The canned pump in the barrel pump unit

2024-11-23

In the barrel pump unit device, the refrigerant is transported by using a canned motor pump. The canned motor pump is a sealed device in which a centrifugal pump and a canned motor are coaxially combined together, and it is abbreviated as the canned motor pump. The canned motor pump has no shaft seal and doesn't require any mechanical seal packing. It only has static seals and has the advantages of no leakage, compact structure, high efficiency, low energy consumption, less vibration and noise, simple installation and convenient use. It is widely used in industries such as petroleum, chemical engineering, refrigeration, and pharmaceuticals that transport flammable, explosive, and highly toxic liquids without solid particles. When the canned motor pump is used in ammonia and fluorine refrigeration systems, it can maintain a normal liquid supply state under low-temperature working conditions of around -50 °C.

The canned motor pump belongs to the category of centrifugal pumps and is a special type of centrifugal pump. The common centrifugal pump is driven by connecting the impeller shaft of the pump with the motor shaft through a coupling, so that the impeller rotates together with the motor to work. In the canned motor pump, the impeller part constitutes the rotor of the motor, and the stator of the motor is arranged along the pump shaft inside the pump body, with the pump casing outside. There is an annular gap of about 0.7 mm between the stator and the rotor. When the pump is working, part of the liquid pumped passes through this annular gap to cool the motor. Some differences between centrifugal pumps and canned motor pumps are mainly as follows:

Transported medium: The medium transported by the canned motor pump is liquid with high requirements for being toxic, harmful, flammable, explosive, and there are strict limitations on leakage; the medium transported by the centrifugal pump has no strict limitations on leakage.
Structure of the pump body: In the canned motor pump, the motor and the rotor are integrated, and there is only static seal at the end, without leakage; in the centrifugal pump, the pump and the motor are two independent parts, and they are connected by a coupling. In this way, there is packing or mechanical seal on the motor side of the rotor, and a small amount of leakage will occur during use.
Bearings: The canned motor pump generally has a bearing monitoring system, and the wear degree of the bearings can be seen intuitively; the centrifugal pump will replace the bearings according to conditions such as shaft vibration and lubricating oil temperature.
Transport capacity: Generally speaking, the transport capacity of the canned motor pump is not as high as that of the centrifugal pump.

That is to say, the difference between the canned motor pump and the ordinary pump lies in the fact that a shielding device is designed in the motor, that is, shielding sleeves are installed on the stator and rotor components to ensure that the stator and rotor cores and windings are not corroded by the transported medium. The shielding sleeves are welded from thin stainless steel sheets with high strength, corrosion resistance, high resistance and non-magnetism. Special equipment and processes are used for welding and shaping. After completion, a helium leak detection test is carried out on the welds to ensure that the shielding sleeves do not leak.

In addition, in the canned motor, graphite sliding bearings treated by impregnating with epoxy resin are used, which have good corrosion resistance, high wear resistance and heat resistance, and a low friction coefficient. In addition, there are lubrication and cooling grooves on the inner holes and end faces of the graphite bearings so that the medium can lubricate and cool them. Since the canned motor pump adopts a circulating cooling system equipped with a filtering device, it ensures that the graphite bearings of the motor always have good lubricity and cools the motor at the same time, thus greatly extending the service life of the motor. In terms of hydraulic design, the impeller is made by precision casting and undergoes strict dynamic balance tests to ensure smooth operation.

When using the canned motor pump to transport the refrigerant, the following points should be noted:

Before the pump is running, it should be fully precooled. Otherwise, refrigerant gas may remain, resulting in the pump failing to draw liquid and having low outlet pressure.
The inlet pipeline of the pump is prone to blockage, resulting in insufficient liquid supply, low outlet pressure, and heating of the pump body.
The refrigerant liquid at the inlet of the pump can only be sucked in when it is in a subcooled state. Otherwise, cavitation will occur and the ability to transport liquid will be lost.
If the pump body is not insulated, the refrigerant will vaporize and cavitation is likely to occur.
Special gas extraction pipes for 导出气体 should be added to the inlet and outlet of the pump. Before the pump is running, the compressor should first extract the gas, and the pump should be run after it is filled with liquid.
During the operation of the pump, differential pressure protection between the inlet and outlet and liquid shortage protection should be carried out for the pump.

Common faults and maintenance of the canned motor pump:

When disassembling the pump, observe the wear of components and analyze and judge:

If there is wear on the pump body ring and the impeller ring, it indicates that the inner diameter of the bearing is excessively worn and the bearing needs to be replaced.
If there is wear between the front cover plate of the impeller and the pump body, it indicates that the end face of the front bearing is excessively worn and the bearing needs to be replaced.
If the inner diameter of the bearing is worn, the shaft sleeve and thrust disc are worn and their colors turn black, it indicates that the gas in the motor cavity has not been completely discharged, there is a lack of liquid or the pump is idling, and spare parts need to be replaced.
If the end face of the front bearing and the surface of the thrust disc are worn, while the rear bearing and thrust disc are not worn, it indicates that the axial thrust is forward and the pump is in a liquid-deficient state. Check whether the inlet filter is blocked, whether the system has insufficient liquid supply, and whether the pump has cavitation.
If the end face of the rear bearing and the surface of the thrust disc are worn, while the front bearing and thrust disc are not worn, it indicates that the axial thrust is backward and the flow rate may be too large. Check whether the outlet valve is opened too wide.
If the shielding sleeves of the stator and rotor are worn, while the inner diameter size of the bearing does not exceed the limit value, it indicates that foreign matter has entered the motor cavity.
Check whether the inlet filter is damaged.

Analyze and judge by analyzing the vibration and noise conditions when the pump is running:

If the pump vibrates and the vibration source comes from the positions of the two bearings in the motor part, it indicates that the bearings are worn and the wear condition of the bearings needs to be checked.
If the pump vibrates and the vibration source comes from the pump body, the outlet valve is closed and the pump pressure is normal, but when the valve is opened, the pressure gauge indication suddenly drops and the running current is lower than the normal value, it indicates that the pump is in a cavitation state. It is necessary to increase the height of the inlet liquid level to meet the requirement that the cavitation margin of the device = 1.3 × the cavitation margin of the pump.
If the pump vibrates, makes noise, the TRG (bearing monitor) gauge indication is in the red zone, the pressure of the pump outlet pressure gauge is low, and the pump fails to deliver liquid, it indicates that the pump is rotating in reverse. It is necessary to adjust the phase sequence of the power line.

Analyze and judge according to the indication of the TRG (bearing monitor) gauge:

If the gauge indication is in the yellow zone or red zone (other conditions are normal), it indicates that the bearings are worn and need to be replaced.
If the gauge has no reaction after the pump is started, but the outlet pressure, current, etc.
are all normal, it indicates that the gauge is damaged and a new gauge needs to be replaced.

During maintenance, when it is found that the pump cannot be repaired completely, it is necessary to select and replace it according to the flow rate, head, rotation speed and motor power of the original pump.