Corrosion and other factors reduce the cross-sectional area of pipes. Most of the conventional NDT (non-destructive testing) methods measure the remaining pipe wall accurately, but over a small area only. Since it is uneconomical therefore to inspect 100 % of the pipe, a pragmatic approach is adopted, and the fitness for purpose of the pipe is determined by measuring the wall thickness at a number of points. However, this approach generally requires unrestricted access to the outer pipe surface, which is not possible when the pipe is insulated, has protective coatings, or is buried. A new method was developed for pipe damage detection and monitoring that allows the inspection of a long range of pipe with one sensor using the ultrasonic characteristics of so-called 'guided waves'. This technique has drastically reduced the work of insulation removal, scaffolding and restauration, and the inspection job itself. Ultrasonic guided waves are excited from easy-to-acccess locations on the pipe and will propagate along the pipe for many meters, and the returning echo indicates the presence of corrosion or other pipe features. Principles, methodology, applications, and limitations of this new LRUT (long-range ultrasonic testing) technique are described, including the inspection of petroleum and petrochemical pipelines at inaccessible locations such as below road crossings and under culverts. The reflectance of the guided wave occurs at points/locations with a change of the acoustic impedance expressed by the section area, density and sound velocity of the material. At constant density and sound velocity, the acoustic impedance only depends on the section area, and the waves are reflected from locations where the section area changes. The method does not give finite sizes of defects, cannot resolve small pits, and cannot differentiate between internal and external corrosion. Under ideal conditions, 160 m of pipe can be inspected in a single test (80 m in both directions), while this length is reduced to 40 m in 30-year-old pipes with some general corrosion and to 10 m in heavily corroded pipes with bitumen wrapping. The length of inspection extends over six welds or to the first flange or second bend or branch. Some experiences with inspections of pipes in an Indian refinery are briefly described.
Inspecting inaccessible piping. Long-range ultrasonic testing is a useful technique
Hydrocarbon Processing ; 87 , 11 ; 77-81
2008
4 Seiten, 2 Bilder, 4 Quellen
Article (Journal)
English
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