Facility maintenance is an important part of the engineering industry.
This means that facility managers must stay on top of the latest inspection and testing codes for all of the parts, equipment, and processes in their respective facilities.
But how is this accomplished without interfering with daily processes or risking the integrity of the equipment in question?
Magnetic particle testing is a non-destructive method of facility integrity testing. It is performed by using a magnetic field to locate areas of deterioration.
Nondestructive testing measures have been adopted in many facilities for their reliable and accurate results alongside the stability of the testing process. This testing method does not damage the components of the equipment, meaning that regular testing is more efficient. Facility managers can feel confident in knowing that their facilities are up to code and that employees are being kept safe.
One example of nondestructive testing (NDT) is magnetic particle testing which is used to identify defects on the surface of magnetic materials.
Magnetic Particle Testing
During magnetic particle testing, an object or surface is tested for surface and subsurface flaws. Ferromagnetic materials such as iron, nickel, and cobalt are commonly tested using magnetic particle testing.
Magnetic particle testing is a two-step process: magnetize the object, then spread metal particles on the object while the magnetic current is still live.
For a surface to undergo magnetic particle testing, inspectors must magnetize the surface first via a magnetic current. This will reveal potential defects in the surface.
In direct magnetization, an electric current is passed directly through the object or surface being inspected. This creates a direct magnetic field in the ferromagnetic object.
Indirect magnetization creates a magnetic field externally, using an outside source rather than passing an electric current through the object.
Inspectors need to consider several points when choosing the proper magnetic current for magnetic particle testing including the object’s shape, types of defects being looked for, the type of material, and how deep the current needs to be for defects to be found.
The magnetic current will remain consistent through the surface of the object if there are no surface flaws; however, if even a slight surface defect is detected, the magnetic current will be interrupted and cause the magnetic current to spread out from the point of defect, creating a flux leakage field (where the flaw in the surface is located).
Once a flux leakage field is created, inspectors spread magnetic particles over the area to make the defect visible to the naked eye. The particles will gather over any defects in the surface.
Wet Magnetic Particle Testing
In wet magnetic particle testing (also known as wet suspension testing), the magnetic particles are spread out while suspended in a liquid carrier.
This is usually done using a horizontal stroke (i.e. a brush technique), but spray cans can also be used. This yields several benefits, especially when detecting smaller surface flaws.
Wet magnetic particle testing spreads an even layer of magnetic particles over the field more efficiently than dry magnetic particle testing. Liquid particles also move more fluidly for a longer period of time, allowing for smaller surface defects to be visible when smaller leakage fields form on the surface of the object.
Dry Magnetic Particle Testing
Dry magnetic particle testing uses dust particles to test for surface defects. This is mostly used to detect small subsurface flaws on rough surfaces or underground welds.
As the field is magnetized, the current creates a pulse that allows the dust particles to move across the surface. The excess powder is lightly blown off while the magnetic current is still applied. Like wet magnetic particle testing, the particles will gather over the surface defects, making the defects more visible.
Pros of Magnetic Particle Testing
- Portable and efficient
- Immediate and easily visible results
- No strict pre or post-cleaning requirements
- Sensitivity to shallow and fine surface flaws
- Can be used with irregular object shapes and surfaces with other objects on them
Cons of Magnetic Particle Testing
- Only ferromagnetic materials can be tested (nickel, cobalt, and other alloys)
- Only surface and subsurface defects can be detected
- Difficulty in demagnetizing the surface post-inspection
- Only small sections can be inspected at a time
- Paint (or sealants) must be removed if thicker than 0.005” for it to work
- Exact alignment between indications/defects and the flux leakage field
Magnetic Particle Testing Code
Inspectors are required by law to follow certain codes when conducting magnetic particle testing and other forms of nondestructive testing for their facilities and equipment parts.
The American Society of Testing and Materials designated standards for magnetic particle testing in ASTM E709-21 (Standard Guide for Magnetic Particle Testing). This covers both wet and dry particle testing as well as standards for preparation, procedure, and techniques for magnetic particle testing.
Achieve Industry Standards with NDT Tanknicians
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At NDT Tanknicians, we embrace the tough challenge set in place to be certified by entities such as the Environmental Protection Agency (EPA,) American Petroleum Institute (API,) Steel Tank Institute (STI,) American Society for Nondestructive Testing (ASNT,) and the National Association of Corrosion Engineers (NACE,) to name a few.
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