What is NDT techniques?
NDT (Non-destructive testing) techniques are in common use to check the integrity of pressure equipment materials and components. The main applications are plate, forgings, castings, and welds.
Non-destructive testing is used in manufacturing and in-service inspections to ensure product integrity and reliability, manufacturing procedures are monitored, production expenses are reduced and quality levels are maintained.
NDT is used during construction to ensure the quality of materials and joining processes during the manufacturing and erection phases, and in-service NDT inspections are used to ensure that the products in use continue to have the integrity necessary to ensure their utility and public safety.
The five most frequently used test methods are:
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Visual examination
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Dye penetrant (DP) testing
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Magnetic particle (MP) testing
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Ultrasonic testing (UT)
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Radiographic testing (RT)
Types of NDT techniques:
Visual examination:
Visual testing is the industry’s most frequently used test method. Because most test techniques require the operator to look at the surface of the inspected portion, visual inspection is intrinsic in most other test techniques.
Close visual examination can reveal surface cracks and defects of about 0.1 mm and above.This is larger than the ‘critical crack size for most ferrous materials.
VT inspections may be performed by direct viewing, using line-of-sight vision, or improved with the use of optical tools such as lenses, mirrors, boroscopes, CCDs and computer-assisted viewing systems.
Dye penetrant (DP) testing:
This is an enhanced visual technique using three aerosols, a cleaner (clear), penetrant (red), and developer (white). Surface defects appear as a thin red line.
Useful standards:
• ASTM E 165: 1982: Dye penetrant examination.
• BS EN 571–1: 1997: Penetrant testing – general principles.
• BS EN 1289: 1998: Penetrant testing of welds – acceptance levels.
• BS EN ISO 3452–4: 1999: NDT penetrant testing – equipment.
Magnetic particle (MP) testing:
This works by passing through the fabric a magnetic flux while spraying magnetic ink on the surface. An air gap in a surface defect creates an ink-attracting discontinuity in the field, thus making the crack visible.
Because magnetic flux lines do not move well in air, the particles will be taken into discontinuity when very fine colored ferromagnetic particles (“magnetic particles”) are applied to the surface of the part, decreasing the air gap and generating a noticeable sign on the part’s surface.
Ultrasonic testing (UT):
Different practices are used for plate, forgings, castings, and welds. The most common technique is the ‘A-scope pulse-echo’ method.
Sound is introduced into the part using an ultrasonic transducer (“probe”) that converts electrical impulses from the UT machine into sound waves, then converts returning sound back into electrical impulses that can be displayed on a digital or LCD screen (on older machines, a CRT screen) as a visual representation.
Radiographic testing (RT):
Radiography is widely used for NDT of pressure equipment components and welds. Industrial radiography includes exposing a test object to radiation penetration so that radiation moves through the object being inspected and a recording medium positioned on the reverse side of the object being inspected.
X-rays are effective on steel up to a thickness of approximately 150 mm.
Gamma (γ) rays can also be used for thickness of 50–150 mm, but definition is not as good as with X-rays.