Crack problems in inspection of pressure pipes of boiler pressure vessels

Crack problems in inspection of pressure pipes of boiler pressure vessels

In order to ensure that the boiler pressure vessel can be maintained in a normal operating condition, the inspection of the boiler pressure pipe should be done well and handled as an important work content. And in the actual work of the boiler pressure vessel, due to the internal environment and the impact of the external environment, there will be cracking problems, and this cracking problem is easy to be ignored by the staff, however, this cracking phenomenon will have a serious impact on the normal operation of the boiler pressure vessel, which hides a great hidden danger of operation. Therefore, the pressure pipe cracking problem should be standardized, so as to ensure that the service life of the boiler pressure vessel and the service life will not be affected.

Basic overview of boiler pressure vessel and pressure piping

Pressure vessel

In the current social environment, the production of pressure vessels and manufacturing, has been supported by the policy from the national level, and in the background of this policy strongly encouraged, the pressure vessel industry has also been more perfect development optimization, especially in the development process in recent years, China’s metal pressure vessel production technology and R & D technology, has reached the world-class level, which also makes the metal pressure vessel in China has a very good development prospects. Pressure vessel itself can withstand a certain degree of pressure, and has a more excellent confinement, which also makes it in addition to the industrial field, also applicable to the military and civilian fields and other fields, and even in the process of scientific research, pressure vessels can still play an important role. However, despite the wide range of applications of pressure vessels, but in most cases, it is still used in the chemical industry and petrochemical industry and other industries, usually, the pressure vessel will present a large tank, the main use is to carry liquid or gas, and in the production stage of manufacturing to become the main carrier of energy supply, in the specific manufacturing process, the pressure vessel application function of the material Mainly for the reactor pressure vessel base material, bolts and nuts and welding materials.

Pressure piping

Pressure piping and pressure vessels are basically the same in nature, it is also an important part of the pressure pot, in the boiler’s daily work process, the key role played by pressure piping is to transport liquids and gases, and pressure piping is part of the transport pipeline, can play a good role in the undirected flow of fluid transport, distribution and discharge. In most of the current pressure boilers, most of the internal pressure piping is composed of flanges, bolted connections, valves, fittings and pipe bodies, etc. In addition to these basic parts, the pressure piping is also equipped with supporting parts. In recent years, the development process, the pressure pipe has a more mature assembly method and production process, can play a good role in ensuring the efficiency and safety of the pressure boiler, and the characteristics of the pressure pipe in the following aspects: first, for the systemic characteristics, each pressure pipe, there is a certain correlation between the pressure pipe, one of the pressure pipe problems, will affect other Second, the composition of more components, compared to other types of pipelines and pressure vessels, pressure piping in the support and composition of a relatively large variety of parts, a variety of materials also have very high quality needs and technical requirements; Finally, the pressure pipeline leakage points are more, compared to the pressure vessel, there are five leakage points in each valve parts.

The main reasons for cracks in the boiler pressure vessel pressure pipe

Stress corrosion problem

Due to the actual operation of the pressure vessel in the process, the items loaded inside it are strongly alkaline, which also makes it under a certain chemical action will produce a reaction, resulting in a more significant potential difference, which is the problem of stress corrosion. At the same time, under the guidance of the role of particles, stress corrosion will also be in the current corrosion surface of the radiation spread, thus triggering the emergence of cracks. In another perspective, pressure vessels and pressure piping in the internal and external stress, the largest part of its will also be due to caustic embrittlement makes the original cracks appear a state of radiation from the inside to the outside, long-term so will lead to more serious safety accidents. Usually, due to stress corrosion and the emergence of cracks, which are presented in a radial state, its appearance is more similar to the spider web, with the characteristics of gradual radiation from the inside out, which requires staff to carry out safety inspection work in the process, to maintain a high degree of attention to the stress corrosion problem, to avoid additional economic losses to industrial enterprises, and cracks are usually with the direction of the main crack and Gradually spread outward, when the corrosion reaches a certain level will trigger a qualitative change, the main crack and the branch crack pattern is very clear, but this phenomenon of cracks due to stress corrosion, which does not lead to pressure boiler plate deformation and other problems.

