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The common defects encountered in pressure vessels in chemical plants are corrosion, cracks, and deformation. Operators must conduct regular technical inspections to identify potential problems early and prevent these defects from worsening and affecting production. 01 Corrosion: Corrosion is the most common defect that occurs in pressure vessels during use, especially in chemical processing vessels. It is caused by chemical or electrochemical reactions between the metal and the medium it comes into contact with. Types of corrosion: The corrosion of containers can be uniform corrosion, pitting corrosion, intergranular corrosion, stress corrosion, and fatigue corrosion. Regardless of the type of corrosion, it can lead to the failure or destruction of the container in severe cases. Corrosion can occur on both the inner and outer surfaces of pressure vessels. The outer wall of the container is generally subject to atmospheric corrosion, which is closely related to factors such as location and season. In dry areas or seasons, atmospheric corrosion is much less severe than in humid areas or rainy seasons. Corrosion on the outer wall of pressure vessels occurs mostly in areas that are frequently moist and prone to the accumulation of water or moisture. Corrosion tends to occur at the contact surface between the container and the bracket, as well as in the areas where the container comes into contact with the ground. Corrosion on the inner wall of the container is mainly caused by the working medium or the impurities it contains. Generally, for containers in which the working medium has a significant corrosive effect, anti-corrosion measures are taken during design, such as using corrosion-resistant materials, applying surface treatments or coatings, or lining the inner walls. Therefore, corrosion on the inner walls of these containers is often caused by the failure of anti-corrosion measures. Corrosion of the inner wall of the container can also be caused by disruptions in the normal operating conditions. For example, dry chlorine does not cause corrosion to steel containers; however, if chlorine contains moisture, or if the container filled with chlorine is not dried after a hydrostatic test, or if moisture gets inside for some other reason, then chlorine reacts with water to form hydrochloric acid or hypochlorous acid, which causes severe corrosion of the container’s inner wall. Structural factors can also cause or exacerbate corrosion; for example, containers with corrosive deposits, or discharge pipes that are positioned above the bottom surface of the container, allow corrosive deposits to accumulate over time at the bottom of the container, thereby leading to corrosion. In addition, welds and heat-affected zones, as well as the areas around rivets in riveted containers and the joint areas, are places where corrosion tends to occur easily. Since the corrosion of the outer wall of the container is generally uniform or localized corrosion, it can be detected by visual inspection. For containers with an outer wall coated in a paint protective layer, if the protective layer is intact and no other suspicious signs are detected, it is generally not necessary to remove the protective layer in order to check for corrosion of the metal wall. For containers with insulation layers or other covering layers on the outside, if the insulating material does not cause corrosion to the wall material of the container, or if the container shell has an anti-corrosion coating, the insulation layer can remain in place as long as it is intact. However, if leaks or other signs that may lead to corrosion are detected, part of the insulation layer at the suspected area should be removed for inspection. The inner wall of the container may suffer from various forms of corrosion. Uniform corrosion and localized corrosion can also be detected through visual inspection. For intergranular corrosion and fracture corrosion (stress corrosion and fatigue corrosion), except that severe intergranular corrosion can be detected by hammer testing, it is generally difficult to identify through visual inspection; metallographic examination, chemical composition analysis, and hardness testing are commonly used. Generally, the lining must undergo a gas-tightness test, and any components that may interfere with this test should be removed. When uniform or localized corrosion is detected on the inner or outer wall of the container through visual inspection, the remaining thickness of the corroded area should be measured in order to determine the thickness of corrosion on the vessel wall and the corrosion rate. Treatment methods: The handling of corrosion defects depends on the specific usage conditions of the container. The general principles are as follows: (1) If defects such as intergranular corrosion or stress corrosion are detected on the inner wall, it is not advisable to continue using the container. If the corrosion is mild, use under the original operating conditions with modifications may be permitted depending on the specific circumstances. (2) When pitting corrosion is detected but it does not interfere with operational procedures (no cracks are present and the corrosion depth is less than half of the calculated wall thickness), the defect may be left untreated and the component may continue to be used. (3) For uniform corrosion and local corrosion, it is determined whether to allow continued use, reduce the inspection interval, operate at reduced pressure, or declare the material unusable, based on the principle that the remaining thickness is not less than the calculated thickness. 