The mechanism, influence and elimination measures of the exciting force of centrifugal pumps

As an important fluid conveying equipment, centrifugal pumps are widely used in industrial, agricultural and civil fields. However, in actual field operation, centrifugal pumps are often affected by exciting forces, resulting in vibration, noise and equipment damage. This article will discuss the generation mechanism of centrifugal pump exciting forces, their impact on pump operation and effective elimination methods from multiple aspects.

• Factors that generate exciting force in centrifugal pumps

1. Hydraulic imbalance
The rotation of the impeller in a centrifugal pump causes centrifugal force to be generated in the fluid, but the distribution of the fluid inside the impeller may be uneven due to the limitations of the impeller design, manufacturing or operating conditions. This unevenness can lead to hydraulic imbalance, which in turn generates periodic or non-periodic exciting forces. Especially in multi-stage pumps, the interaction between the impellers is more likely to cause hydraulic excitation.
2. Mechanical imbalance
Mechanical imbalance is one of the important sources of exciting forces in centrifugal pumps. During the manufacturing or installation of the impeller, there may be problems such as uneven mass distribution and axial deviation, which lead to uneven distribution of centrifugal force. When rotating at high speed, this imbalance can cause periodic exciting forces, which in turn cause vibration.
3. Fluid dynamic interference
The design of the inlet and outlet flow channels of the centrifugal pump, the interaction between the impeller and the volute/guide vanes, and the backflow and vortex phenomena in the pump can all cause fluid dynamic interference. These interferences can cause pressure pulsation and fluid vibration, thereby forming exciting forces. For example, the interaction between the impeller and the volute/guide vanes can cause vibrations related to the blade passing frequency (BPF).
4. Cavitation
Cavitation is a common problem in the operation of centrifugal pumps. Cavitation occurs when the pressure in a local area of ​​the pump is lower than the saturated vapor pressure of the liquid, causing the liquid to vaporize and form bubbles. When the bubbles burst in the high-pressure area, an instantaneous high-pressure shock wave will be generated, causing vibration and noise. This phenomenon will not only lead to the generation of exciting forces, but may also cause erosion damage to the impeller surface of the pump, further exacerbating the generation of exciting forces.
5. Interaction of the piping system
The interaction between the centrifugal pump and the piping system will also cause exciting forces. For example, the pressure fluctuation of the liquid in the pipeline will be transmitted to the pump body through the inlet or outlet of the pump, forming a periodic external excitation force. In addition, the layout of the pipeline and the support structure, the stiffness of the base/foundation, and the alignment of the pump and the motor will also affect the manifestation and intensity of the exciting force.

• Effect of exciting force on centrifugal pump

1. Cause vibration and noise
The exciting force directly causes the vibration of the pump body and related components, accompanied by a large noise. Excessive vibration will lead to a decrease in the operating stability of the pump, increase maintenance costs, and affect the normal operation of peripheral equipment.
2. Aggravate the damage of components
Due to the periodic action of the exciting force, the bearings and seals of the pump are susceptible to fatigue damage, resulting in a shortened life. Especially in the case of high-frequency vibration, these key components may fail prematurely, causing unexpected equipment shutdown.
3. Reduce operating efficiency
The exciting force will cause turbulence in the fluid flow, increase energy loss, and thus reduce the efficiency of the centrifugal pump. In addition, the vibration caused by cavitation may also cause damage to the impeller surface of the pump, further reducing the performance of the pump.
4. Cause structural damage
The long-term exciting force will cause structural fatigue of the pump body and key components, and even lead to serious consequences such as cracks and fractures. This damage will not only cause unexpected system shutdowns, but may also cause serious safety accidents.

• Measures to eliminate exciting forces

1. Optimize impeller design
Hydraulic imbalance can be effectively reduced by optimizing the impeller geometry and the number of blades. For example, by increasing the number of blades or changing the outlet angle of the blades, the amplitude of fluid pulsation can be reduced. In addition, the use of a more symmetrical impeller design can reduce fluid-induced vibration.
2. Improve flow channel design
Optimize the pump's inlet and outlet flow channels and volute design to reduce fluid dynamic interference. For example, the use of a double volute design can effectively reduce hydraulic imbalance; the reasonable design of the guide vane shape and its number of blades can reduce the pressure pulsation caused by the blade passing frequency. In addition, avoiding sharp changes in diameter and bending in the flow channel can reduce eddy currents and backflow phenomena.
3. Dynamic balancing correction
During the impeller manufacturing and installation process, dynamic balancing correction should be strictly performed to ensure that the impeller mass is evenly distributed and the axis is centered. Dynamic balancing correction can significantly reduce the exciting force caused by mechanical imbalance.
4. Select damping device
During the installation of the pump, damping devices such as spring damping pads or dampers can be added to reduce the transmission efficiency of the exciting force. In addition, optimizing the connection between the pump and the piping system and avoiding rigid connection can also help reduce vibration.
5. Avoid cavitation
In order to avoid cavitation, the pump inlet pressure should be reasonably designed to ensure that it is higher than the saturated vapor pressure of the liquid. In addition, the selection of materials and coatings with excellent anti-cavitation performance can also reduce the damage of cavitation to the pump.
6. Optimize operating conditions
The operating conditions of the centrifugal pump have a direct impact on the magnitude of the exciting force. For example, whether the pump speed, flow rate and head are reasonably matched will significantly affect the vibration level. Therefore, during the selection and operation of the pump, its operating point should be as close to the optimal efficiency point as possible.
7. Strengthen foundation and pipeline design
Improve the stiffness and damping performance of the pump foundation and reduce the transmission of external vibration. In addition, reasonably design and arrange the pipeline and its bracket and fixed point position (such as the length of the straight pipe section, avoid the generation of air pockets, etc.) to avoid the influence of the pipeline on the pump body.
8. Monitoring, diagnosis and maintenance
By installing vibration sensors and monitoring systems, the vibration status of the pump can be monitored in real time, abnormal conditions can be detected in time and measures can be taken. For example, spectrum analysis can be used to identify the source of vibration, so that the problem can be solved in a targeted manner. In addition, regular maintenance and replacement of wearing parts such as bearings and seals can effectively extend the service life of the equipment.