What is Remote Field Testing (RFT)? Principle & Applications

Table Of Content

1. What is Remote Field Testing (RFT)?
2. The Principle of Remote Field Testing: Through-Transmission
3. Transmitter and Receiver Coils
4. The Two Coupling Paths
5. Direct Path
6. Indirect (Remote) Path
7. Why the Field Crosses the Wall Twice
8. Reading Wall Loss by Phase and Amplitude
9. Phase
10. Amplitude
11. RFT Probes and Equipment
12. Applications of Remote Field Testing
13. Where RFT Fits: RFT vs ECT, NFT and IRIS
14. When to Choose RFT
15. When Another Method May Be Better
16. Limitations of Remote Field Testing
17. Conclusion
18. Frequently Asked Questions (FAQs)

What is Remote Field Testing (RFT)?

An electromagnetic nondestructive testing (NDT) method mainly utilized to examine ferromagnetic pipes and tubes, known as Remote Field Testing (RFT). The magnetic field passes through the tube wall twice in the remote field testing method, which gives the same sensitivity to defects on the inside diameter (ID) and outside diameter (OD) of the tube as conventional eddy current testing.

This special feature provides RFT NDT as one of the most dependable methods of evaluating heat exchanger tubes of carbon steel, boiler tubes, feed water heaters, pipelines and cast iron water mains.

The Principle of Remote Field Testing: Through-Transmission

The key feature of remote field eddy current testing is that it is through transmission. Conventional eddy current testing uses electromagnetic fields that stay near the probe, and remote field testing is based on a magnetic field that has gone through the wall of the tube twice before it is measured.

This double-walled crossing makes a change to the way the inspection works and is the reason RFT provides both internal and external defect sensitivity at the same time.

Transmitter and Receiver Coils

An inspection probe for RFT is comprised of:

1. A coil of wire known as a transmitter (exciter) coil.
2. One or more receiver coils
3. The probe body is in the center of the tube.

In remote field testing the receiver coil is placed about 2 tube diameters away from the transmitter coil as opposed to using two coils close together as in conventional ECT testing.

This spacing is important because it results in the receiver not receiving the strong direct electromagnetic field from the transmitter, but only the weak remote field.

The Two Coupling Paths

When a low frequency alternating current is sent through the tube, two magnetic field paths are formed.

Direct Path

The first path is the normal path from the transmitter to the receiver.

Initially, this field is extremely strong.

Carbon steel, however, has high magnetic permeability and electrical conductivity so that the direct field is quickly decayed by:

1. Eddy current losses
2. Magnetic absorption
3. Skin effect

The direct field is very weak in a short distance.

Indirect (Remote) Path

The second path is very different.

The magnetic field does not remain inside the tube, but rather:

1. Leave the transmitter.
2. Travels outside of the tube wall
3. Rides on the edges of the tube.
4. Re-enters into the wall of the tube.
5. Connects to the receiver coil.

This magnetic field has passed through the wall twice, and thus contains information about the condition of the tube from both sides.

The indirect field is stronger than the direct field at the receiver location, this area is referred to as the remote field.

Why the Field Crosses the Wall Twice

This is why the double wall crossing is unique from all other electromagnetic tube testing methods.

In the process of transmission, the magnetic field:

1. Crosses the wall (after leaving the tube).
2. Does not remain in the tube.
3. Goes over the wall again before he/she reaches the receiver.

Quenching points on both the internal and external surfaces of the crossing cause an effect that is more or less equally pronounced for the received signal.

There is little inner surface bias as compared to conventional eddy current testing.

RFT NDT is well suited for detecting:

1. External corrosion
2. Internal corrosion
3. General wall thinning
4. Erosion
5. Pitting
6. Baffle wear

Reading Wall Loss by phase and amplitude

Two attributes of a signal are tested by remote field testing:

Phase

The phase shift depends on the thickness of the wall.

The thickness of the wall is reduced as the wall becomes corroded or eroded, thus the magnetic field is not slowed down as much as it goes through the wall.

As a result, phase measurements can give a reliable measure of the wall thickness remaining.

Generally, it is recognized that the best parameter for sizing gradual wall loss is phase.

Amplitude

Signal amplitude is affected by defects, which affect the strength of the magnetic field.

Measurable amplitude variations occur due to localized defects that may be pits, grooves or extreme levels of corrosion.

Amplitude is particularly useful for seeing defects that are isolated and to verify indications seen on the phase data.

The measurements have been combined in modern inspection software, to provide better defect characterization and minimize false calls.

RFT Probes and Equipment

A typical remote field eddy current testing system is composed of:

1. Instrument
2. Exciter (driver) coil
3. Receiver coil(s)
4. Probe centering device
5. Data acquisition software

Systems can be configured with:

1. Single driver probes for standard tube inspection
2. Double-driver probes for better signal in challenging situations
3. Large asset coverage configurations using arrays

An advantage of RFT inspection over conventional eddy current inspection is that it is relatively insensitive to variations in fill-factor. Probe centering is not as critical as with earlier designs, and small changes have little effect on the inspection results.

A detailed discussion of probe design, array technology and Remote Field Array (RFA) systems is beyond the scope of this article and is best presented in dedicated articles.

Remote Field Array (RFA) extends conventional remote field testing by combining array technology with RFT for improved defect characterization and faster ferrous tube inspections.

