1. Operating status before the event
Unit load 335MW, total coal capacity 162t/h, main steam/reheat pressure 17.83/4.0 MPa, main steam/reheat temperature 525/541°C. Mills A, B, C, and E of the milling system are in operation, and mill D is in standby; A and C electric water pumps are in operation, and electric pump B is in standby; B sealing fan is in operation, and A sealing fan is in standby.
2. The incident
At 21:25 on a certain day, when the unit was not operating, the operator found smoke coming from the drive end bearing of the B sealed fan motor. He immediately tried to start the A sealed fan but failed. The switch cabinet reported a "negative sequence first stage protection" alarm, and the smoke from the B sealed fan bearing intensified.
The operating personnel urgently put in oil guns and air preheaters to blow soot, and stopped the E, B, C, and A coal mills in sequence. After the DCS shut down the B sealing fan and failed, the accident button was manually pressed. 21:36 unit load drops to 45MW.
21:50 detected that the insulation of sealed fan motors A and B was normal. On-site inspection found that the cooling water pipe of sealed fan A was frozen, and maintenance personnel responded urgently. Subsequently, primary fans A and B and electric pump C were stopped.
The next day 00:50, the A sealing fan cooling water pipe was temporarily reconnected to the D grinding motor cooling water return pipe and slag ditch for maintenance. 01:03 Turn the B sealing fan switch to the A sealing fan and start it successfully. Gradually restore the pulverizing, primary air and other systems, switch the plant power, and put denitrification into operation. 01:11 The unit load began to recover, and the oil guns were gradually withdrawn. 02:00 The load returned to normal.
3. Cause analysis
Direct cause:
The cage of the NU217 roller bearing in the front bearing assembly (NU217+6217 double-row bearing) of seal-air fan B's motor was damaged. Dismantling revealed a large quantity of grease in the motor stator cavity that had not deteriorated, while the bearing itself lacked lubricant. The clearance between the oil baffle cover and motor shaft greatly exceeded the allowable limit, so grease was squeezed through this gap into the stator cavity during regreasing, with only a small amount entering the bearing chamber. The motor was a high-speed, two-pole motor (2970 r/min) that consumed grease relatively quickly. Operation with insufficient grease caused the bearing temperature to rise, with no temperature measuring point installed. The grease melted and escaped, ultimately causing cage wear and deformation.
Indirect reasons:
The B and C phase vacuum tubes of the original A sealed fan switch were stuck due to low temperature, and the contacts were not reset properly. The poor conduction after closing caused the motor to run without phase, triggering a "negative sequence first stage protection" alarm, causing the backup fan to fail to start normally.
4. Expose problems
There are loopholes in equipment management, and maintenance personnel have insufficient ability to identify and handle hidden dangers;
The motor oil cover is worn and not replaced in time, causing grease leakage and shortening the operating cycle;
Cooling water pipes are prone to clogging due to sediment accumulation and freezing in severe cold environments;
Important motor bearings lack temperature monitoring means.
5. Preventive measures
Strengthen maintenance management and technical training to improve the quality of inspections and the ability to identify hidden dangers;
Use major and minor repair opportunities to thoroughly inspect and replace worn sealing parts to ensure reliable sealing of the bearing chamber;
Strictly implement the regular lubrication system, regularly clean the oil injection channel and establish a maintenance ledger;
Contact the manufacturer to conduct a comprehensive inspection of the switch cabinet to prevent the recurrence of vacuum tube jamming;
Clean cooling water pipes regularly to prevent blockage and freezing;
Consider installing temperature monitoring devices for important motor bearings.
6. Lessons from the incident
This incident was caused by aging equipment and inadequate maintenance. The hidden defects in the switches of the backup equipment were superimposed, which led to the expansion of the incident. All units should strengthen equipment status tracking and regular maintenance, pay attention to regular testing and maintenance of standby equipment, and effectively improve equipment reliability.
(Source: Thermal Power and Electric Power Co., Ltd.)