Boiler Operation and Maintenance (Objective)
- 2. Start up & Operation By PRASHANT PATIL
- 3. BOILER START UP Boiler startup can be of two types: • Startup of a cold boiler. • Startup of warm / hot boiler
- 4. START UP OF COLD BOILER • The boiler has been idle for more than three days. • There is no pressure in the steam drum. Drum Pressure is less than 2 Kg/cm 2 (g) • Drum metal temperature is less than 70 Deg. C. START UP OF WARM / HOT BOILER • Starting of the boiler immediately after trip-Hot Startup pressure >10 kg/cm2 • When the boiler is still in Hot / Warm with the drum pressure more than 03 Kg / cm2 g - Warm Startup.
- 5. Boiler Cold Start Up- Essentials • HT Power (If Applicable) • LT Power • 24 v/110v Power, DCS,SCADA Panel Readiness • UPS • Instrument air • Service air • Cooling water • Fuel • Panel & Field Operation team
- 6. Boiler Start Up Sequence • Walk Down checks • System line up • Water filling • Burner Checks • Fuel gas charging • Equipment (aux) in service • Boiler start and pressurizing. • Pressurizing and loading. • Boiler auto operation
- 7. Burner Start Up & Shut Down • Burner Management System (BMS) • Preinterlocks to be satisfied • Purge interlock to be satisfied • Main interlocks to be satisfied • Burner ready to start • Burner on • Flame monitoring • Putting Controllers in auto mode • Boiler Loading • Burner stop/trip • Gun Clearing • Boiler shut down
- 8. BOILER OPERATING PROCEDURES This subsection describes the boiler operation as applicable for the following boiler conditions: Startup of a cold boiler. Startup of warm / hot boiler. Boiler shutdown.
- 9. STARTUP OF A COLD BOILER Boiler startup can be termed as cold startup when there is no pressure or when pressure is less than 2-kg/cm2 G in the steam drum. In a cold start up, possibilities of some inspection or maintenance works having been done is presumed. A walk down check is required and the boiler and its auxiliaries are to be prepared meticulously for a start up from the DCS of the control room. Before a walk down check ensure that all work permits have been returned, tags are removed and maintenance permission for boiler start up is available.
- 10. WALK DOWN CHECKS Using powerful torches or low voltage inspection lamps, inspect the furnace and the second pass of the boiler and ensure the following: BOILER Furnace and second pass are clear, all maintenance personnel have been withdrawn and no scaffolding or inspection devices have been left inside. Furnace floor, water walls and second pass convection tubes are clean and there is no evidence of any water drips, slag or any other deposits. Steam and water drums are clear and manholes are closed properly.
- 11. BOILER….. Verify that all access doors, inspection doors of furnace and wind box are tightly closed. Verify that all peepholes have been tightly closed and sealing/ cooling air connections have been made and the air valves are open. Verify that the air duct, wind-box, external economiser and flue gas duct to stack is clear and that all maintenance personnel have been withdrawn. Ensure that the manhole doors are closed properly. Verify that the safety valves are not gagged. Verify that the illuminators of local level gauges are 'ON'. Inlet valves from steam and water-side are open and their drain valves are
- 12. BOILER…. •Verify whether the Nitrogen purging valves are open (connected to the steam drum and main steam piping). If they are open, they have to be closed before boiler light up. BURNER Verify that the pilot & fuel gas, fuel oil and atomizing media flexible hoses are connected to the burner at the proper locations. Verify that the Igniter has been inserted in the burner housing and that the HT cable has been connected to the igniter.
- 13. BURNER…… •Verify that the Flame Scanners have been installed properly, cooling air hoses are connected and valves are open. Verify that the wind-box dampers fully open and air supply to actuator is available. Verify physically and in DCS that the fuel shut off valves is fully closed and bleed valves are open. The manually operated fuel isolating valves are to be verified for shut. These valves have to be opened just before boiler start up.
- 14. FD FAN Check that the fan inspection doors are closed. Ensure that the bearing lubrication is done properly. Check that the coupling guards are installed. Ensure that the damper actuator is properly fixed to the operating lever and air supply is open to actuator and I/P Converter.
