Basic knowledge of shielded pumps

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I. Working principle of the canned pump 1. The working principle of the canned pump is the same as that of a centrifugal pump. The rotation of the impeller generates centrifugal force, which separates the liquid inside the impeller from it, converting kinetic energy into pressure energy, thereby enabling the transport of the medium. The key to the operation of a shielded pump lies in utilizing the interaction characteristics between the motor stator and rotor. The rotor and impeller are connected by a shaft, while the coils of the stator and rotor are separated from the fluid being transported by thin shielding sleeves, thereby converting the dynamic seal into a static seal. Due to the pressure difference, the medium flowing between the stator and rotor forms a circulation that cools the motor and bearings as well as lubricates them; therefore, a shielded pump does not require external lubrication. 2. Like centrifugal pumps, it also relies on centrifugal force to pump liquids. The difference is that a centrifugal pump is driven by an electric motor, while a shaded pole motor is itself a motor, its stator being covered with coils. When electricity is applied, a magnetic field is generated, which drives the rotor to rotate; the impeller and the rotor work together to pump the liquid. 3. The pump head and motor of the canned pump are sealed within a pressure vessel filled with the pumping medium, using only static seals. The impeller of the shielded pump and the rotor of the motor are fixed on the same shaft, while the rotor and stator of the motor are separated by a shielding sleeve. The rotor operates within the medium being transported, with power being transmitted to it through the stator’s magnetic field. II. Structure of the shielded pump 1. Sliding bearings: Due to the long length of the rotor, a shielded pump requires two moving bearing seats. The two bearings require precise alignment. If the alignment is poor, the bearing can easily break. Due to the relatively soft nature of graphite, graphite is often used for the sliding bearings in shielded pumps. The stone mill sliding bearing forms a friction pair with a shaft sleeve made of hard alloys such as tungsten, chromium, and cobalt deposited on the surface, or hard alloys like silicon nitride produced by plasma spraying; its service life can exceed one year. Some canned pump manufacturers, having successfully resolved the alignment issue of the two sliding bearing housings, can also use purely sintered grade A silicon carbide material. If used properly, the service life of pure sintered grade A silicon carbide sliding bearings can exceed three years. 2. Shielding sleeves: A shielded pump usually has two shielding sleeves, namely the stator shielding sleeve and the rotor shielding sleeve. It is used to prevent the working medium from penetrating into the stator windings and rotor core, with a thickness generally ranging from 0.4 to 0.7 mm. Due to the presence of the shielding sleeve, the gap between the stator and rotor of the motor increases, which leads to a decline in the performance of the shielded motor. At the same time, eddy currents are generated within the shielding sleeve, increasing power losses. For shielded pumps, the shielding sleeve should be made of a non-magnetic material with good corrosion resistance and high strength; Hastelloy is the preferred material for the stator shielding sleeve. Rotor shielding sleeves are generally made of Hastelloy or austenitic stainless steel. 1. The liquid conveyed does not leak, making it suitable for transporting liquids that are harmful to humans, highly corrosive, flammable and explosive, expensive, or radioactive. It does not draw in air or other substances from the outside, making it suitable for use in vacuum systems and in situations where it deteriorates upon contact with external air ； 2. No need to inject lubricant or sealant, with low noise level ； 3. Suitable for transporting liquids at high temperatures, extremely low temperatures, and high melting points; it takes advantage of the shaft-seal-free design of this pump to handle such special liquids that are difficult to manage with pumps equipped with auxiliary seals ； 4. The motor and pump are integrated into one unit, adopting a very compact structure; as a result, it has a small size, low weight, and requires little space. It doesn’t need alignment and is easy to install. 5. Since shielded pumps use sliding bearings and are lubricated by the medium being transported, media with poor lubricity are not suitable for transportation using shielded pumps. 6. The efficiency of shielded pumps is usually lower than that of centrifugal pumps with single-end mechanical seals, but it is roughly comparable to that of centrifugal pumps with double-end mechanical seals.