Powerful PAC Upgrades Wastewater Treatment Centrifuge
Since its original construction in 1952, the Interceptor Sewer System has served Chattanooga and surrounding metropolitan areas. The Moccasin Bend Wastewater Treatment Plant (WWTP) is part of this system and includes the liquid handling, solid handling, pump stations, and plant maintenance divisions. The plant receives an average flow of around 50 million gpd (gallons per day), and has a peak treatment capacity of 220 million gpd.
The facility’s two centrifuge units remove water from the thickened sludge produced by primary clarification, anaerobic digestion, and activated sludge processes. The sludge is fed into the centrifuges at a concentration of 2% or less solid material. The centrifuges discharges de-watered material, which is combined with lime and a polymer additive to raise its pH and achieve desired stabilization, while preventing the growth of harmful bacteria.
The solids concentration of de-watered material and lime is around 30%. This mixture is applied to farmland to supply nutrients for nonfood crop production. Removing water from the sludge makes the bio-solid material easier to handle in the plant, transport to farms, and apply over farmland.
We needed to upgrade the control system for a number of reasons. The older control system was installed in the mid-1990s and consisted of a programmable logic controller (PLC) and a CRT-type HMI to control both centrifuge units. We had already replaced the HMI once in the past five years, and the current unit was beginning to show early signs of trouble, so we were faced with a choice: purchase a new HMI, or construct a new control system with updated and enhanced functions for each centrifuge.
After evaluating PLCs, PACs (programmable automation controllers), and HMIs, we selected, designed, and installed a PAC-based control system. The Productivity3000 PAC is a compact logic controller that combines the features and capabilities of a PC-based control system with that of a typical PLC. We also selected a C-more HMI. Both the PAC and HMI were supplied by AutomationDirect, a supplier of controls and automation components.
We decided to replace the failing HMI and the older PLC with the PAC and HMI before the CRT-based HMI failed completely. If it failed, there would be no way to operate the two centrifuges. We chose to install one HMI and one PAC per centrifuge, instead of one complete system to control both units as with the existing automation system.
This would allow us to take one unit completely out of service for maintenance without affecting the other unit in operation. It would also give us greater flexibility as one centrifuge could now act as a redundant backup for the other.
Our operators start the centrifuge, monitor it during operation, and control production through it by using the HMI and PAC. The PAC system monitors sludge flow into the centrifuge as well as bearing temperature, centrifuge vibration, and motor current.
Monitoring these parameters enables our operators to judge how well the material is being de-watered, and to watch the solids loading. This constant monitoring is critical as the solids concentration can change throughout the day and week. Our operations staff makes flow rate adjustments to the material fed to the centrifuge based on solids concentration to maintain optimal final bio-solids quality.
The control system allows the operator to change the feed material flow rate, and the flow rate of the supplemental lime material which is used to aid in de-watering. The operator enters flow-rate percentages via the HMI touch panel. The PAC adjusts the VFDs (variable frequency drives) to control pump speeds according to flow rate values.
An RTD inserted through the bearing retainer and in contact with the stationary bearing race measures the centrifuge bearing temperature. A transmitter converts the RTD output into a 4-20 mA signal. The bearings are pressure-lubricated, and if the temperature rises to a preset level, an alarm sounds. If the temperature continues to increase, the centrifuge is shut down to prevent bearing damage.
We use a magnetic flowmeter to measure sludge flow into the centrifuge. The HMI displays this flow rate, and the PAC totalizes the flow to provide per-shift production metrics. The operator adjusts the flow at the HMI touch screen, and the PAC in turn controls the pump speed via the VFD.
The PAC also monitors vibration. The plant’s solids operations supervisor and the maintenance staff review the data to determine when the centrifuge requires repair, extensive cleaning, or overhaul. PAC alarm functions include bearing temperature, lube oil temperature, drive motor current, conveyor system status, and lime addition. Table 1 lists all of the centrifuge parameters monitored and/or controlled by the PAC.
