Magnetic Normalizing

Sucker Rod Magnetic NormalizingWhen a string of used sucker rods is received at one of TRC plants from the field, the rods typically have very inconsistent residual magnetic properties.  By magnetically normalizing each sucker rod prior to our initial inspection process, we greatly increase our ability to detect injurious defects.

TRC has a patent pending for this unique and innovative process.

Permeability will vary greatly within a used sucker rod in areas of localized stress (handling damage, bending, etc.), fatigue (cyclic loading), improper normalization of the upset runout, etc.

By normalizing the magnetic properties of sucker rods prior to directing them through an energized eddy current test coil, the induced eddy current field is greatly intensified by any permeability changes in the rod.  If the magnetic field strength produced by the eddy current test coil varies even slightly, these small variations have a large effect on the impedance of the coil.  And, they are often very large in comparison to the changes caused by changes in conductivity or dimension.

Permeability

Permeability is described as the ease with which a material can be magnetized.  For non-ferrous metals such as copper, brass and aluminum, the permeability is the same as that of “free space.”  In other words, the relative permeability (mr) is one.

For ferrous metals, however, the value of mr may be several hundred, and this has a significant influence on eddy current response.  In addition, permeability will vary greatly within a metal part due to localized stresses, heating effects, and other factors.  Permeability will vary in a used sucker rod due to handling and bending (localized stresses), fatigue (cyclic loading), and improper normalization in the upset runout (heating effects).

Eddy Current Testing

When an energized eddy current test coil is placed on non-magnetized ferromagnetic material, the field is greatly intensified by the magnetic properties of the material so that a large change in the impedance of the test coil occurs.

If the magnetic field strength at various locations varies even slightly, these small variations have a large effect on the impedance of the coil.  These changes in the impedance of the coil are often so large (in comparison to the changes caused by changes in conductivity or dimension) that they mask all other changes.

Normalization Process

The process for normalizing the magnetic properties of a sucker rod prior to eddy current inspection comprises the following steps:

  • The sucker rod is conveyed through a magnetizing coil with the power supply adjusted such that the sucker rod is magnetized to or near saturation with a longitudinally oriented magnetic field.
  • The magnetized sucker rod is subsequently conveyed through a demagnetizing coil with the power supply adjusted such that the magnetic field induced by the magnetizing coil is reversed and reduced to a near zero gauss level.

Following this normalization process, the sucker rod will contain a minimal strength, longitudinal, magnetic field that is consistent throughout the length of the sucker rod, with the exception of those areas of varying permeability caused by localized stress, fatigue, improper normalization of the upset runout, etc.