- 15. GENERAL Ensure that the battery limit valves are open and following inputs are available: Instrument air and Service air. Feed water up to level control station. Power up of feeders for FD fan motor and all the other drives. Fuel gas and pilot gas up to terminal point isolation valve. Chemical dosing supply at terminal point. Cooling water supply at terminal point. Refer the respective pump & fan vendor manuals for the preparation and pre start up checks to be done on the equipment.
- 16. SYSTEM LINE UP PRELIMINARY REQUIREMENTS Power supply: ensure that the power supply is switched 'ON' and available for all the motors and panels. Operating station - DCS is to be ensured for readiness. As instrument air is necessary for the operation of most of the valves and actuators, charge the instrument air header, the branch lines and instruments supply lines elsewhere. Root valves of all Instruments (Pressure gauges, Pressure transmitters, DP transmitters, Level gauges, etc.,) must be kept open and their drains if any are to be kept closed. Ensure that all the transmitters are lined up
- 17. BOILER ECONOMISER Keep the air vent on the outlet headers of all the Economizer modules open. The vent has to be closed as soon as air is vented from modules and water comes out continuously during initial filling of the boiler. Ensure that the drain valves of bottom headers are kept closed. STEAM AND WATER DRUM Ensure that the drum vent valves are kept open. These valves will have to be closed when the drum pressure attains 2 kg/cm2 G. Open the CBD line isolation valve and keep
- 18. STEAM AND WATER DRUM…. Open the IBD line isolation valves and keep close the IBD valve. Keep the phosphate dosing line isolation valve open. However, the dosing will be started after the boiler lightup. Verify that drains and vents of Standpipes provided for level transmitters, indicator & gauges are closed. Ensure that the drain and vent valves of the rear wall panel header are closed. Open the drain valves of drain header. Once there is no water/ Condensate coming out, these valves have to be closed. The isolation valve from the drain header to the common BD tank is to be kept open. Ensure that the boiler fill-up line is kept closed.
- 19. SUPERHEATER…. Ensure that the drain valves of the outlet & intermediate headers of the superheater & the attemperator are kept open. This is to ensure that the superheater coils are totally drained before lightup of boiler. Keep open the startup vent isolation valve. The control valve has to be kept open 100%. Ensure that the MSSV and pressure equalization valves are in close position and remote selected. Keep the battery limit isolation valve close. Open the drain valves on the steam line and let the condensate get drained.
- 20. FEEDWATER SYSTEM FEEDWATER AND SPRAY WATER LINE Select the drum level and feed water flow controllers in manual mode and close the 30% & 100% control valves. Open the manual isolation valves of the feed- water Flow Control Valves. Keep close MOV’s Open the 30% Control Valve Motorized isolation valve open, when CV is to be taken in line. Select the steam temperature controller into manual mode and keep the output fully closed.
- 21. FEEDWATER AND SPRAY WATER LINE... Open the manual isolation valves of the spray water control valve. Keep the bypass valve close. The drain valves on the feed water and the temperature control stations have to be kept closed. SAMPLE SYSTEM All the sample lines isolation valves have to be closed. Sample coolers can be taken into service once the boiler is pressurized (approx. 2-kg/cm2 ).
- 22. FILLING WATER IN BOILER The following procedure is adapted for filling the boiler: During water filling, for assurance of correct steam drum water level, post an attendant at the drum level to monitor the local gauges and to communicate to the control room when the level of '50 mm' below NWL is reached. Operator to ensure that the feed water control valves are closed and the controller is in manual mode. The boiler fill-up line on the drain header is opened and the isolation valve to the BD tank on the drain header is closed. The drain valves on the front & rear wall panel bottom headers have to be kept opened for the boiler fill-
- 23. FILLING WATER IN BOILER……. • Request an attendant to close the air vents on front & rear wall panel top header as soon as water comes out from it continuously. Water rises to the steam drum through the furnace front & rear wall header. It will take minutes to fill water to the required level. As water level in steam drum reaches '50 mm' below the NWL on local level gauge (confirmation by the attendant) and in level indicator, water filling can be discontinued. Close the fill-up valve on the drain header and also close the drain valves on the furnace front & rear wall
- 24. FILLING WATER IN BOILER……. Caution: Filling water temperature should not be more than 38 deg-C of the boiler metal temperature. During filling of a cold boiler, the ambient temperature should be used as an indicator of the boiler metal temperature. Assuming an ambient temperature of 40 deg C, the maximum temperature of filling water shall be 78 deg C.
