The city of Lake Alfred, FL, faced a dilemma — the town was planning to build 11 new subdivisions and was facing new regulations from the Florida Department of Environmental Protection. With an entire citywide operating budget of $12.3 million, how could it afford a crucial wastewater plant upgrade?
The original plant relied on relays, chart recorders, and a simple illumination-based annunciator. While this had served the town since 1989, it was no longer a viable solution for the future. In addition, the plant needed a better way to monitor and gather data while lowering operating costs.
The original plant upgrade design proposed a main PLC cabinet in the central building interconnected with a traditional motor control center in the blower building using traditional I/O. This required 400-500 wires underneath a roadway in several conduits or a duct bank, a process that would entail expensive and inconvenient road construction.
Ethernet Solution
Darwin Thomson of Unitron Controls®, a subsidiary of Barney's Pumps, envisioned a different scenario.
"We proposed a Model 6 iMCC (Intelligent Motor Control Center) from Schneider Electric that gathers data from itself via the Ethernet network. External data also is collected from a separate panel and transmitted via the Ethernet network. Then, the information gathered from both the MCC and panel is combined and transmitted together via a single Category 5 cable that is routed under the road via an existing spare conduit, so there was no need to tear up the road," said Thomson.
The city and contractor were pleased with this option. It decreased construction time by several days, reduced labor, concrete, wiring, and conduit costs, and also increased data monitoring capabilities.
The installed system consists of the iMCC, three Square D® brand M-Flex™ enclosed drives, a control cabinet in the blower building, and a new annunciator panel in the main building.
The iMCC includes various drives and starters for the plant equipment. The system gathers motor operating data from each starter and drive, and reports this over Ethernet. The same Ethernet cable controls the motors and drives. The iMCC was completely wired and tested before arrival, so the contractor only had to wire the power cables and connect a single Ethernet cable.
The iMCC uses Motor Logic® Plus II overload relays, which allow monitoring of a variety of motor parameters over Ethernet. Plus, it offers inputs for connection to external devices, a feature that Unitron and Lake Alfred found useful because seal fail and thermal contacts could be connected to the network without the need for an additional PLC.
The enclosed drives operate the 50-HP positive displacement blowers in the aeration basin. The drives reduce the motor speed based on the dissolved oxygen level in order to modulate air supply to the exact needs of the process. This modulation helps control electricity costs, lowering the city's operating expenses. These Ethernet-based drives are controlled and monitored in a similar fashion to the iMCC. A Modicon® Momentum™ PLC with two Ethernet switches controls these drives and is housed in the control cabinet.
Finally, the new annunciator panel, located next to the old one, houses a Momentum PLC, a Magelis® iPC panel-mounted computer and an Ethernet switch.
"We decided to use the Momentum PLC even though it is not usually thought of as a process control PLC," said Thomson. "The Momentum PLC had enough power and control capability to handle the tasks required at Lake Alfred while keeping component costs down and panel real estate to a minimum."
Enhanced Data Access
Due to the graphical intensity needed, the team chose 15"-screen Magelis iPC displays for the entire facility. These displays give operators a view of any data point in the facility. Operators can receive information ranging from instrument readings, such as dissolved oxygen, to motor data from each motor, such as current.
In the original plant's control configuration, operators could only see a few process parameters that were logged by a chart recorder or displayed on several digital meters. Motor data was not available. Outside of the chart recorders, permanent information was only gathered by hand written logs.
"With the new system, we can log all data that the state requires in five minutes. Data can be loaded to a data stick that we insert into the front of the iPC terminal, so we no longer have to open the panel door, or incur the cost of chart recorder paper. This makes it safer and less costly to operate than the old system. Also, we can now keep a year's worth of data. We update Unitron Controls on a monthly basis with data that helps ensure our system is working properly," said Rob Robertson, Lead Operator for the City of Lake Alfred.
The design features two separate PLC's, one in the annunciator, and another in the control cabinet in the blower building. Under normal circumstances, the annunciator PLC controls the facility, while the PLC in the control cabinet runs other routines. In case of a communication failure between the two PLC's, the PLC in the control cabinet can assume control, although not instantaneously. While this is not true redundancy, this level of backup is applicable to the needs of the city, and kept project costs to a minimum.
Remote Access Capability
The new system allows operators to log into the PLCs remotely. This means that Unitron Controls can do a brief examination of the situation before sending someone to the plant. For quick resolution items, this saves the cost of a service call.
While the use of Ethernet avoided road construction and sped up the installation, there were some unexpected challenges. An unforeseen piping change required the control system to be completely brought online about two weeks ahead of schedule. This led to challenges in bringing the MCC online.
Unitron Controls looked to the Schneider Electric's Water Wastewater Competency Center for help.
"During start-up we had some challenges. It was not until the Water Wastewater Competency Center got involved that the situation was identified and rectified. I now have these guys on speed dial," said Kevin Perdomo, project engineer for Unitron Controls.
Today, Lake Alfred's Robertson is optimistic.
"During start-up we had our concerns. This was a new technology for the city. The unknowns, plus the challenges of going live early, made us uncomfortable. Unitron has helped us through these challenges, we have created easy to follow procedures. We are now comfortable with the system, the added data it provides, and the ease of operation compared to the old system."
About the Author:
Grant Van Hemert, P.E., is an automation and control applications engineer at the Schneider Electric Water Wastewater Competency Center
