The Structure and Application of Magnetic Drive Centrifugal Pump

1.Structure of Metal Magnetic Drive Centrifugal Pump

The magnetic drive centrifugal pump consists of four main components: the housing, rotor, connecting parts, and transmission system. It is available in two configurations: direct-coupled and non-direct-coupled. The direct-coupled design features a magnetic coupling (external magnet) directly connected to the motor shaft, eliminating the need for external shafts, rolling bearings, or coupling components, as illustrated in Figure 1-12.

 

 

Figure 1-12  Schematic Diagram of Direct-Coupled Magnetic Drive Centrifugal Pump

 

1—Pump body; 2—Impeller; 3—Pump shaft; 4—Shaft sleeve; 5—Sliding bearing; 6—Pump cover;7—Inner magnetic rotor; 8—Isolation sleeve; 9—Outer magnetic rotor; 10—Electric motor

The non-direct-connected magnetic drive centrifugal pump, also known as the standard magnetic drive centrifugal pump, features an external shaft with a magnetic coupling (external magnet) connected to the motor via a bearing housing and coupling. The schematic structure of this pump is illustrated in Figure 1-21.

(1) Shell section

The shell part is composed of the pump body (pump shell), pump cover, isolation sleeve, etc. It bears all the working pressure of the pump.

(2) Rotor section

The rotor assembly consists of two main components: the rotating parts mounted on the pump shaft and those installed on the drive shaft. The pump shaft's rotating components include the impeller, bearings, thrust ring assembly, inner magnetic rotor, and the shaft itself, forming the rotor section that interfaces with the medium. The drive shaft's rotating parts comprise the outer magnetic rotor, rolling bearings, drive shaft sleeve, and the shaft itself, constituting the rotor section that contacts the air.

(3) Connection section

It is composed of connecting frame, bearing box and other parts, which play the role of connecting and supporting.

(4) Transmission section

The connection section refers to the coupling between the pump and the drive unit. Magnetic drive centrifugal pumps employ two connection methods: (1) connecting the pump's internal magnetic coupling to the drive unit's magnetic coupling (external magnetic coupling); (2) using a diaphragm-type extended coupling component to connect the pump's external shaft magnetic coupling to the drive unit. This design allows pump maintenance by simply removing the coupling's intermediate section bolts and diaphragm, eliminating the need to disassemble the drive unit for servicing, thus ensuring convenient maintenance.

2. Main Components and Their Functions of Metal Magnetic Drive Centrifugal Pump

(1) Main Components of Metal Magnetic Drive Centrifugal Pump

The key components of a metal magnetic drive centrifugal pump include: impeller, shaft, suction chamber, pump body (housing), isolation sleeve, bearing housing, and port ring. Some models may also incorporate guide vanes, induction wheel, and balance disc. The flow passages consist of the suction chamber, pump body (housing), and impeller, each serving the following functions.

① Inlet chamber The inlet chamber is located at the front end of the impeller inlet, where the liquid is drawn into the impeller through the suction port. It is required that the flow loss of the liquid passing through the inlet chamber be minimal, and the velocity of the liquid entering the impeller should be uniformly distributed.

②Impeller The rotating impeller converts energy by drawing in liquid, imparting pressure energy and kinetic energy to the liquid. The impeller is required to maximize energy transfer to the liquid while minimizing flow loss.

(2) Functions of Key Components in Metal-Magnetic Drive Centrifugal Pumps

① Pump body (pump housing)

The pump body, also known as the pump casing, comes in two types: axially split and radially split, serving as a component that withstands liquid pressure. Most single-stage pumps feature a volute casing, while multi-stage pumps typically use annular or circular casings. Its primary function is to contain the liquid within a defined space, channel the liquid ejected from the impeller's flow passages into discharge pipes, and convert part of the liquid's kinetic energy into pressure energy, thereby increasing its pressure.

The pump body generally has the following three types:

a. The volute pump body (shell) resembles a snail shell in appearance (Figure 1-22). Inside the volute, there are flow channels with gradually expanding cross-sections. The shape and dimensions of these channels significantly influence the pump's performance.

b. Pump body (housing) with guide vane assembly. The pump body (housing) is a rotating structure, housing the impeller's outer component.

The flow channel is surrounded by several guide vane structures.

c. Double-layer pump body (shell) A pump body (shell) with an additional cylindrical outer casing is called a double-layer pump body (shell).

