A variety of quality control measures can be used to locate steel pipe flaws. These inspection methods are performed by trained technicians who have the experience and knowledge necessary to accurately prep the tested materials and read the testing results. Below is a list of quality control techniques used to locate pipe defects.
Testers use visual inspection to quickly assess the quality and condition of a weld or component. Visual inspection can help detect potential failures. It is sometimes used in conjunction with other surface inspection methods that allow the detection of defects not visible to the naked eye.
Visual inspection is the ideal method used in many quality control programs. It is relatively easy to perform and inexpensive because specialized equipment is not necessary. Visual inspection requires appropriate lighting, good vision and a strong knowledge of what needs to be identified and corrected. Technicians can use a variety of tools, such as borescopes and magnifying glasses, to enhance their ability to detect defects. Television camera systems are often used during inspection.
The inspected specimen must be free of grease, dirt, dust or other debris that may interfere with the inspection. Surface preparation can involve wiping, blast cleaning and chemical treatment.
X-ray can be used to inspect steel components beyond what the human eye can assess. High voltage x-ray machines are required for a material as dense and heavy as steel. Sometimes the steel material is so thick that even an X-ray is insufficient. In these cases, gamma rays are used. Gamma rays are produced by such radioactive isotopes as Iridium 192. Sources of gamma are often portable, making them appropriate for use on construction sites.
When inspecting with X-ray and gamma rays, the film is placed opposite of the test specimen. The radiation passes through the tested material and exposes the film. The film is then processed, producing a series of images, which can be read by a trained evaluator.
Welding flaws can be detected using soundwaves. This type of inspection is called ultrasonic inspection and is performed and interpreted by highly skilled technicians. During an ultrasonic inspection, pulsed beams of short wavelength and high frequency are applied, using strategically placed transducers. Defects throughout or within the thickness of the specimen will reflect the sound back to the transducer, thus revealing useful information, such as distance, flaw size, and reflectivity.
Magnetic Particle Testing
A certain degree of magnetism can be present in steel pipe. Magnetic particle testing is used to detect flaws within the material. In magnetic particle testing, the tested surface is sprayed with iron particles. The iron particles can be applied in a variety of forms, including colored, fluorescent, dry, wet or suspended in liquid. Ultraviolet lights can also be applied during magnetic particle testing for enhanced visibility. Once the iron particles attach to surface flaws, technicians will be able to locate the flaws and determine the flaw size.
Magnetic particle inspection is mostly used for locating surface flaws, but sometimes it can also be used to detect some types of subsurface flaws. Careful preparation of the surface is required for accurate results.
Dye Penetration Inspection
Surface breakage is a flaw identifiable using a low-cost technique called dye penetration inspection, or dye penetrant inspection. This type of quality control testing can be used on any material once it is cleaned thoroughly. The penetrant is then applied and allowed time to soak into the flaws. The “dwell time” (usually 5 to 30 minutes) vary depending on the tested materials and the size of the flaw(s). Smaller flaws require longer dwell times.
Excess dye is removed using a method specific to the type of penetrant used. Excess penetrant must be removed properly to achieve accurate results. After this is done, a white developer is applied to the specimen. This enhances the visibility of the defects. These areas of “bleed out” identify the location and orientation of the defect, as well as the possible types of defect. A technician then uses visible light and ultraviolet radiation to inspect the test surface.
Eddy Current Testing
An electromagnetic technique, the Eddy current test is used exclusively on conductive materials. Eddy current testing is used to identify cracks, size variations, and material variations. During this form of testing, an energized coil is placed in close proximity to the surface of a metal component. The changes that emerge once the eddy currents are induced help interpret flaw types and material conditions.
Flux Leakage Testing
Flux leakage tests are used to assess larger diameters and heavier wall tubes. Magnetic energy is used in flux leakage testing. The magnetic lines of flux, freely flowing around the wall and through its full thickness is studied. Impediments will cause the lines of flux to be transferred to each side of the imperfection, and if the imperfection is large enough, some of the flow will leak into the atmosphere. Using special electronic instruments, trained technicians can find these flux leaks, determine the severity, and locate the area of discontinuity in the tube wall.