POWER March 2017 - 17

3. Graphic results. The condenser
tubesheet map shown here is color coded
with eddy current testing results. Courtesy:
Conco Services Corp.
The Eddy Current Process at a
Glance. After the condenser or heat exchanger
tubes have been cleaned and are
prepared for testing, a certified nondestructive
testing technician chooses one
of a variety of probes that is appropriate
for the testing application. Once inserted
into the tube, an alternating current
flows through a wire coil in the eddy current
probe and it generates an oscillating
electromagnetic field. As the probe and
its magnetic field are brought close to
the conductive material of the tube, the
circular flow of electrons, also known as
the eddy current, begins to move through
the tube like water swirling through a
pipe.
The interaction of the eddy currents
fective tube cleaning must first be performed.
Fill factor is the volume at which
the eddy current probe occupies the inner
diameter of the tube being tested, and it
is an important variable in the quality of
an eddy current inspection. Fill factor is
reduced when tubes are not clean.
For an inspection to have high accuracy
and repeatability, the eddy current
probe must be able to achieve a fill factor
of 85% or higher, with the probe occupying
as much of the diameter of the tube
as possible. High fill factor provides better
magnetic coupling with the tube and
superior data acquisition. Fill factor that
is too low will reduce the quality of the
data and increase the likelihood of false
signals and masked defects. Along with
fill factor, dirty tubes increase the signalto-noise
ratio that is a vital component
of the eddy current inspection (Figure 2).
Whatever cleaning method is utilized,
condenser and heat exchanger tubes
should be cleaned as close to the tube
surface as possible. The silver lining for
the plant is that the return on investment
associated with tube cleaning is
significant. Tube fouling and corrosion
can cause major loss of heat transfer and,
if left unattended, even tube failure.
In preparation for eddy current testing,
a common practice is for plants to
contract with a tube-cleaning specialist.
Tube cleaners are inserted into the
tubesheet in the inlet waterbox. Then,
pressurized water is used to shoot the
cleaners through the tubes. Debris and
obstructions are flushed from the tubes,
making them ready for eddy current testing.
Typically, metal tube cleaners can be
used eight to twelve times before they
become ineffective, so an investment in
mechanical cleaners will usually be recouped
quickly.
March 2017 | POWER
flowing through the tube metal and the
magnetic field of the coil will provide the
valuable data points that tell the story of
the tube's condition. Changes or variations
in the eddy currents are caused by
defects in the tube wall, such as nearsurface
cracks and inconsistencies in
metal thickness. The eddy current instrument
records the pattern of variations
and they are analyzed to draw conclusions
about the condition of the tubes.
Over time, results can be compared and
any trends or progression in the defects
can be observed, which will guide maintenance
decisions (Figure 3).
Remote Field Testing
While both eddy current and remote field
testing (RFT) are nondestructive examination
methods that rely on the science
of electromagnetic induction, they are
very different in operation and application.
Eddy current testing relies on direct
coupling
between the inspection
coil and the test material. It is effective
with nonferrous tubing, such as copper,
brass, and titanium. RFT is a method used
to inspect ferrous tubes, such as carbon
steel and ferritic stainless steel. It was
developed to overcome the permeability
effects in ferromagnetic tubing.
The RFT inspection process uses slower
probe speeds, and it is less sensitive
to local defects than conventional eddy
current testing. Yet, it still provides
valuable information on tube condition.
Higher fill factors are not as critical in
RFT as in eddy current testing, with 60%
to 70% fill factor being commonplace.
Although RFT can detect both internal
and external defects, it cannot distinguish
between them.
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POWER March 2017

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