The production efficiency of boilers is closely related to the quality of pressure vessels and pipelines. If the
Pressure Vessel and pressure pipe are broken or damaged, it will definitely affect the safe operation of the boiler. Therefore, it is necessary to conduct a comprehensive inspection of the pressure vessel and pipeline of the boiler, strengthen the operation and management of the boiler, and ensure that the cracks in the pressure vessel and pipeline can be resolved in time. To analyze the causes and types of cracks in pressure vessels and pipes, it is necessary to formulate specific treatment measures in combination with practical and theoretical knowledge to ensure the safe and stable operation of the boiler.
1 Common crack analysis of pressure vessel piping
1.1 Stress corrosion cracking
Corrosion cracking is a common crack type, which is related to the joint action of internal and external stress and corrosive medium. Commonly found in boiler steam pipes and container seats, mainly tied outside the pipes. According to the research, due to the influence of the hot water medium on the pipeline itself, after the boiler system has been in operation for a long time, the pipeline is susceptible to directional corrosion by the internal hot water, and the flowing water medium has a certain pressure. When the pressure value on the outer surface of the pipe exceeds the stress limit of this area, it will inevitably cause external deformation and cracking of the pressure pipe.
1.2 Mechanical fatigue cracks
Mechanical fatigue cracks are common in rotating mechanical parts such as impellers, blades, and shafts in boiler auxiliary systems. This kind of crack formation mainly includes two stages, which are specifically divided into the initial stage of long crack propagation and the late stage of tangential crack formation. Therefore, the formation of mechanical fatigue cracks is related to the operating time of the boiler. In the initial stage, it is mainly manifested by the accumulation of strain response, leading to long mechanical fatigue cracks with obvious external features.
1.3 Creep cracks
Due to the long-term effects of high temperature and stress bonding, the pressure pipeline is prone to creep cracking, which leads to the damage of the metal structure of the pipeline and the deformation of some areas of the pipeline, which reduces the safety of the pressure pipeline. Creep cracking usually occurs in the heat-affected zone of the header, high-temperature steam pipes, heating expansion pipes and other areas. The creep characteristics are very obvious, the crack direction is mostly perpendicular to the maximum tensile stress, the crack direction is usually more tortuous, the crack area is larger, and most of the cracks are arranged in parallel, and most of the main cracks are located in the middle of the cracks. Creep problems also have some subtle features, such as creep holes, some irregular holes will be distributed in the crack area, and the shape of crack holes is generally elliptical. In addition, the arrangement of the cracks in the damaged area of the weld is quite special, and most of them are distributed parallel to the direction of the weld. The crack will extend along the weld surface to the weld content, and the parallel microcracks on both sides of the main crack will distribute along the grain and expand to the boundary of the main crack to form a crack zone.
1.4 Welding crack
At present, most of the boilers, pressure vessels and pressure pipes used in my country are welded with metal plates, so the welding cracks of the pressure-bearing equipment of the boiler are prone to cracks after being subjected to pressure, and the influencing factors of cracks are mostly concentrated in the reduction of the structural strength of the pipeline under high temperature environment , and thermal cracking after stress. Usually, after the boiler is manufactured, it needs to be left to cool before it can be put into use. Therefore, when the boiler is manufactured, it will also bring cold cracks, which will affect the safety of the boiler operation. Since the hydrogen element will be in contact with the metallographic structure during welding, the austenite will slowly transform into iron element during welding cooling. At this time, the hydrogen element in the pipeline structure will gradually approach the middle, forming hydrogen cracks during welding cooling.
2 Causes of Welding Cracks in Pressure Vessel Pipelines
2.1 Causes of cracks
When welding pipes with pressure vessels, the weld metal will solidify. If the temperature is too high, it will directly lead to cracking. The main reason is that such cracks mainly cause cracks between crystals. The main reason for this kind of crack is that the pipe is heated evenly and cooled, and the molten pool crystals are formed under the influence of thermal stress. Therefore, in different crystallization time ranges, different crystals will produce different impurities, and the metal that crystallizes first has a higher purity. But as time goes by, the purity of the metal will decrease, and a large amount of metal impurities will be formed during the crystallization process. These impurities have the characteristics of agglomeration, and when they come into contact with it, an intergranular film will appear, and the zone will appear and thermal cracks will appear .
2.2 Materials and environment
The welding material itself contains many chemical components in the pipeline welding area. If the material does not meet the corresponding standards or the local application is too concentrated, such quality hidden dangers will directly affect the operation quality of the pipeline, so the material environment should be the primary work before construction. . From an environmental point of view, on-site construction is significantly affected by climatic conditions. Long-term exposure to low temperature environments will cause the risk of cracks in the welded parts of the pipes; long-term exposure to high humidity environments will significantly increase the moisture in the air. , so that there are quality problems such as air holes in the welding part.
