POWER October 2011 - 21

in different operating environments have different failure mechanisms,
making some of them more suited than others for inspection
by APR. Finned tube heat exchangers are a typical example
of heat exchangers particularly conducive to APR inspection.
The reason APR is particularly useful on finned tube heat exchangers
is that it is purely an internal pipe inspection method.
This is a limitation where tubes are susceptible to external damage
by corrosion or abrasion from support plates. In finned tube
exchangers, however, the tubes often rest on top of each other,
supported by the fins. There are no support plates, and in cases
where the external fluid is forced air, there are no hostile chemicals
on the exterior of the tubes.
Dual-cycle geothermal power plants often employ this type
of heat exchangers. In this type of geothermal plant, hot brine
heats an organic liquid having a low boiling temperature, turning
it into vapor. This higher-pressure gas expands through the
turbine, producing power. The lower-pressure gas is then condensed
using finned tube heat exchangers (condensers in this
case), which are cooled by air forced across the highly finned
heat exchanger tubes by large fans. Thoroughly inspecting these
heat exchanger pipes has been problematic, until now.
How APR Works
The theory behind APR is to inject a wideband acoustic pulse into
a pipe. This pulse acts as a form of " virtual probe. " As long as
the pulse encounters no discontinuities, it continues to propagate
down the tube. Whenever a discontinuity is encountered-
such as a blockage, expansion (due to wall loss, for example), or
hole-a reflection is created. The reflected waves propagate back
5. Different discontinuities have different signatures.
In tubes belonging to heat exchangers, any sensed discontinuity represents
a fault. Source: AcousticEye
Impinging pulse
Reflection from
a constriction
Reflection from
a dilation
Reflection from
a leak
down the tube, where they are recorded for analysis (Figure 5).
The ultimate purpose of tube inspection should be to examine
tubes as rapidly as possible and then analyze the measurements
both rapidly and using objective criteria. Keeping this in mind,
APR is very well-suited to this task on both counts. The pulse
acting as a probe travels through the tube at the speed of sound,
resulting in inspection rates much faster than those possible
with other techniques. Measurement of a single tube takes only
several seconds, and there is no physical probe to push through
the tubes or become stuck. The resultant measurements can then
be analyzed by appropriate signal processing software, which is
faster and more objective than human analysis.
CIRCLE 13 ON READER SERVICE CARD
October 2011 | POWER
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21

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POWER October 2011

Table of Contents for the Digital Edition of POWER October 2011

Contents
POWER October 2011 - Cover1
POWER October 2011 - Cover2
POWER October 2011 - Contents
POWER October 2011 - 2
POWER October 2011 - 3
POWER October 2011 - 4
POWER October 2011 - 5
POWER October 2011 - 6
POWER October 2011 - 7
POWER October 2011 - 8
POWER October 2011 - 9
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POWER October 2011 - Cover3
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