Detailed explanation of the internal structure and principle of magnetic drive pumps

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Magnetic drive pump, also known as magnetic pump. Similar to canned pumps, there are only static seals and no dynamic seals in terms of structure. Thus, there is no leakage when transporting liquids. Used in petrochemical systems to transport flammable, explosive, volatile, toxic, corrosive, and valuable liquids. The main components of a magnetic pump include the pump body, impeller, inner magnet, outer magnet, isolation sleeve, internal shaft of the pump, external shaft of the pump, as well as sliding bearings, rolling bearings, couplings, etc. The key component is the “magnetic coupling”. It consists of an inner magnetic steel (including a guide ring and a sleeve), an outer magnetic steel (including a guide ring), and a isolation sleeve. The choice of materials for the internal and external magnets has a significant impact on the efficiency and reliability of magnetic pumps. Currently, the commonly used materials are as follows: Rare-earth cobalt is a new type of permanent magnetic steel. Such as samarium-cobalt, aluminum-nickel-cobalt, praseodymium-cobalt, mixed rare-earth cobalt, and cobalt-copper alloys. Its magnetic energy product can reach 80–240 kl/m; it features high efficiency in magnetic drive and possesses extremely strong resistance to demagnetization. Its coercivity HCT is 360~1200 kA/m. The operating temperature can reach 300°C. 　　Neodymium-iron-boron is also a type of rare-earth permanent magnet. Its basic properties are similar to those of samarium-cobalt, but its magnetic energy product is higher than that of samarium-cobalt; the downside is that its operating temperature is only 120 degrees. C, and its magnetic stability is relatively poor. 　　Ferrites were once used as materials for magnetic pumps, but due to their magnetic energy density of only 10–30 kJ/m3, their operating temperature being limited to 85°C, and the high losses associated with magnetic drive, they are now rarely used. 　　The main structure of this pump type is still a centrifugal pump. However, the drive uses magnetic transmission principles. The motor is connected to the external magnet through a coupling. The impeller is connected to the internal magnet. A fully sealed isolation sleeve is placed between the inner and outer magnetic magnets, completely separating them and keeping the inner magnetic magnet immersed in the medium. Meanwhile, the shaft of the motor directly drives the impeller to rotate synchronously through the mutual attraction between the magnetic poles of the magnets. 　　In metal isolation sleeves, eddy current losses occur within the sleeve; in order to reduce these losses and improve transmission efficiency, high-resistance materials such as Hastelloy C are typically used. In this case, eddy current losses account for approximately 5% to 25% of the total transmission efficiency. Non-metallic isolation sleeves can improve transmission efficiency due to the absence of eddy current losses within them, but they have a low temperature tolerance, typically only 100–120°C. 　　Since magnetic pump completely eliminates shaft seal leakage, it also provides automatic overload protection during power transmission. In industries such as petrochemicals and pharmaceuticals, its use is gradually increasing as the required driving power is low. The current performance characteristics of stainless steel magnetic pump are as follows: flow rate of 0.2~200Ⅱi3/h, head < 125m, motor power ≤75kW, and operating temperature range of -20~100℃. Related products: Magnetic pump series. The performance of magnetic pump is similar to that of a centrifugal pump, except that its efficiency is lower. During the operation of a magnetic pump, if the operating temperature exceeds the allowable temperature for the magnets, demagnetization will occur. The typical operating temperature for domestic magnetic pumps is 100°C. The medium conveyed by magnetic pumps must not contain iron-containing, ferromagnetic impurities or hard impurities. To prevent the bearings and isolation sleeves from overheating or being damaged, running them without load must be avoided, and operation at a flow rate lower than 30% of the rated value is not permitted. Magnetic drive utilizes the property that magnets can attract ferromagnetic materials, as well as the magnetic force that exists between magnets or magnetic fields; non-ferromagnetic materials have little or no effect on the magnitude of this magnetic force. Therefore, power transmission can take place without contact, through non-magnetic conductors (insulation sleeves). 　　Magnetic drive can be designed as synchronous or asynchronous. Most magnetic drive pumps use a synchronous design. The motor is connected to the external magnet through an external coupling, while the impeller is connected to the internal magnet. A fully sealed isolation sleeve is placed between the outer magnetic steel and the inner magnetic steel, thereby completely separating them and keeping the inner magnetic steel submerged in the medium. The shaft of the motor drives the impeller to rotate synchronously through the magnetic attraction between the poles of the magnetic steels. 　　Asynchronous design magnetic drive, also known as torque ring magnetic drive. A torque ring with a squirrel-cage structure is used to replace the internal magnet, and the torque ring rotates at a slightly lower speed under the attraction of the external magnet. Due to the absence of internal magnets, its operating temperature is higher than that of magnetically driven systems with synchronous operation. 　　2. Structure of magnetic pump 1) Magnetic coupler Magnetic drive is accomplished through a magnetic coupler. A magnetic coupler mainly consists of components such as an inner magnet, an outer magnet, and a isolation sleeve, and it is the core component of a magnetic pump. The structure of the magnetic coupler, the design of its magnetic circuit, and the materials used for its various components are related to the reliability, magnetic drive efficiency, and lifespan of the magnetic pump. The magnetic coupler should be suitable for outdoor starting and continuous operation under specified environmental conditions, without any decoupling or demagnetization occurring. 　　