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Keywords: oil temperature; Viscosity; Flow resistance; Cut off oil
1. Equipment Introduction
The boiler of Unit 2 in a certain factory is equipped with four BBD-4060 low-speed double inlet and double outlet steel ball coal mills, with a ball loading capacity of 77 tons. The two ends of the mill are supported by two bearing shells coated with melted black gold on the surface. The bearing shells at both ends are lubricated by the oil station shown in Figure 1. The lubricating oil is 150 industrial lubricating oil, which is supplied from the outlet of the oil station through a diameter of about 3 centimeters and a length of about 6 meters to the driving end bearing shells, and through a diameter of about 3 centimeters and a length of about 14 meters to the non driving end bearing shells. Both bearing shells are equipped with high temperature protection, which triggers an alarm when the bearing shell temperature reaches 50 ℃ and trips the grinding machine when it reaches 55 ℃.
Figure 1
2. Accident process
At 04:50 on March 2nd, the main operator of the boiler in Unit 2 of a certain factory ordered the boiler patrol operator to switch the cooling oil cooler of the C coal mill lubricating oil station from the bypass to the main circuit (previously, due to the low ambient temperature and low oil temperature, the cooling oil cooler was in bypass operation) in order to control the temperature of the non driving end bearing due to the high temperature of the non driving end bearing of the C mill bearing at 44.5 ℃ (according to the ambient temperature at that time, it should normally be around 36 ℃, and the grinding machine would trip when it reached 55 ℃) and the upward trend. By reducing the oil temperature, the cooling oil cooler was switched from the bypass to the main circuit, The lubricating oil temperature of the C coal mill gradually decreased. At 06:05:12, the temperature of the non drive end bearing of the C coal mill was high (50 ℃) and an alarm was triggered. The boiler operator immediately started the top shaft oil pump of the C coal mill, opened the cold air valve at the inlet of the C coal mill, closed the hot air valve at the inlet, and controlled the temperature of the C coal mill bearing. At 06:07:15, the temperature of the non drive end bearing rose to 55 ℃, and the high temperature protection of the C coal mill bearing was activated. The mill tripped. After the mill tripped, the temperature of the non drive end bearing continued to rise to 70.18 ℃ and then began to decrease. After inspection, it was found that the non drive end bearing had slight wear.
3. Accident analysis
After the high temperature of the grinding machine bearing tripped, personnel were immediately sent to the site for inspection. It was found that the lubricating oil supply pump at the lubrication station was running normally, and there were no signs of abnormal movement in the valves of the oil station system. The passage was normal, and the oil supply pressure slightly increased. There was no oil flowing out of the lubricating oil distribution pipe of the non drive end bearing and the oil had been cut off. There was oil flowing out of the lubricating oil distribution pipe of the drive end bearing and the oil volume was relatively small. At the same time, by checking the historical trends of the oil supply system on DCS, it was found that the oil temperature decreased from 25.92 ℃ to 12.69 ℃ between 04:50 and 06:07, while other parameters showed no significant changes. The following table shows the changes in oil temperature and bearing temperature after the operation of the oil cooler:
Due to the fact that the operating personnel only operated the lubricating oil station cooler before the accident occurred, and there were no abnormal movements or malfunctions in the oil station system, based on the above inspection results and the operational adjustments at that time, the main reasons for the high temperature tripping of the grinding machine bearing are analyzed as follows:
(1) After the oil cooler of the lubricating oil station switches from the bypass to the main circuit operation, the lubricating oil temperature gradually decreases under the action of the oil cooler. At this time, the characteristics of the lubricating oil gradually change, and the viscosity increases with the decrease of oil temperature. The frictional force between the lubricating oil and the oil pipeline gradually increases, and the resistance along the way gradually increases. The amount of lubricating oil gradually decreases, and Z ultimately causes the output of the oil supply pump to be unable to overcome the resistance along the way, resulting in the interruption of the lubricating oil of the non drive end bearing of the mill. The lubrication between the mill shaft and the bearing is lost, and the bearing temperature rises to the protection action value, which ensures correct protection action. Due to its proximity to the lubricating oil station (the oil pipeline is about 6 meters long, which is about 8 meters shorter than the non driving end), the driving end of the mill only experiences a decrease in oil volume without cutting off oil when the oil temperature decreases and the resistance along the way increases. Therefore, the temperature rise of the driving end bearing shell is not significant.
(2) The opening of the lubrication station's oil supply valve is too small. This valve is a ball valve with an opening of about one-fifth. When the oil temperature is high and the viscosity is low, the lubricating oil can pass smoothly. When the lubricating oil temperature decreases and the viscosity increases, the local resistance increases, and the flow is blocked. After the mill trips, a slight increase in lubricating oil pressure can be checked on site to prove this point. The lubrication pressure measuring point is in front of the oil supply valve. When the flow at the oil supply valve is blocked, the oil supply pressure will increase.
(3) The operating personnel have insufficient business skills and inadequate consideration. After the oil cooler was put into operation, they did not fully consider the impact of oil temperature reduction and viscosity increase on flow resistance. They did not arrange personnel to track the operation of the adjusted lubricating oil system, resulting in a decrease in lubricating oil volume and failure to detect oil interruption in a timely manner. After the high temperature alarm of the bearing was sent on the DCS panel, the cause of the high temperature of the bearing was not analyzed in a timely manner, which delayed the processing time.
(4) The installation of the lubricating oil pressure measuring point is not scientific. The measuring point is installed in front of the oil supply valve. When the oil supply decreases or even interrupts due to blockage such as accidental closing of the oil supply valve, the oil pressure will increase instead, and the low oil pressure protection will not operate at all.
4. Preventive measures
(1) Measures are issued to regulate the lubricating oil temperature of the coal mill, specifying the control range of the lubricating oil temperature in each season, controlling the rate of oil temperature change when adjusting the lubricating oil temperature, and strengthening monitoring after adjustment until the oil temperature and tile temperature are normal and stable, and the oil supply is normal and stable.
(2) Change the pressure measuring point of the lubricating oil station of the grinding machine from the front of the oil supply valve to the back of the oil supply valve, so that the high and low oil pressure can truly reflect the amount of lubricating oil entering the grinding machine bearing. When the amount of lubricating oil decreases due to various reasons, a low lubricating oil pressure alarm can be issued, and the protection of the grinding machine can trip correctly when the pressure is low, preventing serious oil cut and bearing burning accidents.
5. Summary
The above accident analysis fully demonstrates that when the temperature of lubricating oil changes significantly, its viscosity will undergo a significant change, which will alter the resistance distribution of the entire oil pipeline and be sufficient to change the flow of lubricating oil in the pipeline, especially in areas with severe throttling, where the reflection is more obvious. Therefore, when adjusting the lubricating oil temperature, it is necessary to fully consider the impact of changes in the flow resistance caused by changes in the viscosity of the lubricating oil on the equipment, in order to prevent accidents caused by equipment loss of lubrication and damage.
2024 August 1st Week KYOCM Product Recommendation:
Rotary bearings consist of an inner ring and an outer ring, one of which usually contains a gear. Together with the connecting holes in the two rings, they enable optimized power transmission through simple and fast connections between adjacent machine parts. Bearing raceways are designed with rolling elements, cages or gaskets to accommodate loads acting individually or in combination in any direction.
Features and advantages:
High carrying capacity
High stiffness for rigid bearing applications
Low friction
Long service life
Surface protection and corrosion resistance
Integrate other features including:
Driving mechanism
Control device
Lubrication system
Monitoring system
Sealed cassette tape
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2024-08-02