3 Prevention and control measures for pressure vessel pressure pipe cracks
3.1 Improve the scientificity of production materials and production
One of them is to strengthen the control of production materials. High-pressure pipelines are in a state of high-intensity operation for a long time, so the requirements for their own materials are relatively high. Purchasing personnel must strengthen material selection and analysis, and choose reliable large-scale suppliers to match the pipe pressure. Secondly, the production of pressure pipes should be strengthened. On the one hand, it is necessary to strictly check the design drawings. The corresponding technicians should analyze the scientificity and validity of the pressure piping design drawings according to the specific design conditions. Drawings and use of pressure piping. On the other hand, it is necessary to improve the ability of production personnel. During the manufacturing process, it must be carried out in strict accordance with the manufacturing process or manufacturing standards. In the manufacturing process, it is also necessary to analyze the cracks that often appear in the discharge line to improve the quality of the discharge line and reduce the occurrence of cracks.
3.2 Enhance the production quality of pressure vessel and pressure piping
It is also very important to strengthen the effective quality control of the boiler. Therefore, in the process of testing the boiler piping system, reliable mutual inspection procedures and self-inspection procedures should be established, and a strict monitoring system should be established. Once a problem occurs, a set of effective solutions can be formulated quickly, and at the same time, the possibility of cracks in the inspection of boilers, pressure vessels and pressure piping can be minimized [4]. In addition, it is necessary to strengthen the quality control during the boiler installation process, conduct x-ray inspections on the main pressure components welded, and find out the problems caused by various situations in the welding process. The general problem should be fixed, and the second non-conforming artifact will be prohibited from use. This work should be supervised by local boiler inspectors, and the next step can be carried out smoothly only after the film image is qualified. Dock and assemble qualified gas cylinders and accessories, complete all tasks before final assembly, make assembly records according to production process requirements, and fill in relevant information before assembly and commissioning of boiler components.
3.3 Strengthen the control of temperature
Due to the continuous operation and high temperature of the pressure pipes of boilers and pressure vessels, as well as the influence of some other factors, cracks are prone to occur. Therefore, in the process of dealing with cracks in pressure pipes, we should strictly control temperature changes and improve the operating environment, such as installing corresponding cooling equipment in the production area, so as to avoid cracks due to temperature rise. At the same time, in the actual inspection process, if the temperature is high, the operation of the container should be stopped in time, and the temperature should be lowered in time. Only in this way can the occurrence of cracks be better controlled.
3.4 Control of strengthened crack area
First, when taking protective measures, cold cracks can be used as the basis for crack protection. To help increase the ductility of the pipe, alkaline covered electrodes can be used to suppress the amount of molecular hydrogen in the pipe to shrink it. In this process, the stress distribution must be carried out in strict accordance with the relevant standards and regulations, the post-weld heat treatment should be strengthened, and the cooling rate of the metal should be reduced. Before welding on pipes, preheating techniques need to be used intelligently. After the welding work is completed, it must be accepted. Turn on the slow cooling mode to ensure that the welding material is dry. During the welding process, the joints must be thoroughly cleaned of contaminants. After the welding is completed, appropriate measures must be taken to remove hydrogen molecules according to the welding situation. In the process of dealing with cracks, we must proceed from the following aspects. We must follow the welding sequence and use alkaline flux to reduce impurities in the weld. Rely on thermally sensitive materials to lower preheat temperatures and reduce solder stress.
4 Application of non-destructive testing techniques commonly used in pressure vessel pipeline crack inspection
4.1 Ultrasonic detection technology
Ultrasonic testing technology is a technology that uses ultrasonic testing, that is to say, it uses sound waves with a frequency higher than 20KHZ for testing. If there is a defect in the workpiece, an interface of different media must be formed between the defect and the metal material. The impedance is different. When the transmitted ultrasonic wave encounters this interface, it will be reflected, and the reflected energy will be received by the probe again. We evaluate the inspected workpiece according to the position of the reflected signal on the screen.
This detection technology is widely used in the crack detection of pressure vessels and pressure pipes. In order to improve its detection effect, the following points should be mastered when applying this technology: 1. The coupling quality of the receiving surface of ultrasonic detection is relatively large. The angle in the workpiece is the same as or close to the angle at which the probe releases ultrasonic waves, which may cause the echo signal to be weak or even undetectable. Therefore, when detecting high-temperature, high-pressure, and high-risk medium pipeline containers, try to choose multi-angle probes for detection during the detection process. ;3. The peaks of the crack reflection echo are sharp, and many crack signals will produce bifurcation signals at the end angles. Although the nature of the defect can be preliminarily judged from experience, it is impossible to identify whether the defect signal must be a crack signal from the principle. When determining the nature of the defect, other non-destructive testing methods can be used to determine the nature of the serious defect, such as X-ray/γ-ray inspection.
