Inspectioneering Journal

Magnetostrictive Sensor Technology - How it's Grown in the Last 10 Years

By Hegeon Kwun at Southwest Research Institute, and Glenn Light at Southwest Research Institute. This article appears in the January/February 2005 issue of Inspectioneering Journal.

Nearly ten years ago the magnetostrictive sensor (MsS) technology was reported in this journal (July/August 1996 Issue, Volume 2 Issue 4) as a method to detect corrosion in insulated piping. At that time, the MsS technology consisted primarily of the longitudinal guided wave mode introduced into the pipe with a coil wrapped around the steel pipe and a number of large magnets setting up an axially oriented magnetic biasing field in the area of the coil. The longitudinal mode worked well for dry, unfilled pipe. However, in liquid filled pipes, the longitudinal mode didn’t work well because it interacts with the liquid, producing extraneous signals that, in turn, cause difficulty in analyzing data.

To provide a little background and historical perspective, we’ll start this article with an excerpt from the 1996 issue:

“SwRI uses magnetostrictive sensors (MsS) which are electromagnetic coils that produce magnetic fields in carbon steel pipes. When these coils are excited with alternating current, the changing magnetic fields in the pipe cause small changes in the physical dimensions of the pipe. This phenomenon is the magnetostrictive effect and it can be used to generate elastic waves in frequencies up to a few hundred kHz. These elastic waves can examine the entire cross-sectional area of a pipe and are sensitive to very small OD and ID circumferential cracking and corrosion defects.

In October 1995, SwRI initiated a joint industry project to develop the capability to detect corrosion in insulated piping systems using MsS. The first phase of the project has been very successful and is nearing completion. In the first phase of the project, SwRI demonstrated that MsSs can detect an isolated pit ( 1/4- inch diameter and 1/8-inch dep) in a 4-inch, schedule 40 pipe from a distance of 40 feet from the sensor.

SwRI is ready to begin limited field trials on strainght lengths of piping to evaluate the capabilities and limitations of the laboratory prorotype system to detect corrosion in insulated piping. In Phase 2 of the project, SwRI will continue the development of the technology, build proprotype instrumentaiton, and develop test procedures for more complex piping system configurations......” End of excerpt.

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