Eddy Current testing is a method of nondestructive testing that is based on the principles of electromagnetism. When an eddy current probe is energized at a given alternating frequency while in close proximity to a conductive test specimen eddy currents are induced in the test specimen by means of electromagnetic induction. This inspection technique does not require couplant or constant contact with the material being examined. The presence or absence of flaws in the test specimen can be determined by monitoring changes in the impedance of the eddy current coil. Changes in coil impedance are displayed as voltage changes in terms of signal amplitude and signal phase. These variations in phase angle and/or signal amplitude are correlated to flaw conditions, such as percent wall loss and volumetric wall loss. Therefore, ET is a comparative method as opposed to an absolute method. Eddy current testing is a widely used and well-understood inspection technique for flaw detection and can provide information on a relative part or coating thickness and specimen conductivity.
How does Eddy Current Testing work?
Eddy currents are created when an energized A/C coil nears a conductive material and opposing currents (eddy currents) are formed. When there is a defect in the material, the flow of the eddy currents change, and this way the defect can be detected by measuring the impedance changes that occur in the A/C coil. Eddy current testing is a very efficient non-destructive method for finding defects in the heat exchanger and condenser tubing.
What is Eddy Current Testing of Tubes?
A common application for eddy current testing is tubing inspection in heat exchangers and condensers. Eddy current testing is a non-destructive method for discovering defects in the tubing, is effective on a number of different tubing materials, and can detect defects that can create larger issues for the heat exchangers and condensers. Eddy current testing is extremely sensitive and can detect extremely small defects in the tube wall. Eddy Current Testing is only one type of Nondestructive testing (NDT). Other NDT testing techniques include visual testing, liquid penetrant examination, ultrasonic testing, magnetic flux leakage, and magnetic particle testing.
Several types of defects can be detected in tubing using Eddy Current Testing:
- Inner-diameter (ID) and Outer-diameter (OD) pitting
- Wear (from support structures, other tubes, and loose parts)
- OD and ID Erosion
Eddy Current Testing Applicability
What kinds of Materials can be Tested with Eddy Current Testing?
Eddy current testing can successfully test any conductive, non-magnetic, or mildly ferromagnetic materials. This would include materials such as nickel alloys and ferritic chromium-molybdenum stainless steel.
What kind of Inspections can be performed using the Eddy Current Testing method?
Eddy current inspection techniques include. but are not limited to:
- Inspecting the integrity of welds and bolts
- Checking heat-treated materials by testing conductivity
- Inspecting metal surfaces for defects
- Determining whether the metal has corroded
- Inspecting metal tubing
Eddy current testing is very useful for testing the integrity of structures made of conductive material like copper, steel, and aluminum metals because the results are immediate and you do not need to touch the actual metal. Eddy Current is used to detecting flaws in the metal. If you need to verify that a conductive material like a pipe is not cracked, pitted or corroded, eddy current testing is one of the best methods. You can use eddy current to determine how thick a non-conductive coating, such as paint, is and determine metal hardness. Verifying that heat exchangers are not corroded is an example where eddy current testing is often used.
How effective is Eddy Current Testing?
Eddy current testing is extremely sensitive to defects in the tube wall and is the most efficient method for detecting and depth sizing tubing flaws. Accurate depth sizing requires the use of calibration standards that mimic the damage mechanism. Eddy current testing is best used as a method to assess the condition of a component, particularly the tubing in a component, but other information on component condition can be identified. As part of the condition assessment program, degradation detected by eddy current testing can be re-inspected over time to determine if degradation is ongoing and to trend degradation through the life cycle of the component.
How often should Eddy Current Testing be performed?
Consider performing eddy current testing during an annual inspection. The actual amount of time between testing periods will rely on a number of factors, such as the amount of damage that was identified in the heat exchangers or condensers. Ideally, eddy current testing will occur approximately every 3 to 5 years, possibly more frequently if there is active damage occurring that should be monitored. Including eddy current testing into an annual inspection ensures that testing occurs with the proper regularity. The Merrick Group has over 25 years of experience performing eddy current testing. Contact The Merrick Group to schedule your eddy current inspection today!
What Types of Probes are used in Eddy Current Testing?
Simple single-coil probes can be used for simple surface or bolt hole exams while array probes with over 50 coils are often used for tubing and surface inspections. The expected damage mechanism and the geometry of the test specimen determine the coil configurations that are used.
John Merrick is the owner and president of The Merrick Group, Inc. He has over 25 years of experience with coordinating national and international sales efforts for nuclear and fossil fuel related maintenance projects. He has successfully completed projects in over 30 nuclear and fossil fuel sites.