T9074-AS-GIB-010/271 Rev 1
A.5.2 Reference Standard Location. The reference standard shall be located at the same distance from the radiation source and imaging plate as the pipe wall of interest (the tangent of the pipe insulation). The reference standard shall be aligned such that the curved surface of the reference standard is oriented with respect to the X-ray beam in the same direction as the curved surface of the pipe. The procedure shall include requirements for the amount of reference standard misalignment and location error permissible and still provide for a qualified thickness measurement. Reference standard location error is evidenced by the image of the reference standard in the insulation. A.5.3 Minimum Source to Imaging Plate Distance . The minimum source to imaging plate distance shall be five times the nominal pipe diameter. When accessibility does not permit the minimum source to imaging plate distance, the greatest possible source to imaging plate distance shall be used and reported to the NDT Examiner for disposition. For each specific case, the NDT Examiner shall determine whether the exposure is still feasible and of sufficient accuracy. A statement summarizing the limited accessibility condition and impact of the condition on the accuracy of the pipe wall measurement shall be annotated in the inspection report. Alternatively, the NDT Examiner may include predefined limits of shorter source to imaging plate distance in the inspection procedure. These conditions shall still be annotated in the inspection report. A.5.4 Source/Imaging Plate Alignment. The imaging plate cassette shall be in contact with the insulation whenever possible and aligned perpendicular to the axis between the source and the region of interest and parallel to the axis of the pipe. The source shall be centered at the midwall of the insulation thickness such that the geometric enlargement is the same for the pipe wall and the reference standard (see figure A-1). Other source locations will result in inaccuracies in the pipe wall measurement due to the different level of geometric enlargement between the reference standard and the pipe wall of interest. This error includes measurements made along the pipe axis as well as measurements of the other pipe wall when the entire pipe diameter is imaged on the same imaging plate. Other source alignments, such as centering the source over the pipe with a reference standard adjacent to each wall to evaluate both walls at once, may be used as long as they are documented in the procedure and qualified during the procedure qualification. Specific cases where these source alignment requirements cannot be met shall be reported to the NDT Examiner for disposition. The NDT Examiner instructions of whether the exposure is still feasible and a statement summarizing the impact of the condition on the accuracy of the pipe wall measurement for each specific case shall be annotated in the inspection report. Alternatively, the NDT Examiner may include predefined limits of misalignment and imaging plate to insulation contact in the inspection procedure. These conditions shall still be annotated in the inspection report.
Figure A-1. Alignment of Radiation Source and Imaging Plate.
A.5.5 Radiographic Contrast. The inspection procedure shall include specific direction to determine whether an exposure has sufficient radiographic contrast at both the inner and outer pipe wall edges to result in a reliable measurement. The requirement shall be based on quantitative pixel value measurements on each edge of the pipe wall edge image, or alternatively, the procedure shall include specific direction and example images to use as direction to the inspector as to
A-3
Made with FlippingBook - Share PDF online