(1) Inner and outer magnets: The inner magnet is firmly fixed to the guide ring using an adhesive, and a sleeve is used to isolate the inner magnet from the dielectric. The minimum thickness of the sleeve should be 0.4 mm; its material should be non-magnetic and suitable for the medium being transported. 　　The outer magnetic steel is also firmly fixed to the outer magnetic steel ring using an adhesive. To prevent damage to the outer magnet during assembly, its inner surface should also preferably be covered with a sleeve. 　　Synchronous magnetic couplers should use rare-earth magnetic materials such as samarium-cobalt and neodymium-iron-boron ； The torque ring actuator can be made of rare-earth magnetic materials such as samarium-cobalt and neodymium-iron-boron, or alnico magnetic materials. Neodymium-iron-boron has a higher magnetic energy product than samarium-cobalt, but its drawback is that it can only be used at temperatures of 120°C, and its magnetic stability is relatively poor. Samarium-cobalt has high magnetic transmission efficiency and magnetic energy product, as well as excellent resistance to demagnetization. There are generally two types of samarium-cobalt used in magnetic pumps: Sm1Co5 of grade 1.5 and Sm2Co17 of grade 2.17. Samarium-cobalt grade 1.5 contains 35% samarium and 65% cobalt; its maximum operating temperature is 250 °C, and its Curie temperature is 523 °C℃ ； Samarium-cobalt grade 2.17 contains 25% samarium, 50% cobalt, and 25% titanium, iron, etc. Its maximum operating temperature reaches 350 °C, with a Curie temperature of 750 °C. 　　(2) Isolation sleeve: The isolation sleeve, also known as an isolation cover or sealing sleeve, is located between the inner and outer magnetic steel pieces; it completely separates them from each other, with the medium being enclosed within the isolation sleeve. The thickness of the isolation sleeve is related to the operating pressure and temperature; if it is too thick, it increases the gap between the inner and outer magnetic cores, thereby affecting the efficiency of magnetic drive ； If it is too thin, it will affect strength. 　　Isolation sleeves come in metal and non-metal types; metal isolation sleeves incur eddy current losses, while non-metallic isolation sleeves do not. Metal isolation sleeves should be made of materials with high electrical resistivity, such as Hastelloy or titanium alloys; austenitic stainless steel can also be used, with a thickness generally equal to or greater than 1.0 mm. For low-power magnetic pumps operating at relatively low temperatures, non-metallic materials such as plastics or ceramics can also be considered for the isolation sleeve. 　　2) Sliding bearings (1) Silicon carbide ceramics Magnetic pumps generally use silicon carbide ceramic bearings. To prevent free silicon ions from entering the medium, pure sintered α-grade silicon carbide is generally required. Silicon carbide sliding bearings have high load-bearing capacity, as well as excellent resistance to erosion, chemical corrosion, wear, and good heat resistance; they can be used at temperatures above 500°C. The service life of silicon carbide sliding bearings is generally over 3 years. 　　(2) Graphite Graphite possesses good self-lubricating properties and can withstand short periods of dry operation; its operating temperature can reach 450°C. However, its wear resistance is relatively poor. The service life of graphite sliding bearings is generally over 1 year. 　　3. Pump protection system (1) Bearing condition monitor If desired, some internationally renowned manufacturers can provide contactless bearing condition monitors to prevent issues such as bearing wear and failure, coupling disengagement, rotor jamming, and power system failures. 　　(2) Motor power monitor The motor power monitor prevents low flow or dry running by monitoring the motor power. 　　(3) Temperature probe – A temperature probe (RTD) is used to monitor the temperature of the isolation sleeve, in order to reflect changes in the pump’s operating condition. It prevents dry running of the pump, wear of internal and external bearings, severe cavitation, pump seizure, pump jamming, and system overheating, etc. 　　(4) Differential pressure switch A differential pressure switch is used to monitor changes in pressure at the pump outlet, which helps prevent situations such as dry running of the pump, severe cavitation, pump blockage, and other issues. It is especially suitable for container emptying/truck unloading, etc. 　　(5) Second-level protection a. Pressure-resistant, hermetically sealed magnetic coupling enclosure An external isolation sleeve surrounds the magnetic coupling enclosure, as shown in the dashed area in Figure 1. When transporting certain highly toxic or flammable chemicals at high system pressures, this enclosure must be a pressure-resistant sealed container, with design and test pressure values identical to those of the hydraulic end of the pump ； Furthermore, a throttle bushing and a mechanical seal (commonly known as a secondary seal) should be installed between the pump’s outer shaft and the magnetic coupling housing. 　　b. Dual isolation sleeve structure (6) Liquid leakage probe For magnetic pumps that utilize a second layer of protection, a liquid leakage probe should be installed. In magnetic pump systems with a pressure-resistant, sealed magnetic coupling chamber structure, the probe will trigger an alarm when the isolation sleeve breaks or when liquid enters the magnetic coupling chamber for some other reason ； In magnetic pumps with a double isolation sleeve structure, the probe will trigger an alarm when the inner isolation sleeve ruptures, or when liquid enters the cavity between the inner and outer isolation sleeves for some other reason. This article is from: http://www.ahtlbf.com/Article/clbnbjgylt_1.html