A Real (Valve) Turn-On

Aug. 15, 2002
Predictive maintenance can cost five times less than preventive maintenance and 10 times less than corrective maintenance

Thanks to embedded digital controllers and communications capabilities, modern control valves are doing much more than just handling fluid flows. Today, they are participating in control as front-line data-acquisition equipment, serving as maintenance and diagnostic systems, and even performing standalone control when necessary.

Valves themselves have not gotten any smarter. The valve bodies and control elements,ball, gate, plug, globe, diaphragm, etc.,have changed a great deal since the first Egyptian farmer figured out how to make a gate valve to divert water from the Nile into his fields some 5,000 years ago. But the most significant changes in recent years have been in valve positioners, actuators, and controllers. They've gone digital.

"Although valve positioners have had microprocessors in them for some time, their use has been limited," says Jonas Berge, engineer at Smar Corp. (http://www.smar.com). "It is only now that the new-architecture control systems have the networking required to efficiently access, display, and manage the data." In other words, digital valve controls have been around for some time, but few users could take advantage of the technology because the manufacturers kept to their proprietary communications systems.

"Over the past three or four years, a virtual flood of digital valve positioner products have hit the market," says David Clayton, senior analyst at ARC Advisory Group (http://www.arcweb.com). "Putting intelligence at the valve makes valve tuning, semiautomatic calibration, and data collection possible." Clayton also says the digital actuators provide plant-floor data to higher levels such as ERP and control systems. Accordingly, almost every one of the new actuators has a network attachment of some sort.

"Valves are transgressing their traditional role as the mechanical workhorses of manufacturing operations and increasingly becoming intelligent components of open control systems," says Clayton.

Having a microprocessor at the valve opens up many interesting possibilities for local control. The Burkert Top Control actuator, for example, has PID and on/off control algorithms and accepts inputs from local temperature, pressure, flow, and level sensors. It has eight built-in flow-control algorithms and lets the user program it with up to seven flow characteristics. With the ability to design your own flow curves, you eliminate the need for various internal valve trims; with the built-in control algorithms, you eliminate the need for an external controller.

While digital positioners and actuators can make life easier for a control engineer, they make maintenance people deliriously happy. No more chasing down a valve with a handheld device. No more taking perfectly good valves out of service for unnecessary maintenance. No more valves failing prematurely because of bad tuning. No more failures without warnings. Smart actuators, when used properly with diagnostic or asset management software, end all those maintenance nightmares.

ARC estimates as much as 40% of manufacturing revenues are devoted to maintenance, and as much as 60% of scheduled maintenance checks on valves and motors are unnecessary. Clayton says that a good answer for valve users is predictive maintenance. "The cost of performing predictive maintenance can be up to five times less expensive than preventive maintenance and 10 times less expensive than corrective maintenance," he says, even when you don't factor in the costs of downtime. One key to predictive valve maintenance is, of course, smart valve positioners because they gather information about valve operations and conditions.

Watching operational statistics also lets operators spot problems. Berge says, "A hunting valve is easy to catch because you will see a great number of reversals. This tells you the positioner or loop is poorly tuned."

"Modern valve actuators can record information from torque-sensing and position-sensing mechanisms and constantly monitor the actuator's torque output," says Chris Warnett, vice president, Rotork Controls (http://www.rotork.com). "When a motor-operated valve is first commissioned, the initial torque demand is recorded in a datalogging device as a footprint. This serves as a reference point for future torque readings so comparisons can be made and the condition of the valve diagnosed."

Digital positioners have exploded onto the scene in the past few years, and vendors are still sorting out what features end users want and need. We expect that all positioners in the future will offer a choice of connectivity, so they are covered for whatever networking scheme finds favor.

Smar's Berge predicts that manufacturers will adopt NAMUR, VDI/VDE, and IEC standard mechanical mounting methods so that their actuators and positioners can easily be used on any valve in the industry.

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