Chemical Engineering October 2010 - 42

Feature Report
Level
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
FIGURE 2. This graphs shows the response
of a control valve to a step input
(reprinted with permission from EnTech
Control Valve Dynamic Specification V3.0)
that poor control-valve performance
negatively impacted control loop performance
on 30% of these loops [2].
Important aspects of control valve
performance include the static response,
the dynamic response and the
valve trim size and flow characteristics.
The International Society of Automation's
(ISA) technical reference,
ANSI-ISA-TR75-25-02, explains the
concepts of control-valve static- and
dynamic-response metrics [3]. ISA
standard ANSI-ISA-75-25-01 is a test
procedure to measure the static and
dynamic response of a control valve
system [4]. However, this standard
does not prescribe what response requirements
should be specified in
order to achieve the desired controlvalve
performance for a particular application.
The EnTech Control Valve
Dynamic Specification V3.0 provides
guidance on specifying the static- and
dynamic-response parameters and the
valve trim size and flow characteristics
to achieve the desired process control
performance [5].
Static response. So, what are these
control-valve response parameters
and what do they mean to process
control performance? Static response
refers to measurements that are made
with data points that are recorded
after the device has come to a rest. Key
static-response parameters for control
valves include travel gain, dead band
and resolution [3]. Travel gain, Gx, is
the change in closure member position
divided by the change in input signal,
both expressed in percentage of full
span. The closure member is the por30
~
120 days
FIGURE 4. In the plant example, the reactor
level was not well-controlled
R-1
LC-1
LV-1
LC-2
R-2
LV-2
FIGURE 3. In this plant example, the
level controller output went directly to
the control valve positioner
tion of the valve trim that moves to
change the flow through the valve.
If there is no signal characterization
inside the valve system, the travel
gain should be 1.0. In other words,
travel gain is a measure of how well
the valve system positions its closure
member compared to the input signal.
Dead band is defined as " the range
through which an input signal may be
varied, with reversal of direction, without
initiating an observable change
in output signal " [6]. 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.
Another key static-response parameter
is resolution. Resolution is defined
as " the smallest step increment of
input signal in one direction for which
42 ChemiCal engineering www.Che.Com oCtober 2010
movement of the output is observed "
[3]. 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 for the amount of
resolution after completing the previous
step in the same direction. Similar
to dead band, a larger resolution
will negatively impact control performance.
Figure 1 illustrates dead band
and resolution.
Dynamic response. The second aspect
of control valve performance
is the dynamic response. Dynamic
response is the time-dependant response
resulting from a time-varying
input signal [4], and includes dead
time, step response time and overshoot.
The ISA technical reference
ANSI-ISA-TR75-25-02 [3] provides
the following definitions for these dynamic
response parameters:
* Dead time - The time after the initiation
of an input change and before
the start of the resulting observable
response
* Step response time - The interval of
time between initiation of an inputsignal
step change and the moment
that the dynamic response reaches
86.5% of its full, steady state value.
The step response time includes
the dead time before the dynamic
response
* Overshoot - The amount by which
a step response exceeds its final,
steady state value (refer to Figure
24 of ANSI/ISA-51.1-1979 (R1993)).
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
Final steady state
average values
input = 37.84,
stem = 37.65
Variation of
valve position
represents
variation
in flow to
downstream
reactor
variation
~ 20%
Input, stem %
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Chemical Engineering October 2010

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