Eddy Current testing is a method of nondestructive testing that is based on the principals of electromagnetism. When an eddy current probe in the close proximity of a test material is energized at a given oscillating frequency, eddy currents are induced in the conductive test material by means of electromagnetic induction. This technique requires no couplant or constant contact with the material being examined. The presence or absence of flaws can be evaluated by monitoring changes reflected back in the electrical characteristic of an eddy current coil, referred to as coil impedance. Any changes in the 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 instead of an absolute method.Eddy current testing is a widely used and well understood inspection technique for flaw detection, as well as thickness and conductivity measurements.
How does eddy current testing work?
When an eddy current probe in the close proximity of a test material is energized at a given oscillating frequency, eddy currents are induced in the conductive test material by means of electromagnetic induction. This technique requires no couplant or constant contact with the material being examined.
What is the principle behind eddy current testing?
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 nondestructive method for finding defects in the heat exchanger and condenser tubing.
What is eddy current testing of tubes?
The main application for eddy current testing is tubing inspection in heat exchangers and condensers. Eddy current testing is extremely sensitive and can detect extremely small defects in the tube wall. Eddy current testing is a nondestructive method for discovering defects in 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.
Several types of defects can be detected in tubing:
- Inner-diameter (ID) pitting
- Outer-diameter (OD) pitting
- Wear (from support structures, other tubes, and loose parts)
- OD and ID Erosion
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 was prior damage that should be monitored. Factoring 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 testing today!
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.