01 Cause Analysis and Judgment Treatment

During the operation of the vertical mill, the motor drives the grinding disc to rotate through the main reducer, which drives the rotation of the grinding disc to rotate the three grinding rollers (120° uniform distribution), and the raw coal falls into the center of the grinding disc through the feeding pipe for grinding and grinding. The company coal internal work through three roller mill grinding roller and grinding disc extrusion to grinding of raw coal, roller through the bearing connection on a tripod, tripod through external three supporting rod, the hydraulic cylinder by setting load pressure to control the rise and fall of tie rod, thus to achieve control of the roller lift to carry on the work, as shown in figure 1. According to the use experience of the equipment manufacturer, when the limit of the tie rod is at zero, the grinding roller and the grinding disc are in “zero contact”. In the normal pulverized coal grinding process, the tie rod position is at 20-30 scale. At this time, the mill runs smoothly and the grinding effect is good.                           

Fig. 1 Schematic diagram of internal work done in coal mill

The causes of mill vibration are generally caused by foreign matter entering, unstable material layer, hydraulic machinery and other factors. Since the vibration of the mill started, no foreign bodies such as iron were found to be discharged from the slag spitting mouth. Meanwhile, the effect of stabilizing the material layer was not achieved by adjusting the water spray of the mill. When the mill vibrates in the field, it is found that the limit of #1 tie rod of the mill is at 40 ~ 50 scale, the limit of #2 and #5 tie rod is at 0 scale, and the work done by the three tie rods is unbalanced. According to the on-site inspection, it is preliminarily determined that the #1 pull rod hydraulic cylinder has a problem. Because the drop of #1 pull rod is not in place, one end of the tripod will lift and the other two fulcrum will drop, which will cause uneven work done by the mill roller and cause vibration of the mill. Therefore, it is suspected that the hydraulic system of the coal mill has problems. Therefore, we began to deal with the hydraulic cylinder of #1 tie rod. However, the vibration of the mill did not change after the replacement of the hydraulic cylinder, and the imbalance of the three tie rods still appeared. Due to vibration and slag spitting problems were not solved, the mill had to be stopped again for inspection.

When the grinding was stopped again, it was found that the external surface of the tripod, grinding roller and grinding disc were normal, and there was no foreign body such as iron block in the grinding. Rolls A and C can rotate normally after being pressed by external force, while Rolls B cannot. The inspection of B roller found that the bearing connection sealing pipe was seriously worn and cracked. Moreover, the bearing of B roller is full of pulverized coal, and the long time accumulation of pulverized coal causes the roller to fail to rotate normally. Therefore, the cracked place is repaired by welding, and then the pulverized coal in the bearing of B roller is cleaned for the first time with clean water, and then the bearing of B roller is cleaned thoroughly with diesel oil to ensure that the roller can rotate freely. After the treatment, the vertical coal mill is opened, the limit of the three tie rods is in the normal position, the material layer is stable, the vibration value of the mill returns to normal, and at the same time, the slag of the coal mill returns to normal.

The grinding plate and the grinding roller produce certain friction in the process of extrusion, and the grinding roller rotates at the same time by extrusion force and friction force. The grinding roller can press the flowing material to form a certain material layer in the process of rotation. When there is a certain material layer between the grinding roller and the grinding plate, the material can be ground reasonably. If in the process of squeezing roller can’t free rotation, so below cannot be rotating roller can’t form stable material layer, because of the roller is not rotating scraper ACTS as a plate in the top of the mill, the flow of material in the side of the roller, can’t form a continuous and stable material layer, thus caused by intermittent contact with mill rollers mill vibration. It can be seen that the direct cause of recent mill vibration is that a stable material layer cannot be formed, and the fundamental reason is that the bearing of roller B cannot rotate normally due to material blockage.
02 conclusions

Through the handling of this accident, a conclusion is drawn that the vibration of the mill may be caused by foreign bodies such as iron block under the condition of balanced tie rod; In the case of unbalanced tie rod mill vibration frequently occurs, generally due to the instability of the material layer. When the vibration of the mill is caused by the instability of the material layer, the reasons should be found from the following aspects: material moisture, mechanical hydraulic pressure, and the rotation of the mill roller, among which the rotation of the mill roller is the most easily neglected factor. When similar problems are encountered in production, attention should be paid to the wind pressure of the sealed fan, because the main function of the sealed fan is to protect the roller bearings and the separator bearings inside the mill and prevent the pulverized coal from entering the chamber. When the pipe pressure of the sealing fan is less than the normal control value, the pipe wear should be checked and treated in time, and the roller bearings should be cleaned with clean water and diesel oil to ensure the safe and stable operation of the equipment.

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