Common defects in pressure vessels and how to deal with them

Pressure vessels in chemical plants are often encountered with defects such as corrosion, cracks and deformation. Operators must carry out regular technical inspections to eliminate hidden problems as early as possible and prevent defects from continuing to develop and expand, which can have an impact on production.

Corrosion is one of the most common defects in pressure vessels during use, especially in chemical vessels. It is caused by chemical or electrochemical changes in the metal and the medium it is in contact with.

Corrosion types

The corrosion of the container can be uniform corrosion, pitting corrosion, intergranular corrosion, stress corrosion and fatigue corrosion. No matter which kind of corrosion, when serious, will lead to the failure or destruction of the container.
Corrosion can occur on both the internal and external surfaces of the pressure vessel. The outer wall of the container is generally the corrosion of the atmosphere, the corrosion of the atmosphere and the region and the season, etc. have a close relationship, in dry areas or season, the corrosion of the atmosphere than the humid areas or rainy season is much lighter.

Corrosion of the outer wall of the pressure vessel is often produced in a wet state and easy to accumulate moisture or moisture parts. In the container and the contact surface of the support, the container and the ground contact part is easy to produce corrosion. The corrosion of the inner wall of the container is mainly due to the working medium or it contains the role of impurities and generated. Generally speaking, the working medium has obvious corrosive effect of the container, design are taken to prevent corrosion measures, such as the choice of corrosion-resistant materials, surface treatment or surface coating, lining the inner wall, etc.. Therefore, the corrosion of the inner wall of these containers is often caused by the destruction of anti-corrosion measures.

Corrosion of the inner wall of the container may also be due to the normal process conditions are destroyed and caused, for example, dry chlorine on the steel container does not produce a corrosive effect, and if the chlorine gas contains moisture or filled with chlorine gas container because of the water pressure test is not dry, or for other reasons into the moisture, the chlorine gas and water to generate hydrochloric acid or hypochlorous acid, the inner wall of the container to produce a strong corrosive effect.

Due to structural reasons can also cause or exacerbate the corrosion effect, for example, with corrosive deposits of the container, the discharge pipe is higher than the bottom plane of the container, so that the bottom of the container long-term accumulation of corrosive deposits, thus producing corrosion.

In addition, welding seams and heat-affected areas, riveted containers around the rivets and seam areas are more likely to produce corrosion.

As the outer wall of the container corrosion is generally uniform corrosion or local corrosion, with the visual inspection method can be found. Outer wall painted protective layer of the container, if the protective layer is intact, and no other suspicious signs found, generally do not need to remove the protective layer to check the corrosion of the metal wall.

Outside there is insulation or other covering layer of the container, if the insulation material on the wall material no corrosion effect, or container shell with corrosion protection layer, in the case of insulation intact, can also not remove the insulation layer, but if you find leakage or other signs of corrosion may cause, then at least in the suspicious place to remove part of the insulation layer for inspection.

There may be various forms of corrosion on the inside of the vessel. Uniform and localised corrosion can also be inspected by visual inspection. Intergranular corrosion and fracture corrosion (stress corrosion and fatigue corrosion), in addition to serious intergranular corrosion can be found with hammering inspection, generally with visual inspection is difficult to determine, commonly used metallographic examination, chemical composition analysis and hardness determination. General lining for gas tightness test, test when there are obstacles to test the components should be removed.

By visual inspection found that the inner or outer wall of the container has uniform corrosion or local corrosion should be measured where the remaining thickness of the corrosion, so as to determine the corrosion thickness of the wall and corrosion rate.

Treatment method

The treatment of corrosion defects should be based on the specific use of the container, the general principles are.

(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, allowed according to the specific circumstances, in changing the original working conditions used.

(2) when found scattered pitting corrosion, but does not hinder the process operation (no cracks, corrosion depth less than half of the calculated wall thickness), the defect can be continued to use without treatment.

(3) uniform corrosion and local corrosion according to the principle of the remaining thickness is not less than the calculated thickness, determine its continued use, reduce the inspection interval period, reduce the pressure to use or sentenced to waste.

