POWER February 2016 - 45

OPERATIONS & MAINTENANCE
5. Walking the line. A Pure Technologies technician walks the electromagnetic sensor and
data logger along the circulating water line on a stabilized conveyance that had to be lowered in
eight pieces through a manhole and reassembled below. Courtesy: NAES
lated, eddy currents are induced through the
insulation into the wall itself.
When the eddying currents reach their
maximum value after saturating the wall
material, they begin to decay back to a neutral
state. This decay is detected by the PEC
system, which translates the duration of this
active field into a distance, which in turn is
calculated as an average wall thickness. The
eddy current generation and subsequent decay
varies with the thickness of the material under
the " footprint " of the probe. The software
determines a comparative result, relating the
thickness displayed to a known (or assumed)
wall thickness at a selected reference point.
The main limitation of this technique is
that PEC averages the wall thickness over its
" footprint, " a roughly circular area where the
eddy currents flow. The footprint's diameter
approximately equals the distance between
the probe and the metal surface. PEC is thus
well-suited to determining wall loss over a
relatively large area but not to detecting very
localized damage.
PEC is based on relative values, which is
do this inspection looked something like an
oversized sawhorse built of machined aluminum
with bicycle wheels mounted on the legs
(Figure 5). We had to lower it in eight pieces
through a manhole and then wing-nut it together
down below. The technician mounted
his sensors and laptop on the cross-member
and adjusted the angle of the wheels so he
could keep it stable as he walked it through
the supply and return pipelines.
This was the first time in 10 years of operation
we'd had a look at this line. We knew
that if it failed, it would fail catastrophically-likely
resulting in a lengthy and expensive
unplanned outage and even more costly
environmental remediation.
Once the Pure team had analyzed the data
for wire damage and its severity, they could
use it to project a likely failure date, which
would give us a leg up on procuring materi6.
Do no harm. Mistras Group technicians
use pulsed eddy current (PEC) technology
deployed on a submersible robot to inspect
the interior of the plant's large water storage
tanks without harming the insulation or epoxy
liners. Courtesy: NAES
als in advance. After examining a total of 40
PCCP sections spanning the 708-foot combined
length of the two lines, however, they
reported only slight indications of wire damage
in two sections.
PEC Inspection of the Water Tanks
We brought in a team from the Houston-based
Mistras Group to inspect for deterioration inside
the 500,000-gallon demineralized and
800,000-gallon makeup water tanks (Figure
6). They recommended visual inspection-
using a submersible robot of the type used
to inspect ship hulls (Figure 7)-as well as
pulsed eddy current (PEC) inspection to measure
the tanks' carbon-steel wall thickness.
How exactly does PEC work? It's like
something out of an old sci-fi movie but without
the plot getting in the way. Basically, it's
a volumetric screening tool that can inspect
insulated carbon-steel components for internal
and external corrosion. It uses a stepped
signal-as opposed to the sinusoidal signal
used by conventional eddy current testing-
so it can measure several different frequencies
within a single step. This allowed the Mistras
team to measure wall thickness and liner flaws
indirectly without disturbing the insulation.
The PEC system consists of a pulser/receiver,
a laptop computer, and a transmitter/
receiver coil (sensor or probe). Instead of using
a sinusoidal AC signal, as with conventional
eddy current (EC) testing, PEC excites
the coil with a pulse of DC voltage. When this
current is switched off, the sudden change in
the magnetic field induces eddy currents in
any conductive component near the transmitting
coil. When a tank wall is lined or insuFebruary
2016 | POWER
www.powermag.com
another limitation. It can detect differences in
the thickness of a single object, such as a tank
wall. This will suffice for many applications,
and when you need a quantitative result, you
can calibrate at a given point within the grid
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POWER February 2016

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