In this paper, they describe a technique to internally inspect HRSG finned tubes via the header’s inspection ports. A Near Field Array (NFA) is used to generate C-scans, Absolute traces and Lissajous patterns characterise the internal integrity of HRSG finned tubes.
Despite promising results, electromagnetic NDT techniques deployed to externally inspect finned tubes were shown to be limited by access due to HRSG design. We therefore determined to develop an internal NDT technique which can be remotely deployed via headers through standard inspection stubs. The initial challenge for inspecting finned tubes in HRSGs is first to be able to deploy the NDT probe at a chosen location.
The Eddy Current (EC) technique selected was the Near Field Array (NFA) technique. In its basic form, the Near Field Technique (NFT) relies on a driver and pickup coil probe that measures the lift-off, and converts it to amplitude based signals. The receiver coil is close to the transmitter coil in order to measure the variation of the near[S1] -field zone of the driver coil. The magnetic field in the near field has very low penetration and is therefore not affected by features such as fins, hangers etc. NFT performs well on the internal surface of finned tubes since the eddy currents do not penetrate the tube wall. NFT is specifically suited to detecting corrosion and erosion inside carbon steel tubing. Recent improvements have made array technology more affordable and Near Field Arrays are now commercially available. The technology is based on the same principle as NFT. However, in this case the single pick-up coil is replaced by a ring of smaller coils acting as individual receivers. The coils behave therefore like an array allowing for C-scan imaging.
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