A.11.1 Equipment description
A power plant has two condensing steam turbine generator sets installed, with a single unit capacity of 300MW. The boiler ash discharge method is pneumatic ash conveying. The ash removal system is equipped with 5 screw air compressors, with air compressor specifications of 220kW, air-cooled units, and exhaust volume: 36m³/min. The instrument air system uses 3 screw air compressors, with air compressor specifications of 220kW, air-cooled units, and exhaust volume of 36m³/min. The air compressors all use IC board control panels, and the system uses PLC centralized control.
A.11.2 Accident description
During the operation of the ash conveying and instrument air compressor systems, all air compressors stopped operating, and they could not be automatically started in time when the compressed air system pressure was low. Due to timely discovery, the air compressors were manually restarted without causing serious consequences.
A.11.3 Cause Analysis
After multiple tests and careful analysis by the technical department, it was found that the reason for the complete shutdown of the air compressors was due to a problem with the start and stop time setting of the air compressors in the PLC centralized control system. The high-pressure shutdown pressure value of the air compressor was set too low, and the maximum delay of the high-pressure shutdown time was 10 minutes. In this way, when the compressed air system used less and more gas and was stable, it remained in a high-pressure state for more than 10 minutes, causing all the air compressors to stop.
A.11.4 Preventive measures
(1) Increase the upper limit of the shutdown pressure of the air compressor in the PLC control. This value is close to the rated pressure of the air compressor. The pressure value is generally set higher than the upper limit of the action of the pressure switch of the air compressor, so as to first realize the capacity adjustment of the air compressor or realize the idling operation of the air compressor. However, after analysis, the factory believes that it is more economical to have the joint control cabinet send a shutdown signal directly instead of running the air compressor idling for 20 minutes until it stops. Therefore, the high pressure shutdown pressure value of the air compressor is set 0.02MPa lower than the upper limit of the pressure switch of the air compressor. That is, when the pressure of the compressed air system is high, the joint control cabinet will first send a shutdown signal to the single machine. This is an incorrect setting method and should be reset.
(2) Adjust the time interval of the high pressure shutdown of the air compressor host and its downstream machines respectively.
(3) Find the leakage point of the compressed air system, reduce the system leakage, reduce the pressure fluctuation of the compressed air, and reduce the number of starts and stops of the air compressor.
