As the computer and process automation world continues to take giant leaps forward, suppliers are challenged to bring advanced functionality to market, support legacy products, and provide a cost effective and plausible transition path that requires minimal effort to commission.
In 1999, the City of Dallas Central Wastewater Treatment Plant presented this problem to various SCADA suppliers. The plant is permitted for a peak two-hour flow of 350mgd. It had a partially functioning, 11-year-old process control system. The original equipment included 49 RTU cabinets housing Bristol Babcock UCS 3380 distributed controllers. These controllers were passing data throughout the plant on over five miles of coax cable to six redundant DEC MicroVAX II mini-computers with more than 40 amplifying repeaters. This system was handling more than 40,000 I/O points. Servicing the older equipment was proving to be too costly, or even impossible in some cases, as replacement parts simply were no longer available.
The Central Wastewater plant also had concerns about Y2K. The Bristol Babcock UCS3380s were Y2K compliant; however the MicroVAX II was not. All things considered, management decided that it was time to upgrade. They received pricing for completely new systems including hardware and PC based HMI systems ranging from $5 to $8 million. Bristol Babcock proposed an upgrade to the current version of the 3380, the 3335 DCS with a PC-based HMI for only $1.8 million.
To facilitate this change and be cost effective as well as functional, Bristol Babcock designed and created mimic boards that connected to, and emulated the old I/O termination boards and had mass termination ribbon connectors to the current Bristol Babcock remote terminated I/O. This saved months of labor-hours rewiring the 40,000 I/O points in the plant. The application programs in the controllers were rebuilt in the newer program interface, ACCOL Workbench, and the I/O was remapped. This process took only days, whereas new code would have taken months to write and then debug.
The next obstacle was the communication network. The project team replaced the existing proprietary network with a redundant star topology fiber optic 10 Mb Ethernet network. The new DPC3335 unit could interface to this Ethernet network as well as the new OpenEnterprise HMI PC-based host that replaced the MicroVAX II system. Screens were created to mirror the older screens for operator ease of transition, and newer screens were also created including screens for troubleshooting I/O points and cards, as well as communications. The OpenEnterprise system has a redundant server and nine workstations. From any workstation, an operator can log in and see the entire process and make operator changes. The HMI interfaces to a SQL server database that allows the plant to generate any needed reports and record all needed plant data.
This system was designed, installed and operational within one year at a savings of $3 to $5 million. According to Steven Plummer, department technology analyst for the Dallas Water Utilities, ?This experience taught us about the value of selecting a vendor with longevity in the marketplace and one who understands the need for generation compatibility.?
