testing (NDT) is a sustainable option. NDT makes it possible to accurately assess a pipe’s condition for validation of expected rates of corrosion, and/ or to detect unexpected corrosion. Areas of concern can then be corrected before a breakdown or Figure 1. The visual schematic of a single element transducer (left) compared to a dual element catastrophic failure occurs. transducer (right). As a result of nondestructive inspection analysis, operators and organisations can determine the need for localised repair or more involved pipeline replacement. This promotes cost control while sustainably maintaining pipeline operations and high safety standards.
How ultrasonic NDT helps inspectors monitor for corrosion
Figure 2. An example A-scan waveform display showing a thickness measurement of 0.500 in.
Many times, an inspector only has access to the outside surface of a pipe, leaving them blind to the inside conditions. The use of ultrasonic technology can provide a nonintrusive monitoring of corrosion in pipelines. Ultrasonic NDT is used to accurately measure pipe wall thickness. Ultrasonic NDT using dual element transducers can help inspectors measure the remaining wall thickness of pipes to determine a pipe’s condition more accurately.
Theory of operation: how ultrasonic NDT technology works
Figure 3. The area in red illustrates the presence of corrosion and the thinning effect that it has on pipe wall thickness.
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Single element ultrasonic NDT technology works by sending high-frequency sound waves into the pipe through a transducer. The transducer contains a piezoelectric element, which is excited by a short electrical pulse to generate a burst of ultrasonic waves. The sound waves travel through the pipe until they encounter a back wall or other boundary. They are then reflected back to the transducer, which converts the sound energy back into electrical energy. In thin-walled materials, such as those used to construct pipeline infrastructures, this energy can sometimes generate noise that can result in false indications and lead to unnecessary pipeline repair or replacement. Dual element transducers, commonly used in corrosion survey applications, differ from single element in that they have separate transmitting and receiving elements separated by a sound barrier. The dual element transducers also have integrated delay lines with a slight roof angle (Figure 1).
