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A large cement clinker production enterprise uses ZGM123G - III medium speed roller coal mill produced by Beijing Electric Power Equipment Factory for coal vertical mill. Its specifications and models are as follows: the coal mill has a production capacity of 15-60 t/h, 3 grinding rollers, the maximum particle size of raw coal entering the mill is ≤ 80 mm, and the fineness of the finished product is 80 μ M sieve residue ≤ 10%, moisture content of raw coal entering the mill ≤ 15%, moisture content of finished coal powder ≤ 1%, outlet temperature of powder selection machine ≤ 80 ℃, rated power of main motor 900 kW, rated current 68.6 A, voltage 10 kV, rated speed of main motor 991 r/min, output speed of reducer 30.9 r/min.
ZGM123G-III coal mill is a medium speed roller disc machine, and its grinding part is composed of a rotating grinding ring and three grinding rollers that rotate fixedly along the grinding ring. The raw coal falls into the center of the grinding disc from the coal feeder on the upper part of the coal mill, and the rotating grinding disc uses centrifugal force to throw the raw coal onto the grinding working surface, which is then ground by the grinding roller and grinding disc. Three grinding rollers are evenly distributed along the circumferential direction on the working surface of the grinding disc, and the grinding force is generated by the hydraulic loading system. Through a static three-point system, the grinding force is uniformly applied to the three grinding rollers. The grinding and drying of raw coal are carried out simultaneously. The primary air evenly enters the surrounding area of the grinding ring through the nozzle ring, and the milled coal powder mixture is transported to the separator at the upper part of the coal mill for separation. The coarse powder is separated and returned to the grinding plate for re grinding. The qualified fine powder is carried out of the separator by the primary air. Seemingly simple operation can lead to many problems in daily production. Below is a brief introduction to the accident of coal mill roller damage in a certain company.
1. Problems that arise
On March 12, 2015, at 8:00 am during the day, the rotary kiln was fed 540 t/h of raw material normally, with a kiln speed of 3.7 r/min, and the coal mill was fed 50 t/h. The fineness of the coal powder produced during the grinding process was 10.8% (80%) μ M square hole sieve, with a loading pressure of 10 MPa during the operation of the coal mill. At 10:06 on that day, the current of the coal mill instantly decreased from 48 A in normal operation to 25 A, and then the vibration of the coal mill increased from 0.6 mm/s in normal operation to about 3.5 mm/s. The outlet temperature of the mill and the feeding amount of the coal feeder did not show significant changes. Immediately notify the on-site coal mill position to check if the loading pressure is normal, and the on-site position feedback that the loading pressure is normal. The operator of the central control room kiln carried out the following operations based on the problem phenomenon: first, stop feeding the coal mill, lift the grinding rollers of the coal mill, notify the on-site inspection post of the coal mill to stand by around the coal mill, and after the current of the mill drops to a stable (no-load) state, feed again and lower the grinding rollers. At this time, the main motor current of the coal mill in the central control room fluctuates greatly compared to before, about 30-45 A, and the vibration fluctuation is also large, about 3-4 mm/s. According to the on-site coal mill post, the loading pressure of the mill is normal, but there is a large amount of slag discharge at the slag discharge port of the coal mill, and from the slag discharge, it is mostly small coal blocks. At this point, the kiln operator in the central control room immediately stopped the coal mill, cooled the inside of the mill, and then reported to the workshop technical personnel. The workshop technicians requested to lift the grinding rollers of the coal mill and then inspect them inside. Due to the large diameter of the grinding rollers, two people were required to enter the coal mill to manually rotate the rollers. After on-site inspection of the grinding rollers, it was found that one of them did not rotate, while the other two did. As a result, the coal powder bin was burned out and the kiln was shut down to deal with the damaged grinding rollers.
2. Cause analysis
1) From the operation screen in the central control room, it can be seen that during the normal operation stage of the coal mill, the outlet pressure of the sealing fan remains above 7000 Pa for a long time, and the pressure of the sealing fan is greater than the primary air pressure at the inlet of the coal mill (the manufacturer's specified pressure is 2000 Pa). At the same time, during the start and stop of the coal mill, the sealing fan is always running, indicating that there is no problem with the work done by the sealing fan itself and the central control operation. However, during the on-site inspection of the outlet pipeline of the sealed fan, it was found that there were loopholes in the sealed pipeline, and the rubber rings at the threading holes and connections of the sealed pipeline were leaking air. The presence of the above two air leaks will inevitably lead to insufficient air flow in the sealing of the grinding roller bearings, causing the accumulation of coal powder near the grinding roller bearings.
2) Upon reviewing recent coal mill operation screens, it was found that since the major overhaul of the coal mill rollers in February, there have been some abnormal phenomena in the operation of the coal mill. The mill will occasionally experience instantaneous vibration (with a maximum vibration value of 5 mm/s), and when the mill vibrates, there is no significant change in the outlet temperature of the mill, the loading pressure of the mill, the feeding amount of the coal feeder, the operating current of the mill, and the operating current of the powder selector. At the same time, there are no iron blocks at the slag discharge port. Due to the absence of any early signs during the vibration of the mill, operators can only make adjustments after the vibration of the mill. Based on this, it can be concluded that after the major overhaul of the coal mill rollers in February, there were problems with the mill rollers, which were not caused by process operations.
