Created on:2021-07-17 10:30

Boiler hydraulic fault diagnosis and maintenance_ Analysis on vibration of coal mill

Boiler hydraulic fault diagnosis and maintenance_ Analysis on vibration of coal mill

The expansion project of a power plant consists of two imported 300MW units. The boiler is produced by Babcock company of Germany. It is a tower type once through boiler with double slag chambers, liquid slag discharge, one reheat and subcritical parameters. The pulverizing system is direct blowing type, with 100% fly ash reburning. Each slag chamber is equipped with two coal mills, and each coal mill provides four Swirling Burners. The maximum load of single slag chamber boiler is 195MW.

Mps-225 type coal mill is produced by Shenyang Heavy Machinery Works in cooperation with Babcock company. It is designed to burn Shenmu coal. The coal mill is driven by three-phase asynchronous motor, which drives torque and reduces speed through reducer. The grinding force is controlled by the hydraulic loading device. Hot air is injected from the nozzles around the grinding plate to transport and dry the grinding products. After the pulverized coal is separated by the rotary separator, the coarse powder returns to the pulverizer for further grinding, and the fine powder is sent to the burner through the pipeline. The following table shows the main operating parameters of the mill.


Main operation parameters of coal mill


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Since the unit was put into operation, there have been many problems with coal mills, and each stage has its own characteristics. In the initial stage of production, the start-up and shutdown time of the coal mill is long, the mill trips frequently and the start-up is difficult. Through the optimization of the start-up and shutdown procedure of the coal mill, the times of the mill trips and the fuel consumption during the start-up and shutdown are greatly reduced. However, with the increase of operation time, serious vibration of the coal mill and a series of problems caused by it appear.

Mps-225 coal mill adopts hydraulic variable loading system, whose function is to automatically adjust the loading force of grinding roller with the change of coal mill operation load, that is, to increase the loading force at high load and to reduce the loading force at low load. The coal mill can automatically adjust the loading force according to the changing characteristics of coal quality (such as grindability and the size of coal particles). The above functions are realized by changing the pressure on the loading piston, and the pulverized coal fineness is realized by controlling the rotating speed of the rotary separator. The hydraulic variable loading system of coal mill has the advantages of low power consumption, idling and small impact on the main motor during starting.

After the mill is put into operation, the loading pressure is too high, especially when the load is low and the amount of coal is small. With the increase of the operation time of the coal mill, the wear of the grinding roller and the grinding pad causes the vibration of the coal mill. Due to the vibration of the coal mill body, the coal feeding pipe and foundation platform also vibrate, the coal feeding pipe leaks, the hydraulic loading rod of the coal mill breaks frequently, and the coal mill is forced to shut down for many times. During the shutdown inspection, it was found that the fracture parts of the pull rod were mainly concentrated on the piston rod of the hydraulic cylinder, and located in the pull rod M100 × 4 at the start of the thread. Through the pull rod stress test during the operation of the coal mill, it is determined that the main reason of the pull rod fracture is the frequent alternating stress. There have been 12 faults in only one year.

When the load of double slag chamber liquid slag removal boiler is 195 ~ 220MW, the coal mill is in low load operation state, and the vibration of the main body is large. This is mainly due to the need to start the third coal mill when the unit load is greater than 195MW. In order to ensure the smooth and safe operation of slag flow in the slag chamber of the third coal mill, its output must be greater than 8.5kgf/s. To ensure that the output of the third coal mill is greater than 8.5kgf/., The output of the first and second coal mills must be reduced to less than 6.0kgf/s. Although the minimum output of mps-225 coal mill is 5.5-6.0kgf/s, the long-term operation of the coal mill under low load condition will bring unsafe risks.

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