Chemical Engineering May 2013 - 33

Control Valve
Performance
Department Editor: Scott Jenkins
M
inimizing process variability is an
important component of a plant's profitability.
The performance of control
valves within process control loops has a
significant impact on maintaining consistent
processes. This refresher outlines some of the
important aspects of control valve performance,
including parameters of both the
static response and the dynamic response.
Static response
A valve's static response refers to measurements
that are made with data points
recorded when the device is at rest. Key
static-response parameters for control valves
include travel gain, dead band and resolution
(Figure 1).
Travel gain (Gx). This term represents the
change in position of the valve closure
member divided by the change in input
signal. Both quantities are expressed as
a percentage of the full valve span. The
closure member is part of the valve trim (the
combination of flow-control elements inside
a valve). Travel gain measures how well the
valve system positions its closure member
compared to the input signal it receives.
Without signal characterization in the valve
system, the travel gain should be 1.0. [1]
Dead band. This term can be defined as
the range through which an input signal
may be varied, with reversal of direction,
without initiating a response (an observable
change in output signal). With respect to
control valve performance, if the process
controller attempts to reverse the position of
the control valve, the valve will not begin
to move until after the controller output has
reversed an amount greater than the dead
band. A large dead band will negatively
impact control performance.
Resolution. This term can be defined as the
minimum amount of change in valve shaft
position when an input is applied. Resolution
will cause the control valve to move
in discrete steps in response to small, step
input changes in the same direction. This
occurs as the valve travel sticks (when the
starting friction on the valve shaft is greater
than the friction when the shaft is in motion).
Similar to dead band, a larger resolution
will negatively impact control performance.
Dynamic response
Dynamic response for a control valve is the
time-dependent response resulting from a
time-varying input signal.
Dead time. This term refers to the time after
the initiation of an input change and before
the start of the resulting observable response.
Step response time. This term represents
the interval of time between initiation of an
input-signal step change and the moment
that the dynamic response reaches 86.5%
of its full, steady-state value [1]. The step
response time includes the dead time before
the dynamic response.
Overshoot. This term is the amount by which
a step response exceeds its final, steady-state
value. Overshoot is usually expressed as a
percentage of the full change in steady-state
value. Figure 2 shows the dead time, step
response time and overshoot
for a control valve
response to a step input
change. In this case,
stem position in percent
of travel is used as the
control valve " output. "
Step-change size. The
dynamic response of
a control valve varies
depending upon the
size of the input step
change. Four " ranges "
of step sizes to help
understand the staticand
dynamic- response
metrics are defined by
ANSI/ISA standards:
* Small input steps (Region
1) that result in no
measurable movement
of the closure member
within the specified
wait time
* Input step changes
that are large enough
to result in some
control-valve response
with each input signal
change, but the response
does not satisfy
the requirements of
the specified time and
linearity (Region 2)
* Step changes that are
large enough to result
in flow coefficient
changes, which satisfy
both the specified
maximum response
time and the specified
maximum linearity
(Region 3)
Output
b
a
a < resolution ≤ b
Input
c ≤ dead band < d
c
d
Time
Dynamics are not shown
FIGURE 1. Dead band and resolution, illustrated here, are
key static-response parameters for control valves
39
Initial overshoot to 38.11 = 23%
37
38
36
35
Stem
Input
Travel gain = 0.91
Time to steady state, Tss = 18.3 s
86.5% of response, T86 = 2.06 s
Dead time Td =1.6 s
Initial steady state average values, input and stem = 35.67
20
10
Time, s
* Input steps larger than
FIGURE 2. This graph shows the response of a control valve
to a step input (reprinted with permission from EnTech Control
Valve Dynamic Specification V3.0)
in Region 3 where the specified magnituderesponse
linearity is satisfied but the specified
response time is exceeded (Region 4)
Region 1 is directly related to dead band
and resolution. Region 2 is a highly nonlinear
region that causes performance problems and
should be minimized. Region 3 is the range of
input movements that are important to control
performance [1].
Process gain
Process gain is the ratio of the change in
a given process variable to the change in
controller output that caused the change.
To achieve effective process control, the process
gain should ideally fall within a certain
range, and should be consistent throughout
the operating range of the valve. When the
process gain is too high, valve non-linearities
are amplified by the process gain and
process control performance deteriorates.
When the process gain is too low, the
range of control is reduced. Changes in the
process gain over the range of operation
result in poorly performing regions in the
closed-loop controller response.
Two control-valve features impact process
gain: the size of the valve trim and the inherent
flow characteristic of the valve. If the
valve trim is oversized, the process gain will
be higher than it would be for an appropriately
sized valve. The valve's flow characteristic
refers to the curve relating percentage of
flow to percentage of valve travel. Inherent
flow characteristic applies when constant
pressure drop is maintained across the valve.
Typically linear, quick opening or equal percentage,
this will impact both the magnitude
and the consistency of the process gain over
the operating range [1]. Good control-valve
performance depends on proper valve sizing
and trim characteristics.
References
1. Beall, James, Improving Control Valve Performance,
Chem. Eng., Oct. 2010, pp. 41-45.
2. Emerson Entech, Control Valve Dynamic Specification,
Version 3.0, November 1998.
3. Hoop, Emily, Control Valves: An Evolution in
Design, Chem. Eng., August 2012, pp. 48-51.
4. Ruel, M., A simple method to determine control
valve performance and its impacts on control
loop performance, Top Control Inc., Swanton,
Vt., white paper, 2001.
5. International Society of Automation (ISA) and
American National Standards Institute (ANSI).
ANSI-ISA-TR75-25-02-2000, Control Valve Response
Measurement from Step Inputs, 2000.
6. Neles-Jamesbury Inc., " The Valve Book, " NelesJamesbury,
Worchester, Mass., 1990.
7. Skousen, Philip L., Valve Handbook, McGraw
Hill, New York, 1998.
Editor's note: Portions of this page were adapted
from the article in Ref. 1.
30
Final steady-state
average values
input = 37.84,
stem = 37.65
Input, stem %
Amplitude
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