Shielded pump

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Shielded pumps feature advanced design; thanks to their complete leak-free operation, they are energy-efficient and help to prevent environmental pollution. These pumps are widely used in industries such as petroleum, chemicals, refrigeration, defense, aerospace, and nuclear energy, and they are the ideal choice for transporting flammable, explosive, highly toxic, radioactive, or valuable liquids. 1 Structural principle of the shielding system: There is no sealing device between the pump and the motor; therefore, a shielded pump is also known as a sealless pump. Its structural feature is that the pump and the motor are directly connected, with the impeller fixed directly to the motor rotor shaft; both are housed within a sealed casing, without any couplings or shaft sealing devices, thereby completely eliminating any leakage of liquid, as shown in Figure 1. To prevent the liquid being transported from coming into contact with the electrical components, thin-walled cylindrical shields made of corrosion-resistant, non-magnetic materials are sufficient to isolate the shield sleeves and rotor shield sleeves from the liquid being transported. 1. The bearings of bearing-sealed pumps are generally made of non-metallic materials such as graphite. It is primarily used to support the rotor assembly and bear the axial forces generated by the pump during operation. Figure 1 Schematic diagram of the shielding pump structure 1 – Pump body ； 2-Impeller ； 3-Front bearing housing ； 4-Front bearing ； 5-Forward thrust plate ； 6-Bearing Monitor ； 7-Rear bearing housing ； 8-Rear bearing ； 9- Rear thrust disc ； 10-axis ； 11-Stator ； 12-Rotor ； 13-Filter: 2. Bearing monitor: The tail end of the pump motor is equipped with a bearing monitor. During pump operation, when the bearings wear out or when the stator shield and rotor shield suffer severe corrosion, the sensor probes of the bearing monitor get worn through, resulting in pressure release inside the monitor; the pressure gauge pointer then moves to the red area. Therefore, when the pointer enters the red area, the pump should be stopped to check for wear on the bearings, as well as corrosion of the stator and rotor shielding sleeves, and to inspect the wear on the sensor probes. 3. The cooling of the motor of the shielding pump in the cooling system, as well as the lubrication and cooling of the bearings, are achieved by using the natural circulation of the liquid being transported to carry away heat. When transporting liquids at room temperature, a circulation pipe is generally used at the pump outlet to send liquid to the rear bearing of the motor; from there, it flows through the gap between the rotor and stator and back to the pump via the front bearing, thus forming an external closed-loop circulation system. When transporting high-temperature liquids, there is a separate circulating cooling circuit equipped with heat exchange equipment to cool the liquid taken from the pump outlet before sending it to the motor. Due to the very small gaps between the bushings and bearings, as well as between the rotor and stator, filters should be connected in series in the circulation circuit when transporting liquids containing solid particles. 2 Characteristics of shielded pumps 1. Advantages The advantage of shielded pumps is the absence of external leakage ； With a compact design, the vertical small-pipe shielded pump can be installed directly on the pipeline without the need for a foundation ； Without a coupling, there is no need to align it when assembling or disassembling ； Bearings do not require additional lubricants or greases, making them easy to operate and maintain. 2. The disadvantage is that the shielding sleeve is difficult to manufacture and costly. The efficiency of shielded pumps is low because the motor rotor rotates in a liquid, which increases frictional resistance; furthermore, the pump must supply circulating coolant to the motor, and the seal gap of the impeller is relatively large. 3 Maintenance of shielded pumps 3.1 Inspection and assembly adjustment of components 1. Inspection of components (1) The rotor shield and stator shield should be free from scratches, swelling, and corrosion ； (2) The pump body, impeller, and inducer should be free of scratches and corrosion ； (3) The working surfaces of the bushing and thrust disc shall be free from damage, and the surface roughness shall meet the requirements ； (4) The shaft shall be free from scratches and corrosion, and its bending degree must meet the requirements specified in the user manual ； (5) Once the bearing exceeds its wear limit, it causes contact between the rotor shield and the stator shield, resulting in damage to these shields. Therefore, the radial clearance C = B – A of the bearing and the wear amount of the total bearing length L must not exceed the values specified in the operating instructions, as shown in Figure 2. 