POWER February 2011 - 50

INSTRUMENTATION & CONTROL
Heat Transfer Analysis
To investigate this problem further, several
finite element analysis (FEA) models of
the existing and more common thermowells
were developed to analyze the time
response of the heat transfer from the fluid
outside of the thermowell to the TC element.
ANSYS Mechanical (an FEA platform
from ANSYS Inc.) was used as the
simulation environment for these studies.
Effects included in this study were:
■ TC design-exposed, grounded, and ungrounded
TCs
■ Thermowell design-straight-shank and
tapered-shank designs
■ TC sheath diameter and thermowell bore
diameter
■ Contact of TC tip to thermowell
■ Convection heat transfer from fluid to
thermowell
■ Conduction heat transfer through thermowell
■ Contact heat transfer from thermowell to
TC sheath
3. Irreconcilable differences. Common historical practice is for the thermocouple diameter
to match the thermowell bore diameter. The impact on the response time of a thermocouple
sheath/thermowell bore diameter mismatch is shown in this chart. Courtesy: Southern
Company Generation
3/8-inch diameter thermocouple (proper fit)
1,010
1,009
1,008
1,007
1,006
1,005
1,004
1,003
1,002
1,001
1,000
1/4-inch diameter thermocouple (improper fit)
■ Convection heat transfer from thermowell
to TC sheath (when there was a gap)
■ Conduction heat transfer from the TC
sheath through the TC insulation to the TC
junction
Given this article's space constraints,
the three most significant of the many possible
arrangements are discussed below.
Thermocouple Sheath/Thermowell
Bore Diameter Mismatch. Among the
many design choices for instrumentation
and control designers are the selection of
the TC sheath diameter and thermowell
bore diameter. Common historical practice
is for the TC diameter to match the thermowell
bore diameter. For example, for
a thermowell with a 0.385-inch diameter
bore, a TC with a 0.375-inch (3/8th-inch)
sheath diameter would be selected. This
combination would provide a relatively
large contact area between the TC and
thermowell, greatly improving heat transfer
characteristics. Conversely, a known
adverse impact is that the relatively close
fit would make the TC more difficult to
replace. The impact of this mismatch for
a 10F increase in steam temperature is
shown in Figure 3.
As shown, the measurement time con0
20
40
60
80
100
Time
(seconds)
4. Just the right amount of exposure. The impact on response time of different thermowell
designs using properly fitted exposed-tip thermocouples is shown. Courtesy: Southern
Company Generation
Wika thermowell
1,010
1,009
1,008
1,007
1,006
1,005
1,004
1,003
1,002
1,001
1,000
Bailey thermowell
120 140
160 180
stant increases from approximately 19
seconds to 64 seconds. The following assumptions
are relevant to these results: the
3/8-inch TC is in intimate contact with the
thermowell throughout its length and the
¼-inch TC contacts the thermowell only
at the base of the thermowell. Arguably,
these two assumptions place the mismatch
in the most adverse light, but the results
are indicative of the large response time
differences that can be obtained.
Thermowell Design. As discussed
previously, the " robust " straight-shank
thermowell design has significant adverse
impacts on the desuperheater outlet response
time. The relative impact for the
exposed type of TCs is shown in Figure
4. As shown, the time constant for the
straight-shank design is more than twice
that of a more typical tapered design.
Thermocouple Not Properly Seated.
As might be expected, if the TC is not seated
properly, the response time will increase
substantially. Analysis showed the time
constant increasing by an order of magnitude
(~30 seconds to ~300 seconds).
50
20
40
60
80
100
Time (seconds)
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120 140
160 180
Lab Tests Verify Effects
Lab tests using precision thermowells and
TCs were used to verify and refine the
FEA results. Wika Instruments provided
two threaded, tapered thermowells-one
with a ¼-inch bore and one with a 3/8-inch
POWER | February 2011
Temperature (F)
Temperature (F)

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POWER February 2011

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