THE FOUR MAIN TYPES OF NDT

THERE ARE FOUR MAIN TYPES OF NDT  METHODS USED IN POWER PLANTS

Ultrasonic testing

Ultrasonic testing uses high-frequency sound waves called ultrasonic pulse waves to detect flaws or other imperfections or defects in metal components, and it can also be used to identify and monitor changes like shifts in thickness within the components that may result in failure or other problems in the future. One of the most common uses of ultrasonic testing is to evaluate corrosion in pipelines and other enclosed structures or components. Ultrasonic testing is most commonly performed on metals and metallic alloys, but it can also be used to test concrete and composites, and sometimes even wood. Although the method offers the advantage of deep penetration to evaluate problems within components, ultrasonic testing requires a high degree of training and competence to apply the method and interpret the results. Results also may be affected by surface imperfections or irregularities.

Magnetic particle testing

Also called magnetic particle inspection (MPI), this technique can be used in metals that have magnetic properties (also called ferromagnetic materials) such as iron, cobalt, nickel and some alloys. MPI works by issuing a magnetic field into the component being tested – essentially magnetizing the component. Next, iron particles are applied to the surface either in a dry form or suspended in liquid. Because even the tiniest cracks and fissures will allow some of the magnetism to “leak” out, these particles become attracted to the tiny cracks, building up and providing evidence of a leak, or “indication.” The component may be magnetized by an electrical current, called direct magnetization, or indirectly by applying a magnetic field to the outside of the component. Like ultrasonic testing, this approach requires considerable training and expertise

Liquid penetrant testing

Liquid penetrant testing is one of the oldest methods of non-destructive testing. In this method, two solutions are used – a penetrant that is first applied to the surface of the component to be tested and a developer that combines with the penetrant to help reveal tiny cracks or other imperfections. After the penetrant is applied, it is allowed to “soak in” for a prescribed period of time to help reveal defects that extend below the component’s surface. Liquid penetrant testing is used in a wide variety of industries and is one of the most popular techniques of non-destructive testing because it is relatively inexpensive, requires no costly special equipment and relies on limited experience and training, making it widely accessible to companies of all sizes.

Radiographic testing

Radiographic testing uses x-rays or radioactive isotopes to evaluate components in much the same way a doctor uses an x-ray to evaluate the structures inside a human body. This type of testing works by sending the radiation through the component and measuring the amount that emerges on the opposite side. The result is a photographic-type image that reflects areas of different densities, including areas where material may be missing or thinned, such as in cracks or areas of weakness. Radiographic testing can be difficult to use on components with irregular surface areas since those irregularities can influence the thickness of the materials that are being evaluated.