Anecdota

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Seeing Cracks in Heat Exchanger Tubes


Circumferential cracking is very difficult to detect and
characterize with standard eddy current bobbin probes. That’s mainly because eddy currents
generated by bobbin probes flow parallel to circumferential cracks. Bobbin probes are also sensitive to ferromagnetic interference generated by tubesheets and
support plates of heat exchangers. The large signals generated
by these structures usually mask small signals from
circumferential cracks. Bobbin probes are
also sensitive to liftoff such as the one found
in expansion transitions. This is where the patented DefHi® eddy
current array probe comes in. The DefHi probe is designed to
efficiently detect and size all cracks in tubes, including circumferential
cracks, wherever they are. DefHi probes incorporate
standard bobbin coils and an array of oval coils. The array increases the probe’s overall
coil density and sensitivity to defects. Multiplexing is key here. The coils are driven sequentially in a clever pattern. This enables the DefHi probe to examine the
entire circumference of the tube with no mutual interference between the coils. The result: high-definition data. The DefHi probe can therefore detect and size
defects typically detected by bobbin probes but also circumferential defects. In a nutshell, the DefHi probe’s multichannel design yields performances similar to rotating probes but at bobbin probe speeds and in a single pass. To demonstrate the capabilities of the DefHi probe, lets see this modified ASME calibration tube. The tube has a variety of simulated defects,
including circumferential notches. First, let’s perform an acquisition without
any ferromagnetic interference to see what we can find. As you can see, Magnifi® displays standard ASME defects detected with a bobbin probe. But you can also see that the array yields very clear imaging of the axial and circumferential crack-like defects in the form of an easy-to-read C-scan. Now, let’s simulate a tubesheet
with this carbon steel ring placed right next to our circumferential notch and perform the acquisition again. You can see that the ferromagnetic interference
induces a strong negative response, but the defect remains visible. The C-scan clearly shows the
circumferential extent of the crack, the phase is used to assess the depth. As you’ve seen, the DefHi solution allows you
to detect and size circumstantial cracks and quantify the extent, detect and size axial cracks, characterize cracks close to tubesheets, offer personnel without extensive ECT knowledge the necessary confidence about their inspection results. Thank you for watching. For more information about eddy current
array technology, visit our Website and follow us on YouTube, Google+, LinkedIn, and SlideShare.

One thought on “Seeing Cracks in Heat Exchanger Tubes

  1. Seeing Cracks in Heat Exchanger Tubes

    Circumferential cracking is very difficult to detect and characterize with standard eddy current bobbin probes for various reasons. Eddyfi has developed the patented DefHi eddy current array probe to help detect some cracks. Discover how by watching our latest video.

    http://youtu.be/TicErOSsdgk

    #heatexchanger   #eddycurrentarray   #nondestructivetesting  

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