Pulsed Eddy Current
What is Pulsed Eddy Current(PEC)?
PEC is an advanced electromagnetic inspection technology used in detecting flaws and corrosion in ferrous materials typically hidden under layers of coating, fireproofing, or insulation.
The phenomenon of pulsed eddy current (PEC) works as follows. A special coil is placed on top of the thermal insulation (PEC is often used on insulated components). This coil consists of a transmitter coil and a receiver coil. The current through the transmitter coil is pulsed. First the transmitter coil is activated, current is flowing, and a magnetic field is established in the steel wall. Then the current is switched off and the magnetic field vanishes. As a result, eddy currents are induced in the top side of the steel wall. These eddy currents then diffuse into depth and decay with a certain rate. Once they arrive at the back wall the eddy currents decay more rapidly.
With the receiver coil, this time of arrival at the back wall is sensed. At places exhibiting wall loss, the arrival time will be earlier than at places with no wall loss. The average wall thickness is calculated from this timing. A change in wall thickness indicates the presence of corrosion or erosion. PEC is a static method. A measurement takes a few seconds during which the coil does not move. Per day, hundreds of measurements can be acquired. Typical fields of application are detection of corrosion under insulation (CUI) and erosion but it can also be applied for measuring wall thickness through most coatings / liftoff.
Where can Pulsed eddy current can be used?
- On outer surfaces with or without insulation or aluminum, stainless and galvanized steel weather jacket, blistering scabs, or fireproofing
- Near pipe elbows, supports, valves, and other metallic structures such as nozzles, and flanges
- Through concrete, polymer coatings, metallic mesh, and rebars
Recent improvements in the PEC technology have seen reduced inspection times, less operator dependency and a more intuitive user interface, less affect by liftoff variations, weather jacket overlap, straps, an improved detection of small defects, a wider application spectrum to include galvanized steel weather jackets, scanning over corrosion byproducts such as scabs and blisters and the ability to scan over concrete, polymer coatings, chicken wire, around elbow bends, near nozzles, flange & pipe supports.
Some limitations of PEC Technology
- Unable to distinguish between near-side and far-side defects
- Used as a screening solutions to be followed by an alternative NDT method such as ultrasound for a comprehensive inspection
- Undersizes flaws smaller than the averaging area of the probe
- Difficult to use on elbows fitted on pipes smaller than 200 mm (8 in) in diameter
- Unable to detect small-volume pitting