Goodbye Guesswork

Prior to completion of the Fox Metro Water Reclamation District (originally named the Aurora Sanitary District) in 1929, waste from the surrounding community was discharged directly into the Fox River. Since then, Fox Metro has met the needs of the expanding community through innovation, forethought and technology. Today, Fox Metro is a highly efficient facility capable of routinely treating 42 million gal of wastewater per day and serving more than 250,000 people.

Prior to completion of the Fox Metro Water Reclamation District (originally named the Aurora Sanitary District) in 1929, waste from the surrounding community was discharged directly into the Fox River. Since then, Fox Metro has met the needs of the expanding community through innovation, forethought and technology. Today, Fox Metro is a highly efficient facility capable of routinely treating 42 million gal of wastewater per day and serving more than 250,000 people.

Upgrade Woes

The plant was recently faced with upgrading some of its aging equipment and turned to engineering firm Walter E. Deuchler Associates. The main pump building housed three constant-pressure piston pumps that maintained flow of primary and thickened sludge to other parts of the plant. The sludge was being pumped 1,800 lateral ft through pipes originally rated for 150 psi.

The piston pumps could not maintain constant pressure in the pipes, so the pressure would often peak around 100 psi, causing breaks in the shear pins of the piston pumps. Evidence of these breakages can still be seen on the ceiling some 25 ft above where the pumps are located, a testament to the severity of the maintenance headaches they caused.

The pumps had reached the end of their useful life, and the Fox Metro treatment plant began seeking a replacement for the existing configuration that would offer greater reliability and reduced maintenance requirements. Deuchler Associates recommended replacing one of the piston pumps with a rotary lobe pump connected to a 125-hp VLT 8000 AQUA VFD. The drive, which is housed in a NEMA 4X air-conditioned enclosure, runs the motor based on feedback from a Danfoss Drives pressure transmitter and pressure switch near the outfeed of the pump.

How They Work

“We can now maintain an optimal flow rate at around 55 psi,” said Ryan Cramer, the project engineer at Deuchler Associates responsible for this project. The pressure transmitter communicates with the VFD to ensure that the pressure in the discharge piping does not exceed 90 psi by allowing the VFD to speed up or ramp down accordingly. A Danfoss pressure switch also lets the drive know if the system reaches 120 psi, indicating a blockage. In the event that pressure reaches this level, the drive will shut down the pump to prevent damage to the piping. The drive can then slow or stop the pumps to prevent damage to the pipes.

“The drive takes all the guesswork out of it,” Cramer said. The implementation of the new equipment was so successful in reducing maintenance requirements that the remaining two piston pumps were replaced with identical setups.

Attached to each VFD is a control panel that contains the timing relays and alarms that control the sludge pump sequence. When the level of sludge in the wet well triggers the control panel to begin pumping, sealwater is initiated to pressurize the mechanical seals in the sludge pump. After a timing relay times out to ensure that the sealwater is fully pressurized in the seals, the drive begins rotating the sludge pump motor to run full speed or maintain 90 psi, whichever comes first. The sludge in the wet well is lowered to a level that triggers the drive to stop the motor. As the drive slows the motor to a stop, another timing relay begins to ensure that the sealwater remains pressurized until after the sludge pump has stopped pumping.

Each of the sludge pumps has been supplied with a variety of alarms: high discharge pressure, low sealwater flow, high vibration, low current and VFD failure. A lockout wired into the control panels allows only one sludge pump to operate at any time to prevent overpressurizing and damaging the main discharge piping.

“Because only one sludge pump can run at a time, this is not a lead-lag system,” Cramer said. “One lead pump runs for an entire week, then the next week a different pump is set to be the lead pump and so on. We have seen that the new peak flow rates are high enough that a lag pump is not needed and one operating pump has more than enough capacity to perform sufficiently.”

Bonus Benefits

In addition to reduced maintenance, VLT 8000 Series drives also provide greater flexibility to the system. “The VFDs give us a measure of control that we didn’t have previously,” Cramer said. With new lobes, the pumps can be run at slower speeds, he explained. “The drives currently run the motors at 45 Hz, but as the lobes wear, we can increase it to 60 Hz to match the existing flow rate.”

The drives also provide a 30-second initial ramp of the motors to pressurize the system more gently.

One of the secondary benefits of the new system is the reduction of risk of injuries to maintenance personnel. The old piston pumps required regular lubrication of their moving parts. The lubricant made the floor extremely slippery, increasing the likelihood of slips and falls. With a more stable and reliable system, maintenance personnel are safer and can focus their efforts elsewhere, and the Fox Metro treatment facility is better able to serve its community.

Chad Larson is communications editor for Danfoss Drives. Larson can be reached at 800.621.8806 or by e-mail at salesinformation@danfoss.com.

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