There are several difficulties encountered during ferromagnetic tubes testing. The remote field testing (RFT) can help to alleviate these difficulties. The RFT testing method offers a lot of benefits, one of which is allowing almost equal sensitivities of detection at the external and internal surfaces of a ferromagnetic tube. Also, this testing method is known to be sensitive to changes in wall thickness. It is not very sensitive to fill-factor changes between the tube and coil.
What is RFT Testing?
RFT means remote filed testing. It is a testing method carried out on ferritic materials. These materials make use of alternating current to cause an electromagnetic field. The remote field testing is a reliable method of testing ferritic materials.
Remote Field Testing is an electromagnetic testing method that discovers discontinuities ferromagnetic (Carbon steel) tubes and pipes. RFT is a through transmission test method in which the receiver coil discovers electromagnetic field that traversed via the tube wall. Also, this method is a non-destructive electromagnetic testing technique integrated in finding defects in tubes and steel pipes.
General Concept of RFT Testing
- The basic probe comprises two receiver coils and one exciter coil.
- There are two main fields that couple the energy between receivers and exciter
- Also, the direct field centers around the exciter coil. Therefore, the direct field attenuates relatively to the distance down the tube.
- The tube wall diffuses the indirect field outward. It then propagates along the tube axis and rediffuses via the tube wall.
- In the remote field zone, the indirect field remains dominant. Also, this zone is usually available at a distance which is more than two tube diameters.
How Does RFT Work?
The remote field testing method makes use of RFT probes. These probes make use of one or more transmitter coils positioned apart from the receiver coil. Furthermore, the transmitter coil creates a magnetic field which passes through and out of the tube wall, towards the receiver. After this, the magnetic field needs to pass through the tube wall to get to the receiver. This is what we refer to as through-transmission. The trough transmission enables the detection of internal and external defects with equal sensitivity. This is what defines remote field testing.
Between the receivers and transmitters are two coupling paths. There are circumferential eddy currents in the wall of Inside the tube which attenuate the direct path. The indirect coupling path comes from the magnetic field of the transmitter that diffuses radially through the wall. Also, the field rapidly spreads along the tube with little attenuation. After this, it re-diffuses back via the pipe wall. There can be changes in the received signal’s phase and magnitude due to anomalies anywhere in the indirect path. Therefore, this can help to detect defects.
RFT testing enables electromagnetic energy to get through the tube walls by integrating a low frequency signal. Therefore, this helps to detect internal and external errors with almost the same sensitivity. The transmitter and the receiver coils have two travel paths of the magnetic field between them.
When is RFT Testing Necessary?
RFT testing is extremely ideal for ferrite pipes inspection. This testing method helps in detecting and quantifying wall thickness reductions. Also, it can detect both internal and external flaws, but it cannot distinguish them. RFT testing is not ideal testing method for finned pipes.
In contrast to eddy current testing (ECT), RFT is an ideal testing method for ferromagnetic materials like tubes and pipes available in heat exchangers and boilers. It is difficult to use conventional ECT to inspect the full thickness of a tube wall. This is because of ferromagnetic materials feature strong skin depth effect.
For instance, remote field testing can inspect wall thicknesses of about 12 mm at a pulling speed range of 0.1 m/s to 0.3 m/s. As regards technique sensitivity, RFT provides nearly the same detection sensitivity for external and internal flaws.
Moreover, RFT testing method is known to be very sensitive to wall thicknesses variations and can be less sensitive to fill factor, centering, wobbling, and probe lift-off. RFT testing is ideal for detecting changes in the thickness of a wall and wall loss.
RFT testing method can’t determine if there is wall loss outside or inside the tube wall since this method is equally sensitive to both OD and ID defects. Furthermore, RFT testing integrates a through-wall transmission technique in which the magnetic field goes through the tube wall two different times. Firstly, at the exciter location and secondly, at the receiver location. The receivers will feel an effect regardless of the alignment of a defect.
Characteristics of RFT Testing
- It achieves same sensitivity for both external and internal defects
- RFT is ideal for ferromagnetic pipes and tubes like those in heat exchangers.
- This testing method is relatively insensitive to wobble or probe lift-off.
- RFT features a lower fill factor which allows the inspection of boiler tubes with bends and swages.
- It can examine thick tube wall.
- Probe centralization is not as crucial as it is for ECT inspections.
- The typical pulling speed ranges between 0.10 m/s and 0.3 m/s.
- In RFT testing, the coils can only measure the material’s total thickness, therefore, it cannot differentiate between OD or ID defects.
- Sensitive to variations in wall thickness
What is the Difference Between RFT testing and ECT testing?
While remote field testing and eddy current testing are two types of testing methods. These two testing methods works based on different principles. In this section, we will look at the difference between these two testing methods.
RFT testing inspects condensers tubing and ferromagnetic heat exchanger since conventional ECT have can’t inspect the full thickness of the tube wall. This is because of the strong skin effect in ferromagnetic materials.
Eddy Current Testing (ECT) offers the most accurate and fastest means to inspect non-ferrous heat exchanger tubing. On the other hand, you can only perform RFT testing on ferrous tubing with effective detection of wall loss. RFT is ideal for detecting wall thinning that results from wear, baffle cuts, erosion, and pitting.
While RFT testing and ECT testing can both inspect ferromagnetic tube. ECT is a more cost-effective method of inspecting tubes. Although RFT and ECT testing methods depend on electromagnetics, there are crucial differences between these two. The frequency of alternating current used is one critical difference.
More Differences
RFT testing makes use of low frequencies which is usually less than several hundred hertz. On the other hand, Eddy current testing (ECT) integrates frequencies of about several thousand hertz. Also, another distinguishing factor between these two testing methods is the types of flaws each of them detects. While Remote field testing RFT can detect defects that are on the internal diameter and external diameter of ferritic material, eddy current testing (ECT) is extremely ideal for inspecting nonferritic material tubing.
Furthermore, Remote Field Testing is an electromagnetic testing method that discovers discontinuities ferromagnetic (Carbon steel) tubes and pipes. RFT is a through transmission test method in which the receiver coil discovers electromagnetic field that traversed via the tube wall.
Eddy current testing measures changes that occur in probe impedance as the probe goes over the defects. ECT estimates the depth of the defect by comparing the ECT signal with the ones received from machined calibration defects.
Benefits of RFT testing
RFT testing has a lot of benefits which include:
- It is suitable for ferromagnetic materials
- RFT testing offers equal sensitivity at the outer and inner surfaces
- This testing method is very sensitive to variations in wall-thickness
- It can be used lower fill factors
- RFT can use adequate probes to measure pit size and wall loss
What is the Typical Defect Response in RFT Testing
There are two effects when the probe goes over a defect in the tube.
Receiver effect
- The first receiver generates a signal when it goes over the defect, and it a similar signal from the receiver coil follows thereafter.
- Also, the removal of receiver signals produces the “S” shaped differential signal.
Exciter effect
- The exciter coil usually goes beyond the defect, and as such it affects the field the receivers sense.
- The two receiver coils can detect the exciter effect and as well generate almost identical signals.
- If there is a large defect, the differential signal can be affected by the exciter. This is because the two receiver coils have a different response.
Conclusion
RFT testing is extremely ideal for ferrite pipes inspection. This testing method helps in detecting and quantifying wall thickness reductions. Also, it can detect both internal and external flaws, but it cannot distinguish them. RFT testing is not ideal testing method for finned pipes.
This testing method offers more benefits than eddy current testing. RFT testing makes use of low frequencies which is usually less than several hundred hertz. Having discussed remote filed testing and how it works, we hope this article has widened your knowledge on RFT testing.
