| Interruption of plant operation can be avoided if non destructive testing inspections are performed on-line at operating temperatures, which may be up to several hundred degrees Celsius in a petrochemical or electric power generating plant. However, there are operational temperature limits of the phased array transducers and associated plastic wedges used for ultrasonic inspections. In addition, there is a major gap in terms of professionally-approved high-temperature inspection techniques. In this project, the design and operation of an ultrasonic phased array system are described for inspections of engineering components such as pipe welds at elevated temperatures of up to 350°C. Wedges are built from plastics resistant to high temperature degradation, and equipped with a cooling jacket around the array. A model of the ultrasonic beam skew pattern due to thermal gradients inside a wedge is developed. The model is used in a separate algorithm to calculate transmission and reception time delays on individual array elements for generation of plane waves or focused beams in a hot test piece, while compensating for thermal gradient effects inside the wedge. The algorithm is also used to investigate the magnitude of thermal gradient effect on the calculated time delays of the phased array elements.;The algorithm results for inspections of test pieces at 150°C demonstrate that application of conventional element time delays can lead to serious phase errors. This results in major distortion of the desired beam profile, and very poor imaging resolution. However, experimental trials indicate that plane waves and focused beams can be generated in a hot test piece using the new focal law algorithm with appropriate timing delays applied to all active array elements. |