Temperature runaway

In the process of boiler production, the professional level of the operator and work experience put forward high requirements to ensure that the production process does not appear in the various non-conforming operations to ensure that the boiler production to meet the relevant standard requirements. Errors in the mechanical operation can make the pressure furnace appear more subtle cracks, and in the continuous working condition gradually spread, thus leading to safety problems. For example, in the welding process, if the temperature exceeds the standard, cracks will appear under the action of high temperature, and after the welding is completed, most of the containers need to be pre-cooled, then if the time conditions and temperature conditions are not well controlled, it will make the cracks in the cooling state further spread. In the boiler production stage, cold cracks and hot cracks are two more common types of cracks, the main reason for the appearance of these two cracks is the operator’s operation is not standardized enough to accurately control the temperature, however, although the cause of these two cracks is due to the lack of professional ability of the operator, but hot cracks in the actual process, due to the contact between the air and the bottle body and Appears more obvious color, security personnel through the naked eye will be able to see, however, the color of cold crack but there is a large difference with the hot crack, it is difficult to observe with the naked eye, must use mechanical equipment to assist detection.

Specific measures to prevent cracking in the inspection of boiler pressure vessels and pressure pipes

Standardize quality inspection procedures

Quality inspection of raw materials has a very important role, and even directly affect the quality of the production of boiler pressure vessels, through the quality control of raw materials, to avoid the problem of cracking at the source. Therefore, this requires the selection of raw materials, raw material transportation and raw material processing and other aspects of the work of strict testing, and clear each raw material application standards, the premise of good inspection of parts to ensure that each part can be strictly tested to avoid the existence of a small part of the problem and the overall function of the pressure vessel damage. Therefore, from the perspective of parts production and processing links, not only need to clarify the basic quality inspection procedures, but also the production standards as the main basis for the standardization of operating procedures to prevent adverse effects on the quality of the boiler due to operational problems. In addition, in order to ensure the quality of inspection work, you can also find a third-party institutions in the community to conduct another inspection to avoid the emergence of inspection loopholes, through the multiple inspection to effectively exclude the existence of safety risks.

Improve the skill level of the operator

From an overall point of view, pressure vessels and pressure pipes in the process of operation of the crack, which is not only due to the influence of internal and external pressure, but also, and the staff’s own operating level is closely linked. Therefore, in order to effectively reduce the probability of cracking, we must focus on improving the operation level of the operator to ensure that the boiler in the start-up phase to the subsequent shutdown phase, are able to achieve a smooth transition, the temperature in the furnace for scientific and reasonable control, to prevent overcooling or overheating and other problems, so as to significantly reduce the problem of cracking caused by temperature changes. Therefore, the operator should strictly follow the requirements to ensure that the boiler can be in good operating condition, so as to effectively extend the specific life of the boiler, and the relevant industrial enterprises should also improve the threshold of boiler operation, for those who have just joined the staff to provide the necessary technical training to help them clarify the factors that can cause cracks, so as to further reduce the cracking of the boiler pressure vessel to produce adverse damage.

Increase the intensity of daily maintenance work

Daily maintenance work belongs to the key to ensure the normal operation of the boiler pressure vessel, which requires a more standardized maintenance and inspection system according to the actual situation. Firstly, the quality inspection of the production process should be standardized, and a management system with guiding and binding effects should be established; secondly, since it is difficult to prevent cracking in the working condition of the pressure pipe, it is necessary to focus on improving the emergency handling ability of the operators, releasing the pressure when the boiler is in a shutdown state, and making the necessary checks; finally, in the daily maintenance work, a Lastly, in the daily maintenance work, a detailed periodic inspection plan should be made to replace each part of the boiler pressure vessel on a regular basis.

Conclusion

In order to ensure the stable operation of the boiler pressure vessel, the necessary inspection work should be done, although in the process of boiler pressure vessel work, due to the influence of objective factors will appear different cracking problems, but still need to grasp the quality at the source, focus on improving the professional level and operating skills of the operator, to establish a more perfect daily maintenance mechanism, so as to further reduce the probability of the occurrence of cracking problems.

Authors: Wang Kegang, Wang Donghu

Source: China Pressure Vessels Manufacturer – www.secmachinery.com

References:

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• [3] Yang Qiuping, Huang Yanan research on cracking in the inspection of pressure piping of boiler pressure-excited vessels [J]China Metal Bulletin,2020(12):167-168.