Apparatus and method for inspecting the profile of the inner wall of a tube employing a wall follower and an eddy current probe

摘要

Both an apparatus and method for accurately inspecting the profile of the inner wall of a conduit, such as a heat exchanger tube in a steam generator, is disclosed herein. The apparatus comprises a cylindrical probe body that is insertable within the tube, a probe assembly disposed within the probe body that includes an eddy current sensing coil and a copper plate which are movable with respect to one another, and a wall follower assembly including a stylus on one end and which is linked to the probe assembly on its other end for converting changes in the radius of the tube wall into changes in the distance between the eddy current sensing coil and the copper plate. A ferrite core is disposed within the sensing coil for focusing the electromagnetic flux into the copper plate. Additionally, the sensing coil is serially connected to a balancing coil in order to provide an accurate zero point for the sensing coil. In the preferred embodiment, the sensing coil is stationarily mounted within the probe body, while the copper plate is connected to the radially movable stylus assembly. The probe body and all of the components of the wall follower assembly including the springs which resiliently urge the stylus toward the tube wall are formed from nonconductive plastic materials so as to maximize the electromagnetic coupling between the eddy current sensing coil and the copper plate. The apparatus is capable of accurately detecting not only localized flaws within the inner tube wall, but also tube ovality and wall thinning which are typically caused by the deposition of sludge in the crevice regions of tubesheets and support plates of steam generators.