Engineers and manufacturers have worked to develop technologies that address common problems with closed impellers.
by Flemming Lykholt-Ustrup (Grundfos)
November 21, 2015

While closed impellers have existed for decades, they have faced issues with vibration, clogging and inefficiency in wastewater applications. In recent years, however, engineers and manufacturers have worked to develop impeller technologies that address these common problems.

One type of impeller allows a non-compromised, free spherical passage throughout the pump. The design improves pump efficiency over traditional channel impellers by as much as 20 percent.

This specific impeller design also tackles the problem of vibration. With a multi-vane impeller, fluid and pressure are more evenly distributed around the complete circumference of the impeller, whereas the pressure distribution is asymmetrical around a single-channel impeller. The traditional and well-known single-channel impeller has the strong advantage of offering high hydraulic efficiency, but it can pose potential problems with vibration.

Image 1. This technology includes a free spherical passage throughout the pump. (Courtesy of Grundfos)Image 1. This technology includes a free spherical passage throughout the pump. (Courtesy of Grundfos)

The Challenge of Vibration

Vibration has always been an issue with impellers, and it presented a significant challenge as the manufacturer's engineers developed the new impeller. In early lab tests, the impeller was efficient and robust, but it vibrated excessively. Vibration can cause premature wear of components, damage to mechanical shaft seals and copper windings, loss of power and even pump failure.

Typically, the acceptable vibration levels for pump impellers vary between 3 and 7 mm/sec. These standards, however, apply to multi-vane impellers and pumps using clean water. No standards exist for wastewater single-channel impeller pumps. As a result, wastewater pumps often face higher vibration levels because of the hydrodynamic characteristics of single-channel pumps, the varying solids and air content of wastewater, and variations in the incoming flow. Although not ideal, the wastewater treatment industry has largely tolerated higher vibration levels. New technologies help change these standards by reducing vibration to levels less than those of wastewater pumps with multi-vane impellers.

Wet Balancing

One specific type of vibration is imbalance, or the vibration caused by rotating machinery. Reducing a pump's vibration level and, consequently, noise level to a minimum ensures the longest life for vital components such as bearings and the shaft seal faces.

The traditional approach to reducing imbalance and running the motor smoothly is to minimize the imbalance in the rotating components and, thereby, avoid vibration. This, however, is not sufficient due to the unsteady pressure forces.

To reduce imbalance in pumps, the manufacturer took another approach: wet balancing. Wet balancing is a statically and dynamically balancing method similar to what is used when balancing the tires on cars. The difference is that for wet balancing, the impeller is in perfect balance when the pump housing and the impeller are submerged in or filled with water. Being in perfect balance under water ensures the optimum operating conditions. In some cases, vibration levels below those known for standard norm motors are met.

Wet balancing involves introducing a deliberate mechanical unbalance to counteract the unsteady dynamic forces originating from pressure fluctuations within the pump. This approach cannot address the full spectrum of unsteady forces but does deal with a major source of excitation, according to Lykholt-Ustrup.

To reduce the basic excitation in the pump, the new impeller combines the unsteady hydraulic forces with the unsteady mechanical forces that come from wet balancing in order to obtain a state of equilibrium. It is common to calculate the forces acting on the impeller when it is pumping—the direct force and the torque—and those obviously change, depending on how much flow is going through the pump. All these forces cannot be made to go to zero. This is something that needs to be balanced out. So, we have made a little cookbook for how to do it—how to put the impeller off balance.

Tests showed a significant reduction in vibration. By introducing wet balancing in its single-channel impellers, the manufacturer reduced vibration levels by 50 to 70 percent. These results mean sturdier and more reliable pumps because the dynamic balancing of the impellers and rotors is essential to prolonging the reliability and functioning of the pump system.