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The function of the centrifugal pump’s mouth ring is to create a narrow, tortuous passage between the impeller and the pump casing, thereby increasing the flow resistance of the fluid and reducing fluid leakage. The installation of the centrifugal pump’s mouth ring also serves to protect the main components of the pump from wear. When the mouth ring wears out, it can be repaired or replaced with a new one, thereby restoring the normal assembly clearance – which is both cost-effective and convenient for maintenance. The structural forms of the impeller ring in centrifugal pumps typically include the radial-clearance ring type, double-ring stepped type, double-ring labyrinth type, and axial-clearance type. The integrity of the centrifugal pump’s mouth ring and the radial clearance between it and the impeller are items that should be checked first when disassembling the pump. If the impeller ring of a centrifugal pump has defects such as grooves, is damaged or cracked, or if the gap exceeds the specified value, it should be replaced with a new ring or the original ring should be repaired by welding. During operation of the centrifugal pump, the mouth ring of the pump and the corresponding circumference of the impeller wear down simultaneously, which causes the gap to increase beyond the specified value. This is because the inner diameter of the new mouth ring should be determined based on the outer diameter of the impeller inlet, in accordance with the regulations regarding the radial gap between the impeller and the pump’s mouth ring. During the repair process, it is important to keep this gap as small as possible in order to improve the pump’s efficiency and extend its service life. The material for mouth rings can generally be gray cast iron HT200, tin bronze, or a tin bronze-bearing alloy (Babbitt). For copper mouth rings fitted with bearing alloy, when the gap widens, it is sufficient to reinstall the alloy without the need to replace the ring. When the wear of the original alloy is minimal and there is no peeling or detachment, it can be repaired by patch welding. The steps for patch welding are as follows: ① Clean and activate with 5% hydrochloric acid. ②Soak it in caustic soda solution at 90°C with a mass fraction of 10% for 10 minutes, and then rinse it in clean water at 90°C. ③Repair welding alloy. Preheat the mouth ring to around 100°C, use gas welding to appropriately melt the existing alloy on the mouth ring, prepare electrodes made from an alloy of the same grade as the original alloy, and apply welds evenly along the circumference or lengthwise direction of the centrifugal pump’s mouth ring (repeated welding is not allowed). After welding is complete, mechanical processing can be carried out to achieve the desired standard dimensions. If the alloy experiences peeling, detachment, or excessive wear to the extent that repair by welding is not possible, the alloy must be poured anew. After the new mouth ring is installed, check whether its clearance with the impeller meets the requirements in Table 1. At the same time, it is necessary to check for any friction between the two components; this is done by applying red lead powder to the rotor, then rotating the rotor – if red lead powder adheres to the collar, further repair is required. Table 1: Clearance of the impeller ring in centrifugal pumps (unit: mm). Nominal diameter of the impeller ring; allowable radial clearance, and radial clearance after wear: 50–80, >80–120, >120–150, >150–180, >180–220, >220–260, >260–290, >290–320, >320–360. Values: 0.06–0.36, 0.06–0.38, 0.07–0.44, 0.08–0.48, 0.09–0.54, 0.10–0.58, 0.10–0.60, 0.11–0.64, 0.12–0.68, 0.48, 0.48, 0.60, 0.60, 0.70, 0.70, 0.80, 0.80, 0.80