We were able to replace a single system that controlled both centrifuges, with a PAC and HMI for each individual centrifuge. Including spares, for each PAC there are 156 I/O points, 128 discrete and 28 analog. Each PAC has five 16-point ac input cards, three 16-point relay output cards, two 8-channel analog input modules, and two 6-channel output modules.
We designed the new centrifuge control system in-house. While starting up and commissioning the system, we verified each signal and function. We found that many of the features of the existing controls had quit working, requiring the operations and maintenance staff to find workarounds. We were able to performs repairs and restore these features during startup.
The original system had an auto-sequence feature that started the centrifuge without operator interaction, but we never fully used this feature. Our operators never felt comfortable allowing the process to start without the operator being in full control of the equipment and each step of the process. Now that we have means to change the PAC logic, we can make the auto-sequence more flexible and user-friendly, so that we can begin to use it with more confidence.
The Productivity Suite software for the PAC took some getting used to, particularly the tag-name creation. I was used to addressing the program elements manually, but the software takes care of this for you. All the programmer has to do is provide a name for the memory reference. Once I got used to this, program development became much quicker.
We chose this PAC because of its modular I/O, powerful CPU, and flexibility. We feel that the processing power and flexibility of the PAC made it a great choice over other controller offerings. The on-board memory and processor speed were large pluses, especially at this price point. Table 2 summarizes our rationale for choosing this particular brand of PAC.
We use many AutomationDirect DL-06 PLCs in other applications. These PLCs offer a block of inputs and outputs combined with a processor and power supply. This configuration works fine for small I/O-count applications, but on larger installations such as the centrifuge, the more modular and expandable PAC is a better solution.
Cost was a motivating factor as well. I estimate that the PAC system implementation saved us several thousand dollars over our other two options. One option was to order a replacement HMI and have a contractor install the original applications. The other option was to use another brand of PLC or PAC to build a new system.
While price was a large part of the decision, we were also particularly interested in the data-logging feature, VFD integration, and e-mail capabilities of the prospective new control system. Although we have not implemented the data logging feature or the e-mail function, we plan to in the future. VFD integration is being used now, and it’s a much needed feature for our application.
What we can’t quantify is the time savings we have realized in being proactive and installing hardware to operate centrifuge units individually. If we hadn’t done so and if the HMI failed, we would have been out of service for many weeks awaiting a replacement unit.
Operational and maintenance cost savings attributable to the new PAC system haven’t yet been documented. As far as up-front costs go, we do know that the PAC hardware, enclosure/sub-panel, HMI, Ethernet switch, dc power supply, wiring, and terminal blocks cost slightly less than just a replacement HMI from the vendor of our old control system.
In addition to the PAC’s modularity, power, flexibility, and affordability, we also greatly benefit from its scalability. We know that we can accommodate any additional features that might be added to the centrifuge to comply with EPA regulations, to improve operations and automate manual functions.
We were able to get the hardware we needed quickly, and with the short lead time, we know we can install any required replacement hardware without a long wait, minimizing downtime.
Room to grow
The PAC allowed us to upgrade our centrifuge controls quickly and easily, and it will allow us to implement future control projects. We intentionally designed the system with about 30% spare I/O points for future needs. These will likely include data logging functions to track productivity, along with using the PAC’s ability to support multiple PID loops. Currently, all control functions are open-loop and require regular manual operator intervention, but we want to automate these repetitious operator tasks. Towards that end, we’re looking at our manually controlled sub-processes and planning ways to automate them.
For example, the sludge and polymer feed rates are currently controlled manually. We’ll likely convert these open-loop controls to closed-loop functions in future revisions. Closed-loop control of these feed rates will allow us to compensate for pump wear and process variations.
We also expect to add closed-loop control for lime dosing, which will save lime costs and improve product quality. Lime dosing is currently adjusted manually at a local control station. Other plans include automating the centrifuge startup sequence and automatically distributing de-watered cake into the truck or transport container.
The upgraded centrifuge control system has met all of our expectations, and the supplier has lived up to its promises. We plan to make use of more of the PAC and HMI features in the future, further increasing the value of the automation system upgrade.
Jeremy Poarch, P.E., is plant engineer at Moccasin Bend WWTP, Chattanooga, TN.
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