- 25. BURNER CHECKS Verify that the natural gas spuds of Burners has been manually cleaned and inserted in the burner housing. Verify that the natural gas pipeline has been connected through flexible hoses to burner. Verify that the Ignitor has been inserted in the burner housing and connected to the pilot gas line through the flexible hose. Verify that the HT cable has been connected to the ignitor. Verify that cooling air, sealing air connections to the burner components. Verify that the (UV+IR) Scanner has been inserted and cooling air hose connected and valve is open. Verify on DCS that natural gas shut valve,pilot gas admission valve is open (DCS-BMS program). Also verify the valve positions on BMS local panel. The manually operated isolating valves are to be verified shut. These valves are opened just before burner light up. Carry out similar checks for other Burners.
- 26. CHARGING OF NG Natural gas lines must be checked carefully before charging to ensure that there are no possibilities of leak through flange joints, valve bonnet, pressure gauge tapings etc., During charging if any minor leaks are noticed, maintenance personnel must be called to attend to the leaks. In case of serious leaks, charging must be abandoned, line to be depressurized, purged and leaks to be attended before proceeding further. If gas leak detector is available, make use of it.
- 27. CHARGING OF NATURAL GAS The charging of the natural gas can be done as follows - •Close the Inert gas isolation valve. Verify that the root valves of all the instruments are open. Verify that all the fuel gas and Pilot gas trip valves (shut off valves) are in ‘close’ position - physically at field and in DCS. Open the Inlet and Outlet isolating valves of the fuel gas Pressure Control Valve and keep close the Flow control valve. With information to the plant, crack open the terminal point isolation valve, while keep open the down stream valve.
- 28. CHARGING OF Natural GAS …... •Observe gas flow upto Main Fuel Trip Valve in the pressure indicators. Observe for leaks if any. Then slowly increase the opening of the terminal point isolation valve. If the gas line upto burners is filled with inert gas, it has to be purged out through the header vent valve with the fuel gas just before burner light up. This can be done by force opening the Main Fuel Trip and start up PCV. Once it is ensured that the fuel gas is available. Normalize the Main Fuel Trip Valve & start up PCV. Then open the burner natural gas manual isolation valves for the burner to be fired. Keep isolated other burners. Open the isolation valve of pilot gas line. Observe that the gas pressure is read by the indicators and pilot gas pressure low alarm extinguishes. Burner inlet pilot gas manual isolation valves shall be opened.
- 29. PLACING FD FAN IN SERVICE Check lubrication of the fan bearings. Check that the fans are free to rotate. Check that the flaps on their discharge ducts are in the closed position and are free to swing. Check that the fan inspection doors are closed. Check that the suction dampers are in the closed position. Ensure that the power supply is available for the feeder in MCC (Motor driven FD Fan). Ensure the oil level in the bearing housing. FD fan can be started either from DCS or from local burner panel. The remote / local selector switch to be positioned according to the startup location decided. Start the FD fan after selecting the airflow controller
- 30. PLACE FD FAN IN SERVICE…. On giving START command Observe that the fan runs up to speed and comes into service. Observe that there are no excessive vibrations (by feel) in the bearings and no abnormal noise from the fan, turbine & gear-box or motor. Observe the bearing temperature stabilize. Observe that the motor casing temperature is normal. Now the fan is ready for furnace purge. Purge start command may be given if required.
- 31. CHARGING OF PILOT GAS If the Natural Gas is available then the isolation valve for pilot gas line is to be opened and the pilot gas to be charged.The discharge pressure of the PCV is to be maintained to around 1 Barg. (This will be again set finally as per reqt during commissioning). If required, suitably adjust the self-acting PCV. LPG back-up has been provided in case NG is not available and burner needs to be started with oil firing.
- 32. BOILER STARTUP AND PRESSURISING The preparations for a boiler start up so far described can be summarized as below: Walk down checks of the boiler, economiser, FD fan, air & flue ducts and pipelines. System/ valves line up of the boiler & economiser, feed water system, steam line and drain lines. Filling the boiler with feed water and normalizing the drum level. Charging the fuel oil , fuel gas & pilot gas lines. Now the boiler is ready for light up. Before light up it is necessary to understand BMS logic.
