Consider the control valve. We write about controllers, about networks, about distributed control systems, PLCs, programmable automation controllers, advanced control strategies, field sensors and transmitters all the time, but we seem to sometimes forget the final element that all that control stuff is aimed at—the control valve that produces the end result.
“The notable thing,” says fieldbus guru Dick Caro of CMC Associates, “is that so few of them are smart. The clear majority of control valves do not have Foundation fieldbus, Profibus-PA or even HART.”
The reason for that is partly the installed base of valves, which dwarfs that of most types of sensors. Valves usually last a long time and are difficult to replace. Even actuators, where the “smarts” reside, are long-lived devices.
Yet, Caro continues, “I have read many user papers on the value of smart valves when used with predictive maintenance. Most control valves are used until they fail or are finally discovered to be the source of VERY bad control loop performance.”
Most suppliers agree and are focusing their development efforts on asset management, rather than improving valves and actuators themselves.
“The hottest topics are not on the design side,” says Eric Fillion, of Metso Automation. “They center on the gains to be achieved through supply chain management and ROI. Another big area—the way data is handled—tying asset management systems into enterprise management.”
But Fillion goes on to talk about some new developments that aren’t software-related. “Big news right now in valves is around materials—seat materials and surface coatings—and using better materials,” he says. “The focus is on looking at better ways to achieve tighter shutoff with materials.”
This of course is due to the worldwide crackdown on fugitive emissions. Valve packing and seals are one of the premier sources of fugitive emissions in a process plant environment.
But if we’re not talking about materials, what else is there?
According to control guru Terry Blevins of Emerson Process Management, “As an integral part of our adaptive technology development we did quite a bit of work in the area of valve diagnostics. In particular, Dr. Willy Wojnis (also from Emerson) developed a technique for automatically quantifying valve resolution and deadband based on normal operating data…i.e., no special plant tests required.”
Why Is This Important?
Greg McMillan, Control columnist, responds: “The valve resolution and deadband are the major contributors to valve dead time (dead time is resolution or deadband divided by the rate of change of controller output). Resolution creates limit cycles in all loops, and deadband causes it in cascade loops and integrating processes, whenever the controllers have reset action.”
He continues with an example. “The resolution limit in pH control sets the number of neutralization stages needed. For a large valve, it would be good to know the per-stroke dead time associated with trying to move air in and out of the large actuator. The ability to identify the installed valve characteristic online is another important step forward,” McMillan says, “because you can use this to linearize the valve or at least know the contribution of the valve gain to the process gain.”
Sandro Esposito of Masoneilan reports that asset management using a variety of fieldbuses is the future of control valves. At the Yokogawa Tech Fair in 2006, he showed a variety of valve actuators, including some intended for safety instrumented systems, and noted that proper asset management can improve the ability of valves to operate, and even lengthen the period between required partial stroking in safety system applications.
So while suppliers continue to trumpet asset management and smarter valves, and end users continue to not buy them, quietly these same suppliers continue to improve the mechanical performance of valves.
George Gassman, senior principal technologist at Emerson Process Management’s Fisher Division, notes, “Valve mounted position controllers will continue to evolve with increased emphasis on safety system compatibility and online detection of valve performance. Valve health will be indicated at the valve with a visible beacon or display, and also communicated to the host system using HART, fieldbus or wireless communications.”
Gassman believes that there are a number of mechanical or physical changes coming. None of them are by themselves earthshaking, but taken together they will provide better valves.
“We will see a turn from mechanical position feedback linkages that have been subject to wear, corrosion and the effects of severe mechanical vibration to linkage-less plus non-contacting position feedback designs,” he says.
He also thinks about fugitive emissions. “Quiescent pneumatic consumption at the valve-mounted instrument will continue to be reduced as valves and controllers, especially those operated on natural gas supplies, will be improved to minimize emissions and operating costs.”
And what about what’s inside the controller? Gassman predicts, “Application-specific firmware for the valve controller will become more prevalent as the full capability of the onboard processor is optimized for specific valve functions. One example is the pressure control mode currently available in Emerson’s Fieldvue digital valve controllers. On spring-assisted actuators, where the valve controller has detected that the stem travel feedback is bad, the position controller is automatically replaced by an actuator pressure controller that allows the actuator pressure to track the set point without the benefit of stem position feedback. This type of override can keep the valve in service and avoid a costly valve shutdown.”