Chemical Engineering October 2010 - 41

Feature Report
Improving Control
Valve Performance
The control valve
is a focal point for
improvements to both
process performance
and economics
James Beall
Emerson Process Management
I
mprovements to the performance
of basic, regulatory control systems
have a great return on investment
and are some of the least
expensive control improvements to
make [1]. Control valves have a major
impact on control loop performance
and, therefore, improvements in valve
performance can have significant economic
benefits. This article shows how
poor control-valve performance can
be identified and corrected to achieve
these benefits. A plant example is
used to demonstrate these methods.
In addition to corrective actions,
comprehensive control-valve specifications,
based on process requirements
for new applications, can
provide quicker plant startups and
immediate economic savings.
The control valve system
In this article, the control valve is considered
to be a dynamic system, from
the input signal through to the flow
coefficient that determines the fluid
flowrate through the pipe. The control
valve system includes the valve,
actuator, motion conversion mechanism,
stem or shaft, closure member
Output
b
a
a < resolution ≤ b
Input
c ≤ dead band < d
c
d
FIGURE 1.
Dead band and
resolution, illustrated
here,
are key staticresponse
parameters
for control
valves
Time
Dynamics are not shown
(such as plug, ball or disc) and other
valve accessories. Examples of valve
accessories include current to pressure
transducer (I/P), positioner, air
booster relay, dampener and air set.
So, when a change of the input signal
to the control valve occurs, the I/P and
positioner must respond to move the
actuator, which must move the motion
conversion mechanism, which must
move the stem or shaft, which must
move the closure member, which must
change the flow coefficient. As you
can see, there is a lot of opportunity
for problems.
The key to control valve performance
is creating a measurable change in
flow through the valve in response
to small, input step changes (1% and
less). A change in flow indicates that
the valve's flow coefficient has actually
changed in response to a change
in the input signal. If the actual flow
through the control valve is not available
or is not measured, then the valve
stem, shaft or actuator movement may
be used to estimate the response of the
valve. However, the movement of the
valve stem, shaft or actuator may not
be an accurate representation of the
actual change in the valve flow coefficient
for all changes of the input signal
to the valve. For example, the inboard
end of the shaft of a rotary valve
might move in response to a change
in the input to the control valve but
the actual flow coefficient might not
change because shaft windup occurs
and the valve closure member does
not move. In some cases the actuator
position may be used to measure the
control valve response and this adds
yet another potential discrepancy between
the input to the control valve
and the actual change in the valve
flow coefficient. However, it is important
to note that the response of the
valve flow coefficient can be no better
than that of the stem, shaft or actuator
position.
Control valve performance
The performance of a control loop will
be limited by the poorest performing
component in the loop, such as the
transmitter, the controller tuning or
the final control element. Note that
for most control loops, the final control
element is a control valve. One study
of over 5,000 control loops revealed
ChemiCal engineering www.Che.Com oCtober 2010 41
Amplitude
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Chemical Engineering October 2010

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