② impeller

The impeller, a key component of a pump, drives liquid transfer through high-speed rotation. Typically consisting of three parts—the hub, blades, and cover plate—the impeller has two types of cover plates: the front cover plate on the inlet side and the rear cover plate on the opposite side.

Magnetic drive centrifugal pumps convey liquids primarily through the action of the impeller installed within the pump body. The size, shape, and manufacturing precision of the impeller significantly influence the pump's performance. Based on structural configuration, impellers can be classified into three types: closed, open, and semi-open (Figure 1-23).

a. enclosed impeller

A disc impeller typically consists of a cover plate, blades, and a hub. The front cover plate is located on the suction side, while the rear cover plate is on the opposite side, with the blades positioned between them. There are 4 to 6 blades between the two cover plates, and these blades are generally backward-curved, as shown in Figure 1-23(a). Closed impellers are highly efficient and widely used, particularly for conveying clean liquids without solid particles or fibers. They come in two types: single-suction and double-suction. The double-suction impeller, as illustrated in Figure 1-24, is suitable for high-flow pumps and offers better cavitation resistance.

b. open impeller

The impeller has no cover plates on either side, with blades connected to the hub via stiffeners, as shown in Figure 1-23(b). This impeller design is simple and easy to manufacture, but has low efficiency, making it suitable for conveying liquids with high solid suspended matter or fibrous content.

c. semiclosed-type impeller

This impeller features only a rear cover plate, as shown in Figure 1-23(c). It is designed for transporting liquids prone to sedimentation or containing solid suspended matter, with an efficiency that falls between open and closed impellers.

 

 

 

Figure 1-23 Impellers of Magnetic Drive Centrifugal Pump

 

Figure 1-24 Double-suction Impeller

There are two types of impeller blades for centrifugal pumps: straight blades and twisted blades.

Straight blades are those whose entire width aligns parallel to the impeller shaft, as illustrated in Figure 1-23.

The twisted blades feature a section that deviates from the impeller axis, as illustrated in Figure 1-25. For low specific speed impellers, the blades are circular with narrow flow channels, facilitating manufacturing. In contrast, high specific speed impellers employ wider flow channels, enabling easier twisting. Such blades enhance the pump's cavitation resistance, reduce impact losses, and ultimately improve overall efficiency.

When the blade bending direction is opposite to the impeller rotation direction, it is called a backward-curved blade; otherwise, it is called a forward-curved blade. Due to the higher efficiency of backward-curved blades, they are generally used for impellers.

③ choma

The sealing ring, also known as the gland, is typically mounted on the pump body and forms a minimal clearance with the impeller suction inlet's outer circumference (Figure 1-26). Since the liquid pressure inside the pump body exceeds the suction inlet pressure, the fluid tends to flow toward the impeller suction inlet. The primary function of the sealing ring is to prevent liquid leakage between the impeller and pump body. Additionally, it serves as a friction-bearing component. When excessive wear occurs in the clearance, replacing the sealing ring prevents the impeller and pump body from being scrapped, thereby extending their service life. Consequently, the sealing ring is classified as a pump's wear-prone component. The clearance dimension between the sealing ring and the impeller suction inlet's outer circumference is generally determined by the diameter of the impeller gland.

④ Isolation sleeve

In a magnetically driven centrifugal pump, the isolation sleeve primarily functions as a shaft seal, serving as the sole component that ensures leak-proof operation. Unlike conventional centrifugal pumps, the rotating shaft is not externally protruding from the stationary pump housing. Instead, the isolation sleeve replaces the traditional shaft seal, effectively preventing both high-pressure fluid leakage and air ingress into the pump chamber (as illustrated in Figure 1-27). This design rationale explains the inclusion of a sealing mechanism in such pumps. The shaft and pump housing are physically separated by the isolation sleeve, which replaces the conventional shaft seal assembly.

⑤ Magnetic Coupling

A magnetic coupling consists of an inner magnet (featuring a magnet holder and a magnet sleeve) and an outer magnet (with a magnet holder). The isolation sleeve, positioned between the inner and outer magnets (Figure 1-28), is a key distinguishing feature of magnetic pumps and serves as their core component. The magnetic coupling's structure, magnetic circuit design, and material selection of its components directly impact the pump's reliability, magnetic drive efficiency, and service life.

