Failure analysis of MPS pulverizer in mid life

Analysis of the mid-term operation failure of MPS pulverizer since the 1990s, medium speed pulverizers have been used in 300350mw units successively. At the initial stage of production, the pulverizer operation is relatively stable, but after a period of operation, the problems of pulverizer vibration and sharp increase in the amount of pebble coal have occurred, which have a great impact on the normal operation of the boiler, This paper analyzes the problems and solutions of MPS pulverizer in phase II unit of Dalian Power Plant in the middle of operation, for reference to other power plants

1 overview of coal mill

the second phase unit of Huaneng Dalian Power Plant adopts MPS89G medium speed roller coal mill produced by Canada BW company. At the initial stage of production, the pulverizer operates very stably, the minimum stable workload can reach 15t/h, and the discharge cycle of pebble coal is basically more than 10h. However, after the pulverizer operates to about 2500h, several pulverizers have the problems of severe vibration when the coal volume is less than 40t/h and sharp increase of pebble coal volume, resulting in the pulverizer rod control mode adopting full digital pulse control, multiple fractures, pebble coal blockage The continuous operation of the pebble coal treatment system leads to the fracture of the high-pressure seawater pump shaft and other faults, which brings great hidden dangers to the safe operation of the unit

2 factors affecting the operation of the coal mill

2.1 influence of primary air volume

the output of the pulverizing system includes coal grinding output, drying output and ventilation output. In order to maintain the large output of the pulverizing system during operation, it must be adjusted reasonably to make the coal grinding output, drying output and ventilation output balance each other under the best working conditions. With the increase of primary air volume, the coal grinding output will increase and the amount of pebble coal will decrease under other conditions unchanged. See Table 1 for the influence of primary air volume change on the operation condition of coal mill

through the analysis of test data, the adjustment of primary air volume is not the direct reason for the increase of pebble coal, but increasing air volume can reduce the amount of pebble coal

2.2 influence of coal quality

after the coal mill was abnormal, the coal mill was tracked and observed, and the coal types were statistically analyzed in detail. Table 2 is part of the statistical data

through the above data analysis, we can speed up the development of the whole industrial chain of biological materials in our province. It can be seen that coal with grindability coefficient greater than 56 generally has a sharp increase in the amount of pebble coal, and reaches the condition of continuous emission. The continuous alarm condition is not gb/t 17898 (1) 999. The stress corrosion test method of stainless steel in boiling magnesium chloride solution disappears, resulting in the damage of the pebble coal alarm device of the coal mill. When the load is 235mw, the amount of coal for two coal mills of unit 3 reaches 52t/h, and it is not enough to fully open the primary air baffle. The maximum differential pressure of the grinding bowl of the coal mill reaches 6.94kpa. In this case, all kinds of regulating means are basically ineffective, and the pulverizer is almost blocked, which poses a considerable threat to the safe operation of the unit

at the initial stage of operation, when grinding the coal listed in Table 2, they all operate normally, but with the extension of operation time, the wear of the coal mill intensifies, and the adaptability of the coal mill to the coal gradually weakens. For some coal types, the vibration intensifies and the amount of pebble coal increases

2.3 influence of the separator baffle at the outlet of the coal mill

the separator baffle of the coal mill directly determines the circulation amount of pulverized coal inside the coal mill and is an important adjustment device for the operation of the coal mill. The adjustment scale of MPS89G pulverizer is 0 ~ 5.5, 0 corresponds to the smallest scale of pulverized coal, and 5.5 corresponds to the coarsest scale of pulverized coal. The separation baffle at the outlet of each pulverizer is set to 2.75 in the boiler adjustment test

2.3.1 adjust the test process

for 3C pulverizer, the opening of separation baffle is 3.5, the amount of pebble coal is normal, and there is no vibration. Manually close the outlet separation baffle from 3.5 to 2.75. When the coal grinding volume is 40t/h, the vibration occurs immediately. After reopening, the vibration disappears immediately

for 4B coal pulverizer put into operation after a long period of shutdown, when the opening of manual baffle is 3.0, because it is started after a long period of shutdown, the pebble coal is discharged continuously, and the outlet baffle is adjusted to 3.5, the amount of pebble coal is reduced rapidly

for 4A coal pulverizer, adjust the opening of manual baffle from 3.5 to 2.75, and the stone coal hopper that has been without alarm for 8h will alarm 1h after washing; After manual adjustment back to 3.5, the alarm is eliminated, and no alarm occurs again

the boiler is tested for thermal efficiency. When the load is 350MW, the total coal volume of the pulverizer is 139t, and there are two pulverizers discharging pebble coal continuously, so the alarm cannot be eliminated. The opening of the separator baffle at the outlet of the coal mill was adjusted from 2.75 to 3.5, and the coal feeding rate of the boiler soon decreased from 139t to 131t and operated stably

2.3.2 result analysis

from the test process, the vibration of the pulverizer is accompanied by the phenomenon of current increase and current fluctuation, and the large amount of pebble coal in the pulverizer is accompanied by the increase of the differential pressure of the pulverizer; When the two abnormalities are both present, the two phenomena also occur at the same time, and this abnormality can be effectively eliminated by increasing the opening of the separation baffle at the outlet of the coal mill