. Drum pressure(4 seconds delay) 5. Siphon ash temperature >450deg.C 6. Root blowers trip 7. Start up time exceeded When coal feeder are in ON 8. Bed height>1400mmwc or <600mmwc 9. PA pressure to APH(4 seconds delay)>1900mmwc 10. PA flow-total PA flow<14kg/s & both PA flow to HGG 1&2<7kg/s 11. Flue gas pressure before ID Fan <-500mmwc 12. SA pressure to APH > 720mmwc 13. Coal feeder trip on bed temperature>920deg.C or <450deg.C 14. Total air/fuel ratio less than minimum End notes: - Pre purge permissive condition checks and furnace purge are to be initiated by the operator from the BMS - The total furnace air flow shall not be reduced below the purge rate air flow. Reducing air flow below the limits will lead to a MFT, and a new furnace purge will be required S.V.NAGESH Head-Technical Cubex Energy Private Ltd 9962666980