Introduction To Unsealed Centrifugal Pumps

Feb 18, 2025

One summary
Sealed centrifugal pumps, also known as leak free centrifugal pumps, can be divided into magnetic driven centrifugal pumps (hereinafter referred to as magnetic pumps) and shielded pumps. They have only static seals in structure and no dynamic seals, so they can ensure that no drop leaks when transporting liquids. With the continuous improvement of environmental protection requirements, the application of unsealed centrifugal pumps is becoming increasingly widespread. In order to facilitate the rational selection of unsealed centrifugal pumps, this article introduces the types, principles, and structures of unsealed centrifugal pumps, compares the characteristics of magnetic pumps and shielded pumps, and summarizes some issues that should be noted when selecting unsealed centrifugal pumps.
II Magnetic pump
1. Working principle of magnetic pump
Magnetic transmission is the use of the characteristic that magnets can attract ferromagnetic materials and there is magnetic interaction between magnets or magnetic fields, rather than non ferromagnetic materials that do not affect or have little effect on the magnitude of magnetic force. Therefore, power transmission can be carried out through non-magnetic conductors (isolation sleeves) without contact.
Magnetic transmission can be divided into synchronous or asynchronous designs. Most magnetic pumps adopt synchronous design. The electric motor is connected to the outer magnetic steel through an external coupling, and the impeller is connected to the inner magnetic steel. There is a fully sealed isolation sleeve between the outer magnetic steel and the inner magnetic steel, which completely separates the inner and outer magnetic steels, keeping the inner magnetic steel in the medium. The motor shaft directly drives the impeller to rotate synchronously through the suction force of the magnetic poles between the magnetic steels.
Asynchronous design magnetic transmission, also known as torque ring magnetic transmission. Replace the inner magnet with a squirrel cage structure torque ring, which rotates at a slightly lower speed under the attraction of the outer magnet. Due to the absence of internal magnetic steel, its operating temperature is higher than that of synchronous magnetic drive.
2. Structure of magnetic pump
1) Magnetic coupler
Magnetic transmission is accomplished by a magnetic coupler. Magnetic couplers mainly include internal magnetic steel, external magnetic steel, and isolation sleeves, and are the core components of magnetic pumps. The structure, magnetic circuit design, and materials of each component of the magnetic coupler are related to the reliability, magnetic transmission efficiency, and lifespan of the magnetic pump. Magnetic couplers should be suitable for outdoor start-up and continuous operation under specified environmental conditions, and should not exhibit decoupling or demagnetization phenomena.
(1) Internal and external magnetic steel
The inner magnetic steel should be firmly fixed on the guide ring with adhesive and isolated from the medium with a sleeve. The minimum thickness of the package should be 0.4mm, and its material should be non-magnetic and suitable for the medium being transported.
The outer magnetic steel should also be firmly fixed to the outer magnetic steel ring with adhesive. To prevent damage to the outer magnetic steel during assembly, it is recommended to cover the inner surface of the outer magnetic steel with a sleeve.
Synchronous magnetic couplers should use rare earth magnetic materials such as samarium cobalt and neodymium iron boron; The torque ring transmission can be made of rare earth magnetic materials such as samarium cobalt, neodymium iron boron, or aluminum nickel cobalt magnetic materials. The magnetic energy product of neodymium iron boron is higher than that of samarium cobalt, but the disadvantage is that the operating 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 anti demagnetization ability. There are usually two types of samarium cobalt used for magnetic pumps, samarium cobalt grade 1.5 Sm1Co5 and grade 2.17 Sm2Co17. Samarium cobalt grade 1.5 contains 35% samarium and 65% cobalt, with a maximum operating temperature of 250 ℃ and a Curie temperature of 523 ℃; Samarium cobalt grade 2.17 contains 25% samarium, 50% cobalt, and 25% titanium, iron, etc. Its maximum operating temperature is 350 ℃, and its Curie temperature is 750 ℃.
(2) Isolation sleeve
Isolation sleeve, also known as isolation cover or sealing sleeve, is located between the inner and outer magnetic steel, completely separating them and enclosing the medium inside the isolation sleeve. The thickness of the isolation sleeve is related to the working pressure and operating temperature. If it is too thick, it will increase the gap size between the inner and outer magnetic steels, thereby affecting the magnetic transmission efficiency; If it is too thin, it will affect the strength.
There are two types of isolation sleeves: metal and non-metal. Metal isolation sleeves have eddy current losses, while non-metal isolation sleeves have no eddy current losses. The metal isolation sleeve should be made of materials with high electrical resistivity, such as Hastelloy, titanium alloy, etc. Austenitic stainless steel can also be used, and its thickness should generally be greater than or equal to 1.0mm. For low-power magnetic pumps and when used at low temperatures, non-metallic materials such as plastic or ceramic can also be considered for their isolation sleeves.
2) Sliding bearings
(1) Silicon carbide ceramics
Magnetic pumps generally use silicon carbide ceramic bearings. To prevent free silicon ions from entering the medium, it is generally required to use pure sintered alpha grade silicon carbide. Silicon carbide sliding bearings have high load-bearing capacity and strong resistance to erosion, chemical corrosion, wear, and good heat resistance. They can be used at temperatures above 500 ℃. The service life of silicon carbide sliding bearings can generally reach more than 3 years.
(2) Graphite
Graphite has good self-lubricating properties, can withstand short-term dry operation, and can be used at temperatures up to 450 ℃. The disadvantage is poor wear resistance. The service life of graphite sliding bearings can generally reach more than 1 year.
3. Pump protection system
(1) Bearing condition monitor
If required by users, some internationally renowned manufacturers can configure non-contact bearing condition monitors (high-temperature pumps) to prevent bearing wear and 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 operation.
(3) Temperature probe
Use a temperature probe (RTD) to monitor the temperature of the isolation sleeve to reflect changes in the pump's operating state. It can prevent dry operation of the pump, wear of internal and external bearings, severe cavitation, pump blockage, pump jamming, and system overheating.
(4) Differential pressure switch
Using a differential pressure switch to monitor pressure changes at the pump outlet can prevent dry operation, severe cavitation, pump blockage, and pump jamming of the pump. Especially suitable for container emptying/tanker unloading, etc.
(5) Second layer of protection
A Pressure sealed magnetic coupling box
The isolation sleeve is surrounded by a magnetic coupling box. When transporting certain highly toxic or flammable chemicals under high system pressure, the container should be a pressure sealed container with the same design and test pressure values as the hydraulic end of the pump; And a throttling liner and mechanical seal (commonly known as a secondary seal) should be installed between the pump outer shaft and the magnetic coupling box.
B Double isolation sleeve structure
(6) Liquid leakage probe
For magnetic pumps with second layer protection, liquid leakage probes should be installed. For magnetic pumps with pressure sealed magnetic coupling box structures, when the isolation sleeve ruptures or liquid enters the magnetic coupling box due to other reasons, the probe will sound an alarm; For magnetic pumps with dual isolation sleeves, when the inner isolation sleeve ruptures or liquid enters the cavity between the inner and outer isolation sleeves due to other reasons, the probe will sound an alarm.