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Basic knowledge about screw pumps: A pump that is formed by the meshing of two or three screws is called a screw pump. For a twin-screw pump, its flow rate is given by: Q=3Ω/16(D²-d²)tn, where D is the outer diameter of the screws ； d——inner diameter of the screw ； t——pitch ； n——RPM. Ω≈3.14 This formula can be understood as follows: Ω/4(D²-d²) represents the area of the annular region between the inner and outer diameters of the screw. When the screw rotates one full turn, its end position remains unchanged, but the liquid moves forward by one pitch inside the screw. Therefore, when the twin screws rotate one full circle to transport liquid forward, the volume of liquid transported is Ω/4(D²–d²). Considering that the meshing area accounts for 1/4 of this volume, the volume of liquid transported is 3/4 • Ω/4(D²–d²)t. Multiplying by the rotational speed n gives the flow rate per minute. For triple-screw pumps, the same principles can be applied for calculations, but it is necessary to understand the design of the screws. In some cases, the outer diameter of both the driving and driven screws is the same, while in others (the majority of cases), the driven screw has a smaller diameter than the driving screw; as a result, the actual flow rate differs. Like gear pumps, the flow rate of screw pumps is a constant value. Its characteristic curve is a vertical line. To prevent the outlet pressure from rising excessively, a safety valve (with a structure similar to that of a pressure relief valve) should also be installed at the outlet. When a screw pump is in operation, the rotor is subject to axial thrust; therefore, some screw pumps place the suction chamber at both ends of the screw, while the discharge chamber is located in the middle of the screw ； The threads on both sides of each screw are in opposite directions; therefore, as it rotates, oil enters at both ends of the screw and exits from the middle, thereby balancing the axial thrust. To eliminate the radial load acting on the driving screw during the operation of a twin-screw pump, the pump can be designed as a triple-screw type, with the driving screw located between two driven screws. Screw pumps are characterized by good self-priming capability, silent operation, long service life, and a slightly higher efficiency than gear pumps (0.70~0.95). Screw pumps are used as oil pumps in power plants, such as the main oil pump of turbines and the main oil pumps of some steam-driven feedwater pumps. The clearances measured during the maintenance of screw pumps are as follows: 1. The radial clearance between the screw and the housing, which is generally set at 0.10~0.15 millimeters. Measure with a feeler gauge. 2. The clearances at the top and sides of the area where the screws mesh are generally 0.10 to 0.15 millimeters. 3. The axial clearance between the driven screw shaft head and the housing is generally set at 2±0.5 millimeters. 4. The bearing shell clearance is 0.08~0.12 millimeters.