RFT’s Limitations for Ferromagnetic Tubing Inspection 

Remote-Field Testing (RFT) has been a reliable method for inspecting ferromagnetic materials due to its ability to detect general corrosion, pitting, and wall loss in tubing. However, RFT is not without its limitations. In ferromagnetic materials, the electromagnetic field used for detection can become distorted or attenuated when interacting with interfering components, such as support plates or baffles. These distortions and attenuations create "blind zones" where defects can be difficult or impossible to detect clearly.  

The conventional RFT method also faces challenges in other areas. For instance, when multiple defects or indications are present at the same circumferential position in the tube, RFT cannot differentiate between them. This results in merged signals, making it impossible to assess the severity of each individual defect.  

The Emergence of RFA  

Eddyfi Technologies’ Remote Field Array (RFA) probe (Figure 1) was developed to address the limitations inherent in conventional RFT. Powered by a patent pending low-frequency multiplexer, the RFA probe combines a conventional dual-driver RFT with a high-density array of coils positioned circumferentially around the probe. These coils are grouped into two rows that are multiplexed in a specific sequence to allow the probe to gather a broader range of data.  

Figure 1: Animated overview of Eddyfi’s RFA probe family, highlighting probe design features and benefits. 

The sensor arrangement in the RFA probe offers a combination of bobbin and array data in a single pass to give inspectors access to both RFT and RFA signal responses for a more thorough analysis. By multiplexing the array coils, RFA generates absolute and differential array channels. This configuration in the RFA probe enables inspectors to achieve three key benefits:  

• Better insight into defect morphology: The RFA probe offers detailed visualization of the tube wall thickness through color-coded C-scan imaging in both 2D and 3D representations. Such comprehensive visualization enhances the ability to assess and understand defect morphology, leading to more informed decision-making. 

• Detection of small defects: The sensitivity of the array channels enables the RFA probe to detect smaller defects that might be missed with the conventional RFT probe.  

• Detecting defects near or beneath support structures: One of the challenges in inspecting ferromagnetic tubing is detecting defects that are located close to or beneath support structures. The array sensors implemented in the RFA probe's configuration allow it to overcome this obstacle. This is particularly important for maintaining the integrity of the tubing, as defects near these critical areas can lead to more severe damage if left undetected (Figure 2). 

Figure 2: Typical representation of RFA C-scan imaging (both 2D and 3D views) for inspecting a tube with a through-wall hole (TWH) near a support plate. 

Improved Detectability and Reduced Blind Zones with RFA

The array configuration not only reduces blind zones but also enhances defect detection, even near components like support plates, where conventional RFT struggles. In RFT, a single or dual receiver coil typically captures the signal, which limits its ability to pinpoint small or closely spaced defects. RFA, on the other hand, employs multiple sensors positioned in an array, each of which receives independent signals. This arrangement increases the ability to distinguish between signal variations caused by defects and those caused by support structures.  

A laboratory study (Figure 3) performed using a 13-millimeter (mm) RFA probe on a 19.05mm [0.75 inch (in)] x 2.11mm (0.083in) carbon steel tube with a square support plate mockup of 635mm (0.25in) thick and 101mm (4in) side length demonstrates that the signal length of the support plate visualized in the RFA C-scan is only one-third of that observed in the RFT strip chart. It's important to note that these measurements were obtained using an encoder, which ensures precise positioning and accurate reading. This reduction in signal length directly correlates to a higher detectability of defects using RFA compared to RFT, and marks improvement is mainly due to the use of array sensors in RFA. 

Figure 3: Comparison of support plate signal length in RFA C-scan vs. RFT strip chart. 

Case Study: Comparing RFT and RFA Results on Real Defective Tubes 

To illustrate the effectiveness of RFA in detecting defects beneath support plates, a case study was conducted on a vertical/straight Heat Exchanger (HX) bundle consisting of 2,789 carbon steel tubes (Figure 4). Each tube had an outer diameter (OD) of 25.4mm (1in) and a wall thickness (WT) of 2.10mm (0.083in). The tubes were measured 3.6 meters (12 feet) in length, with two tubesheets located at both extremities and single-segmental support plates distributed along the tube length. 

