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How to repair the canned motor pump

How to repair the canned motor pump?

Shielded pump is composed of centrifugal pump and three-phase asynchronous shielded motor. It is suitable for conveying all kinds of toxic, harmful and valuable liquids. It is widely used in chemical, pharmaceutical, nuclear industry, aerospace and other devices.
The shield pump shall be overhauled regularly to ensure reliable operation. With the increase of service life and quantity, the damage is also increasing, and the number of restorative overhaul is increasing year by year.
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Structural features and damage

Canned motor pump is a product integrating pump and motor. There is a sleeve made of special metal materials in the stator and rotor of the motor. They are sealed respectively and do not contact the transmitted liquid medium, so as to prevent the iron core and winding of the motor from corrosion and maintain good insulation performance of the stator winding.
The shielded pump adopts graphite bearing and depends on the transmitted liquid for lubrication. Bearing wear is very important for reliable operation. In order 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 of the monitor dial will point to the red area, indicating “alarm”. At this time, stop the operation immediately for inspection. If the wear of the bearing has exceeded the limit value, replace it with a new graphite bearing, otherwise it may cause the stator and rotor shielding sleeves to rub against each other until the shielding sleeve is damaged, resulting in the erosion of liquid medium to the stator winding and damage to the motor.
Shielded pumps for chemical industry are mostly used in explosion-proof places, so their junction boxes are made of explosion-proof structure. A temperature relay is embedded at the coil end of the shielded motor. When the motor winding is overheated, it plays the role of overheating protection. The operating temperature of the temperature relay is different according to the insulation grade used by the motor. Some canned motor pump casings are equipped with heat exchangers with serpentine tubes inside. The high-temperature medium is cooled through the serpentine tubes and then lubricated by the graphite bearing of the motor. At the same time, the cooling water in the jacket can also cool the motor.
The canned motor pump is mainly damaged in the following ways:
(1) Graphite bearing, shaft sleeve and thrust plate are worn or damaged due to dry grinding due to lack of lubricating fluid.
(2) Stator and rotor shielding sleeves damaged
The main reason for the damage of the shielding sleeve is that the bearing is damaged or the wear exceeds the limit value, resulting in the friction and damage of the stator and rotor shielding sleeve; Secondly, leakage occurs at welds due to chemical corrosion.
(3) Stator winding damage
In addition to the causes of stator winding damage caused by overload, turn to turn short circuit and ground breakdown, as well as the dielectric erosion of motor winding caused by the damage of stator shielding sleeve.
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Regular maintenance of shield pump

In order to avoid and reduce the sudden damage accident of the shield pump, the shield pump needs regular maintenance. In case of “alarm” of bearing monitor, maintenance shall be carried out immediately. Chemical plants generally operate continuously, and the periodic maintenance of shield pump can only be carried out when the plant is planned to stop. Most canned motor pumps can be overhauled once a year.
The maintenance method of shielded pump is: disassemble the shielded pump: clean all parts first, and then visually check them for abnormalities. Then measure the dimensions of key parts and conduct electrical inspection on the motor winding.
(1) Mechanical inspection
Measure the bore diameter of graphite bearing and the shaft diameter of shaft sleeve, and observe the finish of their mating surfaces. If the fit clearance between graphite bearing and shaft sleeve exceeds the maintenance standard (0.55-11kw fit clearance, diameter difference is 0.4mm, 15-45kw fit clearance, diameter difference is 0.5mm) or the finish of the fit surface is poor, the bearing, shaft sleeve or thrust plate shall be replaced according to the situation.
Measure and check the dimensions of the upper and lower outer stops of the impeller, the retaining ring matched with them and the inner diameter of the pump seat, and whether these two fit clearances are within the scope specified in the maintenance standard. If they exceed the tolerance, replace the parts or take other measures (such as surfacing and sleeve inlay) to make the fit clearance meet the specified requirements. Otherwise, the performance, flow, head and axial balance force of the pump will be affected.
Observe and check the appearance of stator and rotor shielding sleeves, especially pay attention to whether there are abnormalities at the welds, and conduct flaw detection 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 impeller for dynamic balance test.
(2) Electrical inspection
DC resistance inspection: the unbalance of three-phase resistance shall not exceed 2.
Insulation resistance inspection: the insulation resistance of motor winding of shielded pump can generally reach more than 100m Ω. If it is lower than 5m Ω, it is necessary to analyze the cause, whether the insulation is damp, or whether the shielding sleeve has leakage points, etc. if there is no problem through the leakage detection of the stator shielding sleeve, it is purely insulation damp and needs to be dried. If there is a problem with the stator shielding sleeve, it is necessary to replace the shielding sleeve.
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Restorative overhaul of shield pump