 

Figure 1-28 Schematic Diagram of Magnetic Coupling Structure

1—Outer magnetic base；2—Outer magnetic steel block；3—Isolation sleeve；4—Inner magnetic steel enclosure；5—Inner magnetic steel block；6—Inner magnetic base

L — Length of magnetic steel block；a — Coating thickness；b — Thickness of isolation sleeve；c — Air gap

a.Internal magnetic steel

The inner magnetic steel is bonded to its base with adhesive. To isolate the inner magnetic steel from the medium, a protective sleeve must be applied to its exterior. The sleeve is available in two types: metal and plastic. Metal sleeves are welded, while plastic sleeves are injection-molded (when the material is metal, non-magnetic austenitic stainless steel must be used).

b.External magnet

The outer magnet and the outer magnet seat are connected by adhesive.

c.Isolation sleeve

The isolation sleeve, also known as the sealing sleeve, is positioned between the inner and outer magnets to completely isolate them, with the medium enclosed within the sleeve (Figure 1-29).

The thickness of the isolation sleeve is related to the working pressure and operating temperature. If it is too thick, the gap between the inner and outer magnets will increase, which will affect the efficiency of magnetic drive. If it is too thin, the strength will be affected. There are two kinds of isolation sleeves: metal and non-metal. The metal isolation sleeve has eddy current loss, while the non-metal isolation sleeve has no eddy current loss.

⑥ sleeve bearing

The pump shaft of a magnetically driven centrifugal pump is supported by a sliding bearing. Since the sliding bearing relies on the transported medium for lubrication, it should be fabricated from materials with excellent wear resistance and self-lubricating properties. Commonly used bearing materials include silicon carbide, ceramics, graphite-based materials, and polytetrafluoroethylene (PTFE) filled composites.

The lubrication of sliding bearings relies on their own fluid flow, which requires the bearings, bushings, and thrust discs to possess excellent self-lubrication, wear resistance, and corrosion resistance. For instance, both SSiC and YWN8 exhibit outstanding wear resistance, corrosion resistance, and self-lubrication properties, with SSiC having higher relative hardness than YWN8. When paired with thrust bearings, the combination of soft and hard materials forms an optimal friction pair, significantly extending bearing service life. Practical tests have shown that the service life of paired bearings made from these materials (SSiC and YWN8) can be up to 10 times longer than that of graphite bearings or SiC bearings paired with the same material. As critical components in magnetic pumps, extending the service life of sliding bearings directly enhances the overall lifespan of the magnetic pump. Therefore, material selection is crucial for ensuring stable and long-term operation of magnetic pumps.

⑦ equalizer

In a magnetically driven pump, the forces acting on both sides of the impeller are unequal, as shown in Figure 1-30. When the pump is momentarily started by the drive mechanism, an axial force is exerted on the impeller toward the suction side. If this axial force is not eliminated, axial movement of the rotating parts will occur, leading to wear, vibration, and overheating, which prevents the pump from operating normally. Therefore, a balancing device must be used to prevent axial movement. The most common types of axial balancing devices include balancing holes, balancing pipes, and balancing discs.

 

 

Figure 1-30 Schematic Diagram of Pump Axial Force

a. balance hole

The same sealing ring is added to the rear cover of impeller, and several holes are opened on the rear cover (balance holes) to make the pressure at the rear cover equal to the suction inlet pressure, so as to balance the axial force.

b. balance pipe

A pipe is connected to the pump body and leads to the suction inlet, ensuring pressure balance on both sides of the impeller. These two devices have simple structures but may cause liquid backflow, reducing efficiency. Additionally, 10%-25% of the axial force remains unbalanced, typically requiring a thrust disk to absorb the residual axial force.

c. balance disk

Figure 1-31 illustrates a schematic of a balance disc assembly, primarily used in multi-stage pumps where it is fixed to the final-stage impeller on the same shaft. An axial clearance exists between the balance disc and the pump body. During operation, high-pressure liquid flows through this clearance into the balance chamber on the right side of the balance disc. The balance chamber is connected to the suction inlet, maintaining equal pressure. This creates a pressure differential across the balance disc, with the opposing thrust and axial forces counterbalancing each other. The pump's rotating components can move laterally, and the balance disc automatically maintains equilibrium during operation. Additionally, methods such as using double-suction impellers or symmetrically arranged impellers can also help balance partial axial forces.