What are the structural differences between pumps made of metallic materials and those made of non-metallic materials?

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Due to the differences in properties between non-metallic and metallic materials, the structural design of non-metallic pumps has the following particularities. 　　1. Clearance at the impeller seal and between the back blades and the pump body: Since the linear expansion coefficients of plastics and fiberglass are 2 to 6 times higher than those of steel, using the same clearance values as in metal pumps can lead to interlocking, thereby damaging the pump. An excessive gap will also reduce the pump’s efficiency and cause an imbalance in the axial force of the pump. Since the radial clearance is not adjustable, an axial clearance can also be considered, allowing the operating clearance to be checked and adjusted without having to disassemble the pump. 　　2. Connection method between the impeller and the shaft ① Metal components are embedded within the impeller; these components are connected to the shaft using keys, or the impeller is directly connected to the shaft via keys, with the end of the impeller protected by an impeller nut. This structure is derived from metal pumps, and due to its numerous sealing surfaces, it has poor reliability. After long-term use, the shaft is prone to corrosion by the medium. 　　② Metal components are embedded within the impeller and connected to the pump shaft via threads, eliminating the need for an impeller nut. This structure provides complete protection for the shaft, preventing it from being corroded by the medium. The disadvantage is that high precision is required for thread machining, and it is strictly prohibited for the motor to run in reverse. 　　3. Thickness of the pump body lining layer and its fixation According to the standards set by pump manufacturers, the thickness of the lining layer is generally 5–8 mm. If the lining layer is too thin (e.g., 1.5–2 mm), local defects in the lining may occur during the pressing process, which reduces the allowable operating temperature and service life of the pump. It also increases the likelihood that the lining layer will peel off from the base metal of the pump body and rear cover. 　　The lining layer can be secured by providing dense annular dovetail grooves on the base metal, which ensures a firm fixation of the lining layer while requiring less machining. It is also possible to use a method of arranging numerous small holes (or threaded holes) in the base metal; however, the lining layer is not as firmly fixed as with dovetail grooves, and more machining is required. 　　To prevent non-metals from falling off or cracking the pump casing due to temperature changes, it is possible to weld turtle-shell wire mesh to the pump casing, thereby allowing plastic-lined pumps to be manufactured in larger sizes, with an inlet diameter of up to DN600. 　　4. Axial balancing force Most plastic pump impellers contain metal components, and it is generally not appropriate to use balance holes to balance the axial force. Back vanes are widely used at home and abroad to balance axial forces. 　　5. Interfaces There are currently no relevant standards specifying the stress requirements for the pipes at the inlet and outlet of non-metallic pumps; these requirements are generally set by the design team, depending on the manufacturing capabilities of the factory itself. For the overall non-metallic pump casing, a swivel flange is generally used for connection to the pipeline, with the swivel flange being connected to the end covers; it is also possible to integrate the flange and the covers into a single unit for connection to the pipeline.