4.2 X-ray/γ-ray detection
Radiographic method is one of the main methods to detect pressure pipeline welds. The principle is to use the difference in the absorption rate of rays between the defect and the surrounding metal to change the transmission intensity of X-rays or γ-rays, so as to detect the defect and its position. At present, the central transillumination method is mainly used for radiographic flaw detection of the main road ring weld. The radiation source is mounted on a self-propelled tracked vehicle. Its main advantages are high sensitivity and high work efficiency. External irradiation equipment is used to weld joints and elbows, rework and test small diameter welds. At present, X-ray film flaw detection is mainly used for welding of high-pressure vessel pipes. Although this method is relatively intuitive, it requires a large number of X-ray films. Designs are mostly done in the field or outdoors, which makes filming and development more difficult. Moreover, there are radiation hazards to the human body. After the pipeline construction is completed, a large amount of space is required to store these films, resulting in the consumption of a large amount of manpower and material resources. With the mature application of DR (X-ray digital imaging) technology in the industrial field, DR technology has been used to replace film ray inspection in some pressure vessel pipeline construction abroad. In terms of domestic pipeline girth weld detection, DR technology has not been applied on a large scale in pipeline engineering, and only industrial tests and small-scale trials have been carried out in some projects. At present, DR detection technology is planned to be widely promoted in the China-Russia Eastern Route Project, but There are also problems such as high equipment investment cost, large equipment volume, lower on-site efficiency than traditional film ray inspection, non-uniform inspection data format, and inability to upload data on-site. The ray method is mostly intuitive for the detection of cracks, and can solve the problem that the nature of defects cannot be determined in ultrasonic testing. The following points should be paid attention to in the detection of cracks by X-ray method: 1. Under the premise of ensuring the penetrating power, the X-ray detection should follow the principle of low voltage and long time as far as possible to ensure the sensitivity; 2. High-quality film should be used in gamma-ray detection according to the standard 1. In the darkroom processing, the film washing solution should be replaced after a certain period of time or after washing a certain number of films. It cannot be used to save costs. Although the number of films to be processed is not large, the films should be washed after long-term storage, which will affect the quality of the film.
4.3 TOFD and phased array detection
TOFD (Ultrasonic Time-of-Flight Diffraction) and phased array testing are both ultrasonic testing, but the difference between TOFD and traditional ultrasonic testing is that TOFD uses ultrasonic diffracted waves for testing, and ultrasonic testing uses ultrasonic reflected waves for testing. The characteristics of diffracted wave detection are: high sensitivity, accurate depth and height for defect location, non-directional reception at any position, and the ability to determine the nature of defects to a certain extent.
The principle of phased array detection technology is the same as that of traditional manual ultrasonic detection, which uses ultrasonic reflection method to detect the workpiece to be inspected. The difference is that traditional ultrasonic detection uses a single probe to deflect the sound beam. In some cases, dual element or single element focusing probes are also used to reduce blind spots and improve resolution. But anyway, the ultrasonic field propagates along the axis at an angle in the medium. Single-angle scanning limits the possibility of qualitative and quantitative ultrasound detection in different directions. Therefore, the most "effective" standard requires the use of multi-angle beam scanning to increase inspection speed. However, it is difficult to achieve detection for complex geometries, large wall thicknesses, or limited probe scanning space, so multi-element phased array probes and electronically focused acoustic beams are required to meet the detection requirements in the above cases. The phased array probe can realize angular deflection and dynamic focusing, which makes the ultrasonic detection move from two-dimensional direction to one-dimensional direction movement, and an external encoder can be installed to realize image storage. The analysis of D-scan images can effectively detect various planar defects and volume defects. The test results can also be displayed in three-dimensional images, which provides rich information for defect location, quantification, qualitative and grading. The above-mentioned TOFD detection technology can also realize image storage, which provides conditions for subsequent review and verification, making the detection results effective and queryable.
Conclusion
There are more and more pressure vessels and pipelines involved in industrial production in my country, and the operating conditions are quite harsh. Long-term exposure to high temperature and high pressure will lead to safety hazards. Once cracks appear, it may cause explosion accidents in later use, so we need to pay attention to containers and pipelines Crack detection work, in-depth analysis of the causes of pressure vessel and pipeline cracks, and effective measures to control vessel and pipeline cracks. In order to improve the quality of equipment and standardize the operation of personnel, it is necessary to adopt advanced non-destructive testing technology when applying testing technology, and master its testing points, reasonably select testing methods, ensure the accuracy of testing results and the safety of industrial production, and further promote my country's industrialization development of.
December 26, 2022
