Magnetic drive pump

Thread Content

Magnetic drive pumps, like shielded pumps, use static seals in place of dynamic seals, thereby solving the problem of leakage. Therefore, it is widely used in industries such as petroleum, chemicals, pharmaceuticals, and non-ferrous metal smelting to transport toxic, harmful, flammable, explosive, corrosive liquids, as well as valuable liquids. The magnetic drive pumps that are widely used in production today include magnetic drive centrifugal pumps, while vortex pumps, gear pumps, and screw pumps are still in the trial or pilot production phase. The following introduces the commonly used magnetic-driven centrifugal pumps at present. 1 Working Principle and Structure of Magnetic Drive Centrifugal Pumps Magnetic drive centrifugal pumps operate by taking advantage of the ability of magnets to attract magnetic materials, as well as the magnetic force that exists between magnets or in a magnetic field. As shown in Figure 1, the motor is connected to the outer magnetic rotor through a coupling, while the impeller is connected to the inner magnetic rotor. A fully sealed isolation sleeve is placed between the inner and outer magnetic rotors; this sleeve is fixed to the pump cover, thereby sealing the medium transported by the pump within the pump body in a static-sealing manner, so as to prevent the medium from leaking out. When the outer magnetic rotor rotates driven by the motor, the mutual attraction and repulsion between the magnetic poles of the permanent magnets in the inner and outer magnetic rotors cause the inner magnetic rotor to rotate as well, thereby driving the pump shaft to rotate and achieving the purpose of transporting liquid. Based on the way in which the rotating components inside the pump are supported, the structural types of magnetically driven centrifugal pumps can be divided into fixed-shaft and rotary-shaft types. In the fixed-axis type, the shaft is fixed to the pump body and does not move; the internal magnetic rotor rotates together with the impeller. This design is compact, reducing the axial dimensions, and it is generally used in applications requiring low power. The rotary shaft type can be further divided into two categories. One type features a motor that is directly connected to the external magnetic rotor; this type of pump eliminates components such as an external shaft, rolling bearings, and couplings, resulting in a compact structure and small size. It is generally used in applications requiring low power, as shown in Figure 2 ； Another type features the motor connected to the bearing support frame of the external magnetic rotor; the external magnetic rotor is mounted on this bearing support suspension, whose flange end is connected to the pump body, while the shaft is connected to the motor. The structure is shown in Figure 1. This type of pump offers good stability, but it has a relatively large size and a more complex structure, making it suitable for high-power applications. 1. The pump body and impeller: The pump body and impeller of a magnetically driven centrifugal pump are similar to those of a centrifugal pump with a seal. 2. Internal and external magnetic rotors. Figure 1: Structure of a magnetically driven centrifugal pump with a non-directly connected drive shaft. Figure 2: Structure of a magnetically driven centrifugal pump with a directly connected drive shaft. 1 – Pump body ； 2-Impeller ； 3-Sliding bearing ； 4-Shaft inside the pump ； 1- Pump casing ； 2-Impeller ； 3-Partition ； 4-Isolation sleeve ； 5-Isolation sleeve ； 6-Inner Magnetic Rotor ； 7-External magnetic rotor ； 8-Rolling bearing 5-External magnetic rotor ； 6-Inner Magnetic Rotor ； 7-Pump shaft 9-Outer pump shaft ； 10-Coupling ； 11- Motor ； 12- The base is used to completely or partially isolate the magnetic material of the rotor from the outside environment, thereby preventing the magnetic material from being oxidized or corroded. Generally, the outer surface of the rotor is covered with metal or plastic; metal covering is achieved through welding, while plastic covering is accomplished via injection molding. 3. The isolation sleeve, also known as a sealing sleeve, is located between the inner magnetic rotor and the outer magnetic rotor; it completely separates the two rotors and encloses the medium within the isolation sleeve. It maintains a certain gap between the outer circumference of the inner magnetic rotor and the inner circumference of the outer magnetic rotor, thereby preventing friction between the inner and outer magnetic rotors during operation from causing cracks or damage to the enclosure, which could ultimately weaken the magnetic properties of the magnetic rotors. 