Continued part 1
Inspection of storage tanks according to American Petroleum Institute (API) standards are important to prevent not only catastrophic failures, but also to avoid problems related to the mechanical integrity of the assets affected mainly by corrosion in floors, walls and ceilings.
In this article the different elements/techniques/equipment to perform inspections according to API-653 and in out of service condition are presented in a summarized and graphical way.
Internal tank inspection (floor inspection)
Magnetic Flux Leakage (MFL) is a widely used technique for non-destructive testing (NDT) of large steel structures such as above ground storage tanks (AST), this technique has a proven track record for decades and is considered an effective, robust and reliable approach. It is oriented to inspect AST floors due to their ability to cover vast areas quickly, they can cover hundreds ofm2 allowing to ideally find and determine the size of any material leaks with diameters in mm.
This article outlines the principles of the technique, benefits/limitations and other tips related to this effective tool widely used by many inspection companies.
Principles of the MFL technique
The basic principle of MFL inspection of a ferrous sample is to properly saturate the local area of interest with a magnetic field.
Near the defect or imperfection, the reluctance R (ratio of magnetomotive force to magnetic flux) of the induced magnetic field increases and, if high enough, that of the magnetic field diverges in the absence of material. This field can bypass the defect within the surrounding material and “leak” out of it.
The amount of magnetic field leakage can be measured by appropriately placed magnetic sensors that are normally located near the surface.
The magnetic circuit is generated with the yoke arrangement shown in Figures. and comprises two magnets, a bridge and two pole pieces. The magnetic yoke is located close to the inspection surface at a height of approximately 4 mm. Traditionally, the lateral position of the magnetic sensors to capture the MFL is located at an equidistant position between the two poles. The height of the MFL sensors from the sample surface can be used to amplify the MFL signal when it is closer to the surface.
Top-to-bottom defect discrimination (STARS technology)
STARS (Surface Topology Air Space Reluctance Sensors) is a patented technology (EddyFi Technologies courtesy). STARS data is collected along with MFL data during an inspection and is used by the SIMS software to automatically discriminate between soil-side and product-side corrosion.
STARS improves MFL size accuracy. The amplitude of an MFL signal from a defect on the top side is different from the bottom side; STARS in combination with MFL helps to reduce this ambiguity.
STARS operates on the air-iron reluctance principle. When scanning over areas without the MFL Sensor product side corrosion, the gap will remain mostly constant. Magnetic flux lines will travel vertically, taking the path of least reluctance between the pole and the plate surface. They travel parallel to the STARS sensor, do not cross it, and there is no STARS response.
In the images on the left, there is a strong response from the MFL sensors, but no response from the STARS sensors. Floormap decision = corrosion is at the bottom.
A difference in the air gap, caused by corrosion on the product side, changes the reluctance of the magnetic path.
The magnetic field lines take the shortest path, the path of least reluctance and move away from the vertical. towards the edges of the defect, intersecting the STARS sensor.
This causes a STARS response as illustrated in the images on the right. A strong response from the MFL sensor is observed, and also a strong response from the STARS sensors. Floormap decision = corrosion is superior side. does it look complicated, but it really is not.
Benefits of using MFL/STARS technologies
- Best discrimination defect, up/down
- STARS improves MFL size accuracy. The amplitude of an MFL signal from a defect on the top side is different from the bottom side; STARS in combination with MFL helps to reduce this ambiguity
- Improved estimation of percent loss of material by detecting defect size around 2 to 3 mm with 10% LPE.
Conclusion
The most recent improvements in MFL/STARS technologies make these techniques one of the most important floor inspection tools in the world.
Don’t miss the next article about our NDT experiences!
If you want to know more about these and other techniques, get in touch with us!
Loading...
