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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.zaDRIVES, 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-
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
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
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.
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.
• 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.
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July ‘16
Electricity+Control