Radiographic Evaluation of Wall Thickness, Corrosion and Deposits in Pipelines

ABSTRACT

Non-destructive testing (NDT) is a group of inspection techniques used to detect, locate and assess flaws in materials without affecting, in any way, their continued usefulness or serviceability. NDT has the ability to inspect castings, weldments and measure wall thicknesses in an accurate and comprehensive manner. In this study, NDT techniques were used to inspect large diameter pipes that are used for transporting substance in the form of liquids and gases in industries. The aim of the investigation was to use NDT techniques to inspect pipes for corrosion and deposit and to evaluate the remaining wall thickness of the pipes. The double wall technique (DWT) and tangential radiographic technique (TRT) were used to evaluate remaining wall thickness, deposits and corrosion in the pipes. The DWT technique was used to inspect corrosion and deposits in the pipes while TRT technique was used to evaluate the penetrated wall thickness of the corrosion attack in tangential position. Two steel pipes having known varying wall thicknesses ranging from 4.00 mm to 13.00 mm with diameter of 150.00 mm were examined to authenticate the accuracy and reliability of the tangential method that was used to measure the remaining wall thickness. The tangential configuration resulted in a higher material thickness, which therefore required more time of exposure compared to the DWT method. The exposure angle of the source to the tangential part of the specimen was approximately seventy-seven degree (77o ). This angle was calculated using the values of source-to-film distance and the outer diameter of the experimental pipe. The measurements for the remaining wall thickness was done with the TRT. This is irrespective of whether the test pipes were insulated or not insulated. The film generated was compared with a normal pipe piece that was not machined to serve as control. From the radiograph obtained, six (6) and seven (7) xiii indications rounded depicting pitting corrosion were revealed on the radiograph of the internally fabricated pipe and externally fabricated pipe respectively. The interpretations obtained from radiographs after the TRT was employed showed that recorded wall thickness obtained from the film is about twice the value of the calculated true wall thickness. The maximum standard deviation of the measured thickness from the radiograph was 0.1414 and a standard error of 0.07 for the externally fabricated pipe. And a maximum standard deviation of 0.085 for the internally fabricated pipe. From this research, the DWT and TRT were successful in the evaluation of corrosion, deposits and wall thickness of the pipes used.