NFT is specifically suited to detecting corrosion, erosion, and pitting inside carbon steel tubing. NFT is perfect for fin-fan tube heat exchangers because eddy currents do not go through the wall of the tube. NFT is also much more sensitive to defects close to structures such as support plates and tubesheets.

1. The principles of Near field testing (NFT):

NFT technology uses two coils - a transmitter and a receiver. Typically the receiver coil is close to the transmitter coil, taking advantage of the transmitter’s near-field zone that is, the zone where the magnetic field from the transmitter coil induces strong eddy currents, axially and radially, in the tube wall.

The basic probe is made with one driver coil and three receiver coils.

The driver coil operates within the same frequency range as a remote field testing (RFT) probe.

The receiver coils are placed relatively close to the driver, in a region where the direct magnetic field and eddy currents are restricted to the tube ID.

Two coils form the differential signal and one coil the absolute signal.

The amplitude of the absolute and differential signal is proportional to the defect depth and size.

Hình 1: Nguyên lý phương pháp kiểm tra từ trường gần (NFT)

2. Advantages and disadvantages of NFT:

- Advantages:

• Fast when used as screening technique
• Possible to examine carbon steel (finned) tubes
• Both pits and overall wall-loss can be detected (diam. Pits > 3 mm)
• Detection of internal thinning and pitting based on sensitivity of eddy current lift off
• Cleaning of tubes less critical

- Disadvantages:

• Cannot detect external defects
• Sizing defects limited (based on signal volume)
• Less suitable as standalone technique