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Eddy Current Testing (ET)

INTRODUCTION
Eddy Current inspection is one of several NDT methods that use the principle of electromagnetism as the basis for conducting examinations. Several other methods such as remote field testing (RFT), flux leakage and Barkhausen noise also use this principle.
THE Applus+ SOLUTION
Eddy currents are created through a process called electromagnetic induction. When alternating current is applied to the conductor, such as copper wire, a magnetic field develops in and around the conductor. This magnetic field expands as the alternating current rises to maximum and collapses as the current is reduced to zero. If another electrical conductor is brought into close proximity with this changing magnetic field, current will be induced in this second conductor. Eddy currents are induced electrical currents that flow in a circular path. They get their name from Eddies, which are formed when a liquid or gas flows in a circular path around obstacles when conditions are right.
ID probes, which are also referred to as Bobbin probes or feed-through probes, are inserted into hollow products, such as pipes, to inspect from the inside out. ID probes have a housing that keep the probe centered in the product and the orientation of the coil(s) somewhat constant relative to the test surface. The coils are most commonly wound around the circumference of the probe so that the probe inspects an area around the entire circumference of the test object in one go.
 
Applus+ offers five inspection methods for heat-exchanger tubing systems:
  • ECT - Eddy Current Testing
  • RFT - Remote Field Testing
  • NFT - Near Field Testing (Fin Fan Testing)
  • IRIS - Internal Rotary Inspection System.
  • MFT – Magnetic Flux Leakage Testing
Choosing the appropriate inspection method for your equipment depends on your tube material and specific inspection needs. All our crews are trained to use all techniques so they can perform complementary inspections, providing the most comprehensive service possible.
 
The best crews in the business. The key Applus+ differentiator is the high level of training received by our crews, who work efficiently and report quickly.
 
They are unique in the industry in that they consist of:
  • A two-person team to perform the inspection
  • An additional technician to analyse results on-site
 
As a result, we can typically provide:
  • An initial report on the day of inspection
  • A final report that is delivered in days, not weeks
 
Thorough reports, fully explained. Reports are only useful when the customer understands them fully. Applus+ ensures our customers understand our reports by:
  • Explaining the initial reports on the day of inspection
  • Providing a timeline for final report delivery
  • Conducting an exit interview to answer all questions
 
The Applus+ goal is to provide excellent service and exceed the industry standard.
TARGET CUSTOMERS
Eddy current testing is of relevance in any sector that involves the use of heat-transfer systems, including the petrochemical, power generation, industrial air-conditioning and commercial heating unit industries.
 
 
KEY CUSTOMER BENEFITS
Benefits of the Applus+ eddy current testing service include:
  • Increase of inspection speed to approximately 60 feet (18m) per minute
  • Differentiation between ID and OD flaws
  • Reliability and accuracy of test results
  • Detection of gradual wall thinning and localised flaws
  • Provision of both phase and amplitude information
  • Inspection of U-bend tubes with some radius limitation
  • Permanent records available on test results
  • Accurate identification and evaluation of flaws under the support plates (baffles) using multi-frequency techniques
Advanced techniques
The alternating current field measurement (ACFM) technique is an electromagnetic technique capable of both detecting and sizing (length and depth) surface-breaking cracks in metals. The basis of the technique is that an alternating constant current in a tangential solenoid, remote from the test surface, induces electric currents in the sample surface which are uni-directional and of uniform strength over a localised area under the solenoid. When no defects are present in this area, these electric currents will be undisturbed. If a crack is present, the uniform current is disturbed and the current flows around the ends and down the face of the crack. A standard PC is used to control the equipment and display results. ACFM is unique in the way data is displayed.
RTD INCOTEST (INsulated COmponent TESTing) is based on the pulsed eddy current (PEC) principle and is a reliable way to survey ferrous pipes and vessels through their thermal insulation and protective coatings. The in-depth PEC technology is an excellent tool for prioritising further inspections. In this technique, the sending coil generates eddy currents at the surface of the material. As they diffuse, they generate a magnetic field that is detected by the receiving coil in the probe. The average remaining wall thickness within the enclosed magnetic field is proportional to the decay time of the received signal.
Magnetic flux leakage (MFL) is a tube-testing technique primarily designed for the rapid testing of ferromagnetic tubes with non-ferromagnetic fins wrapped around them, such as in air fin coolers. Two strong magnets generate a static magnetic field that saturates the tube wall (Fig. 1 ). When a flaw (pitting, wall loss, etc.) is located between the two magnets, the magnetic flux in the tube wall is disturbed and a small amount of flux will leak into the inner tube. This leakage of flux is detected by the coils placed between the magnets. The variation of the flux leakage induces current in the coils, thereby causing a signal output. This signal output can be used to provide information on any wall-thickness reduction in the tube. Magnetic flux leakage (MFL) is mainly applied in the inspection of air fin coolers, but it can also be used for inspecting bare tubes with diameters of one inch (2.5cm) and above.
Remote field testing or RFT is one of several electromagnetic testing methods commonly employed in the field of non-destructive testing. Other electromagnetic inspection methods include magnetic flux leakage, conventional eddy current and alternating current field measurement testing. Remote field testing is associated with eddy current testing and the term 'remote field eddy current testing' is often used when describing remote field testing. However, there are several major differences between eddy current testing and remote field testing. RFT is primarily used to inspect ferromagnetic tubing since conventional eddy current techniques have difficulty inspecting the full thickness of the tube wall due to the strong skin effect in ferromagnetic materials. For example, using conventional eddy current bobbin probes to inspect a steel pipe 10 mm thick (such as what might be found in heat exchangers) would require frequencies around 30 Hz to achieve adequate I.D. to O.D. penetration through the tube wall. The use of such a low frequency results in a very low sensitivity of flaw detection. The degree of penetration can, in principle, be increased using partial-saturation eddy current probes, magnetically biased probes and pulsed saturation probes. However, because of the large volume of metal present as well as potential permeability variations within the product, these specialist eddy current probes are still limited in their inspection capabilities. The difficulties encountered in the testing of ferromagnetic tubes can be greatly alleviated with the use of the remote field testing method. The RFT method has the advantage of allowing nearly equal sensitivities of detection at both the inner and outer surfaces of a ferromagnetic tube. The method is highly sensitive to variations in wall thickness and tends to be less sensitive to fill-factor changes between the coil and tube. RFT can be used to inspect any conducting tubular product, but it is generally considered to be less sensitive than conventional eddy current techniques when inspecting non-ferromagnetic materials. Near Field Testing (NFT) technology is a rapid and cost-effective 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 is specifically suited to the detection of internal corrosion, erosion or pitting in carbon steel tubing. The NFT probes measure lift-off or 'fill factor' and convert it to amplitude-based signals (no phase analysis). Because 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 tube.
Eddy Current Testing (ET)
 
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