Screw pump is a positive displacement rotor pump, born in 1931. Unique structure, self-priming capacity, high efficiency, small size, reliable work, can transport a wide range of viscosity media. Screw pump is divided into single screw pump, twin-screw pump, three screw pump, etc., what is their working principle? What are the characteristics? Do you know that the selection methods and techniques are different?

The working principle of screw pump is to use the rotation of screw to suction and discharge liquid. The middle screw is the driving screw, which is driven by the prime mover. The screw on both sides is the driven screw, which rotates in the opposite direction with the driving screw.

Each screw is meshed with each other, and the screw is closely matched with the inner wall of the liner, so that one or more sealing spaces will be separated between the suction and discharge ports of the pump. With the rotation and meshing of the screw, these sealing spaces are continuously formed at the suction end of the pump, and the liquid in the suction chamber is sealed in it, and the liquid enclosed in each space is continuously discharged from the suction chamber along the axial direction of the screw to the discharge end.

From the above working principle, we can clearly understand the advantages of screw pump

Wide range of pressure and flow;

A wide range of liquid types and viscosities are transported;

Because the inertia force of the rotating parts in the pump is low, it can use a high speed;

It has good suction performance and self-priming ability;

The flow is uniform and continuous, the vibration is small and the noise is low;

Compared with other rotary pumps, it is not sensitive to the entering gas and dirt;

Solid structure, easy installation and maintenance.

Accordingly, the disadvantages of screw pump are obvious: the screw processing and assembly requirements are higher; the performance of the pump is more sensitive to the change of liquid viscosity.

Screw pump family

Although the screw pump family is not huge, but according to the screw standard, it can also be divided into different types. This paper focuses on the most commonly used single screw pump, twin-screw pump and three screw pump.

Single screw pump

Single screw pump is a new type of internal meshing rotary positive displacement pump. The main working parts are eccentric screw (rotor) and fixed bushing (stator).

Compared with other pumps, single screw pump has its own unique advantages

Compared with centrifugal pump, single screw pump does not need to be equipped with valve, and the flow is stable linear flow.

Compared with plunger pump, single screw pump has better self-priming ability.

Compared with diaphragm pump, single screw pump can transport various kinds of mixed impurities, medium containing gas and solid particles or fibers, as well as various corrosive substances.

Compared with gear pump, single screw pump can transport high viscosity material.

Different from plunger pump, diaphragm pump and gear pump, single screw pump can be used for reagent filling and metering.

Double screw pump

Single from the structural point of view, twin-screw pump is the external mesh screw pump. It uses two mutual meshing, non-contact screws to pump liquid.

In terms of structure, the twin-screw pump is also very complete, including horizontal, vertical, with heating sleeve and other types, which can transport low viscosity or high viscosity medium with particles, adjust screw spacing according to particle size, select correct materials, and even transport many corrosive media.

Features: twin screw pump as a positive displacement pump, pump suction chamber and discharge chamber should be closely separated. This requires the pump body and screw cylindrical surface and screw and screw clearance should be as small as possible. At the same time, screw and pump body, screw and screw between each other to form a sealing cavity, to ensure the airtight, otherwise, there may be liquid back flow back from the gap.

The unique structure of twin-screw pump makes it possible to transport various kinds of media without stirring, pulsation and stability; because the pump body structure ensures that there is always pumped liquid in the working elements of the pump as sealing liquid, the double screw pump has strong self-priming ability and can transport vapor-liquid mixture. The special design of twin screw pump also ensures high suction performance, i.e. very small NPSHr value.

Twin screw pump can be divided into two types of bearings: internal bearing and external bearing.

In the built-in bearing structure, the bearing is lubricated by the conveyor. External bearing structure of the twin-screw pump is working cavity and bearing separate.

Features of twin screw pump with external bearing: there is side clearance between the structure of the pump and the screw, and the external bearing with independent lubrication allows it to transport various non lubricating media. In addition, the synchronous gear is adjusted so that the screw does not contact and half of the output torque is transmitted to the driven screw.

Like all screw pumps, the external bearing type twin-screw pump also has self-priming capacity, and most of the pump conveying elements are arranged symmetrically with double suction, which can eliminate the axial force and have a large suction height.

The advantages of the structure of the twin-screw pump with external bearing greatly broaden the application range of the twin-screw pump, that is: in addition to conveying the medium with good lubrication, it can also transport a large number of non lubricating media, media with various viscosities (up to 3 * 106mm / s) and corrosive (acid, alkali, etc.) and abrasive liquids;

In addition, due to the existence of constant clearance and the characteristics of the profile line, the twin screw pump belongs to the non sealed positive displacement pump. Therefore, in addition to conveying pure liquid, it can also transport the mixture of gas and liquid, that is, vapor-liquid mixed transportation, which is one of the unique advantages of the twin-screw pump;

Twin screw pump can also be dry running: because the moving parts do not contact each other during working, the short-term dry running will not damage the pump components. This feature provides great convenience for the automatic control process, but the dry running time is limited by many factors, which is generally very short.

In addition, the twin screw pump has no shearing and emulsifying effect in the conveying process, so it will not damage the molecular chain