Balance holes should not be excessively open, as this can significantly affect the pump's efficiency. The following explains some common methods used to balance axial thrust in pumps.
In single-stage pumps, there are several typical approaches for balancing axial forces:
1. **Balancing Hole**
A few small holes are drilled near the hub of the rear cover of the impeller, and a sealing ring is installed on the rear cover. The outer diameter of the sealing ring matches that of the impeller's suction inlet. During operation, the liquid behind the rear cover connects with the suction inlet, resulting in similar pressure levels. The area outside the seal ring on the rear cover is equal to the area outside the front cover’s suction port, so the total force from the liquid pressure on both sides of the impeller is nearly balanced, leaving only a small residual axial force that is absorbed by the bearing. Under normal conditions, this method effectively balances axial force. However, it causes more leakage, which interferes with the normal flow of liquid into the impeller, reducing pump efficiency by approximately 2–5%. This method is typically used in small single-stage centrifugal pumps.
2. **Balancing Pipe**
This approach is similar to the balancing hole method. A mouth ring is placed on the rear cover of the impeller, aligned with the suction inlet. A balancing pipe then directs the liquid from the sealed space back to the pump inlet, helping to balance the axial force. The cross-sectional area of the balancing pipe must be equal to or larger than the flow area of the mouth ring gap by 4–5 times.
3. **Balancing Vanes**
Radial vanes are symmetrically placed on the rear side of the impeller. As the impeller rotates, these vanes accelerate the liquid on the backside, increasing centrifugal force and reducing pressure. This helps balance the pressure on both sides of the impeller. The effectiveness of this method depends on the size of the vanes and the clearance between them and the pump casing. However, it also reduces overall pump efficiency.
4. **Double Suction Impeller**
In single-stage or some multi-stage centrifugal pumps, a double-suction impeller is used. This design inherently balances the axial thrust due to the symmetrical flow on both sides. However, it is challenging for manufacturers to ensure perfect symmetry in the geometry of both flow components, so a small residual axial force still exists. This force is typically supported by a single-row radial ball bearing at one end of the shaft.
Additionally, for multi-stage pumps, other axial force balancing methods include symmetrical impeller arrangement, balanced hub design, and the use of balancing discs. These techniques help manage the cumulative axial forces generated across multiple stages, ensuring smooth and stable pump operation.
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