Radio-based SCADA System Links 100 Lift Stations
Albany is a city of 78,000 people located on the Flint River in the southwestern part of the state. The collection system captures and sends to the city's one treatment plant about 20 million gallons a day (mgd) of wastewater. The network contains more than 80 unattended lift stations scattered about the city. In addition, 18 similar stations are devoted to storm water management. Each station operates with three or four pumps.
Revere Control Systems, Inc.E="1">Competitive Proposals in Two Stages
To secure a variety of alternative concepts and preserve flexibility in selecting a vendor for the new system, the Albany staff requested proposals based on general requirements, rather than requesting bids on tight specifications. Six contractors from Alabama, Florida and Wisconsin were invited to send representatives to an initial joint meeting and a tour of the facilities early in 1996.
Based on the responses and on further meetings with all competitors, the city refined its requirements and requested a second round of proposals. After review, <%=company%> of Birmingham, AL, was selected to design and install the new system.
Revere's concept uses Motorola MOSCAD remote terminal units (RTUs) with two-way FM radio communication channels. The firm was not the only contractor whose proposal centered on incorporating the MOSCAD equipment. Reasons for the final selection included the projected cost-effectiveness, and the good fit with requirements. In particular, the Albany team had been satisfied with the PLC-based controls provided by Revere for a recent upgrade and expansion of the wastewater plant.
Work on the new SCADA system began in July of 1996 and was finished in about six months. Since then, more RTUs have been added as new lift stations were constructed.
According to Albany's Superintendent Larry Wert, the staff decided to install "a sort of bare-bones installation to begin with," but they wanted the capability for expansion. He said the program and equipment now in place have the capacity to accomplish much more than what is being asked of them at present.
Revere Control Systems, Inc.E="2">An Optimal System for the Present
Most of the lift stations in the collection system have centrifugal pumps, but a few use air eductors. Several handle lifts of more than 20 ft. At some stations (see photograph) centrifugal pumps and motors are submerged in the wet well that feeds them, and are mounted on rails for removal and servicing. Thus the adjacent dry well equipment space contains only manual valves.
At other stations, centrifugal pumps are mounted at the bottom level of the dry well. Some are submersible units, and others have vertical shafts extending to motors mounted above the maximum floodwater level. Motor controls are located in weather-resistant enclosures, or in a building over the dry well.
Regardless of such variations, five discrete (on-off) signals are monitored at each station as follows:
- Wet well high level from a float switch near the top, indicating that the well may soon overflow.
- Dry well high level from a float switch at the floor of the lowest level, indicating that a space is beginning to flood.
- An open door or hatch, indicating that the space is beginning to flood.
- A key-operated switch, indicating that an authorized person has entered the building or dry well.
- AC power failure, indicating that the radio RTU is operating on emergency battery backup.
In addition, two stations pumping combined wastewater and storm water have electrically actuated gate valves to divert the water to the nearby Flint River during a flood emergency. The RTU at each of those stations had to provide two more discrete inputs to monitor gate position switches, and two discrete outputs to open and close the gate. However, those gates are no longer used, and combined sewers are being separated.
Revere Control Systems, Inc.E="3">Flexibility and Room to Grow
The central communication processor is a Motorola MCP-M located in the wastewater treatment plant. It communicates via a Modicon, Inc., MODBUS connection with an Intel Pentium-based PC used by operating personnel in the main building.
The computer is dedicated to a SCADA host and operator interface system configured by the contractor in U.S. Data's Factory Link under Microsoft Windows NT software. Via an Ethernet connection, the same Factory Link system is accessed by a similar computer located in a nearby maintenance office. That computer is used by personnel of <%=company%>, who operate and maintain the treatment plant and lift stations under a contract with the city.
The RTUs report changes in monitored variables soon after occurrence. In addition, the central unit polls all remotes every 15 to 20 minutes to verify communication.
One major advantage of the Motorola RTUs is that they have a "store-and-forward" capability for relaying messages to and from designated peers. Thus, for example, having an RTU near the top of a hill can save the expense of a relay station for other RTUs in a valley beyond.
The MOSCAD units act as programmable controllers (PLCs) with built-in two-way radio communication capability. They can readily be expanded in modular fashion to incorporate additional discrete and analog signals representing such parameters as flow rate, motor temperature, and vibration. That was a major requirement called for in this application. The RTUs can also handle a variety of intelligent local control functions that might be implemented in the future, such as periodic actuation of ventilator fans. The network can accommodate up to 246 RTUS.
U.S. Filter's site manager Brian Johnson summed up the project this way. He said, "Any kind of power surge or transmitter lock-up would take our old system out for a while, and it was hard to fix because everything was proprietary. We were looking for something we could depend on, with off-the-shelf parts. The new system fits our profile for now, and it's flexible, so we can expand it over a period of time."