2. Assembly and adjustment (1) Bearing assembly: When installing bearings, the front and rear bearings must not be swapped. Before installing the bearing in the bearing housing, a flat steel washer should be placed at the cutout on the outer circle of the bearing, as shown in Figure 3. After the bearing is installed in the housing, the set screw should be tightened to the position of the steel washer, using a tightening force that complies with the requirements specified in the instruction manual. (2) Assembly of the thrust disc: When assembling the thrust disc, the coated side should face the axial working surface of the bearing. After assembling the rear shaft sleeve and thrust disc, be sure to properly lock the stop washer in place. (3) Adjustment of the total axial clearance of the rotor: The total axial movement of the rotor should meet the requirements specified in the user manual; if these requirements are not met, the thickness of the shims between the rear bearing and the rear bearing housing can be adjusted. (4) Adjustment of the axial clearance between the impeller and the front bearing housing: The axial clearance between the impeller and the front bearing housing should meet the requirements specified in the user manual; if it does not meet these requirements, the shim between the impeller and the shaft sleeve can be adjusted. (5) After the impeller and inducer are assembled, the retaining shims must be tightened. 3.2 Trial operation of shielded pumps and fault handling 1. Preparations before startup (1) Check the maintenance records to ensure that the data is correct, and prepare all the necessary record forms for the trial operation ； (2) Clean the area around the pump ； (3) Check whether all various accessories of the equipment are complete and in good working condition, and whether the bolts are secure ； (4) For shielded electric pumps with cooling water jackets, check whether the cooling system is unobstructed ； (5) Open the pump suction valve to fill the pump with liquid ； (6) Open the exhaust valve to fully release air ； (7) Fully open the pump’s discharge valve, then close it ； (8) Contact an electrician to check the motor resistance and supply power. 2. Start up (1) Press the start button, and pay attention to whether the TRG indicator value is within the green range ； (2) After the pump outlet pressure stabilizes, slowly open the outlet valve to control the outlet pressure ； (3) Ensure that the motor current does not exceed the specified value ； (4) Check the sound and vibration of the pump, and make records as required. 3. Stop the pump: (1) Close the outlet valve, open the bypass valve, and press the pump stop button ； (2) Close all valves related to the pump, except for the flow control valve, which remains open ； (3) Keep proper records of pump shutdowns, and maintain cleanliness of the equipment and its surrounding area. 4. Precautions (1) Operation without load is strictly prohibited ； (2) Continuous reverse operation is not permitted ； (3) It must not be operated under cavitation conditions ； (4) When the TRG gauge indicates a red area, operation must be stopped ； (5) If abnormal noises or vibrations are detected during operation, the cause must be promptly identified and the fault resolved ； (6) It should not operate when the flow rate is below the minimum value ； (7) The plant shall not be started up or kept operating if the flow rate in the cooling water jackets, heat exchangers, and circulation pipelines is below the specified value. 2. Fault handling of shielded pumps: The common fault symptoms, causes, and handling methods for shielded pumps are shown in Table 1. Table 1 Common Fault Causes and Solutions for Shielded Pumps
Serial Number | Fault Symptoms | Fault Causes | Solutions
1 | Easy bearing wear | Bearing corrosion, wear, shaft bending, foreign objects entering the pump, low flow rate, poor bearing lubrication, vacuum or cavitation, unbalanced rotor | Replace the shaft sleeve, straighten or replace the motor rotor, remove foreign objects, make process adjustments, balance the rotor
2 | Excessive vibration | Excessive bearing wear, shaft bending, friction between the impeller and the pump casing, unbalanced rotor, poor piping or pipeline shocks, vaporization or vacuum inside the pump, foreign objects entering the pump | Replace the bearings, correct the shaft or replace the rotor, adjust the clearance, balance the rotor, reconfigure the piping, eliminate stress or pipeline system vibrations, make process adjustments, remove foreign objects
3 | Abnormal noises | Excessive bearing wear, friction between the impeller’s ring and the pump body’s ring, shaft bending, foreign objects entering the pump, imbalance, air in the pump body or inlet pipe, cavitation, excessive or insufficient flow rate | Replace the bearings, adjust the clearance to meet specifications, straighten or replace the rotor, disassemble to remove foreign objects, balance the rotor, adjust process operations
4 | Low outlet pressure | Impeller blockage, incorrect rotation direction, excessive wear in the gap between the impeller’s ring and the casing’s ring, damaged impeller, medium viscosity exceeding design values, excessive gas in the incoming fluid, clogged inlet filter, low opening of the inlet valve | Remove blockages, check the inlet filter, rewire the system, replace the impeller or the casing’s ring, replace the impeller, make process adjustments, remove gas from the incoming fluid, remove debris, make process adjustments
5 | Motor overload | Damaged thrust bearings, too small a clearance between the impeller and the pump casing resulting in friction, foreign objects entering the pump, excessive flow rate, excessive radial bearing clearance causing the rotor to drag, high medium viscosity | Adjust the impeller’s balancing force, replace the thrust bearings, adjust the clearance values, disassemble to remove foreign objects, adjust the opening of the inlet and outlet valves, replace bearings and rotors, make process adjustments
6 | Motor overheating | Improper design of the motor and pump, insufficient or no cooling water | Redesign the system, increase the amount of cooling water or restart the cooling system