Abstract: This paper analyzes the basic types and harmfulness of sliding bearing failure of magnetic pump, analyzes the Failure cause of sliding bearing, and summarizes the countermeasures to reduce the sliding bearing of magnetic pump.
Keywords: magnetic pump; Sliding bearings; Failure
1. Overview of the application of magnetic pumps
A certain unit's 80000 ton/year catalytic dry gas to ethylbenzene/styrene plant uses 45 magnetic pumps produced by Sundyne Corporation in the United States to transport toxic media such as benzene, ethylbenzene, and styrene. During the operation of the device, multiple failures of the sliding bearing of the magnetic pump have occurred, which not only affects the safety of the pump operation but also reduces production efficiency. Magnetic pumps similar to other sister units in the same industry often experience sliding bearing failures, which has a certain universality.
2. Hazards of Failure of Magnetic Pump Sliding Bearings
The sliding bearing (Figure 6) of the magnetic pump is installed on the bearing body of the magnetic pump, and is matched with the shaft sleeve on the inner magnetic rotor. The material is mostly silicon carbide. Its flushing, lubrication, and cooling rely on its own medium for circulating flushing and lubrication. It introduces the high-pressure liquid flow from the back of impeller 2 into the isolation sleeve, with a portion of the liquid flow flowing into the balance hole on the back of the blade hole through the intermediate bearing 7, and a portion of the liquid flow passing through the gap between the isolation cover and the inner magnetic rotor.
1.Pump body; 2. Impeller; 3. Pump cover; 4. Isolation sleeve; 5. Pump shaft; 6. Bearings; 7. Shaft sleeve; 8. Thrust ring; 9. Inner magnetic rotor; 10. External magnetic rotor; 11. Connection frame; 12. Suspension body; 13. Transmission shaft
When the sliding bearing fails and breaks, small pieces of silicon carbide fragments will enter the isolation sleeve and inner magnetic rotor of the magnetic pump with the flushing lubrication circuit medium. Due to the high hardness of silicon carbide, which is between the isolation sleeve and the internal magnetic rotor, due to high-speed rotation, it will cut open the isolation sleeve and the internal magnetic rotor like a turning tool, posing great harm.
3. Analysis of Failure cause of sliding bearing of magnetic pump
3.1 Insufficient exhaust before starting the magnetic pump
Due to the use of an internal flushing cooling system in the magnetic pump, unlike the external flushing system, which has a clear exhaust system, it is necessary to exhaust the remaining gas in the pump before starting the pump. If the gas in the pump is present, it will further reduce the flushing cooling effect, forming a vicious cycle, which will eventually lead to the fracture and failure of the sliding bearing of the magnetic pump.
3.2 Low flow operation of magnetic pump
The flow rate of the flushing and lubrication circuit of a magnetic pump is usually 2% -3% of the pump design flow rate. When the actual flow rate is less than the pump design Z small flow rate, the flow rate of the flushing and lubrication circuit of the pump sliding bearing will also decrease. When the magnetic pump operates at low flow rate for 3 minutes, it is possible to cause an increase in the liquid temperature in the flushing chamber of the centrifugal pump. The increase in medium temperature will cause partial flushing and lubrication medium gasification, causing dry friction of the pump sliding bearing A series of faults such as damaged sliding bearings and demagnetization of magnetic cylinders.
3.3 Impurities in the medium
Due to process pipeline or operational reasons, some pumps contain impurities. For example, a certain pump (P113) contains a large amount of coke powder due to process reasons, and the lifespan of the sliding bearing is about 50% lower than that of other pumps. At the same time, if the pipeline cleaning before the device starts is not clean, it will also cause an increase in the failure rate of the sliding bearing. Therefore, regular inspection and cleaning of the inlet filter screen of the magnetic pump is one of the means to ensure the smooth operation of the magnetic pump.
4. Measures to reduce the failure rate of magnetic pump sliding bearings
4.1 Adding a magnetic pump anti dry rotation controller
The anti dry rotation power monitor is a load monitoring device based on measuring the motor current of the magnetic pump, which has both measurement and protection functions. It measures and displays the actual power in kilowatts (kW) or percentage of power (%) of the pump motor, usually set to 30% of the rated current of the measured pump. When the operating current of the motor is lower than the set value, the monitor can automatically disconnect the magnetic pump motor. Avoid magnetic pumps operating under low or overload conditions.
4.2 Add a return line to the outlet pipeline of the pump
For magnetic pumps that are prone to operating at low flow rate Z, in order to ensure long-term operation of the magnetic pump and prevent frequent tripping of the anti dry rotation controller, a return line is added to the pump outlet pipeline, which directly flows back from the pump outlet pipeline to the pump inlet pipeline, ensuring that the pump operates within the safety protection zone. The method of adding a return line has been validated in P120 (anti alkylation material pump), and the service life of the sliding bearing of the pump has been significantly improved by adding a return line.
4.3 Strengthen daily inspections and do a good job in equipment management
The sliding bearing faults of magnetic pumps are often related to temperature rise, and strengthening temperature rise monitoring of operating pumps can prevent 90% of equipment failures in magnetic pumps. By measuring the pump body temperature and medium temperature, and combining them with the pressure gauge at the pump outlet, it can be determined whether the magnetic pump is working properly. At the same time, we also have the following measures to ensure the long-term operation of the magnetic pump sliding bearings.
Regularly clean the inlet filter screen of the pump, check if there is any internal leakage in the inlet filter screen, and do not allow ferromagnetic impurities or carbon powder accumulation in the medium.
Before starting the machine, fully exhaust the pump and ensure that the pipeline is unobstructed;
Before starting the pump, check the direction of rotation of the motor and do not allow reverse rotation.
The heat pump is not allowed to drive in a cold state to prevent thermal cracks in the sliding bearing. The damage rate of the sliding bearings of the heat pump that starts without preheating reaches over 60%.
4.5 Regular inspection of magnetic pumps
Due to the characteristics of silicon carbide bearings, they have higher hardness and are less prone to wear. When many sliding bearings crack, there is no significant change in the vibration of the magnetic pump due to no wear. Therefore, we need to regularly check the operation of the magnetic pump every six months to prevent malignant events such as damage to the isolation sleeve and inner magnetic rotor. Through our regular inspection of the pumps, 95% of accidents can be eliminated in the Mengya state.
5. Summary
Compared with ordinary centrifugal pumps, magnetic drive pumps have low vibration, low noise, and less obvious faults than ordinary centrifugal pumps, which requires long-term careful observation by operators and maintenance personnel in working practice to eliminate equipment accidents in the bud state. At the same time, planned maintenance and regular sampling inspection are also required to ensure the safe, stable and long-term operation of the equipment.
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