Near-field array (NFA) is the multiplexed evolution of near-field testing (NFT). Thanks to its sizing capabilities, it is an excellent alternative to IRIS in fin-fan air coolers and ferromagnetic heat exchanger tubing.

Near-field array working principle

How it Works

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 tubesheet, 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, magnetic flux leakage (MFL), partial saturation eddy current testing (PSEC), internal rotary inspection system (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 and size small, volumetric defects 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.

Benefits

  • 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 (no magnet = easy to push and pull)

Detection Capabilities According to Defect Type

Defect/Tech ECT ECA IRIS RFT NFT NFA MFL PSEC
ID pitting

Excellent

Excellent

Excellent

Acceptable, but limited

Acceptable, but limited

Excellent

Excellent

Excellent

OD pitting

Excellent

Excellent

Excellent

Acceptable, but limited

Not suitable 

Not suitable

Acceptable, but limited 

Acceptable, but limited 

Axial cracking

Acceptable, but limited

Excellent

Not suitable

Acceptable, but limited

Acceptable, but limited

Acceptable, but limited

Not suitable

Acceptable, but limited

Circumferential cracking

Acceptable, but limited

Excellent

Not suitable

Not suitable

Not suitable

Acceptable, but limited

Acceptable, but limited

Acceptable, but limited

ID corrosion

Excellent

Excellent

Excellent

Excellent

Excellent

Excellent

Excellent

Excellent

OD corrosion

Excellent

Excellent

Excellent

Excellent

Not suitable

Not suitable

Acceptable, but limited

Acceptable, but limited

At tubesheet

Acceptable, but limited

Excellent

Excellent

Acceptable, but limited

Not suitable

Not suitable

Acceptable, but limited

Acceptable, but limited

Excellent

Acceptable, but limited

Not suitable

Sizing Capabilities According to Defect Type

Defect/Tech ECT ECA IRIS RFT NFT NFA MFL PSEC
ID pitting

Good

Excellent

Excellent

Good

Not suitable

Excellent

Not suitable

Not suitable

OD pitting

Excellent

Excellent

Excellent

Good

Not suitable 

Not suitable

Not suitable 

Not suitable

Axial cracking

Good

Excellent

Not suitable

Not suitable

Not suitable

Good

Not suitable

Not suitable

Circumferential cracking

Not suitable

Excellent

Not suitable

Not suitable

Not suitable

Good

Not suitable

Not suitable

ID corrosion

Good

Excellent

Excellent

Excellent

Not suitable

Excellent

Not suitable

Not suitable

OD corrosion

Excellent

Excellent

Excellent

Excellent

Not suitable

Not suitable

Not suitable

Not suitable

At tubesheet

Good

Good

Excellent

Not suitable

Not suitable

Not suitable

Not suitable

Not suitable

Excellent

Good

Not suitable

Suitability According to Tubing Material

Material/Tech ECT ECA IRIS RFT NFT NFA MFL PSEC
Non-ferromagnetic Tube

Yes

Yes

Yes

No

No

No

No

No

Integral finned tube

Yes

Yes

Yes

No

No

No

No

No

Low ferromagnetic Tube

No

No

Yes

Yes

Yes

Yes

Yes

Yes

Integral finned tube

No

No

Yes

Yes

Yes

Yes

Yes

Yes

Ferromagnetic Tube

No

No

Yes

Yes

Yes

Yes

Yes

Yes

Integral finned tube

No

No

Limited

Yes

Yes

Yes

Yes

Yes

Aluminum finned tube

No

No

Yes

No

Yes

Yes

Yes

Yes