Pulsed eddy current (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.
How it Works
A magnetic field is created by an electrical current in the coil of a probe. When the probe is placed on the insulation, fireproofing, or coating, the field penetrates through all the layers (including sheeting, if present) and stabilizes in the component thickness, and then the electrical current in the transmission coil is turned off, causing a sudden drop in the magnetic field. As a result of electromagnetic induction, eddy currents appear in the component wall. The eddy currents diffuse inward and decrease in strength. The decrease in eddy currents is monitored by the PEC probe and used to determine the wall thickness. The thicker the wall, the longer it takes for the eddy currents to decay to zero.
PEC is therefore the analysis of transient eddy currents in a conductive component following a sharp electromagnetic transition.
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
To analyze transient eddy currents, pulsed eddy current comprises three phases:
The probes injects a magnetic field penetrating and stabilizing in the component wall.
The probe interrupts emission abruptly and strong eddy currents are induced in the component wall.
The sensors in the probe measure the decay rate of eddy currents as they diffuse inside the component wall.
How Pulsed Eddy Currents Evaluate Thickness
This new generation of PEC adapts the width of pulses to the thickness of components under test, guaranteeing the probe’s magnetic field completely penetrates the thickness. It also offers the capacity to fine tune when the probe “listens to” the eddy current decay rate, making it easy to isolate the decay rate from other undesirable signals, such as the one from metallic cladding (in the case of CUI) and eliminating the need for any contact with the surface under test. Unlike eddy current testing, with PEC, the part under test is not under constant excitation from the probe. The background “noise” generated by an ECT probe is therefore absent when using PEC, which allows for greater amplification and less noise in the received signal, making it perfect to detect the very weak contribution from corrosion under insulation (CUI), for example.
Early on, the eddy current decay rate in a conductive material under insulation and cladding follows a power law, where a relative change in voltage results in a proportional relative change in time and produces a rapid drop in a Log-Lin scale graph. Later on, as the eddy currents reach through to the other side of the component, their decay rate follows an inverse exponential distribution that produces a straight line in a Log-Lin scale graph.
Where V is the voltage, t represents time, and τ is the decay rate. Different wall thicknesses generate different decay rate curves:
Determining the decay rate is performed by the software used to analyze PEC signals and varies from product to product. Eddyfi’s Lyft® reinvents pulsed eddy current calculating signal decay rate, which makes for shorter acquisition times and mostly immunizing against probe liftoff variations.
Smallest Detectable Defects in Relation to PEC Probe Footprints and Averaging Area
The concepts of footprint and averaging area of a probe are key to understanding what a PEC can and cannot detect. The footprint is affected by the size of the probe and the distance from the component or structure being examined from the probe (liftoff). The footprint (FP) is of utmost importance as it is the decisive factor in determining the dimension of the inspection grid, edge effect, and the smallest detectable volume or defect.
The FP is defined as the full width at half maximum (FWHM) of the response detected by the probe, ensuring a 50% signal overlap between each point on the grid map.
The averaging area is the surface that a probe can “see” on a part under test. The wall thickness is the average wall thickness within the averaging area. As a result, flaws smaller than the averaging area are underestimated. The Lyft probe’s averaging area is 1.8 times the probe footprint (AvgA = 1.8 × FP). Further, the volume of the smallest detectable defect is 15% of the footprint’s volume. Defects of smaller diameters can be detected if the target depth is increased to maintain a minimum volume ratio of 15% in relation to the footprint.
Smallest Detectable Defect Dimensions
To find the minimum diameter:
- Minimum volume ratio: VolRatio(%) = 15%
- Specific defect depth: DefDp(%)
DefDiam = FP × √(VolRatio(%) / DefDp(%))
To find the minimum depth:
- Minimum volume ratio: VolRatio(%) = 15%
- Specific defect diameter: DefDiam
DefDp(%) = (FP / DefDiam)2 × VolRatio(%)
Advantages of Reinvented PEC Technology
Reduced Inspection Time
- Faster than any other PEC system
- Grid mapping mode 2 to 10 time faster (usually <1 s)
- Dynamic scanning mode (unique to Lyft)—probe speed up to 75 mm/s (3 in/s)
Reliable and Repeatable Results
- Less operator dependent (SmartPULSE™)
- Less affected by liftoff variations, weather jacket overlap, straps
- Better detection of small defects (dynamic scanning mode)
- Unaffected by structures above probes
Wider Application Spectrum
- Galvanized steel weather jackets
- Can be used on corrosion byproducts (scabs/blisters)
- Can be used through concrete, polymer coatings, chicken wire
- Inspection near nozzles, flange, pipe supports
Easy to Learn and Use
- C-scan imaging
- Intuitive user interface
- Embedded inspection workflow
- All-in-one solution
Current Limitations of Reinvented PEC Technology
- Edge effect approximately the size of one probe’s footprint near metallic structures
- Not designed to distinguish between near-side and far-side defects
- Not designed to detect small-volume pitting
- 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