Gas-Fired Cogeneration Plant Replaces Aging DCS

Background
Ripon Cogeneration, a subsidiary of Veresen holding company, sells power to one of the nation's largest utilities, helping supply energy to homes and businesses throughout most of Northern and Central California. The turbine at the 50-megawatt-capacity plant, located in the San Joaquin Valley, also generates steam that is utilized for the production of Distilled Water that is then used at the adjacent process manufacturer.

In 2012, owners of the 25-year-old plant decided to retire their obsolete distributed control system (DCS).

“Our aging system could no longer maintain performance and it was too costly to repair,” said Brett Weber, operations and maintenance manager, Ripon Cogeneration. “Also, very few people even understood the legacy system anymore, and we had limited vendor support. We had to do something.”

Challenge
The major problem: Multiple fail-safes were hard-coded into the DCS, so even small deviations in process variables – for example, temperature – would trip the entire system offline and force the plant to shut down. Operators and technicians could only react to the safety trips, which occurred much too often.

Re-starting the gas turbine and the critical subsystems that supported it – including the chiller, reverse osmosis and distilled water plant – required about an hour each time the plant shut down, cutting into the plant's production.

The multiple startups also put the plant in danger of exceeding its emission limits. Just as a car expels more exhaust when a driver cranks the engine, power plants emit more combustibles during startups. Power-plant emissions and other potential environmental impacts are closely regulated in California. In all, Ripon operates under permits from 12 different regulatory bodies that monitor everything from groundwater quality to ammonia levels around the plant.

Operating with a nearly antiquated DCS was complicated by the lack of local support for repairs or expert maintenance.

“Every time there was a major issue with the system, we had to wait for support, sometimes up to three weeks,” Weber said. The maintenance and repair issues inevitably became more frequent, more costly and more time-consuming.

To top things off, in 2012, Ripon completely lost its process historian when the plant server died. In addition to a failing DCS system, Ripon lost all of its data. Just fixing the server and historian would have been a $20,000 project with no guarantee that the data would be recovered.

In addition, daily production reports were being developed manually – plant operators would record data from the control-system meters and manually transcribe data from Web pages, insert the data into a production report, and send the report to the vice president in a remote location. The manual report was taking one to two hours every day to produce.

“When the new COO of our company visited our plant and saw what we were working with, that was the deciding factor,” Weber said. “It only took him 30 minutes until his decision was made – to approve the plant's recommendation to put in a new control system with an integrated information solution.”

Solution
The new system includes an information-enabled, scalable, multidiscipline control platform that combines process and safety control with communication and state-of-the-art I/O. The system is equipped with 750 I/O points and is able to collect up to 1,000 points of process data. The plant's old proprietary network was replaced with EtherNet/IP™, allowing easy installation of the new system and smooth integration with the existing plant subsystems.

To eliminate the plant's error-prone, manual data-collection process, the PlantPAx™ system includes data historian software, as well as a visualization, analysis and reporting portal that provides role-based dashboards with real-time insight into production. The supervisory-level visualization capabilities included within the PlantPAx system provide operators with optimal insight into production information and diagnostics perfectly incorporated into Ripon's re-designed and upgraded operator control room. The PlantPAx system will leverage all historical data from the process system and automate daily production reports, allowing plant operators to focus more closely on system operations than manual reporting each day.

In the past, Ripon was unable to fully understand what was causing system aberrations. Case in point: the plant's gas-turbine scenario. There is a specific control point in the gas turbine, referred to as T2 – turbine inlet air temperature.. The PlantPAx system allows operators better insight into T2 temperature and what affects it, so they can proactively respond to changes, improving productivity.

Results
The PlantPAx system provides a single, cohesive, open communication protocol and has enhanced overall plant performance at Ripon. Project benefits include:

• Significant reduction in shutdowns – Nuisance fail-safes are down nearly 90 percent with the new system.
• Regulatory compliance – Because unplanned shutdowns are few, the plant starts up just once per day, keeping its emissions well within check. “Implementing the PlantPAx system has indirectly helped us comply with all other regulatory issues because the controls are automated and operators can monitor conditions in real time,” Weber said.
• Reduced startup time – Instead of needing an hour to re-start the plant, the new system takes approximately 45 minutes.
• Controllable process variables – Personnel now can easily monitor system variables through the PlantPAx dashboards and decide whether to operate around small aberrations or shut down the system if necessary.
• A single, open network – Ripon can more easilyexpand the plant and take advantage of new Ethernet- based equipment and devices. Furthermore, the nonproprietary network simplifies the hiring and training process because new engineers do not need specialized expertise to operate and troubleshootthe system.
• Production intelligence – Plant operators will be able to see and leverage real-time system data in order to address issues as they arise and to make sure the plant is rarely running at full steam. Local vendor support – Both Maverick Technologies and Rockwell Automation specialists can respond quickly if the need arises.

“From an aesthetic standpoint, Ripon's fully integrated automation control system offers a whole new operator experience,” Weber said.

“Now when you're in the operator seat, it feels like you are driving one complete car – rather than one with a Chevrolet body, Ford engine and Toyota dashboard,” Weber said. “This project has set the standard for all future projects at our plant and is a model for Veresen plant design.”