[introduction] defects of pressure vessel equipment manufacturing plant in the often encountered is corrosion, cracks and deformation, the operator must conduct regular inspection, eliminate hidden dangers and prevent defects as soon as possible, continue to expand, the impact on production.
First, corrosion
Corrosion is one of the most easily produced defects in the use of pressure vessels, especially in chemical containers. It is caused by the chemical or electrochemical action of the metal in contact with the medium.
Types of corrosion
Corrosion of the container can be uniform corrosion, pitting corrosion, intergranular corrosion, stress corrosion and fatigue corrosion. No matter what kind of corrosion, serious when will lead to the failure or failure of the container.
1, the inner and outer surfaces of the pressure vessel can be corroded. The outer wall of the container is generally the atmospheric corrosion, the corrosive action of the atmosphere and regional and seasonal and so on have a close relationship, in dry area or season, atmospheric corrosion is much more than the wet area or rainy season.
2, pressure vessel wall corrosion arising from often in the wet state and easy accumulation of water or damp place. Part of the contact surface between the container and the bracket, the container and the ground contact is prone to corrosion. The corrosion of the inner wall of the container is mainly caused by the action of the working medium or the impurities contained in it. Generally speaking, the working medium has obvious corrosion of the container, design are to take anti-corrosion measures, such as the selection of corrosion-resistant materials, surface treatment or surface coating, in the inner wall and lining, etc.. As a result, the corrosion of the inner walls of the containers is often caused by the destruction of the corrosion protection measures.
3, the corrosion of the inner wall of the container can also be caused by the destruction of the normal conditions, such as dry chlorine on the steel container has no corrosive effect, and if the chlorine containing water or filling containers for chlorine after hydraulic test without drying, or due to other reasons, into the water, chlorine and water effect the generation of hydrochloric acid or sodium hypochlorite, has strong corrosive effect on the inner wall of the container.
4, because of the structure can cause or exacerbate corrosive effects, for example, container with corrosive deposits, discharge pipe is higher than the bottom plane of the container, the container at the bottom of the long-term accumulation of sediment corrosion, resulting in corrosion.
5, in addition, the weld and heat affected zone, rivet joints and joints around the joint area are relatively easy to produce corrosion places.
processing method
The treatment of corrosion defects should be based on the specific use of the container, the general principle is that the:
1, the inner wall found intergranular corrosion, fracture, corrosion and other defects, it is not easy to continue to use. If the corrosion is slight, allow the use of the original operating conditions under the conditions of the specific circumstances.
2, when it is found that the dispersion point of corrosion, but does not interfere with the process of operation (no cracks, corrosion depth is less than half the thickness of the calculation of the wall), can not be treated to continue to use the defect.
3, uniform corrosion and local corrosion is not less than the thickness of the residual thickness of the principle of calculation, to determine its continued use, reduce the test interval, the use or the use of.
Two, crack
The crack is one of the most dangerous defects in the pressure vessel, which is the cause of the brittle failure of the container, and also promotes the generation of fatigue cracking and corrosion cracking.
Types of crack
Cracks in pressure vessels, according to their formation process, can be roughly divided into two categories, namely, the production of raw materials or containers in the production of cracks and containers used in the process of crack or crack growth. The former includes the rolling crack of steel plate, the drawing crack of the vessel, the welding crack and the elimination of stress heat treatment crack. The latter includes fatigue crack and stress corrosion crack.
2, the raw material of rolling crack is due to loose, metal material itself has the defects such as shrinkage and non metallic inclusions together, linear defects generated by rolling. This defect can be in the interior of the material, can also be in the surface, without a certain direction and fixed parts. Similar cracks are often found in small - sized high - pressure vessels.
3, welding crack is mainly produced in the process of container manufacturing, which is due to the container manufacturing factory quality inspection is not strict, or the original defects were slightly not found in the use of the process of development.
4, the elimination of stress heat treatment crack is a kind of branching intergranular crack, is in the welding process to eliminate stress after heat treatment, but also in the use of.
