Common Crack Types of Pressure Vessels and Pipelines and their NDT Techniques
Common crack types of pressure vessels and pipelines:
Stress corrosion cracking
Stress corrosion crack is a common type of crack, which is related to the interaction of internal and external stress and corrosion medium. Common in the boiler steam pipe and container seat, mainly tied in the pipeline outside. When the pressure on the outer surface of the pipeline exceeds the stress limit of the region, it will inevitably lead to the external deformation and cracking of the pressure pipeline.
Mechanical Fatigue Crack
Mechanical fatigue cracks are common in rotating machinery parts such as impeller, blade and shaft in boiler auxiliary system. This kind of crack mainly includes two stages, namely, the early stage of long crack propagation and the late stage of tangential crack formation. Therefore, the formation of mechanical fatigue crack is related to the running time of boiler. In the initial stage, the accumulation of strain response leads to long mechanical fatigue cracks with obvious external characteristics.
Creep Crack
Because of the long-term effect of high temperature and stress bonding, the pressure pipeline is prone to creep cracking, which leads to the destruction of the pipeline metal structure and the deformation of part of the pipeline, and reduces the safety of the pressure pipeline. The creep cracking usually occurs in the heat affected zone of header, high temperature steam pipe, heating expansion pipe and other areas. In addition, the crack arrangement in the damaged region of the weld is quite special, and most of them are parallel to the direction of the weld. The cracks propagate along the weld surface to the contents of the weld, and the parallel micro-cracks on both sides of the main crack distribute along the grain and propagate towards the boundary of the main crack to form the crack zone.
Welding Crack
Usually, after the boiler is manufactured, it can be put into use only after it is cooled, and when the boiler is manufactured, it will bring cold cracks, which will affect the safety of the boiler operation. Because the hydrogen element will contact with the matrix during welding, the austenite will slowly transform into iron during welding cooling, then the hydrogen element in the pipeline structure will gradually approach to the middle, forming hydrogen crack during welding cooling.
Precautions:
Improve the scientific nature of production materials and manufacturing
One is to tighten Means of production controls. The high-pressure pipeline is in the state of high strength for a long time, so the requirement of its own material is higher. Purchasing staff must strengthen material selection and analysis, select reliable large-scale suppliers to match the pipe pressure. The second is to strengthen the control of manufacturing. On the one hand, to strictly check the design drawings. According to the specific design conditions, the corresponding technical personnel should analyze the scientific and effective design drawings of pressure pipelines. On the other hand, to improve the capacity of production personnel. In the manufacturing process, must be in strict accordance with the manufacturing process or manufacturing standards. In the manufacturing process, it is also necessary to analyze the common cracks in the discharge line in order to improve the quality of the discharge line and reduce the occurrence of cracks.
Enhance quality control
It is also very important to strengthen the effective quality control of the boiler. Therefore, in the process of testing the boiler pipeline system, it is necessary to establish a reliable mutual check program and self-check program, and establish a strict monitoring system. When a problem occurs, an effective solution can be developed quickly, while minimizing the possibility of cracks occurring during inspection. In addition, it is necessary to strengthen the quality control during the installation of the boiler, the main pressure parts of the welding X-ray detection to find out the welding problems.
Strengthen the temperature control
Because the pressure vessel and the pipeline run continuously and the temperature is high, as well as some other factors influence, easy to produce the crack, therefore should control the temperature change strictly, and carries on the improvement to the operating environment, such as in the production area to install the corresponding cooling equipment, so as to avoid temperature cracks. At the same time, in the actual inspection process, if the temperature is high, it should be timely stop the vessel operation, timely cooling treatment, only in this way, can better control the emergence of crack.
Strengthen the control of crack area
When taking protective measures, the cold crack can be taken as the basis of crack protection. To help increase ductility of the pipe, we can use an alkaline covering electrode to suppress the amount of hydrogen molecules in the pipe. In this process, the stress distribution must be strictly in accordance with the relevant standards and regulations, strengthen the post-weld heat treatment, reduce the metal cooling rate. Turn on slow cooling mode to ensure the welding material is dry.
NDT techniques commonly used in pressure vessel pipeline crack inspection:
Ultrasonic testing technology
The ultrasonic testing technology uses the frequency above 20 khz sound wave to carry on the detection, if the workpiece has the flaw, then the flaw and the metal material must form a different medium interface, the acoustic impedance of the interface is different. This detection technique is widely used in the crack detection of pressure vessel and pressure pipeline. In order to improve the detection effect, the following key points should be grasped in the application of this technique:
(1)The ultrasonic detection is limited by the surface coupling, the surface of the detected workpiece needs to be smooth and clean, and the probe surface can not have foreign body;
(2)If the defect exists in the workpiece at the same angle or close to the angle of the ultrasonic wave released by the probe, the echo signal may be very weak or even undetectable, therefore, when detecting high-temperature, high-pressure and high-risk media pipeline containers, multi-angle probes should be selected as far as possible in the detection process.
(3) The reflection echoes of cracks have more sharp peaks, and the end angles of many crack signals can also produce bifurcation signals, although the nature of defects can be judged by experience, it is not possible to identify whether the defect signal must be a crack signal in principle, such as X-ray/γ-ray detection.
X-ray/γ-ray detection
X-ray method is one of the main methods to detect the welding seam of pressure pipeline. The principle is to detect the defect and its position by changing the transmission intensity of X-ray or γ-ray, which is different from the absorption rate of the surrounding metal. At present, the method of central radiography is mainly used to detect the circumferential weld of main pipeline. The radiation source is mounted on a self-propelled tracked vehicle. The utility model has the advantages of high sensitivity and high working efficiency. External irradiation equipment is used for welding joints and elbows, reworking and testing small diameter welds. At present, X-ray film flaw detection is mainly used in high pressure vessel pipeline welding.
X-ray method is more intuitive for crack detection, it can solve the problem that the nature of defects can not be determined in ultrasonic detection. The following points should be paid attention to in X-ray crack detection:
(1) the X-ray detection should follow the principle of low voltage and long time to ensure the sensitivity;
(2) high quality film should be used in the γ-ray detection according to the standard;
(3) in darkroom processing, the film-developing liquid should be changed after a certain time or a certain number of films have been developed. In order to save the cost, although the number of films developed is small, the quality of the films will be affected by the re-development after a long time storage.
TOFD and phased array detection
TOFD (ultrasonic diffraction time difference method) and phased array detection are also ultrasonic detection, but TOFD and traditional ultrasonic detection is different, TOFD is to use ultrasonic diffraction wave detection, ultrasonic testing is the use of ultrasonic reflection wave testing. The characteristics of diffraction wave detection are: high sensitivity, high accuracy of defect location and depth, no directivity can be received at any position, and the defect properties can be determined to a certain extent.
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