Magnetic transmission is the use of magnets to attract ferromagnetic materials and the magnetic force between magnets or magnetic fields. Non-ferromagnetic materials do not affect or rarely affect the magnitude of the magnetic force, so it can pass through non-magnetic conductors (isolation sleeves) without contact ) Perform power transmission.
Magnetic drive can be divided into synchronous or asynchronous design. Most magnetic pumps use synchronous design, as shown in the figure below. The motor is connected to the outer magnetic cylinder through an external coupling, and the impeller is connected to the inner magnetic cylinder. There is a fully sealed isolation sleeve between the outer magnetic cylinder and the inner magnetic cylinder, which completely separates the inner and outer magnetic cylinders, so that the inner magnetic cylinder is in the medium, and the rotor shaft of the motor directly drives the impeller synchronization through the attraction of the magnetic poles between the magnetic cylinders. Rotate.
Asynchronous design magnetic transmission, also called torque ring magnetic transmission, uses a cage-shaped torque ring to replace the inner magnetic cylinder. The torque ring is similar to the rotor of an asynchronous motor. The induced electromotive force and the magnetic field are generated in the rotating magnetic field generated by the outer magnetic cylinder. Induces a magnetic field to rotate at a slightly lower speed. Since there is no permanent magnet in the inner magnetic cylinder, its use temperature is higher than that of the synchronous drive magnetic drive.
1. Magnetic coupling
The magnetic drive is completed by a magnetic coupling. The magnetic coupler mainly includes parts such as inner magnetic cylinder, outer magnetic cylinder and isolation sleeve, which is the core component of magnetic pump. The structure of the magnetic coupling, the design of the magnetic circuit and the materials of its parts are related to the reliability, magnetic transmission efficiency and life of the magnetic pump. The magnetic coupling is suitable for outdoor startup and continuous operation under the specified environmental conditions, and there should be no decoupling and demagnetization.
(1) Inner and outer magnetic cylinder
The inner magnetic cylinder should be firmly fixed on the guide ring with adhesive, and the inner magnetic cylinder is separated from the medium with a sleeve. The minimum thickness of the sheath should be 0.4mm, and the material should be non-magnetic and suitable for the conveyed medium.
The outer magnetic cylinder should also be firmly fixed on the outer magnetic cylinder guide ring with adhesive. In order to prevent atmospheric corrosion, damage to the isolation sleeve by the outer magnetic cylinder and possible collision sparks under accident conditions, and to prevent damage to the outer magnetic cylinder during assembly, the inner surface of the outer magnetic cylinder should preferably be covered with a cover. The sleeve should be a material that does not produce sparks.
Synchronous magnetic coupling should use rare earth magnetic materials such as samarium cobalt and neodymium iron boron; torque ring actuators can use rare earth magnetic materials such as samarium cobalt and neodymium iron boron. To adapt to high temperature conditions, aluminum nickel cobalt magnetic materials can also be used. The magnetic energy product of neodymium iron boron is higher than that of samarium cobalt. The disadvantage is that the use temperature is only 120 ℃ and the magnetic stability is relatively poor. Samarium cobalt has high magnetic transmission efficiency and magnetic energy product, and has extremely strong demagnetization resistance. There are usually two types of samarium cobalt used in magnetic pumps, Ming cobalt 1.5 grade Sm1Co5 and 2.17 grade Sm2Co17. Samarium cobalt grade 1.5 contains samarium 35%, cobalt 65%, the highest operating temperature is 250℃, and the Curie temperature is 523℃; Ming cobalt grade 2.17 contains 25% for shirts, 50% cobalt, 25% titanium, iron, etc., and its maximum operating temperature is up to 350℃, Curie temperature 750℃. See the figure below for details.
