The electro-pneumatic positioner was introduced in the 1970s to correctly align and stabilize control valve position under variable instrument and process conditions. In subsequent decades, the electro-pneumatic positioner evolved into the more capable digital valve controller, but all the new features these devices provided often made configuration and troubleshooting more complicated.
My cover story in the March 2024 issue of Processing, titled “Evaluating Digital Valve Controller Options,” discusses a new generation of digital valve controllers that address these and other issues.
Prior to the invention of the electro-pneumatic positioner, fast and accurate valve control was very difficult to achieve. Pressure drop across the valve, varying instrument air pressure and valve deadband caused valve position to vary from the commanded position by everchanging amounts.
Electro-pneumatic valve positioners were designed to eliminate these deviations by continuously measuring the actual valve position and adjusting actuator pressure as needed to maintain the commanded position. These valve positioners were succeeded by fully digital valve controllers in the 1990s, which progressively offered many improvements—including additional sensors, the removal of troublesome mechanical linkages, and modular components—increasing reliability and performance.
Manufacturers then turned their attention to software improvements, with a goal of utilizing the sensors and valve position information to detect developing control valve problems, and to alert plant personnel before control performance degraded significantly. Digital valve controllers had now become a continuous source of data, which could be mined and acted upon to maximize uptime and avoid unplanned shutdowns.
These advances, while welcome, came with increased configuration complexity, and because the valve monitoring software remained separate from the valve controller, any pause in communication would result in data loss.
As microchip processing power has increased, many useful features have been developed and deployed in digital valve controllers. One simple improvement is the ability for users to select from a multitude of languages on the controller interface, allowing for ease of use worldwide.
Additionally, configuration has been simplified with interactive calibration procedures that are largely automatic, requiring users to answer just a few basic questions.
Onboard data storage prevents data loss in the event of communication failure, and onboard analytic capabilities enable these controllers to identify specific issues and offer suggested solutions right at the device.
End users should demand certain features when selecting valve controllers for a new project or upgrade:
Modern digital valve controllers with onboard valve monitoring and diagnostics reduce unplanned outages and increase profitability. End users choosing controllers should carefully evaluate the features offered to take advantage of the significant advancements seen in the last few decades.
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