Shielded pump maintenance process

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A shielded pump is composed of a centrifugal pump and a three-phase asynchronous shielded motor mounted coaxially. It requires no mechanical seal and thus causes no leaks; it is suitable for transporting various toxic, hazardous, and valuable liquids, and is widely used in facilities in the chemical, pharmaceutical, nuclear, aerospace, and other industries. Shielded pumps need to be regularly maintained to ensure reliable operation. As the years of use increase and the number of units in use grows, the degree of damage also rises, leading to a steady increase in the number of major repair operations required. 1. Structural features and damage conditions: A shielded pump is a device that combines a pump and a motor. The stator and rotor parts of the motor are each enclosed in a sleeve made of special metallic material, which seals them off from the liquid medium being transported. This prevents the motor’s core and windings from being corroded, and it helps to maintain good insulation properties for the stator windings. Shielded pumps use graphite bearings and are lubricated by the liquid being transported. The wear condition of bearings is crucial for reliable operation. To monitor bearing wear, mechanical or electromagnetic bearing monitors are generally installed. When the wear of the bearing exceeds the specified allowable value, the pointer on the monitor dial will point to the red area, indicating an \"alarm\". The operation should be stopped immediately at this point to conduct an inspection. If the wear level of the bearing has exceeded the limit value, a new graphite bearing should be installed; otherwise, it may lead to friction between the stator and rotor shielding sleeves, eventually causing damage to these sleeves. This can result in the liquid medium penetrating the stator windings and other components, thereby damaging the motor. Most chemical process shielded pumps are used in explosion-proof areas, so their junction boxes are designed with an explosion-proof structure. The coil ends of shielded motors are equipped with temperature relays, which serve as a means of overheat protection when the motor windings overheat. The operating temperature of these temperature relays varies depending on the insulation class used in the motor. Some casing sections of the motors in shielded pumps are equipped with heat exchangers that contain coiled tubes; the high-temperature medium is cooled through these coiled tubes before being used to lubricate the motor’s graphite bearings. Meanwhile, the cooling water inside the jacket can also serve to cool the motor. Shielded pumps mainly suffer from the following types of damage: (1) Wear of the graphite bearings, shaft sleeves, and thrust plates, or damage caused by dry friction due to a lack of lubricant. (2) Damage to the stator and rotor shielding sleeves. The main cause of damage to these shielding sleeves is bearing failure or wear that exceeds the allowable limits, resulting in friction between the stator and rotor shielding sleeves and subsequent damage ； Secondly, leaks occur in areas such as welds due to chemical corrosion. (3) Stator winding damage: In addition to the causes of stator winding damage that are common to ordinary motors, such as overload, inter-turn short circuits, and ground breakdown, there is also damage to the winding insulation caused by the erosion of the motor windings due to damage to the stator shielding cover. 2. Regular maintenance of canned pumps: To avoid and reduce sudden failures of canned pumps, it is necessary to carry out regular maintenance on them. If the bearing monitor gives an “alarm”, maintenance must be carried out immediately. Chemical processing units generally operate continuously, and the regular maintenance of shielded pumps is only carried out during planned shutdowns of the units. For most shielded pumps, an annual inspection is sufficient. The maintenance method for shielded 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 carried out. (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 fit clearance between the graphite bearings and bushings exceeds the specifications set for maintenance purposes (for 0.55–11 kW units, the clearance should be 0.4 mm in terms of diameter difference; for 15–45 kW units, it should be 0.5 mm), or if the surface finish of the fitting surfaces is poor, it is necessary to replace the bearings, bushings, or thrust plates as appropriate. Measure the dimensions of the upper and lower external 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 tolerances fall within the ranges specified in the maintenance standards; if they are out of range, the parts need to be replaced or other measures taken (such as surfacing or sleeve installation) to bring the fit tolerances back to the required levels. Otherwise, it will affect the pump’s performance, flow rate, head, axial balance force, etc. 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 impellers for a dynamic balance test. (2) Electrical inspection: DC resistance check: The imbalance of the three-phase resistances shall not exceed 2. Insulation resistance check: 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 analyze the cause: whether the insulation is damp or if there are any leakage points in the shielding sleeve. If leak testing of the stator shielding sleeve shows no problems, then the issue is simply damp insulation, which requires drying treatment. If there are problems with the stator shielding sleeve, then it needs to be replaced. 3. Major repair of shielded pumps: Shielded pumps with damaged windings or shielding sleeves require major repair. The damage can be broadly divided into two types: in one type, the stator shielding is intact, but the stator windings suffer from breakdown to ground or between phases, inter-turn short circuits, and the windings burn out due to overload. Another cause is the intrusion of medium into the stator windings due to damage to the stator shielding sleeve, which leads to damage to the stator windings. In either case, the stator coil and shielding sleeve need to be replaced. Due to the special structure of the canned pump, replacing the stator windings is relatively complex; it is necessary to remove the stator shield and the end plates in order to take out the stator windings. After repairing the winding, new shielding sleeves and end plates must be made again. Its materials require special specifications, and high precision is also needed for its fabrication. (1) Stator winding replacement: The stator winding replacement process is similar to that of ordinary motors, except that the insulation grade is higher; H-class insulation is 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 the windings is to repair them as they were originally; in particular, the number of coil turns must not be changed arbitrarily, as such changes will significantly affect the motor’s main performance. The wire diameter only needs to be close to the original value, and neither the winding configuration nor the coil pitch should be altered. (2) Replacement of the shield sleeve: Replacing the shield sleeve is the challenging and distinctive aspect of repairing shielded pumps, and it is key to ensuring the quality of such repairs. 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 stator shield loss PB 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 mechanical strength; it also should not be too thick, as excessive thickness increases the loss in 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 requirements mentioned above. Its resistivity coefficient 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) Fabrication of the shielding sleeve: The process of manufacturing the shielding sleeve is likely to be the challenge and key aspect in the major overhaul of shielded pumps. High requirements are placed on the dimensional accuracy and shape tolerances of the shielding sleeve, as well as on the welding quality of the welds. There are certain technical challenges involved in tasks such as calculating the dimensions after material cutting, cutting the material, performing straight-seam welding, rounding it into a perfect circle, pressing it together, carrying out ring-seam welding, and checking for leaks. 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 leak detection of the stator: After welding the stator end plates and casing, inserting the shielding sleeve, and completing the ring welds at both ends of the shielding sleeve, an overall leak detection check should be performed on the 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 the repair of the stator and rotor is completed, final assembly can be carried out once qualified graphite bearings, shaft sleeves, thrust plates, seals, etc. are ready. After assembly, turn the rotor by hand; the rotation should be even and smooth. The rotor should have a certain amount of axial movement, and this amount should fall within the limits specified in the maintenance standards: For pumps with a power rating of 0.55–3.75 kW, the axial movement ranges from 0.9–1.5 mm, 1.4–2.0 mm, and 1.8–2.5 mm respectively. After completing the overall assembly, check the DC resistance and insulation resistance. Once it is confirmed that the electrical performance is normal, submerge the entire pump in the water tank (make sure the wiring box remains above the water level so that water does not get inside 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 test conditions allow for conducting performance tests on the pump.