Magnetic Flux Leakage (MFL) tools identify and measure metal loss (corrosion, gouges etc…) through the use of an axially oriented applied magnetic field. An axial MFL tool induces a magnetic field in the pipe by using powerful magnets to magnetize the pipe wall to saturation. When corrosion or other degradation of the wall occurs, the pipe wall thickness is reduced. At areas of corrosion (reduced wall thickness) the amount of magnetic flux carried is less than that with full wall thickness. This means that there is leakage of magnetic flux at these areas. The reduction in the wall thickness at both outer and inner walls of the pipe can cause the same effect. The tool locates and records magnetic flux anomalies in the pipe and the recorded magnetic flux data is converted to provide an indication of metal loss in the pipe.
The measured flux leakage is dependent on the material characteristics of the pipe, wall thickness, stresses in pipelines, and dimensions of the flaw. Apart from the reduction of wall thickness, a crack in the pipe wall can also cause flux leakage, however, axial MFL employs flux lines that are parallel to the axis of the pipe and can therefore not detect axial stress corrosion cracks or seam weld cracks.
There are two types of MFL pigging tools, high resolution MFL and standard resolution MFL. The main difference between the two is in the number of sensors and the amount of resolution. Most MFL tools can determine the location and o'clock position of a metal loss anomaly and detect if a corrosion anomaly is internal or external to the pipe wall. It also provides data of each corrosion anomaly including its length and maximum pit depth, which allows for calculation such as ASME B31G to determine the pipe's remaining strength.
MFL tools require special shipping procedure as the tool's magnets may interfere with navigation systems and MFL tools are much larger, longer and heavier than UT tools, therefore mobilization cost should be considered when evaluating the overall cost of in-line inspection and comparing UT method with MFL method.
MFL tools can be used with or without liquid making them suitable for inspection of most pipelines and they are able to capture data whilst travelling at much greater speeds than UT tools and over a longer distance. Data captured from MFL tools needs to be analyzed and interpreted by experts which can be time consuming for long pipelines whereas the data from UT tools is presented in a simple c scan format making analysis and interpretation very quick and easy.
Traditionally MFL pigging has always been considered a cheaper option than UT intelligent pigging however with Dacon's advancement's in technology and far lower mobilization costs and logistics Dacon’s UT solution is proven to be a significantly cheaper solution that provides more accurate results.