About the axial force of the centrifugal pump

Axial force generation and harm

When the centrifugal pump is in normal operation, its main shaft generates axial force. Due to the fluid flow in the pump chamber, a dynamic reaction force is inevitably generated on the main shaft, and thus an axial force generated during operation of the pump is unavoidable. Under the action of the axial force, the rotor generates axial displacement, causing the static and dynamic parts to grind and collide with each other, resulting in serious damage to the water pump. The presence of axial force can cause the pump to run smoothly for a long time, reducing its service life and overall performance, and even endangering the safety of the operator in severe cases. Therefore, balancing the axial force of the pump is the key to improving the performance of the pump shaft and thus improving the overall performance and safety of the pump.


In addition to the above inevitable factors causing the axial force of the pump rotor, other unreasonable factors will also lead to axial forces, mainly the following:

1、When the pump is in normal operation, the pressure at the impeller inlet is P1, the pressure on the back of the impeller is P2, and P2 > P1. Therefore, a thrust F1 is generated along the axial direction of the pump.

2、After the liquid flows through the impeller, the dynamic reaction force F2 is generated due to the change of the flow direction. In a multistage centrifugal pump, the fluid usually flows axially into the impeller and flows radially. The change in flow direction is due to the force of the impeller, so the liquid also reacts to the impeller with an equal and opposite force. Axial force F due to asymmetrical pressure distribution on the blade

3、The blade working face pressure is stronger than the pressure on the back of the blade, and the pressure difference formed by it will also generate axial force.

4、The axial force F4 due to the asymmetrical pressure distribution in the impeller runner.

5、For a vertical pump, the internal rotor is gravity, which becomes part of the axial force; for horizontal pumps, this axial force does not exist.

6、The front and rear covers of the impeller are asymmetric;

7、There are unreasonable factors in the structural design of the shaft step and the shaft end; 8. Other factors cause the rotor to generate axial force, such as radial flow in the pump chamber. Among the many factors that generate axial force, the dynamic reaction force of the fluid in the pump chamber and the asymmetry of the front and rear cover plates of the impeller are the main reasons for the axial force generated by the rotor.

Pump axial force balance method
There are various methods for balancing the axial force of the pump rotor. For example, a thrust bearing is arranged outside the pump, a balance hole or a balance pipe is arranged on the pump cavity to reduce the pump pressure, and the impeller is designed to adopt a back blade, a double impeller, a symmetric distribution of the impeller, and the like. Form, as well as the use of balance plates, balance drum structures, etc. Among them, the balance wheel and the balance drum structure are used to balance the axial force of the rotor.
The balance disc is widely used in the axial force balance of the multi-stage pump, and is located behind the impeller of the pump stage. The structural principle is shown in Figure 1. The balance device has two gaps, radial and axial, and the pressurized liquid flowing out from the final stage flows into the cavity in front of the balance disc through the radial gap to form a high pressure state. A balance pipe is opened on the cavity on the rear side of the balance plate, and communicates with the inlet of the water pump, so that the pressure in the cavity is substantially the same as the pressure at the inlet of the pump. Since the pressures in the two cavities before and after the balance disc are not equal, a pressure difference is formed, and a balance force which is opposite to the axial force is generated to achieve a balance effect. When the balance plate structure is used to balance the axial force of the pump rotor, the balance force changes as the axial force changes continuously. Therefore, the working process is a dynamic balancing process. The balance disc automatically adjusts its variable gap size by the rotation of the rotor, thereby adjusting the balance force, and can fully balance the axial force of the rotor without relying on external thrust bearing assistance, so the balance structure can omit the external thrust bearing.

