Assessing the Health of Air-Operated Control Valves

Oct. 18, 2019
Keeping critical assets in a functioning state is easy with new tools.

Keeping critical assets in a functioning state is easy with new tools

Air-operated control valves are the primary control element in most water treatment plant flow applications. These flow control workhorse devices are often overdesigned and specified for millions of cycles, opening and closing to regulate flow rates and levels in the water treatment process. They open in conjunction with pump motors to move the untreated and partially treated wastewater from process to process until a large control valve is opened to release treated water into rivers, lakes and salt water or applications that utilize treated water.

After performing their task for years dependably, air-operated control valves may have cycled for millions of openings and closings and will often require attention. Smart versions of control valves — those with positioners that have digital protocols built in, such as HART™ — have cycle counters built in that let maintenance staff connect to the valve and see how many times a valve has cycled. Cycle count, though indicative of how many times a valve has operated, is not a finite indicator of the health of the valve. A control valve installed in rough service applications will age faster than one installed in light service applications. Due to this inherent variability, other methods to assess valve health are needed in addition to monitoring cycle counts.

New intelligent loop calibration diagnostic tools like the Fluke-710 can generate the 4–20 mA signals that, when applied to the input of a control valve, will move a forward-acting valve from open to closed and back to open by varying the applied mA signal from 4 mA to 20 mA and back to 4 mA. With an analog control valve, the technician can compare the valve position by observing a position indicator’s movement compared to a graduated indicator scaled from 0 to 100%.

Many critical control valves have a feedback element that outputs a 4–20 mA signal that mirrors the input signal as the valve moves. If they agree, the valve will appear to be functioning as desired. In HART control valves, the feedback is digital and communicated via some variable. By interrogating this variable, as the mA input signal to the valve changes, the technician can see if the valve is moving as expected.

Loop calibrators with built-in HART functionality often have a logging function and apply a changing mA signal to a valve, interrogating the valve’s position via the digital position feedback and recording the values. This recorded information, combined with a diagnostic built-in test, will compare the expected values and give a quick “Good, Marginal, Bad” assessment of the valve. The logged information can then be uploaded to a PC and plotted for further analysis and archiving. With this type of information, a determination can be made as to whether a valve requires additional attention or maintenance can be deferred. WW

Fluke offers test, measurement and monitoring products and software that are used in electronic design, manufacturing, and network troubleshooting, as well as in electrical, industrial, medical, process and calibration applications. Learn more at www.fluke.com.

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