The PE4332 Pulsed Eddy Current Flaw Detector detects metal corrosion under insulation with thickness up to three hundred millimeters. The device does not need any surface preparation including insulation removal.
The method of pulse eddy current testing enables assessment of metal volume ratio in a space underneath the sensor. This allows detecting the common area corrosion and pitting located both on the outer and inner metal surfaces.
To validate the method, let us measure test steel discs with thicknesses ten, five, and two millimeters respectively. The measurements will be carried out through insulation pieces with thicknesses from fifty millimeters to three hundred millimeters. We will check whether the device see difference in steel thicknesses of these three discs. After test pieces, we will proceed with real pipes where casings will be installed in addition to insulation.
Before taking measurements, let’s talk a little about the method itself.
The measurements begin by applying a sufficiently long current pulse to the sensor coil. This current generates a magnetic field that completely magnetizes the test object. When the current is interrupted, eddy currents arise in the test object, preventing demagnetization of the object. The thicker the metal under the sensor, the longer the demagnetization of such object. Thus, the method is based on dependence of the test object demagnetization rate on the object thickness.
The device displays a diagram of the test object demagnetization over time, and the metal wall thickness calculated according to the demagnetization rate
The sensor has a working aperture comparable to the size of the sensor itself. About ninety per cent of this area provides a response signal to the receiver of the device transducer. Due to this measurement method, we will not be able, on the one hand, to see small cracks and defects that are significantly smaller than the sensor surface. But on the other hand, the device perfectly measures large areas of corrosion through the insulation and casing.
Let us skip directly to the measurements, starting with a small sensor and a small insulation thickness of fifty millimeters.
Before taking the measurements, it is important to set the parameters of the test object and to carry out a quick calibration. The calibration must be carried out on each new test object; in addition, we recommend to carry out calibration when there is a clear change in the physical parameters of the test object, for example, a visible change in the thickness of the insulation. Let us carry out a calibration measurement where the metal thickness is known.
When changing the sensor, we also recommend to carry out a one-time calibration away from the metal objects, for example, by raising the sensor by at least fifty centimeters above the pipe.
The calibration is completed and we proceed directly to measuring the thickness of the disks under the insulation models.
As the device does not measure directly the wall thickness, the thickness measurement accuracy is ten per cent; however, in practice, we see good reproducibility of results both for fifty millimeters and for one hundred millimeters thick insulation and an easy-to-view change of the demagnetization diagram and measured wall thickness in such areas where the pipe thickness actually changes.
For insulation thicknesses up to two hundred millimeters, use a middle-size sensor. Do not forget to calibrate. The demagnetization curve depends on the insulation thickness, so it is important to set the correct calibration parameters for correct calculation of the test object wall thickness.
For insulation thicknesses up to three hundred millimeters, use a large sensor. During real work, select the sensor based on the insulation thickness of the test object and on the corrosion areas you would like to detect. As we see, the sensor successfully got through the three hundred millimeters insulation.
The PE 4332 Pulse Eddy Current Flaw Detector has demonstrated good results on test steel disks and we can proceed to measurements of a real pipe with insulation and casing.