Practical advantages of EMAT Ultrasonic Testing (UT) include dry inspection, an imperviousness to surface conditions, and unique wave modes such as shear waves with horizontal polarization (SH waves). In this article, I will cover the practical advantages of EMAT for in-service applications using Medium Range Ultrasonic Testing (MRUT).
As a review, EMAT, or Electro Magnetic Acoustic Transducer, is a UT technique that generates the sound in the component inspected instead of the transducer. As illustrated in Figure 1, EMAT generates ultrasonic waves into a test object using electromagnetic induction with two interacting magnetic fields. A relatively high frequency (RF) field generated by electrical coils interacts with a low frequency, or static, field generated by magnets to produce 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.
Because the sound is generated in the part inspected instead of the transducer a liquid couplant is not required. EMAT is a completely non-contact technique that has significant advantages for in-service applications over more conventional piezoelectric inspection techniques. More specifically, in this article, we will discuss the applications and techniques deployed when using guided waves with a MRUT system.
The two most common methods for in-service inspections with guided waves are long range UT (LRUT) and medium range UT (MRUT). LRUT is used almost exclusively for pipeline inspection in reflection mode to cover long distances (tens of meters) from a fixed ring of sensors. It normally works with low frequencies (sub-100kHz). Typical detection capability/limitation is 10% of cross-sectional area wall loss. MRUT is used in both attenuation and reflection mode to cover shorter distances (0.1-5m). The sensors are mounted on scanners to inspect long stretches of pipe or tanks. It typically works with frequencies from 100kHz to 1MHz, and can detect small pits (being approximately 10 times more sensitive than LRUT).
Modern MRUT equipment uses high frequency guided waves with a typical inspection range between 4” (0.1m) and 16’ (5m) to detect corrosion, cracks, and discontinuities on exposed tubes, gas lines, oil pipelines, and storage tanks. The system uses high-power EMAT technology to perform 100% scanning at speeds of up to 150mm/s on pipe diameters from 2.0” (50mm) to 46.0” (1168mm) with 0.5” (13mm) or less wall thickness. The inspection can be performed on rough and corroded surfaces, when covered with thin wraps or coatings (<3mm).
The equipment can be configured with a hand-held instrument and scanner (Figure 2) for smaller, easy-to-access jobs, or a high-speed portable system with an automated crawler (Figure 3) for fast scanning and climbing on pipes and tanks horizontally and vertically. The hand-held instruments are designed to be used with permanent magnet sensors, while the high-speed system can be used with permanent or pulsed magnet sensors for superior signal-to-noise performance. Both equipment options are suitable for axial and circumferential scanning techniques.
MRUT equipment includes sensors and software configurations to excite guided wave modes for different thicknesses and environmental conditions. With the use of higher frequencies and a shorter range, the MRUT technique detects isolated pitting and wall loss with up to 10 times better lateral resolution than LRUT systems with minimal or no dead zone.
Pipe & Tank Inspection – The MRUT technique using shear horizontal and lamb guided waves is an excellent choice for quick screening and detection of cracks, pits, and corrosion on in-service pipes and tanks. Applications include:
* Inspection under supports - One of the most problematic areas for corrosion. It works for both pipelines that are welded or those resting on supports.
* Air-to-soil interfaces - Using non-leaking guided waves, the technique can be used for inspection of the first 1-2m of an inaccessible structure (buried or hidden).
* Tanks - When mounted on a crawler, it permits covering large sections of the tank (1m) for quick screening and assessment.
* Free-standing pipeline - Detects smooth corrosion, pitting, and cracks on free-standing pipelines with minimum insulation (less than 3mm).
On relatively thin structures, it is possible to generate guided waves that fill up the material and permit a complete, volumetric inspection. The most common types of volumetric waves are Lamb waves. The sensors can be configured in either a Pitch-Catch or Pulse-Echo configuration depending on the type of scanning technique in use. There are two types of scanning techniques that can be employed with an MRUT system: axial and circumferential.
Axial scanning is a preferred method for scanning long distances of continuous pipe. As illustrated in Figure 4, it is a through transmission technique that achieves 100% volumetric pipe inspection through rapid ultrasonic sampling of the pipe circumference as the sensor is moved along the length of the pipe. Ultrasonic sampling is provided at up to 400 readings per second for fast scanning and identification of defects.
A pulse-echo sensor configuration measures ultrasonic transmissions around the pipe circumference. The transmitting and receiving elements are positioned less than 180 degrees apart around the pipe outer surface. A volumetric guided wave is produced and sent in both directions: clockwise and counterclockwise. The strength of both transmissions are measured by the receiver element. Defects, such as cracks, pits, and wall loss, provide observable attenuation of the sound or phase shift due to time of flight change.
Circumferential scanning is the preferred method when scanning around obstructions or around air to soil interfaces. As illustrated in Figure 5, it is a through transmission technique that achieves 100% volumetric pipe inspection through rapid ultrasonic sampling of the pipe as the sensor is moved around the pipe circumference. Ultrasonic sampling provides up to 400 readings per second for fast scanning and identification of defects.
A pulse-echo sensor configuration measures ultrasonic reflections down the length of the pipe. Defects, such as cracks, pits, and wall loss, provide observable reflections of the sound.
As a non-contact UT technique, EMAT has distinct advantages that make it the technique of choice for many applications. When combined with MRUT sensors and methods, the advantages of EMAT translate into an effective high-speed scanning system for in-service applications.
