Rapid long-range inspection of chemical plant pipework using guided waves

Corrosion in pipework is a major problem in the oil. chemical and other ind ustries. Many pipes are insulated, which means that even external corrosion cannot be seen, without removing the insulation, which is prohibitively ex pensive. Particularly severe problems are encountered at road crossings whe re the pipe cannot be inspected without excavation. Ultrasonic guided waves in the pipe wall provide an attractive solution to this problem because th ey can be excited at one location on the pipe and will propagate many metre s along the pipe, returning echoes indicating the presence of corrosion or other pipe features. Guided Ultrasonics Ltd have now commercialised the tec hnique and this paper describes the results of an extensive set of field tr ials using the method, together with the results of systematic laboratory a nd theoretical investigations of the influence of defect depth and circumfe rential extent on the guided wave reflectivity: It is shown that propagatio n distances of over 25 m in pipe diameters from 2 to 24 inch can be obtaine d using a dry-coupled piezoelectric transducer system. The defect detection sensitivity is generally set to the removal of 10% of the cross sectional area of the pipe at a single location, but it is often possible to find sma ller defects if required. The technique was originally designed To work on pipes that were either uncoated or covered with, for example, epoxy paint. Recent tests have shown promising results with more attenuative coatings an d these are discussed. The results show that the technique has wide applica tion in pipe systems in the chemical and other industries.