Electricity + Control July 2016

DRIVES, MOTORS + SWITCHGEAR VALVES + ACTUA ORS

can provide the input mA signal to drive the control valve across its operating range. When applying a 3,8 mA input signal to a normally closed valve, the valve should be hard closed. It should remain closed at 4,0 mA and move slightly off its seat at 4,2 mA. At the other end of its operation, at 19,8 mA, it should be nearly fully open. At 20,0 mA it should be fully open, and hard open at 20,2 mA (resting on the travel stop). Tests of this nature will deter- mine if the valve is opening and closing correctly, but still fall short of testing the valve across the entire range where it provides control. Many valves, including those that are ‘smart,’ have a feed-

valves needs to be established. Using existing maintenance intervals is a starting point. If there is no established maintenance interval, the service location of the valve needs to be evaluated. Rough service applications dictate a shorter maintenance interval than light duty service, for example. An interval of six months to a year to start (unless the service location is very hard on the valves) is a reasonable starting point. Some devices installed in safety and shut-down systems need to be checked every three months regard- less of service location.

back element built in that outputs the actual position as a percentage of open/close. This output can be a 4 to 20 mA signal or a digital HART variable that repre- sents 0 to 100% of control valve operating span. Applying a varying mA signal, while si- multaneously monitoring the output mA or percentage of span signal, gives a technician a means to see whether a control valve is operat- ing correctly over its range. By recording simultaneously the applied mA signal and the output mA signal or PV percentage of span, the valve’s performance can be documented. This

Conclusion Once baseline valve signature data is recorded, the valves need to be tested at the defined intervals and the signatures recorded. The signatures can be compared to the baseline signatures to determine changes in their performance. If the output response curve has developed a nonlinear signature or has aberrations in the curves, the valve may be developing excessive stiction or hysteresis that may require it to be removed for service.

documented test and result is often called a valve’s ‘signature.’ The output should smoothly mirror the applied mA input signal. Any deviation from the applied signal is a potential indication of aberrant behavior by the valve. Maintenance strategy can reduce costs A best-in-class maintenance strategy for control valves can reduce costs by both minimising the number of valves pulled physically from processing and minimising failure risks. To establish such a practice, the baseline condition of the valve at a known good state needs to be documented. Ideally the documentation occurs when the valve is commis- sioned or after it is overhauled. The technician records the signature of the valve in the ideal state, plotting the output mA or percentage of span signal versus applied input signal, and stores this information with the time, tag number of the valve, and the date the activity is performed. Calibration management software can be used to man- age this information. Once the baseline performance signatures of the valves are recorded, a maintenance interval for testing the performance of the

• Sophisticated valve performance tests require removing the valve and testing its performance on a ‘valve prover’. • The documented test and result is called the valve’s ‘signature’. • Once the baseline performance signature of the valve is recorded, a maintenance interval for testing its performance must be established.

take note

Jim Shields – Fluke product marketing manager - manages the product marketing mix for field calibration, incorporates customer VOC (Voice of Customer) into new product innova- tions, and handles ‘Best Practice’ in managing instrument maintenance and calibration for Fluke Corporation (Dana- her Company), Seattle in the USA. Enquiries: Comtest. Tel. +27 (0) 10 595 1821 or email sales@comtest.co.za

July ‘16 Electricity+Control

35