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To enhance the service life and operational safety of shielded pumps, they are typically equipped with several protective features. First, a **bearing wear monitor** is commonly used to detect bearing degradation. These monitors can be mechanical, electrical, or electronic, depending on the detection method. During operation, the bearing wear monitor continuously checks the condition of the bearing. If excessive wear is detected, the pump should be stopped for inspection, repair, or replacement. In case of sudden bearing failure, immediate shutdown is necessary.
Mechanical monitoring systems offer the most direct and reliable way to detect bearing wear, but they are not adjustable and lack warning signals. On the other hand, electronic or electrical types allow for on-site display and can output 4–20mA signals as well as alarm switches. Modern shielded pumps often use electronic bearing wear monitors, as shown in Figure 2-15.
Second, **current protection** is another critical feature. When a shielded pump runs dry, it can cause serious damage. A significant drop in flow rate leads to a noticeable decrease in current, which triggers the current protector to send a signal to stop the pump. Similarly, if a magnetic pump experiences excessive load, the current increases, and the protection system will automatically cut off power to prevent motor damage.
Third, **motor over-temperature protection** is essential. Temperature sensors are embedded in the stator’s three-phase windings to monitor temperature levels. This helps avoid exceeding the insulation limits of the motor and ensures compliance with explosion-proof standards. There are three main types of temperature sensors: thermistors (PTC), thermal switches, and resistance temperature detectors (RTDs) like PT100. The first two are cost-effective and widely used, but they provide only on/off alarms with fixed thresholds. RTDs, however, allow real-time temperature monitoring through compatible instruments and are often used in critical applications or special winding configurations.
In addition to these protections, shielded pumps may also include features such as **heat exchange capacity monitors**, **liquid level monitors**, or **internal pressure protectors**, depending on the application. These additional safeguards ensure that the pump operates safely under various conditions, meeting diverse industrial requirements.