Whenever a wintry cold front whips through metropolitan Pittsburgh,
employees at Equitable Gas Co. go into action quietly behind the scenes.
They click a few buttons on their computer screens to start up the engines driving the reserve compressors that will force more gas through underground pipes into 275,000 homes and businesses in southwestern Pennsylvania and north central West Virginia.

Their response to such surges in demand for natural gas is immediate, thanks to a supervisory-control-and-data-acquisition (SCADA) system that lets them monitor and control the utility’s assets throughout the city. The company installed the system a few years ago, thereby joining the growing ranks of utilities and other companies that are relying on SCADA for managing assets spread over large geographic landscapes.

Automating the compressor stations to give the operators remote control over the delivery apparatus is an important part of the utility’s operating strategy. “We make money by pumping gas,” explains Wan Jani, director of compressor engineering. The company incurs an opportunity cost whenever its response lags behind demand.

In the past, teams of operators at each of the 10 main compressor stations oversaw the compressors manually around the clock. It could take as long as three hours for them to notice a surge in demand and to dispatch technicians to start the 3,000 to 4,000-horsepower engines. Conversely, it could take just as long to shut the engines off when demand waned, or in cases when the company needed to halt the flow until its crews could solve a problem, such as a burst pipe.

To eliminate this lag, Jani automated the engines and compressors with programmable logic controllers (PLCs) and the WinCC SCADA human-machine interface (HMI) from Siemens Energy and Automation Inc., in Alpharetta, Ga. All of the process sensors, electric actuators and other devices in the system plug directly into a Profibus network, which eliminated much of the wiring to the PLCs.

This simplification was crucial because management wanted to minimize the time that each of the 32 compressors was out of service. Management, moreover, gave Jani and his team only 18 months to automate them and to install the SCADA system. “We wouldn’t have been able to wire all 32 to the PLCs with a conventional wiring scheme in just 18 months,” says Jani. Each engine was out of service for only one month, rather than three.

The payoff was that the company can respond more quickly with fewer people. “We had about 85 controls people working in the compressor stations before SCADA,” says Jani. “We were able to cut that number to 35.” In addition to displaying the status of the gas delivery system in real-time data, the software generates alarms when something goes wrong, sending messages to the appropriate people by computer and cell phone.

Enabling technology

The chief enabling technologies for this kind of wide-area SCADA have been advancements in security, information storage and retrieval at the remote locations, and Web enabling, according to Phil Aponte, HMI SCADA product manager at Siemens. “Using our Web-client function, Equitable Gas can publish to the Web and make certain components available for remote access by individual operators,” he says.

Advances in distributed intelligence also have helped to encourage greater use of SCADA over large geographical areas. “Critical tasks such as high-speed counting and PID (proportional integral derivative) control can be distributed at the I/O (input/output) level,” says Arun Sinha, director of business development at Opto 22, a Temecula, Calif., automation supplier. “These are time-critical tasks that would otherwise have to happen back at the control room.”

These benefits attracted the attention of McMinnville Water & Light, the small utility serving the 25,000 people living in and around McMinnville, Ore. The company’s old SCADA system was not monitoring its grid and evaluating the effects of corrective actions on power quality in real time. The dial-up data modems and leased phone lines that it was using to communicate with its six substations were too slow in transmitting crucial information between the PLCs and the operations staff. The system, moreover, was capturing only a portion of the pulses from meters used for calculating bills.

To improve the system’s response and reliability, the company installed a Snap Ethernet I/O control system from Opto 22 and strung a fiber-optic line to link the substations with the main office. “With our extended wide-area network, we can look at just about every field device connected to it without our having to leave our desks,” says Jon Spence, the technician at McMinnville who does all of ...