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NDT Flaw detection- A comparative study on applicability

Updated: Oct 30, 2020



Abstract: There are several methods of NDT flaw detection techniques which can be employed for flaw detection in pipes, heat exchanger tubes, tanks and pressure vessel’s shell. Some of them will bring excellent result but at the same time miserably fail in other applications. This document covers a comparative study of such NDT techniques with a view to employ right technique for specific application.


ECT (Eddy Current Testing): This is a proven method for detecting flaws in metal structures which is typically used to detect defects or flaws metal tubing in heat exchangers. Such testing can detect corrosion or any other damage that causes metal thinning.


Basic Principle of ECT

Eddy currents are generated through a process called electromagnetic induction influenced under alternating current (AC) applied to the conductor, such as copper wire and magnetic field is developed around the conductor. This magnetic field expands as the alternating current rises to maximum and is reduced to zero when field is contracted. In changing magnetic field, if any other electrical conductor is placed in vicinity, current will be induced in the test conductor. Change or variation in eddy current depends on the electrical conductivity and magnetic permeability of the test object, and the presence of defects. The change in eddy current and corresponding change in phase and amplitude which is being detected by eddy current probe indicates presence of defects.

Typical applications for encircling/sector ECT sensors are:

  • Can be used in magnetic or non-magnetic objects and even on ferrous and non-ferrous materials.

  • Detect short surface and some subsurface defects on wire, bar and tube.

  • ID or OD defects in the weld zone can be detected on welded tubes.

  • Objects with uniform cross sectional material, including squares, rectangles, hex and round can be tested.

  • Continuity and locate welds in single and multi-conductor insulated wire and cable can be detected.

  • Can be employed in wire drawing, parts forming, re-spooling operation to inspect cut lengths etc.

  • Used for longitudinal surface defects on test parts, such as small shafts and bearings.

EDDY CURRENT ARRAY TECHNIQUE (ECA)

The eddy current array technique (ECA) is a variant of conventional eddy current testing and shares the same electromagnetic/inductive principles of eddy current.

As it uses “array” (adjoining eddy current coils are forming a coherent eddy current assembly) it covers larger area in comparison to ECA making it a a faster and more cost efficient scanning tool.

The Eddy Current Array technique uses a multiplexer to excite the single coil elements in a pre-defined scheme to leverage the probe’s width. Multiplexing also minimizes the interference between coils in close proximity and maximizes the resolution of the probe. Another advantage is that each coil element is able to transmit and/or receive eddy current signals, i.e. different circuit modes can be realized which enables the technique to detect cracks or corrosion independently of its orientation in a single pass. An expressive full coverage 2D or 3D C-Scan report can be generated for all tested areas which is another advantage of the ECA technique.


Features of ECA

  • High inspection speed, reducing the time for large no. of tubes.

  • High probe coverage, high test reliability.

  • Independent for defect orientation through intelligent multiplexing.

  • Probes can be adapted to complex geometry.

  • Different inspections in one pass.

  • High reproducibility.

Near Field Testing (NFT)

Near Field Testing (NFT) technology is a rapid and cost-effective solution intended specifically for fined 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 suitable for 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.

Near Field Array (NFA)

Near-Field Array (NFA) is the multiplexed evolution of Near-Field Testing (NFT). Thanks to its increased probability of detection, it is an excellent alternative to IRIS in fin-fan air coolers and ferromagnetic heat exchanger tubing.

How it Works

Since magnetic field of remote-field testing (RFT) propagates through and outside the tube wall, the technique cannot be used in aluminum-finned tubes as external aluminum fins of these tubes greatly influence the quality of inspection signals thus, aluminum-finned carbon steel tubes are most difficult tubular components to inspect.

NFA technology functions in transmit-receive mode. A single bobbin coil acts as the transmitter to generate the near field, an absolute bobbin receiver coil detects and sizes the general internal wall loss, and up to two rows of multiplexed receiver coils cover the entire inner surface of aluminum-finned tubes (full 360°). With up to 30 optimized coils NFA is capable of generating high-quality signals yielding a very good signal-to-noise ratio (SNR) that allow detecting circumferential and axial cracking. The coil configuration of NFA also enables C-scan imaging despite a scan speed equivalent to NFT at 300 mm/s (12 in/s) in tubes ranging from 19.1 mm to 38.1 mm (0.75–1.50 in) in diameter. So doing, NFA technology gives probes the necessary resolution to reliably detect small volumetric defects of approximately 3.2 mm (1/8 in) in a single pass.

