DCS/SCADA System Improves Water Utility`s System Performance
By Gary Durney, St. Louis County Water Co.
Maintaining four individual water treatment plant sites located throughout St. Louis County is no small task for St. Louis County Water Co. However, thanks to a new distribution control system (DCS)/SCADA system, the job is certainly easier than it once was. The Hathaway TIS-4000 Info/Mation DCS/SCADA system installed in 1994-95 allows more flexibility and greater control over the system than ever before. It has made more complete and timely data available to management, aiding in decision making and trouble shooting. Info/Mation has also bridged several platform and protocol differences between process control systems and management information systems (MIS).
St. Louis County Water manages its water distribution system with a frame relay wide area network (WAN), connecting each treatment plant and the distribution load control center (DLCC). The DLCC not only manages the entire distribution system, but also allows operation of any distribution site from any other site. Should there be a failure at any one site or at the DLCC, any other site or the DLCC can back it up while the rest of the sites operate in stand-alone fashion. The system employs Hathaway`s proprietary software across the network, providing access to process information on a real-time basis.
Not only is there now more operating flexibility, the new DCS/SCADA system also allows more complex system control. For example, with the old telemetry systems, tank fills occurred at set times that were controlled by electro-mechanical time clocks and relays in the field. The operator had the ability to hold off fills during set times, but could not start a fill outside of those times. With the Hathaway system, the operator can start a fill at any time, allowing more flexibility in dealing with day-to-day operating conditions. Now instead of reducing plant pumpage rates to match reductions in consumer demand, the operator can create a demand in the system by opening a tank.
Another advantage of the new system is that operators can remotely select the backfill pressure used to fill each tank. The backfill pressure is the system pressure that is maintained by the modulating fill valve. With the old system, an operator had to manually change a valve`s time clock or pressure switch in the field. However, the new system now allows the operator to enter pressure limit set points and the system ensures compliance. Should a set point need to be changed, an individual with appropriate security clearance can make the change through the SCADA system. As a result, changes are made more frequently to accommodate special operating conditions.
Another particularly valuable system feature is that it allows programmable logic controllers (PLC) to be reprogrammed and the modifications downloaded across the network. This was not possible with the old system, which required modifications only through field wiring changes. The new system not only adds flexibility to change control schemes, but also saves many trips to the distribution site. Another advantage of the SCADA system is that more data is monitored, alarmed and collected. Maximum and minimum pressures, as well as changes in equipment status and operator actions are recorded, allowing for better trouble shooting and problem correction.
Hathaway`s Info/Mation technology provides basic plant data in the St. Louis water system, such as high service pump status and flows. It allows the DLCC operator to better track plant changes affecting distribution system operation. Flow data trending has also been automated, making it easier to manipulate the data.
Several network service outages and PLC network failures have been handled effectively with the system`s ability to hand off control to each distribution site`s local control system. In many cases, the DLCC operator was isolated from the rest of the system, but the plant operators were able to take control of and operate their respective distribution sites until the problem was corrected. In other instances, where there was a problem on the leased circuit from one master to the PLCs, the partner master automatically took control. The system`s ability to operate each site independently has also improved hardware and software upgrades without disrupting operations. Network equipment and parts can now be taken down without disrupting normal operations.
The St. Louis County Water Co. DCS/SCADA system consists of the control center, four plant sites and a distributed network of 45 RTUs located at tank and booster pump sites throughout St. Louis County. The control center and the remote plant sites are connected by a WAN based on Cisco Routers and Southwestern Bell frame relay service operating at 56 and 128 Kbps. The WAN provides for multiple routes between systems in the event of a local communications outage. The WAN is connected to the MIS system operating on an IBM token ring network. This connection provides any computer on the token ring access to operational information contained in the SCADA system including archived historical data.
System software consists of the standard operating systems provided with the hardware, Hathaway`s Info/Mation System software, TIS-4000 control system software and database software such as dBase and Lotus. Hathaway provided a special application that allows the PLCs to be reprogrammed and the new logic downloaded over the communications link. No separate communications link or port for PLC configuration is required. This software is now a standard part of the Hathaway package. The DCS/SCADA system at the central control site consists of redundant CPUs, each with three monitors. Both systems are fully functional and capable of independent operation. In addition, there is the usual complement of alarm and event printers. There is also a projection screen system which can display any of the system graphics and is ideal for operator training.
Even though the physical network consists of local Thinwire ETHERNET, phone company frame relay service and high-speed phone service to the MIS token ring, all sites are equally accessible and appear to be on a single local network. This permits an operator at one of the plant sites to call up trend data archived at the central control center as easily as if the data were stored locally. Similarly, an administrator with a PC on the token ring network can access reports and archived data on any of the plant or central control systems. It is even possible to call up system graphics and monitor on-line system operation and system alarms from the administrator`s PC.
The Hathaway DCS/SCADA system RTUs are based on Modicon 984 PLCs. They are designed to handle normal start and stop sequencing based on operator initiated commands. In the event of a communications timeout, the PLC logic is designed to perform automatic control for an indefinite period until communications are restored. In order to facilitate maintenance and changes in control strategy and equipment, the system provides for PLC control program downloading over the WAN to the local RTU.
Robert G. (Gary) Durney is St. Louis County Water Co.`s production and systems operation vice president. He was closely involved with the DCS/SCADA system implementation.
Since the implementation of the Info/Mation DCS/SCADA system, St. Louis County Water Co. has easier control over its four water treatment plants, including Central Plant, pictured here. Photo courtesy of St. Louis Water Co.