Dry running of a centrifugal pump is unacceptable. Many users are terrified of this, largely due to a lack of understanding of its causes and how to handle it properly. However, brief dry running (less than one minute) will not cause damage.
Causes of dry running in a centrifugal pump include: insufficient inlet medium or inadequate inlet pressure, blockage in the inlet pipe, or incomplete purging of air from the pump chamber.
When a centrifugal pump is dry, the impeller has little or no contact with the medium, rendering it ineffective and preventing proper liquid delivery. This is all wasted effort. Because there is no effective work, the load on the motor is very small, resulting in minimal current and minimal impact on the motor itself. However, dry running can be fatal to the pump itself. A water pump running dry is prone to cavitation, damaging the pump body and flow components. Whether using a mechanical seal or a packing seal, a dry-running water pump will lack proper lubrication, leading to dry friction and rapid damage. The rotor and pump body of a dry-running pump will overheat; without liquid cooling, the narrow gaps (such as sealing rings) are easily seized due to thermal expansion and contraction. In a multi-stage centrifugal pump, the balance disc will lack lubrication and quickly burn out, resulting in damage.
To prevent and address dry-running in centrifugal pumps, it is recommended to upgrade the pump design:
1. Improve the clearances of the centrifugal pump, widening them as needed to prevent seizing;
2. Perform special processing on the pump shaft, such as tempering and heat treatment, to increase hardness and use low-flexibility materials to prevent excessive oscillation and seizing during dry running;
3. Use mechanical seals and packing cavities that do not rely on liquid lubrication;
4. Select enclosed, self-lubricating bearings that require no lubrication.
The pump chamber of a horizontal self-priming centrifugal pump has a larger volume compared to other centrifugal pumps, providing a space for water storage. Before normal operation, the chamber must be filled with liquid. There is a self-priming period before operation, during which air is continuously expelled from the pump body; this period is considered partial idling.
An idling sensing system is installed. Modern equipment monitors the pump in real time. When idling occurs, an alarm is immediately triggered and the pump automatically shuts down to protect the unit's safety.
Precautions for use:
1. Installation Instructions
1. During installation, the weight of the pipeline should not be placed on the pump; each pipeline should have its own support to prevent deformation that could affect operational performance and lifespan.
2. The pump and motor are an integrated structure, requiring no alignment during installation, making installation very convenient.
3. The foundation bolts must be tightened during installation to prevent vibration during startup from affecting pump performance.
4. Before installing the water pump, carefully check the pump flow channel for any hard objects (such as stones, iron particles, etc.) that may affect pump operation, to avoid damage to the impeller and pump body during operation.
5. For ease of maintenance and safe operation, install a regulating valve on each of the pump's inlet and outlet pipes, and a pressure gauge near the pump outlet to ensure operation within the rated head and flow range, ensuring normal pump operation and extending pump lifespan.
6. For pumps used in suction lift applications, a foot valve should be installed, and the inlet pipe should not have excessive bends, and there should be no water or air leaks.
7. If a check valve is used on the discharge pipe, it should be installed outside the gate valve.
8. After installation, rotate the pump shaft; the impeller should not produce friction noise or jamming. If so, the pump should be disassembled to check the cause.
9. Pump installation methods include rigid connection and flexible connection.
2 Centrifugal Pump Installation Sequence
1. When the unit arrives on site, the pump and motor are already aligned with the base. Leveling the base does not require removing the pump and motor, making installation very convenient.
2. Place the base on the foundation and place wedges near the anchor bolts to raise the base by 20-40 mm. This is for leveling and filling with cement grout later.
3. Use a level to check the base's levelness. After leveling, fill the base with cement grout. Check the levelness again after the cement dries.
4. When the unit has a large power output, the pump, motor, and base may be packaged separately for ease of transport. In this case, the user needs to install and align the pump unit themselves. The method is as follows:
a) Clean the supporting surface of the base, the pump feet, and the motor feet of the pump unit thoroughly. Place the pump and motor on the base. 1) Install the pump on the base.
b) Adjust the pump shaft to be level. After leveling, secure the pump to the base with bolts to prevent movement.
c) Lift the motor, align the pump coupling with the motor coupling, and lower the motor to the corresponding position on the base.
d) Adjust the gap between the two couplings to approximately 5mm, and check if the center lines of the motor shaft and pump shaft coincide. This can be done by placing a straightedge on the coupling; the outer circumference of both couplings should be flush with the straightedge. If they do not coincide, adjust the relative position of the motor or pump, or use thin iron sheets for adjustment.
e) To check the installation accuracy, measure the gap between the two coupling planes at several different positions on the circumference of the coupling using a feeler gauge. The difference between the maximum and minimum gaps on the circumference of the coupling plane should not exceed 0.3mm, and the difference between the vertical or horizontal center lines at both ends should not exceed 0.1mm.
5. When the unit does not have a base, it needs to be installed directly on the foundation. The method is similar to 4, but more careful calibration is required.