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This post was last edited by Dream 100 on 2020-6-4 19:05. Summary: This article diagnoses and analyzes the problems and faults that often occur during the operation and use of the control station of a large ball mill thin oil station of a new materials company. It uses German Siemens S7-200 series PLC to transform and design the control station, carry out targeted control loop transformation and improve the corresponding protection procedures to improve the operation rate and safety of the equipment. The thin oil station is a special lubrication supporting equipment for ball mills. It is mainly used for the lubrication of the ball mill equipment bearings. The thin oil station consists of a lubricating oil tank, a heater, a filter, two low-pressure oil supply pumps, two high-pressure oil supply pumps, pipeline valves, and oil delivery and oil return pipelines. When the mill is running, the low-pressure oil supply pump runs throughout the entire process, supplying oil to the upper part of the hollow shaft of the mill and completely drenching it with oil; when the mill is started and stopped, the high-pressure oil pump runs, supplying oil to the space between the large tiles and the hollow shaft of the mill to form an oil film, and lift the mill; after the flow, oil pressure and other data of the thin oil station are normal, a ready signal is triggered to prepare to start the mill. During the operation of the mill, if abnormal signals such as low oil pressure, low flow, oil pump stop, or low oil tank level occur at the thin oil control station, an alarm will be issued and a ball mill shutdown signal will be output. This new material company's large-scale ball mill thin oil station control system is provided by the manufacturer. Due to many problems in the design and matching, abnormal parking and failure to start the mill often occur. Many factors affect the normal production organization and pose a safety threat to the main equipment of the ball mill. After comprehensive investigation and analysis, Siemens S7-200 series PLC was used to improve the design of the thin oil station control station system. The original electrical and control components were retained and utilized as much as possible, and the control and protection functions were improved through software and hardware measures. 1 Frequently occurring problems and fault manifestations Due to the harsh on-site working conditions, the thin oil station is in a dusty and oily environment, and the membrane switch panel of the original thin oil station control box often fails. Secondly, the ball mill often shuts down abnormally due to the failure of the dilute oil station. After inspection, there is no specific fault, and sometimes it returns to normal after restarting. After many investigations and analysis of this kind of fault, it was concluded that the on-site mill vibrated heavily during operation, resulting in poor contact of the relay contacts of the dilute oil station. The instantaneous signal disturbance caused the mill to lose the start-up signal, causing the mill to trip and stop. Due to the instantaneous poor contact, the abnormal state could not be sustained, making it difficult to accurately determine the fault point, and repairs often required a lot of time. 2 Improvement methods 2.1 Improve the electrical components and materials of the electric control box to adapt to harsh environments. The original design uses an electric control box composed of two integrated printed circuit board relays, micro components, membrane switch panels, etc., and is made of carbon steel. Although the internal miniaturization and integration of the electric control box can reduce wiring, the actual operating conditions of the on-site equipment are not considered. The environment of the thin oil station is harsh, with vibration, noise, oil pollution, etc. The carbon steel shell of the control box is not tightly sealed after long-term corrosion and deformation, resulting in frequent short-circuit faults in the printed circuit board, contacts, etc., which prevents normal operation and is difficult to replace and repair. In view of this situation, the original carbon steel control box is replaced with a control box made of 304 stainless steel with a protection grade of IP65 and structurally sealed electrical components are used to replace printed circuit board relays and membrane switch panels. This can effectively reduce poor contact caused by dust and oil. It not only adapts to harsh working conditions, facilitates maintenance work, but also greatly reduces the failure rate. 2.2 Use PLC output channels to drive directly to reduce intermediate links. In the original control scheme, all input and output channels are isolated by intermediate relays, and a self-maintaining control loop is installed next to the ball mill. This results in cumbersome and complicated wiring. There are many external series connection points in the control loop, and phenomena such as virtual terminal connections and poor contact of intermediate relay contacts often occur, making it difficult to find fault points. Secondly, the distance between the control cable of the dilute oil station and the high-voltage power cable of the ball mill is relatively close, so the induced voltage value of some control cables has been measured as high as 100V. Considering the on-site environment's characteristics of heavy dust, severe vibration, long lines and interference from high-voltage cables, a PLC with a channel voltage of AC220V was selected to eliminate intermediate relay links as much as possible to enhance the ability to resist electromagnetic interference. Therefore, in addition to retaining three interlocking signals with the ball mill (fault shutdown, oil station ready, and normal power supply), the thin oil control station sets intermediate relay isolation, and the rest adopts the output channel 220VAC direct drive method. S7-200PLC should pay attention to channel load distribution. The PLC contains three common terminals and ten output points. Therefore, the load must be distributed reasonably to avoid burning the common terminal due to excessive load concentration. The control input point of the thin oil station equipment