What would it mean to you if you could extend the service life of your floating surface aerators and mixers, perhaps doubling their effective lifetimes? What if, at the same time, you could significantly reduce the operating and maintenance expense of these same units? It turns out that both of these benefits can be realized by focusing on an operational quality that often goes overlooked: vibration.
Excess vibration: Myriad causes, bad outcomes
Excessive vibration can stem from a great many factors. Bad motor bearings can allo
w excess shaft movement. An unbalanced propeller, in which the pitch or rake of the blades is asymmetric, can lead to an unbalanced hydraulic flow through the volute of the mixer or aerator. A motor shaft that is bent, even very slightly, will also cause vibration. Most often, these kinds of root causes co-occur and reinforce one another. An asymmetric propeller will lead to turbulent flow through the volute, causing excess shaft movement, which can lead to bending of the shaft over time. A bent shaft will vibrate the motor bearing, applying axial force between the ball bearings and the raceway. This shows up as telltale "false brinnell" marks on the raceway and can lead to improper or incomplete rotation. This prevents the correct formation of the lubricant film, which can lead to raceway wear, allowing for further improper motion and damage. Like dominos, a problem with one part of the complete system will spread rapidly, taking years off the life of the equipment.
This kind of damage can also dramatically impact equipment performance, even before it requires complete replacement. An asymmetric propeller and the resulting turbulent flow can increase wear on the propeller blades, causing them to fail prematurely. Worn propellers are less effective at both mixing and aeration, reducing process performance and raising the risk of missing effluent requirements. This sort of turbulence can also increase the load on the motor, resulting in increased energy usage. In short, this kind of vibration can lead to headaches and heartburn long before it leads to complete replacement.
Simple steps to reduce vibration
The good news is that there are a number of simple and critical steps that can be taken to reduce vibration or catch and correct vibration precursors prior to installation. The first is establishing a quality cutoff for motor runout, evaluating the runout of every motor received and rejecting any that demonstrate eccentricity beyond the accepted limit. Once an acceptable motor has been identified, it should be mated with a propeller that has been measured and verified as acceptable in terms of symmetric pitch and rake, but even this is not sufficient. The very best way to confirm the propeller characteristics that will be seen in the field is hydraulic verification, or "wet testing." Only by testing a specific motor/propeller combination in a test tank can accurate power consumption, vibration and turbulence characteristics be obtained. This also means that propellers and motors should be replaced in tandem, as without the hydraulic testing there is no real way to know if a good motor and a good propeller will remain vibration-free when in combination. Finally, whenever possible, influent screening should be utilized to prevent stringy, fibrous material or "rags" from entering the basin. These can accumulate on the propeller over time, leading to vibration and propeller/bearing damage even when all other causes have been checked for and eliminated.
Excess vibration resulting from lower- quality motors and propellers, damaged bearings or bent motor shafts can dramatically reduce both the performance and service life of aerators and mixers, causing increased capital, operating and maintenance costs. Fortunately, a few simple steps can go a long way toward minimizing vibration to the greatest extent possible, dramatically extending equipment life and preventing process problems with a minimal up-front investment.
For more information, contact Erich DeLang at (815) 639-4562 or edelang@aqua-aerobic.com.