- 33. BOILER PRESSURISATION AND LOADING Once the burner is ON, Boiler will be pressurized according to the pressurizing curve. After establishing flame in burner, the operator takes care of the following: Operator has to take care of proper fuel to air supply. Operator has to inspect the flame in the furnace through peepholes. Clear furnace with proper flame is important. Monitor the steam drum water level. Monitor the parameters, which can cause a boiler trip as explained in BMS logic. Line up the phosphate dosing to steam drum at a slow rate.
- 34. BOILER PRESSURISATION AND LOADING…. • Operator has to be on alert for swelling of drum level when the boiler water temperature builds up to 90 0 C. Open the IBD valve and regulate such that the water level is within limits. IBD Valve is to be closed once the water level is normalized. Modulate the firing / fuel flow so that the boiler pressurization is following the pressurization curve. Firing rate has to be increased manually from DCS console. Positive air fuel ratio has to be maintained during modulation. In the process second burner may be started. Start vent/ pressure control valve may have to be throttled in the process of pressurization. Aim is to attain rated steam flow with the rated steam pressure and temperature.
- 35. BOILER PRESSURISATION AND LOADING…. At 2 kg/cm2 G drum pressure drum vent valves have to be closed. At 2 kg/cm2 G drum pressure superheater drain valves have to be closed. Once the swelling is over and the drum level normalizes, select the drum level controller into single element control and into auto mode with set point corresponding to the NWL. Monitor that the drum level controller is maintaining the level automatically. Operator shall have a walk down check of feedwater line, fuel gas line, the boiler and the valves. Any water, steam leakages (through valve glands, bonnets, flanges) and other minor abnormalities noted can be corrected when the boiler pressure is less than 5 kg/cm2 G.
- 36. BOILER PRESSURISATION AND LOADING…. When the boiler is in service, the Operator has to be alert to the alarms appearing on DCS. When an alarm registers, the Operator has to check the cause for the alarm. Corrective steps have to be taken wherever necessary to restore normal values. Quick action by the Operator prevents further deterioration of values to the boiler trip level. Steam temperature controller shall be put into auto mode in DCS console with the set point. At around rated super heater outlet pressure MSSV bypass valve can be opened to warm-up the main steam line to plant or to equalize the pressure. The trap units in the main steam piping to be lined up.
- 37. BOILER PRESSURISATION AND LOADING…. Line up of battery limit isolation valve. If the downstream of the battery limit valve is in pressurised condition, pressurize the upstream by opening the MSSV bypass valve. Once the pressure is equalized on both the sides of, MSSV can be opened. If the downstream of is not under pressurised condition, it can be opened just before opening the MSSV. When the boiler main steam line pressure reaches rated pressure and steam flow above 25 % MCR, MSSV can be opened to connect the boiler with plant main. As the MSSV is opened the start-up vent valve shall be closed. MSSV bypass pressure equalization valve shall be closed. Once the steam flow is reached above 30% MCR, drum level control shall be changed over to 3-Element
- 38. Now, Combustion control shall be taken into auto mode (Boiler master pressure control. ) Depending on requirement, required no. of burner can be started. Other burners can be started either from DCS or from LBP by keeping the combustion control either in auto mode or temporarily keeping it in manual mode. Operator to adjust flow through SH steam, saturated steam and boiler water sample coolers so that a small stream of cold samples are obtained from them. Call for a laboratory analysis of the SH steam, boiler water and boiler feed water sample. Necessary water & steam parameters to be maintained by adjusting the chemical dosing rate BOILER PRESSURISATION AND LOADING….
- 39. COLD STARTUP PRESSURE RAISING CURVE
- 40. HOT STARTUP Restarting a boiler immediately after a trip when the boiler is still hot is termed as a hot restart. Restart of the boiler above a steam pressure of 10 kg/cm2 G is termed as a Hot startup. In a Hot restart, as the boiler was in service till the trip out, the required valve line up (as stated for the cold start up) will be available and can be quickly checked by visual inspection. Hot restart requires operations to be done fast, within the maximum permissible firing rates so as to obtain quickly the required main steam pressure and temperature conditions. Following are the steps involved in the hot startup: •Normalize the steam drum level by modulating the feed water flow control valve.