still uses PLC DC 24VDC. The input point of the electrical control signal from the ball mill uses AC 220V power supply to be directly input through the expansion module channel. At the same time, in order to simplify the control loop and reduce intermediate links, the self-maintaining control loop next to the ball mill is removed. After the improved design was implemented, the 15 isolation intermediate relays were reduced to 3, the control loop was simplified, and hidden fault points were reduced. * * Reduce the number of abnormal shutdowns of the ball mill. 2.3 Add lubricating oil flow judgment conditions to ensure lubrication safety. The oil volume of the original thin oil station is controlled according to the electric contact pressure gauge, and there is an interlock to allow the ball mill to start when the oil volume is high and to stop the ball mill when the oil volume is low. The oil pressure value of the thin oil station can be adjusted through the valve in the oil pump pipeline. Therefore, in order to quickly reach the pressure value required to start the ball mill, the operator sometimes artificially adjusts the oil valve opening and ignores the reasonable oil supply. The operator's deliberate operation may cause insufficient lubrication of the grinding tiles of the mill when the oil flow is low, or the oil flow may be too large, causing the tile box oil to overflow, and the safety of lubrication cannot be ensured. Therefore, a flow switch was installed during the transformation, and the electrical contact pressure gauge contacts were used as input conditions at the same time to improve the reliability and safety of lubrication. 2.4 Reduce unnecessary low oil temperature shutdown interlock. The factory is located in the south, where the average temperature is relatively high. Moreover, the oil pressure and flow interlock has been set up at the thin oil station, which has provided sufficient protection. During operation, the vibration of the thin oil station caused the disconnection of the temperature measuring element and caused an abnormal parking accident. Therefore, the over-protection of the low oil temperature interlock can be removed. 2.5 Anti-shake delay delay is applied to the oil pressure contact. During the operation of the thin oil station, the mechanical vibration is large, which will cause the contact jitter of the oil pressure electric contact pressure gauge of Shanghai Automation Instrument No. 4 Factory. When the temperature is high in summer, the oil temperature in the oil tank will sometimes be high, and the height difference will generate bubbles during oil return, causing unstable oil supply. Contact jitter will cause the oil pressure electric contact switch signal to be unstable, which will cause low-pressure malfunction and cause shutdown, or the oil pressure will not reach the PLC program set time, which will not meet the oil station's ready signal, causing the ball mill host to be unable to start and stop normally. During the transformation, it is necessary to eliminate the impact of vibration when the equipment is running. The collected oil pressure signals are fully filtered and anti-shake through software and hardware means to minimize the rejection and misoperation caused by the jitter of the oil station contact signal. Therefore, the low oil pressure anti-shake delay trigger of 2s is added to the PLC internal editing program. After a 2s delay, the fault signal is output. At the same time, the intermediate relay on the output side is cancelled, and the electrical side is directly driven by 220VAC. 2.6 The startup procedure of the main/standby oil pump should be handled to prevent competition. In the thin oil station system, the startup procedure of the main/standby oil pump should be handled to prevent competition. The main/backup oil pump starting requirements are: start the main oil pump first, and if the detected oil pressure cannot reach the specified value after 2 seconds, the backup oil pump will start. The two main/standby oil pumps cannot be started at the same time, and the program should try to avoid frequent starting of the standby oil pump. If the main oil pump stops abnormally when the oil station is running, the backup oil pump is immediately programmed to start to maintain the working oil pressure. If the oil pressure is still lower than the working pressure, it is not allowed to start and an alarm signal is issued. 2.7 Implement power-off protection when the PLC working power supply is abnormally powered off. When the power supply of the control cabinet of the thin oil station is normal, and the PLC detects a fault of the thin oil station during operation, the PLC will interlock to implement shutdown protection of the ball mill. However, if the power supply of the control box of the dilute oil station is interrupted, the PLC cannot drive the electrical side fault shutdown coil, which may cause major equipment accidents. To this end, the power-off protection function needs to be added to the program. The S7-200PLC flag SM0.0 (internal special flag) is used as the trigger judgment condition to drive the output relay. On the electrical side circuit, the power monitoring normally closed point and the fault shutdown normally open contact are connected in parallel to meet the functional requirements. 3 Conclusion This improved design solves the control loop and method of the original thin oil station, and improves the overall operation rate and equipment safety of the ball mill. According to statistics, the number of abnormal shutdowns of the ball mill has been reduced from the original monthly average of 5 times to 1 time, and the ball mill failure shutdown caused by the thin oil station has been basically eliminated. In this improved design, the advantages of PLC's own software and hardware are fully utilized, hidden fault points are reduced, and peripheral circuits are simplified. Combined with the harsh environment of the thin oil station, we improved the more appropriate 220AC control voltage and protection procedures, diagnosed and predicted possible special situations such as power interruptions, and improved the oil supply safety function of the thin oil control station as much as possible, achieving obvious results. author: Zhang Liang