Now days, we are hearing more on explosion in Boilers & related auxiliaries. This is harming man, machines & system and creating unrecoverable situations in power plant. Upon thinking on this following major causes or reasons come into picture.
The main reasons for Boiler explosions are;
- Operating the Boiler more than the design pressure for long time
- Operating the Boilers at lower water level in drum
- High furnace pressure
- Poor water quality
- Poor maintenance of Boilers in shutdowns
- Ignoring the aging factors of Boiler parts
- Operating the Boilers at higher main steam or metal temperature
1-What are the reasons for operating the Boilers at more than operating pressure?
- Malfunction of pressure transmitter or leakages in its impulse line
- Faulty local pressure gauges
- Faulty safety valves or safety valves set at higher pressure
- Improper control of combustion system
- Operating the Boiler at higher pressure set point
- Varying fuel quantity & quality
- Frequent load fluctuation on Boiler
- Improper spreading of fuel or no control on fuel feeding system
2-What are the potential reasons for low or no water level in the boilers?
- Feed water pump is not running, but showing running indication in DCS
- Mis-operation of feed water control valve or control valve is not opened as per requirement
- False reading from feed water or steam flow meters
- Malfunction of drum level transmitters/controllers
- Low water level in Deaerator or mal function of Deaerator level sensors
- Feed water discharge pump is closed
- More load on Boiler than design
- Under capacity feed water pumps
- Cavitation or steaming in feed water pumps
- Steaming in Economizers
- Long time lifting of Boilers safety valves-(Higher blow down of Safety valves)
- Wrongly set Boiler safety valves (Super heater line safety valve should be set at lower pressure than drum safety valves)
- Heavy passing in blow down valves
- Bypassing the instrument process interlocks like alarm value, low level trips etc
3-How does no water in pressure parts lead to explosion?
No water in pressure parts for long time leads into overheating of pressure parts & eventually failure.
Sudden entry of cold water into no water operating parts leads to flashing of water resulting into high volume steam. This high volume steam suddenly lead to explosion, as there is no sufficient space for steam to expand.
4-How do wrongly set Boiler safety valves lead to explosion?
Requirement is SH outlet Steam line safety valve is to be set at lower pressure than steam drum safety valves.
If drum safety valves set at lower pressure, on lifting of drum safety valves before lifting of SH line safety valves lead to starvation of super heater coils leading to overheat & eventually explosion.
5-What are the reasons for high furnace pressure?
- High furnace pressure is due to;
- No control on FD fans
- Bypassing or not working of Fans interlocks
- Sudden closure or miss-operation of ID fans dampers
- Variation of fuel moisture
- Improper spreading of fuel into furnace
- Wrongly set over fired air
- Draught transmitter malfunction & its impulse pipe leakages
- Choke up of open portion transmitter
6-How the Boiler does lead into explosion due to high furnace pressure
High furnace pressure leads into damages to the sealing, buck stay & membranes if exceeds leads to furnace & duct explosion.
7-How does the poor quality of feed water lead into Boiler explosion?
Poor quality of feed water leads to scaling & corrosion of pressure parts.
Scaling will lead into poor heat transfer & overheating of tubes, this over heating of tubes cause failure of pressure parts followed by explosions.
8-What are the reasons for overheating of pressure parts?
- Low water level
- Continuous flame impingement on particular area of pressure parts
- Localized heating
- Fuel (Coal) deposition on pressure parts area
- Pressure parts choke up
- Poor material quality
- More Excess air than requirement
- Internal scaling
- Lower feed water temperature at economizer inlet
9-What is the significance of shutdown maintenance of Boilers to avoid explosions?
- In order to take proper care on Boilers to avoid damages & explosion, need to concentrate on following listed areas of maintenance
- Thorough cleaning of pressure parts
- Inspection of pressure parts thickness & replacement of less thickness
- Proper setting of air & gas nozzles
- Proper setting of mechanical spreaders to avoid direct hitting of coal particles to rear wall tubes
- Inspection of steam line spring hangers
- Cleaning & inspection of buck stays
- Safety valves maintenance & proper floating
- Removing all obstacles & temporary supports in boilers for boilers free expansion
- Proper setting of burners & air nozzles to avoid localized heating & flame impingement
- Overhauling of steam drum gauge glasses
- Maintenance & calibration of pressure & level transmitters, pressure gauges & temperature gauges
- Proper maintenance of soot blowers
Lance tubes if not rotating may damage the pressure parts by continuous impingement of steam jet.
10-What are the reasons for Boiler back end flue gas ducts explosion?
It is due to secondary combustion & formation of carbon monoxide at ducts such as APH & ESP.
During start up or abnormal operation of Boilers, fuel will carry through flue gas & settle into APH & ESP ducts, where its incomplete combustion starts & form CO gas.CO is very explosive gas, when external Oxygen combines with this gas results into explosion.
11-What precautions can be taken to avoid high metal temperature operation?
- Installation of metal skin temperature sensors for Super heater coils
- Avoiding wet fuel intake into the furnace
- Avoiding more excess air than design for long time
- Maintain required or designed feed water temperature at economizer inlet
- Operate boilers more than 50% of its MCR
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