Chemical Engineering January 2023 - 31

Part 2
Cover Story
Non-Invasive Temperature
Measurement in the CPI
While thermowells have become well-established, they have potential drawbacks that can
affect their performance. The benefits of alternative non-invasive temperature sensing is
presented here
Guruprasad Sosale
ABB Measurement & Analytics
T
emperature is one of the
most important parameters
for industrial processes,
providing a key indicator of
system safety, product quality and
overall process efficiency , as well
as equipping operators with the information
they need to ensure their
plant is running at optimum performance.
As such, temperature should
ideally be measured as widely as
possible to provide the best possible
overview of process conditions.
This article discusses the limitations
of conventional thermowell
technology and how non-invasive
temperature measurement has advanced
to become a cost-competitive
alternative.
Thermowell issues
In the chemical process industries
(CPI), thermowells are often
selected as the preferred method
for temperature measurement. Essentially
metal sheaths that protect
the measurement sensor against
the process being measured, thermowells
have become commonplace
throughout the CPI, being
used in everything from food-andbeverage
through to oil-and-gas
processing. While thermowells can
be effective in protecting the temperature
sensor, their use is not
without problems.
Foremost among these problems
is the need for them to be in physical
contact with the process medium,
which means first shutting
down the pipeline and then drilling
a hole through which the probe can
be inserted (Figure 1). This may
also require the addition of flanges
or fittings to help maintain
pipeline integrity
and satisfy safety requirements.
While these
devices have typically
offered accurate and reliable
performance, the
time, cost and disruption
associated with their installation
has tended
to limit the widespread
measurement of temperature
in a process.
Aside from cost and
time, the installation of
thermowells can also
pose safety risks, especially
in applications
where liquids may be
flowing at high speeds,
high pressures or both,
which could cause injury
if the pipeline is not properly
shut down.
Shutterstock
FIGURE 1. Thermowells require boring a hole in the pipe and bolted
flanges to support the temperature sensor
Another challenge has been the
potential complications arising from
the characteristics of the medium
being measured. Substances can
be liquid, gaseous or in viscous or
semi-viscous states, or, in some
cases, a combination of these,
flowing at different rates. These
substances can vary in their characteristics,
ranging from benign liquids,
such as water, to aggressive
chemicals or abrasive mixtures. In
each case, the design of the temperature
device used needs to be
able to withstand the rigors of the
application to provide reliable and
safe operation.
Where thermowells are used
to protect temperature sensors,
they must also be designed for
the characteristics of the medium
being measured to help safeguard
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temperature sensor against
chemical and mechanical stress.
An example is abrasive components
suspended in a process.
When moving through piping at
high speeds, abrasive particles can
subject the thermowell to scouring
that can reduce its operational
life over a prolonged period. While
using special thermowell materials
can help to tackle this, it can
also add substantially to the overall
price tag. To protect against the
effects of abrasive and chemically
aggressive media, thermowells
must be regularly inspected and
replaced if necessary.
To avoid problems such as premathe
ture
wear and potential failure, thermowells
must be regularly inspected
and replaced if necessary, adding to
the costs. These inspections require
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Chemical Engineering January 2023

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