Understanding the Balance Hole of a Split Case Pump Impeller
In split case pumps, the impeller plays a critical role in moving the fluid efficiently. One often overlooked but essential design feature of the impeller is the balance hole, also known as the return port. The primary function of the balance hole is to reduce the axial thrust generated during pump operation, thereby minimizing wear on the bearings and thrust discs and improving the overall longevity and performance of the pump.
What Is a Balance Hole in a Split Case Pump Impeller?
The balance hole is a small opening designed into the impeller of a split case pump. When the impeller rotates, it imparts energy to the liquid, causing it to move from the center (eye) of the impeller toward the periphery through the flow passage between the blades. This process increases both the pressure and velocity of the fluid, which in turn generates a forward axial force. This axial force pushes against the thrust components of the pump, which can cause premature wear if not managed properly.
To counteract this, the balance hole allows a small portion of the high-pressure fluid from the impeller periphery to flow back to the lower-pressure region near the impeller eye. This backflow helps to balance the axial thrust and protect critical components like the bearings and thrust plates from undue stress.
Design Considerations and Performance Impact
The effectiveness of the balance hole in reducing axial force depends largely on its size and number. A well-designed balance hole can significantly prolong the life of a split case pump by reducing the mechanical load on the axial support components. However, this comes at a trade-off. The fluid that passes through the balance hole represents a small loss of efficiency—typically between 2% and 5% of the design flow.
In addition, the leakage flow through the balance hole can disrupt the smooth flow of fluid entering the impeller, especially when it collides with the main inlet flow. This disruption can deteriorate the pump’s anti-cavitation performance, making it more susceptible to cavitation damage under low-pressure conditions.
Flow Behavior at Non-Rated Conditions
At flow rates different from the rated conditions, the internal flow characteristics change significantly. For instance, at low flow rates, pre-rotation effects can cause the pressure at the impeller eye to drop below that at the outer periphery, increasing leakage through the balance hole. While this may increase the head of the split case pump slightly, the pressure in the chamber beneath the sealing ring remains low, leading to further reduction of axial force.
Conversely, at high flow rates, the head of the pump drops, which also results in a decrease in axial thrust. Thus, the balance hole design should be optimized not only for normal operation but also for off-design conditions.
Recommended Design Ratio
Research and practical experience indicate that the total area of the balance hole should be approximately 5 to 8 times the gap area of the impeller wear ring (also known as the mouth ring). This ratio helps achieve an optimal balance between axial thrust reduction and efficiency retention, ensuring the split case pump performs reliably across a range of operating conditions.
Conclusion
Understanding the purpose and design of the balance hole in a split case pump impeller is essential for engineers and maintenance professionals aiming to optimize pump performance and longevity. While it contributes to minor efficiency losses, the balance hole is a crucial element in managing axial thrust and protecting internal pump components. For anyone operating or designing a split case pump system, paying close attention to the balance hole’s size, location, and interaction with other components like sealing rings can significantly improve pump reliability and reduce maintenance costs.
