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    Hi – I wanted to kick off by discussing the capability of a NDT method. Should we know what accuracy we are likely to get when we employ an Ultrasonic thickness test or what size of surface defect might we be able to detect with MPI or DPI ?
    I think most people would agree, why inspect if we don’t know what the method is capable of finding and how accurate it is. The understanding of a NDT’s method’s capabilit    y probably applies more to in-service inspection than new construction but you could still argue that an understanding of what can effect NDT accuracy would assist both sectors of industry. In this Blog I am going to look at BS7910 which has some interesting data on NDT capability. This British Standard was published in 2013 so not a new document but tucked away at the back is some data on NDT capability and it does not provide any comfort for Engineers carrying out fitness-for-service critical assessments of planar type defects.

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    The document indicates for MPI (DPI is similar) testing a weld with a poor profile your detection capability is 20mm and your accuracy is +/- 10mm. Even with some local dressing the figures only become 10mm and +/- 5mm. Not very encouraging for NDT capability and for the Engineer trying to put forward their calculation for continued use. My experience is we can do better than this but improvement is based on improving the surface finish prior to the MPI or DPI examination.
    Ultrasonic Thickness Testing (UTT) is another test where this method’s capability is important. For example if you indicated that your UTT accuracy was +/- 0.1mm then the fitness-for-service critical assessment of the UTT readings should take this into consideration and reduce all UTT readings by 0.1mm. That change would be considered fairly minor but in-service UTT on aging plant the UTT +/- accuracy is most likely to be much higher and when applied could have a significant impact on the Engineers continued fitness for purpose calculations.

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    September 2020

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