Boiler safety protection

Boiler safety protection

1. Boiler Safety Protection

2. PROTECTION AND INTERLOCKS Introduction :- In modern power plant continuous and reliable operation of various auxiliaries if a unit becomes necessary. Extensive protection are provided to trip the boiler or turbine or any individual auxiliary in case of faults so that the faulty equipment is isolated without causing further damage. Interlocking is provided to switch on the reserve equipment in case of tripping of the running equipment to enable continuity of functions .

3. Boiler Safety Protection Boiler protection are provide to detect the abnormal condition by constantly measuring the various parameter under normal and faulty condition. If any abnormality happened trip the boiler to prevent from major damage.

4. Boiler Steam Drum water Level High and low Main Steam Pressure High Main Steam Temperature High and Low Bed Temperature High and Low Furnace Draft High Furnace Explosion Flame Failure Water wall / Screen tube / Evaporator tube Failure Super heater tube failure Coal feeder failure APH-PA and APH-SA tube failure Boiler feed Pump failure Fan Failure

5. Drum Level Low Cause:- Failure of boiler feed pump. Failure of drum level controller. Inadvertent opening of C.B.D. Change in load. (Sudden reduction in load) Water tube failure. Effect :- Overheating of water wall tubes due to starvation Action :- 1.Run the boiler if drum level is within safe limit (Low alarm and lo-lo alarm came). If it goes beyond safe limit allow boiler to trip. 1.1.Attempt to control with feed valve. 1.2.If feed valve is stuck up open parallel line valve or bypass valve. 1.3.Take care while increasing feed flow that running B.F.P. do not trip on feed flow high. If low level is due to tripping of feed pump start stand by pump, if it is on manual. 2. If water level is below safe level, allow to trip the boiler on drum level lo-lo to protect the boiler drum and tubes.

6. Drum Level High Cause:- Failure of drum level controller. Extra ordinary increase in load Sudden increase in firing rate Effect :- Carry over with sharp fall in super heater temperature Water may enter turbine and serious damages to turbine may occur Super Heater deposit and fouling Turbine Deposit and increase in sp. Steam consumption Action :- 1.Run the boiler if drum level is within safe limit (drum level high and high high alarm came). If goes beyond limit allow boiler to trip. 1.1. Attempt to control with feed valve. 1.2. If feed valve is stuck up close isolating valve. 1.3. Attempt to control drum level by opening E.B.D. 1.4. Close Economizer inlet valve and allow feed pump to run in recirculation. 1.5. If above measures fail trip the feed pump. 2. If level is due to sudden rise in load. If high level rises above normal level allow boiler to trip on drum level high high . 2.1. If main steam temperature drop takes place open steam pipe drains. 2.3. Trip the turbine prime mover when steam temperature falls.

7. Main Steam Temperature - High Cause:- Low feed water temp HP Heater not in service at constant firing / load. Sudden increase in firing rate to increase steam pressure. In adequate spray. Lodging of soot on the water walls considerably. Delayed combustion Effect :- Rise in (+ ve ) positive turbine expansion. Creep rate increase in tube metal, turbine parts, steam piping Increase of heat load on condenser Action:- Check feed water temp. If HP heaters are available, put in service and if not, reduce the load. Slow down firing rate. Reduce excess air, if more. Check Spray control Use tilting or cut down upper level burner

8. Main Steam Temperature - Low Cause:- Dirty super heater Inadequate air flow. High Spray Too high feed water temp. Sharp increase in load and pressure drop. High Drum level Faulty Separators Effect :- Turbine expansion may be negative. May induce thermal stresses in S.H. Chance of turbine LP blade damage due to water hammer Action :- Check air flow – increase, if necessary. Reduce spray, if more. Check feed water temp. Avoid sharp rise in load to boiler pressure drop. Check health of separators at the next possible opportunity If, it is due to high drum level – allow unit to trip and open main steam drains Check health of separators at the next possible opportunity.

9. Main Steam Pressure - High Cause:- Sudden drop in load Sudden drop steam flow. Un-controlled fuel entry. Turbine trips. Effect :- Disturbance in drum water level. Safety valves may life if pressure rise is up to that extent. Boiler may trip on high pressure. Action :- Open start up vent valve. Control fuel input and drum level. Check coal feeder feeding rate. Adjust the load on unit, if it has dropped. If turbine has tripped first allow boiler to trip but safety valve may lift.

