Packaged Scada, RTUs Keep Water System Running
A radio-based SCADA system that relies on factory-built, PLC-based Master Terminal Units (MTUs) and Remote Terminal Units (RTUs) has improved operations at the 25,000-customer Chelmsford Water District, Chelmsford, Mass.
Thee MTU and RTU packages - designated RTU-PAKs(tm) - were custom manufactured for each Chelmsford site by the Industrial Applications Group, Schneider Automation Inc., Raleigh, N.C. Included are a primary MTU at water district headquarters, a secondary MTU (at the towns tallest and main water tank) which acts as a radio repeater and backup, and 20 slave RTUs at scattered pumping station, tank, and valve vault sites.
Each fully assembled and wired RTU-PAK consists of a Modicon(r) PLC, discrete and analog I/O points, I/O interposing relays, terminal blocks ready for field and 120 VAC wiring hookup, 450 MHz UHF radio and modem, and a UPS. The components are mounted in a NEMA enclosure appropriate to the environment at each site.
RTU-PAK software includes a Modicon RTU communications function block preloaded into the MTUs plus Modsoft(r) ladder logic programming software. All MTU and RTU logic for communicating with and operating the sites, as well as the Human/Machine Interface (HMI) programming and graphics, were provided by Dynalektrix, Plymouth, Mass., the SCADA system supplier. Dufresne-Henry, Westford, Mass., engineering consultant to the water district, designed the radio/PLC based SCADA system and developed the bid specifications. The design also included the I/O list, operator interface and communications requirements, and the system hardware/ software components.
The new SCADA system replaced proprietary pulse and tone equipment that relayed simple pump status and tank levels via metallic phone lines to a now discontinued pumping station.
The water district didnt want to be locked into sole source telemetering equipment again. The engineering consultant therefore specified that general purpose, off-the-shelf components available from multiple sources be used. The system also had to use open protocols programmable and serviceable by multiple sources.
Monitoring and Control
A PC HMI, located at the water district offices and running a Windows(r) 95 process monitoring and control package, provides the window into the SCADA system. The HMI ties to the primary MTU in an adjacent room over a Modbus Plus network.
Monitoring and Control
Through the RTUs at the districts 11 pumping stations and 16 wells, the HMI operator has full manual/off/auto control of each pump and the associated process via a custom designed graphic display for each site. Included on the graphic are:
Monitoring and Control
- Pump status, fail
Monitoring and Control
- Discharge pressure, flow, totalized output, and pH,
Monitoring and Control
- Continuous well level,
Monitoring and Control
- Pump run hours,
Monitoring and Control
- KOH tank level,
Monitoring and Control
- Data logging and trend charts of all variables on Massachusetts-compliant reporting forms,
Monitoring and Control
- Pumphouse ambient temperature
Monitoring and Control
- Intrusion, flooding, fire, chlorine, pH, eyewash activation, and other alarms.
Monitoring and Control
Robert Doak, Superintendent, Chelmsford Water District, noted that because continuous well level information is now available the water system is much easier to balance. Wasteful pumping also is eliminated, such as satellite wells overpumping into main wells. In one case, satellite pumping has been reduced to a quarter of what it was in the past.
Monitoring and Control
The new SCADA system has reduced manual data logging and number of trips to each site on a daily basis. Other SCADA features, such as historical trending, alarms, and maintenance logs provide the water district with great flexibility in operation and maintenance of the water distribution system.
Monitoring and Control
For tank locations, the HMI operator continuously receives realtime levels and flows, while motor-operated valves in vaults are remotely controlled from the HMI. Valve position feedback is provided. Formerly, operators had to open manholes to open and shut valves.
Monitoring and Control
Because the RTU-PAKs and sensors are backed up by an uninterruptible power supply, all sites have at least 10 minutes power available to transmit power failure alarms and site status to the HMI. The MTUs, HMI, and main tank site additionally have 24-hour battery backup systems to permit them to remain in radio contact with field sites to follow and broadcast tank levels. Tank levels drive automatic control of the entire SCADA system.
Monitoring and Control
HMI update time is approximately 50 seconds for all sites. The primary MTU initiates polling; the secondary MTU actually performs the polling and takes over as the primary MTU if the true primary or its communications fails. The PLC time/date clocks are resetable from the HMI for seasonal time changes.
Critical Alarms Are Broadcast
Every variable in the system has a setpoint and alarm tolerance range adjustable from the HMI. Critical out-of-range variables are transmitted during off hours to a telephone company paging system by an autodialer. The autodialer continuously calls a list of district individuals until the page is acknowledged. To assist service personnel, more than 80 alphanumeric statement groups define failure modes and locations.
Critical Alarms Are Broadcast
Doak carries a laptop with built-in modem to access the new SCADA system remotely. He can troubleshoot and operate the system as easily from home as he can at the HMI. During offhour storms, the superintendent typically connects to the system to watch for signs of trouble, such as loss of power to a pump or rapid drop in the water storage tanks. In the past, low tank levels and water restrictions often resulted from district personnel being unaware of offhour problems.
PLC Logic Gives Range of Operating Choices
Local intelligence in the pumping station RTU-PAK PLCs includes logic to shut down and lock out pumps if high pressure and no flow occurs (valve probably closed), low pressure and excess flow (line break), high pH (excessive potassium hydroxide injection), etc. An operator normally visits a station to reset locked out equipment, although the equipment can be reset from the HMI. Station logic automatically portions potassium hydroxide according to flow and pH sensor feedbacks. The new SCADA systems improved safety in portioning potassium hydroxide, which is injected as a buffer for acidic water, was extremely important to the Chelmsford Water Commissioners.
PLC Logic Gives Range of Operating Choices
Other PLC logic delays pump starts to off-peak utility pricing hours, with overrides assuring immediate pump activation if tanks are low or if levels are falling rapidly. For stations having easily depleted wells, other logic prevents pumps from operating more than a certain number of hours per day during dry seasons.
PLC Logic Gives Range of Operating Choices
Two stations have pump variable speed drives. PLC logic here adjusts pump speeds to keep well levels constant. If desired, optional logic allows the pumps to meet constant flow or constant discharge pressure setpoints. Compound control logic even allows a user-selectable mix of flow, pressure, level, and level rate-of-change to determine pump activation and speed.
PLC Logic Gives Range of Operating Choices
Water tank logic includes alarming if level falls more than one foot in five-minutes, indicating a pipe break, open fire hydrant, etc.