When there are pits on the surface of the flange, there are generally two commonly used inspections: one is the leakage magnetic flux method for detecting the flange, and the other is the ultrasonic testing method for detecting the flange. Let's first explain that the propagation process of ultrasonic waves, mechanical vibrations in a medium, is called waves. The human ear can sense elastic waves with frequencies above 20 Hz and below 20000 Hz, so elastic waves within this frequency range are also called sound waves. Elastic waves with frequencies less than 20 Hz are also called infrasonic waves, while elastic waves with frequencies above 20000 Hz are called ultrasonic waves. Both infrasound waves and ultrasound cannot be felt by the human ear. The ultrasonic testing method utilizes the pulse reflection principle of ultrasonic waves to detect the thickness of corroded pipe walls.
During the inspection, the probe is vertically emitted ultrasonic pulses towards the inner wall of the flange. The probe first receives reflected pulses from the inner surface of the pipe wall, and then the probe receives reflected pulses from the outer surface of the pipe wall. The distance between this pulse and the reflected pulses on the inner surface reflects the thickness of the pipe wall.
The advantages of ultrasonic testing for flanges are large detection thickness, high sensitivity, fast speed, low cost, harmless to human health, and the ability to locate and quantify defects. Ultrasonic testing is not intuitive in displaying defects, and the testing technology is difficult, easily affected by subjective and objective factors, and the testing results are not easy to save. Ultrasonic testing requires a smooth working surface, requiring experienced inspectors to distinguish the types of defects and be suitable for inspecting thick parts, making ultrasonic testing also have its limitations.
Secondly, the basic principle of magnetic flux leakage testing is based on the high magnetic permeability of ferromagnetic materials. The magnetic permeability at the corrosion defect of the flange is much smaller than that of the flange, and the flange is magnetized under the action of an external magnetic field. When there are no defects in the flange, the majority of the magnetic field lines pass through the steel pipe, and the magnetic field lines are evenly distributed at this time; When there are defects inside the flange, the magnetic field lines bend and a portion of the magnetic field lines leak out of the surface of the steel pipe. By detecting the leakage magnetic flux that escapes from the surface of the magnetized flange, the presence of defects can be determined.