→ Read: Remote-Field Array (RFA) Technology for Ferrous Tubing Inspections

Applications of Remote Field Testing

The potential to inspect ferromagnetic materials is the reason why remote field testing is broadly used in power generation, petrochemical, water distribution, and manufacturing.

Typical inspection priorities are:

1. Carbon steel heat exchanger tubes
2. Boiler tubes
3. Feedwater heater tubes
4. Condenser tubes made from ferromagnetic materials
5. Carbon steel pipelines
6. Cast iron water mains
7. Ductile iron pipelines
8. Industrial process tubing

Common flaws that are found are:

1. General wall loss
2. Internal corrosion
3. External corrosion
4. Pitting
5. Erosion
6. Flow-assisted corrosion
7. Baffle plate wear
8. Localized metal loss
9. Manufacturing defects that involve changes in wall thickness.

Since the inspection can be done on both external and internal surfaces of tubes, RFT is frequently chosen when surface corrosion occurs outside the tube but cannot be seen.

Learn how Remote Field Array technology is used to detect corrosion and wall loss in large ferrous pipes, extending the capabilities of conventional remote field testing.

→ Read: Corrosion Detection in Large Ferrous Pipes – Remote Field Array Technology

Where RFT Fits: RFT vs ECT, NFT and IRIS

Selecting the correct inspection technique depends on tube material, expected damage mechanism, accessibility, and inspection objectives.

When to Choose RFT

Opt for remote field testing when inspecting carbon steel or other ferromagnetic tubes when both internal and external corrosion is a concern. RFT is especially useful for the detection of overall thinning and corrosion underneath support structures or baffle plates.

When Another Method May Be Better 

Where the material to be inspected is non-ferromagnetic (e.g. stainless steel, copper alloys), and small cracks are to be detected, ECT is still the preferred choice. When the focus is on inner surface defect, NFT is designed to be helpful, but for detailed corrosion assessment with lower inspection speeds IRIS is very helpful.

Limitations of Remote Field Testing

There are several merits of RFT NDT, but there are also a number of drawbacks to consider.

1. Only applicable to ferromagnetic materials.
2. More resistant to very fine cracks than traditional ECT.
3. Low spatial resolution due to a large inspection field.
4. Complex interpretation of signals around small defects.
5. The speed of inspection may be slower than some array-based technologies.
6. Signals can be affected by support structures in the tubes and need to be interpreted.
7. Is unable to differentiate overlapping indications multiple times without additional examination techniques.
8. Usually used in conjunction with IRIS or visual inspection for a thorough evaluation of condition.

Conclusion

One of the most critical electromagnetic inspection methods for ferromagnetic tubes and pipes that cannot be solved by the conventional eddy current testing is Remote Field Testing. Far field testing measures a magnetic field that passes through the tube wall twice so it is very sensitive to both the internal and external wall loss, and is effective for detecting corrosion, erosion, pitting, and overall thinning.

While not ideal for every inspection situation, remote field eddy current testing is used where needed for accurate wall-loss detection on carbon steel heat exchangers, boiler tubes, feedwater heaters, cast iron pipelines and other ferromagnetic assets. RFT together with other complementary techniques like NFT and ECT enables the asset owner to make informed maintenance decisions, while minimizing the chances of unforeseen failures.

Frequently Asked Questions (FAQs) 

What is Remote Field Testing (RFT)?

An electromagnetic NDT method for the inspection of ferromagnetic tubes and pipes is called Remote Field Testing (RFT). It operates by measuring a magnetic field passing through the tube wall twice, making it almost equally sensitive to corrosion, erosion, pitting and wall thinning on the inside and outside surface of the tube.

How is Remote Field Testing different from conventional Eddy Current Testing? 

Conventional Eddy Current Testing is best suited for non-ferrous materials and more sensitive to surface defects close to the probe. However, remote field eddy current testing is a specialized eddy current technique that is only applicable to ferromagnetic materials and is used to detect both internal and external wall-loss defects in a balanced manner, based on a unique through-transmission principle.

Which materials can be inspected using RFT?

Remote Field Testing is mainly applicable to carbon steel, low alloy steel, cast iron and ductile iron. It is used widely on heat exchanger tubes, boiler tubes, feedwater heaters, pipelines, water distribution mains where corrosion monitoring is important.

What sort of defects can RFT find?

RFT inspection can identify a variety of volumetric defects such as general wall loss, internal corrosion, external corrosion, pitting, erosion, flow-assisted corrosion, baffle-plate wear and localized metal loss. It is especially useful for detection of slow thinning of walls which may compromise the structural integrity.

What are the benefits of Remote Field Testing?

The greatest benefit of testing in the field is that their sensitivity to surface and subsurface defects in ferromagnetic tubes is similar. It is also able to accept moderate positioning variations of the probe, is efficient for long lengths of investigation, and is suitable for investigating inaccessible locations for corrosion.

When should RFT be used instead of IRIS or NFT? 

RFT is the recommended method to use for examining ferromagnetic tubes for wall loss on the inside and outside wall surfaces. IRIS is used when there is a need for very precise wall-thickness measurement and NFT is more appropriate when identifying defects near the inner surface. Several inspection programmes integrate these techniques for a total inspection.