3) Remove the damaged grinding roller to the ground for inspection. When the lubricating oil inside the grinding roller was drained and the oil drain hole was opened, it was found that there was no lubricating oil inside, and it was all small particles of coal powder, which blocked the oil drain hole. Insert the welding rod into the interior, but there is still no lubricating oil coming out. Therefore, it is judged that the sealing components of the grinding roller (including oil seal and dust seal) are damaged, and only then will coal powder enter the interior of the grinding roller. Inquiring about on-site positions and searching for maintenance records of coal mill rollers in February, it was found that there was indeed a prolonged oil leakage at the sealing area of the rollers, and no oil replenishment was carried out. At the same time, equipment management personnel did not pay attention, resulting in small coal powder entering the interior of the rollers from the damaged sealing area of the rollers.
4) Open the pressure cover above the grinding roller and the sealing parts (oil seal, dust seal) of the grinding roller bearing. It was also found that there was no lubricating oil inside, and it was all small coal powder. Wrap the grinding roller bearing tightly. When inspecting the sealing components of the grinding roller (oil seal, dust seal), it was found that both sealing components had varying degrees of aging and damage. After cleaning the internal coal powder, it was found that the retainer of the double row self-aligning bearing of the grinding roller was damaged and detached.
3. Handling methods and preventive measures
After identifying the cause of the accident, targeted measures were taken. Due to the long repair cycle of the damaged grinding rollers, in order to install them quickly, the spare grinding rollers inside the warehouse were transported to the site and replaced according to the standard process. At the same time, the following preventive measures have been formulated.
1) Strengthen the ideological awareness of equipment management personnel and pay more attention to equipment that operates continuously for a long time. When there is a precursor issue with the device, it should be resolved in a timely manner.
2) Strengthen the basic inspection of equipment. Every time the coal mill roller sleeve is repaired and replaced, the quality of the lubricating oil inside the roller should be checked to see if there are iron filings, coal powder, and dirty oil inside the lubricating oil. At the same time, check whether the sealing components (oil seal, dust seal) of the grinding roller bearing are damaged. A two-step inspection method is adopted. The first step is to visually inspect whether there is lubricating oil leakage at the sealing components of the grinding roller. The second step is to address minor damage to the sealing components that cannot be observed by the naked eye. The following operation is taken: empty the lubricating oil inside the grinding roller, install a pressure gauge in the oil discharge hole of the grinding roller, and install a shut-off valve at the back. Connect nitrogen gas behind the shut-off valve, increase the nitrogen pressure inside the grinding roller to 0.18-0.25 MPa, and maintain it for 20-40 minutes. When the pressure is not less than 0.13 MPa, it indicates that the sealing performance of the grinding roller is good, the sealing components are not damaged, and can be used normally.
3) Ensure that there is no air leakage in the outlet pipeline of the sealing fan. Block the threading port on the outlet pipeline of the sealing fan with adhesive materials such as blue glue to restore the sealing effect of the rubber ring at the connection of the outlet pipeline of the sealing fan. During the normal production and operation stage of the coal mill, each shift's on-site position should conduct two comprehensive inspections of the sealing fan outlet pipeline, keep inspection records, and promptly solve any problems such as air leakage.
4) Strictly control the quality of spare parts, including procurement quality and installation quality. The procurement of spare parts must first choose raw materials from the original manufacturer, followed by manufacturers with large production capacity and good reputation. During the installation of spare parts in key parts of the equipment, equipment technicians should supervise the installation steps on site throughout the entire process to ensure the quality of the equipment in the later stage.
4. Conclusion
There are many reasons why coal powder enters the bearing of the coal vertical mill roller, and the above case is just one of them. This issue should be taken seriously. This accident caused the company to shut down the kiln for up to 120 hours, resulting in a loss of approximately 40000 tons of clinker production and a significant increase in maintenance costs. Every accident is not accidental, and there are some signs before the accident occurs. This requires strengthening personal ideological awareness and basic equipment management work in daily work to ensure the safe and efficient operation of equipment.
2024 March 4th Week KYOCM Product Recommendation:
A spherical roller bearing is a rolling-element bearing that permits rotation with low friction, and permits angular misalignment. Typically these bearings support a rotating shaft in the bore of the inner ring that may be misaligned in respect to the outer ring. The misalignment is possible due to the spherical internal shape of the outer ring and spherical rollers.[1] Despite what their name may imply, spherical roller bearings are not truly spherical in shape. The rolling elements of spherical roller bearings are mainly cylindrical in shape, but have a (barrel like) profile that makes them appear like cylinders that have been slightly over-inflated.
2024-03-22