What is radiographic inspection? Radiographic inspection of welds. Radiographic examination: GOST
At the heart of radiation monitoring is the abilitynuclei of certain substances (isotopes) to decay with the formation of ionizing radiation. In the process of nuclear decay, elementary particles are emitted, 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 heavyhelium 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 distinctive properties: a small penetrating ability and powerful energy.
Neutron radiation consists of a neutron flux. These particles do not have their own electric charge. Charged ions are formed only in the interaction of neutrons with the nuclei of the irradiated substance, therefore, in the case of neutron radiation, a secondary induced radioactivity is formed in the irradiated object.
Beta-radiation occurs during reactions inside the nucleuselement. This is the transformation of a proton into a neutron or vice versa. In this case, electrons or their antiparticles, positrons, are emitted. These particles have a small mass and extremely high speed. Their ability to ionize matter is small, in comparison with alpha particles.
Ionizing radiation of a quantum nature
Gamma radiation accompanies the abovethe emission of alpha and beta particles in the decay of the isotope atom. There is an ejection of the photon flux, which is electromagnetic radiation. Like light, gamma radiation is of a wave nature. Gamma particles move with the speed of light, respectively, have a high penetrating ability.
X-ray radiation is also based on electromagnetic waves, so it is very similar to gamma radiation.
With the radiographic method of nondestructivecontrol mainly use gamma and X-rays, which are of electromagnetic wave nature, as well as neutron. For the production of radiation using special instruments and installations.
X-ray radiation is obtained with the help ofX-ray tubes. This is a glass or cermet welded cylinder, from which air is pumped to accelerate the movement of electrons. On both sides electrodes with opposite charges are connected to it.
The cathode is a spiral of tungsten filament, whichdirects a thin beam of electrons to the anode. The latter is usually made of copper, has an oblique cut with an angle of inclination from 40 to 70 degrees. In the center of it there is a plate of tungsten, 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.
Gamma and neutron radiators
The 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 radiators are performed in a similar way, only they use the neutron flux energy.
By the method of detecting the results,radioscopic, radiometric and radiographic monitoring. The latter method is different in that the graphic results are recorded on a special film or plate. Radiographic monitoring takes place by applying radiation to the thickness of the monitored object.
For detection, plates of special materials, film, X-ray paper are used.
The advantages of controlling weld seams by radiographic method and its disadvantages
When checking the welding quality in the mainuse magnetic, radiographic and ultrasonic testing. In the oil and gas industry, the weld joints of the pipes are particularly carefully checked. It is in these industries that the radiographic method of control is the most sought after due to the undoubted advantages over the other control methods.
Another of its dignity is its unique accuracy. When carrying out ultrasonic or ferroprobe control, there is always a possibility of false detector triggering due to the contact of the searcher with the unevenness of the welded seam. With contactless radiographic inspection, this is excluded, ie, unevenness or inaccessibility of the surface is not a problem.
Thirdly, the method makes it possible to control various materials, including nonmagnetic materials.
Finally, the method is suitable for working in complexweather and technical conditions. Here radiographic control of oil and gas pipelines remains the only possible one. Magnetic and ultrasonic equipment often malfunctions due to low temperatures or design features.
However, he also has a number of drawbacks:
- the radiographic method for monitoring welded joints is based on the use of expensive equipment and consumables;
- specially trained personnel are required;
- work with radioactive radiation is hazardous to health.
Preparation for control
Preparation. The radiators are X-ray machines or gamma-ray flaw detectors.
Checking the sensitivity level. In the areas the standards for testing the sensitivity are laid out:
- wire - on the seam itself, perpendicular to it;
- grooving - having deviated from the seam not less than 0.5 cm, the direction of the grooves is perpendicular to the seam;
- lamellar - having deviated from the seam not less than 0.5 cm or on the seam, marking marks on the standard should not be seen in the picture.
Technology and schemes of radiographic controlWelds are developed, based on the thickness, shape, design features of the products to be controlled, in accordance with NTD. The maximum permissible distance from the monitoring 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 monitored surface is calculated according to the NTD for various types of welds and the thickness of the material.
Evaluation of results. The quality of radiographic control directlydepends on the detector used. When using a radiographic film, each lot must be checked for compliance with the required parameters before use. Reagents for image processing are also tested for suitability in accordance with the normative document. Preparation of the film for inspection and processing of the finished images should be carried out in a special darkened place. The finished images should be clear, without superfluous stains, the emulsion layer should not be disturbed. Images of standards and markings should also be viewed well.
To assess the results of monitoring, to measure the size of detected defects apply special templates, magnifiers, rulers.
Based on the results of the inspection, a conclusion is made on the suitability, repair or rejection, which is documented in the journals of the established form on the normative document.
Application of filmless detectors
Today digital technologies are becoming more activeare introduced into industrial production, including in the radiographic method of nondestructive testing. There are many original developments of domestic companies.
With a digital data processing system in the processradiographic 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 controlling 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 the film and equipment of a special room for this;
- You do not need to buy film and reagents for it;
- The exposure process takes a little time;
- instant image acquisition in digital quality;
- fast archiving and storage of data on electronic media;
- possibility to use the plates many times;
- the energy of irradiation under control can be reduced by half, and the depth of penetration increases.
That is, there is a saving of money, time and a reduction in the level of radiation, and therefore, a danger to the staff.
Safety during the radiographic inspection
In order to minimize the negativethe impact of radioactive rays on the health of the worker, it is required to strictly observe the safety measures when performing all the stages of radiographic inspection of welded joints. Basic safety rules:
- all equipment must be serviceable, have the necessary documentation, performers - the required level of training;
- In the control zone, persons not related to production are not allowed to stay;
- when the radiator is operating, the installation operator must be on the side opposite to the radiation direction by at least 20 m;
- the radiation source must be equipped with a protective screen that prevents dispersion of rays in space;
- It is forbidden to stay in the zone of possible irradiation for more than the maximum permissible norm of time;
- The level of radiation in the area of people's presence must be constantly monitored with the help of dosimeters;
- The venue should be equipped with means of protection against the penetrating effects of radiation, such as lead sheets.
Normative and technical documentation, GOSTs
Radiographic inspection of welded jointsis carried out in accordance with GOST 3242-79. The basic documents for conducting radiographic inspection are GOST 7512-82, RDI 38.18.020-95. The size of the marking marks must comply with GOST 15843-79. The type and power of radiation sources is chosen depending on the thickness and density of the irradiated substance in accordance with GOST 20426-82.
The sensitivity class and type of standard are regulated by GOST 23055-78 and GOST 7512-82. The processing of radiographic images is carried out in accordance with GOST 8433-81.
When working with radiation sourcesshould be guided by the provisions of the Federal Law "On Radiation Safety of the Population", SP 126.96.36.19912-10 "Basic Sanitary Rules for Ensuring Radiation Safety", SanPiN 188.8.131.523-09.