Eddy Current Testing on Finned Tubes

[Ilovescience]

All,

I been reading conflicting information on the questions below as well as finding different answers online:

1. Will there be issues with inspecting a finned tube (Adm brass tube with aluminum fins) with ET? Will there be interference issues?

2. Does ET or IRIS give false readings at tubesheets?

3. How come ET can be used to find cracks but IRIS can't?

 

[EdStainless]

IRIS is looking at the surface of the tube and giving you dimensional information.
There is no penetration of the material.
You can ET finned tubes.
You have to set up filters to remove the signal from the fins.
The effect of tubesheets on ET depends on the alloy of the tubes and tubesheets.
Many combinations have very limited sensitivity in the tubesheet (and maybe support plates as well).
Make sure that your ET standard matches your tubes.
The alloys, size, Mfg methods must all match your real tubes.
And make sure that the artificial discontinuities are representative of what you are actually looking for.
The default ones in ASME do not work for tubes (boilers maybe but not HX tubes).

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P.E. Metallurgy, consulting work welcomed

 

[Rubje]

i been wondering the same thing as the OP.

I'm assuming to detect cracks there has to be some penetration into the material then?

Also when you said "The default ones in ASME do not work for tubes (boilers maybe but not HX tubes)" what's the reason for this? why doesn't the default values work with tubes?

 

[EdStainless]

The ASME discontinuities are much too large, and not usually of the same geometry that you would find in the tubes.
For example in SS tubes where pitting is a concern, we would make standards with a series of 0.030" diameter flat bottom holes (20% thru 100%).
If you use larger holes and try to size the indications you will likely be off by +/-50% on depth.
ET finds cracks because they disturb the magnetic field.
If you are doing ET you need to follow up by removing a tube with indications.
That is the only way to verify the analysis.

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P.E. Metallurgy, consulting work welcomed

 

[Ilovescience]

Thank you EdStainless!