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SERVICES >> ADVANCED NDT HEAD E&B TUBES

Eddy Current Testing (ECT)

Eddy Current Technique (ECT) is based on measuring the impedance of a coil. The impedance of the coil changes as the electromagnetic field interacts with the material. Initially, the coil is placed in the tube and balanced on the sound material. The probe is then pulled and variations in coil impedance recorded. The impedance changes are related to the type and size of defect.

ECT can be used to inspect Non-Ferrous Tubes (Eg.SS304/316) Brass, Titanium, Inconel, Monel Copper-nickel etc.

It is well suited for the detection and reliable sizing of the following types of Defects

  • Pitting
  • Localized corrosion
  • Cracks

It may required to use different types of probes based on the types of tube damage expected. Very fast inspection speeds of up to 2 meters/second may be achieved using automated probe pullers.

Signal Display

 

Remote Field Electromagnetic Testing (REFT)

RFET is a Low Frequency Technique that accurately measures wall loss especially in Ferromagnetic tubes. It uses a relatively simple instrument and is not affected by high magnetic permeability of ferromagnetic material assuming that it is uniform in the length of the tube.

Two primary paths exist for coupling the energy between the exciter and the receivers. The direct field which carries no useful information of the tube wall condition is rapidly attenuated with distance down the pipe and is undetectable beyond about two pipe diameters. The indirect field diffuses radically outward through the pipe wall, moves along the pipe, and re-diffuses radically back through the pipe wall. The zone in which this indirect field is dominant is named the "remote field zone".

Any discontinuity present in the material will affect the Remote Field signal . These changes in the Remote field can be compared to those measured on known machined flaws in a calibration tube with properties similar to the one being tested. A comparative analysis provides the value of tube wall loss on the tube being tested.  RFET can be used to inspect Ferro-magnetic tubes in

  • Shell and Tube Heat Exchangers
  • Boilers 

It is well suited for the detection of the following types of corrosion

  • General Corrosion / Erosion
  • Localized Corrosion and Pitting
  • Support Plate Fret wear extending beyond the baffle

It has limitations in the detection and sizing of

  • Under Baffle Corrosion
  • Isolated pitting / pin holes

Signal Display

 

Magentic Flux Leakage ( MFL )

MFL is a tube testing technique primarily designed for rapid testing of ferromagnetic tubes with non-ferromagnetic fins wrapped around them as in Air Fin Coolers. Two strong magnets generate a static magnetic field that saturates the tube wall. When a flaw (Pit, 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 in the inner tube. This leak of flux is detected by the coils placed between the magnets. The variation of the flux leakage induces the current in the coils, thereby causing a signal output. This signal output can be used to provide information on wall thickness reduction, if any in the tube.

Magnetic Flux Leakage (MFL) is mainly applied for the inspection Air Fin Coolers; however it can be used for inspecting bare tubes also.

Advantages

  • Not affected by the presence of Aluminum Fins.
  • Relatively High Speed Inspection. (up to 1meter/sec).
  • Very good sensitivity for both Pitting and Circumferential Grooves.

Limitations

  • Presence of residual magnetism hampers tube repulls
  • More a detection tool & is not very useful for flaw sizing since the signal represents volumetric change and not the change in depth of the flaw
  • Signal sensitivity depends on pull speeds & isolated Pin Hole type of flaws cannot be detected

Signal Display

 

Internal Rotating inspection System (IRIS)

IRIS is a pulse-echo based tube inspection technique. A transducer excited by a high frequency pulse producers an ultrasonic wave that propagates into water. A mirror deflects the wave to produce a normal incidence beam on the inner diameter (ID) of the tube. Echoes reflected from each metal water interface are digitized and processed to extract the time of flight and amplitude of the frontwall and backwall echoes. Further processing is applied to calculate the tube ID, outer diameter (OD) and wall thickness (WT).

Complete tube inspection is obtained by rotating the mirror. A hydraulic turbine (IRIS) or an electric motor produces the driving force. Synchronization of the rotation can be obtained by various methods like ultrasonic targets or encoders.

The Olympus MS5800 IRIS system can accommodate these synchronization modes in order to display in real time, the data either on a cross-section thickness display (B-scan), or as a surface area thickness map (C-scan).

IRIS- Applications / Capabilities

  • IRIS can be applied on all types of heat exchanger and boiler tube materials.
  • It can be applied on tubes with diameter of 0.75 inch and thickness greater than 1 mm for best results

Advantages

  • Only technique that gives exact tube wall thickness
  • Will provide information on flaw profile and location i.e. if present on the ID or OD of the tube
  • With real time C-scan capabilities during data collection, pits as small as 1 mm diameter can be easily detected

Limitations

  • Very slow speed of operation
  • Requires extensive tube cleaning
  • Limited to minimum wall thickness measurement of 0.8mm for Carbon steel tubes

Signal Display