As we near the end of this year's 25th anniversary retrospective of Control's coverage of topics ranging from analyzers and asset management through optimization and safety systems, control valves appear to have changed the least. Our fourth issue, in March 1990, already commented earnestly about minimizing fugitive emissions and improving valve integrity through better design and materials, ongoing work to standardize a fieldbus for digital valve control, and the exciting prospect of a "smart" control valve.
The smart valve would include "electronic pressure sensors near the upstream and downstream flanges, effectively measuring and sending the pressure differential that exists under flowing conditions across the valve orifice. Another sensor would transmit the valve stem position to an attached microprocessor," allowing the microprocessor to calculate the mass flow "at any given moment" and on detecting an upset, to "reposition the valve stem to reestablish the previously desired flow rate."
Over the next few decades, valve technology has stayed well ahead of the engineering, purchasing and maintenance disciplines, which have contributed the most to valve deficiencies in process control by ignoring the latest innovations, specifying the wrong sizes and trims, buying the cheapest items that might barely work, leaving off the positioners and consistently over- and under-maintaining the installed base.
As editor-in-chief Walt Boyes reported in March 2007, "While suppliers continue to trumpet asset management and smarter valves, and end users continue not to buy them, quietly these same suppliers continue to improve the mechanical performance of valves."
A November 1998 discussion of rotary- vs. rising-shaft valves noted that advances in valve and actuator technologies hadn't yet completely overcome the inherent characteristics of the two main alternatives. Control valves are "not just pig-iron devices to be thrown in as an afterthought," said J.B. Arant of DuPont. "This is your final control element and can make all of the fancy and high-performance sensors and computers to no avail."
As we progress through this decade, innovative actuators, positioners and algorithms are letting less-expensive rotary valves perform like pricier globes and gates. Control system and software developers are closing the gap between valve potential and actual performance with diagnostics that can allow users not only to pinpoint problem valves and their exact valve problems, but to overcome them with software remedies that can make an ailing valve perform well enough to maintain control until the underlying backlash, slip or stiction problem can be corrected.
Today, the bad news is that our writers continue to find that the same old valve problems—and apparently in many cases, the same old valves—remain major contributors to problems with loop tuning and process stability. The good news is, such problems can be and have been solved by savvy Control' readers who understand the pitfalls, then engineer and maintain them out of their plants by applying some of the abundant solutions they've found on these pages.