What is the function of the balance pipe of a centrifugal pump? How to balance the axial force?

Centrifugal pumps are widely used fluid conveying devices in industry, agriculture, municipal engineering, and daily life. Their performance stability and reliability are crucial. In the structural design of centrifugal pumps, the balance pipe, as a key component for balancing axial forces, plays a crucial role.

1. Basic Structure of Centrifugal Pumps and Axial Force Issues

The basic structure of a centrifugal pump consists primarily of an impeller, pump body (casing), pump shaft, bearings, sealing rings, and a stuffing box. The impeller is the core component of the centrifugal pump, responsible for transferring mechanical energy to the fluid, imparting kinetic and static pressure energy. However, during operation, the fluid acting on the impeller generates an axial force directed toward the pump's suction port. This axial force pushes the rotor toward the suction side. In severe cases, it can cause friction or collision between the impeller and the pump casing, compromising safe operation.

2. Position and Function of the Balance Pipe

To address axial force issues, a balance pipe is incorporated into centrifugal pump designs. The balance pipe is typically located on the outer side of the impeller disc, near the high-pressure side of the hub, and connected to the centrifugal pump's suction port. Specifically, the balance pipe extends from the pump's outlet sealing ring and connects to the pump's suction port via external piping. Its primary function is to balance the pump's axial thrust, reduce axial movement of the rotor, and prevent friction between the impeller and the casing.

When the pump is operating, high-pressure water is ejected from the impeller outlet, with a portion flowing to the rear of the impeller, keeping the pressure behind the impeller essentially equal to the outlet pressure. The front of the impeller, the suction side, is at much lower pressure, creating a large pressure differential across the impeller and generating axial thrust. The function of the balance pipe is to direct the pressurized water behind the impeller to the suction side, balancing the pressure across the impeller and thus eliminating or reducing axial thrust.

3. Balancing Principle and Effect of the Balance Pipe

The balancing principle of the balance pipe is to direct liquid from the high-pressure side to the low-pressure side, balancing the pressure differential across the impeller and thus balancing the axial force. However, it should be noted that the balance pipe cannot completely balance the axial force; the remaining axial force must be borne by a specially installed thrust bearing. Furthermore, the use of a balance pipe may cause liquid backflow, reducing pump efficiency. Despite this, due to its simple structure and ease of implementation, the balance pipe is still widely used in centrifugal pump design.

4. Other Methods for Balancing Axial Forces in Centrifugal Pumps

In addition to balancing tubes, centrifugal pumps can balance axial forces in a variety of ways, including:
1. Balancing Holes in the Impeller
Several small holes (called balancing holes) are drilled symmetrically on the impeller disk near the hub. Seals are placed on the pump casing and impeller disk to minimize the pressure difference between the liquid on both sides of the impeller, thereby reducing axial forces. This method is commonly used in single-stage, single-suction centrifugal pumps.
2. Using a Double-Suction Impeller
Due to their symmetry, double-suction impellers theoretically generate no axial forces. However, due to manufacturing quality and differences in fluid flow on both sides of the impeller, axial forces cannot be completely balanced.
3. Using a Balanced Impeller
Radial ribs are installed on the outside of the impeller disk to balance axial forces. With these ribs, the high-pressure liquid inside the impeller rotates under the force of the ribs. Centrifugal force reduces the pressure of the liquid in this cavity, thereby balancing the axial forces on both sides of the impeller.
4. Installing a Thrust Bearing
For small pumps with low axial forces, thrust ball bearings are often used to balance axial forces. However, radial thrust bearings require directional installation. Improper installation will fail to balance axial forces and could even cause accidents.
5. Balancing Methods for Multistage Centrifugal Pumps
Multistage centrifugal pumps typically use balancing discs, balancing drums, or a combination of these devices to balance axial forces. These methods are relatively complex but effective.

In summary, the centrifugal pump's balancing tube is located between the high-pressure side of the impeller disc, near the hub, and the suction port. Its primary function is to balance the pump's axial thrust and prevent friction between the impeller and the casing. While the balancing tube cannot completely balance axial forces and may cause liquid backflow and reduce efficiency, its simple structure and ease of implementation make it crucial in centrifugal pump design. Depending on the specific type and operating conditions of the centrifugal pump, other balancing methods can be used to ensure safe and stable pump operation.