According to the number of screw rods in the screw pump, the screw pump is usually divided into four types: single screw pump, twin screw pump, three screw pump, and five screw pump (the four screw pumps that are rarely produced and used at present are generally not listed separately as a category).
According to the tightness of the spiral groove space formed when the screw meshes in the pump, the screw pump is usually divided into a sealed screw pump and a non-sealed screw pump.
What are the basic types of screw pumps?
The most commonly used screw pumps are single screw pumps, twin screw pumps, and triple screw pumps.
1. Single screw pump:
The single screw pump is a new type of internal meshing rotary positive displacement pump. The main working parts are the eccentric screw (rotor) and the fixed bush (stator).
Compared with other pumps, single screw pump has its own unique advantages:
Compared with centrifugal pumps, single screw pumps do not need to install valves, and the flow rate is a stable linear flow.
Compared with plunger pumps, single screw pumps have better self-priming capabilities.
Compared with diaphragm pumps, single screw pumps can transport various mixed impurities, media containing gas, and solid particles or fibers, and can also transport various corrosive substances.
Compared with gear pumps, single screw pumps can deliver high-viscosity substances.
Unlike plunger pumps, diaphragm pumps, and gear pumps, single screw pumps can be used for the filling and metering of pharmaceuticals.
Structurally speaking, the twin-screw pump is an externally engaged screw pump. It uses two screw rods that mesh with each other and do not touch each other to pump liquid. In terms of structure, twin-screw pumps are also very complete. There are various types such as horizontal, vertical, and heating jackets. They can transport low-viscosity or high-viscosity media with particles. Even many corrosive media can be conveyed.
The twin-screw pump is a positive displacement pump, and the suction chamber in the pump should be tightly separated from the discharge chamber. This requires that the pump body and the outer surface of the screw and the gap between the screw and the screw should be as small as possible. At the same time, the screw and the pump body, and the screw and the screw form a sealed cavity with each other to ensure airtightness, otherwise liquid may flow back from the gap.
The unique structure of the twin-screw pump enables it to convey various media without stirring, without pulsation, and smoothly; because the structure of the pump body ensures that the pumping liquid is always stored in the working components of the pump as the sealing liquid, the twin-screw pump has a strong Excellent self-priming ability and can be mixed with gas and liquid. The special design of the twin-screw pump also ensures that the pump has high suction performance, that is, a small NPSHr value.
Twin-screw pumps can be divided into two types: built-in bearings and external bearings. In the version with built-in bearings, the bearings are lubricated by the conveyed material. In the twin-screw pump with an external bearing structure, the working chamber is separated from the bearing. The structure of this pump has side clearance between the screw and the independently lubricated outboard bearings allowing it to deliver a variety of non-lubricating media. In addition, the synchronous gear is adjusted so that the screws do not touch, and half of the output torque is transmitted to the driven screw at the same time. Like all screw pumps, the external bearing twin-screw pump also has the self-priming ability, and most of the pump delivery components themselves are double-suction symmetrically arranged, which can eliminate axial force and have a large suction height.
The advantages of the twin-screw pump structure with external bearings greatly broaden the scope of use of the twin-screw pump, that is: in addition to conveying medium with good lubricity, it can also convey a large amount of non-lubricating medium with various viscosities (the highest viscosity can reach 3 *106mm/s) media and corrosive (acid, alkali, etc.) and abrasive liquids; in addition, twin-screw pumps are non-sealed positive displacement pumps due to the existence of constant gaps and the characteristics of the molding line, so in addition to transporting pure liquid, it can also transport a mixture of gas and liquid, that is, vapor-liquid mixed transmission, which is also one of the very unique advantages of twin-screw pumps; twin-screw pumps can also run dry: because the moving parts do not touch each other during work, so 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 and is generally very short. In addition, the twin-screw pump has no shearing and emulsification during the conveying process, so it will not damage the molecular chain structure and the specific fluid properties formed in the working process, and because the transmission relies on synchronous gears, the pump runs with low noise and vibration, work smoothly.
The various characteristics of the external bearing twin-screw pump make it widely used in the oilfield chemical industry and the shipbuilding industry. It can also be made of different materials such as ordinary cast iron and stainless steel according to various usage conditions; the conveying temperature can reach 250°C; and it has different heating structures, and the theoretical flow rate can reach 2000 cubic meters per hour.
3. Three-screw pump
The three-screw pump is a rotor-type volumetric pump, which uses the principle of screw meshing, relying on the rotating screws to mesh with each other in the pump sleeve.
It is mainly composed of a bush (pump cylinder) fixed in the pump body, a driving screw inserted in the pump cylinder, and two driven screws meshing with it. Three screw rods meshing with each other form a sealed chamber according to each lead in the pump cylinder, resulting in a seal between the suction and discharge ports.
The three-screw pump seals the conveyed medium in the meshing cavity, and pushes it to the outlet continuously and at a constant speed along the screw axis to provide stable pressure for the system. When working, the two ends act on the suction and discharge pressure of the liquid respectively, which generates axial thrust on the screw. In addition, high-pressure oil is introduced into the bottom of each screw sleeve through the central oil hole of the active screw, so that a balanced thrust opposite to the direction of the axial thrust will be generated at the lower end of the screw. Therefore, for small pumps with a pressure difference of less than 10 kg/cm2, thrust bearings can be used. Since the two driven screws mesh symmetrically with the driving screw, the radial force acting on the driving screw of the three-screw pump is completely balanced, and the driving screw does not bear a bending load. The radial force on the driven screw is supported by the pump cylinder liner along its entire length, without additional bearings at the outer end, and basically does not bear bending loads. During operation, a layer of oil film is formed between the outer surface of the screw and the inner wall of the pump cylinder, which can prevent direct contact between metals and greatly reduce the wear of the screw tooth surface.
Three-screw pumps, like other positive displacement pumps, when the discharge port of the pump is completely closed, the pressure in the pump can rise to a dangerous level that will damage the pump or overload the motor. Therefore, a safety valve must be installed at the suction and discharge ports of the pump. The shaft seal usually adopts a mechanical shaft seal and can take different forms according to the working pressure.
The remarkable features of the three-screw pump are simple structure, slight pressure pulsation, stable flow, stable and reliable operation, high speed allowed, low noise, high efficiency, long life, and self-priming ability. According to the needs of the conveyed medium, the screw pump unit can provide a heating or cooling structure. It is suitable for conveying various non-corrosive oils and similar oils and lubricating liquids. The viscosity range of the transported liquid is generally 3.0~760mm2/s (1.2~100°E). High-viscosity media can be transported after heating to reduce the viscosity, and the temperature generally does not exceed 150°C. Its flow range is generally 0.2~250m3/h, and the maximum working pressure can reach 4MPa.
Three-screw pumps are widely used in various industries such as petroleum, chemical fiber, metallurgy, machinery, electric power, machine tools, ships, glass, and roads. Three-screw pumps are suitable for conveying lubricating liquids such as fuel oil, lubricating oil, hydraulic oil, crude oil, asphalt, and other similar oils. The three-screw pump is used as a lubrication pump in the industrial field, as a hydraulic pump in the hydraulic system, as a delivery and booster pump in the fuel system, and as a delivery and refueling pump in the oil delivery system.