4. For bearing magnetic drive pumps, sliding bearings, rolling bearings, and combined rolling/sliding bearings are used as bearing types. The shaft inside the pump is supported by sliding bearings; since these bearings operate while immersed in the medium being transported, their lubricity is relatively poor. Therefore, sliding bearings should be made from materials with good wear resistance and self-lubricating properties. Common bearing materials include tin-antimony bearing alloy, lead-antimony bearing alloy, graphite, polytetrafluoroethylene, silicon carbide ceramics, etc. 5. The coupling comes loose; it’s the same as with pumps that have seals ； Use a flexible coupling. 6. Electric motor: Like sealed pumps, standard electric motors are used. 2 Characteristics of magnetically driven centrifugal pumps 1. Advantages L-skid (1) Uses static sealing instead of dynamic sealing, thereby preventing the medium from leaking out and achieving complete sealing. (2) Low vibration and noise. Magnetic drive pumps exhibit low vibration transmission, low noise generation, and smooth operation due to magnetic driving, which allows for an extended service life of the pump. (3) Motor overload protection function. When the pump is in operation and overload occurs, the internal and external magnetic rotors of the pump will automatically move relative to each other, preventing the pump shaft or motor from being damaged or burned out. (4) It has a self-cooling effect. Magnetic drive pumps are designed with a self-cooling circuit that uses their own working medium for cooling, thereby keeping the pump temperature within acceptable safe limits; as a result, no external cooling system is required for components such as bearings, magnetic drives, and isolation sleeves. (5) Maintenance and repair work take very little time. 2. Disadvantages: (1) Its efficiency is lower than that of ordinary centrifugal pumps. (2) High requirements are placed on the material and manufacturing of the isolation sleeve. (3) Due to limitations in materials and magnetic drive, magnetic centrifugal pumps are generally used in China only for transporting fluids at temperatures below 100°C and under a pressure of 6 MPa. (4) Since the wear resistance of the isolation sleeve material is generally poor, magnetic centrifugal pumps are usually not used to transport media containing solid particles. (5) The coupling requires high alignment; when the alignment is poor, it can lead to damage to the bearings at the inlet and wear of the isolation sleeve. 3 Maintenance of the magnetically driven centrifugal system 3.1 Inspection and assembly adjustment of components 1. Inspection of components (1) The instrument body and impeller should be free from any scratches or corrosion. (2) The inner and outer magnetic rotors shall be free of cracks, breaks, holes, etc. (3) After the isolation sleeve is removed, it should be carefully inspected for any cracks. For those made of metallic materials, flaw detection is necessary to ensure that they possess the required burst pressure and safety factor. (4) Once the bearings exceed their wear limits, it can cause vibration in the pump; therefore, during maintenance, the inner and outer surfaces of the bearings should be inspected to ensure that there are no scratches on them and that the clearance between the bearings and the shaft is within the specified range. (5) The shaft shall be free from scratches and corrosion, and its straightness must meet the requirements specified in the instruction manual. 2. Assembly (1) When assembling the magnetic rotor with the power equipment, the maximum end face runout of the outer magnetic rotor and the isolation sleeve shall not exceed 0.25 mm, and the maximum radial runout shall not exceed 0.50 mm. At the same time, it is necessary to ensure that the radial clearance between the inner and outer magnetic rotors and the isolation sleeve meets the requirements specified in the user manual. (2) For paired sliding bearings, the axial clearance between the thrust plate and the end face of the graphite bearing must meet the requirements specified in the operating instructions; if no such requirements exist, this clearance shall not exceed 1 mm. The radial clearance of the bearings should meet the specified requirements; if the clearance is too small, the bearings are prone to thermal expansion during operation, which can lead to shaft seizure or high frictional loads, thereby affecting efficiency ； If the gap is too large, it will accelerate bearing wear and vibration. (3) For the impeller and mouth ring, the impeller usually adopts a molded, streamlined, integral cast structure; advanced designs incorporate internal magnets within the impeller. Mouth rings are generally made of CFRIT or silicon carbide materials. Since both the mouth ring and the impeller are wear-prone components, and the clearance between the mating dimensions required for assembly is relatively large, it is necessary to ensure that the clearance of the mouth ring meets the requirements specified in the user manual during assembly. (4) After the external magnetic rotor is connected to the motor, its radial and axial runout should be less than 0.01 mm. (5) During assembly, it is not allowed to use hard tools or objects to strike the graphite bearings, as well as the inner and outer rotors, isolation sleeves, etc. 3.2 Trial operation and fault handling of magnetically driven pumps 1. Trial operation of magnetically driven centrifugal pumps. 