Knowledge popularization: Balance holes of impellers

Thread Content

Balance holes (return ports) are primarily used to counteract the axial forces generated during the operation of the impeller. They help to reduce wear on the bearing surfaces as well as on the thrust disk. When the impeller rotates, the liquid contained within it is forced, under the effect of centrifugal force, from the center of the impeller along the flow channels between the blades toward the periphery of the impeller. Due to the action of the blades on the liquid, both its pressure and velocity increase, resulting in an axial force directed forward. Holes in the impeller are created to reduce this axial force, thereby protecting the bearings and thrust disk and helping to control pump pressure. The extent to which the axial force is reduced depends on the number of holes and their diameter. It is worth noting that the sealing ring and the balance holes complement each other. The disadvantage of using this balancing method is efficiency loss (the leakage rate from the balance holes is generally 2% to 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 cavitation resistance. At non-rated flow rates, the flow state changes. At low flow rates, due to the effect of pre-rotation, the pressure in the central part of the impeller inlet is lower than that at the periphery, resulting in increased leakage through the balance holes. Although the head increases, the pressure in the cavity beneath the sealing ring remains low, thereby further reducing the axial force. At high flow rates, the head decreases, and thus the axial force also decreases. Research results show that achieving better performance is possible when the total area of the balance holes is 5 to 8 times the area of the ring gap. However, it is obvious that the inclusion of balance holes in the impeller of a centrifugal pump reduces the pump’s volumetric efficiency and hydraulic efficiency.