This article was printed in CONTROL's October 2009 edition.
By Tom Pagakis
The scenic beach town of Carlsbad, Calif., just north of San Diego is known for its beautiful beaches, upscale homes and quaint shopping district. The SCADA system employed by the city monitors current status and provides remote control abilities for all of Carlsbad's water and reclaimed water. The key components for these systems are SNAP controllers from Opto 22 (www.opto22.com). The city began using the hardware as an experiment. Years ago, outdated technologies like tone telemetry were in place, and then later the city used a mix of I/O modules and systems from Grayhill (http://grayhilloem.com) and other vendors. Special drivers had to be created to make everything communicate, and the system became very complicated and unwieldy.
The SNAP systems—which communicate with each other and back to their command and control center through a combination of radio, serial and Ethernet links—are used to monitor, control and acquire data, as the City of Carlsbad's Municipal Water Department regulates all water transport, and an independent water treatment plant and controls the chemical dosing of ammonia and chlorine that cleans the water. In total, these activities incorporate more than 1,000 I/O points distributed across the control center and numerous remote sites. The department's responsibilities center on two distinct areas of operations: sourcing, transport and distribution of purchased water from lakes and reservoirs to the city, and reclamation of treated water.
Water Transport and Distribution
Water purchased from San Diego County's water authority is transported through piping to Carlsbad, where it's distributed to businesses and residences. Delivery of the potable water requires little to no pumping, as the water travels via a gravity feed that brings it from storage tanks as large as 9 million gallons located at higher elevations (around 700 feet) all the way down to sea level.
The SCADA system has been carefully configured to control equipment and monitor the water of both systems throughout this entire process. Specifically, analog and digital input and output modules connect to:
- Level transducers that track water levels both in large reservoirs and tanks,
- Pressure transducers in pipes that measure pressure within each zone, and ensure that water is pumped and is flowing efficiently as it's being delivered,
- Flowmeters that measure how many gallons of water per minute are being pumped,
- Pressure-sustaining, pressure-reducing and various other valves that open and close incrementally to adjust the rate at which the water flows.
Currently, the SCADA system encompasses three large reservoir impounds, 14 reservoirs and 20 pressure reducing stations used to monitor more than 500 miles of distribution pipeline.
Significantly, all of the monitoring and data acquisition functions are performed, not by a single (or even multiple) central controllers, but by multiple local I/O processors (or "brains") distributed at pump stations and several other points across the city's infrastructure. These brains communicate to I/O located on the same rack and perform functions normally reserved for an expensive PLC. For example, the brain takes valve readings from the analog input modules, and performs scaling calculations to convert a voltage measurement into a percentage. (Thus, for a typical 4-20mA analog input module, a measurement of 12 volts would result in a reading of 50%.) To have this scaling and engineering unit conversion take place locally at the I/O level relieves the main controller from extra processing, while also giving the control center personnel the convenience of being able to view and interpret easy-to-understand units of measure (for example, percentages as opposed to voltages.)
Distributing intelligence in this manner also provides a safeguard in the event there's an interruption in the operation of one of the main controllers. The brain can be instructed to continue to record measurements and perform calculations, and operators can continue to view this data from the control center's HMI.
The SCADA system's HMI is Wonderware's (www.wonderware.com) InTouch, which is integrated neatly with the Opto 22 hardware, and allows control center personnel to view operational variables and conditions relating to pressure, flow, levels, equipment status and more. The department also uses InTouch to generate bar and line graphs (indicating such things as tank levels, pump status and valve positions) and also for performance and historical trending. For example, the system generates a 24-hour trend for each storage tank along with reports for the city on how much water is pumped each day, week and month.
The HMI is configured to alarm on a number of I/O point readings. By defining a "normal" or "safe" operational state for specific I/O points, if conditions ever deviate from those parameters, the control center receives a visual alarm. The alarms are also integrated into a pager system, so the on-call operator receives a cell phone message, and he or she can acknowledge the alarm immediately and handle any crisis appropriately. For example, as the control center monitors downstream and upstream water deliveries, it receives alarms on any dramatic changes in pressure due to factors such as a fire hydrant being used or damaged—something that occurred regularly when the region recently experienced a number of large wildfires.
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.