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Regular maintenance of shielded pumps & To avoid and reduce sudden failures of shielded pumps, they require regular maintenance. If the bearing monitor gives an “alarm”, maintenance must be carried out immediately. Chemical plants generally operate continuously, and the regular maintenance of shielded pumps is only carried out during planned plant shutdowns. For most shielded pumps, an annual inspection is sufficient. The maintenance method for canned pumps is to disassemble the pump: first clean each component, and then conduct a visual inspection to check for any abnormalities. Then, the dimensions of the key parts are measured, and an electrical inspection of the motor windings is performed. (1) Mechanical inspection: Measure the bore diameter of the graphite bearing and the shaft diameter of the sleeve, and check the finish of their mating surfaces. If the clearance between graphite bearings and bushings exceeds the limits specified in the maintenance standards (for 0.55–11 kW, the allowable clearance is such that the diameter difference is 0.4 mm; for 15–45 kW, it is 0.5 mm), or if the surface finish of the mating surfaces is poor, then it is necessary to replace the bearings, bushings, or thrust plates as appropriate. Measure the dimensions of the upper and lower outer flanges of the impeller, as well as the retaining rings that fit with them and the inner diameter of the pump base. Check whether these fit gaps fall within the ranges specified in the maintenance standards; if they are out of range, the parts need to be replaced or other measures (such as surfacing or sleeve installation) must be taken to bring the fit gaps within the required limits. Otherwise, it will affect the pump’s performance, flow rate, head, axial balance force, and so on. Inspect the appearance of the stator and rotor shielding sleeves; pay special attention to any abnormalities at the welds. If necessary, conduct flaw detection and leak testing. After long-term operation, the balance of the rotating parts may change. Therefore, it is necessary to assemble the rotor together with rotating parts such as the impeller for a dynamic balance test. (2) Electrical inspection – DC resistance check: The imbalance of the three-phase resistances shall not exceed 2%. Insulation resistance test: The insulation resistance of the motor windings in shielded pumps generally exceeds 100 MΩ. If the value is below 5 mΩ, it is necessary to determine the cause: whether the insulation has become damp, or if there are any leakage points in the stator shield. If no leaks are detected in the stator shield during inspection, then it’s simply due to moist insulation, which requires drying treatment. However, if there are problems with the stator shield itself, it must be replaced. For a shielded pump that requires a major restoration repair, if its windings or shielding sleeve are damaged, such a major restoration repair is necessary. The types of damage can generally be divided into two categories: in one case, the stator shield remains intact, while the stator windings experience breakdown to ground or between phases, inter-turn short circuits, and overheating, ultimately leading to winding burnout. The other cause is that damage to the stator shield allows the medium to penetrate into the stator windings, resulting in their damage. In any case, the stator coil and shield sleeve must be replaced. Due to the special structure of the shielded pump, replacing the stator windings is relatively complex; it is necessary to remove the stator shielding sleeve and the end plates in order to take out the stator windings. After repairing the windings, it is necessary to remake a new shielding sleeve and cover plate. Its materials have special requirements, and the manufacturing precision also needs to be relatively high. Regular maintenance of canned motors (1) Replacement of stator windings: The replacement process for stator windings is similar to that of ordinary motors; however, the insulation class is higher, with Class H insulation being commonly used. For QY polyimide enameled wire, materials with H-class insulation must be used for the slot insulation, slot wedges, insulating sleeves, lead wires, and impregnating varnish. The principle for replacing windings is to repair them as they originally were. In particular, the number of coil turns must not be changed arbitrarily, as any change in the number of turns significantly affects the motor’s main performance characteristics. The wire diameter may vary provided that the total cross-sectional area remains approximately the same; moreover, the winding configuration and coil span should also remain unchanged. (2) Replacement of the shield sleeve Replacing the shield sleeve is the difficult and distinctive aspect of repairing canned pumps; it is the key to ensuring the quality of the repair. To reach the original machine’s level, attention must be paid to the following aspects. (a) Material selection for the shielding sleeve: Among the losses in a shielded pump motor, the losses associated with the stator shielding sleeve are significant; sometimes they amount to 2–3 times the iron loss. The losses of the repaired shielding sleeve should be maintained at the original level. The loss pb of the stator shield is proportional to the thickness of the shield and inversely proportional to the resistivity ρ of the shield material. The material of the shielding sleeve must be a non-magnetic material with good mechanical properties and corrosion resistance; to reduce eddy current losses, a material with a high resistivity should be chosen. The thickness should not be too thin, as this would affect the mechanical strength; nor should it be too thick, as excessive thickness would increase the losses of the shielding sleeve. The shielding casings of imported shielded pumps are mostly made from a special stainless steel material called Hastelloy-C, which contains over 50% nickel; the properties of this material meet all the aforementioned requirements. Its resistivity is twice that of ordinary stainless steel 1Cr18Ni9Ti, meaning that the loss in the shielding sleeve is half that of ordinary stainless steel materials. (b) Manufacturing of the shield sleeve: The process of manufacturing the shield sleeve is likely to be the challenge and key aspect in the major repair of shielded pumps. High requirements are placed on the dimensional accuracy and shape tolerances of the shielding sleeve, as well as on the quality of the welds. There are certain technical challenges involved in aspects such as the calculation of the unfolded dimensions after material cutting, cutting, straight-seam welding, rounding, pressing, ring-seam welding, and leak testing. The effective gap between the stator and rotor of a shielded motor is generally only 0.5–1.0 mm; therefore, it is required that the shielding sleeve not only be able to be pressed smoothly into the stator and rotor cores but also fit tightly against them, as this is necessary to ensure there is a sufficient effective gap between the stator and rotor. (c) Overall leakage test of the stator: After completing the welding of the stator end plates and casing, as well as the pressing in of the shield sleeve and the circumferential welding at both ends of the shield sleeve, a leakage test should be conducted on the entire stator. The method is as follows: fill the inner cavity between the frame and the stator shield with a pressure of 0.5 kg/c㎡, then submerge the entire stator in clean water; if there are any leakage points, bubbles will appear in the water. This is a simple and effective method for detecting leaks. (3) Final assembly and inspection tests: After completing the repair of the stator and rotor, final assembly can be carried out once qualified graphite bearings, bushings, thrust plates, sealing rings, etc. are prepared. After assembly, turn the rotor by hand; the rotation should be even and smooth, and the rotor should have a certain amount of axial movement, with this amount falling within the limits specified in the maintenance standards: For pumps with a power rating of 0.55–3.7 kW, the axial movement is 0.9–1.5 mm; for pumps with a power rating of 5.5–11 kW, it is 1.4–2.0 mm; and for pumps with a power rating of 15–45 kW, it is 1.8–2.5 mm. After completing the overall assembly, check the DC resistance and insulation resistance. Once the electrical performance is found to be normal, submerge the entire pump in a water tank (make sure the wiring box remains above the water level so that water does not enter it), and conduct a test by powering it on (the pump’s outlet can be covered at this time). Observe whether there are any abnormalities in terms of current, operating noise, or vibration. It would be even better if the test conditions allowed for a performance test of the pump.