In regenerative type air heater in boiler when the electrical motor trips always the air motor start on auto. Failure of this will lead to a major breakdown of air pre-heater and loss of availability of boiler.
Regenerative air pre-heaters are used in large capacity boilers due to the design's compactness. These air pre-heaters rotate at slow revolutions per minute to enable the air pre-heater heat transfer elements to pick up heat from flue gases and dissipate it to the combustion air. If the air pre-heater rotation stops even for a very short time, the rotor will bend and the boiler will have to be shutdown. To prevent such damage designers provide air motors to handle such a situation. In large capacity boilers there will be two regenerative air pre-heaters- one on left and the other on the right side. When both regenerative air pre-heater motors trip, the boiler control room operator and the local operator will have to handle the situation and keep the unit running.
The causes for such an occurrence, the plant response during this period, how the boiler desk operator should respond, and the local operator action are discussed. The situation can be that the air motors start on auto or fail to start. If the air motors fail to start, then the operator has no option but to trip the unit to avoid major damage to the air pre-heaters. The local operator will have to immediately organize to hand rotate the air pre-heaters.
Causes for air pre-heater electrical motors trip
- 0.4 KV supply failure
- Heavy Seal rubbing leading to overload of motor
- High gas temperature entering the air pre-heater leading to high deflection of the rotor
Immediate plant response
- Both air pre-heater motor trip alarms come on in the control room
- Air motor start annunciation comes
- Slow fall in air pre-heater air outlet temperature
- Slow increase in the flu gas temperature leaving air pre-heater
Immediate boiler desk operator action
- Check reason for motor trip
- Restart electrical motor at the earliest
- Reduce boiler load depending upon how the air outlet temperature drops and how flue gas temperature increases
Immediate local operator action
- Check for seal noise and heavy rubbing
- Check rotor deflection
- Check for correct opening of air pre-heater dampers
- Get electric motor to restart condition and inform desk operator
Adopting good operating practice always leads to improved availability and efficiency.
- Check seal clearances at regular intervals as per OEM recommendation
- Maintain boiler parameters as per or close to design
- Check air pre-heater leakage on weekly basis and attend to reason attributing to leakage
- Make sure all the combustion air goes through the air pre-heater to increase the heat recovery duty
- Check the air pre-heater pressure drop and take corrective action
Keep in mind all the operating concerns on air pre-heater