- 41. HOT STARTUP…. • As soon as the burner is started open the Start up vent valve. • Once the flame is stabilised, increase the firing rate according to the rate of pressurisation specified in the pressurisation curve. • Maintain the drum level. • Once rated pressure and temperature is reached connect the boiler to the main header and close the start up vent. Note: • On a trip of Boiler, it is not always necessary to close the main steam valve stop valve. A non-return valve ensures that there will be no back flow of HP steam from the plant mains to Boiler. However during restart, care must be taken to build steam temperature along with steam pressure so that, when steam starts flowing to the plant mains it is of acceptable temperature.
- 42. HOT START PRESSURISING CURVE
- 43. WARM START-UP Re-start of the boiler with steam pressure not less than 03-kg/cm2 (g) is termed as a warm start-up. The preparations and valve line up required is as for Hot restart. However as the HRSG has been idle for several hours, the valve line up requires to be verified Super heater drain operations as for hot restart Opening HP steam line drain, start up valve, dump valve as for hot restart. The time taken for a warm start may be more than that hot start but less than that of a cold start all other operations as for cold start apply.
- 44. WARM START-UP PRESSURISING CURVE
- 45. BOILER OPERATION WALK DOWN CHECK LIST BOILER • Check for unusual noises. This may be from steam or water leakages. Check for steam or water leaks from valves, connections and fittings. Check for unusual traces of water on floor. Check for air / flue leakage from windbox, boiler casing and ducting. Walk around the furnace exterior and observe for any hot spots or gas leaks. Check position of fan suction and discharge damper and check for obstruction. Check for passing from safety valves at normal operating pressure. Check that the drain lines and drip pans are not plugged.
- 46. BOILER….. Check for clear & bright flame and clear stack Check to see that proper water level is being shown by the direct water level gauge. Check for water or steam leak from ports or drain connections, which will cause a false water level in the gauge glass. Inspect the glass for discoloration or fouling Check for any obstruction for thermal expansion ROTARY EQUIPMENTS Listen for any unusual noises Feel and inspect for vibration regularly Inspect bearing temperature Inspect for lubrication level Check for any abnormality in couplings Check current and voltage of motors
- 47. LOG SHEET It is suggested to record the boiler parameters during startup and normal operation. Observed abnormalities (if any) recorded can be used for analysis, troubleshooting and maintenance purposes. Log sheet to be filled once in every hour by the operating staff. Feed water, boiler water quality are also to be noted once in four hours. Total gas consumption and steam production of a day to be noted. Logbook should furnish the details about Boiler trips with reasons and time. Boiler running hours. Boiler shut down details (forced or planned, outage hours, jobs carried out, etc.,)
- 48. EMERGENCY PROCEDURES LOW WATER LEVEL CAUSES Feedwater control system failure. BFP failure Tube leak ACTION • Compare control room indication with gauge glass level. If the water level falls out of sight due to momentary failure of water supply system, due to negligence of the operator, due to momentary fluctuations that might occur with extraordinary changes in load, appropriate action should be taken at once to trip the fuel. Any decision to continue to operate, even if only for a short time at a reduced rating, would have to be made by someone in authority who is thoroughly familiar with the circumstances that led to the emergency and positively certain that the water level can be restored
- 49. LOW WATER LEVEL….. In the absence of such a decision Stop the burner immediately Shut off steam flow Simultaneously, if feedwater has become available and the operator is assured that no pressure part has been damaged Take the feedwater control system into manual mode Allow the water flow to boiler gradually to normal water level. (Do not hurry up which may lead to sudden quenching and tube leak) if pressure part damage is suspected Reduce the steam pressure gradually Open the drum air vent when the pressure drops below 2 kg/cm2
- 50. LOW WATER LEVEL…. • Cool the boiler so as to examine the extent of damage Drain the boiler after cooling If any tube rupture and bulging is observed rectify the same If any tube leakages were observed in convection bank near drums, re-roll the leaking tubes After the repairs conduct hydrotest Determine the cause of low water
- 51. HIGH WATER LEVEL CAUSES Feedwater control malfunction Operator error Instrument air supply failure Foaming ACTION Take the drum level control loop into manual mode Reduce the water level immediately by operating the intermittent blow down to maintain the drum level Reduce the steam discharge rate, if necessary Start the stand by compressor if required