2 Crack

Crack is one of the most dangerous defects in the pressure vessel, it is a factor leading to brittle damage of the vessel, while promoting fatigue rupture and corrosion rupture.
Types of cracks

Cracks in pressure vessels, according to their generation process, can be broadly divided into two categories, namely cracks generated in the manufacture of raw materials or vessels and cracks or extended cracks generated during the use of the vessel. The former includes rolling cracks in steel plates, pulling cracks in containers, welding cracks and stress relief heat treatment cracks; the latter includes fatigue cracks and stress corrosion cracks.
Raw material rolling cracks are linear defects generated by rolling due to the accumulation of defects such as looseness, shrinkage and non-metallic inclusions that exist in the metal material itself. This defect can be in the interior of the material, but also on the surface, without a certain direction and a fixed part. Similar cracks are often found in some of the small high-pressure vessels that are drawn.
Welding cracks are mainly produced during the manufacturing process of the container, which is due to poor quality inspection of the container manufacturing plant, or the original defects are not found slightly and developed during the use of the process.
Stress relief heat treatment crack is a branched intergranular crack, which is produced during the post-weld stress relief heat treatment and can also be extended in use.
Fatigue cracks are caused by poor structure or material defects in the vessel, resulting in high local stresses and cracks that occur after the vessel has been repeatedly pressurised or depressurised several times, and can be found in some pressure vessels that are opened and stopped frequently.
Corrosion crack is a corrosive medium in certain working conditions, corrosion of the material and the gradual formation of such cracks are often related to stress. Because the stress and corrosion of both promote each other, the latter in the material surface to form a gap to produce stress concentration, or weaken the metal intergranular bond, while the former accelerates the progress of corrosion, so that the surface gap to the depth of development.
Although cracks in the pressure vessel may exist in various parts of its internal and external surfaces, but generally the most likely place to produce cracks is the weld and welding heat-affected zone and the local stress is too high parts.

Treatment
Cracks can be inspected by visual inspection and non-destructive testing. Signs of cracking are generally found or initially detected by visual inspection and then further confirmed by non-destructive testing. Non-destructive testing, whether it is liquid penetrant testing, fluorescence testing and magnetic testing, has a high utility for checking surface cracks and can be used appropriately according to the specific situation.

When the pressure vessel is found to have a crack defect, the first should be based on the crack where the part, number, size, distribution and working conditions of the container and other analysis of the reasons for the crack, if necessary, can be metallurgical examination, to determine the crack is a raw material defects, or container manufacturing left behind, or the use of the process generated. Then according to the severity of the defects and the specific circumstances of the container to determine the defects or the presence of defects in the container processing method.

Micro-cracks left behind by rolling or drawing the container due to the material are generally shallow and can be ground away with hand files or grinding wheels etc. Welding cracks should be eradicated when they are found during inspection.

Cracked parts due to poor structure and high local stress are generally not suitable for further use. Containers with corrosion cracks should not be used after the cracks have been removed or welded.

In special cases, due to the manufacture of containers or raw materials left behind by the crack is indeed difficult to eliminate, after qualified pressure vessel defects assessment unit inspection and identification, and according to the analysis and calculation of fracture mechanics, to confirm that the crack will not expand, and has a sufficient margin of safety, the container can take reliable guardianship measures, continue to use, but to shorten the inspection interval, close monitoring of the development of cracks.

3 Deformation

Deformation refers to the vessel in use after the overall or local place to change the geometry, this defect is generally in the pressure vessel is relatively rare.

Deformation types

The deformation of the vessel can generally be expressed as a local depression, bulging, overall flat deflated, overall expansion and several other forms.

Local depression is a local area of the container shell or head by the impact of external forces or extrusion and thus the occurrence of surface depression, this deformation can generally only be produced in the shell wall is thin on the small container, it does not cause the container wall thickness change, but only to make a local surface lost the original geometric shape.

Drum package is a part of the container pressure surface due to severe corrosion, wall thickness significantly thinned, and thus the role of the internal pressure occurred under the outward convex deformation. Individual cases can also be due to the local temperature of the container is too high, resulting in the mechanical properties of the material to reduce the drum, this deformation will make the container this area of the wall thickness to further reduce the thinning.

Overall flat deflated because of the external pressure of the shell wall thickness is too thin, so that the loss of stability under pressure, the loss of the original shell shape, this deformation only occurs in the container by the external pressure components, such as jacketed containers of the inner tube.

Overall expansion deformation is because the container wall thickness is too thin or overpressure use, resulting in the whole container or some cross-section yield deformation and caused by. This deformation is generally slow and can only be detected under special monitoring.

Processing methods

The inspection of deformation is generally available for visual inspection, less serious deformation can be detected by gage inspection.
Deformation defects of the container, in addition to less serious local dents, other than the general should not continue to use. Because after the plastic deformation of the container, the wall thickness is always different degrees of thinning, and deformation of the material will also be due to strain hardening and reduce the habit toughness, corrosion resistance is also poor.
For the slight drum package deformation, if the deformation area is not too large, and did not affect the other parts of the container, then in the case of container material weldability is good, you can consider the use of digging patch processing. That is, the part of the local drum bag digging, and then use the same shape and material of the plate to fill the welding, welding according to the original technical requirements of the container technical inspection of the weld.
Source: Chemical Equipment Man
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