Figure 4: A captured-field photo of the tubesheet from top view. There were no tubes obstructed or mechanically plugged prior to inspection. 

RFT Test Results 

The bundle was initially inspected using an 18mm (0.709in) RFT probe. The inspection revealed a group of tubes potentially experiencing wall loss beneath a support plate. In these tubes, the defect was barely visible on RFT impedance plane because the shape of the signal for the support plate with wall loss closely resembled that of the nominal support plate signal. Figure 5 presents a comparison between the strip charts and impedance planes for the support plate signal with and without the defect. As shown, the signal for the support plate with wall loss underneath does not significantly differ from the nominal support plate signal. The similarity in signal shapes makes it challenging to distinguish the defect from the support plate signal and to accurately size it. 

Figure 5: RFT data comparison between a nominal support plate (a) and a support plate signal with defect underneath (b) showing a close similarity in signal shape on the impedance plane. 

RFA Test Results 

To confirm the existence of the defect identified during the RFT inspection, an 18mm (0.709in) RFA probe was used to rescan the tubes where RFT results were ambiguous. Figure 6 shows a comparison of RFT and RFA results for a tube with wall loss under the support plate. As shown, RFA can clearly confirm the presence of wall loss beneath the indicated support plate, which is visible on the C-scan and impedance plane corresponding to the RFA section.  

Figure 6: Comparison of RFT and RFA results on a tube with wall loss beneath a support plate highlighting RFA’s improved detectability and clearer visualization on both C-scan and impedance plane. 

The results shown in Figure 6 confirm the findings from the RFA inspection provide much clearer visualization of the defect for detection and sizing. This is attributed to the presence of multiple sensors around the probe's circumference which allow for the collection of more detailed data around blind zones.  

To further highlight the defect detection capabilities of RFA, Figure 7 presents the application of RFA on two additional tubes where RFT results were inconclusive. As shown in these instances, the wall loss under the support plate signal is confirmed by a very clear and sharp indication on the differential array channel. These indications correspond to severe wall loss beneath the support plate. This highlights how RFA can reveal defects that would otherwise go unnoticed or misinterpreted with RFT. 

Figure 7: Clear indication of wall loss under support plates using RFA on two different tubes where RFT results are uncertain. 

The results obtained from RFA testing were also instrumental in verifying findings from RFT inspections. As depicted in Figure 8, the RFT portion of the results indicates a potential defect from steam erosion near the tubesheet on the Voltage plane representation of the ABS channel. However, when examining the RFA results, the RFA C-scan displayed on the ABSA-LargeSignal channel provides additional clarity and confirms that the observed defect is indeed attributed to steam erosion.  

It's worth noting that the generic RFA setup implemented in Magnifi offers three distinct channels with unique C-scan configurations: ABSA, ABSA-LargeSignal, and DIFA. This differentiation improves RFA's ability to provide inspectors with precise data for analysis. Unlike the ABSA channel, the ABSA-LargeSignal channel has the median filter disabled, which makes it especially effective for detecting extended defects such as general corrosion or erosion. By not applying the median filter, this channel preserves the continuity and true representation of elongated indications to offer a clearer depiction of such defects. 

Figure 8: RFA C-scan imaging providing confirmation of RFT testing results. 

The results presented in this blog post illustrate how RFA outperforms RFT in detecting wall loss close to support plates. RFA offers clear and conclusive results when RFT data remains uncertain, especially in challenging areas such as near or beneath baffle plates. By employing an array of sensors positioned circumferentially around the probe, RFA delivers high-resolution C-scan images and improved data collection. As a result, the RFA probe offers a more comprehensive view of the tubing, even in areas where the presence of support plates would hinder defect visualization. This advanced capability enables inspectors to detect and size defects with much greater precision. Consequently, the use of RFA not only improves the reliability of inspection results but also supports more informed maintenance decisions for industries relying on ferromagnetic tubing.  

For more information on Eddyfi Technologies’ tubing probes, visit our website where you can access our Tubing Probe Creator tool to help you find the right tool. Looking for a more personalized experience? Contact us to be put in touch with an NDT expert ready to keep you Beyond Current!