What is radiographic inspection? Radiographic inspection of welds. Radiographic inspection: GOST

Radiation monitoring is based on the ability of the nuclei of certain substances (isotopes) to decay with the formation of ionizing radiation. In the process of nuclear decay, an emission of elementary particles occurs, which is called radiation or ionizing radiation. The properties of radiation depend on the type of elementary particles emitted by the nucleus.

Corpuscular ionizing radiation

Alpha radiation appears after the decay of heavy helium nuclei. The emitted particles consist of a pair of protons and a pair of neutrons. They have a large mass and low speed. This is due to their main distinguishing properties: small penetrating ability and powerful energy.

Neutron radiation consists of a neutron flux. These particles do not have their own electric charge. Only when neutrons interact with the nuclei of the irradiated substance do charged ions form, therefore, with neutron radiation in the irradiated object, secondary induced radioactivity is formed.

Beta radiation occurs during reactions inside the nucleus of an element. This is the conversion of a proton into a neutron or vice versa. In this case, electrons or their antiparticles - positrons - are emitted. These particles have a low mass and extremely high speed. Their ability to ionize matter is small compared to alpha particles.

Quantum Ionizing Radiation

Gamma radiation accompanies the above processes of emission of alpha and beta particles during the decay of an isotope atom. A photon flux is emitted, which is electromagnetic radiation. Like light, gamma radiation has a wave nature. Gamma particles move at the speed of light, respectively, have high penetrating power.

X-ray radiation is also based on electromagnetic waves, so it is very similar to gamma radiation.

Radiographic inspection
Also called bremsstrahlung. Its penetrating ability directly depends on the density of the irradiated material. Like a light beam, it leaves negative spots on the film. This feature of X-ray is widely used in various fields of industry and medicine.

The radiographic method of non-destructive testing mainly uses gamma and X-ray radiation, which have an electromagnetic wave nature, as well as neutron. For the production of radiation using special devices and installations.

X-ray machines

X-rays are obtained using x-ray tubes. This is a glass or metal-ceramic sealed cylinder from which air is pumped out to accelerate the movement of electrons. On both sides, electrodes with unlike charges are connected to it.

A cathode is a tungsten filament coil that directs a thin beam of electrons to the anode. The latter is usually made of copper, has an oblique slice with an angle of inclination from 40 to 70 degrees. In the center of it there is a tungsten plate, the so-called focus of the anode. An alternating current of 50 Hz is applied to the cathode to create a potential difference at the poles.

Radiographic inspection method
The electron flow in the form of a beam directly hits the tungsten plate of the anode, from which the particles sharply slow down the movement and electromagnetic oscillations occur. Therefore, X-rays are also called braking rays. In radiographic testing, it is mainly X-ray radiation that is used.

Gamma and neutron emitters

A source of gamma radiation is a radioactive element, most often an isotope of cobalt, iridium or cesium. In the device, it is placed in a special glass capsule.

Neutron emitters are performed in a similar manner, only they use the energy of the neutron flux.

Radiography

By the method of detecting the results, there are distinguished radioscopic, radiometric and radiographic control. The latter method is characterized in that the graphic results are recorded on a special film or plate. Radiographic control occurs by supplying radiation to the thickness of the controlled object.

Radiographic inspection of welded joints
An image appears on the detector located below the control object, on which possible defects (shells, pores, cracks) appear, stains and stripes, consisting of voids filled with air, since the ionization of substances of different density during irradiation occurs non-uniformly.

For detection, plates of special materials, film, x-ray paper are used.

Advantages of weld inspection by radiographic method and its disadvantages

When checking the quality of welding, mainly magnetic, radiographic and ultrasonic testing are used. In the oil and gas industry, places for welded pipe joints are especially carefully checked. It is in these sectors that the radiographic control method is the most popular due to the undoubted advantages over other control methods.

Radiographic inspection of pipelines
Firstly, it is considered the most obvious: on the detector you can see an exact photocopy of the internal state of matter with the locations of the defects and their outlines.

Its other advantage is its unique accuracy. When conducting ultrasonic or fluxgate control, there is always the possibility of false positives of the detector due to the contact of the finder with irregularities of the weld. With non-contact radiographic control, this is excluded, i.e. surface roughness or inaccessibility is not a problem.

Thirdly, the method allows controlling various materials, including non-magnetic ones.

And finally, the method is suitable for working in difficult weather and technical conditions. Here, radiographic monitoring of oil and gas pipelines remains the only possible one. Magnetic and ultrasound equipment often malfunctions due to low temperatures or design features.

