POWER February 2013 - 37

INSTRUMENTATION & CONTROLS
Impact on Control Loop Performance
The February 2011 article on this topic explained
how a simple simulation was done
to evaluate the effect of a faster temperature
response on the performance of a cascade
proportional integral derivative (PID)-based
steam temperature control loop. That simulation
showed that there was a significant (but
not huge) improvement in the disturbance rejection
capability of the controls with a faster
temperature measurement.
As a result of another research project
on the steam temperature control, a model
predictive controller was in service during
the time the thermowell was replaced,
and that controller did not utilize the desu2.
Test responses. Step responses of desuperheater outlet temperature to a change in
spray valve position are shown. The response to a step change open is shown in the top two
graphs, and a step change close is shown in the bottom two. Source: Southern Company
70
65
60
55
50
3,650
785
780
775
770
3,650
70
65
60
55
50
4,160
790
785
780
775
770
765
4,160
February 2013 | POWER
4,185
4,210
4,235
Time (sec)
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37
4,260
4,285
4,310
4,185
4,210
4,235
Time (sec)
4,260
4,285
4,310
3,675
3,700
3,725
Time (sec)
3,750
3,775
3,800
3,675
3,700
3,725
Time (sec)
3,750
3,775
3,800
perheater outlet temperature. This made
it impractical to try to assess the impact
that the faster response had on the control
system performance. Recently, a modified
PID-based control strategy was installed
for testing that does use the desuperheater
outlet temperature, so it may be possible
with this system to make a control performance
comparison between the fast and
slow temperature measurement responses.
After the old thermowells were removed,
the bores of the wells were inspected using
a small borescope. Considerable debris was
found in the bore of one thermowell, and
some of it appeared to be metallic (Figure
3). Some of this debris may have fallen into
the thermowell when the thermocouple was
removed during removal of the thermowell.
This discovery required determining
the nature of the debris and what impact it
might have had on the slow response time
observed with the old thermowell.
A plan was developed to test the old thermowell
using the same temperature calibration
facility described in the earlier article. Tests
were to be conducted first in the as-found
condition and later with the bore cleared of all
debris. Unfortunately, the thermowell with the
3. Bore debris. This debris was found in the
bottom of a bore in one of the old (removed)
thermowells. Courtesy: Southern Company
4. Molten salt test. Field-testing was impractical,
so a laboratory setting with a molten
salt bath at 350F was used to assess performance
of the nontapered thermowell and to
confirm earlier test results. The Hart Scientific
heat bath with thermocouple tray used for
testing thermocouple response times (L) and
the underside of the tray holding test thermowell
and reference thermocouples (R) are
shown. Source: Southern Company
Temperature (F)
Valve position demand (%)
Temperature (F)
Valve position demand (%)

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POWER February 2013

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