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If the axial force acting on the impeller (and transmitted to the shaft) is not eliminated or balanced, this force will pull the rotor axially, causing it to come into contact with fixed components, which will result in damage to the pump’s components and render it non-functional. The following methods are generally used to balance the axial force of pumps. 1. Thrust bearings: For small pumps with low axial forces, using thrust bearings to handle these forces is usually a simple and economical approach. Even when other balancing devices are used, thrust bearings are sometimes installed considering that there is always some residual axial force. 2. Balance hole or balance tube: As shown in Figure 1, a sealing ring is attached to the rear cover plate of the impeller; the diameter of this sealing ring is generally equal to that of the front sealing ring. Meanwhile, an opening is made in the lower part of the rear cover plate, or a dedicated connecting tube is provided to connect with the suction side. Due to the pressure loss caused by the flow of liquid through the gap in the sealing ring, the pressure of the liquid below the seal decreases, thereby reducing the axial force acting on the rear cover plate. The degree to which the axial force is reduced depends on the number of holes and their diameter. Under these conditions, there is still 10–15% of unbalanced axial force. To fully balance the axial force, it is necessary to increase the diameter at which the sealing ring is located further. It should be noted that the sealing ring and the balance hole complement each other; having only a sealing ring without a balance hole cannot balance the axial force ； By using only balance holes without seal rings, the leakage is significant, and the degree of balancing the axial force is very low. http://www.b-fz.com/UploadFiles/%E7%A6%BB%E5%BF%83%E6%B3%B5%E5%B9%B3%E8%A1%A1%E8%BD%B4%E5%90%91%E5%8A%9B%E7%9A%84%E7%BB%93%E6%9E%84%E6%9C%89%E5%93%AA%E4%BA%9B1.png Figure 1: Schematic diagram of the balance hole. This balancing method can reduce the pressure on the shaft seal; however, its drawback is an increase in volume loss (the leakage rate through the balance hole is generally 2–5% of the design flow rate). Furthermore, the leakage flow passing through the balance hole collides with the main fluid flow entering the impeller, disrupting the normal flow pattern and reducing the pump’s cavitation resistance. To this end, some pump bodies are provided with openings that connect to the suction pipe through pipelines, but this makes the structure more complex. Using the aforementioned balancing method, it is not possible to achieve complete balance of the axial force; the remaining axial force must be borne by the pump’s bearings. Structures that use balance holes to balance axial forces are widely used, not only in single-stage centrifugal pumps but also in multi-stage centrifugal pumps. However, since the axial forces cannot be completely balanced, thrust bearings are still required. Moreover, the addition of an extra ring increases the axial dimensions of the pump; therefore, it is used only in pumps with low head and small size. 3. Double-suction impeller: When a single-stage pump uses a double-suction impeller, the impeller is symmetrical, which allows the axial forces on both sides of the impeller to cancel each other out. But in reality, due to differences in the sealing clearances on both sides of the impeller, or due to misalignment of the impeller relative to the center of the volute, there is still a small residual axial force, which must be borne by the bearings. 4. Back vanes: The pump back vanes are installed on the outside of the rear cover; in other words, they represent an additional semi-open impeller that is positioned on the back side of the main impeller, in a direction opposite to that of the suction flow, as shown in Figure 2. To facilitate casting, such back vanes are usually designed to be radial in shape, though they can also be made curved. After adding back vanes to the impeller, these vanes force the fluid to rotate, increasing its angular velocity; this change in the rotational speed alters the pressure head distribution on the rear cover plate, thereby reducing the unbalanced forces. The remaining axial force still needs to be borne by the bearings. http://www.b-fz.com/UploadFiles/%E7%A6%BB%E5%BF%83%E6%B3%B5%E5%B9%B3%E8%A1%A1%E8%BD%B4%E5%90%91%E5%8A%9B%E7%9A%84%E7%BB%93%E6%9E%84%E6%9C%89%E5%93%AA%E4%BA%9B2.png Figure 2: Schematic diagram of the back blade. In addition to balancing axial forces, the back blade also helps to reduce the pressure of the liquid in front of the shaft seal. The head of a pump equipped with a backward-curved vane pump increases by about 1–2%, resulting in a decrease in the pump’s efficiency of 2–3%. The back blade also serves to prevent impurities from entering the shaft seal, and is commonly used in pumps that transport liquids containing impurities. 