Eddy
current inspection is often used to detect corrosion, erosion,
cracking and other changes in tubing. Heat exchangers and steam
generators, which are used in power plants, have thousands of
tubes that must be prevented from leaking. This is especially
important in nuclear power plants where reused, contaminated water
must be prevented from mixing with fresh water that will be returned
to the environment. The contaminated water flows on one side of
the tube (inside or outside) and the fresh water flows on the
other side. The heat is transferred from the contaminated water
to the fresh water and the fresh water is then returned back to
is source, which is usually a lake or river. It is very important
to keep the two water sources from mixing, so power plants are
periodically shutdown so the tubes and other equipment can be
inspected and repaired. The eddy current test method and the related
remote field testing method provide high-speed inspection techniques
for these applications.
A
technique that is often used involves feeding a differential bobbin
probe into the individual tube of the heat exchanger. With the
differential probe, no signal will be seen on the eddy current
instrument as long as no metal thinning is present. When metal
thinning is present, a loop will be seen on the impedance plane
as one coil of the differential probe passes over the flawed area
and a second loop will be produced when the second coil passes
over the damage. When the corrosion is on the outside surface
of the tube, the depth of corrosion is indicated by a shift in
the phase lag. The size of the indication provides an indication
of the total extent of the corrosion damage.
A tube inspection using a bobbin probe is simulated
below. Click the "null" button and then drag either
the absolute or the differential probe through the tube. Note
the different signal responses provided by the two probes. Also
note that the absolute probe is much more sensitive to dings and
the build up of magnetite on the outside of the tube than the
differential probe is.
Eddy
current inspection is often used to detect corrosion, erosion,
cracking and other changes in tubing. Heat exchangers and steam
generators, which are used in power plants, have thousands of
tubes that must be prevented from leaking. This is especially
important in nuclear power plants where reused, contaminated water
must be prevented from mixing with fresh water that will be returned
to the environment. The contaminated water flows on one side of
the tube (inside or outside) and the fresh water flows on the
other side. The heat is transferred from the contaminated water
to the fresh water and the fresh water is then returned back to
is source, which is usually a lake or river. It is very important
to keep the two water sources from mixing, so power plants are
periodically shutdown so the tubes and other equipment can be
inspected and repaired. The eddy current test method and the related
remote field testing method provide high-speed inspection techniques
for these applications.
A
technique that is often used involves feeding a differential bobbin
probe into the individual tube of the heat exchanger. With the
differential probe, no signal will be seen on the eddy current
instrument as long as no metal thinning is present. When metal
thinning is present, a loop will be seen on the impedance plane
as one coil of the differential probe passes over the flawed area
and a second loop will be produced when the second coil passes
over the damage. When the corrosion is on the outside surface
of the tube, the depth of corrosion is indicated by a shift in
the phase lag. The size of the indication provides an indication
of the total extent of the corrosion damage.