Common defects and treatment methods for pressure vessels
Pressure vessels in chemical plants often encounter defects such as corrosion, cracks and deformation. Operators must conduct regular technical inspections to eliminate hidden dangers as early as possible to prevent defects from continuing to develop and affect production.
Corrosion is one of the most prone to defects in the use of pressure vessels, especially in chemical containers. It is caused by chemical or electrochemical changes in the metal and the medium it is in contact with.
The corrosion of the corrosive type container may be uniform corrosion, pitting corrosion, intergranular corrosion, stress corrosion, and fatigue corrosion. Regardless of the type of corrosion, it can cause failure or damage to the container in severe cases.
Corrosion can occur on both the inner and outer surfaces of the pressure vessel. The outer wall of the container is generally corrosive to the atmosphere. The corrosive effect of the atmosphere is closely related to the area and the season. In dry areas or seasons, the atmospheric corrosion is much milder than in wet areas or rainy seasons.
Corrosion of the outer wall of the pressure vessel is often caused by a portion that is often in a wet state and is prone to accumulation of moisture or moisture. Corrosion is likely to occur at the contact surface of the container with the holder and the portion of the container in contact with the ground. Corrosion of the inner wall of the container is mainly caused by the working medium or the impurities it contains. In general, containers with a corrosive effect on the working medium are designed to be protected against corrosion, such as the use of corrosion-resistant materials, surface treatment or surface coating, and lining the inner wall. Therefore, the corrosion of the inner walls of these containers is often caused by the destruction of corrosion protection measures.
Corrosion of the inner wall of the container may also be caused by the destruction of normal process conditions. For example, dry chlorine does not corrode the steel container, and if the chlorine contains moisture or the container filled with chlorine is not dried after the hydrostatic test. Or, if it enters moisture for other reasons, chlorine gas reacts with water to form hydrochloric acid or hypochlorous acid, which has a strong corrosive effect on the inner wall of the container.
Corrosion can also be caused or exacerbated by structural reasons, for example, a container with corrosive deposits, which is higher than the bottom plane of the container, causing long-term accumulation of corrosive deposits on the bottom of the container, thereby causing corrosion.
In addition, welds and heat-affected zones, rivets around the riveted container, and seam areas are relatively susceptible to corrosion.
Since the corrosion of the outer wall of the container is generally uniform or localized, it can be found by visual inspection. The outer wall is painted with a protective coating. If the protective layer is intact and no other suspicious signs are found, it is generally not necessary to remove the protective layer to check the corrosion of the metal wall.
A container with an insulating layer or other covering layer on the outside. If the insulating material has no corrosive effect on the material of the wall, or the container shell has an anti-corrosion layer, the insulating layer may not be removed if the insulating layer is intact, but if the leak is found, Or other signs that may cause corrosion, at least in the suspect location to remove part of the insulation for inspection.
The inner wall of the container may have various forms of corrosion. Uniform corrosion and localized corrosion can also be checked by visual inspection. For intergranular corrosion and fracture corrosion (stress corrosion and fatigue corrosion), in addition to severe intergranular corrosion, it can be found by hammer inspection. It is difficult to judge by visual inspection. Metallographic examination, chemical composition analysis and hardness are commonly used. Determination. Generally, the lining shall be tested for hermeticity, and the members that obstruct the inspection shall be removed during the inspection.
After visual inspection, it is found that the residual thickness of the corroded part should be measured when the inner wall or the outer wall of the container is uniformly corroded or partially corroded, thereby determining the corrosion thickness and corrosion rate of the wall.
Treatment methods Corrosion defects are treated according to the specific use of the container. The general principles are:
(1) When defects such as intergranular corrosion and fracture corrosion are found on the inner wall, it is difficult to continue to use. If the corrosion is slight, it is allowed to be used under the original working conditions depending on the specific situation.
(2) When the dispersion point corrosion is found, but does not hinder the process operation (no crack, the corrosion depth is less than half of the calculated wall thickness), the defect can be left untreated.
(3) Uniform corrosion and local corrosion According to the principle that the remaining thickness is not less than the calculated thickness, it is determined to continue to use, reduce the inspection interval period, reduce the pressure or use it.
Cracks are one of the most dangerous defects in pressure vessels, which are factors that cause brittle fracture of the vessel, while at the same time promoting fatigue cracking and corrosion cracking.
Crack Types The cracks in the pressure vessel can be roughly divided into two categories according to the formation process, that is, cracks generated in the manufacture of raw materials or containers and cracks or extended cracks generated during the use of the container. The former includes rolling cracks of steel sheets, cracking of containers, weld cracks, and stress-relieving heat treatment cracks; the latter includes fatigue cracks and stress corrosion cracks.
The raw material rolling crack is a linear defect generated by rolling due to the accumulation of defects such as looseness, shrinkage cavities and non-metallic inclusions existing in the metal material itself. This defect can be in the interior of the material or on the surface without any directionality or fixed location. Similar cracks are often found in some small, high-pressure containers that are drawn.