10. Bed Temperature - High Cause:- High CV and low ash coal used Low P.A. flow / S.A. flow Sudden change in load Faulty bed temp thermocouple Ash recirculation system trouble Faulty Bed Ash Draining Effect :- Chances of clinker formation Chances of refractory failure Chances of screen tube failure Action ;- Control bed temp by recirculation ash through recirculation feeder Start sand feeder if bed height permits. Increase P.A. flow up and increase S.A. flow. Reduce the load by cutting coal feeder. Coal feeders trip, if temp rises above limit and allow boiler to trip, if temp exceeds beyond safe limit to save bed from clinkerisation. Check the bed temp thermocouple.

11. Bed Temperature - Low Cause:- High PA with respect to load / high SA flow w.r.t . load. Low CV high ash coal used. Coal interruption or coal feeder trips or overfeeding of bed material into furnace. Recirculation ash feeder speed may increase. Faulty bed temp. thermocouple. Water tube / screen Effect:- Boiler steam flow will reduce. Super heater temp drops. Furnace draft will fluctuate. Action:- Reduce P.A. Fan flow, if excessive / reduce S.A. flow, if excessive. Reduce speed of recirculation ash feeder. Stop bed material supply, if running. Check bed temp thermocouple. Check any leakage sound from furnace. Check coal feeder and coal supply.

12. Furnace Draft High Cause:- Due to faulty operation of fan control. Disturbed combustion Un-controlled fuel entry Effect :- Boiler may damage due to high furnace pressure. Weak parts of furnace (ductings and enclosures) may explode due to high furnace pressure. Action :- If it is due to faulty operation of I.D./P.A./S.A. fan Control, take it on manual and maintain the furnace in negative. If furnace pressure has increased beyond limit allow boiler to trip on furnace draught high.

13. Furnace Explosion Cause:- Accumulation of un-burnt fuel during lit up / Hot gas path/start up of boiler. When un-burnt fuel gets sufficient air, it cuts fire instantly in a confined space causing an explosion. Improper burning. Inadequate air. Secondary combustion. Over riding Purge permissive during restart after flame failure Effect:- Furnace explosion Can cause extensive damage. Action:- Always purge the boiler with 40% full load air for about five minutes. No cut short in purging allowed. Adjust fuel air ratio. Ensure the idle burner fuel shut off valve is closed. Regularly check that the furnace safety system and the trip interlocks are functioning properly. Analyze flue gas regularly. Flue gas should not contain carbon monoxide. Maintain proper furnace draft. Boiler should have an auto trip interlock in case of high furnace pressure.

14. Flame Failure Cause:- Dirty (Quality) Oil Improper atomization Faulty Flame Scanner Failure of Blower Furnace Pressure High Low Combustion Air Effect:- Boiler will trip on flame failure Chances of furnace explosion, if un-burnt oil / coal mixture entered in furnace. Steam pressure fall. Super Heater Steam Temp. drop sharply. Variation observed in drum level. Action:- Purge the boiler before putting burner back and purge burner as per cycle time given by supplier. Check the flame scanner and clean the photocell, if found dirty. Check the sparking circuit and high voltage transformer supply. Check spark plug, if found dirty, clean it and set the gap. Clean the burner tip, if nozzles (holes) are found chocked. Check the air blower and maintain air fuel ratio. Check the explosion doors, close it if opened.

15. Water wall / Screen tube / Evaporator tube Failure Cause :- Starved water wall due to low drum level Block tube, eroded tube, pitted tube, salt deposits Circulation affected due to open low point drains Effect :- Hissing steam leakage noise from boiler. Drop on Boiler Water Conductivity Unstable flame fluctuating draught. Bed temperature drops sharply. High feed water flow for given steam generation / increase in make up water. Action :- Every shift check low point drain valve are fully closed. Every shift check the furnace to detect steam noise. At first diagnostics of tube failure, start load reduction; trip out unit before damage become serious. Quicker shutdown reduces extensive damage. Try to locate tube at low level. Don’t allow the drum level to go beyond a danger level. Start another BFP, if drum level is not maintained by running BFP.

16. Super heater tube failure Cause :- Sustaining high metal temp due to water wall slogging or inadequate steam flow and high gas temp during hot start . Erosion on tubes due to high excess air. Blocked tubes. Starvation of tubes. Salt deposition due to high water level in drum. Poor quality of spray water. Effect :- Flue gas temp drops. High feed water consumption compared to steam flow. Over loading on I D Fan Erosion of other tubes and damages to other SH tubes. Action :- As soon as leakages noticed start reducing the load and trip the boiler. Listen the SH region for steam leakage. Try to locate leakage through manholes before the boiler is depressurized.

