Knowledge



Electromagnetic Testing

Eddy-current testing (also commonly seen as Eddy Current Testing and ECT) is one of many electromagnetic testing methods used in nondestructive testing (NDT) making use of electromagnetic induction to detect and characterize surface and sub-surface flaws in conductive materials.


ECT principle

In its most basic form, the single element ECT probe, a coil of conductive wire is excited with an alternating electrical current. This wire coil produces an alternating magnetic field around itself in the direction ascertained by the right-hand rule. The magnetic field oscillates at the same frequency as the current running through the coil. When the coil approaches a conductive material, currents opposed to the ones in the coil are induced in the material, which follow circular paths. These circular currents are called eddy currents.


Variations in the electrical conductivity and magnetic permeability of the test object, and the presence of defects causes a change in the flow pattern, intensity and phase of eddy currents.In turn acorresponding change in phase and amplitude of the coupled magnetic field that can be detected by measuring the impedance changes in the coil. This is a telltale sign of the presence of defects and the basis of standard ECT (using pancake coil).


Applications:

The two major applications of eddy current testing maybe broadly classified as surface inspection and tubing inspections.


Tubing inspection is generally limited to non-ferromagnetic tubing and is known as conventional eddy current testing. Conventional ECT is used for inspecting steam generator tubes in nuclear plants and heat exchanger tubes in power and petrochemical industries. The technique is very sensitive to detect and size pits. Wall loss or corrosion can be detected but sizing is not accurate.


Advantages:

  • Sensitivity to surface defects
  • Can detect through several layers
  • Can detect through surface coatings
  • Accurate conductivity measurements
  • Can be automated
  • Little pre-cleaning required
  • Portability

Disadvantages:

  • Very susceptible to magnetic permeability changes
  • Only effective on conductive materials
  • Will not detect defects parallel to surface
  • Not suitable for large areas and/or complex geometries
  • Signal interpretation required
  • No permanent record (unless automated)

Techniques of Electromagnetic Testing


Tube inspection can be performed by techniques which employelectromagnetic phenomena (as in Eddy Current Testing) as detailed below:


Conventional Eddy Current Testing (ECT)


Surface Testing



Surface inspection is used extensively in the aerospace industry wherein very sensitive ECT techniques are implemented to detect tight cracks. Surface inspection can be performed both on ferromagnetic and non-ferromagnetic materials.


Tube Testing


Eddy current testing is a noncontact method used to inspect nonferromagnetic tubing. This technique is suitable for detecting and sizing metal discontinuities such as corrosion, erosion, wear, pitting, baffle cuts, wall loss, and cracks in nonferrous materials.



  • Two coils are excited with an electrical current, producing a magnetic field around them. The magnetic fields penetrate the tube material and generate opposing alternating currents in the material. These currents are called eddy currents
  • Any defects that change the eddy current flow also change the impedance of the coils in the probe
  • These changes in the impedance of the coils are measured and used to detect defects in the tube


Tube Inspection with Remote Field Testing (RFT)


Remote field testing (RFT) is being used to successfully inspect ferromagnetic tubing such as carbon steel or ferritic stainless steel. This technology offers good sensitivity when detecting and measuring volumetric defects resulting from erosion, corrosion, wear, and baffle cuts. Remote field probes are used all around the world to successfully inspect heat exchangers, feed-water heaters, and boiler tubes.


  • RFT with up to four different frequencies and real-time mixes

This feature provides more flexibility for mixing and defect validation. The detection and sizing of flaws at the support plate is made easier with multifrequency inspections and dual-driver operations.


  • RFT with frequencies ranging from 20 Hz to 250 kHz

The high frequency available in the market extends RFT inspection to thin materials with low permeability, such as 400-series stainless steel, and other ferromagnetic alloys.


Tube Inspection with Near Field Testing (NFT)


  • Air coolers
  • Carbon steel heat exchangers

Near field testing (NFT) technology is a rapid and inexpensive solution intended specifically for fin-fan carbon-steel tubing inspection. This new technology relies on a simple driver-pickup eddy current probe design providing very simple signal analysis.


NFT probes can be successfully used to inspect carbon steel heat exchangers, and air cooler tubes.


NFT is specifically suited to the detection of internal corrosion, erosion, or pitting on the inside of carbon steel tubing. The NFT probes measure lift-off or "fill factor," and convert it to amplitude-based signals (no phase analysis). Because the eddy current penetration is limited to the inner surface of the tube, NFT probes are not affected by the fin geometry on the outside of the tubes.


Tube Inspection with Magnetic Flux Leakage (MFL)



Magnetic flux leakage (MFL) is a fast inspection technique, suitable for measuring wall loss and detecting sharp defects such as pitting, grooving, and circumferential cracks. MFL is effective for aluminum-finned carbon steel tubes, because the magnetic field is almost completely unaffected by the presence of such fins.


NFT probes can be successfully used to inspect feed water heater tubes, air cooler tubes and carbon steel heat exchanger tubes