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Structural features and applications of twin-screw pumps Structural features and applications of twin-screw pumps 1. Structural features of twin-screw pumps Twin-screw pumps are external meshing screw pumps that use two screws that mesh with each other but do not come into contact to pump liquids. As a positive-displacement pump, the suction chamber of a twin-screw pump must be tightly separated from the discharge chamber. Therefore, the clearance between the pump body and the outer surface of the screw, as well as between the screws themselves, should be as small as possible. At the same time, the screw forms sealed chambers with the pump body and with other screws, ensuring airtightness; otherwise, liquid might flow back through the gaps. Double-screw pumps can be divided into two types: those with internal bearings and those with external bearings. In the structural design with built-in bearings, the bearings are lubricated by the conveyed material. The working chamber of a twin-screw pump with an external bearing structure is separated from the bearings. Due to the structure of this pump and the lateral clearance between the screws, it can transport non-lubricating media. Furthermore, the synchronizing gear is adjusted so that the screws do not come into contact, while half of the output torque is transmitted to the driven screw. Like all screw pumps, external-bearing twin-screw pumps also possess self-priming capability. Moreover, the pumping elements of most such pumps are arranged in a double-suction symmetric configuration, which eliminates axial forces and allows for a high suction height. These properties of the pump enable its widespread use in the oilfield chemicals industry and the shipbuilding industry. External-bearing twin-screw pumps can be manufactured from various materials such as ordinary cast iron and stainless steel, depending on the specific application requirements. The transfer temperature can reach 250ºC. The pump features various heating structures, and its theoretical flow rate can reach 2000 cubic meters per hour. Features of twin-screw pumps: no mixing, no pulsation, and steady transfer of various media. Thanks to its design, the pump always maintains a supply of liquid within its working components as a sealing fluid; all such pumps have strong self-priming capabilities and can transport liquids mixed with vapor. The special design of the pump ensures high suction performance, namely a very low NPSHr value. External bearings with independent lubrication are used, allowing the transport of various non-lubricating media. Available in various configurations such as horizontal, vertical, and those with heating jackets, it can transport a wide range of clean fluids that contain no solid particles, whether they have low or high viscosity. By using the appropriate materials, it is even capable of transporting many corrosive fluids. 2. Applications of twin-screw pumps: As can be understood from the principle of twin-screw pumps, in those models with external bearings, the bearings ensure that the two screws do not come into contact with each other inside the bushing; a constant gap is maintained between the tooth flanks (the value of this gap is determined by the operating conditions and the specifications of the pump), and a constant gap also exists between the outer surface of the screws and the inner surface of the bushing. The transmission of the two screws is accomplished by synchronous gears. The gearbox has independent lubrication, separated from the pump’s working chamber. The advantage of this design is that it **expands the range of applications for twin-screw pumps; that is, in addition to transporting media with good lubricity, they can also handle a wide variety of non-lubricating media, as well as fluids of various viscosities (with the highest viscosity reaching 3*106 mm/s), and liquids that are corrosive (such as acids and bases) or abrasive. Due to its constant clearance and the characteristics of its profile, the twin-screw pump is a type of non-sealed positive-displacement pump. As such, in addition to transporting pure liquids, it can also transport mixtures of gases and liquids, that is, gas-liquid mixtures – and this is one of the very unique advantages of twin-screw pumps. Due to its uniquely designed structure, the twin-screw pump can achieve self-priming without the need for a separate priming device. Moreover, because the axial flow velocity of the conveying shaft is low, it has a strong priming capacity, resulting in a very low NPSHr value. Double-screw pumps can also run dry. Since the moving parts do not come into contact with each other during operation, short periods of dry running will not damage the pump components. This feature greatly facilitates automated control processes, but the duration of dry running is limited by various factors and is generally very short. Furthermore, twin-screw pumps do not cause shearing or emulsification during transportation; as a result, they do not damage the molecular chain structure nor the specific fluid properties that are formed during operation. Moreover, since transmission is achieved through synchronous gears, these pumps operate with low noise, minimal vibration, and smooth performance.