Shield pumps are a combination of a centrifugal pump and a three-phase asynchronous shielded motor, mounted coaxially. They eliminate the need for mechanical seals, making them ideal for handling toxic, hazardous, and expensive liquids. These pumps are widely used in industries such as chemicals, pharmaceuticals, nuclear energy, and aerospace due to their leak-free operation and high reliability. During my time working at Shanghai Petrochemical, I participated in several overhauls involving imported shield pumps. These included both routine maintenance and full recovery overhauls. As these pumps age and their usage increases, the frequency of damage and subsequent overhaul has also risen. I have repaired various foreign shield pumps with power ratings ranging from 0.55 kW up to 130 kW, gaining extensive hands-on experience in their maintenance and repair. **1. Structural Characteristics and Common Failures** A shield pump integrates a pump and a motor, where the stator and rotor are enclosed within special metal sleeves. This design prevents direct contact between the liquid medium and the motor components, protecting the windings from corrosion and ensuring good insulation properties. The pump uses graphite bearings that rely on the pumped liquid for lubrication. Monitoring bearing wear is crucial for reliable operation. Most shield pumps are equipped with either mechanical or electromagnetic bearing monitors. When the bearing wear exceeds the allowable limit, the monitor indicates an alarm, signaling the need for immediate inspection. If not addressed, excessive wear can lead to rubbing between the stator and rotor shields, causing damage and potential failure. In chemical environments, junction boxes are typically explosion-proof. Temperature relays are embedded in the motor coil ends to protect against overheating, with different action temperatures based on the insulation class. Some shield pumps also feature cooling systems, where a serpentine tube circulates the hot medium to cool the graphite bearings and the motor itself. Common failures include: - **Wear or dry friction of graphite bearings, bushings, and thrust plates** - **Damage to the rotor shield due to excessive bearing wear or chemical corrosion** - **Stator winding damage caused by erosion from leaking media or electrical faults like short circuits or overload** **2. Routine Maintenance Procedures** To prevent unexpected failures, regular maintenance is essential. If the bearing monitor triggers an alarm, immediate overhaul is required. In chemical plants, which operate continuously, maintenance is usually performed during scheduled shutdowns. Most shield pumps require annual servicing. The maintenance process involves disassembling the pump, cleaning all parts, and conducting visual inspections. Key measurements are taken, including the clearance between the graphite bearing and shaft, as well as electrical checks on the motor windings. **Mechanical Inspection:** - Measure the bore diameter of the graphite bearing and the sleeve shaft to check for clearance and surface finish. - Inspect impeller and pump seat clearances to ensure proper alignment and performance. - Check the rotor shield for weld integrity and leaks. - Perform dynamic balancing tests if necessary. **Electrical Inspection:** - Check DC resistance to ensure it is balanced within 2%. - Test insulation resistance, which should be above 100 MΩ. If lower, determine whether it's due to moisture or a shielding issue. **3. Recovery Overhaul Process** If the stator windings or shield are damaged, a full recovery overhaul is needed. There are two main types of damage: one where the stator shield is intact but the windings are damaged due to short circuits or overload, and another where the shield is compromised, allowing the liquid to reach the windings. Replacing the stator windings requires careful attention to insulation levels, typically H-class materials. The number of turns, wire diameter, and winding pattern must match the original to maintain performance. **Shield Sleeve Replacement:** This is one of the most challenging parts of the overhaul. The shield material must have high resistivity and good corrosion resistance, often using specialized alloys like Hastelloy-C. The production process demands precision in cutting, welding, and fitting to ensure tight tolerances and leak-proof performance. After replacing the shield, a complete leak test is conducted by pressurizing the cavity and submerging the stator in water to detect any leaks. **4. Assembly and Testing** Once all components are replaced and inspected, the pump is reassembled. The rotor should rotate smoothly with appropriate axial movement, within specified limits. After assembly, electrical tests like DC resistance and insulation resistance are performed. A final test run is conducted to check current, sound, vibration, and overall performance. Regular maintenance and timely repairs are crucial to extending the life and efficiency of shield pumps, especially in critical industrial applications.

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