How to select control valves, Part 3

When it comes to selecting and sizing control valves, gain and stability are key characteristics. The non-commercial valve selection chart in this article serves as a reference tool you can download!

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Split ranging of control valves does not necessarily require the use of positioners, because one can also split-range the valves through the use of different spring ranges in the valve actuators.

If the need is only to increase the speed or the thrust of the actuator, it is sufficient to install an air volume booster or a pressure amplifier relay, instead of using a positioner. If the goal is to modify the valve characteristics on fast processes, this should not be done by the use of positioners, but by installing dividing or multiplying relays in the controller output.

Smarter Smart Valves
Much improvement has occurred and more is expected in the design of intelligent and self-diagnosing positioners and control valves. The detection and correction for the wearing of the trim, hysteresis caused by packing friction, air leakage in the actuator, and changes in valve characteristics can all be automated. If the proper intelligence is provided, the valve can compare its own behavior with its past performance and when the same conditions result in different valve openings, it can conclude, for example, that its packing is not properly lubricated, or the valve port is getting plugged. In such cases, the valve can automatically request and schedule its own maintenance.

A traditional valve positioner serves only the purpose of keeping the valve at the opening that corresponds to the control signal. Digital positioners can also collect and analyze valve-position data, valve operating characteristicsand performance trends and can enable diagnostics of the entire valve assembly. The control signals into smart positioners can be analog (4-20 mA) or digital (via bus systems). The advantages of digital positioners relative to their analog counterparts include increased accuracy (0.1% to 1% versus 0.3%-2% for analog), improved stability (about 0.1% compared to 0.175%), and wider range (up to 50:1 compared to 10:1).

Smart valves should also be able to measure their own inlet, outlet and vena contracta pressures, flowing temperature, valve opening (stem position) and actuator air pressure. Valve performance monitoring includes the detection of “zero” position and span of travel, actuator air pressure versus stem travel, and the ability to compare these against their values when the valve was new. Major deviations from the “desired” characteristic can be an indication of the valve stuffing box being too tight, a corroded valve stem or a damaged actuator spring.

Additional features offered by smart valves include the monitoring of packing box or bellows leakage by “sniffing” (using miniaturized chemical detectors), checking seat leakage by measuring the generated sound frequency, or by comparing the controller output signal at “low flow” with the output when the valve was new. Another important feature of digital positioners is their ability to alter the inherent characteristics of the valve.

Valves Serve as Flowmeters
A control valve can also be viewed as variable area flow meter. Therefore, smart valves can measure their own flow by solving their valve-sizing equation. For example, in case of turbulent liquid flow applications, where the valve capacity coefficient Cv can be calculated as      the valve data can be used to calculate flow. This is done by inserting the known values of Cv, Gf, ΔP and the piping geometry coefficient (Fp) into the applicable equation for Cv. Naturally, in order for the smart valves of the future to be able to accurately measure their own flow, they must be provided with sufficient intelligence to identify the applicable sizing equation for the particular process. (See Section 6.15 in Volume 2 of the Instrument Engineers’ Handbook for valve sizing equations.)


PICKING THE RIGHT CONTROL VALVE

When it comes to selecting and sizing control valves, this unique and completely non-commercial valve selection chart not only helps you pick the right control valve for the job, but also serves as a fantastic reference tool you can download!

Control Valve Selection

CLICK HERE to open an enlarged pdf version for easy viewing chart, or click the Download Now button at the end of this article to save the chart for future reference.

 


  About the Author
Béla Lipták is the editor of the Instrument Engineer’s Handbook. He is  recipient of ISA’s Life Achievement (2005) and Control’s Hall of Fame (2001) awards. He can be reached at liptakbelaieh4@aol.com.
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