Balanced disc structure defects:
(1) When the pump rotor is started at a lower speed, the pump generates less thrust and cannot push the balance disc away from the balance seat, resulting in contact between the two during work, causing grinding and causing wear. After reaching a certain limit, it needs to be replaced and lowered. Balance the life of the disc.
(2) The axial gap of the balance disc is extremely small, which makes it limited in its application range. It is not suitable for working conditions such as liquid sand in the pump body, dry running of the pump or liquid near the pump.
(3) Since the balance disc relies on the balance tube to leak the liquid in the pump, causing a pressure difference between the front and rear to balance the axial force, the leakage will cause the efficiency of the pump to decrease. The balance drum is widely used in the field of axial force balance of centrifugal compressors. Its structure is shown in Figure 2. There is no axial gap in the balance drum. The pressurized liquid enters the balance chamber through the radial gap of the balance drum to form a high pressure. A balance tube is disposed on the inner wall of the cavity to communicate with the inlet of the pump, so that the pressure in the chamber is always greater than or equal to the inlet pressure, thereby achieving the purpose of balancing the axial force. The balance drum structure balances the axial force, and the balance between the static and dynamic parts of the balance drum structure is established at the time of manufacture, and the balance force cannot be automatically adjusted. Therefore, it is necessary to externally install the thrust bearing to balance the excess balance force, and the advantage is that The parts are non-contact, wear-free and therefore have a long service life.
When the balance plate is used to balance the axial force of the pump, the axial gap is very small. When the liquid flow in the pump changes, or the working state of the pump changes instantaneously, the balance plate and the balance ring are easily attached or clamped. Both of them produce grinding and damage, which greatly reduces the life of the balance disc and affects the normal operation of the pump. Therefore, in order to improve the performance of the axial force balance mechanism of the water pump, the balance drum structure can be used for axial force balance. However, when the balance drum structure is used for axial force balance alone, the pump body will have a large leakage, which greatly reduces the pump delivery. effectiveness. Therefore, in order to improve the overall performance of the balance device, combined with the advantages of both, the balanced force of the balance plate and the balance drum can be used to balance the axial force of the pump rotor.

The advantage of adopting this structure is that:
(1) When the balance drum is used to balance the axial force, it is necessary to install a thrust bearing on the outside. When the joint structure is used, the thrust bearing can be omitted due to the existence of the balance disc.
(2) When the balance drum balances the axial force, it can balance the balance force generated by a part of the balance disc, reduce the pressure difference between the balance disc and the front and back, make the balance disc and the support ring easier to separate, and not easy to cause wear, so that the performance of the entire balance device is reliable.
(3)When the pump is required to idling, a thrust bearing needs to be installed on the pump, and the presence of the joint mechanism can greatly reduce the load of the thrust bearing. In this paper, the water pump is applied to the auxiliary feed water of the nuclear power plant, and the motor auxiliary feed pump specification specifies that the balance disc is not allowed to be used as the axial force balance device. Therefore, the test object is intended to adopt a single balance drum or balance disc and balance drum joint mechanism as the pump axial direction. Force balance device.

Calculation of relationship between flow and axial force
The calculation of the relationship between the axial force of the pump rotor and the fluid flow is complicated, and the form is diverse. Different types of balance structures are used, and the calculation method of the balance force is also different. In this paper, the test object is proposed to adopt two different structures: single balance drum or balance disc and balance drum joint mechanism as the pump axial force balance device, and the best performance is selected as the final balance scheme. According to two different structures, the relationship between the total axial force and the flow rate after balancing by the balancing device is determined. 1 The single balance drum is used as the axial force balance device. When this structure is used, the relationship between the fluid flow in the pump body and the total axial force of the rotor is calculated. 2 The balance disc and the balance drum joint mechanism are used as the axial force balance device, and the relationship between the fluid flow in the pump body and the axial force of the rotor when calculating this structure is calculated.

Axial force test
The existing axial force prediction methods are generally divided into two types: the experimental measurement is calculated by the empirical formula. The axial force precision and applicability obtained by the empirical method and the theoretical calculation method are poor, and the centrifugal pump for important purposes is needed. Perform the actual measurement of the axial force to ensure the correct design of the axial force balance device, and select the appropriate axial thrust bearing to prevent failures caused by the axial force balance device or bearing failure. At the same time, accurate axial force values can be obtained through testing to correct and modify empirical calculation formulas and theoretical calculation models. A variety of axial force testing methods at home and abroad can be divided into the following categories:

1、Installation pull pressure sensor

2、Hydraulic force measuring mechanism

3、Directly attach the strain gauge

4、Force measuring elastic element

5、Spring scale measuring device

6、Eddy current effect measuring device