Remote-field testing (RFT) is a non-destructive electromagnetic testing technique mainly used to find defects in steel pipes and tubes.

How it Works

RFT probes use one or several transmitter coils positioned two tube diameters apart from the receiver coil. The magnetic field created by the transmitter coil travels through and out of the tube wall, radially and axially, towards the receiver. The magnetic field must travel through the tube wall again to reach the receiver. This is called through-transmission and is what defines RFT. Through-transmission allows external and internal defects to be detected with equal sensitivity.

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 rediffuse 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.

Benefits

RFT has several benefits over other electromagnetic testing techniques:

  • Suitable for ferromagnetic materials
  • Equal sensitivity at the inner and outer surfaces
  • Highly sensitive to wall thickness variations
  • Can be used with lesser fill factors than ECT

Detection Capabilities According to Defect Type in Tubing

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 in Tubing

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