(2) Isolation sleeve
The isolation sleeve is also called an isolation cover or a sealing sleeve. It is located between the inner and outer magnetic cylinders and completely separates the inner and outer magnetic cylinders, and the medium is enclosed in the isolation sleeve. The thickness of the isolating sleeve is related to the working pressure and operating temperature. If it is too thick, the gap size between the inner and outer cylinders will be increased, the magnetic transmission torque will be reduced, and the eddy current loss in the isolating sleeve will be increased, thereby affecting the efficiency of the magnetic transmission; Affect strength and stiffness.
There are two types of isolation sleeves: metal and non-metal. The metal isolation sleeve has eddy current loss, and the non-metal isolation sleeve has no eddy current loss. The metal isolation sleeve should be made of materials with high resistivity, such as Hastelloy, titanium alloy, etc., or austenitic stainless steel, whose thickness is generally 1.0~1.2mm. For low-power magnetic pumps, and when the operating temperature is low, non-metallic materials such as plastic or ceramics can also be considered for the isolation sleeve.
2. Sliding bearing
(1) Silicon carbide ceramics
Magnetic drive pumps generally use silicon carbide ceramic bearings. In order to prevent silicon carbide from containing free silicon, thereby reducing its corrosion resistance, it is generally required to use pressureless sintered α silicon carbide. Silicon carbide sliding bearings have high load carrying capacity, strong erosion resistance, chemical corrosion resistance, abrasion resistance and good heat resistance. The service temperature can reach above 500℃. The service life of silicon carbide sliding bearings can generally reach more than 3 years.
Silicon carbide bearings are usually mounted on metal parts. Since the thermal expansion coefficient of silicon carbide is much smaller than that of general metal materials, special inlay structures (such as metal corrugated pads) or special metal materials (such as titanium alloys) should be used when transporting high temperature or low temperature media.
(2) Graphite
Graphite has good self-lubricating properties, can withstand short-term dry operation, and the use temperature can reach 450°C. The disadvantage is poor wear resistance. The service life of graphite sliding bearings is generally more than 1 year.
In order to improve the service life and the safety of operation, the magnetic pump can provide one or more of the following safety protection devices.
1. Bearing condition detector
If users need, some internationally renowned manufacturers can configure non-contact bearing condition monitors to prevent bearing wear failure, coupling decoupling, rotor jamming and power system failures.
2. Motor power monitor
The motor power monitor monitors the motor power to avoid low flow or dry running.
3. Temperature probe
Use a temperature probe (RTD) to monitor the temperature of the isolating sleeve to reflect the changes in the pump's operating state. It can prevent the pump from dry running, internal and external bearing wear, severe cavitation, stuffy pump, pump jam and system overheating.
4. Differential pressure switch
Use the differential pressure switch to monitor the pressure change of the pump outlet, which can prevent the pump from dry running, severe cavitation, stuffy pump, pump jam, etc. Especially suitable for hand container emptying/tank truck unloading, etc.
5. Second layer of protection
(1) Pressure-tight magnetic coupling box
Outside the isolation sleeve is a magnetic coupling box. When transporting certain highly toxic or flammable chemicals under high system pressure, the tank should be a pressure-tight container, and its design and test pressure are the same as the hydraulic end of the pump; and the outer shaft of the pump and the magnetic coupling tank There should be a throttling bushing and a mechanical seal (commonly known as a secondary seal). API685 recommends this type.
(2) Double isolation sleeve structure
When the double isolation sleeve structure is adopted, the design and test pressure value of the outer isolation sleeve should also be the same as the hydraulic end of the pump.
6. Liquid leak probe
For the magnetic pump with the second layer of protection, a liquid leakage probe should be installed. For a magnetic pump with a pressure-tight magnetic coupling box structure, the probe will alarm when the isolating sleeve is broken or liquid enters the magnetic coupling box for other reasons; for a magnetic pump with a double isolating sleeve structure, when the inner isolating sleeve is broken Or when liquid enters the cavity between the inner and outer isolation sleeves due to other reasons, the probe will alarm.
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