Inspectioneering
Inspectioneering Journal

Complete Volumetric Inspection of In-Service Vessels and Piping Using EMAT Ultrasound

By Paul K. Davidson at WIS, Inc.. This article appears in the May/June 1999 issue of Inspectioneering Journal

EMATs (Electro-Magnetic Acoustic Transducers) have been used for over six years for field service inspection of in-service piping. Recent advances in technology have allowed us to inspect new types of on-stream piping.

This paper presents a brief introduction to EMATs and then discusses recent advances in EMAT inspection techniques as applied to chemical and petrochemical plants. The paper presents a variety of new applications and presents live field data which clearly show the benefits of EMATs. Among those benefits are the ability to rapidly inspect 100% of in-service and on-stream piping, tanks, pressure vessels and boilers.

Basic EMAT Theory

In a simplified view, an EMAT can be considered to be a transducer that uses the interaction of a magnetic wave and eddy currents to couple ultrasound into test pieces. Unlike piezoelectric transducers, which generate the ultrasonic wave in a crystal, an EMAT actually generates the ultrasound in the body of the test piece. This has many advantages for work in the field.

In addition to being non-couplant, EMATs will work at high temperatures and pass through and under various coatings while scanning at high speeds. Because there is no incidence angle or crystal selection required, EMATs are less prone to operator error and will produce more repeatable results. Additionally, EMAT’s have the very real advantage of being able to easily generate a variety of ultrasonic wave modes. Besides the familiar longitudinal and SV (shear vertical) waves, EMATs can also generate SH (shear horizontal) waves and Lamb waves.

A magnetic field is applied to the test piece and the EMAT circuit is placed in the magnetic field. A high power AC toneburst is then driven through the EMAT. The toneburst passes into the test piece as eddy currents. The eddy currents interact with the static magnetic field to produce stresses in the metal leading to the generation of ultrasonic waves. The frequency of the current, the shape of the EMAT transducer, its position in the magnetic field and the thickness of the test piece combine to determine the mode of ultrasound generated. The wave mode and frequency determine the type of analysis to be performed. Figure 1 shows the vector diagrams for Lorenz force generation of SV and SH waves. The frequency of the current, the shape of the EMAT transducer and its position in the magnetic field determine the type (or mode) of ultrasound generated. The wave mode and frequency determine the type of analysis to be performed.

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