17. Loss of Coal feeder Cause :- Electrical supply failed V.F.D. fault Both PA Fan TRIP. Bed Temperature high(940 DegC ) Bed Temperature low(400 DegC ) Furnace Pressure High(>100 mmwc) Furnace Pressure Low(- 150 mmwc) Drum level Low(<10 %) Drum level High(> 75 %) Syphone Temperature (450 DegC ) Main Steam Temperature High(955 DegC ) Effect :- Boiler pressure may fall down SH steam temp fall sharply. Bed temp. will decrease. Variation in furnace pressure(Low) Variation in drum level (Low) Action :- Control boiler pressure by reducing load on turbine /prime mover or process. Control super heater steam temp by closing spray valve. Reduce PA / SA flow to control bed temp. Control furnace draft and drum level. Check electrical fault or emergency stop push button & restart coal feeder. Check VFD fault, if any. Restart coal feeder after checking cause of failure.

18. APH-PA and APH-SA tube failure Cause :- Erosion of tube. Corrosion of tube. Effect :- Flue gas temp after APH will fall down. Increases in O ₂ percentage at ESP inlet. PA / SA Fan overloaded. Bed temp may be increased. Action :- Control flue gas temp by passes PA-APH. Reduce coal feeding to maintain O ₂ percentage. Control bed temp. If leakage of tubes are more stop the boiler and plug / replaced.

19. Boiler Feed Pump Failure Cause :- Electrical Motor protection relay operates Lube oil Pressure Low Discharge flow high Motor Wdg Temperature high(140DegC) Motor Bearing Temperature High(>90DegC) Pump Bearing Temperature high(>110DegC) Pump Vibration High(>9.3 mm/sec) DP Across Strainer High(2500 mmwc) Deaerator level low(< 20%) Motot Bearing Oil Pressure Low(0.1) Effect :- If standby pump is available, it will start on auto or start it manually. If standby pump does not start on auto & manual, drum level may go very low, which may trip the boiler. Action :- Start the stand by pump, if it does not start on auto. Adjust the load to maintain the drum level as well as to prevent the tripping of this pump on high feed water flow. See the proper operation of recirculation valve and C.W. Valve. Analyze and rectify the fault in the main feed pump. Put it on auto. Restore the bus supply, if it has lost and make the pump available. Check the Deaerator level and if it falls below safe limit, stop the feed pump, immediately. Check the BFP suction strainer and clean it, if found chocked.

20. Loss of ID Fan Cause :- Electrical motor protection Furnace Pressure Low(- 150 mmwc) Fan bearing temperature becomes high Motor bearing temp becomes high Drive fault (MCC fault) Effect :- Furnace may get pressurized Boiler will trip on boiler furnace pressure high. Action :- Check emergency or field stop. Rectify electrical fault, if any. Check cause for boiler trip & normalize it. Check fan / motor-bearing temp. Restart I.D. Fan after checking cause of tripping and taking corrective actions.

21. Loss of PA Fan Cause :- Electrical motor protection. PA O/L Pressure High(>1800 mmwc) Motor Wdg Temperature high(>140 ) Motor Bearing Temperature High(>150) Drum Level High.(> 75%) Drum Level Low (< 10%) Furnace Pressure High(>100 mmwc) MCC fault. Run feed back off. ID Fan(Both) trip. SA Fan(Both) trip. Effect :- Boiler will trip on Furnace draft low P.A. flow low. Action :- Rectify electrical fault, if any. Check cause for boiler trip and normalize it. Check fan / motor-bearing temp. Restart the P.A. Fan after checking the cause of tripping and taking corrective actions.

22. Loss of SA Fan Cause :- Electrical motor protection. Drum Level High.(> 75%) Drum Level Low (< 10%) Furnace Pressure High(>100 mmwc) Furnace Pressure Low(- 150 mmwc) MCC fault. I.D. Fan (Both)trip. Effect :- 1. Boiler will trip on low furnace draft. Action :- Rectify electrical fault, if any. Check cause for boiler, trip & normalize it. Check fan / motor-bearing temp. Restart the S.A. Fan after checking the cause of tripping and taking corrective actions