Eddy Current Testing (ECT) is a specialized non-destructive testing (NDT) technique that has several applications in the power (Balance of Plant) and petrochemical industries.
 

Principle

Eddy Current method is based on the principle of measuring changes in the impedance of a electromagnetic coil as it is scanned over a surface of conductive material.

The test is performed by a electromagnetic coil that is placed over a conductive material. The coil produces a magnetic field that is induced in the material. To counter the coil's primary magnetic field, eddy currents are produced in the material. Eddy currents produce a secondary magnetic field to oppose the coil's primary magnetic field. When the coil is scanned over a defect, the secondary magnetic field is distorted thereby changing the loading on coil. Changes in coil loading directly affect the coil impedance. These changes in coil impedance are related to the defect.

Flaw detection with eddy currents is limited to the penetration depth. Penetration depth is inversely proportional to the square root of conductivity, frequency and permeability. For most applications, the penetration depth in non-ferromagnetic material is limited to approximately 5 mm (0.20 inches). In case of ferromagnetic materials, such as carbon steel, the penetration depth is extremely shallow because of high permeability. Inspection of ferromagnetic material is therefore limited to surface flaws only.

For further details on ECT theory, click here.

Surface ECT Calibration on EDM notches. Instrument: Nortec 1000
 

ECT Techniques

Following is a summary of ECT techniques

Defect detection in tubes. This method is applied in inspection of heat exchanger tubes. The inspection is performed with a bobbin coil that produces a electromagnetic field in the tube. When the probe is pulled across a defect, the electromagnetic field is distorted. This distortion in magnetic field changes the coil impedance that is related to the defect. ECT method detects pits, wall loss and cracks in tubes.

Surface crack detection. Surface Eddy Current is applied for detection of surface cracks in both non-ferromagnetic (stainless steel , inconel etc.) and ferromagnetic materials (4340 steel, carbon steel). The inspection is done with pancake coil probes. Some common probes are spot probes and pencil probes.

Material Sorting. Changes in Material Conductivity affect the eddy currents produced in the material. These changes affect the coil impedance that is related to the material. Inspection is done with spot probes.

Clad overlay measurement. This method is applied for measurement of non-ferromagnetic material clad over a ferromagnetic base. eg. stainless steel/inconel clad over carbon steel. A change in clad thickness changes the impedance of the ECT coil. This change in impedance is correlated to the clad thickness. ECT is a fast and reliable method for this application. Inspection is done with spot probes.

Measuring Stainless Steel/Inconel Clad on carbon steel. Instrument: Hocking Phasec 2200

Paint thickness measurement. The method is based on the principle of measuring the lift-off of ECT probe over the surface. Handheld ECT machines are available for this application. Inspection is done with spot probes.

Coating thickness measurement. The method is based on the principle of measuring the lift-off of ECT probe over the surface. Handheld ECT machines are available for this application. Inspection is done with spot probes.

Wear measurement. ECT method can measure a wear/wall loss in non-ferromagnetic materials. The method is applicable if the depth of penetration is greater than the material thickness. ECT depth of penetration can be controlled by varying the frequency. Depth of penetration can be increased by lowering the frequency.
 

ECT APPLICATIONS

Following is a brief summary of ECT applications.

ECT of Heat Exchangers. By far inspection of Heat Exchangers is the No. 1 application of ECT in power plants and petrochemical plants. Heat exchangers include condensers, general petrochemical plant exchangers, feedwater heaters, air coolers and lube oil coolers. Both the conventional ECT and remote field ECT are used for inspection. Conventional ECT is used for inspection of non-ferromagnetic tubing such as stainless steel, copper-nickel alloys, titanium etc. Remote Field ECT is applicable for inspection of ferromagnetic tubing such as carbon steel and nickel. Conventional ECT is a fast, reliable and accurate method for detection of detects in tubing. Inspection can be done at pulling speeds upto 6 ft/ sec. RFECT is limited to detection of larger defects and inspection speed is limited to approximately 1 ft/sec. A specialized version of conventional ECT is Full Saturation ECT. This technique is applicable for thin ferromagnetic tubes such as seacure in condenser tubes and partially ferromagnetic tubing materials such as Monel and Alloy 2205.

Turbine ECT. ECT has several applications in turbine inspections. The technique is used for inspection of blades, disks and rotor bore. Following is a list of turbine applications.

• Crack detection at the trailing edges of blades
• Crack detection in the steeples of blades
• Wear measurement at the cooling holes of gas turbine blades
• Crack detection is disk steeples
• Crack detection in turbine bore

Vessels in Petrochemical Plants. The two major applications of ECT in vessel inspection are crack detection and clad thickness measurement. Crack detection is performed on the ID surface of stainless steel vessels. The technique is used as an alternate to penetrant testing (PT) as it can detect tight cracks and the technique is significantly faster than PT. 
Clad measurement is performed on vessels with carbon steel shell and stainless steel clad. ECT is a rapid and accurate technique for measuring any loss of SS clad over the carbon steel shell. NDE Assocaites, Inc uses the Hocking Phasec 2200 and the Nortec 1000 for such inspection.

Piping Systems. Stainless steel is susceptible to stress corrosion cracking. The cracking can occur in presence of chlorides and moisture. One source of chlorides is insulation with more than acceptable levels of chlorides. ECT is a highly effective technique for detection of SCC cracking on the OD surface of SS piping. The technique detects tight cracks that can be missed by penetrant testing. ECT will also detect cracks that are just below the sue surface and within the eddy current skin depth.

Bolt Hole Inspection. Bolt holes are be inspected for cracks. Cracks in bolt holes ca be caused by fatigue and corrosion. Inspection is done with a bolt hole scanner interfaced to a ECT machine. Bolt hole ECT a very common application in the aerospace industry.

Bellows. ECT is applied on bellows for crack detection. Cracks in bellows are caused by stress corrosion. Both stainless steel and inconel bellows are inspected for detection of ID and OD surface cracks.

Crack detection in bellows

Boiler tubes. Remote Field ECT is applied for detecting wall loss in Boiler tubes. Inspection is done by passing a RFECT probe through the boiler tubes. Access is usually obtained from the steam drum. Inspection is done at very low frequencies.

Above is a list of major applications of ECT. Other than that, the technique is used where it can be configured for the specific NDT test.

NDE Associates, Inc. conducted condenser inspections onboard "Voyager of the Seas"

Inspection of Turbine Steeples with ECT

NDE Associates, Inc.
515 Tristar Drive
Webster, TX 77598
Phone: 281-488-8944    Fax: 281-488-8485