EDDY CURRENT INSPECTION OF TUBES.Eddy current
Eddy Current is a very suitable technique for the inspection of tubes in e.g. heat exchangers, condensers, evaporators and airfin coolers made of non-ferrous materials.
Eddy Current inspection will be used to quickly reveal the condition of the tubes in a heat exchanger. Local defects as well as overall wall-loss can be detected and quantified. EC can detect both internal and external defects and can distinguish between them. Cracks can be detected depending on their size and orientation. By applying Multi frequencies, defects under support plates can be detected and to some extent quantified.
The probe used in Eddy Current examination contains a coil which generates a changing magnetic field. When the probe is inside a tube of a conductive material this magnetic field will cause eddy currents to flow in the tube material. The amount of eddy currents that can flow in the tube depends on the condition of the tube at the location of the coils. The Eddy currents will in their turn generate a magnetic field which opposes the original magnetic field of the coil. The resultant of the two opposing magnetic fields influences the impedance of the coil in the probe. This means the impedance of the test coil depends on the condition of the tube. The signals on the computer screen represents the changing in the impedance of the coil, and thus the condition of the tube. During signal analysis, the signals acquired during a heat exchanger inspection will be compared to the signals from reference defects. Reference defects are defects with known depth and shape and are machined into a calibration standard. The calibration standard needs to be of the same material and dimensions as the tubes to be examined.
After an inspection an “on-site” report detailing the condition of each tube will be presented to the client.
For Eddy Current inspection Dacon Inspection uses the Corestar Omni 100 tester. This tool is designed for metal loss assessment of tubes in the diameter range from 1/4” to 3”. The omni 100 is capable of performing multi-frequency and multi-channel measurements. The eddy current signals are presented on a laptop computer and all inspection data is automatically stored to hard disc. The equipment operates on 220 V, 50 Hz.
Overview of possibilities and limitations of Eddy Current
- EC testing is a fast method for inspecting tubes of non-ferrous material (stainless steel, copper etc.). EC can also be applied on finned non-ferrous tubes.
- Holes with a diameter of 0.5 mm are detectable or pits with the same volume as a 0.5 mm hole. This counts for tubes with an internal diameter smaller than appr. 30 mm. In bigger tubes sensitivity goes down a little bit. How much depends a lot on the situation but detecting a 1 mm hole should be possible. Most of the times sensitivity is better than this.
- Overall wall-loss is detectable from 10% of the nominal wall thickness and up.
- Defect depths are being reported in defect classes with a width of 10% of the nominal wallthickness (e.g. defect class 3: 30- 40 % wall-loss).
- EC can detect both internal and external defects and can distinguish between internal and external defects.
- It's possible to detect and quantify defects under support plates.
- Non volumetric defects like cracks can be detected depending on their orientation and size.
Remark: Named values concerning sensitivity and accuracy have been generalised. In particular sensitivity is very dependend on the signal to noise ratio during the examination.