02 Cracks: Cracks are the most dangerous type of defect in pressure vessels; they are responsible for causing brittle failure in these vessels, and they also contribute to the occurrence of fatigue fractures and corrosion-induced failures. Types of cracks: Cracks in pressure vessels can be roughly divided into two categories based on their formation process: cracks that arise during the use of raw materials or during vessel manufacturing, and cracks that occur or spread during the vessel’s operation. The former includes rolling cracks in steel plates, drawing cracks in containers, welding cracks, and stress-relief heat treatment cracks ； The latter includes fatigue cracks and stress corrosion cracks. Rolling cracks in raw materials are linear defects that arise during rolling due to the accumulation of defects present in the metal material itself, such as porosity, shrinkage cavities, and non-metallic inclusions. This defect can be located inside the material or on its surface, with no definite direction or fixed location. Similar cracks are also often found in some drawn small high-pressure vessels. Welding cracks mainly occur during the container manufacturing process, either due to lax quality control by the manufacturers or because minor existing defects go unnoticed and develop over time during use. Stress-relief heat treatment cracks are branch-like intergranular cracks that occur during stress-relief heat treatment after welding, and they can also expand over time as the material is in use. Fatigue cracks occur due to poor structural design of the container or defects in the material, which result in excessive local stress; these cracks appear after the container is subjected to repeated cycles of pressurization and depressurization. Such cracks can be found in pressure vessels that are frequently opened and closed. Corrosion cracks are gradually formed as a result of the corrosion agent attacking the material under certain operating conditions; such cracks are often related to stress. This is because stress and corrosion reinforce each other: corrosion creates notches on the material’s surface, leading to stress concentration or weakening of the intergranular bonds in the metal, while stress accelerates the progression of corrosion, causing the surface notches to deepen. Although cracks in pressure vessels can occur in various areas on their inner and outer surfaces, the areas most prone to cracking are typically the welds and the heat-affected zones of welding, as well as those subjected to excessive local stress. Treatment methods: The inspection of cracks can be carried out through visual inspection and non-destructive testing. Generally, signs of cracks are detected or initially identified through visual inspection, and then confirmed further through non-destructive testing. Non-destructive testing methods, whether it is liquid penetrant testing, fluorescent testing, or magnetic testing, are highly effective for detecting surface cracks, and can be selected appropriately depending on the specific circumstances. When cracks are detected in a pressure vessel, it is first necessary to analyze the causes of these cracks based on their location, quantity, size, distribution, as well as the operating conditions of the vessel. If needed, metallographic testing can be conducted to determine whether the cracks are due to defects in the raw materials, remnants from the vessel’s manufacturing process, or resulting from use over time. Then, the method for dealing with the defect or the container containing the defect is determined based on the severity of the defect and the specific conditions of the container. The micro-cracks left in containers due to material rolling or drawing are generally shallow and can be removed using a hand file or grinder. Weld cracks should be removed as soon as they are detected during inspection. Components that develop cracks due to poor structure and excessive local stress are generally not suitable for continued use. Containers with corrosion cracks should not be used either after the cracks have been removed or welded up. Under special circumstances, when cracks remaining from container manufacturing or raw materials are indeed difficult to eliminate, and after inspection and assessment by a qualified pressure vessel defect evaluation agency, as well as through analysis and calculations based on fracture mechanics, it is confirmed that the cracks will not propagate and that there is sufficient safety margin, the container can continue to be used under reliable monitoring measures; however, the inspection interval must be shortened, and close surveillance of the development of the cracks is required. 03 Deformation: Deformation refers to a change in the geometric shape of a container, either overall or in certain parts, after it has been in use. This type of defect is relatively rare in pressure vessels. Types of deformation: The deformation of containers generally manifests in forms such as local dents, bulges, overall flattening, and overall swelling. Local depression is a surface indentation that occurs in a specific area of a container shell or head as a result of external impact or compression. This type of deformation generally only occurs in small containers with thin shell walls; it does not cause any change in the wall thickness, but rather results in a loss of the original geometric shape of that particular area of the surface. A bulge is an outward protrusion that occurs when the pressure-bearing surface of a certain part of a container experiences severe corrosion, resulting in a significant reduction in wall thickness, and thus under the effect of internal pressure. In some cases, excessively high local temperatures in the container can also lead to a decrease in the mechanical properties of the material, resulting in bulging; such deformation further reduces the wall thickness in that area of the container. The overall flattening occurs because the wall thickness of the shell, under external pressure, is too thin, resulting in a loss of stability and the loss of its original shape; this deformation takes place only in those parts of the container that are subjected to external pressure, such as the inner cylinder of a jacketed container. Overall expansion deformation occurs because the wall thickness of the container is too thin or the container is used under excessive pressure, resulting in yield deformation of the entire container or certain cross-sections. This type of deformation generally occurs slowly and can only be detected through special monitoring. Treatment method: Deformation can generally be checked by visual inspection, and less severe deformations can be detected using measuring tools. Containers with deformation defects, aside from less severe local dents, are generally not suitable for continued use. This is because in containers that have undergone plastic deformation, the wall thickness always decreases to some extent, and the deformed material also loses its toughness due to strain hardening; furthermore, its corrosion resistance is reduced. For minor bulging deformations, if the affected area is not large and it does not affect other parts of the container, patching can be considered provided that the weldability of the container material is good. The area that is about to bulge out is removed, and then a patch made of the same shape and material is welded in its place. After welding, the weld is inspected in accordance with the original technical specifications for the container.