1) Preparatory work before startup (1) Check the maintenance records to ensure that the data is correct, and prepare all the necessary record forms for the trial operation ； (2) Clean the area around the pump ； (3) Check whether all various accessories of the equipment are complete and in good working condition, and whether the bolts are secure ； (4) Magnetic pumps equipped with rolling bearing housings, as well as their lubrication systems, shall have lubricant added in accordance with the specifications provided in the equipment’s technical documentation ； (5) The outer rotor should rotate smoothly by hand, without any uneven weight or abnormal noises ； (6) Open the pump suction valve to fill the pump with liquid ； (7) Open the vent valve to fully exhaust the air ； (8) Fully open the pump’s discharge valve, then close it ； (9) Contact an electrician to check the motor resistance and supply power. .2) Startup: (1) Press the start button, while checking the current and pressure levels ； (2) Once the pump outlet pressure stabilizes, slowly open the outlet valve to control the outlet pressure ； (3) Ensure that the motor current does not exceed the specified value ； (4) Check the sound and vibration of the pump, and make records as required. 3) Stop the pump: (1) Close the outlet valve, then press the pump stop button ； (2) Keep proper records of pump shutdowns, and maintain cleanliness of the equipment and its surrounding area. 4) Operating requirements: (1) Under normal operating conditions, magnetic pumps do not experience demagnetization due to aging over time. However, demagnetization occurs when the pump is overloaded, stalls, or when the operating temperature exceeds the allowable temperature for the magnets. Therefore, the magnetic pump must operate under normal operating conditions. (2) Magnetic pumps must not be operated without load to prevent damage to the sliding bearings and isolation sleeves. The medium transported by magnetic pumps must not contain ferromagnetic or hard impurities. Magnetic pump operation is not allowed at a flow rate lower than 30% of the rated value. (3) For magnetic pump operations on a daily basis, it is necessary to check whether the current, temperature rise, and outlet pressure are within normal ranges, whether there is any leakage during operation, whether the operation is stable, and whether vibration and noise levels are normal. Any abnormal conditions found should be addressed promptly. 2. Fault handling of magnetically driven centrifugal pumps: The common fault symptoms, causes, and solutions for magnetically driven centrifugal pumps are shown in Table 1. Table 1 Common Causes of Failures and Solutions for Magnetically Driven Centrifugal Pumps
Serial Number | Fault Symptoms | Causes | Solutions
1 | Pump fails to start | Foreign objects in the pump; accumulation of impurities in the pump bearings, causing jamming; friction between the inner and outer magnetic rotors; electrical faults | Remove foreign objects; disassemble and clean the pump; inspect electrical components
2 | High noise at the drive end | Damaged bearings; large misalignment of the coupling; poor lubrication; excessive imbalance on the drive side | Disassemble and inspect; replace bearings; realign them to meet specifications; replace lubricant; balance the pump
3 | High noise at the medium side | Friction between the impeller and the pump casing; excessive wear of bearings and shaft journals; improper fixation of the outer magnetic rotor to the drive shaft; damaged bearings; foreign objects in the pump; cavitation; unstable liquid flow | Disassemble and adjust clearances; replace shaft sleeves and shafts; reassemble the outer magnetic rotor; inspect and replace bearings; remove foreign objects; adjust operating procedures
4 | Insufficient flow rate or low output pressure | Low suction head; excessive gap in the mouth ring; gas in the pump; demagnetization of the magnet | Clean the inlet filter; raise the liquid level; replace the mouth ring; exhaust air
5 | Leakage | Loose sealing bolts; damaged isolation sleeves; damaged or failed gaskets | Tighten loose bolts; replace isolation sleeves; inspect and replace gaskets
6 | Excessively high temperature of the isolation sleeve | Demagnetization of the magnet; friction between the inner and outer magnetic rotors and the isolation sleeve; blocked internal return channels | Inspect, replace, and realign components; clear blockages
7 | Excessive current | Foreign objects entering the pump; high viscosity of the fluid; damaged bearings | Remove foreign objects; measure viscosity to ensure it meets requirements; replace bearings