Chemical Engineering January 2023 - 32

ABB
dictated by the boundary layer, a
turbulent flow regime would have
the temperature evenly distributed
throughout the flow, with little difference
between the
temperature
in the center of the pipe and the
temperature at the pipe wall. For a
metal pipe, the inner- and outer-wall
temperature in such a flow case
would have a negligible temperature
difference - in a similar manner to
electricity, metals are extremely good
conductors of heat.
FIGURE 2. Unlike thermowells, non-invasive temperature sensors are mounted outside the pipe, so there
are no issues related to contact with the fluid inside the pipe
at least a partial shutdown, usually
requiring complete emptying of the
system. Thermowells also need to
be cleaned, which can incur further
costs, including those resulting from
having the pipeline shut down while
the probe is removed.
In addition to increased costs,
using thermowells can also have
safety
implications. A thermowell
placed in flowing media, for instance,
can begin to vibrate due to
vortex formation. In extreme cases,
this vibration can lead to the thermowell
breaking, which can have
dramatic consequences, not only
for the system, but also for the
whole operating environment.
Consequently, the standards governing
the stability of thermowells,
such as ASME PTC 19.3 TW-2016,
have become ever more restrictive,
increasing engineering costs and
restricting the ability to use conventional
thermowells in some cases.
Given these factors, it may come
as a relief to know that a thermowell
may not even be necessary in many
cases, due to non-invasive sensor
technology (Figure 2).
Principles behind non-invasive
To understand how non-invasive
temperature measurement technology
works, it is necessary to first
understand the physics of a typical
pipe line where a liquid or liquid-like
mixture is flowing. The behavior of a
flow in a pipeline is known as the flow
profile. There are three main categories
of flow profile, each related to
the velocity of the liquid. These categories
are termed as laminar, transitional
and turbulent.
Laminar flows occur at stable,
32
low flowrates, with the fluid moving
in the same direction at a constant
speed. The fluid settles into streamlined
tiers, which are prevented from
moving by the viscous forces within
the liquid, and the flow velocity increases
in a well-known parabolic
profile from the wall to the center of
the cross section, with a relatively
distinct boundary layer.
Transitional flows occur when the
increased velocity of the fluid causes
distortions in the flow, leading the
tiers within the fluid to become
mixed. This mixing causes the flow
to show both laminar and turbulent
characteristics at different points in
the pipeline.
Turbulent flow occurs at faster
flowrates, where eddies and whorls
cause mass distortions by forming
their own separate sub-distortions.
These mass distortions cause the
tiers within the fluid to blend. The
resulting average velocity profile
is constant across the majority of
the cross section with a very small
boundary layer thickness.
In a simplistic manner, the temperature
difference across the cross
section of the fluid follows the general
flow profile. The difference between
the bulk fluid temperature
and the inner wall of the pipe is
largely determined by the boundary
layer thickness.
As turbulent flow is the most common
type of flow profile in industrial
applications, it is this type of flow
that presents interesting possibilities
for temperature measurement.
Physics models of the temperature
distribution demonstrate that in contrast
to a laminar-flow regime, where
the representative temperature is
The vast majority of existing piping
and in plants to come are and
will continue to be metallic. This
makes non-invasive surface temperature
measurement an increasingly
attractive option for process
measurement applications, where
around 70% of installations feature
turbulent, rather than laminar, flows.
To date, the challenge has been to
accurately capture the surface temperature
of the piping and the dynamic
temperature changes.
Engineering tools that can be
used to calculate the temperature
difference between the surface and
the medium are now available [1].
Advanced features enable factors
such as the effect of flow, density
viscosity, pipe diameter and even
ambient effects to be included to
help determine how good a surface
measurement can be.
The benefits of non-invasive
Devices to measure the surface
temperature piping are not new. Socalled
" skin temperature " sensors,
for example, which measure the
temperature at the pipe wall, have
been available
for a while. These
devices typically employ either thermocouples
or resistance temperature
detectors (RTDs) that can be
mounted on the pipe surface. While
these can be effective, their performance
tends to be hampered by
poor design, poor location, or bad
installation, reducing their accuracy
and response times. If not properly
insulated, they can also be sensitive
to ambient conditions that can
affect their measurement accuracy.
Improving performance also dictates
a mounting along the pipe under the
insulation. In contrast to thermowells,
this can make it even more difficult
and costly to investigate or replace
sensors in the field.
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