OUR TUBE STANDARDS FOR ECT RFT NFT MFL IRIS NFA
15.0. TUBE INSPECTION BY ECT, RFT, NFT, MFL, IRIS,
NFA
15.1. EDDY CURRENT TESTING (ECT):
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.
Advantages of ECT
High examination Rate i.e. 80-100 or more tubes can be
Inspected/per 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.
15.2. REMOTE FIELD TESTING (RFT)
Remote Field
Testing (RFT) is one of the several electromagnetic testing
methods commonly employed in the field of non-destructive testing. RFT may also
refer to as RFEC (Remote Field Eddy Current) or RFET (Remote Field
Electromagnetic Technique). RFT is primarily used to inspect ferromagnetic
tubing since conventional ECT has difficulty in inspecting the full thickness
of the tube wall due to the strong skin effect in ferromagnetic materials.
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)
15.3. NEAR FIELD TESTING (NFT):
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.
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
15.4. MAGNETIC FLUX LEAKAGE (MFL)
Magnetic Flux
Leakage (MFL) uses 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.
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
15.5. IRIS – INTERNAL ROTARY INSPECTION SYSTEM
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 IRIS
IRIS can be applied on almost every 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
15.6. NEAR FIELD ARRAY (NFA):
By design, aluminum-finned
carbon steel tubes are one of the most challenging tubular components to
inspect. The external aluminum fins of these tubes greatly influence the
quality of inspection signals making it difficult to size pitting, cracking at
the tube sheet, tapering, and general wall loss in such tubes. Obviously,
because the magnetic field of remote-field testing (RFT) propagates through and
outside the tube wall, the technique cannot be used in aluminum-finned tubes.
However, inspection companies use other techniques NFT, MFL,
Partial Saturation Eddy Current Testing (PSEC) or IRIS that
do not have this limitation.
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 and thanks to channel multiplexing, 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.