5, because the container structure fatigue crack is bad or material defects caused by local high stress cracks in the container after repeated pressure or pressure relief, pressure vessel in some frequently can be found in the cracks.
6, corrosion cracking is a corrosive medium in a certain working conditions, corrosion of the material and gradually formed, the crack is often related to the stress. Because of both stress and corrosion promote each other, the latter is formed on the surface of the material notch stress concentration occurs, or weaken the metal intergranular binding force, and the former is to accelerate the progress of corrosion, the surface to the deep development gap.
7, the crack of the pressure vessel is likely to exist in all parts of the inner and outer surface, but it is usually the most easy to produce cracks in the weld and weld heat affected zone and the local stress is too high.
processing method
1, crack inspection can be used to visually inspect and nondestructive testing. In general, through the visual inspection found or initially found signs of cracks, and then further confirmed by nondestructive testing. Non destructive testing, whether it is the liquid penetration test, fluorescence detection and magnetic inspection, to check the surface cracks have a high effect, can be selected according to the specific circumstances.
When the pressure vessel crack, cause should be generated according to the crack location, quantity, size, distribution and container work conditions analysis of crack, when necessary for metallographic examination, to determine the crack defects in raw materials, stay or container during the manufacture process, or use in. Then, according to the severity of defects and the specific circumstances of the container to determine the defects or defects in the container handling method.
2, due to micro crack container material rolling or pull out the left, are generally shallow, can be rubbed off by hand filing or grinding wheel etc.. Welding cracks should be eliminated when the inspection found.
3, due to poor structure, local stress is too highCrack components are generally not suitable to continue to use. The presence of corrosion cracks in the container, and should not be eliminated or welded to continue to use.
4, in the special case, because the crack left container manufacturing or raw materials is difficult to eliminate, after unit defect assessment of Pressure Vessels Inspection qualified identification, and according to the analysis and calculation of fracture mechanics, the crack will not confirm the extension, and has enough safety margin, the container can be taken care, reliable to use but, to shorten the inspection intervals, closely monitor the crack development.
Three, deformation
Deformation is the change of the geometry of the vessel after the use of the whole or part of the container, which is relatively rare in the pressure vessel.
Type of deformation
Generally, the deformation of the container can be shown as a part of the form of the local sag, the bulging, the flat flat, and the whole expansion.
1, local depression is a local area vessel or head by external impact or extrusion surface depression occurred as a result of this deformation, usually only in a small container shell with thin wall, it does not cause the container wall thickness changes, but only to a local surface lost the original geometry.
2, the drum package is a part of the pressure side of a part of the pressure surface due to severe corrosion, wall thickness significantly reduced, and thus the internal pressure occurred under the influence of the outward bulge deformation. In some cases, the local temperature of the container is too high, resulting in the reduction of the mechanical properties of the material and the drum package, which will further reduce the thickness of the wall of the container.
3, the whole flat shell wall is subjected to external pressure because the thickness is too thin, resulting in loss of stability under pressure, the loss of the original shape of the casing, the deformation occurs only under external pressure vessel components, such as jacket vessel tube.
4, the overall expansion deformation is because the container wall thickness is too thin or super pressure use, resulting in the entire container or some section yield deformation caused by. This kind of deformation is generally slow, only in the special monitoring can be found.
processing method
1, the deformation of the general inspection can be directly checked, less severe deformation can be found through the measuring tool.
2, resulting in deformation of the container, in addition to less serious local depression, the other is generally not suitable to continue to use. Because the plastic deformation of the container, the wall thickness of a total of varying degrees of thinning, and the deformation of the material will also be due to strain hardening and reduce toughness, corrosion resistance is poor.
3, the slight bulge deformation, if the deformation area is not too large, but also does not affect other parts of the container, the container is in good condition of welding materials, you can consider using combined treatment. Will be part of the drum package to dig, and then the same shape and material plate for repair welding, after welding, according to the original technical requirements of the vessel to carry out technical inspection of the weld.