Compared to other inspection technologies, Near-Field Array technology is easier to deploy. NFA does not incorporate any magnets, so probes are easy to push and pull through tubes, and are not as sensitive to pull speed as MFL probes. NFA probes also do not require water or complex tools, making them much easier to use than IRIS.

Magnetic Flux leakage (MFL)


MFL is a rapid and robust corrosion detection technique. This NDT system accurately detect defects in heavy-wall ferromagnetic tubing which is also used to inspect high-permeability ferromagnetic metals such as carbon steel bar, plate, wire rope and parts. It relies upon strategically placed Hall effect sensors to detect the magnetic leaking field that is created by corrosion. The method can also be used to detect ferritic inclusions in non-ferritic material. MFL employs a DC magnetizing field to create enough flux density to bring the material to near-saturation. A transverse magnetizing field is used to detect longitudinally oriented defects, while a longitudinal field is used to find transverse defects.

Typical applications:

  • For typical OCTG and other heavy wall tube tests.

  • Surface or internal conditions such as cracks, pits, seams and other defects interrupt the flux field and “leak” beyond the product surface.

  • The amplitude and frequency of the voltage generated by the flux sensor in response to a discontinuity is generally indicative of the severity and location of that discontinuity.

  • Combining MFL inspection with Ultrasonic (UT) testing fulfills certain API standards for OCTG pipe that require a second method, at the discretion of the pipe producer, when using UT as the first method.

Advantages

  • Inspect air cooler tube with a high-resolution array, providing intuitive C-scans at NFT speeds.

  • Detecting and sizing internal defects in one pass.

  • Detecting axial and circumferential cracks.

  • Easy to use

Internal rotary inspection system (IRIS)

As UT device essentially requires couplant, being the same IRIS uses water. Therefore, tubes to be tested are filled with water to use this technique. Its transducer generates an ultrasonic pulse parallel to the axis of the tube under test. A rotating mirror directs the ultrasonic wave to the tube wall. A part of the ultrasonic wave is reflected by the inner-diameter (ID) wall, while the rest is reflected by the outer-diameter (OD) wall of the tube. Thickness of tube wall can be known by flight of wave between known distances. When the probe is pulled, the circular motion of the mirror results in a helical scan path. IRIS is generally used in boilers, shell-and-tube heat exchangers, and fin-fan heat exchanger tubes.

Advantage of IRIS

  • Corrosion on OD and ID can be detected.

  • Pitting and wall loss can be detected.

  • Accurate wall thickness measurements are possible.

  • Sensitive to both internal and external defects.

  • Defect position can be located in relation to tube length.

Limitation

· Tubes must be flooded with filtered water making the inspection setup complex and difficult to use.

· It is sensitive to ID / OD deposits and fins, which are not defects.

Partial Saturation Eddy Current (PSEC) AND Full Saturation Eddy Current (FSEC)


PSEC (Partial Saturation Eddy Current) is a technique for the rapid inspection of ferromagnetic tubes and is capable of detecting internal and external defects. Conventional eddy current techniques cannot be applied to ferromagnetic tubes due to their high magnetic permeability that results in low field penetration and high noise levels. The technique relies on partial magnetic saturation of the tube using permanent or electromagnets mounted in the probe.

The presence of defects in the tube causes variations in magnetic flux density that are detected using eddy current sensors. It is more sensitive to pitting and baffle plate defects than RFT, but there is very little phase information, wall loss measurement capability is very limited and it is recommended as a screening tool only.

FSEC is a similar technique to PSEC, designed for Duplex stainless steels and other weakly ferromagnetic tubes such as Monel. However, because these materials have a significantly lower magnetic permeability than Carbon steels, it is possible to fully saturate the material using permanent magnets (dependent on wall thickness & tube size). Duplex then behaves as a non-ferromagnetic material allowing more sensitive and accurate conventional eddy current sizing techniques to be applied.

