Detection of localised flaws in small-diameter carbon steel tubes using multi-NDE techniques

Abstract

This paper proposes multi-NDE techniques for enhanced and reliable detection of localised defects or flaws in small-diameter (19.05 mm outer diameter (OD) and 2.77 mm wall thickness (WT)) low-carbon steel tubes. Multi-NDE techniques comprising magnetic flux leakage (MFL), remote field eddy current (RFEC) and the ultrasonic internal rotary inspection system (IRIS) are used to investigate the detectability and spatial resolution for the detection of localised flaws of different types and sizes, which simulate flaws generally formed in carbon steel tubes. The studies reveal that the MFL and IRIS techniques detect 0.55 mm-deep holes and notches while the RFEC technique detects 1.11 mm-deep holes and 0.55 mm-deep notches. The IRIS technique is capable of resolving axial and circumferential holes (1 mm diameter and 1.39 mm depth) of 2 mm hole-to-hole separation, while the MFL technique can resolve axial holes of 2 mm hole-to-hole separation but not circumferential holes. The RFEC technique can detect axial holes as a single hole but is unable to detect circumferential holes, resulting in no spatial resolution for the axial and circumferential holes. Compared to the MFL and RFEC signals, IRIS readily discerns the spatial extent of the flaws with the possibility of sizing from the images. The study also reveals that the resolution capability of the IRIS technique is influenced by the speed of the turbine rotation inside the tube.