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Several Non-destructive Testing Techniques Commonly Used In Pressure Vessels
Jan 15, 2019

1, radiation detection, radiography is the most widely used radiation detection method in pressure vessels. Radiography refers to the penetration of a test piece by X-ray or γ-ray. In the test piece, the intensity difference occurs due to the influence of the defect on the absorption of the radiation. The defect is detected by measuring the difference, and the film is used as a device for recording information. Detection method. Radiographic equipment can be divided into: X-ray inspection machine, high-energy radiation detection equipment, and gamma-ray inspection machine.


2, ultrasonic testing, mainly used to detect the internal buried defects of the butt weld and the internal surface crack of the pressure vessel weld, also used for the detection of cracks in pressure vessel forgings and high-pressure bolts. Ultrasonic testing can be divided into ultrasonic testing, ultrasonic thickness measurement, ultrasonic measurement of grain size, stress measurement, etc., pulse reflection method based on the echo of the defect and the echo of the bottom surface; the defect is judged according to the shadow of the defect. The penetrating method also has a resonance method in which a standing wave is generated from a test object to judge a defect or to judge a plate thickness. The most commonly used is the pulse reflection method.


3. Magnetic particle detection, magnetic particle detection is a non-destructive detection method based on the interaction between the leakage magnetic field and the magnetic powder at the defect to show the surface and near surface defects of the ferromagnetic material. The principle is that the ferromagnetic material generates a strong magnetic induction intensity inside the magnetization, and the magnetic flux density increases by several hundred times to several thousand times. If there is discontinuity in the material (mainly including the discontinuity caused by the defect and the structure, shape and material) If the discontinuity is caused by other reasons, the magnetic field lines will be distorted, and some magnetic lines of force may overflow the surface of the material and pass through the space to form a leakage magnetic field. The local magnetic pole of the leakage magnetic field can attract ferromagnetic substances.


4. Penetration test, after the surface of the component is applied with a permeate containing a fluorescent dye or a dye, the permeate may penetrate into the surface opening defect after a certain period of time under capillary action; after removing excess permeate from the surface of the component Then, the developer is applied on the surface of the component. Similarly, under capillary action, the developer will attract the permeate retained in the defect, and the permeate will be back-infiltrated into the developer. Under a certain light source, the trace of the permeate at the defect It is displayed to detect the shape and distribution of defects.


5, diffraction time difference method ultrasonic testing (TOFD)

The TOFD detection technique is to generate a waveform at the tip of a discontinuous defect. When it is converted, a diffraction wave is generated. This diffraction wave covers a large angular range, and the diffraction wave detects the defect and records the flying time of the signal. It is possible to measure the height of the defect, and then the defect can be quantified. The defect size is usually defined as the time difference of the flight of the diffraction signal, and the amplitude of the signal is not related to the defect quantitative.


Choose the appropriate non-destructive testing method

When performing non-destructive testing on pressure vessel equipment, all kinds of inspection methods have certain characteristics, and all workpieces and all defects cannot be applied. In order to improve the reliability of the test results, the materials, manufacturing methods, working media, and failure modes of the equipment should be used. And select the type, shape, location and orientation of the defects that may be generated, and select the most suitable non-destructive testing method. In non-destructive testing, it must be recognized that any non-destructive testing method is not omnipotent, and each method has its own advantages and disadvantages. Therefore, in non-destructive testing, if possible, do not use only one non-destructive testing method. Instead, use as many detection methods as possible, and learn from each other to obtain more defect information, so as to have a clearer understanding of the actual situation. Ensure safe long-term operation of pressure vessel equipment.

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