The structural integrity of storage tanks and piping systems for the oil and gas industry is vital to business. Operators must be vigilant in locating cracks, thinning of structure walls or heavy corrosion. Then they must assess whether that pipe or tank can remain in service or must be repaired or even replaced.
Non-destructive techniques allow for engineers to test components or equipment in a way that keeps operational downtime to a minimum.
Some NDT methods are better than others in certain situations. The size of the equipment being tested may come into effect. The material the test subject is made of certainly comes into play. So engineers and operators should be aware of the array of NDT methods that are available and know which ones are best suited for specific circumstances.
Computed radiography is appropriate for pipelines or piping systems. As long as the person conducting the test has access to opposite sides of the test subject – one for the machine emitting the radiation and one for the imaging plate – it’s an excellent test for discovering flaws.
Ultrasonic testing is also excellent for finding pits, voids or cracks in systems and materials. But it has its limitations. The International Atomic Energy Agency wrote that with ultrasonic testing, some surface and subsurface flaws may not be detected because discontinuity may interfere with the initial pulse and signal.
When ultrasonic or computed radiographic testing is not the best option, magnetic flux leakage may be, especially for pipelines, piping systems and storage tanks.
What is magnetic flux leakage?
As the name suggests, this particular type of test uses magnetism to detect flaws in metallic structures. But there is an element of visual inspection to this type of testing as well.
The NDT Resource Center wrote that if a bar magnet is cracked (but not broken all the way through), opposing magnetic poles will form at the edges of the crack. The space in the crack is called the flux leakage field. If iron particles are spread over the bar magnet’s surface, they will be pulled to the edges of the crack. Essentially, the iron particles magnify the crack when the crack itself may not have been visible to the naked eye.
Application of magnetic testing The item to be tested is magnetized with a permanent magnet or an electromagnet.
The IAEA (International Atomic Energy Agency) wrote that this can be done longitudinally for long cylindrical structures like pipes. Or it can be done circularly by introducing an electrical current to the object being tested and letting it flow along the cylindrical object.
Either a direct current or alternating current can induce magnetism into the item being tested, the IAEA noted. However, the use of DC would allow for detection of flaws deeper into the material.
Iron particles are applied after the test object is magnetized and they usually contain a dye so the defects are readily visible.
Also, the IAEA wrote that the magnetic field has to flow perpendicular to the defect for it to be detected. If the field is flowing parallel, the flaws will not be detected.
This testing technique is especially useful for finding defects in the floors of storage tanks. Once the magnetic flux leakage test indicates the cracks or corrosion, sensors can then be used to determine how much of the material has been lost.
Magnetic flux leakage testing is particularly attractive because it is a rather simple test to perform, equipment is portable, and it gives almost instantaneous results. It can be used to detect surface flaws as well as those just beneath the surface. And it doesn’t have to be used only on large metallic objects or expansive piping networks. It is an effective test for smaller componentry as well.
Once the iron particles are cleaned and the test object is demagnetized, it can go right back into service provided the testing found no serious structural flaws. Magnetic flux leakage is a rather simple test in principle. It is an effective method for testing objects in the downstream oil and gas industry as part of an overall asset integrity management plan.