The MFL technique can be used to inspect heat exchanger, but it is very useful in air cooler where the aluminum fins around the tube do not allow the use of RFT. Also, when there is a need for circumferential crack detection in carbon steel material, MFL is about the only technique that can do that with a good sensitivity. As for ECT, this technique requires a good fill factor as well as a good probe centering position to maintain a fair sensitivity to small volume defects. A quick technique for which the pulling speed can go up to 1m/s.

1. The principles of Magnectic flux leaking testing (MFL):

The principle of operation is based on magnetization. Two permanent magnets coupled to a steel core are used to magnetize the tube wall until complete saturation is reach.

For wall loss detection, we use the coil operating in the absolute mode. That coil is wound around the steel core between the two magnets. It measures the strength of the magnetic field produced by the magnets. So where there is general thinning such as erosion, even if no flux leakage occurs, the strength of the magnetic field decreases, and this variation is detected.

To detect pitting, using the lead coil. That coil is wound over the absolute coil, between the two magnets. When the magnetic field comes across pitting, the flux lines are diverted, and theses distortions cannot be contained in the tube wall. Then flux leakage occurs on the inside or outside surface of the tube depending on the pitting location. The lead coil will detect this leakage, but it is not possible to deduce the location (ID or OD) of the defect. To find the pitting location, that need the response from a third coil, the trail one.

Picture 1: The principles of Magnectic flux leaking testing (MFL).

2. Advantages and Disadvantages of RFL:

- Advantages:

• Can inspect ferromagnetic tubes up to 3.5 inches in diameter and 0.120 inches wall thickness.
• Permanent records can be obtained on test results.
• Instrumentation can withstand adverse field conditions.
• Can detect flaws under support plates as well as flaws adjacent to end sheets

- Disadvantages:

• Distinguishes ID from OD flaws.
• Very sensitive to inspection speed. Accuracy of test results can fluctuate with probe speed.
• Instrumentation and probes could be very expensive.
• Tubes must be cleaned. Scale or deposit can fill a flaw which will make it difficult to qualify its depth.
• Requires high inspection skills for data analysis and evaluation.
• Inspection speed up to approximately 15 feet per minute.
• Cannot accurately size discontinuities.
• Cannot inspect U-bend tubes