Germany [Summary country report on coordinated research project on development of protocols for corrosion and deposit evaluation in large diameter pipes by radiography]

Full text: In the framework of the CRP, Germany provided certified stepped film density strips for the calibration of film densitometers. They were used by each participant in his own country to validate the density measurements made in the radiographs prepared from the test specimen. According to the working plan, BAM belongs to Group 2 (8 and 12 inch pipe diameter, Co 60 available). The necessary raw pipe material was ordered and the corresponding drawings for machining were prepared in 2003. Because of weight and handiness it was decided to cut each of the two pipes into 2 halves, i.e. one half with outer steps and the other half with inner steps. In this way, a total number of 4 pipes were produced. As material, steel St-52 was chosen, the maximum wall thickness was 26 mm. The film exposure was finalized in October 2003. As radiation source, Co 60 was used and, as detector, AGFA D7 films with front/back screens of 0.5 mm Fe. Two different inspection methods have been applied: tangential projection technique (TRT) for absolute wall thickness measurements and double wall penetration technique (DWT) for estimation of wall thickness differences in the pipe centre. The combination of Fe screens with AGFA D7 films gives an excellent image quality with very low noise, but needs about double exposure time as compared with Pb screens. After film exposure, the evaluation of the films started. Wall thicknesses and optical film densities have been measured according to the recommended procedures as agreed at the expert meetings. In parallel Excel data sheets were developed for easy data evaluation and for the comparison of results provided by the different participating countries. The final Excel sheets for data evaluation and data collection have been distributed to the other CRP participants at the end of 2003. All films have been digitized at BAM Berlin. The digitized films have been evaluated also by special software, developed together with BASF for the German chemical industry. For film evaluation in TRT mode at higher wall thickness, a new procedure for correct measurement of the outer pipe edges at film densities higher than 4 (the scratching technique) was presented at the expert meeting in 2004. The maximum penetrated steel thickness with Co-60 was determined to be 130 mm, with scratching and higher film densities, this was extended to 155 mm. In the DWT evaluation mode, a constant effective attenuation coefficient at (0.028 +/- 0.004) mm was found. The measurement accuracy is about 0.5 mm. No influence of insulation was found with Co-60 in both techniques. The final comparison with data available by the other CRP partners verified these findings. Procedures and application sheets were drafted for both inspection techniques and discussed with the CRP partners in the final meeting in 2005. The computer based film evaluation allows a better precision at lower wall thicknesses and avoids the additional high density films for TRT, because the inner wall profile can be detected by digital profile evaluation at lower densities as compared to visual viewing at the light box. (author)