"Eddy Currect as an inspection method for evaluating external corrosion in CS tubes exchangers"

Question:

"What do you think about this inspection technique to detect external corrosion in Carbon Steel tubes exchangers? Is a reliability method of inspection?. We actually are not using Eddy Current to inspect this type of exchanges, we remove a couple of tubes and we do an external inspection of this sample to evaluate external corrosion and residual life of tubes. I think Eddy Current could be a good inspection technique in future, now it´s a little expensive method and you can´t apply for all exchangers, reentubing directly in cheaper in many cases."

Answer:

While we do not consider ourselves to be an expert on this subject, we can offer a few comments. It is my impression that more people are using magnetic flux exclusion (MFE) to inspect carbon steel exchanger tubes, using eddy current to inspect non magnetic tubes. Eddy currents are disturbed by a variable magnetic flux, making inspection difficult on ferrous tubes. Thick tubes are especially difficult due to the inability to magnetically saturate the entire wall thickness. The ability to detect external indications to some extent depends on what kind of indications you are looking for, their severity and where they are located. Also, the tubes have to be clean for the inspection to be effective. Many users are using MFE as a non destructive inspection tool for carbon steel exchanger tubes. With larger diameter tubes (1") IRIS (internal ultrasonic inspection) is also being used to inspect carbon steel exchanger tubes. However, we have used MFL (magnet flux leakage) with a great deal of success for I.D. or O.D. corrosion of carbon steel tubes. MFL technique is good for detecting general wall losses not small isolated pitted areas. I have also used IRIS tube inspection on C.S. tubes looing for imppingement areas on tube O.D's with great success. This method is great for measuring the tube wall thickness which can detect fairly small sized pits. However with IRIS the tube I.D.'s have to be real clean where as MFL is more forgiving. The IRIS method is also very slow compared to MFL.

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