Basic knowledge of canned pump maintenance

The canned pump is composed of a centrifugal pump and a three-phase asynchronous canned motor coaxially. It does not require mechanical seals and has no leakage. It is suitable for transporting various toxic, harmful and valuable liquids and is widely used in chemical, pharmaceutical, nuclear industry, aerospace and other equipment.
Canned pumps must be inspected regularly to ensure reliable operation. When I was working at Shanghai Petrochemical (participating in maintenance), many imported canned pumps underwent general maintenance and restorative overhaul. As the service life increases and the number of uses increases, the damage situation also increases, and the number of restorative overhauls increases year by year. I have repaired foreign canned pumps of various sizes. Power range from 0.55 kW to 130 kW.
1. Structural characteristics and damage
The canned pump is a product that combines a pump and a motor. There is a sleeve made of special metal material on the stator and rotor parts of the motor to seal them separately and prevent them from coming into contact with the transported liquid medium, so that the core and windings of the motor are not Corroded, the stator winding maintains good insulation properties. The structure diagram of the canned pump is shown in Figure 1. The canned pump uses graphite bearings and relies on the fluid being transported for lubrication. Bearing wear is very important for reliable operation. In order to monitor bearing wear conditions, mechanical or electromagnetic bearing monitors are generally installed. When the wear amount of the bearing exceeds the specified allowable value, the pointer on the monitor dial will point to the red zone, indicating an “alarm”. At this time, the operation should be stopped immediately and inspected. If the bearing wear exceeds the limit value, new graphite bearings should be replaced. Otherwise, the stator and rotor shielding sleeves may rub against each other until the shielding sleeve is damaged, causing the liquid medium to erode the stator winding, etc., causing damage to the motor. Canned pumps for chemical industry are mostly used in explosion-proof places, so their junction how to see the impact of the rise of blind boxes on 3d printing are made of explosion-proof structure. There is a temperature relay embedded at the end of the coil of the shielded motor. When the motor winding overheats, it serves as overheat protection. Depending on the insulation level used in the motor, the operating temperature of the temperature relay will be different. Some canned pump motor shells are equipped with a heat exchanger with a serpentine tube inside. The high-temperature medium is cooled through the serpentine tube and then lubricated by the graphite bearings of the power supply machine. At the same time, the cooling water in the jacket can also cool the motor.
The canned pump mainly suffers from the following damage situations:
(1) Graphite bearings, sleeves and thrust plates are damaged due to wear or dry grinding due to lack of lubricant.
(2) The stator and rotor shielding sleeves are damaged
The main reason for the damage to the shielding sleeve is that the bearing is damaged or the wear exceeds the limit value, causing the stator and rotor shielding sleeves to rub against each other and be damaged; secondly, chemical corrosion causes leakage in the welds and other places.
(3) Stator winding damaged
In addition to the same reasons as ordinary motors: overload, inter-turn short circuit, ground breakdown, etc. that cause damage to the stator winding, there are also media erosion of the motor winding due to damage to the stator shielding sleeve, causing damage to the winding insulation.
2. Regular maintenance of canned pump
In order to avoid and reduce sudden damage accidents of the canned pump, the canned pump needs regular maintenance. If the bearing monitor “alarms”, it must be inspected immediately. Chemical plants generally operate continuously, and regular maintenance of canned pumps can only be carried out when the plant is scheduled to shut down. Most canned pumps can be inspected once a year.
The maintenance method of the canned pump is: disassemble the canned pump: clean each part first, and then conduct a visual inspection on them to see if there are any abnormalities. Then measure the dimensions of key parts and conduct electrical inspections on the motor windings.
(1) Mechanical inspection
​Measure the hole diameter of the graphite bearing and the shaft diameter of the bushing, and observe the smoothness of their mating surfaces. If the fitting clearance between the graphite bearing and the bushing exceeds the maintenance standard (0.55-11kw fitting clearance, diameter difference is 0.4mm, 15-45kw fitting clearance, diameter difference is 0.5mm) or the mating surface finish is poor, it needs to be replaced according to the situation Bearings, bushings or thrust plates.
Measure and check the dimensions of the upper and lower outer joints of the impeller, their matching retaining rings and the inner diameter of the pump base. Whether these two matching clearances are within the range specified by the maintenance standards, parts must be replaced or other measures must be taken if they are out of tolerance ( Such as: surfacing, inserting) to ensure that the matching gap meets the specified requirements. Otherwise, the performance, flow rate, head, axial balance force, etc. of the pump will be affected.
Observe and check the appearance of the stator and rotor shielding sleeves, pay special attention to whether there are any abnormalities at the welds, and perform flaw and leak detection if necessary.
After long-term operation, the balance of the rotating part may change. Therefore, it is necessary to assemble the rotor together with rotating parts such as impellers for dynamic balancing testing.
(2) Electrical inspection
DC resistance check: The unbalance of the three-phase resistance shall not exceed 2%.
