Twin screw pump is an externally engaged screw pump, which uses two mutually engaged and non-contact screws to pump liquid. The twin-screw pump is a double suction non closed twin-screw pump. The driving screw extending out of the pump at one end is driven by the prime mover. The driving screw and driven screw have threads with different rotation directions (if the former is right-handed, the latter is left-handed). The screw is closely fitted with the pump body. Driven screw is driven by driving screw through synchronous gear. As a positive displacement pump, the suction chamber in the twin-screw pump should be closely separated from the discharge chamber. Therefore, the clearance between the pump body and the external surface of the screw and the screw should be as small as possible. At the same time, sealing cavities are formed between the screw and the pump body, and between the screw and the screw to ensure airtightness, otherwise liquid may flow back from the gap.
Twin screw pump can be divided into two forms: internal bearing and external bearing. In the structure type of built-in bearing, the bearing is lubricated by the conveyor. The working chamber of twin-screw pump with external bearing structure is separated from the bearing. Because of the structure of the pump and the side clearance between the screws, it can transport non lubricating medium. 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, external bearing twin screw pumps also have self-priming capacity, and most pump delivery elements are symmetrically arranged with double suction, which can eliminate axial force and also have large suction height.
These characteristics of twin-screw pump make it widely used in oilfield chemical industry and shipbuilding industry. The external bearing twin screw pump can be made of ordinary cast iron, stainless steel and other materials according to various use conditions. The conveying temperature can reach 250 ℃. The pump has different heating structures, and the theoretical flow can reach 2000 m3/h.
The pump body structure ensures that there is always pumped liquid in the working elements of the pump as the sealing liquid. All pumps have strong self-priming capacity and can be mixed with steam and liquid.
The special design of the pump ensures that the pump has a high suction performance, that is, a very small NPSHr value.
The external bearing with independent lubrication is used to allow the transmission of various non lubricating media.
Horizontal, vertical, with heating jacket and other types of structures are complete, which can transport all kinds of clean low viscosity or high viscosity media without solid particles, select the right materials, and even transport many corrosive media.
It is known from the principle of twin-screw pump that for twin-screw pump with external bearing, the two screws do not contact each other in the bushing through the positioning of the bearing, and a constant clearance is maintained between the tooth sides (the clearance value is determined by the working condition and the pump specification), and a constant clearance is also maintained between the outer circle of the screw and the inner circle of the bushing. The transmission of two screws is completed by synchronous gear. There is independent lubrication in the gear box, which is separated from the pump working chamber. The advantages of this structure greatly expand the application scope of the twin-screw pump, that is, in addition to the medium with good lubrication, it can also transport a large number of non lubricating media, media with various viscosities (zui high viscosity can reach 3 * 106 mm/s), and corrosive (acid, alkali and other properties) and abrasive liquids.
The twin-screw pump is a non sealed positive displacement pump due to the existence of constant clearance and the characteristics of the profile. Therefore, in addition to delivering pure liquid, it can also deliver the mixture of gas and liquid, that is, vapor liquid mixed transportation, which is one of the very advantages of the twin-screw pump.
Due to the design of the structure, the twin-screw pump can self-priming without a special self-priming device, and has a strong suction capacity, that is, a small NPSHr value, due to the small axial flow speed of the axial conveying.
The twin-screw pump can also run dry. As moving parts do not contact each other during operation, short-term dry running will not damage pump components. This feature provides great convenience for automatic control process, but the dry running time is generally short due to various factors.
In addition, the twin-screw pump has no shearing and emulsification during the transportation process, so it will not damage the molecular chain structure and the specific fluid properties formed in the working process. Since the transmission relies on synchronous gears, the pump has low noise, low vibration and stable operation.
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