Repair of pressure vessels in use

This post was last edited by sifangok on 2016-11-3 at 22:41. I haven’t been able to find the reason for the cracks that frequently appear in those pressure vessels; I definitely wouldn’t dare to use them any longer. Check where the cracks appear; if they occur at areas that are not weld points, it may be necessary to discard the item. If cracks appear in large numbers at the weld points as well, discarding it is an option. Regardless of the cause, as long as it does not exceed the design specifications, have the manufacturing and design teams take a look first. Cracks are the most dangerous type of defect in pressure vessels; they are the cause of brittle failure in such vessels, and they also contribute to the occurrence of fatigue fractures and corrosion-induced failures. Cracks in pressure vessels can be roughly divided into two categories based on their formation process: cracks that arise from the raw materials or during vessel manufacturing, and cracks that occur or propagate 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 as well as on its surface, without any specific 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 container 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 during 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 operated and shut down. 　　Corrosion cracks are gradually formed as a result of the corrosion of a material by corrosive agents 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 generally the welds and the weld heat-affected zones, as well as those subjected to excessive local stress. 　　Crack inspection can be performed 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 the appropriate method can be chosen based on 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 with the defect is determined based on the severity of the defect and the specific conditions of the container. The micro-cracks left by rolling or drawing the container from the material are usually 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 generally cannot be used further. Containers with corrosion cracks should not be used either after the cracks have been removed or welded shut. 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.

This post was last edited by sifangok on 2016-11-3 at 23:13. Problems occur frequently; it is recommended to find ways to collect evidence in order to file a claim

To find out the cause, it comes down to three aspects: design, manufacturing, and use. A few years ago, in our plant, extensive cracks appeared in the sections near the lower head of the steam buffer tank for three consecutive days. Initial analysis suggested that frequent start-up and shutdown operations, coupled with delayed drainage, led to increased temperature fluctuations at the interface between steam and liquid. Later, a level tank was installed to automatically drain the condensate, and since then the system has been operating safely for 4 or 5 years.

A few years ago, in our plant, extensive cracks appeared in the sections near the lower head of the steam buffer tank for three consecutive days. Initial analysis suggested that frequent start-up and shutdown operations, coupled with delayed drainage, led to increased temperature fluctuations at the interface between steam and liquid. Later, a level tank was installed to automatically drain the condensate, and since then the system has been operating safely for 4 or 5 years. This of yours is probably a fatigue crack

For pressure vessels that have developed cracks, as long as the damage is not severe, they should be repaired and continued to be used rather than being replaced outright

API 571 (Volumes 1 and 2) Mechanisms of Damage in Refineries

If a pressure vessel develops cracks, it is advisable to seek the help of professional manufacturers to deal with the issue. In addition to manufacturing defects, operating conditions can also be a cause of cracks. Cracks represent the most dangerous type of defect in pressure vessels, and they must be addressed as soon as possible!

Whether the medium used does not match the material of the container, or it is used for a long time at the design threshold

It must be dealt with as soon as possible, and it should be repaired by a qualified company that keeps repair records and takes photographs, etc