Near Field Testing (NFT) is an electromagnetic non-destructive testing (NDT) technique used to inspect conductive materials, particularly thin-walled tubing, for internal defects. It is often compared to Remote Field Testing (RFT), but operates on a different principle.
How does NFT work?
NFT employs a probe with two coils: a transmitter coil and a closely positioned receiver coil. The transmitter coil generates a magnetic field, and the receiver coil measures changes in that field within the “near-field” zone of the transmitter.
- Transmitter Coil: An alternating current is passed through the transmitter coil, creating a magnetic field.
- Near-Field Zone: The receiver coil is positioned close to the transmitter, within the region where the magnetic field is strong and primarily influenced by the transmitter.
- Eddy Currents: The magnetic field from the transmitter induces eddy currents in the inner wall of the tube.
- Receiver Coil Response: The receiver coil detects changes in the magnetic field caused by variations in the eddy currents. These variations indicate changes in the material, such as corrosion or erosion.
- Limited Penetration: A key characteristic of NFT is that the eddy currents do not penetrate deeply through the tube wall. This makes it ideal for detecting internal surface defects.
Key Differences from RFT
- Coil Spacing: NFT uses closely spaced transmitter and receiver coils, while RFT uses a larger separation.
- Field Interaction: NFT measures changes in the near-field, where the magnetic field is strong, and eddy currents are concentrated near the inner surface. RFT relies on measuring a weaker, through-transmitted field.
- Penetration: NFT has limited wall penetration, making it best for inner surface defects. RFT has greater penetration and is sensitive to defects on both the inner and outer surfaces.
- Applications: NFT is well-suited for finned tubes, where external fins interfere with other techniques.
Applications of NFT
NFT is particularly well-suited for inspecting:
- Fin-fan coolers: The external fins on these tubes make other inspection methods challenging.
- Carbon steel tubing: NFT is effective for detecting internal corrosion, erosion, and pitting in these tubes.
- Heat exchangers
Advantages of NFT
- Unaffected by External Fins: NFT is not significantly affected by the presence of external fins, as the eddy currents do not penetrate deeply.
- Sensitive to Internal Defects: NFT is highly sensitive to defects on the inner surface of the tube.
- Relatively Fast: NFT can be a fast inspection method.
- Simpler Signal Analysis: Signal interpretation in NFT can be simpler compared to some other electromagnetic techniques.
- Good Sensitivity Near Support Structures: More sensitive to defects near support plates and tubesheets.
Limitations of NFT
- Limited Penetration: NFT is not suitable for detecting defects on the outer surface of the tube or for measuring overall wall thickness.
- Material Restrictions Best suited for ferromagnetic materials.
Detection of Subsurface Flaws: Not designed to detect flaws that are located deeper within the tube wall.