Intelligent Positioners Offer a Window into Valve and Process Health

Nov. 1, 2008
Increasing demand for fieldbus-based control systems is motivating manufacturers to upgrade to digital valve positioners.
Sales of digital positioners will outpace those of pneumatic and electropneumatic positioners, as users seek to improve their visibility into plant operations to maximize productivity and the availability of production resources. The asset management focus of users is also fueling demand for digital positioners that incorporate powerful onboard diagnostics capabilities, and use digital communication protocols.Asset management initiatives increase demand for data transparency. Much of the data necessary to make informed decisions regarding maintenance, operational performance and financial return of plant assets lies in digital positioners. Digital positioners are proving to be a great enabler to predictive maintenance by providing a Valve Degradation Analysis, which is important for critical valves, particularly in Safety Instrumented Systems (SIS).We Energies' Pleasant Prairie Power Plant is a coal-fired generating plant in Wisconsin. The company's largest plant, Pleasant Prairie has two units, and a total net generating capacity of 1,224 megawatts. The boilers for each of the plant's two units are 20 stories high and generate steam at 995 degrees Fahrenheit and 1,990 pounds per square inch. The cooling system for each unit uses 200,000 gallons of water a minute, pumped between the steam condensers and cooling towers, converting exhaust steam from the turbine back into water for reuse. Pleasant Prairie's first unit became operational in 1980, while the second unit went online in 1985. For many years, the plant used pneumatic positioners for its control valves. Maintenance was generally reactive in nature, in response to an operator reporting a problem with a specific control loop, such as an unresponsive valve. Generally, problems with valves were discovered after it was too late, and maintenance would have to bypass valves manually for repairs during operating hours, often while the plant was under full load.Going digital
The plant took measures to upgrade with a plant asset management (PAM) system and digital positioners connected with a Hart network. This predictive maintenance software provides data that could be used as a guide for operators at Pleasant Prairie to make changes to optimize valve performance. In some cases, the system has helped operators to identify valves that are working harder than they should, and make adjustments. In the past, there would be no warning about a potential problem. The valve positioner would simply compensate for the additional load, and the problem would only become apparent when the valve finally failed. Now, operators can respond proactively and alert maintenance when they notice the valve is overloaded.The combination of digital valve positioners and asset management software has helped the plant shift maintenance from a reactive approach to a predictive, proactive strategy. As a result, plant performance and availability have both improved, and maintenance can assess the health of valves and repair them before operators know about impending problems, instead of the other way around.We Energies saw significant improvement in its maintenance practices and plant availability by installing digital valve positioners and using asset management software to harness the full potential of valve diagnostic data, but asset management goes beyond just positioners. ARC believes that users should consider replacing their full array of instruments with intelligent field devices where appropriate to leverage the full benefits of PAM.Though intelligent positioners and field devices are attracting greater interest, particularly in greenfield projects in developing countries, the majority of installed positioners and field devices in developed regions are unable to provide users with real-time data regarding the health of their valves and instruments. As a result, many manufacturers are missing out on the benefits that intelligent instruments can provide for plant availability and profitability.Allen Avery, [email protected], is an analyst at ARC Advisory Group Inc., in Dedham, Mass.

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