Electro Magnetic Acoustic Transducers (EMAT)

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Electro-Magnetic Acoustic Transducer (EMAT) is an Ultrasonic Testing (UT) technique that generates sound in the component being inspected rather than the transducer. Because the sound is generated in the part inspected instead of the transducer, EMAT is a completely non-contact technique. This gives it significant advantages over more conventional piezoelectric transducers; although it also has some weaknesses, which can limit its applications.

EMAT works by generating ultrasonic waves into a test object using electromagnetic induction with two interacting magnetic fields. A relatively high frequency field generated by electrical coils interacts with a low frequency or static field generated by magnets to generate a Lorentz force in a manner similar to an electric motor. This disturbance is transferred to the lattice of the material, producing an elastic wave. In a reciprocal process, the interaction of elastic waves in the presence of a magnetic field induces currents in the receiving EMAT coil circuit. For ferromagnetic conductors, magnetostriction produces additional stresses that can enhance the signals to much higher levels than could be obtained by the Lorentz force alone.

EMAT is capable of generating all wave modes used in ultrasonic testing, including some modes that are very difficult or impractical with conventional transducers. They are used in a wide variety of applications in the O&G and chemical processing industries such as:

EMAT Inspection of In-Service Piping

One of the most common uses of a field EMAT inspection device is for volumetric inspection of in-service piping. This technique can be used to examine equipment such as large diameter header piping at high temperature, transmission lines for hazardous wastes, and natural gas distribution lines

EMAT Inspection of Tubulars

One of the the most common uses of EMAT inspection in the field is the inspection of tubes. Tubes are tested by generating and propagating ultrasonic Lamb waves circumferentially in the tubular. A Lamb wave is a class of plate waves which means that the entire volume of the pipe is filled with ultrasonic energy.

Remaining Wall Thickness at Pipe Supports

A particular application of employing EMATs on tubulars is for the specific determination of pipe wall thickness at the point of contact with a pipe support. Pipes often suffer localized OD corrosion at the support and a determination of remaining wall thickness is extremely difficult using standard techniques. Our recent work has improved on known analysis techniques.

EMAT Inspection of Vessels

EMAT is proving to be an efficient means of completely inspecting relatively large surface areas of material. The procedure, which has been developed to examine vessels and plates, involves mounting a transmit and a receive EMAT sensor in a fixture, allowing 1 to 2 feet separation. Bulk ultrasonic waves are then generated and transmitted through the material, effecting a volumetric test as the fixture moves across the surface.

As a non-contact UT technique, EMAT has distinct advantages that make it the technique of choice for many applications, including dry inspection capabilities, an imperviousness to surface conditions, and unique wave modes such as shear waves with horizontal polarization (SH waves).

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