- 52. FURNACE PUFFING & BACK FIRING CAUSES Uncontrolled feeding of fuel into the furnace Sudden increase in FD fan air Tube rupture ACTION Take airflow control system into manual mode and maintain the furnace pressure Never increase fuel / air flow suddenly. Always increase air first then gas while taking boiler on load In case of tube failure take the boiler out of service immediately Keep the water wall tubes clean by monitoring the combustion conditions and water quality
- 53. BOILER EXPLOSION CAUSES Accumulation of fuel in the furnace due to incomplete combustion Loss of ignition Fuel block valve leakage Frequent unsuccessful startups In-sufficient purging of furnace With the mixture of unburned fuel with air in explosive proportions and the application of heat sufficient enough to raise the temperature of the mixture to ignition point. ACTION Trip the burner immediately Purge the furnace extensively Analyze the reasons for explosion and rectify the system Evacuate or clean the furnace to the possible extend
- 54. OPERATIONAL PRECAUTIONS FOR SAFETY The handling and burning of any fuel is potentially hazardous. Some fuels ignite more readily than others do. Safe handling and operation demands knowledge of the characteristics of the fuel and careful observance of necessary precautions. Because of the rapid diffusion of gas, any leak of gas into an enclosed space may result in an explosive mixture of air and gas. One of the requirements of good combustion is intimate mixing of fuel and air. In case of gaseous fuels such mixtures are potentially explosive and can be expected to ignite from a single spark. Therefore these mixtures must never be allowed to exist except inside the combustion chamber
- 55. OPERATIONAL PRECAUTIONS FOR SAFETY…. • Never allow fuel to accumulate anywhere, other than in lines that form a part of a fuel supply system. Steps should be taken immediately to clean or purge the area thoroughly and then locate the source. Carry out routine checks to detect leaking fuel Purge the furnace completely before introducing any light or spark Maintain a positive airflow through the burners into the furnace and up the stack Ensure operation staff is alert to all abnormal conditions.
- 56. OPERATIONAL PRECAUTIONS FOR SAFETY… To observe the above rules, automatic control instruction include Purge interlocks e.g. requiring a specified minimum airflow for a specific time period sufficient to purge the setting before the burner is started Flame Detector for the burner connected to an alarm and interlocked to shut-off the fuel to the burner, it serves upon flame failure Closed position switches for burner shut-off valve, requiring that fuel shut- off valve be closed to permit opening the fuel trip valve. Shut-off of fuel on failure of FD fan Shut off of fuel in the event of electric power failure or instrument Air failure
- 57. OPERATIONAL PRECAUTIONS FOR SAFETY…. Operating the boiler with low feedwater temperature will result in corrosion of economizer coils. The raising and lowering of steam parameters should be restricted to the value given in the starting diagram. Exceeding these values will result in reduced fatigue life of pressure parts. In case of tube failure, which can be identified by hearing the noise in the boiler gallery and increase in draught pressure, flue gas and steam temperature, and the boiler should be shutdown at the earliest by regular procedure for maintenance work. Otherwise large number of tubes may fail due to steam erosion and
- 58. OPERATIONAL PRECAUTIONS FOR SAFETY…. Boiler salt in the wet steam will accelerate corrosion. Always use deaerated de-mineralized water for boiler feeding. Carryover of salt in steam occurs either due to mechanical or vapor carryover from steam drum. Efficient drum internals can only reduce mechanical carry over. Silica is always carried over in vaporous form. Continuous monitoring of sodium and silica in boiler water and steam is desirable. Before operating a boiler, ensure complete knowledge of water chemistry. Whenever boiler is started after a shutdown of more than 3 days, check all safety interlocks before boiler start up for proper functioning
- 59. OPERATIONAL PRECAUTIONS FOR SAFETY…. The steam drum should normally be filled up to the point when water is showing in the bottom part of the gauge glass. This is to allow for the swell on heating and to reduce any blowing down resulting from this cause to a minimum. Once the boiler is boxed up, the water level in the steam drum must be raised to the very top of the drum. Filling the drum like this will prevent excessive temperature differentials along the drum wall. The water is then shut-off and the boiler is allowed to cool. While raising pressure on boilers, continuous firing is recommended, since intermittent firing may result in re-condensation in the tube loops.
