How to Detect Pits on Flange Surface
There are two common methods for detecting pits on flange surface, including magnetic flux leakage and ultrasonic wave.
The definition of ultrasonic wave is explained below. The transmission process of mechanical vibration in medium is called wave. In general, humans can hear sounds with a frequency between 20 HZ and 20 kHZ. Such frequency range is called elastic wave or sound wave. Elastic wave with a frequency lower than 20 HZ is also called infrasonic wave. Elastic wave with a frequency higher than 20 kHZ is also called ultrasonic wave. Infrasonic wave and ultrasonic wave cannot be sensed by humans’ ears.
Ultrasonic wave inspection method is used to measure thickness of pipe walls based on pulse reflection principle. During inspection, the probe is used to transmit ultrasonic pulse into internal walls of flanges in a vertical direction. The probe will first receive reflected pulse from internal wall surface and then receive those from external wall surface. Reflected pulse distance between internal wall surface and external wall surface is the thickness of pipe walls.
Ultrasonic wave for detecting pits on flange surface has advantages of precise inspection, quick detection, low cost and no harm to humans’ health, helping to accurately detect position of pits. However, the method is only suitable for parts with relatively large thickness. Flaw detection results of ultrasonic wave are not visualized. It is difficult to apply flaw detection technique. Besides, the inspection is easily affected by subjective and objective factors. The detection records are not easily kept. Experienced inspectors are required to distinguish types of defects. Hence, the method of ultrasonic wave for detecting flaws has its limitations.
The basic principle of magnetic flux leakage is based on high permeability property of ferromagnetic materials. Flanges with corrosion defects have far lower permeability than those with no corrosion defects. Flanges are magnetized under externally-applied magnetic field. As for flanges without defects, magnetic lines of force mostly pass through steel pipes and are well distributed. As for flanges with detects, magnetic lines of force are curved and part of them leak from steel pipe surface. The method of inspect magnetic flux leakage from flange surface can be used to judge whether flanges have defects.