In case of a shielded pump with damaged winding or shielding sleeve, restorative overhaul shall be carried out. There are two kinds of damage: one is that the stator shielding sleeve is good, but the stator winding breaks down to ground and between phases, the coil turns are short circuited, and the winding is burned due to overload. The other is that the medium invades the stator winding due to the damage of the stator shielding sleeve, resulting in the damage of the stator winding. In either case, the stator coil and shielding sleeve shall be replaced.
Due to the special structure of canned motor pump, the replacement of stator winding becomes more complex. The stator shielding sleeve and 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. It has special material requirements and high manufacturing accuracy.
(1) Stator winding replacement
The replacement of stator winding is similar to that of ordinary motor, but the insulation grade is higher, and most of them adopt H-class insulation. QY polyimide enamelled wire, slot insulation, slot wedge, insulating sleeve, lead wire and impregnated paint shall adopt H-class insulation materials. The principle of replacing the winding is: repair as is, especially the number of coil turns can not be changed at will. The change of the number of turns will significantly affect the main performance of the motor. The wire diameter can 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 particularity of repairing the shielding pump and the key to ensure the quality of repairing the shielding pump. To reach the original level, we must pay attention to the following aspects.
(a) Material selection of shielding sleeve
In the loss of canned motor pump, the loss of stator shielding sleeve is very large, sometimes 2 ~ 3 times of iron loss. The loss of shielding sleeve after repair shall be maintained at the original level.
The loss Pb of stator shielding sleeve is directly proportional to the thickness of shielding sleeve and inversely proportional to the resistance coefficient of shielding sleeve material ρ. The shielding sleeve material must be non-magnetic material with good mechanical properties and corrosion resistance. In order to reduce eddy current loss, the material with high resistance coefficient should be selected. The thickness should not be too thin, which will affect the mechanical strength, nor too thick. Too thick will increase the loss of shielding sleeve. Most of the shielding sleeves of imported shielded pumps use a special stainless steel material called Hastelloy-C with nickel content of more than 50%. The performance of this material can fully meet the above requirements. Its resistance coefficient can be twice as large as that of ordinary stainless steel 1cri8nig9ti, that is, the loss of shielding sleeve is 1/2 of that of ordinary stainless steel.
(b) Fabrication of shielding sleeve
The manufacturing process of shielding sleeve should be the difficulty and key of restorative overhaul of shielding pump. The requirements for dimensional accuracy and shape tolerance of shielding sleeve are high, and the welding quality requirements of weld are also very strict. There are certain technical difficulties in the calculation of blanking development size, cutting, straight seam welding, rounding, press fitting, circumferential seam welding, leak detection, etc. The effective clearance between stator and rotor of shielded motor is generally only 0.5 ~ 1.0mm. Therefore, it is required that the shielding sleeve not only can be smoothly pressed into the stator and rotor core, but also make the shielding sleeve close to the core. Only in this way can there be sufficient effective clearance between stator and rotor.
(c) Overall leakage detection of stator
After completing the welding of stator sealing plate and casing, pressing in the shielding sleeve and completing the circumferential seam welding at both ends of the shielding sleeve, the overall stator shall be subject to leakage inspection. The method is: fill the inner cavity between the base and the stator shielding sleeve with a pressure of 0.5 kg / C ㎡, and then immerse the stator in clean water. If there is a leakage point, bubbles will appear in the water, which is a simple and effective leak detection method.
(3) Final assembly and inspection test
After completing the repair of stator and rotor, prepare qualified graphite bearings, shaft sleeves, thrust plates and sealing rings for general assembly. After assembly, rotate the rotor by hand. The rotation shall be uniform and flexible. The rotor shall have a certain axial displacement, which shall be within the range specified in the maintenance standard:

Shield pump power/kw

0.55-3.7

5.5-11

15-45

Axial movement/㎜

0.9-1.5

1.4-2.0

1.8-2.5

After the final assembly, check the DC resistance and insulation resistance. After the electrical performance is considered normal, immerse the shielded pump into the water tank as a whole (the junction box should be on the water surface, and the water cannot enter the junction box) for electric running test (at this time, the pump outlet can be covered) to observe whether there are abnormalities in current, running sound, vibration, etc. If there are test conditions, it is better to do the performance test of the pump.

Source: Network Arrangement – China Pipe Fitting Manufacturer – Yaang Pipe Industry (www.epowermetals.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

If you want to have more information about the article or you want to share your opinion with us, contact us at sales@epowermetals.com

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