The probe used in PSEC examination contains normal eddy current coils. In addition to that the probe contains two coils that are used as an electro magnet which is used to partly cancel out the magnetic properties of the tube material. The electromagnets’ field is mainly active at the inner surface of the tube where it compensates for the skin effect. This allows some eddy currents to penetrate the material. Defects in a tube will cause changes in the permeability and in the amount of eddy currents at that location.

During signal analysis, the signals acquired during an inspection will be compared to the signals from reference defects. Reference defects are defects with known depth and shape and are machined into a calibration standard. The calibration standard needs to be of the same material and dimensions as the tubes to be examined.

Possibilities and limitations of Partial Saturation Eddy Current

  • Can inspect finned ferrous tubes and when small diameter pitting is expected.

  • Holes with a diameter of 2 mm or pits with the same volume as a 2 mm hole are normally detectable.

  • This counts for tubes with an internal diameter smaller than appr. 30 mm. In bigger tubes sensitivity goes down a little bit. How much depends a lot on the situation.

  • Overall wall-loss is detectable from 10% of the nominal wall thickness and up Thickness loss is reported as % of wall thickness.

  • PSEC can detect both internal and external defects and can distinguish between internal and external defects.

  • Non volumetric defects like cracks can be detected, depending on their size, shape and orientation.

  • Defects under support plates are detectable but the accuracy of sizing is limited.

  • For the sizing of internal local defects, phase information is available. Sizing of external local defects is based on defect volume, shape and orientation.

  • For accurate sizing it is desirable to verify a few defect indications using IRIS. The additional information can be used to adjust the PSEC findings.

  • For precise measurement, the maximum possible probe size shall be used and the tubes have to be very clean in order to allow the probe to pass through the tube.

Remote field eddy current testing (RFET) or Remote field testing (RFT)


RFT is used for defect detection in ferromagnetic materials including some ferritic stainless steels. It is good for detection and sizing of cracks, corrosion and mechanical damage in the tubes, however has limitation in detecting gradual wall loss and pits less than 6% of tube wall in tube as this process employs low frequencies. If shallow pits or gradual thinning is suspected, sometimes, Remote Field Testing reading is cross checked with IRIS inspection. With RFT, it is possible to test from 400-420 5-6 meter heat exchanger tubes in a 12-hour shift depending on how clean the tubes are and to some extent the number of defects.

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.

The basic RFT probe consists of an exciter coil and detector coil, the former sends a signal to the later. The exciter coil emits a magnetic field which travels outwards from the exciter coil, through the pipe wall, and along the pipe. The detector is placed inside the pipe two to three pipe diameters away from the exciter and detects the magnetic field that has travelled back in from the outside of the pipe wall. In areas of defect or metal loss, the field arrives at the detector with a faster travel time (greater phase) and greater signal strength (amplitude) due to the reduced path through the steel. Hence the dominant mechanism of RFT is through-transmission.

Two coupling paths exist between the transmitters and receivers. The direct path, inside the tube, is rapidly attenuated by circumferential eddy currents induced in the tube’s wall. The indirect coupling path originates in the transmitter’s magnetic field that diffuses radially outward through the wall. At the outer wall, the field spreads rapidly along the tube with little attenuation and re-diffuse back through the pipe wall and are the dominant field inside the tube at the receiver. Anomalies anywhere in the indirect path cause changes in the magnitude and phase of the received signal, and can therefore be used to detect defects.

It has several benefits over other electromagnetic testing techniques:

  • Equal sensitivity at the inner and outer surfaces

  • Suitable for ferromagnetic materials like MS and CS.

  • Measures wall thickness very effectively.

Conclusion:

Since several NDT techniques have been evolved keeping in view advantages and limitation in specific applications, choosing the appropriate inspection method for your equipment is required to obtain the precise and desirable result. It depends on your tube material, type of defects, location of defects and specific inspection needs. A ready reference of comparative benefits and limitation is enclosed below:


Suitability According to Tubing Material


Suitability According to Defect Type in Tubing


Defect Sizing Capabilities According to Defect Type in Tubing



Content Credit: Twi-Global, Delta Test GmbH, www.eddify.com, www.mac-ndt.com

Title image Credit: Advanced NDT Services LLC.


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