5. Symmetrical arrangement of impellers: This method is mainly used in multi-stage pumps. All the impellers of the pump are evenly distributed in two directions, arranged face to face or back to back in a specific sequence (as shown in Figure 3), thereby allowing the axial forces to balance each other out. http://www.b-fz.com/UploadFiles/%E7%A6%BB%E5%BF%83%E6%B3%B5%E5%B9%B3%E8%A1%A1%E8%BD%B4%E5%90%91%E5%8A%9B%E7%9A%84%E7%BB%93%E6%9E%84%E6%9C%89%E5%93%AA%E4%BA%9B3.png Diagram of symmetrical arrangement of impellers. The principles for arranging impellers are: (1) The flow channels between stages should not be too complex, in order to facilitate casting and reduce flow resistance losses ； (2) A low-pressure stage should be arranged on the shaft seal sides at both ends to reduce the pressure acting on the shaft seals ； (3) The stage difference between two adjacent impellers should not be too large, in order to reduce the pressure difference between stages and thereby minimize leakage between them. The symmetric arrangement of segmented pumps can balance the axial forces, but it increases inter-stage leakage. When impellers are arranged symmetrically, they can achieve complete balance only under conditions of identical structure; when the hub shafts at different stages are not the same, certain axial forces will also be generated. 6. Balance drum: The balance drum is a cylinder that is installed behind the final impeller and rotates together with the rotor. A radial gap is formed between the outer circular surface of the balance drum and the pump body. In front of the balance drum is the rear pump chamber of the final impeller, and behind it is the balance chamber that is connected to the suction inlet. The resulting pressure difference acting on the balance drum creates a balancing force directed to the right, which is used to counteract the axial force acting on the rotor. As shown in Figures 4 and 7, the balance disk can automatically and completely balance the axial force under various operating conditions; therefore, it is widely used in multi-stage centrifugal pumps. As shown in Figure 5, there is a gap between the shaft sleeve and the pump body, and an axial gap bo exists between the disk end face and the pump body. Behind the balance disk is a balance chamber that is connected to the pump’s suction inlet. The pressure before the radial gap b is the pressure p on the back side of the final impeller; after passing through the gap b, the pressure drops to p’. The pressure drop across the radial gap is Δp1 = p – p’. After the liquid passes through the axial gap b0, the pressure drops further to po, with the pressure drop across the axial gap being Δp2 = p’ – po. Here, po is close to the pressure at the pump’s suction inlet. The pressure drop across the entire balance disk assembly is △p=△p1+△p2. In this way, a balancing force is applied to the balance disc, in a direction opposite to the axial force of the pump. http://www.b-fz.com/UploadFiles/%E7%A6%BB%E5%BF%83%E6%B3%B5%E5%B9%B3%E8%A1%A1%E8%BD%B4%E5%90%91%E5%8A%9B%E7%9A%84%E7%BB%93%E6%9E%84%E6%9C%89%E5%93%AA%E4%BA%9B5.png Figure 5: Schematic diagram of the balance disk. The working principle of the balance disk is as follows: when the axial force exceeds the balancing force of the balance disk, the rotating part of the centrifugal pump moves to the left, thereby reducing the axial gap bo. This increases the resistance to fluid flow through that gap, and as a result, the overall resistance coefficient of the entire balancing system also increases. However, since △p remains constant, the leakage amount q decreases; as a result, △p1 decreases while △p2 increases, thereby increasing the balancing force. As the rotating part continues to move to the left, the balancing force keeps increasing, and at a certain point, the balancing force and the axial force reach equilibrium. When the axial force is less than the balancing force, the rotating part moves to the left; this is the reverse of the aforementioned process, and it also brings the centrifugal pump into an axial equilibrium state. Therefore, centrifugal pumps equipped with a balance disc device generally do not come equipped with thrust bearings.