Hydrocarbon Processing - June 2021 - 76

Process Controls, Instrumentation and Automation
FIG. 2. Liquid flow around a thermowell can create vibrations sufficient
to cause fatigue-induced failure.
0.39
0.31
0.24
0.16
0.08
5
10
15
Velocity, ft/sec
FIG. 3. When the frequency of the wake shedding matches the resonant
frequency of the thermowell, the resulting vibration can be violent.
Sometimes, this vortex-induced vibration (VIV) is tolerable,
but when severe, it can lead to two problems: sensor failure
causing a lost reading or fatigue-induced failure of the thermowell
itself. In this situation, the thermowell eventually bends or
cracks, which can lead to process containment loss.
Wire fatigue and breakage. If vibration cannot be eliminated,
one mitigation technique is using redundant sensors.
Some temperature transmitters can process signals from two
sensors, either from separately mounted sensors or from dual
sensors built into a single sheath. If the values differ, transmitter
diagnostics can trigger an alert. Similarly, if one sensor fails,
the transmitter can switch immediately from the primary to the
backup sensor without losing the reading.
The greatest reduction in risk is achieved with two independent
sensors; however, this requires additional process
penetrations. Two independent sensors-even if they are
feeding one transmitter-can reduce the probability of losing
the reading by 80%.
76 JUNE 2021 | HydrocarbonProcessing.com
20
25
30
1.52
RMS tip displacement vs. fluid velocity
Velocity, m/sec
4.57
3.05
6.10
Lock-in region
transverse vibration
8
6
4
2
FIG. 4. Helical square designed thermowellsc
effects, reducing vibration by up to 90%.
break up wake-shedding
7.62
9.14
10
Thermowell durability. When VIV is an issue, the conventional
solution is simply using a massively thick and/or very short
thermowell that can stand up to the vibration. This is a problem
for several reasons, including:
* A heavy cross-section causes significant time lag
for the sensor to respond to changes in temperature
* Large diameter thermowells can be difficult to mount
and can cause increased internal pipe blockage
* A short thermowell may not extend far enough into
the process media stream to deliver an accurate
temperature reading.
This leaves designers trying to determine how thin and long
a thermowell can be and still withstand the VIV. For a given
thermowell installed in each pipe, vibration will change with
fluid velocity, but the two do not have a linear correlation. A
certain fluid velocity will cause the VIV frequency to match the
natural frequency of the thermowell, which can cause the amplitude
to increase significantly (FIG. 3), with accelerated metal
fatigue as the result.
These critical relationships can be analyzed mathematically
using formulas outlined by ASME in its PTC 19.3 TW-2016
Standard. This calculation method has been built into free online
thermowell calculation toolsb
that guide a design engineer
through the sizing process by analyzing the likelihood of vibration
problems for a specific thermowell shape in a specific installation.
Each calculation involves about 20 variables for the
dimensions and operating characteristics, so it is difficult to process
manually. It must also be repeated for each set of operating
characteristics, another reason to use automated online software.
Process data for various operational levels can be entered manually
or uploaded using a template, so a designer can test multiple
thermowell tags at once. The system performs calculations in the
usual way, but also permits users to test what-if scenarios to see if
a thinner thermowell might work or if a stock size already in use in
the plant can do the job. These alternatives are offered as possibilities,
providing designers with the flexibility to make the final call.
Avoiding fluid-induced vibration. Suppressing vibration at
its source generally involves using a thermowell profile designed
to avoid normal wake-shedding problems. Helical square designs
(FIG. 4) disrupt the formation of long vortices, resulting in
far less vibration-up to a 90% reduction in some cases.
Helical geometries have been used successfully with wind
Tip displacement, mm
Tip displacement, in.
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