- 60. TUBE FAILURES Operating the boiler with a known tube leak is not recommended. Steam or water escaping from a small leak at pressure can cut other tubes by impingement and set up a chain reaction of tube failures. Large leaks can be dangerous. The boiler water may be lost, the ignition may be lost and boiler casing can get damaged. Small leaks can sometime be detected by the loss of water in the cycle or system. A loss of boiler water chemicals or by the noise made by the leak. If a leak is suspected the boiler should be shut down as soon as possible by following the normal shutdown procedure. After the exact location of the leak or leaks is located, the leaks may be repaired by replacing the failed tube or by splicing in a new section of tube, conforming to relevant ASME code.
- 61. TUBE FAILURES….. An investigation of the tube failure is very important so that the condition causing the tube failure can be eliminated and future failures can be prevented. This investigation should include a careful visual inspection of the failed tube and in some cases a lab analysis. It is recommended that every effort be made to find the cause of tube failures before operation is resumed.
- 62. BOILER SHUTDOWN A Boiler shut down can be of two types: Planned shut down where the Operator gets advance notice and adequate time to shut down the boiler in an orderly manner, for inspection, maintenance or for cold reserve. Boiler Trip on interlock protection or emergency shutdown by the Operator.
- 63. PLANNED SHUT DOWN On a planned shut down operator to carryout the following activities in the sequence: • Reduce the boiler load gradually. This is to be done in such a manner that while reducing the firing as the oil/gas pressure approaches minimum prescribed value burners are to be withdrawn one by one from top. • Close the MSSV. Take the combustion control loop into manual mode. • Then stop the bottom burner manually from DCS. • Close the start-up vent isolation valve. Ensure that the pressure control valve is closed and select the controller into manual mode. • Carry out the furnace purge. • Stop the FD fan and close the suction damper.
- 64. PLANNED SHUT DOWN ….. • Continue to feed water to the boiler to maintain normal water level. Drum level controller shall be changed to single element mode and maintain the level. Once the level is normalised, take the feed water / drum level controller in to manual mode and close the valve (output as 0 %). • Stop phosphate dosing. Close the CBD valve. • Verify the EBD valve is closed. • Allow the boiler to cool naturally. • Scanner cooling air supply is continued till the furnace temperature falls below 100 0 C or for at least 8 hours. • If the proposed shut down of the boiler is for more than 8 hours, fuel supply has to be isolated. • When the steam drum pressure drops to 2-bar (g), open the steam drum vent and the start-up vent.
- 65. BOILER EMERGENCY TRIP Sometime boiler trips either by a protection or due to a power failure or by an emergency trip command by the operator causing the fuel block valve to close and abruptly stopping the firing. • Under such emergency trip conditions the operator action is - • For protecting the boiler • For investigating the cause of trip and if possible to eliminate the cause If the cause is eliminated, to restart the boiler If the cause cannot be immediately eliminated to continue the shutdown till it is done
- 66. BOILER PROTECTION AFTER A TRIP BOILER PROTECTION AFTER A TRIP • Close main steam stop valve immediately • Continue to feed the water if level requires to be maintained • Check that fuel block valves are closed • Check that there is no flame in the furnace • Carry out post trip purge of the furnace
- 67. BOILER SHUTDOWN DURING SUDDEN TUBE FAILURE • In case of a tube rupture followed by a severe loss of drum water level, it is advised immediately to stop the burners, purge the furnace and also isolate the steam & feedwater system. This is to avoid undue thermal stresses on pressure parts. The drum vent & start up vent have to be opened as the pressure falls to 2 Bar (g). The extent of the damage is to be ascertained after cooling the boiler completely. • In case of a sudden tube failure, if the water level could be maintained and flame is not lost, it is recommended that the boiler be taken out of service as soon as possible. The impingement of steam & water from a leaking tube on adjacent tubes can cause additional failure of tubes. In such cases, the normal planned shutdown procedure is immediately implemented.
- 68. Boiler Operation • Any Question?
- 69. Thank You !!!