Partial saturation Eddy Current (PSEC)

When small defects (diameter pit < 5 mm) are expected to be found in ferrous tubes the sensitivity of RFT might not be sufficient. In this case PSEC should be applied. PSEC is also very suitable to inspect finned ferrous tubes. PSEC is a technique whereby a changeable magnetic field is used to partly cancel out the magnetic properties of the ferrous material. The technique is very suitable for detection and quantification of local defects like pitting. PSEC can detect, and distinguish between, internal and external defects. It is also possible to detect overall wall-loss. To ensure reliability and sensitivity during an examination the maximum possible probe size shall be used. Therefore the tubes have to be thoroughly cleaned in order to allow the probe to pass through the tube. Approximately the same amount of tubes can be examined in one inspection day as with RFT...

PARTIAL SATURATION EDDY CURRENT INSPECTION OF TUBES

Partial saturation Eddy Current (PSEC)
PSEC is a technique used for the inspection of tubes in e.g. heat exchangers and airfin coolers made of ferrous materials. This technique will mainly be applied on airfin cooler tubes. Combined with RVI or IRIS verification this is the ideal technique to assess the condition of airfin coolers. If small diameter pitting is expected the technique can also be applied on other ferrous tubes. This technique is most suitable for the detection and quantification of local defects like pitting in ferrous materials. PSEC can detect and distinguish between internal and external defects and it is possible to detect overall wall-loss.

Theory
The probe used in PSEC examination contains normal eddy current coils. In addition to that the probe contains two coils that are used as an electro magnet. Conventional eddy current can not be applied on ferrous tubes. The permeability causes a very big magnetic field in the tube material to be generated. This magnetic field opposes the magnetic field of the probe which makes it impossible for the probes’ magnetic field and eddy currents to penetrate the material. The electromagnet in the PSEC probe is used to partly cancel out the magnetic properties of the tube material. The electromagnets’ field is mainly active at the inner surface of the tube where it compensates for the skin effect. This allows some eddy currents to penetrate the material. Defects in a tube will cause changes in the permeability and in the amount of eddy currents at that location. The impedance of the test coil will be affected by a combination of changes in permeability and eddy current strength. The signal 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 an 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 a report detailing the condition of each tube will be presented to the client.
Equipment
For Partial Saturation 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 PSEC 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 Partial Saturation Eddy Current

• PSEC is a method to inspect finned ferrous tubes or other ferrous tubes when small diameter pitting is expected.
• Holes with a diameter of 2 mm or pits with the same volume as a 2 mm hole are normally detectable. 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.
• 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 wall thickness (e.g. defect class 3: 30- 40 % wall-loss).
• PSEC can detect both internal and external defects and can distinguish between internal and external defects.
• Non volumetric defects like cracks can be detected, depending on their size, shape and orientation.
• Defects under support plates are detectable but the accuracy of sizing is limited.
• For the sizing of internal local defects, phase information is available. Sizing of external local defects is based on defect volume, shape and orientation. For accurate sizing it is desirable to verify a few defect indications using IRIS. The additional information can be used to adjust the PSEC findings.
• To ensure reliability and sensitivity during an examination the maximum possible probe size shall be used. Therefore the tubes have to be very clean in order to allow the probe to pass through the tube.

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


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