There are many causes that can contribute to boiler flame failure. Operating the boiler with unstable flame is always dangerous and can lead to boiler explosion. Tripping the boiler is the option for all boiler designers and operators.
Maintaining a good flame in a boiler is the key to a good boiler performance. Unstable flame is always a threat for boiler furnace explosion, which can lead to a large outage of the boiler and economic loss. When a flame failure occurs in a boiler, the boiler desk operator and the local operators will have to act immediately and bring the boiler back on line with all safety taken in to consideration. It is always seen that the greatest number of explosions in boilers takes place during start-up and shutdown. It is during this period that the probability for unburned to accumulate in flue gas path of the boiler is very high. Hence it has become a practice of all boiler designers to interlock purging the boiler with boiler start-up. Flame failure in a boiler can be due to many reasons.
The specific causes
- Heavy oil or warm-up oil trip valve closing
- Sudden reduction in mill feeder speed to a minimum
- Loss of ignition energy
- Flame scanner trip due to poor and unstable flame
- Malfunctioning of flame scanner
- Scanner air fan trip and slag build-up
- Sudden coal hang-up in one or more mills
- Coal feeder trip
- Electrical supply failure
- Air flow below 30% of total air flow
- Tripping of all forced draft air fan
- Tripping of all induced draft air fan
- Very high furnace pressure
These causes lead to flame failure individually or in a few a combination. This will depend upon when the flame quality becomes unacceptable.
The plant response
- In many cases it is possible to see fluctuation in furnace pressure
- Flickering of the flame scanner can be seen when flame quality deteriorates
- Tripping of the primary air fan or very low primary air header pressure is more probable to occur just before flame failure
- After flame failure immediately the unit trips
The boiler desk operator immediate action
- The boiler desk operator will have to start boiler purge
- Assess the reason for flame failure
- Light-up the boiler after the purge is complete and local operator gives clearance
Local operator immediate action
- Check furnace and report to boiler control room
- Check flame scanners for any possible slag accumulation
- Check mills and restore them for starting
- Check oil trip valves for proper function
- Make sure oil guns are in position for light up of boiler
It has to be remembered that delay in lighting up the boiler will decay the boiler pressure and temperature. Depending upon the duty of the boiler the time taken to come back on line will vary.