A radio repeater is air-lifted to Cat Mountain in a Pima County. Repeaters include the pole, enclosure, solar panel, regulator, battery, cabling, and antenna. Photo: Richard Sloan.
Identifying alarm conditions
Wastewater plant operators are notified of a blockage after seven consecutive violations sound. Source: Richard Sloan
Human Machine Interface
Aided by software consultant Software Mechanics Inc., Tucson, Ariz., the Pima County Wastewater Management Department developed a data processing and human-machine interface (HMI) software to allow personnel to access the system when away from the plant.
Secure Internet access to the software allows employees to review data right away when an alarm sounds. This is extremely handy in cases of alarms during the typically unmanned third shift, and allows workers to remotely determine if an event is a real alarm or an instrument failure. This avoids deploying personnel to investigate a conveyance problem that may not exist.
“This system has provided our department with a better understanding of the dynamics of our sewer system,” says Mike Bunch, deputy director of conveyance with the Wastewater Management Department. “Our capacity analysis, inflow and infiltration issues, and emergency responses have all benefited from the data and alarms that the system provides.”
The software's main screen shows conveyance system status, where recent alarms are displayed, and any planned, current, or completed conveyance system changes. These are useful whenever alarms are sent and a user wants to check if the alarm is valid or may have been caused by an operational change in the conveyance system (pump-arounds, diversions, etc.). All operational activities that result in flow changes are coordinated with the flow monitoring section so alarms may be adjusted accordingly before the flow changes are implemented.
In addition to serving as a process monitor and alarm system, the software also serves as a data archive and allows users to recall historical data for any meter. The user selects the meter and the time-frame; the information's transmitted to the user via an e-mail attachment in a comma-separated format for easy import into spreadsheet applications.Payment And Payoff
The cost of a SCADA system installation depends on the size of the conveyance system to be monitored and the desired level of meter accuracy. A good rule of thumb is to plan for one meter for every 5 miles of major lines. Meters also can be located in areas of extensive growth and places that historically have had flow problems.
Solar-powered site monitoring locations range from $7000 to $10,000 per site, installed. The range in monitoring location costs arises primarily from using either a highly accurate open-channel meter or a less costly simple level-only instrument.
Typically, hardware for a central control center is about $10,000. The main control center components include a central radio and a data server with battery backup. Network administrators advise tape and CD backup or multiple hard drives in a redundant array of independent disks configuration. This system should securely connect to the Internet so personnel can quickly learn about alarm conditions.
As an alternative to developing SCADA software systems in-house, wastewater utilities can use commercially available software like Wonderware, from Invensys Systems Inc., Lake Forest, Calif., to monitor flow conditions and transmit alarms. A 500-tag Wonderware development license retails for about $3300.
Radio repeaters may be needed to reach points in hilly or more remote areas. Repeater locations are solar-powered and cost approximately $6000 installed per repeater site. Since northern states may not get as much sun as southern states, solar-powered systems may not work as well in some locations.