Working Principle and Characteristics of Submerged Pumps

     A submerged pump is a type of single-screw transfer pump, whose main working components include a screw with an eccentric helical structure (called the rotor) and a screw bushing with a double-line helical inner surface (called the stator).

Its working principle is as follows: when the motor drives the pump shaft to rotate, the screw not only spins around its own axis but also rolls along the inner surface of the bushing, thereby forming sealed chambers inside the pump. For each full rotation of the screw, the liquid in the sealed chamber is pushed forward by one pitch. With the continuous rotation of the screw, the liquid is pressured from one sealed chamber to the next in a helical manner, and finally discharged out of the pump body.

As a new type of liquid-conveying machinery, the submerged pump features a simple structure, safe and reliable operation, convenient installation and maintenance, continuous and uniform discharge, and stable pressure. It is a pump that relies on screw rotation to suck and discharge liquids, and is most suitable for conveying viscous fluids.

The submerged pump sucks and discharges liquids by virtue of screw rotation. The middle screw serves as the driving screw, which is driven to rotate by the prime mover; the screws on both sides are driven screws that rotate in the opposite direction along with the driving screw. Both the driving and driven screws are designed with double-start threads.

Thanks to the mutual meshing of the screws and the tight fit between the screws and the inner wall of the bushing, one or more sealed spaces are separated between the suction port and the discharge port of the submerged pump. As the screws rotate and mesh, these sealed spaces are continuously formed at the suction end of the pump, enclosing the liquid in the suction chamber. The liquid is then continuously pushed axially along the screws to the discharge end, and the liquid sealed in each space is discharged uninterruptedly. This process is analogous to a nut being continuously pushed forward when a threaded rod rotates—and this is the basic working principle of a submerged pump.

Seal Structure of Screw and Casing & Advantages and Disadvantages of Submerged Pumps

The sealing surface between the screw and the casing is a spatial curved surface. On this curved surface, there exist non-sealing areas such as ab or de, which form numerous triangular gaps like abc and def together with the groove parts of the screw. These triangular gaps constitute liquid channels, connecting the groove A of the driving screw with the grooves B and C of the driven screw.

Grooves B and C then wind along their own spiral lines toward the back of the screw, and connect with the grooves D and E on the back respectively. Since there are also triangular gaps like a’b’c’ (similar to those on the front side) on the sealing surface where grooves D, E are connected to groove F (which belongs to another spiral thread), grooves D, F and E are also connected. In this way, the grooves ABCDEA form a sealed space shaped like the symbol "∞" (if a single-start thread is adopted, the groove will spiral along the axial direction of the screw, connecting the suction port and the discharge port and making it impossible to form a seal).

It is easy to imagine that a number of independent "∞"-shaped sealed spaces will be formed on such a screw, and the axial length occupied by each sealed space is exactly equal to the lead t of the screw. Therefore, to enable the screw to separate the suction port from the discharge port, the length of the threaded section of the screw must be at least greater than one lead.

From the above working principle, the submerged pump has the following advantages:

It can transport a wide range of liquid types and viscosities.

Due to the low inertia force of the rotating parts inside the submerged pump, it can operate at a very high rotational speed.

It has excellent suction performance and self-priming capability.

It delivers a continuous and uniform flow with low vibration and noise.

Compared with other rotary pumps, it is less sensitive to incoming gases and contaminants.

It features a robust structure and is easy to install and maintain.

The disadvantages of the submerged pump are that the screw requires high machining and assembly precision, and the pump performance is relatively sensitive to changes in liquid viscosity.

Structural Features and Application Scope of Submerged Pumps

The working part of a submerged pump is submerged in the liquid, ensuring zero leakage at the shaft seal. It also features a small footprint, reliable operation, convenient maintenance, and strong corrosion resistance. Submerged pumps are widely used in industrial sectors such as chemical engineering, pharmaceuticals, papermaking, and petroleum.

The vertical motor of the submerged pump is fastened to the motor base with bolts and directly connected to the pump via a flexible coupling. The pump body, intermediate connecting pipe, pump frame, discharge pipe, and pipe flange are assembled as a single unit with bolted connections and fixed to the base plate. The entire submerged pump is mounted on the vessel through the base plate.

The axial and radial forces of the pump (including the hydraulic pressure generated during pump operation, the weight of the impeller and rotor, etc.) are borne by the single-direction thrust ball bearing, single-row radial ball bearing, and sliding guide bearing installed in the bearing housing to ensure the safe and stable operation of the pump. The bearings are lubricated with grease, while the guide bearings are lubricated by the conveyed liquid. Therefore, the liquid level must be higher than the centerline of the impeller when the submerged pump is in operation.

According to the different lengths of the submerged part extending into the vessel (L), the pump is divided into two structural types: with intermediate guide bearing and without intermediate guide bearing.

Submerged pumps are manufactured in different specifications based on the length of the part extending into the vessel. Since the working part is submerged in the liquid, the axial and radial forces generated during operation are supported by rolling bearings and sliding bearings respectively, resulting in quiet and noiseless operation. There is no liquid splashing at the shaft seal. The stuffing seal is equipped with a cooling system, which can use cooling water to dissipate heat according to the temperature of the medium used by the user.