Pulp and Paper Canada

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Best practices for aerator maintenance and reliability

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May 16, 2024 in News
By Aqua-Aerobic Systems

Wastewater treatment is a vital component in pulp and paper manufacturing. Many mills are confronted with the need for optimization through energy savings, working with reduced number of employees, and increased stringent water effluent requirements.

Most mills use direct-dive surface aerators in aerated stabilization basins (ASBs) to reduce biochemical oxygen demand (BOD) and keep solids in suspension as the most economical equipment choice.

This equipment often provides benefits like lower capital costs compared to other aeration equipment, no required permanent structure, and no need to dewater the basin for install. They can be used in conjunction with other equipment for supplemental aeration and mixing, and are not limited by the geometry of the basin. They are relatively low maintenance and can reduce energy consumption by incorporating variable frequency drives (VFDs) for turndown capabilities.

Photo courtesy of Aqua-Aerobic Systems.

But, there’s more to aeration than just installing the aerators and letting them run 24-7. A preventive maintenance program can optimize the life of the equipment. Here are some best practices to follow to get the most out of your direct-drive surface aerators, while reducing your operating costs.

Walk-through visual inspection
Surface direct-drive aerators don’t require a lot of maintenance compared to other types of equipment, but proactive maintenance is key to maintaining mechanical integrity for longevity and optimum performance.

“I recommend doing a walk-through at least once or twice a week,” says Loryn Martin, product manager at Aqua-Aerobic Systems. Martin advises visually inspecting mooring lines and electrical cables, as they do stretch over time. When replacing an aerator motor, power section or complete unit, Martin suggests replacing the electrical cable, especially if the electrical cable is several years old.

Surface direct-drive aerators are shipped from the factory with the power section dynamically balanced within two mils peak-to-peak horizontal displacement measure at the upper and lower motor bearing to ensure the impeller is balanced with the motor. However, external forces, such as large debris striking the unit shaft or prop, may cause the unit to become unbalanced. “A visual inspection of the equipment in the pond can alert you by sight if the unit is excessively vibrating and or the sound of bearings failing. If a unit is starting to become unbalanced or excessively vibrate, it must be shut down before the motor is destroyed,” Martin advises. 

Mitigating motor failure
Keeping water out of the motors is also critical. “It all starts with the design of the unit,” Martin says. “Surface direct-drive aerators are an axial flow pump bringing water up the motor shaft.  A key component is a lower bearing isolator selection and performance on the drive end of the motor.”

Aqua-Aerobic Systems’ standard is a lip seal, Martin says. “We have found the lip seal to be a more effective bearing isolator in preventing water from getting into the motor due to direct contact with the motor shaft, therefore resulting in fewer instances of motor failure.”

Martin also suggests a fluted labyrinth seal guard, rather than a flat seal guard, to divert water away from under the end-bell of the motor, and having condensate drains adequately sized and not plugged on the bottom.

When using units intermittently, Martin advises including space heaters or trickle charge heating to keep moisture out of the windings during the shut-down of the unit. “Make sure the units are operating with their intake cones on, as removing these in the field may cause premature hydraulic failure of the unit.”

Reducing energy with VFDs

Photo courtesy of Aqua-Aerobic Systems.

Mills are always looking to reduce energy consumption and ASB ponds require a significant amount of energy depending on the flow, loading and size of the basins. In addition to a preventative maintenance program to keep motors at optimum performance, programming options from variable frequency drives can help with efficiency and energy consumption.

“Programming the equipment to meet peak times and loading is essential,” Martin says. “Having turndown capabilities during the off-peak times is key to lowering electrical usage. This also helps with extending the life of the motors.”

Making equipment “good as new”
When a direct-drive surface aerator’s motor fails, mills must consider their options to get a unit back into running conditions. “Some mills just scrap the motor and purchase a new one or send to an electric motor shop to have the motor rewound,” Martin says. “Many mills have stainless steel floats, which can last more than 25 years. An economical solution is to have a few complete power sections built, balanced and ready to be pulled from inventory to attach to their existing float.”

Aqua-Aerobic Systems offers a program called Good As New (GAN), where customers can send in either the old power section or their existing diffusion head. “It is inspected and built with a brand-new motor, wearables and prop. It is then balanced and tested and shipped back to the mill,” Martin says. “If a mill would like us to hydraulically test the unit before shipment, we have on site test-tanks and can give a report showing the results of amperage, voltage and power draw.”

A proactive approach
Having a preventive maintenance program in place can not only extend the life of the equipment, but also prevent a reactive approach to any issues that may arise.

“Following the routine regreasing schedule of the manufacturer, most mills use a 75-horsepower surface direct drive aerator as a standard,” Martin says. “The most difficult process is not the regreasing, but getting to the unit.”

Photo courtesy of Aqua-Aerobic Systems.

To assist with this, Martin points to a development from Aqua-Aerobic Systems called the Saf-T-Float. “The float itself is its own docking station, with a non-slip coating for walking and standing as well as removeable handrails.”

Another step in preventative maintenance is to monitor prop wear by the motor amperage.  Martin says the extent of prop damage can be estimated by amp draw. “By tagging a new power section or unit in the basin and recording the amperage to get a baseline, an operator can continuously monitor to see how much prop wear there is over time. Props are pitched to run at 88 to 94 per cent of full load amps.”

To reduce costs and improve the reliability of aerators, proactive maintenance, a solid understanding of the aerator’s design, and preparation are key.

 “Ultimately, it’s about keeping things simple for the mechanical, electrical instrumentation and maintenance departments, as well as optimizing operational costs to get the final result of quality effluent,” Martin says.

For more information, visit aqua-aerobic.com

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