However, it also has a number of minuses:

  • the radiographic method for controlling welded joints is based on the use of expensive equipment and consumables;
  • specially trained personnel are required;
  • working with radioactive radiation is hazardous to health.

Preparation for control

Training. As emitters, x-ray machines or gamma-ray flaw detectors are used.

Radiographic method for weld inspection
Before radiographic inspection of welds begins, the surface is cleaned, a visual inspection is performed to identify defects visible to the eye, the control object is labeled and marked. The performance of the equipment is checked.

Checking the sensitivity level. On the plots are laid out standards for checking sensitivity:

  • wire - on the seam itself, perpendicular to it;
  • grooves - departing from the seam for at least 0.5 cm, the direction of the grooves is perpendicular to the seam;
  • lamellar - deviating from the seam at least 0.5 cm or to the seam, marking marks on the standard should not be visible in the picture.

The control

The technology and schemes for radiographic inspection of welds are developed based on the thickness, shape, design features of the controlled products, in accordance with the technical documentation. The maximum permissible distance from the test object to the radiographic film is 150 mm.

The angle between the direction of the beam and the normal to the film should be less than 45 °.

The distance from the radiation source to the controlled surface is calculated according to the technical specifications for various types of welds and material thickness.

Evaluation of the results. The quality of radiographic control directly depends on the detector used. When using a radiographic film, each batch must be checked before use for compliance with the required parameters. Reagents for processing images are also tested for suitability in accordance with scientific and technical standards. Preparation of the film for control and processing of finished images should be carried out in a special dark place. Finished pictures should be clear, without extra spots, the emulsion layer should not be broken. Images of standards and markings should also be clearly visible.

radiographic and ultrasound monitoring

To evaluate the results of control, measure the size of detected defects, special templates, magnifiers, and rulers are used.

Based on the results of the control, a conclusion is made on the suitability, repair or rejection, which is made out in the established form magazines according to the normative documentation.

Application of filmless detectors

Today, digital technologies are increasingly being introduced into industrial production, including the radiographic non-destructive testing method. There are many original developments of domestic companies.

In a digital data processing system, radiographic inspection uses reusable flexible plates of phosphorus or acrylic. X-rays fall on the plate, after which it is scanned by a laser, and the image is converted to a monitor. When monitoring, the location of the plate is similar to film detectors.

This method has a number of undoubted advantages, in comparison with film radiography:

  • there is no need for a long process of processing film and equipment of a special room for this;
  • no need to constantly buy film and reagents for it;
  • the exposure process takes little time;
  • instant image acquisition in digital quality;
  • fast archiving and storage of data on electronic media;
  • the ability to use plates repeatedly;
  • the radiation energy during control can be reduced by half, and the penetration depth increases.

That is, there is a saving in money, time and a decrease in the level of exposure, and therefore, danger to the maintenance staff.

Radiographic safety precautions

In order to minimize the negative impact of radioactive rays on the health of an employee, it is necessary to strictly observe safety measures when performing all stages of radiographic inspection of welded joints. Basic safety rules:

Radiographic non-destructive testing

  • all equipment must be serviceable, have the necessary documentation, performers - the necessary level of training;
  • in the control zone is not allowed the presence of persons not associated with the production;
  • when the emitter is operating, the installation operator must be on the side opposite to the direction of radiation by at least 20 m ;
  • the radiation source must be equipped with a protective screen that prevents the scattering of rays in space;
  • it is forbidden to stay in the zone of possible exposure for longer than the maximum permissible rate of time;
  • the radiation level in the area where people are located should be constantly monitored with dosimeters;
  • the venue should be equipped with protection against the penetrating effects of radiation, such as lead sheets.

Normative and technical documentation, GOST

Radiographic inspection of welded joints is carried out in accordance with GOST 3242-79. The main documents for radiographic control are GOST 7512-82, RDI 38.18.020-95. The size of the marking signs must comply with GOST 15843-79. The type and power of radiation sources is selected depending on the thickness and density of the irradiated substance according to GOST 20426-82.

The sensitivity class and type of standard are regulated by GOST 23055-78 and GOST 7512-82. The process of processing radiographic images is carried out in accordance with GOST 8433-81.

When working with radiation sources, one should be guided by the provisions of the Federal Law of the Russian Federation “On the Radiation Safety of the Population”, SP 2.6.1.2612-10 “Basic Sanitary Rules for Ensuring Radiation Safety”, SanPiN 2.6.1.2523-09.

Source: https://habr.com/ru/post/C40016/


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