The Opto-based SCADA system was also programmed to trigger an alarm that is passed on to the control center HMI when valves malfunction, or when a failure occurs in any of the variable-frequency drives (VFDs) that regulate the pumps.
The VFDs the department uses are driving pump motors with large power loads, and these are subject to voltage spikes and unusual waveforms. To deal with this, they were wired up to analog inputs to continuously monitor our VFDs' hertz and motors' RPMs. That way, if either one of them goes, operators know right away.
The strict monitoring and alarming is due in part to the water district's contract with the City of Carlsbad for how much water can be sourced, as well as when and how fast it can be pumped. The city is contracted for seasonal delivery of reclaimed water per day, so the Opto 22 control programs call for the controllers to shut down pumps as soon as this volume is reached.
Furthermore, due to rates imposed by the local power company, San Diego Gas & Electric, the optimal time of day to pump the water is late at night. So, although the system has been programmed to monitor the city's water supply levels and begin automated pumping whenever readings drop below a certain point, a condition written into the program will always delay that pumping until 10:15 p.m. and cease pumping at 5:50 a.m.
Figure 5. The systems generates a 24-hour trend for each storage tank and reports for the city on how much water is pumped each day, week and month.
Water reclamation is a process by which water and sewage from homes and businesses is brought to a treatment plant where it's cleaned using biological and chemical processes, and then returned to the environment. All of the sewage for six nearby cities goes to an independently operated treatment plant monitored from the control center.
The SCADA system can monitor the treatment plant's operations—particularly wet well levels in lift stations—and other remote equipment. Using level transducers, we monitor reclaimed water storage tank levels, and track how much clean water has been output and is currently available. The system was configured so that, whenever the city's water supply starts to dwindle and tank readings reach a certain level, the controllers send analog and/or digital output signals to partially or fully open valves and start pumps. This allows reclaimed water to be brought from the treatment plant, combined with other-sourced water and then distributed. The SCADA system controllers are carefully configured to pump and "mix" water from different sources this way, so that all the stored water in lakes and more than a dozen different storage tanks can be circulated, and water never "stands" in any one place for too long without being refreshed.
Finally, our SCADA system is used for a small amount of water treatment. The system is connected to Hach (www.hach.com) sensors and analyzers that monitor water properties. The system then regulates chemical dosers that inject chlorine and ammonia into the water as needed.
The SCADA system receives input signals from assorted infrared devices, motion detectors and reed switches outfitted on door frames—all of which are used to ensure there is no unauthorized entry into lift stations, booster stations or any other facility. The department plans to add video surveillance to these intrusion-monitoring activities, and it is also closely tracking the development of a proposed desalinization plant that could provide an additional 50 million gallons of drinking water per day for distribution.
Tom Pagakis is a SCADA technician for the City of Carlsbad Municipal Water Department.