Effects of velocity on magnetic flux leakage inspection of gas pipelines. Topical report, April 1994-April 1996

The magnetic flux leakage (MFL) technique is the most commonly used technique to inspect large diameter transmission pipelines. With hundreds of thousands of miles of transmission pipelines, the size of the problem suggests the use of high inspection velocities. Also, the typical product flow velocity in gas transmission lines often exceeds 10 miles per hour. However, high inspection velocities can compromise the ability to accurately detect and characterize defect. In general, changes in the flux leakage signals near metal loss defects can be seen at speeds exceeding a few miles per hour and become more pronounced as speed increases. This report starts with a general theory of velocity that shows that velocity affects the magnetic field at the point where the field enters the pipe and where the field in the pipe is altered by metal loss defects.