Fieldbuses Uplift All Processes

Low-Power, Twisted-Pair Fieldbuses Are Still Replacing Old Point-to-Point Wiring, Delivering Better Data and Using Their Ethernet and Wireless Versions to Support New and Revamped Process Applications Worldwide

By Jim Montague

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In these days of Ethernet, wireless and smarty-pants interfaces, one may suddenly stop and ask, "Whatever happened to all those fieldbuses?" Well, just like any hardworking technology, the major fieldbus communication protocols and their low-power networks of twisted-pair wiring are hiding in plain site, laboring mightily and without fanfare in process applications worldwide. Besides collecting and delivering more sophisticated data than their predecessors, fieldbuses and their own Ethernet and wireless versions continue to replace increasing amounts of hard wiring and gain ground on the huge, point-to-point installed base they've been combating since their earliest days.

For instance, the San Jacinto Tizate geothermal power project is located near Léon, Nicaragua, about 90 kilometers northwest of Managua. The site consists of almost 10,000 acres or just over 40 square kilometers in the Nicaraguan Depression along the 70-kilometer Cordillera de Maribios chain of active volcanoes, and it has an estimated total capacity of 277 megawatts. The San Jacinto project was built in two phases and started up two 36-megawatt power plants in January and December 2012. This generating capacity is needed because Nicaragua has the greatest dependence on oil, lowest per-capita access to electricity and highest energy costs in Central America. But, like any geothermal or other alternative energy project, bringing the plants to commercial operation meant overcoming some unusual problems.

Also Read "Practical fieldbus tools aid predictive maintenance"

The project was originally awarded to Ram Power Corp. in Reno, Nev., and its design engineering firm, Power Engineers Inc. in Hailey, Idaho. Following updated site surveys and technical evaluations, Ram Power and Power Engineers decided to build the two-phase project using single-flash, condensing turbine development, which is designed to be the most efficient and cost-effective method for extracting geothermal energy. After building production wells and supporting steam-field installations, single-flash technology injects high-pressure, high-temperature, two-phase geothermal brine, which flashes into a low-pressure separator. The resulting steam is sent to a Fuji turbine to generate power, and remaining fluid is injected back into the well's geothermal reservoir where it's reheated.

Simplicity and Savings

However, with the long distances between the San Jacinto project's remote steam-field wells and its central control buildings, Ram Power and Power Engineers decided on and designed a decentralized control system to run the plants and opted for a traditional DCS and controllers in a central control room. These controls would communicate via Modbus TCP/IP and fiber-optic Ethernet cabling to FG-110 FF gateway switches from Softing at each of the individual wells. The gateway switches host the wells' remote field devices and their local Foundation fieldbus (FF) segments, and support data exchange between the field devices and the plants' overall control systems. Given the distances between the wells and the plants, these fieldbus-based networks and their control in the field (CiF) functions also save huge amounts of cabling, time and cost compared to old-fashioned hardwiring.

In addition, the FG-110 FFs gateways can each connect up to four FF H1 segments to run as many as 64 field devices, and the gateway and its linking device use Windows-based FF tools to configure individual field devices and define necessary software function block linking and scheduling. Next, the FF configuration, FF Link Active Scheduler (LAS) and FF Time Master cooperate to define and execute CiF.

Finally, San Jacinto also deploys redundant pressure process values to gain more benefits from the gateways' CiF features. Their input selector (IS) function block ensures redundancy, and resulting data is sent to the PID function block to calculate new inputs.

"After researching available options, Ram Power settled on the FG-110 FF gateways, and Power configured and installed them," says Tom McAuliffe, control system engineer at Power Engineers. "There are four FG-110 FF gateways installed in the San Jacinto geothermal well field, and they've been working reliably for the last year. Power also made several recommendations to Softing about how the gateways could be enhanced, and these modifications were incorporated in the next firmware version."

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