Introduction to nondestructive testing of pressure vessels such as boiler steel pipes

Pressure vessels such as boiler steel pipes and pressure vessel components often have defects that are difficult to detect, such as lack of fusion, lack of penetration, slag inclusions, pores, cracks, etc. in the welds. It is impossible to conduct destructive inspections on each boiler or pressure vessel to know the location, size, and nature of these defects. Therefore, nondestructive testing methods must be used. That is, without destroying the structure, physical methods are used to inspect and measure the changes in the physical quantities of the workpiece or structure to infer the internal organization and defects of the workpiece or structure.

Steel pipe nondestructive testing equipment

The purpose of nondestructive testing is:

(1) Improve the manufacturing process and ensure product quality.

(2) In the product manufacturing process, defects can be discovered in advance to avoid product scrapping, thereby saving time and expenses and reducing the cost of product manufacturing.

(3) Improve product reliability, ensure product safety, and avoid accidents.

Apply nondestructive testing to all aspects of product design, manufacturing, installation, use, and maintenance; through a series of tests, determine the quality of design, raw materials, manufacturing process, and operation, and find out the factors that may cause damage, and then improve them, to improve the reliability of the product.

Commonly used nondestructive testing methods are radiographic testing, ultrasonic testing, magnetic particle testing, penetrant testing, and eddy current testing. In addition, there are leak detection, acoustic emission testing, stress testing, visual inspection, etc.

Radiographic testing

Radiographic testing is a method of using the ability of rays to penetrate metal and other materials to check the quality of welds. The basic principle of radiographic testing is the projection principle. When the rays pass through the weld metal, when there are defects in the weld metal (such as cracks, slag inclusions, pores, incomplete penetration, etc.), the rays attenuate differently in the metal and the defects and the sensitivity on the film is also different. The rays attenuate quickly in the metal, and slowly in the defects. Therefore, the size, shape, and position of the defects in the weld can be determined by radiographic testing. Since X-ray flaw detection is based on the projection principle, this method is more sensitive to volume defects (such as slag inclusions).

Ultrasonic flaw detection

Ultrasonic flaw detection is a method of non-destructive testing that uses the reflection characteristics of sound waves when they propagate in the medium and encounter different medium interfaces. Since the elasticity of gas, liquid, and solid media is very different, the influence on the propagation of ultrasonic waves is different, so reflection, refraction, and waveform conversion will occur on the heterogeneous interface. When ultrasonic waves propagate in the weld, if there are defects in the weld, they will be reflected at the interface of the defect, which will be received by the probe and form a waveform on the screen, so that the nature, location, and size of the defect can be judged. Traditional ultrasonic flaw detection cannot record and save the flaw detection results, and the evaluation of defects depends too much on human factors. Therefore, at present, my country uses X-ray flaw detection in low-pressure boilers.

Magnetic particle flaw detection

Magnetic particle flaw detection uses the leakage magnetic field formed at the defect to attract magnetic powder to display defects that are difficult to observe with the naked eye. Magnetic particle inspection first applies an external magnetic field to the weld to be inspected for magnetization. After the weld is magnetized, fine magnetic powder (average particle size of magnetic powder is 5-10μm) is evenly sprayed on the surface of the weld. If there is no defect near the surface of the weld to be inspected, it can be regarded as a uniform body with unchanged magnetic permeability after magnetization, and the magnetic powder is also evenly distributed on the surface of the weld. When there are defects near the surface of the weld, the defects (cracks, pores, non-metallic slag inclusions) contain air or non-metal, and their magnetic permeability is much smaller than the magnetic permeability of the weld metal. Due to the change of magnetic resistance, a leakage magnetic field is generated at the defect on the surface or near the surface of the weld, forming a small magnetic pole. The magnetic powder will be attracted by the small magnetic pole, and the defect will be displayed due to the accumulation of more magnetic powder, forming a defect pattern that can be seen by the naked eye. The surface or near-surface defects of the weld generate leakage magnetic fields due to their low magnetic permeability. When the leakage magnetic field intensity reaches the level that can absorb magnetic powder, the surface or near-surface defects of the weld can be observed. The greater the strength of the applied magnetic field, the greater the strength of the leakage magnetic field, and the higher the sensitivity of magnetic particle inspection.

Coloring flaw detection method

The color of the dye is used to display defects. The dye dissolved in the penetrant should have a bright and visible color. The fluorescence display flaw detection method uses the luminescence of fluorescent substances to display defects. In flaw detection, the fluorescent substance adsorbed in the defect is irradiated by ultraviolet rays, reaches an excited state due to the absorption of light energy, and enters an unstable state. It is bound to return from this unstable state to a stable state, reduce potential energy, and emit photons, that is, emit fluorescence.

Eddy's current flaw detection

A workpiece flaw detection method that uses an excitation coil to generate eddy currents in a conductive workpiece, and measures the change in the eddy current of the object being inspected through a detection coil. The detection coil of eddy current flaw detection can be divided into three types according to its shape: through-type coil, probe-type coil, and insertion-type coil. Through-type coils are used to detect wires, rods, and pipes, and their inner diameter fits on round rods and pipes. Probe-type coils are placed on the surface of the workpiece for local detection. Insertion coils, also called internal probes, are placed inside pipes and holes for inner wall testing.