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Tube Inspection by ECT, RFET, NFT, ECA, MFL, IRIS

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Eddy Current Testing (ECT) works on the principle of electromagnetic induction. In ECT a probe is excited with sinusoidal alternating current to induce eddy current in an electrically conducting material such as stainless steel, aluminium etc. The change in coil impedance that arises due to distortion at regions of discontinuities and associated magnetic flux leakage is measured. This is a surface technique and can readily detect very shallow surface defect and sub-surface defects. ECT is a simple, high-speed, high sensitive, versatile and reliable NDT technique and is popularly used in many engineering industries.

Remote Field Testing (RFT) is one of the several electromagnetic testing methods commonly employed in the field of non-destructive testing. RFT may also referred to as RFEC (Remote Field Eddy Current) or RFET (Remote Field Electromagnetic Technique). RFT is primarily used to inspect ferromagnetic tubing since conventional ECT have difficulty in inspecting the full thickness of the tube wall due to the strong skin effect in ferromagnetic materials.

Near Field Testing (NFT) technology is a rapid and cost-effective solution intended specifically for fin-fan carbon-steel tubing inspection. This 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. 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.

The conventional ECT faces major limitations with respect to detection & sizing of circumferential cracking and extent of volumetric flaws in tubular. Rotating probes is a solution but it reduces the inspection speed. Therefore Eddy Current Array (ECA) along with bobbin probe technology is the solution to perform high speed inspection in a single pass.

Magnetic Flux Leakage (MFL) uses a powerful magnets to magnetize the conductive material under test where a magnetic field is created around defects like corrosion or material loss. The magnetic field induced in the part saturates it until it can no longer hold any more flux. The flux overflows and leaks out of the pipe wall and strategically placed sensors can accurately measure the three-dimensional vector of the leakage field. Because magnetic flux leakage is a vector and that a sensor can only measure one direction, any given probe must have three sensors to accurately measure the axial, radial, and circumferential components of an MFL signal.

Internal Rotary Inspection System (IRIS) is an ultrasonic method for testing of pipes and tubes. The ultrasonic beam allows detection of metal loss from the inside and outside of the tube wall. It is a fairly sensitive technique. The sensitivity achieved will depend on tube dimensions and tube cleanliness. Both ferromagnetic and non-ferromagnetic tubes can be inspected. A three dimensional picture of the defect is obtained, thus the defect profile and its depth is provided. Interpretation of results is easier than in the other techniques

Advantages Of ECT

  • High examination Rate i.e. 80-100 or more tubes can be Inspected/ hour
  • Examination of Ferrous and Non Ferrous material
  • Repeatability comparison between historical and subsequent test results to establish corrosion rate and remaining life
  • ECT discriminates between ID and OD defect orientation
  • ECT is sensitive to gradual wall loss, cracking, small pitting etc.

Advantages Of RFT

  • Detection of wall loss
  • Ideal for ferrous tubes
  • RFT can detect defects away from tube support plate and tube sheet
  • Detect localized corrosion/erosion
  • Inspect ferromagnetic materials (carbon steel and stainless steel)

Advantages Of NFT

  • NFT is faster than electromagnetic testing techniques
  • External reference coil is not required during NFT
  • The results of NFT is unaffected by support plates and Tube sheets

Advantages Of ECA

  • One-pass inspection providing bobbin and array probe signal high definition which provide reliable and repeatable detection of small defects whatever the position around the circumference
  • High sensitivity to pitting and circumferential cracking (ID and OD circumferential crack detected down to 20% depth and 25° extension)
  • More reliable characterization of flaws with C-scan imaging improved detection of circumferential cracking under tube sheets

Advantages Of MFL

  • One of the few methods used to inspect finned tubes
  • Can be used on all ferromagnetic materials
  • Good sensitivity to pitting
  • High-speed inspection

Advantages Of IRIS

  • IRIS can be applied on almost each material
  • Exact determination of location of pitting/corrosion
  • Small changes in thickness measurable
  • No negative effect on results caused by external items like fins or baffles
  • Measurement produces nominal value for wall thickness of pipe
  • No calibration pipes needed
  • IRIS can also be used in steam drums

Source:- https://www.ndts.co.in/tube-inspection-by-ect-reft-nft-eca-mfl-iris/