Insulation resistance check: The insulation resistance of the canned pump motor winding can generally reach more than 100MΩ. If it is lower than 5MΩ, the cause needs to be analyzed, whether the insulation is damp, or whether the shielding sleeve has leakage points, etc. If there is no problem with the stator shielding sleeve, it is purely because the insulation is damp and needs to be dried. If there is a problem with the stator shielding sleeve, The shielding sleeve needs to be replaced.
3. Restorative overhaul of canned pump
​If a canned pump with damaged windings or shielding sleeves is encountered, restorative overhaul is required. Damage situations are generally divided into two types: one is that the stator shielding is good, but the stator winding has breakdown to ground and between phases, short circuit between coil turns, and overload, causing winding burnout. The other is due to the damage of the stator shielding sleeve, which causes the medium to invade the stator winding, causing damage to the stator winding. In either case, the stator coil and shielding sleeve need to be replaced.
Due to the special structure of the canned pump, it becomes more complicated to replace the stator winding. The stator shielding sleeve and the sealing plates at both ends must be removed before the stator winding can be removed. After repairing the winding, a new shielding sleeve and sealing plate must be made again. The material requirements are special and precision machining boron carbide ceramic parts.
(1) Replacement of stator winding
The stator winding replacement is similar to that of ordinary motors, except that the insulation level is higher, and most use H-class insulation. QY polyimide enameled wire, slot insulation, slot wedges, insulating sleeves, lead wires and impregnating paint all need to use H-class insulation materials. The principle of replacing the winding is: repair it as it is, especially the number of coil turns cannot be changed at will. Changes in the number of turns will obviously affect the main performance of the motor. The wire diameter only needs to be close to the original total area, and the winding form and coil span should not be changed. .
(2) Replacement of shielding sleeve
Replacing the shielding sleeve is the difficulty and special feature of repairing the canned pump, and it is the key to ensuring the quality of the repaired canned pump. To reach the original machine level, you must pay attention to the following aspects.
(a) Selection of shielding sleeve materials
Among the losses of the canned pump motor, the stator shielding sleeve loss is very large, sometimes reaching 2 to 3 times the iron loss. The repaired shielding sleeve loss should be maintained at the original level.
The stator shielding sleeve loss PB is proportional to the shielding sleeve thickness and inversely proportional to the resistance coefficient ρ of the shielding sleeve material. The shielding sleeve material must be a non-magnetic material with good mechanical properties and corrosion resistance. In order to reduce eddy current losses, materials with a large resistivity must be selected. The thickness should not be too thin, which will affect the mechanical strength, nor should it be too thick, which will increase the loss of the shielding sleeve. Most of the shielding sleeves of imported canned pumps are made of a special stainless steel material called Hastelloy-c with a nickel content of more than 50%. The performance of this material can fully meet the above requirements. Its resistance coefficient is twice that of ordinary stainless steel 1Cri8Nig9Ti, that is, the shielding sleeve loss is 1/2 of ordinary stainless steel materials.
(b) Production of shielding sleeve
The manufacturing process of the shielding sleeve should be the difficulty and key to the restorative overhaul of the shielding pump. The dimensional accuracy and shape tolerance requirements of the shielding sleeve are relatively high, and the welding quality requirements of the welds are also very strict. There are certain technical difficulties in calculating the unfolded size of the blank, the relationship between sheet metal parts and fiber laser cutting machine, straight seam welding, rounding, press fitting, circumferential seam welding, leak detection, etc. The effective gap between the stator and rotor of a shielded motor is generally only 0.5~1.0mm. Therefore, the shielding sleeve must not only be able to be smoothly pressed into the stator and rotor cores, but also the shielding sleeve must be close to the core. Only in this way can sufficient space between the stator and rotor be ensured. effective gap.
(c) Overall stator leak detection
After completing the welding of the stator sealing plate and the casing, pressing in the shielding sleeve and completing the circumferential seam welding at both ends of the shielding sleeve, the stator should be inspected for leaks as a whole. The method is: fill the inner cavity between the machine base and the stator shielding sleeve with a pressure of 0.5㎏/c㎡, and then immerse the stator as a whole in clean water. If there is a leakage point, bubbles will appear in the water. This is a simple method. And effective leak detection method.
(3) Final assembly and inspection tests
After completing the repair of the stator and rotor, prepare qualified graphite bearings, sleeves, thrust plates, sealing rings, etc. for final assembly. After the assembly is completed, turn the rotor by hand. The rotation should be uniform and flexible. The rotor should have a certain amount of axial movement, and its movement should be within the range specified by the maintenance standards:
Canned pump power kw 0.55~3.7 5.5~11 15~45
Axial movement mm 0.9~1.5 1.4~2.0 1.8~2.5
After completing the final assembly, check the DC resistance and insulation resistance. After deeming the electrical performance to be normal, immerse the whole canned pump in the water tank (the junction box must be on the water surface, and water cannot enter the junction box) and perform an electric rotation test (at this time the pump will The water outlet can be covered) and observe whether there are any abnormalities in current, running sound, vibration, etc. It would be better if there are test conditions to conduct a performance test of the pump.