INSTRUMENTATION
instruments are not working up to their
potential. Unfortunately, that means the
process is degraded before the operator
can detect the poor performance, a situation that is especially true for older
technology instruments.
To see what effects better control of
valves have on the whole unit or plant
takes much effort—you have to tune,
adjust and maintain many valve assemblies to see how they add up to a large
return. At the same time, it only takes a
few, poorly maintained instruments or
valves to keep the whole plant from
realizing the benefits of good maintenance on the rest of the valves.
toring performance continuously. With
this wide range of options, choosing the
right diagnostic tool and deciding how to
implement it can be difficult.
You also have to add to this difficulty
the reality that developing an effective
maintenance plan requires an understanding of the potential benefits of
diagnostics, the types of diagnostic tests
available, and how to specify and implement diagnostics during all phases of
plant operations.
MAXIMIZING
PERFORMANCE WITH
DIAGNOSTICS
The diagnostic tools available to support
legacy analog and new digital equipment
range from handheld devices for calibration and simple monitoring to integrated
asset management programs for moni-
DIAGNOSTIC CAPABILITIES,
THEN AND TODAY
Valve diagnostic devices and techniques
continue to evolve. First-generation
diagnostic tests were limited to valve
signature (which provide insight into
assembly friction, bench set, spring rate
and seat load), dynamic error band
(which combines hysterisis, deadband
and dynamic error) and step response to
an input signal.
A good way to look at the overall pic-
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ture, however, is to look at the performance diagnostics capability of each of the
technologies discussed earlier: Pneumatic instruments, I/P, positioner and digital
valve controller on Foundation fieldbus.
When a plant is using pneumatic
instruments or I/Ps, measuring valve
performance requires a portable flow
scanning device, which has been available for many years. With this technology, the operator is acting as the detector
of variability, and the process is the sensor. In most cases that means plant
operations and performance have
already been impacted before maintenance procedures could be started.
An alternative for keeping the plant’s
valves operating at peak is to rebuild
them during every turnaround. That can
be costly, but performance is guaranteed when the turnarounds are close
enough to each other. In addition, every
valve assembly for turnarounds typically
is removed from the line and rebuilt.
Analyzing the health of analog or
legacy control valves involves portable,
field-based test equipment. These diagnostic devices can be applied to any air-operated valve to allow testing of the
entire valve assembly, including all
accessories, in the installed condition.
Testing is done with the valve out of
service; and along with diagnoses of
operating condition, the field test allows
future baseline comparisons that can
point to developing operating problems.
When a plant is using smart positioners with a compatible distributed control system, there are several ways of
knowing when valve variability is too
great. The most obvious are the same
ways as with the older technology. Now,
however, we have new possibilities. For
example, when the operator is calling
maintenance, plant performance could
be greatly improved if that operator
could see the problem before plant performance is affected. With pneumatic
equipment, I/P or positioners, this can
be very difficult. When the digital valve
controller is part of the system (as
shown in Figure 4), it is much easier for
that controller to provide the operator
or maintenance personnel with early
diagnostic information.
