Author: Ph.D. Yolanda Reyes, October 05, 2022.
Introduction
Non-Destructive Testing (NDT) is key to guaranteeing the quality of metallic materials used in industries: industrial pressure vessel plants, petrochemical plants, power or nuclear power plants, maritime structures, metal structures, bridges and civil works.
What are non-destructive tests?
Non-destructive tests are techniques that do not cause damage to the material being inspected, and allow fast and reliable results to be taken to take preventive or corrective measures. For industrial plants, NDT methods are used for plant components, tank and piping construction, as well as cast and forged components, and metallic and non-metallic structures.
According to the ASNT, “Non-Destructive Testing is called any type of test performed on a material that does not permanently alter its physical, chemical, mechanical or dimensional properties.” These test methods have evolved as an essential element in modern engineering designs, production processes and technologies, maintenance and diagnostics. Therefore, the detection capacity and the reliability of non-destructive procedures are of vital importance for the final use of the inspected item, and respond to the challenges and needs of the manufacturing industry in general.
Importance of NDT
Due to the importance of using this tool, and in order to regulate its activity, non-destructive testing societies exist in most industrialized countries, with the ASNT (American Society For Nondestructive Testing) standing out worldwide. the CHSNDT (Chinese Society For Nondestructive Testing), and the EFN-DT (European Federation For Nondestructive Testing), which brings together the respective European societies of the different countries.
Among the main non-destructive tests are: induced currents or Eddy currents, inspection by penetrating liquids (dyes), industrial radiography, among others.
They are important to ensure plant reliability and safety, because they do not cause damage to components and systems, they offer advantages in terms of safety and profitability.
Industrial plant engineers, operators and non-destructive testing (NDT) technicians must be specialized and certified in NDT to detect the weak points of a wide range of components and systems. Thus guaranteeing compliance with design codes and current regulations both in new construction projects and in the maintenance of existing plants. It is also important to identify the most suitable non-destructive test for each specific problem and guarantee reliable results.
The application of these tests can be carried out on any type of metallic material, as well as its welds, its main applications being the construction of pressure vessels, petrochemical plants, power and nuclear power plants, the transport sector, the shipping industry and civil works ( metal structures and bridges).
Main techniques of Non-Destructive Testing
This article describes three of the main non-destructive tests, namely : TOFD (Time of flight diffraction), Phased Array Ultrasonic Testing (PAUT) and Penetrating Inks.
The Automated Test of TOFD (Time of flight diffraction):
Flight diffraction (TOFD) is a reliable ultrasonic non-destructive testing (UT) method used to look for flaws in welds. TOFD uses the time of flight of an ultrasonic pulse to find the location of a reflector. It can also be used for weld overlays and heat affected zones of other components, such as piping, pressure vessels, lining material, storage tanks, and structural steel.
Like most UT methods, TOFD works by blasting sound waves into a component and measuring the time it takes for them to return. What sets TOFD apart from other UT methods is that instead of just measuring high-amplitude sound waves reflecting off the back of the component, it measures the response time of low-amplitude waves diffracting off the back of the component. crack tips.
This technique uses a pair of ultrasonic transducers, one as a transmitter and one as a receiver. Low frequency waves propagate at an angle and only diffract back to the receiver if they hit a defect. If this happens, the time it takes for both waves to reach the receiver can be used to create a complete image of the weld and identify the size and location of the damage.
TOFD is one of the fastest non-destructive testing methods because, unlike other types of UT methods, only one scan is usually required to find defect information within the weld. It can locate and measure the size of many different types of defects with incredible precision. It also has a high degree of repeatability. Because of this, the growth of any defect can be tracked over time. Finally, it is capable of detecting damage that could traditionally only be detected using other techniques, such as pulse echo or radiography.
It is a powerful tool for welding inspection, widely used worldwide in the petrochemical, mining and nuclear sectors.
The technique allows for very fast and efficient quality control of very thick welds, or with complex bevel configurations, where the PE or PAUT technique can have problems.
The TOFD technique detects discontinuities regardless of their orientation, however, it has 2 dead zones, so the test must be combined with Ultrasonic Techniques such as conventional UT, mechanized UT PE or Phased array.
Phased Array Ultrasonic Test (PAUT):
It is an advanced Non-Destructive Testing technique that uses a set of ultrasonic tests (UT), made up of numerous small elements, each of which is pulsed individually with synchronization by computer programming (“phases”). When these elements are excited using different time delays, the beams can be directed at different angles, focused at different depths, or multiplexed along a long array, creating the beam’s electronic motion.
Phased array probes can be used manually in a free-running mode by rubbing the surface of a component, connected to an encoder to record position, or mounted to a motorized or semi-automated scanner for optimal productivity. Using phased array probes in direct contact with the component, whether mounted on a hard wedge, a water delay line, or even inside a wheel probe, gives inspectors the ability to quickly scan large areas in Look for corrosion, cracks and other defects with high resolution.
Penetrant liquid tests:
This type of test was used in a chlorine-soda chemical plant, specifically, in a chlorine tank, due to an unidentified leak, being used to detect possible porosities, cracks, joints on the cylindrical surface. The equipment used to carry out the field test consisted of solvents, cleaner, a liquid called dye and a suspension of powder in liquid used as developer.
The test began with the cleaning of the surface to be inspected until an area free of paint, welding splashes, paint and oils was achieved, thus avoiding contamination that could prevent the dye from entering discontinuous welds.
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