The term “Non-destructive Testing” (NDT) refers to a set of testing techniques used to ascertain the mechanical and physical features of a product without damaging it. The most common NDT techniques are PMI (positive metal identification, to evaluate the chemical composition of a metal product), penetrating fluids (to detect leakages), ferrite content (to measure the ferritic state of a duplex product), hydrostatic test (to check pressure losses and fluid leakages), magnetic particles (to verify the quality of a metal surface), radiographic examination and UT testing (ultrasonic tests).
The most common non-destructive testing (NDT) for pipe fittings, in brief, are:
- Positive material identification (PMI): identification of the chemical composition of the metal used to manufacture the fitting. Uses PMI sensors, including X-ray fluorescence or optical emission spectrometry. This test is frequently executed for high-value materials, from duplex steel onwards.
- Penetrating liquids: this is a low-cost test to detect cracks or abnormal porosity on the surface of the fittings and is executed by applying a special liquid to its surface.
- Ferrite content: this test aims to measure the ferrite content for duplex, super duplex, and stainless steel materials to make sure the metal is able to achieve the yield strength, fracture toughness and corrosion resistance it is meant to reach. Ferrite content is measured in weld seams by magnetic induction.
- Hydrostatic test / hydro testing (pressure testing): the fitting gets filled with a test liquid (example water) and a pressure is applied to check if any pressure loss occurs (which would indicate manufacturing defects)
- Magnetic particles: by means of a magnetic field, possible surface discontinuities or weakness can be identified
- Macrography: 2 / 3 D pictures of items visible to naked eye.
- Micrography: pictures under the microscope to see the micro-granular structure of the metal and other details not visible to the naked eye
- Pneumatic pressure: the fitting gets filled with pressurized air; measurements are taken to see if there is pressure loss due to any defects.
- Radiography: X-rays / gamma rays to detect imperfections leading to possible quality defects
Out of the above tests, pressure testing is one of the most commonly specified, especially for mission-critical fittings in the pipeline.
NON-DESTRUCTIVE TESTING TYPES
It is essential that the completed weld should not have discontinuities or voids and that the mechanical strength of the weld is equal to or greater than the parent pipe. To reduce the possibility of failure during the hydro-test or, far worse, in-service failure, an inspection plan that applies a variety of nondestructive tests can be implemented to detect any weaknesses in the fabrication. Nondestructive examination means the assessment of a weld without damaging it physically and affecting its pressure sealing characteristics. Several methods are available, with different costs and differing levels of accuracy. Fully qualified personnel, who are in a position to interpret the results and take the appropriate action, must carry out all of these options:
- Visual (surface crack detection for all material).
- Magnetic particle examination (surface-crack detection for carbon steel and any magnetic metals).
- Dye penetrant examination (surface-crack detection for nonmagnetic stainless steels and other nonmagnetic metals).
- Radiography (surface and through the metal).
- Ultrasonic examination (surface and through the metal).
All nondestructive examination of welds must take place before hydro testing of the piping system and painting (if necessary) or insulation (if necessary). This means that if a weld fails the examination test, the bare pipe can be repaired and retested, before the painting or insulation operation.
Different piping systems have different types and levels of inspection, depending on the service fluid, material, temperature, pressure, and location.
Visual inspection is the simplest and the cheapest method, and all welds must be subject to this basic method, using either the naked eye or a magnifying glass to confirm imperfections. All surfaces to be visually examined must be thoroughly cleaned. This method is used only to detect surface imperfections. If these are found, additional tests are employed to discover the extent of the flaw. Even if a weld is to be examined by more-accurate methods of inspection, it should be subjected to the basic visuals, because of their low cost. Also, if imperfections are detected visually, an additional examination can be intensified around this area of concern.
The liquid penetrant (or penetration) examination (LPE) method is used on metals considered to be nonmagnetic, such as austenitic- chromium stainless steel. This technique requires the surface application of a penetrating liquid containing a dye. The liquid is given time to seep into any surface flaws, and excess liquid is removed. The surface is allowed to dry, and the weld is examined. Flaws are indicated by the presence of dye, which is visible to the naked eye.
MAGNETIC PARTICLES EXAMINATION
Magnetic particle examination (MPE) is used to detect surface cracks on ferromagnetic materials, such as carbon steel. Some low alloys are magnetic, however, austenitic-chromium stainless steel is very weakly magnetic and therefore excluded from this type of examination; this is subjected to dye penetrant examination, which is covered later. The MPE method is very useful for detecting fine cracks that are invisible to the naked eye. To carry out the examination, the weld under analysis is first strongly magnetized with an electromagnet, then fine particles of a magnetic material, such as iron or magnetic iron oxide, are applied to the surface. The magnetic powder is attracted to the edges of any surface cracks, making them visible to the naked eye.
ULTRASONIC TEST (UT)
Ultrasonic (UT) waves with a frequency of 500–5000 kHz are transmitted as a narrow beam toward a target. On reaching a metal surface with a flaw, the waves are reflected and returned to a suitable receiver. The time required for the return of the echo is a measure of the length of the path covered by the waves.
If used correctly, the ultrasonic method can approach the accuracy of radiography. The benefit of ultrasonic testing is that the equipment is portable; therefore, UT is useful when the weld is in an awkward location or needs to be examined on site.
RADIOGRAPHIC TEST (RX or RT)
Radiographic (RT) examination is the most useful nondestructive test, as it detects subsurface flaws invisible to the naked eye. This method originally employed X-rays, but today pipe joints can be examined using gamma-rays produced by portable radioactive isotopes. All sources of radiation are potentially dangerous, and exposure over extended periods must be avoided. Personnel protection is often a requirement for technicians carrying out the radiography.
A film is placed on one side of the weld, and on the other side, the weld is subjected to X-rays in the direction of the film. As the X-rays pass through the weld, any imperfections on the surface and through the weld are detected by a dark shadow on the exposed film. No imperfection shows up as clear with uniform shade. The analysis of radiographic films requires considerable experience, and the defects that may be detected include cracks (surface and subsurface) and subsurface cavities caused by oxide film; lack of fusion; trapped slag, flux, or foreign material; and gas pockets (porosity).
Each radiograph must be recorded with the number of the weld to identify the exact location of the weld, and the names of the radiographer and inspector must also be listed. Radiographs are open to interpretation, and it is essential that the personnel used for this activity are suitably qualified.
(Source: Peter Smith – Piping Materials Selection and Applications, 2004)