Welding cracks are mainly produced during the manufacturing process of the container. This is due to the fact that the quality inspection of the container manufacturer is not strict, or the original defects are slightly undetected and developed during use.
Stress-relieving Heat-treated cracks are branched intergranular cracks that are produced during post-weld stress relief heat treatment and can also be extended during use.
Fatigue cracks are caused by poor structure of the container or defects in the material, causing local stresses to be too high. Cracks are generated after repeated pressurization or pressure relief of the container. Such cracks can be found in some pressure vessels with frequent opening and closing. .
Corrosion cracks are formed by corrosion of a corrosive medium under certain working conditions. The cracks are often related to stress. Because both stress and corrosion promote each other, the latter creates a stress concentration on the surface of the material, or weakens the intergranular bonding force of the metal, while the former accelerates the progress of corrosion and causes the surface notch to develop deeper.
Although the crack of the pressure vessel may exist in various parts of its inner and outer surfaces, the most common crack occurrence is the weld and the heat affected zone and the local stress.
Treatment methods Crack inspection can be performed by visual inspection and non-destructive inspection. It is generally found through visual inspection that the signs of cracks are found or initially detected and then further confirmed by non-destructive testing. Non-destructive testing, whether it is liquid penetrating flaw detection, fluorescent flaw detection and magnetic flaw detection, has a high effect on the inspection of surface cracks, and can be appropriately selected according to specific conditions.
When the pressure vessel is found to have crack defects, the cause of the crack should be analyzed according to the location, quantity, size, distribution of the crack and the working conditions of the vessel. If necessary, metallographic examination can be carried out to judge the crack as the defect of the raw material. , or left in the manufacture of the container, or produced during use. The defect or the container handling method for the defect is then determined based on the severity of the defect and the specific conditions of the container.
Due to the microcracks left by the rolling or drawing of the material, it is generally shallow and can be rubbed by hand or by a grinding wheel. Welding cracks should be removed when inspections are found.
Parts that are cracked due to poor structure and excessive local stress are generally not suitable for continued use. Containers with corrosive cracks should not be used after the cracks are removed or repaired.
In special cases, cracks caused by container manufacturing or raw materials are indeed difficult to eliminate, and are inspected and qualified by a qualified pressure vessel defect assessment unit, and according to the analysis and calculation of fracture mechanics, it is confirmed that the crack does not expand and is sufficiently safe. Margins, containers can be reliably monitored and continue to be used, but the inspection interval should be shortened to closely monitor the development of cracks.
Deformation refers to the change in geometry of the container in whole or in part after use. Such defects are generally rare in pressure vessels.
The deformation of the deformed type of container can generally be expressed as several forms such as partial depression, bulge, integral ridge, and overall expansion.
The partial depression is a surface concavity which occurs when a partial area of the container casing or the head is impacted or pressed by an external force, and the deformation is generally only generated on a small container having a thin shell wall, which does not cause the wall thickness of the container. The change, but only the loss of the original geometry of a partial surface.
The bulge is a part of the pressure receiving surface of the container due to severe corrosion, the wall thickness is significantly reduced, and thus the outward convex deformation occurs under the action of internal pressure. In some cases, the bulge may be generated due to the local temperature of the container being too high, resulting in a decrease in the mechanical properties of the material, which deformation will further reduce the wall thickness of the region of the container.
The overall flattening is because the wall thickness of the shell subjected to external pressure is too thin, so that it loses stability under pressure and loses the original shell shape. This deformation only occurs in the externally pressed parts of the container, such as jackets. The inner cylinder of the container.
The overall expansion deformation is caused by the wall thickness of the container being too thin or overpressure, resulting in yield deformation of the entire container or some sections. This deformation is generally slow and can only be discovered with special monitoring.
The inspection of the deformation of the treatment method is generally available for visual inspection, and the less severe deformation can be found by gage inspection.
Containers that produce deformation defects, except for less severe local depressions, are generally not suitable for continued use. Because of the plastically deformed container, the wall thickness always has different degrees of thinning, and the deformed material also reduces the toughness due to strain hardening, and the corrosion resistance is also poor.
For slight bulging deformation, if the deformation area is not too large and does not affect other parts of the container, the excavation treatment may be considered in the case where the container material has good weldability. The part of the partial bulge is to be dug, and the plate of the same shape and material is used for repair welding. After welding, the weld is technically tested according to the original technical requirements of the container.
Source: China Pressure Vessels Manufacturer – www.secmachinery.com
www.secmachinery.com is one of the leading china filtration equipment & sanitary stainless steel pressure vessels manufacturer, with professional factory. We focus on the sanitary pressure vessels, filtration equipment research and development, manufacturing, sales and service since the company been established. Our products is widely used on bio-pharmaceutical, food and beverage, fine chemical industry.
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